# Patent

## Title
High strength steel sheet and high strength galvanized steel sheet excellent in shapeability and methods of production of same

## Abstract

High strength steel sheet and high strength galvanized steel sheet which are excellent in shapeability which secure a tensile maximum strength 900 MPa or more high strength while obtaining excellent ductility and stretch flangeability, which sheets have predetermined compositions of ingredients, have steel sheet structures which contain volume fraction 1 to 20% of residual austenite phases, and which have martensite transformation points of the residual austenite phases of −60° C. or less.

## Background
TECHNICAL FIELD
The present invention relates to high strength steel sheet and high strength galvanized steel sheet which are excellent in shapeability and to methods of production of the same.
BACKGROUND ART
In recent years, there have been increasing demands for higher strength in the steel sheet which is used for automobiles etc. In particular, for the purpose of improving collision safety etc., high strength steel sheet with a tensile maximum stress of 900 MPa or more is also being used. Such high strength steel sheet is inexpensively formed in large volumes by press working it in the same way as soft steel sheet and is being used as structural members.
However, in recent years, along with the rapid increase in strength of high strength steel sheet, in particular in high strength steel sheet with a tensile maximum stress of 900 MPa or more, the problem has arisen of the shapeability becoming insufficient and of working accompanied with local deformation such as stretch-formability becoming difficult. For this reason, in high strength steel sheet with a high tensile maximum stress as well, realization of sufficient workability has become demanded.
PLT 1 discloses, as art for improving the bendability of high strength steel sheet, steel sheet with a tensile strength of 780 to 1470 MPa, a good shape, and excellent bendability which is obtained by taking steel sheet which has a microstructure mainly comprised of bainite or tempered martensite, making the amount of Si which is contained in the steel, by mass %, 0.6% or less, cooling down to a temperature at least 50° C. lower than a predetermined bainite transformation temperature to promote transformation from austenite to bainite or martensite and thereby rendering the volume rate of residual austenite which is contained in the structure and has a martensite transformation point of −196° C. or more 2% or less.
PLT 2 discloses, as art for improving the shapeability of high strength steel sheet, the method of improving the ductility and stretch flangeability by cooling steel sheet which has been hot rolled down to 500° C. or less, coiling it, then reheating it to 550 to 700° C., then successively performing a cold rolling process and continuous annealing process so that a second phase which contains residual austenite and further contains a low temperature transformation phase becomes fine in average particle size and so that the amount of residual austenite, amount of solid solution C in the residual austenite, and average particle size satisfy predetermined relationship formulas.
PLT 3 discloses, as art for improving the stretch flangeability of high strength steel sheet, steel sheet which is reduced in standard difference in hardness inside of the steel sheet and which is given equivalent hardness in the entire steel sheet region.
PLT 4 discloses, as art for improving the stretch flangeability of high strength steel sheet, steel sheet which is reduced in hardness of hard portions by heat treatment and which is reduced in hardness difference with the soft parts.
PLT 5 discloses, as art for improving the stretch flangeability of high strength steel sheet, rendering the hard portions the relatively soft bainite so as to reduce the difference in hardness from soft parts.
PLT 6 discloses, as art for improving the stretch flangeability of high strength steel sheet, steel sheet which has a structure comprised of, by area rate, 40 to 70% of tempered martensite and a balance of ferrite where a ratio between an upper limit value and a lower limit value of a concentration of Mn in the cross-section of the thickness direction of the steel sheet is reduced.
CITATIONS LIST
Patent Literature


    PLT 1: Japanese Patent Publication No. 10-280090A
    PLT 2: Japanese Patent Publication No. 2003-183775A
    PLT 3: Japanese Patent Publication No. 2008-266779A
    PLT 4: Japanese Patent Publication No. 2007-302918A
    PLT 5: Japanese Patent Publication No. 2004-263270A
    PLT 6: Japanese Patent Publication No. 2010-65307A


SUMMARY OF INVENTION
Technical Problem
In the high strength steel sheet which is described in PLT 1, there is the problem that in the steel sheet structure, there is little ferrite and residual austenite for improving the ductility and therefore sufficient ductility cannot be obtained.
The method of production of high strength steel sheet according to PLT 2 requires a large scale reheating apparatus, so there is the problem that the manufacturing cost increases.
In the arts which are described in PLTs 3 to 6 as well, the workability in high strength steel sheet with a tensile maximum strength of 900 MPa or more is insufficient.
The present invention was made in consideration of the above problems and has as its object the provision of high strength steel sheet and high strength galvanized steel sheet which are excellent in shapeability and methods of production of the same by which a tensile maximum strength 900 MPa or more high strength is secured while excellent ductility and stretch flangeability are obtained.
Solution to Problem
The inventors etc. engaged in intensive studies on the steel sheet structure and method of production for obtaining excellent ductility and stretch flangeability in high strength steel sheet. As a result, they discovered that by making the steel ingredients suitable ranges and further by establishing suitable annealing conditions after cold rolling, it is possible to make the ratio of the residual austenite phase in the steel sheet structure a predetermined range while lowering the martensite transformation start temperature of the residual austenite phase and that by producing high strength steel sheet under such conditions and controlling the ratio of the residual austenite phase in the steel sheet structure and the martensite transformation point to suitable ranges, a 900 MPa or higher tensile maximum strength is secured while the ductility and stretch flangeability (hole expandability) are improved and excellent shapeability is obtained.
The present invention was made as a result of further studies based on the above findings and has as its gist the following:
(1) High strength steel sheet which is excellent in shapeability which contains, by mass %, C: 0.075 to 0.300%, Si: 0.70 to 2.50%, Mn: 1.30 to 3.50%, P: 0.001 to 0.030%, S: 0.0001 to 0.0100%, Al: 0.005 to 1.500%, N: 0.0001 to 0.0100%, and O: 0.0001 to 0.0100%, which contains, as optional elements, one or more of Ti: 0.005 to 0.150%, Nb: 0.005 to 0.150%, B: 0.0001 to 0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.01 to 1.00%, V: 0.005 to 0.150%, and one or more of Ca, Ce, Mg, Zr, Hf, and REM: total 0.0001 to 0.5000%, and has a balance of iron and unavoidable impurities, wherein the structure of the steel sheet contains, by volume fraction, 2 to 20% of residual austenite phase, and the residual austenite phase has a martensite transformation point of −60° C. or less.
(2) The high strength steel sheet which is excellent in shapeability according to (1), characterized in that a ratio of the residual austenite phase which transforms to martensite at −198° C. is, by volume fraction, 2% or less of the total residual austenite phase.
(3) The high strength steel sheet which is excellent in shapeability according to (1) or (2), characterized in that the residual austenite phase has a martensite transformation point of −198° C. or less.
(4) The high strength steel sheet which is excellent in shapeability according to any one of claims 1 to 3, characterized in that the structure of the steel sheet further contains, by volume fraction, ferrite phase: 10 to 75%, bainitic ferrite phase and/or bainite phase: 10 to 50%, tempered martensite phase: 10 to 50%, and fresh martensite phase: 10% or less.
(5) High strength galvanized steel sheet which is excellent in shapeability characterized by comprising the high strength steel sheet according to any one of (1) to (4) on the surface of which a galvanized layer is formed.
(6) A method of production of high strength steel sheet which is excellent in shapeability characterized by comprising a hot rolling process of heating a slab which contains, by mass %, C: 0.075 to 0.300%, Si: 0.70 to 2.50%, Mn: 1.30 to 3.50%, P: 0.001 to 0.030%, S: 0.0001 to 0.0100%, Al: 0.005 to 1.500%, N: 0.0001 to 0.0100%, and O: 0.0001 to 0.0100%, which contains, as optional elements, one or more of Ti: 0.005 to 0.150%, Nb: 0.005 to 0.150%, B: 0.0001 to 0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.01 to 1.00%, V: 0.005 to 0.150%, and one or more of Ca, Ce, Mg, Zr, Hf, and REM: total 0.0001 to 0.5000%, and has a balance of iron and unavoidable impurities, directly, or after cooling once, to 1050° C. or more, finishing the rolling at the Ar3 point or more to obtain a steel sheet, and coiling it at 500 to 750° C. in temperature, a cold rolling process of pickling the coiled steel sheet, then cold rolling it by a screwdown rate of a screwdown rate 35 to 75%, and an annealing process of heating the steel sheet after the cold rolling process up to a maximum heating temperature of 740 to 1000° C., then cooling by an average cooling rate from the maximum heating temperature to 700° C. of 1.0 to 10.0° C./sec and by a 700 to 500° C. average cooling rate of 5.0 to 200° C./sec, next holding at 350 to 450° C. for 30 to 1000 seconds, then cooling down to room temperature and, while cooling from the maximum heating temperature to room temperature, reheating from the Bs point or less than 500° C. to 500° C. or more at least once and reheating from the Ms point or less than 350° C. to 350° C. or more at least once.
(7) The method of production of high strength galvanized steel sheet which is excellent in shapeability characterized by producing high strength steel sheet by the method of production of high strength steel sheet according to (6), then galvanizing it.
(8) A method of production of high strength galvanized steel sheet which is excellent in shapeability characterized by producing high strength steel sheet by the method of production according to (6) during the annealing process of which, at the time of cooling from the maximum heating temperature to room temperature, dipping the steel sheet after the cold rolling process in a zinc bath so as to hot dip galvanize it.
(9) A method of production of high strength galvanized steel sheet which is excellent in shapeability characterized by producing high strength steel sheet by the method of production according to (6) after the annealing process of which performing hot dip galvanization.
(10) A method of production of high strength galvanized steel sheet which is excellent in shapeability according to (8) or (9) characterized by performing alloying treatment at 470 to 650° C. in temperature after the hot dip galvanization.
Advantageous Effects of Invention
According to the present invention, high strength steel sheet where a 900 MPa or higher tensile maximum strength is secured while excellent shapeability is obtained can be realized.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1A is a view which shows an example of a cooling pattern in annealing treatment in the method of production of the present invention.
FIG. 1B is a view which shows another example of a cooling pattern in annealing treatment in the method of production of the present invention
FIG. 2 is a view which explains an embodiment of the present invention and a view which shows the relationship between a tensile strength TS and a total elongation EL.
FIG. 3 is a view which explains an embodiment of the present invention and a graph which shows the relationship between a tensile strength TS and a hole expansion rate λ.



DESCRIPTION OF EMBODIMENTS
Below, high strength steel sheet and high strength galvanized steel sheet which are excellent in shapeability and methods of production of the same of embodiments of the present invention will be explained. Note that the following embodiments are explained in detail for enabling the gist of the present invention to be understood better, so unless otherwise indicated, do not limit the present invention.
Note that, in the following explanation, the start temperature at which austenite (γ-iron) transforms to martensite in the process of the drop in temperature in the production of steel sheet will be referred to as the “Ms point” while the start temperature at which the residual austenite in the structure of the high strength steel sheet of the present invention which is produced transforms to martensite will be referred to as the “Msr point”.
First, the structure of the high strength steel sheet of the present invention will be explained.
The steel sheet structure of the high strength steel sheet of the present invention has a 2 to 20% residual austenite phase. The residual austenite phase has an Msr point of −60° C. or less. The residual austenite phase which is contained in such a steel sheet structure of the high strength steel sheet of the present invention is stable even with respect to a plurality of deep cooling treatments.
The structure other than the residual austenite phase is not particularly limited so long as a tensile maximum strength of 900 MPa or higher in strength can be secured, but preferably has, by volume fraction in the steel sheet structure, a ferrite phase: 10 to 75%, bainitic ferrite phase and/or bainite phase: 10 to 50%, tempered martensite phase: 10 to 50%, and fresh martensite phase: 10% or less. By having such a steel sheet structure, the result becomes high strength steel sheet which has a more excellent shapeability.
The phases which can be obtained in the structure of the steel sheet will be explained below:
Residual Austenite Phase
The residual austenite phase has the property of greatly improving the strength and ductility, but in general forming starting points of fracture and greatly degrading the stretch flangeability.
In the structure of the present steel sheet, by reheating two times as explained later, the defects which were present in the residual austenite phase and were liable to form starting sites for martensite transformation are already consumed and only the austenite phase with its high degree of cleanliness selectively remains. As a result, an extremely stable residual austenite phase is obtained. Such a residual austenite phase gradually transforms to martensite along with deformation, so has the property of not easily forming starting points of fracture and causing extremely little deterioration of the stretch flangeability.
As an indicator of the above-mentioned stability, the martensite transformation start temperature (Msr point) of the residual austenite phase may be mentioned. Stable residual austenite in which an austenite phase with high degree of cleanliness remains does not change in amount of residual austenite even with dipping in liquid nitrogen for 1 hour, that is, applying so-called deep cooling treatment. The Msr point is the liquid nitrogen temperature (−198° C.) or less and is extremely stable. Furthermore, in general, by repeatedly apply deep cooling treatment, the residual austenite is gradually decreased, but in the high strength steel sheet according to the present invention, the residual austenite does not decrease and is extremely stable even if treated for deep cooling five times.
The steel sheet of the present invention gives high strength steel sheet with a strength and ductility which are greatly improved and with a stretch flangeability which is extremely small in deterioration by a residual austenite phase with an Msr point of −60° C. or less present in a volume fraction of 2% or more.
From the viewpoint of the strength and ductility, the volume fraction of the residual austenite phase in the steel sheet structure is preferably 4% or more, more preferably 6% or more. On the other hand, to make the volume fraction of the residual austenite phase in the steel sheet structure over 20%, it is necessary to add elements such as C or Mn in over the suitable quantity resulting in the weldability being impaired, so the upper limit of the residual austenite phase is made 20%.
In the present invention, the ratio of the residual austenite phase which transforms to martensite at −198° C. is preferably a volume fraction of 2% or less. Due to this, a more stable residual austenite phase is obtained, so the ductility and stretch flangeability are remarkably improved and excellent shapeability is obtained.
Further, if the Msr point of the residual austenite in the steel sheet structure is −198° C. or less, the result becomes a more stable residual austenite phase, the ductility and stretch flangeability are further remarkably improved, and excellent shapeability is obtained, so this is preferable.
The volume fraction of the residual austenite phase is obtained by examining the steel sheet at the plane parallel to the sheet surface at 1⁄4 thickness by X-ray analysis, calculating the area fraction, and deeming this as the volume fraction. However, the 1⁄4 thickness plane is made the plane obtained by grinding and chemically polishing the base material again after deep cooling treatment to obtain a mirror finish.
Further, considering measurement error, the residual austenite phase is deemed to transform to martensite at the point of time when the relationship shown below is satisfied:

Vγ(n)/Vγ(0)<0.90

where, “n” is the number of times of deep cooling treatment, Vγ(n) is the residual austenite percent after the n-th deep drawing treatment, and Vγ(0) is the residual austenite percent in the base material.

