# Patent

## Title
High strength steel sheet having excellent formability and stability of mechanical properties and method for manufacturing the same

## Abstract

A high strength steel sheet including, by mass, C: 0.03% or more and 0.25% or less, Si: 0.4% or more and 2.5% or less, Mn: 3.5% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% or more and 2.5% or less, N: 0.008% or less, Si+Al: 1.0% or more, the balance being Fe and inevitable impurities. The area ratio of ferrite is 30% or more and 80% or less, the area ratio of martensite is 0% or more and 17% or less, the volume fraction of retained austenite is 8% or more, and the average grain size of retained austenite is 2 μm or less.

## Background
TECHNICAL FIELD
This disclosure relates to a high strength steel sheet having excellent formability and stability of mechanical properties, the steel sheet being suitable as a member for use in industrial fields such as the automotive industry and the electric industry, and a method of manufacturing the steel sheet.
BACKGROUND
Recently, improving fuel economy of automobiles has been an important issue from the viewpoint of global environmental conservation. Accordingly, there have been active movements to reduce the weight of car bodies by increasing the strength of the material of car bodies to thereby allow the thickness of the material to be reduced. However, an increase in the strength of a steel sheet results in deterioration of formability. Thus, currently, development of a material having both high strength and high formability has been anticipated. Moreover, when the high strength steel sheet is formed into a complicated shape such as that of automobile components, the occurrences of cracking and necking at an overhanging portion and a stretch flange portion are big issues. Thus, a high strength steel sheet having both high ductility and good hole expandability, the steel sheet being capable of overcoming the issues of the occurrences of cracking and necking, has been needed. Furthermore, increasing the strength of a steel sheet and reducing the thickness of a steel sheet significantly deteriorate shape fixability. To address this, in press-forming, a method of designing a die while expecting there to be a change in shape after releasing, which is estimated in advance, has been widely employed. However, when the tensile strength (TS) of a steel sheet varies, the deviation from the change in shape estimated by assuming a constant TS increases. Consequently, defects in shape occur and, therefore, individual reworking operations after the press-forming such as shaping by sheet metal processing, are necessary. As a result, mass production efficiency is significantly reduced. Thus, the TS variation of a steel sheet is required to be as small as possible.
For example, Japanese Unexamined Patent Application Publication No. 61-157625 proposes a high strength steel sheet having a tensile strength of 1000 MPa or more, a total elongation (EL) of 30% or more, and markedly high ductility, the steel sheet being manufactured using strain induced transformation of retained austenite. Such a steel sheet is manufactured by causing a steel sheet containing C, Si, and Mn as fundamental components to form austenite, subsequently quenching the steel sheet in the bainite transformation temperature range, and performing isothermal holding, namely, an austempering treatment. Retained austenite is formed due to the concentration of C in austenite caused by the austempering treatment, and formation of a large amount of retained austenite requires a large amount of added C exceeding 0.3%. However, an increase in the C concentration in steel deteriorates spot weldability and, in particular, a C concentration exceeding 0.3% significantly deteriorates spot weldability. Thus, it has been difficult to bring this technique into active use for steel sheets for use in automobiles. In addition, no consideration is given to hole expandability and stability of mechanical properties in JP '625 because the main purpose is to improve the ductility of a high strength thin steel sheet.
In Japanese Unexamined Patent Application Publication No. 1-259120, a good strength-ductility balance is achieved by heat-treating a high Mn steel in the ferrite-austenite dual-phase region. However, in JP '120, no analysis is performed on the improvement of ductility caused by concentrating Mn in untransformed austenite. Thus, there is a room to improve formability. In Japanese Unexamined Patent Application Publication No. 2003-138345, local ductility is improved by hot-rolling a high Mn steel, thereby causing a microstructure to include bainite and martensite, performing annealing and tempering to form fine retained austenite, and causing the microstructure to further include tempered bainite or tempered martensite. However, it is difficult to maintain strength because of the microstructure including a large amount of bainite or martensite formed by tempering at a high temperature. In addition, the amount of retained austenite is limited to improve the local ductility, which results in an insufficient amount of total elongation.
It could therefore be helpful to provide a high strength steel sheet having a steel composition including a low C content, a TS of 780 MPa or more, a TS×EL of 22000 MPa·% or more in addition to excellent hole expandability and stability of mechanical properties and a method of manufacturing the steel sheet.
SUMMARY
We found that it is possible to manufacture a high strength steel sheet having a steel composition including a low C content, a TS of 780 MPa or more, a TS×EL of 22000 MPa·% or more in addition to excellent hole expandability and stability of mechanical properties by adjusting alloy elements properly, performing an annealing treatment by holding in the temperature range of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more and, subsequently, performing cooling to room temperature to form a microstructure which includes ferrite in an area ratio of 30% or more and 80% or less, martensite in an area ratio of 0% or more and 17% or less, and retained austenite in a volume fraction of 8% or more and in which an average grain size of retained austenite is 2 μm or less.
We thus provide:

    
    
        (1) A high strength steel sheet having excellent formability and stability of mechanical properties, the steel sheet having a composition including, by mass, C: 0.03% or more and 0.25% or less, Si: 0.4% or more and 2.5% or less, Mn: 3.5% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% or more and 2.5% or less, N: 0.008% or less, Al+Si: 1.0% or more, and the balance being Fe and inevitable impurities, wherein the steel sheet has a steel microstructure comprises an area ratio of ferrite: 30% or more and 80% or less, an area ratio of martensite: 0% or more and 17% or less and a volume fraction of retained austenite: 8% or more, and an average grain size of the retained austenite is 2 μm or less.
        (2) The high strength steel sheet having excellent formability and stability of mechanical properties described in (1), wherein the steel microstructure further comprises an area ratio of tempered martensite: 5% or more.
        (3) The high strength steel sheet having excellent formability and stability of mechanical properties described in (1) or (2), wherein the composition further includes, by mass, at least one element selected from Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less.
        (4) The high strength steel sheet having excellent formability and stability of mechanical properties described in any one of (1) to (3), wherein the composition further includes, by mass, at least one element selected from Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less.
        (5) The high strength steel sheet having excellent formability and stability of mechanical properties described in any one of (1) to (4), wherein the composition further includes, by mass, at least one element selected from Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.
        (6) The high strength galvanized steel sheet having excellent formability and stability of mechanical properties described in any one of (1) to (5), further comprising a zinc-based coating on a surface of the steel sheet.
        (7) The high strength galvanized steel sheet having excellent formability and stability of mechanical properties described in (6), wherein the zinc-based coating is a hot-dip galvanized coating or an alloyed hot-dip galvanized coating.
        (8) The high strength aluminium-coated steel sheet having excellent formability and stability of mechanical properties described in any one of (1) to (5), further comprising an aluminium-based coating on a surface of the steel sheet.
        (9) A method of manufacturing a high strength steel sheet having excellent formability and stability of mechanical properties, the method including hot-rolling a steel slab having the composition described in any one of (1), (3), (4), and (5), performing pickling, optionally performing cold-rolling, subsequently performing holding in a temperature range of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more, performing cooling to room temperature, and optionally performing a tempering treatment in a temperature range of room temperature to 500° C. under the condition:

exp[160/(580−T)]×ln(t)≦20

        wherein,
        
            T: Average holding temperature (° C.) in the temperature range of room temperature to 500° C.
            t: Holding time (s) in the temperature range of room temperature to 500° C. exp(X) and ln(X) are the exponential function of X and the natural logarithm of X, respectively.
        
        
        