Ferrite Phase
The ferrite phase is a structure which is effective for improving the ductility and is preferably contained in the steel sheet structure in a volume fraction of 10 to 75%. If the volume fraction of the ferrite phase in the steel sheet structure is less than 10%, sufficient ductility is liable to not be obtained. The volume fraction of the ferrite phase in the steel sheet structure, from the viewpoint of the ductility, is more preferably 15% or more, still more preferably 20% or more. The ferrite phase is a soft structure, so if the volume fraction exceeds 75%, sufficient strength will sometimes not be obtained. To sufficiently raise the tensile strength of steel sheet, the volume fraction of the ferrite phase in the steel sheet structure is more preferably made 65% or less, still more preferably made 50% or less.
Bainitic Ferrite Phase and/or Bainite Phase The bainitic ferrite phase and/or bainite phase is a structure with a good balance of strength and ductility and is preferably contained in the steel sheet structure in a volume fraction of 10 to 50%. The bainitic ferrite phase and/or bainite is a microstructure which has a strength intermediate to that of a soft ferrite phase and hard martensite phase and tempered martensite phase and residual austenite phase. From the viewpoint of the stretch flangeability, inclusion of 15% or more is more preferable and inclusion of 20% or more is further preferable. If the volume fraction of the bainitic ferrite phase and/or bainite exceeds 50%, the yield stress will excessively rise and the shape freezability will deteriorate, so this is not preferred.
Tempered Martensite Phase
The tempered martensite phase is a structure which greatly improves the tensile strength and may be included in the steel sheet structure to a volume fraction of 50% or less. From the viewpoint of the tensile strength, the volume fraction of the tempered martensite is preferably 10% or more. If the volume fraction of the tempered martensite which is contained in the steel sheet structure exceeds 50%, the yield stress will excessively rise and the shape freezability deteriorates, so this is not preferable.
Fresh Martensite Phase
The fresh martensite phase has the effect of greatly improving the tensile strength. However, it forms starting points of fracture and greatly degrades the stretch flangeability, so it preferably limited to a volume fraction of 15% in the steel sheet structure. To raise the stretch flangeability, it is more preferable to make the volume fraction of the fresh martensite phase in the steel sheet structure 10% or less, still more preferably 5% or less.
Others
The steel sheet structure of the high strength steel sheet of the present invention may further contain a pearlite phase and/or coarse cementite phase or other structure. However, if the steel sheet structure of high strength steel sheet contains a large amount of pearlite phase and/or coarse cementite phase, the problem arises of the bendability deteriorating. From this, the volume fraction of the pearlite phase and/or coarse cementite phase which is contained in the steel sheet structure is preferably a total of 10% or less, more preferably 5% or less.
The volume fractions of the different structures which are contained in the steel sheet structure of the high strength steel sheet of the present invention can, for example, be measured by the following method:
In measuring the volume fractions of the ferrite phase, bainitic ferrite phase, bainite phase, tempered martensite phase, and fresh martensite phase which are contained in the steel sheet structure of the high strength steel sheet of the present invention, first, a sample is taken using the cross-section of sheet thickness parallel to the rolling direction of the steel sheet as the examined surface. Further, the examined surface of this sample is polished and etched by Nital and the range from 1⁄8 to 3⁄8 of the sheet thickness is observed by a field emission scanning electron microscope (FE-SEM) to measure the area fraction. This was deemed as the volume fraction.
Next, the composition of ingredients of the high strength steel sheet of the present invention will be explained. Note that in the following explanation unless particularly designated otherwise, “%” indicates “mass %”.
C: 0.075 to 0.300%
C is an element which is required for obtaining a residual austenite phase. It is included for achieving both an excellent shapeability and high strength. If the content of C exceeds 0.300%, the weldability becomes insufficient. From the viewpoint of the weldability, the content of C is more preferably 0.250% or less, still more preferably 0.220% or less. If the content of C is less than 0.075%, it becomes difficult to obtain a sufficient amount of residual austenite phase and the strength and shapeability fall. From the viewpoint of the strength and shapeability, the content of C is more preferably 0.090% or more, still more preferably 0.100% or more.
Si: 0.70 to 2.50%
Si is an element which enables the residual austenite phase to be easily obtained by suppressing the formation of iron-based carbides in the steel sheet and is an element which is necessary for raising the strength and shapeability. If the content of Si exceeds 2.50%, the steel sheet becomes brittle and the ductility deteriorates. From the viewpoint of the ductility, the content of Si is more preferably 2.20% or less, still more preferably 2.00% or less. If the content of Si is less than 0.70%, iron-based carbides form after annealing while cooling down to room temperature, the residual austenite phase cannot sufficiently be obtained, and the strength and shapeability deteriorate. From the viewpoint of the strength and shapeability, the lower limit value of Si is more preferably 0.90% or more, still more preferably 1.00% or more.
Mn: 1.30 to 3.50%
Mn is added for raising the strength of the steel sheet. If the content of Mn exceeds 3.50%, coarse MN concentrated parts form at the center of thickness of the steel sheet, embrittlement easily occurs, and the cracking of the cast slab or other trouble easily arises. Further, if the content of Mn exceeds 3.50%, there is the problem that the weldability also deteriorates. Therefore, the content of Mn has to be made 3.50% or less. From the viewpoint of the weldability, the content of Mn is more preferably 3.20% or less, still more preferably 3.00% or less. If the content of Mn is less than 1.30%, a large amount of soft structures are formed during the cooling after the annealing, so securing a 900 MPa or more tensile maximum strength becomes difficult. Therefore, the content of Mn has to be made 1.30% or more. Further, to raise the strength of the steel sheet, the content of Mn is more preferably 1.50% or more, still more preferably 1.70% or more.
P: 0.001 to 0.030%
P tends to segregate at the center of thickness of the steel sheet and has the probability of causing the weld zone to become brittle. If the content of P exceeds 0.030%, the weld zone becomes greatly brittle, so the content of P is limited to 0.030% or less. The lower limit of P is not particularly set so long as the effect of the present invention is exhibited, but if making the content of P less than 0.001%, the manufacturing cost greatly increases, so 0.001% is made the lower limit.
S: 0.0001 to 0.0100%
S has a detrimental effect on the weldability and the manufacturability at the time of casting and at the time of hot rolling. Therefore, the upper limit value of the content of S is made 0.0100% or less. Further, S bonds with Mn to form coarse MnS which causes the ductility and stretch flangeability to fall, so the content is more preferably made 0.0050% or less, still more preferably 0.0025% or less. The lower limit of the content of S is not particularly set so long as the effect of the present invention is exhibited, but if making the content of S less than 0.0001%, the manufacturing cost greatly increases, so 0.0001% is made the lower limit.
Al: 0.005 to 1.500%
Al is an element which suppresses the formation of iron-based carbides and enables residual austenite to be easily obtained. It raises the strength and shapeability of steel sheet. If the content of Al exceeds 1.500%, the weldability deteriorates, so the upper limit is made 1.500%. From the viewpoint of the weldability, the content of Al is more preferably 1.200% or less, still more preferably 0.900% or less. Al is an element which is effective also as a deoxidizing material, but if the content of Al is less than 0.005%, the effect as a deoxidizing material is not sufficiently obtained, so the lower limit of the content of Al is made 0.005%. To sufficiently obtain the effect of deoxidation, the amount of Al is more preferably made 0.010% or more.
N: 0.0001 to 0.0100%
N forms coarse nitrides which cause the ductility and stretch flangeability to deteriorate, so the amount of addition has to be kept down. If the content of Ni exceeds 0.0100%, this tendency becomes more marked, so the upper limit of the content of N is made 0.0100%. N becomes a cause of formation of blowholes at the time of welding, so the smaller the content, the better. The lower limit of the content of N is not particularly set so long as the effect of the present invention is exhibited, but if making the content of N less than 0.0001%, the manufacturing cost greatly increases, so 0.0001% is made the lower limit.
O: 0.0001 to 0.0100%
O forms oxides which cause the ductility and stretch flangeability to deteriorate, so the content has to be kept down. If the content of O exceeds 0.0100%, the deterioration of the stretch flangeability becomes remarkable, so the upper limit of the content of O is made 0.0100% or less. The content of O is more preferably 0.0080% or less, still more preferably 0.0060% or less. The lower limit of the content of O is not particularly set so long as the effect of the present invention is exhibited, but if making the content of O less than 0.0001%, the manufacturing cost greatly increases, so 0.0001% is made the lower limit.
The high strength steel sheet of the present invention may further contain the elements which are shown below in accordance with need:
Ti: 0.005 to 0.150%
Ti is an element which contributes to the rise in strength of the steel sheet through precipitation strengthening, fine grain strengthening by suppression of growth of ferrite crystal grains, and dislocation strengthening through suppression of recrystallization. If the content of Ti exceeds 0.150%, precipitation of carbonitrides increases and the shapeability deteriorates, so the content of Ti is made 0.150% or less. From the viewpoint of the shapeability, the content of Ti is more preferably 0.100% or less, still more preferably 0.070% or less. To sufficiently obtain the effect of the rise in strength by Ti, the content of Ti has to be made 0.005% or more. To raise the strength of the steel sheet, the content of Ti is preferably 0.010% or more, more preferably 0.015% or more.
Nb: 0.005 to 0.150%
Nb is an element which contributes to the rise in strength of the steel sheet through precipitation strengthening, fine grain strengthening by suppression of growth of ferrite crystal grains, and dislocation strengthening through suppression of recrystallization. If the content of Nb exceeds 0.150%, precipitation of carbonitrides increases and the shapeability deteriorates, so the content of Nb is made 0.150% or less. From the viewpoint of the shapeability, the content of Nb is more preferably 0.100% or less, still more preferably 0.060% or less. To sufficiently obtain the effect of the rise in strength by Nb, the content of Nb has to be made 0.005% or more. To raise the strength of the steel sheet, the content of Nb is preferably 0.010% or more, more preferably 0.015% or more.
V: 0.005 to 0.150%
V is an element which contributes to the rise in strength of the steel sheet by precipitation strengthening, fine grain strengthening by suppression of growth of ferrite crystal grains, and dislocation strengthening through suppression of recrystallization. If the content of V exceeds 0.150%, precipitation of carbonitrides increases and the shapeability deteriorates, so the content is made 0.150% or less. To sufficiently obtain the effect of raising the strength by V, the content has to be 0.005% or more.
B: 0.0001 to 0.0100%
B is an element which suppresses phase transformation at a high temperature and is effective for increasing the strength and can be added in place of part of the C and/or Mn. If the content of B exceeds 0.0100%, the workability while hot is impaired and the productivity falls, so the content of B is made 0.0100% or less. From the viewpoint of the productivity, the content of B is preferably 0.0050% or less, more preferably 0.0030% or less. To sufficiently obtain higher strength by B, the content of B has to be made 0.0001% or more. To effectively increase the strength of the steel sheet, the content of B is preferably 0.0003% or more, more preferably 0.0005% or more.
Mo: 0.01 to 1.00%
Mo is an element which suppresses phase transformation at a high temperature and is effective for increasing the strength and can be added in place of part of the C and/or Mn. If the content of Mo exceeds 1.00%, the workability when hot is impaired and the productivity falls, so the content of Mo is made 1.00% or less. To sufficiently obtain higher strength by Mo, the content has to be 0.01% or more.
W: 0.01 to 1.00%
W is an element which suppresses phase transformation at a high temperature and is effective for increasing the strength and can be added in place of part of the C and/or Mn. If the content of W exceeds 1.00%, the workability when hot is impaired and the productivity falls, so the content of W is made 1.00% or less. To sufficiently obtain higher strength by W, the content has to be 0.01% or more.
Cr: 0.01 to 2.00%
Cr is an element which suppresses phase transformation at a high temperature and is effective for increasing the strength and can be added in place of part of the C and/or Mn. If the content of Cr exceeds 2.00%, the workability when hot is impaired and the productivity falls, so the content of Cr is made 2.00% or less. To sufficiently obtain higher strength by Cr, the content has to be 0.01% or more.
Ni: 0.01 to 2.00%
Ni is an element which suppresses phase transformation at a high temperature and is effective for increasing the strength and can be added in place of part of the C and/or Mn. If the content of Ni exceeds 2.00%, the weldability is impaired, so the content of Ni is made 2.00% or less. To sufficiently obtain higher strength by Ni, the content has to be 0.01% or more.
Cu: 0.01 to 2.00%
Cu is an element which raises the strength by presence as fine particles in the steel and can be added in place of part of the C and/or Mn. If the content of Cu exceeds 2.00%, the weldability is impaired, so the content is made 2.00% or less. To sufficiently obtain higher strength by Cu, the content has to be 0.01% or more.
One or More of Ca, Ce, Mg, Zr, Hf, and REM: Total 0.0001 to 0.5000%
Ca, Ce, Mg, Zr, Hf, and REM are elements which are effective for improving the shapeability. One or more can be added. If the content of the one or more of Ca, Ce, Mg, Zr, Hf, and REM exceeds a total of 0.5000%, conversely the ductility is liable to be impaired, so the total of the contents of the elements is made 0.5000% or less. To sufficiently obtain the effect of improvement of the shapeability of the steel sheet, the total of the contents of the elements has to be 0.0001% or more. From the viewpoint of the shapeability, the total of the contents of the elements is preferably 0.0005% or more, more preferably 0.0010% or more. Here, “REM” is an abbreviation for “rare earth metal” and indicates the elements which belong to the lanthanoid series. In the present invention, the REM or Ce is often added as a Misch metal. Sometimes, elements of the lanthanoid series in addition to La or Ce are contained compositely. Further, even when elements of the lanthanoid series other than La and Ce are included, the effects of the present invention are exhibited. Further, even if adding metal La or Ce, the effects of the present invention are exhibited.
Above, the composition of ingredients of the present invention was explained, but so long as in a range not impairing the properties of the steel sheet of the present invention, for example, elements other than the essential added elements may also be included as impurities which are derived from the starting materials.
The high strength steel sheet of the present invention can also be made high strength galvanized steel sheet on the surface of which a galvanized layer or galvannealed layer is formed. By forming a galvanized layer on the surface of the high strength steel sheet, steel sheet which has excellent corrosion resistance results. Further, by forming a galvannealed layer on the surface of the high strength steel sheet, steel sheet which has excellent corrosion resistance and which has excellent coating adhesion results.
Next, the method of production of the high strength steel sheet of the present invention will be explained.
To produce the high strength steel sheet of the present invention, first, a slab which has the above-mentioned composition of ingredients is cast. As the slab which is used for hot rolling, for example, it is possible to use a continuously cast slab or a slab which is produced by a thin slab caster etc. For the method of production of the high strength steel sheet of the present invention, it is preferable to use a process such as continuous casting-direct rolling (CC-DR) where the steel is cast, then immediately hot rolled.
The slab heating temperature in the hot rolling process has to be 1050° C. or more. If the slab heating temperature is low, the finish rolling temperature falls below the Ar3 point. As a result, two-phase rolling of the ferrite phase and austenite phase results, so the hot rolled sheet structure becomes an uneven mixed grain structure. The uneven structure is not eliminated even after the cold rolling and annealing process and therefore the ductility and bendability deteriorate. Further, if the finish rolling temperature falls, the rolling load increases and the rolling becomes difficult or shape defects are liable to be invited in the steel sheet after rolling. The upper limit of the slab heating temperature is not particularly set so long as the effect of the present invention is exhibited, but it is not preferable economically to set the heating temperature to an excessively high temperature, so the upper limit of the slab heading temperature is preferably made 1350° C. or less.
The Ar3 point can be calculated by the following formula:

Ar3(° C.)=901−325×C+33×Si−92×(Mn+Ni/2+Cr/2+Cu/2+Mo/2)+52×Al

In the above formula, C, Si, Mn, Ni, Cr, Cu, Mo, and Al are the contents of the different elements (mass %).
The finish rolling temperature of the hot rolling is made the higher of 800° C. or the Ar3 point as the lower limit and 1000° C. as the upper limit. If the finish rolling temperature is less than 800° C., the rolling load at the time of finish rolling becomes high, the rolling becomes difficult, and shape defects are liable to be invited in the hot rolled steel sheet which is obtained after rolling. If the finish rolling temperature is less than the Ar3 point, the hot rolling becomes two-phase region rolling of the ferrite phase and austenite phase and the hot rolled steel sheet structure will sometimes become an uneven mixed grain structure.
The upper limit of the finish rolling temperature is not particularly set so long as the effect of the present invention is exhibited, but if the finish rolling temperature is made excessively high, to secure that temperature, the slab heating temperature has to be made excessively high. Therefore, the upper limit temperature of the finish rolling temperature is preferably made 1000° C. or less.
The steel sheet after rolling is coiled at 500 to 750° C. If coiling the steel sheet at a temperature exceeding 750° C., the oxides which are formed on the steel sheet surface excessively increase in thickness and the pickling ability deteriorates. To raise the pickling ability, the coiling temperature is preferably 720° C. or less, more preferably 700° C. or less. If the coiling temperature becomes less than 500° C., the hot rolled steel sheet becomes excessively high in strength and cold rolling becomes difficult. From the viewpoint of lightening the load in cold rolling, the coiling temperature is preferably made 550° C. or more. 600° C. or more is more preferable.
The thus produced hot rolled steel sheet is pickled. Due to the pickling, the oxides on the steel sheet surface can be removed. This is important from the point of improving the chemical convertability of the cold rolled high strength steel sheet of the final product or the hot dip coatability of cold rolled steel sheet for hot dip galvanized or galvannealed steel sheet use. The pickling may be just a single treatment or may be divided into a plurality of treatments.
The pickled steel sheet may be supplied as is to the annealing process, but by cold rolling it by a screwdown rate of 35 to 75%, steel sheet with a high thickness precision and excellent shape is obtained. If the screwdown rate is less than 35%, it is difficult to hold the shape flat and the final product becomes poor in ductility, so the screwdown rate is made 35% or more. If the screwdown rate exceeds 75%, the cold rolling load becomes too great and cold rolling becomes difficult. From this, the upper limit of the screwdown rate is made 75%. The number of rolling passes and the screwdown rate for each pass are not particularly prescribed so long as the effect of the present invention is exhibited.
Next, the obtained hot rolled steel sheet or cold rolled steel sheet is subjected to the following annealing treatment.
First, the rolled steel sheet is heated to a maximum heating temperature of 740 to 1000° C. in range. If the maximum heating temperature is less than 740° C., the amount of the austenite phase becomes insufficient and it becomes difficult to secure a sufficient amount of hard structures in the phase transformation during subsequent cooling. If the maximum heating temperature exceeds 1000° C., the austenite phase becomes coarse in particle size, transformation does not easily proceed during cooling, and, in particular, a soft ferrite structure becomes hard to be sufficiently obtained.
The heating up to the maximum heating temperature is preferably performed with a heating rate from the (maximum heating temperature −20)° C. to maximum heating temperature, that is, in the last 20° C. at the time of heating, of 0.1 to 0.8° C./sec. By making the heating rate in the 20° C. up to the maximum heating temperature gradual heating in the above range, the effects are obtained that the rate of advance of reverse transformation to the austenite phase becomes slower and the defects in the initial austenite phase become smaller.
The holding time at the time of heating to the maximum heating temperature may be suitably determined in accordance with the maximum heating temperature etc. and is not particularly limited, but 10 seconds or more is preferable, while 40 to 540 seconds is more preferable.
Next, primary cooling is performed by an average cooling rate from the maximum heating temperature to 700° C. of 1.0 to 10.0° C./sec. By this primary cooling, it is possible to make the ferrite transformation and transformation to bainitic ferrite and/or bainite suitably proceed while leaving a non-transformed austenite phase until the Ms point and transforming all or part to martensite.
If the average cooling rate in the above cooling temperature range is less than 1.0° C./sec, pearlite transformation proceeds during the cooling whereby the nontransformed austenite phase is reduced and a sufficient hard structure cannot be obtained. As a result, sometimes it is not possible to secure a tensile maximum strength 900 MPa or more strength. If the average cooling rate exceeds 10.0° C./sec, sometimes a soft ferrite structure cannot be sufficiently formed.
The holding time in the ferrite transformation temperature region from right after heating to when the steel sheet temperature reaches 700° C. is not particularly limited, but is preferably 20 to 1000 seconds. To make the soft ferrite phase sufficiently form, it is necessary to hold the steel for 20 seconds or more in the ferrite transformation temperature region from right after annealing to when the steel sheet temperature reaches 700° C., preferably hold it there for 30 seconds or more, more preferably hold it there for 50 seconds or more. If the time during which the steel is made to held at the ferrite transformation temperature region exceeds 1000 seconds, the ferrite transformation excessively proceeds, the nontransformed austenite is reduced, and a sufficient hard structure cannot be obtained.
Furthermore, after the above primary cooling, secondary cooling is performed by an average cooling rate from 700 to 500° C. of 5.0 to 200° C./sec. Due to this secondary cooling, the transformation from austenite to ferrite after annealing reliably proceeds. If cooling by a 1° C./sec to 10.0° C./sec average cooling rate similar to the primary cooling from a temperature region exceeding 700° C., the ferrite phase is insufficiently formed and the ductility of the high strength steel sheet cannot be secured.
In the method of production of the present invention, the steel sheet which has been treated to cool in the above two stages is held at 350 to 450° C. in temperature for 30 to 1000 seconds of time. If the holding temperature at this time is less than 350° C., fine iron-based carbides form and concentration of C at the austenite phase does not proceed resulting in an unstable austenite phase. If the holding time exceeds 450° C., the solid solution limit of C in the austenite phase becomes lower and C becomes saturated even at a small amount, so concentration of C does not proceed resulting in an unstable austenite phase.
If the holding time is less than 30 second, the bainite transformation does not sufficiently proceed, the amount of C (carbon) which is discharged from the bainite phase to the austenite phase is small, the concentration of C at the austenite phase becomes insufficient, and an unstable austenite phase results. If the holding time exceeds 1000 seconds, coarse iron-based carbides start to form and the concentration of C in the austenite conversely falls, so an unstable austenite phase results.
Furthermore, in the annealing process of the present invention, as shown in FIG. 1A, when cooling from the maximum heating temperature to room temperature, the steel is reheated from the Bs point (bainite transformation start temperature) or less than 500° C. to 500° C. or more at least once and is reheated from the Ms point or less than 350° C. to 350° C. or more at least once. By performing reheating treatment by such two types of conditions, it is possible to make the austenite phase which has internal defects and easily transforms to other structures in the nontransformed residual austenite phase, that is, the unstable austenite phase, preferentially transform and obtain a bainite phase, bainitic ferrite phase, or tempered martensite phase.
Note that, for example, as shown in FIG. 1B, even if cooling down to the Ms point or less than 350° C., then reheating to 500° C. or more, it is deemed that the reheating from the Ms point or less than 350° C. to 350° C. or more and the reheating from the Bs point or less than 500° C. to 500° C. or more have respectively been performed. Such a pattern of reheating treatment may also be performed.
Further, it is possible to hold the steel at the above-mentioned 350 to 450° C. temperature range between the reheating from the Ms point or less than 350° C. to 350° C. or more and the reheating from the Bs point or less than 500° C. to 500° C. or more.
The Bs point (bainite transformation start temperature) can be calculated by the following formula:

Bs(° C.)=820−290C/(1−VF)−37Si−90Mn−65Cr−50Ni+70Al

In the above formula, VF is the volume fraction of ferrite, while C, Mn, Cr, Ni, Al, and Si are the amounts of addition of these elements (mass %).

The Ms point (martensite transformation start temperature) can be calculated by the following formula:

Ms(° C.)=541−474C/(1−VF)−15Si−35Mn−17Cr−17Ni+19Al

In the above formula, VF is the volume fraction of ferrite, while C, Si, Mn, Cr, Ni, and Al are the amounts of addition of these elements (mass %).

Note that, it is difficult to directly measure the volume fraction of the ferrite phase during production of high strength steel sheet, so in the present invention, a small piece of the cold rolled steel sheet is cut out before running the sheet through the continuous annealing line, that small piece is annealed by the same temperature history as the case of running it through the continuous annealing line, the change in volume of the ferrite phase of the small piece is measured, the result is used to calculate a numerical value, and that value is used as the volume fraction VF of the ferrite. This measurement may be performed using the result of the first measurement operation when producing steel sheet under the same conditions. The value does not have to be measured each time. Measurement is performed again when greatly changing the production conditions. Of course, it is also possible to observe the microstructure of the actually produced steel sheet and feed back the results to the production the next time and on.
In the above-mentioned reheating from the Bs point or less than 500° C. to 500° C. or more, the start temperature is made the Bs point or less than 500° C. so as to cause the formation of bainite nuclei for consuming the defects in the austenite. The reheating temperature was made 500° C. or more so as to deactivate the transformation nuclei and avoid the formation of iron-based carbides induced by transformation excessively proceeding in the high temperature region.
In the above-mentioned reheating from the Ms point or less than 350° C. to 350° C. or more, the start temperature is made the Ms point or less than 350° C. so as to cause the formation of martensite nuclei for consuming the defects in the austenite. The reheating temperature was made 350° C. or more so as to avoid the formation of fine iron-based carbides obstructing the concentration of C at the austenite phase in the martensite and/or bainite due to being allowed to stand at less than 350° C.
The reason why performing the above-mentioned two-stage reheating in different temperature regions results in the residual austenite phase strikingly rising is not fully clear, but it is believed that the bainite nuclei and the martensite nuclei consume respectively different types of defects.
Due to the above processes, defects which can form starting sites of martensite transformation which are present in the residual austenite phase are consumed, only the austenite phase with its high degree of cleanliness selectively remains, and an extremely stable residual austenite phase is obtained. As a result, high strength steel sheet which has high ductility and stretch flangeability and which is excellent in shapeability is obtained.
The annealed steel sheet may be cold rolled by about 0.03 to 0.80% for the purpose of correcting the shape. At that time, if the cold rolling rate after annealing is too high, the soft ferrite phase will be work hardened and the ductility will greatly deteriorate, so the rolling rate is preferably made the above range.
The annealed steel sheet may be electrolytically galvanized to obtain high strength galvanized steel sheet. Further, the annealed steel sheet may be hot dip galvanized to obtain high strength galvanized steel sheet. In such a case, for example, it is possible to cool from the maximum heating temperature to room temperature in the annealing process, for example, down to 500° C., apply further reheating, then dip in a zinc bath for hot dip galvanization.
Further, during the secondary cooling during the above annealing treatment and while holding between 350 to 450° C. or after holding at 350 to 450° C., the steel sheet may be dipped in a galvanization bath to produce high strength galvanized steel sheet.
After the hot dip galvanization, it is possible to further treat the plating layer of the steel sheet surface to alloy it at a temperature of 470 to 650° C. By performing such alloying treatment, a Zn—Fe alloy obtained by the galvanized layer being alloyed is formed on the surface, and high strength galvanized steel sheet which is excellent in rust prevention is obtained.
This heating in the alloying treatment may be performed in place of the reheating from the Bs point or less than 500° C. to 500° C. or more or the reheating from the Ms point or less than 350° C. to 350° C. or more.
In performing the plating treatment, to improve the plating adhesion, for example, it is possible to plate the steel sheet before the annealing process by plating comprised of one or more elements selected from Ni, Cu, Co, and Fe. By performing such plating treatment by this method, high strength galvanized steel sheet which is formed with a galvanized layer on its surface, has high ductility and stretch flangeability, and has excellent shapeability is obtained.
The high strength steel sheet on the surface of which a galvanized layer is formed may further be formed with a film comprised of a P oxide and/or P-containing composite oxide.
EXAMPLES
Below, the high strength steel sheet and high strength galvanized steel sheet which are excellent in shapeability and the methods of production of the same of the present invention will be explained more specifically using examples. The present invention is not of course limited to the following examples and may be suitably changed in a range able to match with the gist of the present invention. These are all included in the technical scope of the present invention.
Slabs which have the chemical ingredients (compositions) of A to AG which are shown in Tables 1 and 2 were cast, then immediately after casting were hot rolled, cooled, coiled, and pickled under the conditions which are shown in Tables 3 to 5. After that, Experiments 5, 14, 19, 24, 29, 34, 39, 44, 49, 54, 59, 98, 102, and 119 left the hot rolled steel sheets as they were, while the other experiments cold rolled them under the conditions which are described in Tables 3 to 6 after pickling. After that, an annealing process was applied under the conditions which are shown in Tables 7 to 14 to obtain the steel sheets of Experiments 1 to 127.















TABLE 1






C
Si
Mn
P
S
Al
N
O


Experiment
mass %
mass %
mass %
mass %
mass %
mass %
mass %
mass %







A
0.107
1.33
1.56
0.020
0.0038
0.043
0.0035
0.0006


B
0.193
1.97
2.49
0.014
0.0009
0.027
0.0021
0.0016


C
0.107
0.99
2.02
0.017
0.0024
0.038
0.0030
0.0025


D
0.247
1.14
1.92
0.019
0.0034
0.005
0.0050
0.0029


E
0.191
1.05
1.41
0.015
0.0029
0.067
0.0030
0.0011


F
0.133
1.89
1.92
0.010
0.0046
0.038
0.0041
0.0022


G
0.203
1.02
1.51
0.014
0.0052
0.073
0.0024
0.0015


H
0.182
0.75
1.87
0.012
0.0037
0.263
0.0037
0.0019


I
0.084
1.51
2.79
0.018
0.0031
0.123
0.0013
0.0020


J
0.260
0.71
2.20
0.019
0.0014
0.302
0.0040
0.0025


K
0.199
1.19
1.89
0.018
0.0027
0.041
0.0060
0.0004


L
0.094
0.90
1.85
0.014
0.0034
0.056
0.0051
0.0013


M
0.183
2.00
1.99
0.007
0.0018
0.045
0.0041
0.0016


N
0.170
1.66
2.59
0.020
0.0008
0.015
0.0037
0.0016


O
0.140
0.74
1.45
0.013
0.0043
0.598
0.0055
0.0004


P
0.099
0.98
1.89
0.020
0.0007
0.044
0.0034
0.0006


Q
0.230
1.24
1.45
0.016
0.0010
0.068
0.0054
0.0028


R
0.119
1.39
2.27
0.016
0.0019
0.031
0.0060
0.0016


S
0.225
1.80
1.52
0.014
0.0042
0.032
0.0029
0.0011


T
0.142
0.99
2.17
0.011
0.0046
0.068
0.0021
0.0011


U
0.194
1.24
1.45
0.011
0.0015
0.053
0.0044
0.0019


V
0.133
2.27
2.55
0.017
0.0051
0.071
0.0056
0.0023


W
0.090
1.44
1.68
0.016
0.0044
0.054
0.0020
0.0007


X
0.101
1.95
1.54
0.009
0.0025
0.062
0.0058
0.0007


Y
0.114
1.62
2.70
0.010
0.0034
0.071
0.0020
0.0013


Z
0.150
1.06
3.16
0.010
0.0036
0.055
0.0018
0.0029


AA
0.015
1.05
2.00
0.013
0.0022
0.027
0.0035
0.0014


AB
0.097
0.06
1.97
0.012
0.0022
0.027
0.0032
0.0008


AC
0.101
1.05
0.52
0.015
0.0021
0.033
0.0033
0.0014


AD
0.093
1.68
2.67
0.002
0.0013
0.033
0.0076
0.0009


AE
0.152
0.75
2.07
0.013
0.0018
0.065
0.0015
0.0005


AF
0.148
1.72
1.55
0.007
0.0025
0.059
0.0080
0.0014


AG
0.209
0.89
2.50
0.007
0.0036
0.039
0.0057
0.0008































TABLE 2






Ti
Nb















Experi-
mass
mass
B
Cr
Ni
Cu
Mo
V
Ca
Ce
Mg
Zr
Hf
REM


ment
%
%
mass %
mass %
mass %
mass %
mass %
mass %
mass %
mass %
mass %
mass %
mass %
mass %






























A














Inv. ex.


B














Inv. ex.


C














Inv. ex.


D














Inv. ex.


E
0.044













Inv. ex.


F

0.022












Inv. ex.


G


0.0019











Inv. ex.


H



0.49










Inv. ex.


I








0.0020





Inv. ex.


J






0.25







Inv. ex.


K







0.105






Inv. ex.


L
0.023
0.013
0.0026
0.39


0.11

0.0032
0.0010




Inv. ex.


M




0.65









Inv. ex.


N





0.24








Inv. ex.


O




1.00
0.60








Inv. ex.


P









0.0025




Inv. ex.


Q










0.0017



Inv. ex.


R











0.0019


Inv. ex.


S












0.0025

Inv. ex.


T













0.0021
Inv. ex.


U
0.069

0.0015











Inv. ex.


V
0.005
0.035


0.23



0.0019





Inv. ex.


W


0.0009






0.0024




Inv. ex.


X



0.67


0.18







Inv. ex.


Y














Inv. ex.


Z














Inv. ex.


AA














Comp. ex.


AB














Comp. ex.


AC














Comp. ex.


AD














Inv. ex.


AE














Inv. ex.


AF














Inv. ex.


AG














Inv. ex.























TABLE 3









Hot







Slab
Ar3
rolling

Cold




heating
transformation
end
Coiling
rolling



Chemical
temp.
point
temp.
temp.
rate


Experiment
ingredients
° C.
° C.
° C.
° C.
%






















1
A
1265
769
915
584
52
Inv. ex.


2
A
1215
769
901
600
52
Inv. ex.


3
A
1185
769
952
612
52
Inv. ex.


4
A
1265
769
952
583
52
Inv. ex.


5
A
1225
769
926
562
0
Inv. ex.


6
B
1195
676
943
618
40
Inv. ex.


7
B
1170
676
910
638
40
Inv. ex.


8
B
1240
676
925
567
40
Inv. ex.


9
B
1185
676
929
528
40
Inv. ex.


10
C
1205
715
912
632
52
Inv. ex.


11
C
1200
715
900
671
52
Inv. ex.


12
C
1175
715
892
695
52
Inv. ex.


13
C
1205
715
885
614
52
Inv. ex.


14
C
1245
715
923
605
0
Inv. ex.


15
D
1190
682
935
660
52
Inv. ex.


16
D
1275
682
904
546
52
Inv. ex.


17
D
1235
682
930
556
52
Inv. ex.


18
D
1250
682
949
613
52
Inv. ex.


19
D
1195
682
905
568
0
Inv. ex.


20
E
1225
747
913
598
38
Inv. ex.


21
E
1240
747
908
682
38
Inv. ex.


22
E
1240
747
898
563
67
Inv. ex.


23
E
1245
747
908
645
67
Inv. ex.


24
E
1270
747
892
620
0
Inv. ex.


25
F
1180
745
944
652
50
Inv. ex.


26
F
1230
745
893
639
50
Inv. ex.


27
F
1215
745
928
542
50
Inv. ex.


28
F
1215
745
894
687
50
Comp. ex.


29
F
1210
745
943
577
0
Inv. ex.


30
G
1170
734
939
681
52
Inv. ex.


31
G
1180
734
933
619
52
Inv. ex.


32
G
1200
734
893
661
52
Comp. ex.


33
G
1230
734
917
594
52
Inv. ex.


34
G
1255
734
931
594
0
Inv. ex.


35
H
1235
686
890
643
38
Inv. ex.























TABLE 4









Hot







Slab
Ar3
rolling

Cold




heating
transformation
end
Coiling
rolling



Chemical
temp.
point
temp.
temp.
rate


Experiment
ingredients
° C.
° C.
° C.
° C.
%






















36
H
1240
686
886
554
38
Inv. ex.


37
H
1225
686
942
572
38
Comp. ex.


38
H
1245
686
929
557
38
Inv. ex.


39
H
1215
686
905
570
0
Inv. ex.


40
I
1205
673
885
528
68
Inv. ex.


41
I
1175
673
951
643
68
Inv. ex.


42
I
1205
673
926
559
68
Comp. ex.


43
I
1265
673
953
566
68
Inv. ex.


44
I
1235
673
910
615
0
Inv. ex.


45
J
1265
642
949
612
36
Inv. ex.


46
J
1215
642
938
618
36
Inv. ex.


47
J
1250
642
898
638
36
Comp. ex.


48
J
1295
642
856
677
52
Inv. ex.


49
J
1215
642
933
588
0
Inv. ex.


50
K
1205
704
930
658
71
Inv. ex.


51
K
1230
704
930
615
71
Inv. ex.


52
K
1195
704
942
672
71
Comp. ex.


53
K
1265
704
914
611
71
Inv. ex.


54
K
1240
704
950
597
0
Inv. ex.


55
L
1190
710
919
616
50
Inv. ex.


56
L
1190
710
950
669
50
Inv. ex.


57
L
1270
710
902
693
50
Comp. ex.


58
L
1200
710
891
679
50
Inv. ex.


59
L
1230
710
924
582
0
Inv. ex.


60
M
1270
697
944
660
52
Inv. ex.


61
M
1180
697
931
581
52
Inv. ex.


62
M
1255
697
883
569
52
Comp. ex.


63
M
1245
697
945
605
52
Inv. ex.


64
N
1185
652
933
675
52
Inv. ex.


65
N
1225
652
895
580
52
Inv. ex.


66
N
1265
652
925
628
52
Comp. ex.


67
N
1220
652
914
681
52
Inv. ex.


68
O
1225
704
949
613
52
Inv. ex.


69
O
1255
704
877
690
52
Inv. ex.


70
O
1220
704
903
671
52
Comp. ex.























TABLE 5









Hot







Slab
Ar3
rolling

Cold




heating
transformation
end
Coiling
rolling



Chemical
temp.
point
temp.
temp.
rate


Experiment
ingredients
° C.
° C.
° C.
° C.
%






















71
O
1215
704
915
684
52
Inv. ex.


72
P
1230
730
932
609
52
Inv. ex.


73
P
1180
730
923
603
52
Inv. ex.


74
P
1230
730
903
639
52
Comp. ex.


75
P
1215
730
927
614
52
Inv. ex.


76
Q
1180
737
890
646
52
Inv. ex.


77
Q
1270
737
934
671
52
Inv. ex.


78
Q
1260
737
913
664
52
Comp. ex.


79
Q
1280
737
947
591
52
Inv. ex.


80
R
1190
701
909
642
40
Inv. ex.


81
R
1245
701
907
629
40
Inv. ex.


82
R
1205
701
886
568
52
Comp. ex.


83
R
1210
701
924
602
52
Inv. ex.


84
S
1215
749
900
648
52
Inv. ex.


85
S
1180
749
918
573
52
Inv. ex.


86
S
1210
749
931
578
52
Inv. ex.


87
S
1265
749
920
589
52
Inv. ex.


88
T
1245
691
942
625
47
Inv. ex.


89
T
1275
691
889
652
47
Inv. ex.


90
T
1275
691
907
585
47
Inv. ex.


91
T
1230
691
897
558
47
Inv. ex.


92
U
1225
748
904
551
67
Inv. ex.


93
U
1190
748
904
608
67
Inv. ex.


94
U
1205
748
897
591
52
Inv. ex.


95
U
1275
748
930
607
52
Inv. ex.


96
V
1185
691
909
678
52
Inv. ex.


97
V
1200
691
899
645
52
Inv. ex.


98
V
1215
691
901
650
0
Inv. ex.


99
V
1230
691
917
582
52
Inv. ex.


100
W
1260
768
888
664
52
Inv. ex.


101
W
1190
768
907
657
52
Inv. ex.


102
W
1195
768
921
564
0
Inv. ex.


103
W
1280
768
914
606
52
Inv. ex.


104
X
1235
755
910
634
52
Inv. ex.


105
X
1275
755
952
604
52
Inv. ex.























TABLE 6









Hot







Slab
Ar3
rolling

Cold




heating
transformation
end
Coiling
rolling



Chemical
temp.
point
temp.
temp.
rate


Experiment
ingredients
° C.
° C.
° C.
° C.
%






















106
X
1210
755
900
621
52
Inv. ex.


107
X
1280
755
939
616
52
Inv. ex.


108
Y
1200
673
886
673
67
Inv. ex.


109
Y
1185
673
925
652
67
Inv. ex.


110
Y
1175
673
940
673
43
Inv. ex.


111
Y
1185
673
953
563
43
Inv. ex.


112
Z
1225
599
929
643
52
Inv. ex.


113
Z
1185
599
915
694
52
Inv. ex.


114
Z
1220
599
902
683
52
Inv. ex.


115
Z
1275
599
735
666
52
Comp. ex.


116
AA
1190
748
935
623
52
Comp. ex.


117
AB
1205
692
889
622
52
Comp. ex.


118
AC
1175
857
894
660
52
Comp. ex.


119
B
1210
676
925
566
0
Inv. ex.


120
AD
1200
682
866
588
50
Inv. ex.


121
AD
1225
682
903
600
50
Inv. ex.


122
AE
1230
689
889
601
50
Inv. ex.


123
AE
1220
689
887
611
50
Inv. ex.


124
AF
1220
770
894
621
50
Inv. ex.


125
AF
1215
770
922
588
50
Inv. ex.


126
AG
1205
634
902
599
50
Inv. ex.


127
AG
1210
634
892
591
50
Inv. ex.





















TABLE 7










First
Second




Heating
cooling
cooling



process
process
process


















Max.
Average
Average






Heating
heating
cooling
cooling



Chemical
Steel
rate
temp.
rate
rate


Experiment
ingredients
type
° C./sec
° C.
° C./sec
° C./sec

















1
A
CR
0.3
920
3.3
34
Inv. ex.


2
A
CR
0.3
812
3.1
32
Inv. ex.


3
A
GA
0.3
816
3.1
28
Inv. ex.


4
A
GI
0.3
821
3.4
29
Inv. ex.


5
A
HR-GA
0.3
812
3.3
33
Inv. ex.


6
B
CR
0.3
819
1.7
27
Inv. ex.


7
B
CR
0.3
825
1.6
28
Inv. ex.


8
B
CR
0.3
826
1.5
31
Inv. ex.


9
B
GA
0.3
823
2.5
27
Inv. ex.


10
C
CR
0.5
846
2.5
13
Inv. ex.


11
C
CR
0.5
836
2.6
11
Inv. ex.


12
C
CR
15  
831
3.2
11
Inv. ex.


13
C
EG
0.5
845
3.4
10
Inv. ex.


14
C
HR
0.4
845
3.1
13
Inv. ex.


15
D
CR
0.7
793
6.2
10
Inv. ex.


16
D
CR
0.7
782
4.5
10
Inv. ex.


17
D
CR
8  
781
3.6
8
Inv. ex.


18
D
GI
0.7
786
4.4
10
Inv. ex.


19
D
HR
0.7
784
3.7
8
Inv. ex.


20
E
CR
0.7
822
2.6
7
Inv. ex.


21
E
CR
0.7
829
3.1
12
Inv. ex.


22
E
GA
0.7
823
2.5
8
Inv. ex.


23
E
EG
0.7
821
2.5
12
Inv. ex.


24
E
HR-GA
0.7
816
2.6
10
Inv. ex.


25
F
CR
0.7
834
2.5
8
Inv. ex.


26
F
CR
0.7
898
3.5
7
Inv. ex.


27
F
CR
0.7
892
2.9
46
Inv. ex.


28
F
CR
0.7
1076 
2.7
52
Comp. ex.


29
F
HR
0.7
898
2.7
55
Inv. ex.


30
G
CR
0.5
793
3.3
31
Inv. ex.


31
G
CR
0.5
789
2.8
35
Inv. ex.


32
G
CR
0.5
730
3.2
32
Comp. ex.


33
G
EG
0.5
783
2.7
30
Inv. ex.


34
G
HR-GA
0.5
800
3.5
30
Inv. ex.


35
H
CR
0.5
780
4.9
33
Inv. ex.





















TABLE 8










First
Second




Heating
cooling
cooling



process
process
process


















Max.
Average
Average






Heating
heating
cooling
cooling



Chemical
Steel
rate
temp.
rate
rate


Experiment
ingredients
type
° C./sec
° C.
° C./sec
° C./sec





36
H
CR
0.5
889
5.3
31
Inv. ex.


37
H
CR
0.5
886
31  
30
Comp. ex.


38
H
GA
0.5
788
5.4
30
Inv. ex.


39
H
HR
0.5
790
2.6
33
Inv. ex.


40
I
CR
0.5
817
3.1
26
Inv. ex.


41
I
CR
0.6
817
3.2
34
Inv. ex.


42
I
CR
0.5
818
0.2
33
Comp. ex.


43
I
GI
0.5
811
2.5
35
Inv. ex.


44
I
HR-GA
0.5
828
3.5
30
Inv. ex.


45
J
CR
0.4
840
2.5
34
Inv. ex.


46
J
CR
0.4
835
3.3
102 
Inv. ex.


47
J
CR
0.4
856
3.6
 1
Comp. ex.


48
J
GI
0.4
835
2.4
33
Inv. ex.


49
J
HR-GA
0.4
846
2.7
31
Inv. ex.


50
K
CR
0.5
810
3.3
59
Inv. ex.


51
K
CR
0.5
793
2.7
65
Inv. ex.


52
K
CR
0.5
804
2.7
57
Comp. ex.


53
K
GI
0.5
796
3.1
61
Inv. ex.


54
K
HR-GA
0.5
799
3.1
63
Inv. ex.


55
L
CR
0.4
821
2.9
56
Inv. ex.


56
L
CR
0.4
837
2.9
61
Inv. ex.


57
L
CR
0.4
828
3.5
58
Comp. ex.


58
L
GI
0.4
837
2.8
66
Inv. ex.


59
L
HR-GA
0.4
826
3.1
53
Inv. ex.


60
M
CR
0.4
829
3.0
57
Inv. ex.


61
M
CR
0.4
824
3.5
11
Inv. ex.


62
M
CR
0.4
823
3.4
59
Comp. ex.


63
M
GA
0.4
815
2.7
64
Inv. ex.


64
N
CR
0.4
827
3.0
27
Inv. ex.


65
N
CR
0.4
821
3.1
31
Inv. ex.


66
N
CR
0.4
810
3.5
29
Comp. ex.


67
N
GI
0.4
818
2.6
31
Inv. ex.


68
O
CR
0.5
953
3.8
 8
Inv. ex.


69
O
CR
0.5
943
3.3
30
Inv. ex.


70
O
CR
0.5
944
3.7
32
Comp. ex.





















TABLE 9










First
Second




Heating
cooling
cooling



process
process
process


















Max.
Average
Average






Heating
heating
cooling
cooling



Chemical
Steel
rate
temp.
rate
rate


Experiment
ingredients
type
° C./sec
° C.
° C./sec
° C./sec

















71
O
GA
0.5
939
2.7
41
Inv. ex.


72
P
CR
0.4
849
7.8
27
Inv. ex.


73
P
CR
0.3
838
2.6
30
Inv. ex.


74
P
CR
0.4
842
2.9
33
Comp. ex.


75
P
GA
0.4
841
2.6
8
Inv. ex.


76
Q
CR
0.4
794
2.8
28
Inv. ex.


77
Q
CR
0.4
801
3.4
30
Inv. ex.


78
Q
CR
0.4
800
2.7
26
Comp. ex.


79
Q
GA
0.4
806
2.8
29
Inv. ex.


80
R
CR
0.4
817
3.2
27
Inv. ex.


81
R
CR
0.4
803
3.3
30
Inv. ex.


82
R
CR
0.4
800
2.6
30
Comp. ex.


83
R
GI
0.4
807
2.7
33
Inv. ex.


84
S
CR
0.4
798
1.9
27
Inv. ex.


85
S
CR
0.4
806
1.8
128
Inv. ex.


86
S
GA
0.4
801
2.4
35
Inv. ex.


87
S
GI
0.4
804
2.1
28
Inv. ex.


88
T
CR
0.4
835
2.1
32
Inv. ex.


89
T
CR
0.4
820
1.6
34
Inv. ex.


90
T
GA
0.5
826
2.4
32
Inv. ex.


91
T
EG
0.4
833
1.9
29
Inv. ex.


92
U
CR
0.4
785
4.8
28
Inv. ex.


93
U
CR
0.4
771
5.4
32
Inv. ex.


94
U
GA
0.4
787
4.7
26
Inv. ex.


95
U
EG
0.4
775
5.2
27
Inv. ex.


96
V
CR
0.4
865
5.0
47
Inv. ex.


97
V
CR
0.4
880
5.1
49
Inv. ex.


98
V
HR
0.4
872
4.8
52
Inv. ex.


99
V
GA
0.4
867
5.2
54
Inv. ex.


100
W
CR
0.2
882
5.1
50
Inv. ex.


101
W
CR
0.2
796
5.3
51
Inv. ex.


102
W
HR-GA
0.2
793
4.6
47
Inv. ex.


103
W
GI
0.2
804
5.1
54
Inv. ex.


104
X
CR
0.5
852
5.4
47
Inv. ex.


105
X
CR
0.5
847
4.9
53
Inv. ex.





















TABLE 10










First
Second




Heating
cooling
cooling



process
process
process


















Max.
Average
Average






Heating
heating
cooling
cooling



Chemical
Steel
rate
temp.
rate
rate


Experiment
ingredients
type
° C./sec
° C.
° C./sec
° C./sec





106
X
CR
17  
853
4.6
53
Inv. ex.


107
X
EG
0.5
857
5.0
47
Inv. ex.


108
Y
CR
0.5
810
4.7
50
Inv. ex.


109
Y
CR
0.5
791
3.1
46
Inv. ex.


110
Y
GA
0.5
803
2.5
50
Inv. ex.


111
Y
CR
18  
807
3.5
43
Inv. ex.


112
Z
CR
0.5
759
2.8
51
Inv. ex.


113
Z
CR
0.5
759
2.7
48
Inv. ex.


114
Z
EG
0.5
747
3.4
51
Inv. ex.


115
Z
CR
0.5
757
2.8
28
Comp. ex.


116
AA
CR
0.5
799
3.1
30
Comp. ex.


117
AB
CR
0.5
795
3.1
27
Comp. ex.


118
AC
CR
0.5
790
3.3
30
Comp. ex.


119
B
HR
0.4
827
1.7
34
Inv. ex.


120
AD
CR
0.4
819
1.8
26
Inv. ex.


121
AD
GA
0.8
842
2.8
27
Inv. ex.


122
AE
CR
0.7
943
3.1
63
Inv. ex.


123
AE
GA
0.7
846
1.9
24
Inv. ex.


124
AF
CR
0.7
899
2.1
33
Inv. ex.


125
AF
GA
0.7
928
2.2
65
Inv. ex.


126
AG
CR
0.7
793
2.1
68
Inv. ex.


127
AG
GA
0.7
809
2.0
61
Inv. ex.



























TABLE 11









Reheating
Reheating
Reheating

Reheating
Reheating

Bainite
Martensite




process 1
process 2
process 3

process 4
process 5

trans-
trans-























Cool-

Cool-

Cool-

Holding
Cool-

Cool-

Alloying
formation
formation




ing
Reheat
ing
Reheat
ing
Reheat
process
ing
Reheat
ing
Reheat
process
start
start




stop
stop
stop
stop
stop
stop
Holding
stop
stop
stop
stop
Alloying
temp.
temp.


Experi-
temp.
temp.
temp.
temp.
temp.
temp.
time
temp.
temp.
temp.
temp.
temp.
(Bs)
(Ms)


ment
° C.
° C.
° C.
° C.
° C.
° C.
Sec.
° C.
° C.
° C.
° C.
° C.
° C.
° C.

























1
464
541


292
360
244





598
412
Inv. ex.


2
458
525


335
379
273


318
378

567
346
Inv. ex.


3
455
520


322
397
240




520
559
370
Inv. ex.


4
456
547


328
397
253





560
360
Inv. ex.


5
460
514


304
414
254




514
571
341
Inv. ex.


6
429
527


273
417
236





440
279
Inv. ex.


7
411
520
421
518
254
421
279
423
518



436
280
Inv. ex.


8
441
516


267
392
243





450
299
Inv. ex.


9
422
534


234
373
56




477
434
271
Inv. ex.


10
483
513


311
385
279





570
402
Inv. ex.


11
490
548


291
421
241





566
403
Inv. ex.


12




312
412
218
397
505



568
387
Inv. ex.


13
495
510


316
379
215





572
395
Inv. ex.


14
459
520


331
370
237





570
392
Inv. ex.


15
454
546


283
358
235





504
295
Inv. ex.


16






245
378
526
318
378

505
280
Inv. ex.


17
475
536


253
387
260





497
280
Inv. ex.


18
482
526


263
390
284





502
291
Inv. ex.


19
476
534


255
362
216





500
284
Inv. ex.


20
476
547


308
400
54





554
321
Inv. ex.


21
458
545


305
406
60





565
324
Inv. ex.


22
489
551


334
400
64
425
520


520
582
345
Inv. ex.


23
484
519


294
369
56





577
334
Inv. ex.


24
453
526


318
388
71
388
504


504
565
326
Inv. ex.


25
477
546


314
424
60





510
342
Inv. ex.


26
464
545


326
359
212





537
368
Inv. ex.


27




344
383
472
407
511



532
378
Inv. ex.


28
478
521


336
404
246





545
384
Comp. ex.


29
471
534


292
416
216





532
371
Inv. ex.


30
463
531


225
362
64





526
265
Inv. ex.


31




254
379
267
411
520



545
290
Inv. ex.


32
483
509


191
366
532





504
219
Comp. ex.


33
488
505


232
358
210





510
236
Inv. ex.


34
466
517
483
518
262
374
261




518
523
292
Inv. ex.


35
463
528


326
414
237





522
331
Inv. ex.


























TABLE 12











Hold-



Bainite
Martensite







ing


Alloy-
trans-
trans-




Reheating
Reheating
Reheating
proc-
Reheating
Reheating
ing
for-
for-




process 1
process 2
process 3
ess
process 4
process 5
process
mation
mation
























Cooling
Reheat
Cooling
Reheat
Cooling
Reheat
Hold-
Cooling
Reheat
Cooling
Reheat
Alloy-
start
start



Ex-
stop
stop
stop
stop
stop
stop
ing
stop
stop
stop
stop
ing
temp.
temp.



peri-
temp.
temp.
temp.
temp.
temp.
temp.
time
temp.
temp.
temp.
temp.
temp.
(Bs)
(Ms)



ment
° C.
° C.
° C.
° C.
° C.
° C.
Sec.
° C.
° C.
° C.
° C.
° C.
° C.
° C.

























36
488
546




276
419
519
318
378

542
355
Inv. ex.


37
476
531


331
399
253





563
376
Comp.

















ex.


38
445
543
451
501
239
368
257




501
469
264
Inv. ex.


39
447
504




216


318
378

500
295
Inv. ex.


40
443
526


338
408
368





464
349
Inv. ex.


41
452
552


312
422
444





481
356
Inv. ex.


42




276
387
431
408
517



451
308
Comp.

















ex.


43
460
511


315
402
339





473
344
Inv. ex.


44
454
543


313
369
388




543
479
352
Inv. ex.


45
482
530


248
409
258





495
260
Inv. ex.


46
471
533


267
416
255
417
525
319
379

499
267
Inv. ex.


47
451
502


285
404
287





338
3
Comp.

















ex.


48
481
532




280


250
377

506
278
Inv. ex.


49
492
532


285
355
2245




476
514
290
Inv. ex.


50
488
545


308
356
357





534
330
Inv. ex.


51
464
536


232
358
225





471
269
Inv. ex.


52
—
—
—
—
—
—
253
—
—
—
—

526
315
Comp.

















ex.


53
466
540


289
399
277





503
305
Inv. ex.


54
478
543


280
386
242
401
522


522
512
292
Inv. ex.


55
472
552
482
512
297
386
74





553
383
Inv. ex.


56
480
510


344
363
74





560
391
Inv. ex.


57
440
510
—
—
—
—
225
—
—
—
—

556
384
Comp.

















ex.


58
462
527


306
381
217





561
396
Inv. ex.


59
493
547


311
372
236
406
503


503
566
398
Inv. ex.


60
441
553


304
361
290





458
307
Inv. ex.


61
431
529


268
392
256





454
289
Inv. ex.


62
—
—
—
—
249
394
216
—
—
—
—

437
251
Comp.

















ex.


63
437
542


248
381
261
375
559


559
440
269
Inv. ex.


64
463
539


327
411
316





481
344
Inv. ex.


65
446
533


296
425
301





470
326
Inv. ex.


66
420
513
433
508
—
—
396
—
—
—
—

454
302
Comp.

















ex.


67
427
523


280
419
339




523
455
320
Inv. ex.


68
494
540


345
367
366





591
370
Inv. ex.


69
495
506


303
422
304





592
371
Inv. ex.


70
—
—
—
—
301
385
332
—
—
304
396

585
361
Comp.

















ex.


























TABLE 13











Hold-



Bainite
Martensite







ing


Alloy-
trans-
trans-




Reheating
Reheating
Reheating
proc-
Reheating
Reheating
ing
for-
for-




process 1
process 2
process 3
ess
process 4
process 5
process
mation
mation
























Cooling
Reheat
Cooling
Reheat
Cooling
Reheat
Hold-
Cooling
Reheat
Cooling
Reheat
Alloy-
start
start



Ex-
stop
stop
stop
stop
stop
stop
ing
stop
stop
stop
stop
ing
temp.
temp.



peri-
temp.
temp.
temp.
temp.
temp.
temp.
time
temp.
temp.
temp.
temp.
temp.
(Bs)
(Ms)



ment
° C.
° C.
° C.
° C.
° C.
° C.
Sec.
° C.
° C.
° C.
° C.
° C.
° C.
° C.

























71
476
526


329
361
299
376
539


537
585
361
Inv. ex.


72
487
513


308
379
273





585
415
Inv. ex.


73
485
510




247


306
373

582
399
Inv. ex.


74
469
530


335
397
7





568
395
Comp.

















ex.


75
460
504


296
414
228
417
528


528
568
389
Inv. ex.


76
486
515
473
523
215
414
238





498
249
Inv. ex.


77
468
513




250


216
371

501
224
Inv. ex.


78
472
554


250
367
3600





528
269
Comp.

















ex.


79
487
545


232
395
269
396
521


521
526
257
Inv. ex.


80
482
537


342
397
453





511
348
Inv. ex.


81
487
536


296
417
431





496
330
Inv. ex.


82
444
534
—
—
—
—
488
440
515
—
—

483
334
Comp.

















ex.


83
458
527


292
409
492





515
353
Inv. ex.


84
453
542


189
425
288





472
219
Inv. ex.


85
489
551
490
520


263
406
509
227
366

504
266
Inv. ex.


86
481
513


232
414
252




513
480
268
Inv. ex.


87
454
535


257
400
244





505
281
Inv. ex.


88
485
535


313
365
255





548
385
Inv. ex.


89




330
411
210
414
506
318
378

529
348
Inv. ex.


90
447
517


316
381
264




517
541
369
Inv. ex.


91
493
505
462
512
291
393
276


332
364

547
380
Inv. ex.


92
484
529


219
400
147





528
226
Inv. ex.


93




248
392
131





514
285
Inv. ex.


94
497
534
459
519
279
384
128




519
544
289
Inv. ex.


95
458
535
469
504
239
402
125
382
513
258
357

532
271
Inv. ex.


96
435
507


311
379
622





444
322
Inv. ex.


97




304
376
456
419
509
330
368

459
344
Inv. ex.


98
427
504


306
364
537





451
337
Inv. ex.


99
415
522




526


312
379
522
438
321
Inv. ex.


100
472
527


289
365
61





591
408
Inv. ex.


101
492
527
451
536
302
362
133





550
330
Inv. ex.


102
459
504


323
359
534




504
551
354
Inv. ex.


103
461
524




246


285
353

545
317
Inv. ex.


104
453
517


288
409
372





521
373
Inv. ex.


105




336
397
353
453
530
340
373

508
361
Inv. ex.


























TABLE 14











Hold-



Bainite
Martensite







ing


Alloy-
trans-
trans-




Reheating
Reheating
Reheating
proc-
Reheating
Reheating
ing
for-
for-




process 1
process 2
process 3
ess
process 4
process 5
process
mation
mation
























Cooling
Reheat
Cooling
Reheat
Cooling
Reheat
Hold-
Cooling
Reheat
Cooling
Reheat
Alloy-
start
start



Ex-
stop
stop
stop
stop
stop
stop
ing
stop
stop
stop
stop
ing
temp.
temp.



peri-
temp.
temp.
temp.
temp.
temp.
temp.
time
temp.
temp.
temp.
temp.
temp.
(Bs)
(Ms)



ment
° C.
° C.
° C.
° C.
° C.
° C.
Sec.
° C.
° C.
° C.
° C.
° C.
° C.
° C.

























106
468
515


291
415
410





521
371
Inv. ex.


107
495
552
465
502
309
381
232
441
539



519
362
Inv. ex.


108
441
523


300
368
783





469
335
Inv. ex.


109
422
519


309
419
625





449
316
Inv. ex.


110
429
535


284
378
655




488
457
301
Inv. ex.


111
441
541
444
518


285


299
378

466
305
Inv. ex.


112
373
542


200
355
272





384
213
Inv. ex.


113
373
540


213
422
220





387
245
Inv. ex.


114
409
550
396
515


270


240
369

412
255
Inv. ex.


115
399
536


218
409
286





410
252
Comp.

















ex.


116
471
506


302
409
232





—
—
Comp.

















ex.


117
477
527


289
377
284





595
401
Comp.

















ex.


118
481
527


275
374
228





648
304
Comp.

















ex.


119
438
540




275


319
375

449
300
Inv. ex.


120
454
511


308
375
101





468
338
Inv. ex.


121




344
379
106
365
503


500
485
366
Inv. ex.


122
464
526




77


275
382

565
383
Inv. ex.


123




357
399
100
380
519


517
558
373
Inv. ex.


124
484
532


298
405
98





575
387
Inv. ex.


125




343
370
87
361
509


503
574
385
Inv. ex.


126
473
536




73


241
357

470
286
Inv. ex.


127




297
380
100
349
508


498
487
314
Inv. ex.









In the annealing process, first, the steel sheets were heated to the maximum heating temperatures described in Tables 7 to 10 by average heating rates between the (maximum heating temperature −20° C.) to maximum heating temperature of the average heating rates described in Tables 7 to 10. Next, in the first cooling process (primary cooling) from the maximum heating temperature to 700° C., they were cooled by the average cooling rates described in Tables 7 to 10. Furthermore, in the second cooling process (secondary cooling) from 700° C. to 500° C., they were cooled by the average cooling rates described in Tables 7 to 10.
After that, the steel sheets were reheated from the Bs point or 480° C. or less to 500° C. or more 1 to 3 times (reheating processes 1, 2, and 4) and, furthermore, were reheated from the Ms point or 350° C. or less to 350° C. or more 1 or 2 times (reheating processes 3 and 5).
After the reheating process 3, the steel sheets were held at 300 to 450° C. in range for exactly the times described in Tables 11 to 14 then were treated by the reheating processes 4 and 5 and cooled down to room temperature.
After being cooled down to room temperature, in Experiments 6 to 49, the steel sheets were cold rolled by 0.15%, in Experiments 60 to 83, the steel sheets were cold rolled by 0.30%, in Experiment 89, the steel sheet was cold rolled by 1.50%, in Experiment 93, the steel sheet was cold rolled by 1.00%, and in Experiments 96 to 118 and 120 to 127, the steel sheets were cold rolled by 0.25%.
The types of steels in the experiments are shown in the tables as cold rolled steel sheet (CR), hot rolled steel sheet (HR), electrolytically galvanized steel sheet (EG), hot dip galvanized steel sheet (GI), hot dip galvannealed steel sheet (GA), and hot rolled hot dip galvannealed steel sheet (HR-GA) (same in tables shown below).
Experiments 13, 23, 33, 91, 95, 107, and 114 are examples in which the steel sheets were electroplated after the annealing process to obtain galvanized steel sheets (EG).
Experiments 4, 18, 43, 83, and 87 are examples in which after the second cooling process, the steel sheets are dipped in a galvanization bath until the holding treatment at 350 to 450° C. in range to obtain hot dipped galvanized steel sheets (GI).
Experiments 48, 53, 58, 98, and 103 are examples in which after the holding treatment at 300 to 450° C. in range, the steel sheets are dipped in a galvanization bath, then are cooled to room temperature to obtain hot dipped galvanized steel sheets (GI).
Experiments 3, 5, 9, 34, 38, 44, 49, 67, 86, 90, 94, 99, 102, and 110 are examples in which after the second cooling process, the steel sheets are dipped in a galvanization bath until holding at 350 to 450° C. in range and then are treated for alloying at the described temperatures to obtain galvannealed steel sheets (GA).
Experiments 22, 24, 54, 59, 63, 71, 75, 79, 121, 123, 125, and 127 are examples in which after the holding treatment at 300 to 450° C. in range, the steel sheets are dipped in a galvanization bath and treated for alloying at the described temperatures to obtain hot dipped galvannealed steel sheets (GA).
Experiments 9, 63, and 90 are examples in which the surfaces of the plating layers are given films comprised of P-based composite oxides.
Tables 15 to 18 give the results of analysis of the microstructures of the steel sheets of Experiments 1 to 127. In the microstructure fractions, the amounts of residual austenite (residual γ) were measured by X-ray diffraction at planes parallel to the sheet thickness at 1⁄4 thickness. The rest gives the results of measurement of the fractions of microstructures in the range of 1⁄8 thickness to 3⁄8 thickness. Sheet thickness cross-sections parallel to the rolling direction were cut out, polished to mirror surfaces, etched by Nital, then examined using field emission scanning electron microscope (FE-SEM).









TABLE 15








Microstructure observation results




Volume fraction



















Chemical
Steel
F
B
BF
TM
M
Residual γ
Others



Experiment
ingredients
type
%
%
%
%
%
%
%




















1
A
CR
12
6
41
32
0
 8
1
Inv. ex.


2
A
CR
55
5
17
18
1
 4
0
Inv. ex.


3
A
GA
54
6
25
10
0
 5
0
Inv. ex.


4
A
GI
54
18
9
15
1
 3
0
Inv. ex.


5
A
HR-GA
57
3
20
13
0
 6
1
Inv. ex.


6
B
CR
36
8
23
21
1
10
1
Inv. ex.


7
B
CR
39
5
17
28
0
11
0
Inv. ex.


8
B
CR
26
4
42
15
0
13
0
Inv. ex.


9
B
GA
41
5
31
8
0
15
0
Inv. ex.


10
C
CR
13
11
31
39
0
 6
0
Inv. ex.


11
C
CR
13
2
42
36
0
 7
0
Inv. ex.


12
C
CR
27
3
28
36
1
 5
0
Inv. ex.


13
C
EG
14
5
28
48
0
 5
0
Inv. ex.


14
C
HR
17
4
50
20
2
 5
2
Inv. ex.


15
D
CR
32
1
21
30
0
15
1
Inv. ex.


16
D
CR
33
2
26
28
0
11
0
Inv. ex.


17
D
CR
32
6
18
29
1
13
1
Inv. ex.


18
D
GI
31
2
17
33
0
17
0
Inv. ex.


19
D
HR
32
7
20
22
0
19
0
Inv. ex.


20
E
CR
43
1
24
20
0
12
0
Inv. ex.


21
E
CR
38
4
20
30
0
 8
0
Inv. ex.


22
E
GA
31
3
21
25
2
16
2
Inv. ex.


23
E
EG
39
1
19
29
0
12
0
Inv. ex.


24
E
HR-GA
42
9
22
15
1
11
0
Inv. ex.


25
F
CR
41
3
18
29
0
 9
0
Inv. ex.


26
F
CR
15
0
29
46
0
 9
1
Inv. ex.


27
F
CR
12
3
51
25
0
 9
0
Inv. ex.


28
F
CR
 0
23
35
35
0
 6
1
Comp. ex.


29
F
HR
14
11
38
30
0
 7
0
Inv. ex.


30
G
CR
56
3
18
9
0
14
0
Inv. ex.


31
G
CR
50
0
22
11
0
17
0
Inv. ex.


32
G
CR
66
0
0
0
0
 0
34 
Comp. ex.


33
G
EG
55
0
23
10
0
12
0
Inv. ex.


34
G
HR-GA
53
3
13
22
0
 8
1
Inv. ex.


35
H
CR
37
7
17
26
0
13
0
Inv. ex.


















TABLE 16








Microstructure observation results




Volume fraction



















Chemical
Steel
F
B
BF
TM
M
Residual γ
Others



Experiment
ingredients
type
%
%
%
%
%
%
%




















36
H
CR
21
7
31
29
0
11
1
Inv. ex.


37
H
CR
 0
6
45
39
0
10
0
Comp. ex.


38
H
GA
58
2
12
15
0
13
0
Inv. ex.


39
H
HR
52
3
14
21
0
10
0
Inv. ex.


40
I
CR
50
4
18
23
0
5
0
Inv. ex.


41
I
CR
43
4
22
26
2
3
0
Inv. ex.


42
I
CR
67
18
2
5
0
0
8
Comp. ex.


43
I
GI
46
3
22
20
3
6
0
Inv. ex.


44
I
HR-GA
41
1
37
15
0
3
3
Inv. ex.


45
J
CR
38
7
14
28
0
13
0
Inv. ex.


46
J
CR
36
6
16
26
0
16
0
Inv. ex.


47
J
CR
73
13
2
5
0
1
6
Comp. ex.


48
J
GI
32
2
17
30
0
19
0
Inv. ex.


49
J
HR-GA
27
10
30
17
0
16
0
Inv. ex.


50
K
CR
28
2
41
14
0
15
0
Inv. ex.


51
K
CR
53
9
18
10
0
10
0
Inv. ex.


52
K
CR
35
0
33
17
3
12
0
Comp. ex.


53
K
GI
42
1
33
13
0
11
0
Inv. ex.


54
K
HR-GA
42
3
17
25
1
11
1
Inv. ex.


55
L
CR
40
13
37
5
0
5
0
Inv. ex.


56
L
CR
26
4
50
15
1
4
0
Inv. ex.


57
L
CR
39
11
36
7
1
6
0
Comp. ex.


58
L
GI
24
5
49
17
0
4
1
Inv. ex.


59
L
HR-GA
26
14
36
20
0
4
0
Inv. ex.


60
M
CR
32
5
36
18
0
9
0
Inv. ex.


61
M
CR
42
5
21
20
0
12
0
Inv. ex.


62
M
CR
49
4
24
7
2
14
0
Comp. ex.


63
M
GA
47
0
19
20
2
12
0
Inv. ex.


64
N
CR
0
1
55
35
0
9
0
Inv. ex.


65
N
CR
19
4
42
25
1
9
0
Inv. ex.


66
N
CR
39
1
23
22
1
14
0
Comp. ex.


67
N
GA
27
0
44
16
0
13
0
Inv. ex.


68
O
CR
36
1
20
34
2
7
0
Inv. ex.


69
O
CR
35
3
43
13
0
6
0
Inv. ex.


70
O
CR
41
5
33
13
2
6
0
Comp. ex.


















TABLE 17








Microstructure observation results




Volume fraction



















Chemical
Steel
F
B
BF
TM
M
Residual γ
Others



Experiment
ingredients
type
%
%
%
%
%
%
%




















71
O
GA
41
3
33
15
0
8
0
Inv. ex.


72
P
CR
0
5
39
46
1
9
0
Inv. ex.


73
P
CR
18
3
42
28
1
8
0
Inv. ex.


74
P
CR
35
4
31
13
8
9
0
Comp. ex.


75
P
GA
34
2
32
28
0
4
0
Inv. ex.


76
Q
CR
50
5
20
11
0
13
1
Inv. ex.


77
Q
CR
54
1
15
20
0
10
0
Inv. ex.


78
Q
CR
51
1
11
28
0
9
0
Comp. ex.


79
Q
GA
46
0
16
23
0
15
0
Inv. ex.


80
R
CR
36
0
30
25
0
7
2
Inv. ex.


81
R
CR
53
7
15
19
0
6
0
Inv. ex.


82
R
GA
55
4
13
20
2
6
0
Comp. ex.


83
R
GI
41
0
18
29
3
6
3
Inv. ex.


84
S
CR
52
10
13
11
0
14
0
Inv. ex.


85
S
CR
43
8
13
24
0
12
0
Inv. ex.


86
S
GA
50
5
15
17
0
13
0
Inv. ex.


87
S
GI
45
1
20
16
2
15
1
Inv. ex.


88
T
CR
3
4
60
27
1
5
0
Inv. ex.


89
T
CR
37
0
22
33
1
7
0
Inv. ex.


90
T
GA
15
2
35
40
0
7
1
Inv. ex.


91
T
EG
12
1
57
21
0
8
1
Inv. ex.


92
U
CR
57
2
17
9
1
13
1
Inv. ex.


93
U
CR
52
6
23
9
0
10
0
Inv. ex.


94
U
GA
46
5
22
16
0
11
0
Inv. ex.


95
U
EG
53
0
21
17
0
9
0
Inv. ex.


96
V
CR
30
5
42
13
2
7
1
Inv. ex.


97
V
CR
14
5
36
36
1
7
1
Inv. ex.


98
V
GI
15
1
30
44
0
10
0
Inv. ex.


99
V
GA
32
4
28
30
0
6
0
Inv. ex.


100
W
CR
17
11
33
30
1
7
1
Inv. ex.


101
W
CR
68
1
9
18
0
4
0
Inv. ex.


102
W
GA
65
5
9
14
0
6
1
Inv. ex.


103
W
GI
65
1
22
9
0
3
0
Inv. ex.


104
X
CR
38
5
20
28
0
7
2
Inv. ex.


105
X
CR
47
1
27
21
0
4
0
Inv. ex.


















TABLE 18








Microstructure observation results




Volume fraction



















Chemical
Steel
F
B
BF
TM
M
Residual γ
Others



Experiment
ingredients
type
%
%
%
%
%
%
%




















106
X
CR
38
5
35
15
1
6
0
Inv. ex.


107
X
EG
47
4
26
19
0
3
1
Inv. ex.


108
Y
CR
41
1
31
18
0
8
1
Inv. ex.


109
Y
CR
54
4
29
8
0
5
0
Inv. ex.


110
Y
GA
52
13
15
14
0
6
0
Inv. ex.


111
Y
CR
48
3
19
19
1
10 
0
Inv. ex.


112
Z
CR
59
3
23
6
0
9
0
Inv. ex.


113
Z
CR
62
1
13
15
0
9
0
Inv. ex.


114
Z
EG
59
5
10
17
0
9
0
Inv. ex.


115
Z
CR
60
7
10
12
0
9
2
Comp. ex.


116
AA
CR
98
0
0
0
0
0
2
Comp. ex.


117
AB
CR
35
31
8
23
0
0
3
Comp. ex.


118
AC
CR
72
15
0
9
0
0
4
Comp. ex.


119
B
HR
31
5
34
13
0
17 
0
Inv. ex.


120
AD
CR
48
7
24
15
0
6
0
Inv. ex.


121
AD
GA
22
21
37
16
0
4
0
Inv. ex.


122
AE
CR
4
31
33
19
1
11 
1
Inv. ex.


123
AE
GA
16
28
38
10
0
8
0
Inv. ex.


124
AF
CR
7
0
31
45
2
13 
2
Inv. ex.


125
AF
GA
9
7
43
28
0
12 
1
Inv. ex.


126
AG
CR
36
6
23
21
0
14 
0
Inv. ex.


127
AG
GA
22
31
22
14
2
9
0
Inv. ex.









Tables 19 to 22 show the results of measurement of the residual austenite fractions and the amounts of solid solution C in the residual austenite after deep cooling treatment tests. These were measured by X-ray diffraction at planes parallel to the sheet thickness at 1⁄4 thickness. The Msr points were measured by preparing liquid nitrogen (−198° C.) and ethanol cooled using liquid nitrogen in 20° C. increments from 0° C. to −100° C., holding the steel sheets at those temperatures for 1 hour, then measuring the residual austenite fractions and using the maximum temperatures at which the austenite fractions fall as the Msr points of the residual austenite phase.









TABLE 19








Residual γ transformation temp.


















Volume








fraction after








liquid
Solid







nitrogen
solution




Chemical
Steel
Msr point
dipping
C amount
















Experiment
ingredients
type
° C.
1st
3rd
5th
%



















1
A
CR
<−198
G
G
G
0.93
Inv. ex.


2
A
CR
<−198
G
G
G
0.88
Inv. ex.


3
A
GA
<−198
G
G
G
0.89
Inv. ex.


4
A
GI
<−198
G
G
G
1.07
Inv. ex.


5
A
HR-GA
<−198
G
G
G
0.89
Inv. ex.


6
B
CR
<−198
G
G
G
0.95
Inv. ex.


7
B
CR
<−198
G
G
G
1.02
Inv. ex.


8
B
CR
<−198
G
G
G
1.07
Inv. ex.


9
B
GA
<−198
G
G
G
0.96
Inv. ex.


10
C
CR
<−198
G
G
G
0.96
Inv. ex.


11
C
CR
<−198
G
G
G
0.90
Inv. ex.


12
C
CR
−100 to −80
P


1.01
Inv. ex.


13
C
EG
<−198
G
G
G
1.01
Inv. ex.


14
C
HR
<−198
G
G
G
0.99
Inv. ex.


15
D
CR
<−198
G
G
G
0.98
Inv. ex.


16
D
CR
<−198
G
G
G
1.03
Inv. ex.


17
D
CR
−100 to −80
P


0.92
Inv. ex.


18
D
GI
<−198
G
G
G
0.88
Inv. ex.


19
D
HR
<−198
G
G
G
0.95
Inv. ex.


20
E
CR
<−198
G
G
G
0.94
Inv. ex.


21
E
CR
<−198
G
G
G
1.05
Inv. ex.


22
E
GA
<−198
G
G
G
1.04
Inv. ex.


23
E
EG
<−198
G
G
G
0.88
Inv. ex.


24
E
HR-GA
<−198
G
G
G
0.96
Inv. ex.


25
F
CR
<−198
G
G
G
1.01
Inv. ex.


26
F
CR
<−198
G
G
G
1.02
Inv. ex.


27
F
CR
<−198
G
G
G
1.05
Inv. ex.


28
F
CR
<−198
G
G
G
0.97
Comp. ex.


29
F
HR
<−198
G
G
G
0.93
Inv. ex.


30
G
CR
<−198
G
G
G
0.98
Inv. ex.


31
G
CR
<−198
G
G
G
1.07
Inv. ex.











32
G
CR
No residual austenite
Comp. ex.















33
G
EG
<−198
G
G
G
1.04
Inv. ex.


34
G
HR-GA
<−198
G
G
G
1.06
Inv. ex.


35
H
CR
<−198
G
G
G
0.99
Inv. ex.


















TABLE 20








Residual γ transformation temp.


















Volume








fraction after








liquid
Solid







nitrogen
solution




Chemical
Steel
Msr point
dipping
C amount
















Experiment
ingredients
type
° C.
1st
3rd
5th
%






36
H
CR
<−198
G
G
G
0.99
Inv. ex.


37
H
CR
<−198
G
G
G
1.06
Comp. ex.


38
H
GA
<−198
G
G
G
0.92
Inv. ex.


39
H
HR
<−198
G
G
G
0.97
Inv. ex.


40
I
CR
<−198
G
G
G
0.91
Inv. ex.


41
I
CR
<−198
G
G
G
0.90
Inv. ex.











42
I
CR
No residual austenite
Comp. ex.















43
I
GI
<−198
G
G
G
1.03
Inv. ex.


44
I
HR-GA
<−198
G
G
G
1.02
Inv. ex.


45
J
CR
<−198
G
G
G
0.94
Inv. ex.


46
J
CR
<−198
G
G
G
1.00
Inv. ex.


47
J
CR
  >0
P


0.98
Comp. ex.


48
J
GI
<−198
G
G
G
0.95
Inv. ex.


49
J
HR-GA
<−198
G
G
G
1.03
Inv. ex.


50
K
CR
<−198
G
G
G
1.06
Inv. ex.


51
K
CR
<−198
G
G
G
0.93
Inv. ex.


52
K
CR
−40 to −20
P


0.99
Comp. ex.


53
K
GI
<−198
G
G
G
0.88
Inv. ex.


54
K
HR-GA
<−198
G
G
G
1.03
Inv. ex.


55
L
CR
<−198
G
G
G
1.06
Inv. ex.


56
L
CR
<−198
G
G
G
0.95
Inv. ex.


57
L
CR
−60 to −40
P


0.98
Comp. ex.


58
L
GI
<−198
G
G
G
0.97
Inv. ex.


59
L
HR-GA
<−198
G
G
G
0.92
Inv. ex.


60
M
CR
<−198
G
G
G
1.01
Inv. ex.


61
M
CR
<−198
G
G
G
1.00
Inv. ex.


62
M
CR
−40 to −20
P


0.93
Comp. ex.


63
M
GA
<−198
G
G
G
1.02
Inv. ex.


64
N
CR
<−198
G
G
G
0.96
Inv. ex.


65
N
CR
<−198
G
G
G
0.91
Inv. ex.


66
N
CR
−40 to −20
P


0.92
Comp. ex.


67
N
GA
<−198
G
G
G
0.94
Inv. ex.


68
O
CR
<−198
G
G
G
0.99
Inv. ex.


69
O
CR
<−198
G
G
G
0.95
Inv. ex.


70
O
CR
−60 to −40
P


0.94
Comp. ex.


















TABLE 21








Residual γ transformation temp.


















Volume








fraction after








liquid
Solid







nitrogen
solution




Chemical
Steel
Msr point
dipping
C amount
















Experiment
ingredients
type
° C.
1st
3rd
5th
%



















71
O
GA
<−198
G
G
G
0.90
Inv. ex.


72
P
CR
<−198
G
G
G
0.93
Inv. ex.


73
P
CR
<−198
G
G
G
1.06
Inv. ex.


74
P
CR
−20 to 0 
P


0.74
Comp. ex.


75
P
GA
<−198
G
G
G
1.02
Inv. ex.


76
Q
CR
<−198
G
G
G
1.04
Inv. ex.


77
Q
CR
<−198
G
G
G
1.02
Inv. ex.


78
Q
CR
−40 to −20
P


0.78
Comp. ex.


79
Q
GA
<−198
G
G
G
0.91
Inv. ex.


80
R
CR
<−198
G
G
G
1.05
Inv. ex.


81
R
CR
<−198
G
G
G
0.97
Inv. ex.


82
R
GA
−40 to −20
P


0.94
Comp. ex.


83
R
GI
<−198
G
G
G
1.01
Inv. ex.


84
S
CR
<−198
G
G
G
0.90
Inv. ex.


85
S
CR
<−198
G
G
G
1.02
Inv. ex.


86
S
GA
<−198
G
G
G
0.99
Inv. ex.


87
S
GI
<−198
G
G
G
1.05
Inv. ex.


88
T
CR
<−198
G
G
G
1.03
Inv. ex.


89
T
CR
<−198
G
G
G
1.03
Inv. ex.


90
T
GA
<−198
G
G
G
1.03
Inv. ex.


91
T
EG
<−198
G
G
G
1.03
Inv. ex.


92
U
CR
<−198
G
G
G
0.89
Inv. ex.


93
U
CR
<−198
G
G
G
1.02
Inv. ex.


94
U
GA
<−198
G
G
G
0.99
Inv. ex.


95
U
EG
<−198
G
G
G
0.92
Inv. ex.


96
V
CR
<−198
G
G
G
0.95
Inv. ex.


97
V
CR
<−198
G
G
G
0.94
Inv. ex.


98
V
GI
<−198
G
G
G
1.05
Inv. ex.


99
V
GA
<−198
G
G
G
0.95
Inv. ex.


100
W
CR
<−198
G
G
G
0.89
Inv. ex.


101
W
CR
<−198
G
G
G
0.93
Inv. ex.


102
W
GA
<−198
G
G
G
1.07
Inv. ex.


103
W
GI
<−198
G
G
G
0.92
Inv. ex.


104
X
CR
<−198
G
G
G
1.05
Inv. ex.


105
X
CR
<−198
G
G
G
1.00
Inv. ex.


















TABLE 22








Residual γ transformation temp.


















Volume








fraction after








liquid
Solid







nitrogen
solution




Chemical

Msr point
dipping
C amount
















Experiment
ingredients
Steel type
° C.
1st
3rd
5th
%






106
X
CR
<−198
G
P

0.89
Inv. ex.


107
X
EG
<−198
G
G
G
1.07
Inv. ex.


108
Y
CR
<−198
G
G
G
0.96
Inv. ex.


109
Y
CR
<−198
G
G
G
0.93
Inv. ex.


110
Y
GA
<−198
G
G
G
0.99
Inv. ex.


111
Y
CR
<−198
G
G
G
1.04
Inv. ex.


112
Z
CR
<−198
G
G
G
1.01
Inv. ex.


113
Z
CR
<−198
G
G
G
0.91
Inv. ex.


114
Z
EG
<−198
G
G
G
1.05
Inv. ex.


115
Z
CR
<−198
G
G
G
0.95
Comp. ex.











116
AA
CR
No residual austenite
Comp. ex.


117
AB
CR
No residual austenite
Comp. ex.


118
AC
CR
No residual austenite
Comp. ex.















119
B
HR
<−198
G
G
G
0.90
Inv. ex.


120
AD
CR
<−198
G
G
G
0.99
Inv. ex.


121
AD
GA
<−198
G
G
G
0.89
Inv. ex.


122
AE
CR
<−198
G
G
G
0.99
Inv. ex.


123
AE
GA
<−198
G
G
G
0.86
Inv. ex.


124
AF
CR
<−198
G
G
G
0.93
Inv. ex.


125
AF
GA
<−198
G
G
G
0.90
Inv. ex.


126
AG
CR
<−198
G
G
G
0.89
Inv. ex.


127
AG
GA
<−198
G
G
G
0.87
Inv. ex.









In the dipping treatment in liquid nitrogen, the operation from dipping the steel sheet in liquid nitrogen for 1 hour, then taking it out and allowing it to stand in the air until reaching room temperature is counted as one treatment. The residual austenite fractions were measured at the points of time of the ends of the first, third, and 10th treatments. Steel sheets with residual austenite fractions which did not change were evaluated as “G (good)” while steel sheets with residual austenite fractions which decreased were evaluated as “P (poor)”.
Tables 23 to 26 show the evaluation of properties of the steel sheets of Experiments 1 to 127. At that time, tensile test pieces based on JIS Z 2201 were taken from the steel sheets of Experiments 1 to 127 and were subjected to tensile tests based on JIS Z 2241 to measure the yield strength (YS), tensile strength (TS), and total elongation (EL).
FIG. 2 shows the relationship between the tensile strength (TS) and the total elongation (EL), while FIG. 3 shows the relationship between the tensile strength (TS) and the hole expansion rate (λ) which serves as an indicator of the stretch flangeability. The steel sheets of the present invention satisfy all of TS≧900 MPa, TS×EL≧17000 MPa·%, TS×λ≧24000 MPa·%. The steel sheets of the comparative examples are not steel sheets which satisfy all of these.










TABLE 23








Material





measurement results
TS ×


















Chemical
Steel
YS
TS
EL
λ
EL
TS × λ



Experiment
ingredients
type
MPa
MPa
%
%
MPa · %
MPa · %



















1
A
CR
943
1026
19
54
19494
55404
Inv. ex.


2
A
CR
503
1004
19
39
19076
39086
Inv. ex.


3
A
GA
474
 982
20
37
19640
36315
Inv. ex.


4
A
GI
492
 998
19
40
18962
40202
Inv. ex.


5
A
HR-GA
493
 941
23
48
21643
45168
Inv. ex.


6
B
CR
777
1102
19
42
20938
46284
Inv. ex.


7
B
CR
729
1143
20
41
22860
46863
Inv. ex.


8
B
CR
872
1426
14
24
19964
34224
Inv. ex.


9
B
GA
674
1163
21
39
24423
45357
Inv. ex.


10
C
CR
835
 980
23
45
22540
44100
Inv. ex.


11
C
CR
846
 992
20
46
19840
45632
Inv. ex.


12
C
CR
778
 969
20
27
19380
26163
Inv. ex.


13
C
EG
959
1074
20
47
21480
50478
Inv. ex.


14
C
HR
707
 937
20
47
18740
43955
Inv. ex.


15
D
CR
906
1213
19
38
23047
46094
Inv. ex.


16
D
CR
873
1205
18
47
21690
56635
Inv. ex.


17
D
CR
830
1193
19
24
22667
28632
Inv. ex.


18
D
GI
1000
1306
17
37
22202
48322
Inv. ex.


19
D
HR
752
1179
20
36
23580
42444
Inv. ex.


20
E
CR
611
1094
17
43
18598
47042
Inv. ex.


21
E
CR
694
1168
19
43
22192
50224
Inv. ex.


22
E
GA
836
1093
19
39
20767
42627
Inv. ex.


23
E
EG
723
1122
16
44
17952
49368
Inv. ex.


24
E
HR-GA
657
1055
20
51
21100
53805
Inv. ex.


25
F
CR
578
1010
23
48
23230
48480
Inv. ex.


26
F
CR
762
1021
21
52
21441
53092
Inv. ex.


27
F
CR
945
1129
19
41
21451
46289
Inv. ex.


28
F
CR
1061
1137
 8
14
 9096
15918
Comp. ex.


29
F
HR
829
1078
18
44
19404
47432
Inv. ex.


30
G
CR
505
1093
22
41
24046
44813
Inv. ex.


31
G
CR
699
1160
18
50
20880
58000
Inv. ex.


32
G
CR
483
 642
15
17
 9630
10914
Comp. ex.


33
G
EG
581
1147
22
50
25234
57350
Inv. ex.


34
G
HR-GA
619
1158
17
48
19686
55584
Inv. ex.


35
H
CR
665
1071
16
46
17136
49596
Inv. ex.



















TABLE 24








Material





measurement results
TS ×


















Chemical
Steel
YS
TS
EL
λ
EL
TS × λ



Experiment
ingredients
type
MPa
MPa
%
%
MPa · %
MPa · %



















36
H
CR
942
1155
18
52
20790
60060
Inv. ex.


37
H
CR
1017
1106
 8
 9
 8848
 9954
Comp. ex.


38
H
GA
477
1103
19
37
20957
40811
Inv. ex.


39
H
HR
590
1095
19
48
20805
52560
Inv. ex.


40
I
CR
546
 935
23
49
21505
45815
Inv. ex.


41
I
CR
513
 925
19
63
17575
58275
Inv. ex.


42
I
CR
460
 861
13
 2
11193
 1722
Comp. ex.


43
I
GI
564
 998
18
48
17964
47520
Inv. ex.


44
I
HR-GA
630
 955
18
59
17190
56015
Inv. ex.


45
J
CR
680
1038
22
43
22836
44634
Inv. ex.


46
J
CR
606
 991
22
50
21802
49550
Inv. ex.


47
J
CR
511
 765
16
 3
12240
 2295
Comp. ex.


48
J
GI
652
 977
24
45
23448
43965
Inv. ex.


49
J
HR-GA
743
1046
21
37
21966
38702
Inv. ex.


50
K
CR
820
1154
20
49
23080
56546
Inv. ex.


51
K
CR
584
1118
20
47
22360
52546
Inv. ex.


52
K
CR
895
1227
13
 1
15951
 1227
Comp. ex.


53
K
GI
720
1142
18
44
20556
50248
Inv. ex.


54
K
HR-GA
766
1141
16
40
18256
45640
Inv. ex.


55
L
CR
615
 998
19
38
18962
37924
Inv. ex.


56
L
CR
687
 925
22
47
20350
43475
Inv. ex.


57
L
CR
656
 964
21
14
20244
13496
Comp. ex.


58
L
GI
736
1024
18
42
18432
42634
Inv. ex.


59
L
HR-GA
732
 998
18
48
17964
47520
Inv. ex.


60
M
CR
1013
1346
16
30
21536
40380
Inv. ex.


61
M
CR
1076
1421
15
28
21315
39788
Inv. ex.


62
M
CR
826
1420
18
 4
25560
 5680
Comp. ex.


63
M
GA
915
1443
13
26
18759
37518
Inv. ex.


64
N
CR
1249
1443
14
29
20202
41847
Inv. ex.


65
N
CR
962
1375
16
31
22000
42625
Inv. ex.


66
N
CR
806
1333
17
 8
22661
10664
Comp. ex.


67
N
GA
932
1353
19
30
25707
40590
Inv. ex.


68
O
CR
681
1019
20
49
20380
49931
Inv. ex.


69
O
CR
655
 980
24
35
23520
34300
Inv. ex.


70
O
CR
615
1021
19
15
19399
15315
Comp. ex.



















TABLE 25








Material





measurement results
TS ×


















Chemical
Steel
YS
TS
EL
λ
EL
TS × λ



Experiment
ingredients
type
MPa
MPa
%
%
MPa · %
MPa · %



















71
O
GA
659
1081
19
42
20539
45402
Inv. ex.


72
P
CR
1002
1099
19
38
20881
41762
Inv. ex.


73
P
CR
791
1034
18
39
18612
40672
Inv. ex.


74
P
CR
707
980
19
 4
18620
 3920
Comp. ex.


75
P
GA
666
963
20
41
19260
39650
Inv. ex.


76
Q
CR
642
1118
17
37
19006
41366
Inv. ex.


77
Q
CR
569
1182
16
40
18912
47280
Inv. ex.


78
Q
CR
598
1206
17
 7
20502
 8005
Comp. ex.


79
Q
GA
601
1101
22
50
24222
55050
Inv. ex.


80
R
CR
709
1162
19
38
22078
44156
Inv. ex.


81
R
CR
525
1070
20
38
21400
40660
Inv. ex.


82
R
GA
582
1134
18
16
20412
18144
Comp. ex.


83
R
GI
732
1128
16
43
18048
48504
Inv. ex.


84
S
CR
607
1228
16
39
19648
47892
Inv. ex.


85
S
CR
724
1209
18
41
21762
49569
Inv. ex.


86
S
GA
622
1211
20
54
24220
65394
Inv. ex.


87
S
GI
740
1238
17
47
21046
58186
Inv. ex.


88
T
CR
1107
1157
16
36
18512
41652
Inv. ex.


89
T
CR
871
1224
17
38
20808
46512
Inv. ex.


90
T
GA
916
1149
19
41
21831
47109
Inv. ex.


91
T
EG
1089
1184
16
44
18944
52096
Inv. ex.


92
U
CR
529
1130
20
54
22600
61020
Inv. ex.


93
U
CR
597
1137
17
43
19329
48891
Inv. ex.


94
U
GA
622
1052
20
52
21040
54704
Inv. ex.


95
U
EG
559
1042
19
40
19798
41680
Inv. ex.


96
V
CR
934
1210
15
41
18150
49610
Inv. ex.


97
V
CR
1055
1247
17
47
21199
58609
Inv. ex.


98
V
GI
900
1150
17
50
19550
57500
Inv. ex.


99
V
GA
795
1155
15
44
17325
50820
Inv. ex.


100
W
CR
878
982
22
45
21604
44190
Inv. ex.


101
W
CR
366
977
23
40
22471
39080
Inv. ex.


102
W
GA
375
934
21
51
19614
47634
Inv. ex.


103
W
GI
391
1013
17
44
17221
44572
Inv. ex.


104
X
CR
764
1104
16
47
17664
51610
Inv. ex.


105
X
CR
626
1112
18
41
20016
45592
Inv. ex.



















TABLE 26








Material





measurement results
TS ×


















Chemical
Steel
YS
TS
EL
λ
EL
TS × λ



Experiment
ingredients
type
MPa
MPa
%
%
MPa · %
MPa · %





106
X
CR
707
1136
17
23
19312
26201
Inv. ex.


107
X
EG
663
1079
20
46
21580
49634
Inv. ex.


108
Y
CR
742
1080
19
42
20520
45360
Inv. ex.


109
Y
CR
619
1128
19
39
21432
43992
Inv. ex.


110
Y
GA
543
1125
17
36
19125
40500
Inv. ex.


111
Y
CR
673
1188
19
47
22572
55836
Inv. ex.


112
Z
CR
606
1198
18
43
21564
51514
Inv. ex.


113
Z
CR
572
1245
17
33
21165
41085
Inv. ex.


114
Z
EG
583
1196
19
34
22724
40664
Inv. ex.


115
Z
CR
554
1152
 4
16
 4608
18432
Comp. ex.


116
AA
CR
323
 424
38
107 
16112
45368
Comp. ex.


117
AB
CR
683
 766
15
28
11490
21448
Comp. ex.


118
AC
CR
398
 834
22
35
18348
29190
Comp. ex.


119
B
HR
833
1167
17
40
19839
46680
Inv. ex.


120
AD
CR
641
 906
22
45
19932
40770
Inv. ex.


121
AD
GA
734
 966
23
43
22218
41538
Inv. ex.


122
AE
CR
953
1156
20
39
23120
45084
Inv. ex.


123
AE
GA
890
1135
18
35
20430
39725
Inv. ex.


124
AF
CR
891
1169
18
56
21042
65464
Inv. ex.


125
AF
GA
879
1234
17
38
20978
46892
Inv. ex.


126
AG
CR
635
1152
19
39
21888
44928
Inv. ex.


127
AG
GA
701
1055
21
46
22155
48530
Inv. ex.









Experiment 115 is an example in which the end temperature of the hot rolling is low. The microstructure is stretched in one direction making it uneven, so the ductility and stretch flangeability are poor.
Experiments 12, 17, 106, and 111 are examples in which the heating rate from the (maximum heating temperature −20° C.) in the heating process is large. The residual austenite phase is unstable and the stretch flangeability is poor.
Experiment 28 is an example in which the maximum heating temperature in the annealing process is high. The soft structure is not sufficiently formed and the ductility is poor.
Experiment 32 is an example in which the maximum heating temperature in the annealing process is low. A large number of coarse iron-based carbides which form starting points of fracture are included, so the ductility and stretch flangeability are poor.
Experiment 37 is an example in which the average cooling rate in the first cooling process (primary cooling) is high. Soft structures are not sufficiently formed, so the ductility and stretch flangeability are poor.
Experiment 42 is an example in which the average cooling rate in the first cooling process (primary cooling) is low. Coarse iron-based carbides are formed, and the stretch flangeability is poor
Experiment 47 is an example in which the cooling rate in the second cooling process (secondary cooling) is low. Coarse iron-based carbides are formed, and the stretch flangeability is poor.
Experiment 52 is an example where no reheating treatment is performed. The residual austenite phase is unstable, and the stretch flangeability is poor.
Experiments 57, 66, and 82 are examples where only reheating from the Bs point or 480° C. or less to 500° C. or more is performed. The residual austenite phase is unstable, and the stretch flangeability is poor.
Experiments 62 and 70 are examples where only reheating from the Ms point or 350° C. or less to 350° C. or more is performed. The residual austenite phase is unstable, and the stretch flangeability is poor.
Experiment 74 is an example where the time of the treatment at 300 to 450° C. in range is short. Carbon does not concentrate at the residual austenite, the residual austenite phase is unstable, and the stretch flangeability is poor.
Next, Experiment 78 is an example where the holding time at 300 to 450° C. in range is long. Iron-based carbides form, the amount of solid solution C in the residual austenite falls, the residual austenite phase is unstable, and the stretch flangeability is poor.
Next, Experiments 116 to 118 are examples where the composition of ingredients deviated from the predetermined range. In each case, sufficient properties could not be obtained.
From the results of the examples which were explained above, it is clear that according to the high strength steel sheet and high strength galvanized steel sheet which are excellent in shapeability and methods of production of the same of the present invention, high strength steel sheet which secures a tensile maximum strength of 900 MPa or more in high strength while is given excellent ductility and stretch flangeability and has sufficiently high shapeability is obtained.
INDUSTRIAL APPLICABILITY
According to the present invention, for example, in applications such as members which are obtained by shaping steel sheet by press working etc., a tensile maximum strength of 900 MPa or more of high strength is secured while excellent ductility and stretch flangeability are obtained and excellent strength and shapeability are simultaneously obtained. Due to this, for example, in particular, by applying the present invention to the field of auto parts etc., in particular by applying it to the field of automobiles, it is possible to fully enjoy the merits of improved safety along with the increased strength of the chassis, improved shapeability at the time of working the members, etc. The contribution to society is immeasurable.

## Claims

1. High strength steel sheet which is excellent in shapeability which contains, by mass %,
C: 0.075 to 0.300%,
Si: 0.70 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.030%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
N: 0.0001 to 0.0100%, and
O: 0.0001 to 0.0100%,

which contains, as optional elements, one or more of
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.005 to 0.150%, and

one or more of Ca, Ce, Mg, Zr, Hf, and REM: total 0.0001 to 0.5000%, and
has a balance of iron and unavoidable impurities, wherein
the structure of the steel sheet contains, by volume fraction,
residual austenite phase: 2 to 20%
ferrite phase: 10 to 75%,
bainitic ferrite phase and/or bainite phase: 10 to 50%,
tempered martensite phase: 10 to 50%, and
fresh martensite phase: 10% or less, and
said residual austenite phase has a martensite transformation point of −60° C. or less,
wherein the residual austenite fraction does not change after 5 cycles, one cycle of the S cycles comprising the steps of dipping the steel sheet in liquid nitrogen for 1 hour, taking the steel sheet out of the liquid nitrogen, and allowing the steel sheet to stand in the air until reaching room temperature.

2. The high strength steel sheet which is excellent in shapeability according to claim 1, characterized in that a ratio of said residual austenite phase which transforms to martensite at −198° C. is, by volume fraction, 2% or less of the total residual austenite phase.

3. The high strength steel sheet which is excellent in shapeability according to claim 2, characterized in that said residual austenite phase has a martensite transformation point of −198° C. or less.

4. High strength galvanized steel sheet which is excellent in shapeability characterized by comprising the high strength steel sheet according to claim 2 on the surface of which a galvanized layer is formed.

5. The high strength steel sheet which is excellent in shapeability according to claim 1, characterized in that said residual austenite phase has a martensite transformation point of −198° C. or less.

6. High strength galvanized steel sheet which is excellent in shapeability characterized by comprising the high strength steel sheet according to claim 1 on the surface of which a galvanized layer is formed.

7. A method of production of the high strength steel sheet which is excellent in shapeability according to claim 1, characterized by comprising:
a hot rolling process of heating a slab which contains, by mass %,
C: 0.075 to 0.300%,
Si: 0.70 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.030%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
N: 0.0001 to 0.0100%, and
O: 0.0001 to 0.0100%,
which contains, as optional elements, one or more of:
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.005 to 0.150%, and
one or more of Ca, Ce, Mg, Zr, Hf, and REM: total 0.0001 to 0.5000%, and
has a balance of iron and unavoidable impurities,
directly, or after cooling once, to 1050° C. or more, finishing the rolling at the Ar3 point or more to obtain a steel sheet, and coiling it at 500 to 750° C. in temperature,
a cold rolling process of pickling the coiled steel sheet, then cold rolling it by a screwdown rate of a screwdown rate 35 to 75%, and
an annealing process of heating the steel sheet after the cold rolling process up to a maximum heating temperature of 740 to 1000° C., then cooling by an average cooling rate from said maximum heating temperature to 700° C. of 1.0 to 10.0° C./sec and by a 700 to 500° C. average cooling rate of 5.0 to 200° C./sec, next holding at 350 to 450° C. for 30 to 1000 seconds, then cooling down to room temperature and, while cooling from said maximum heating temperature to room temperature, reheating from the Bs point or less than 500° C. to 500° C. or more at least once and reheating from the Ms point or less than 350° C. to 350° C. or more at least once.

8. A method of production of high strength galvanized steel sheet which is excellent in shapeability characterized by producing high strength steel sheet by the method of production of high strength steel sheet according to claim 7, then galvanizing it.

9. A method of production of high strength galvanized steel sheet which is excellent in shapeability characterized by producing high strength steel sheet by the method of production according to claim 7 during the annealing process of which, at the time of cooling from said maximum heating temperature to room temperature, dipping the steel sheet after said cold rolling process in a zinc bath so as to hot dip galvanize it.

10. A method of production of high strength galvanized steel sheet which is excellent in shapeability according to claim 9 characterized by performing alloying treatment at 470 to 650° C. in temperature after said hot dip galvanization.

11. A method of production of high strength galvanized steel sheet which is excellent in shapeability characterized by producing high strength steel sheet by the method of production according to claim 7 after the annealing process of which performing hot dip galvanization.

12. A method of production of high strength galvanized steel sheet which is excellent in shapeability according to claim 11 characterized by performing alloying treatment at 470 to 650° C. in temperature after said hot dip galvanization.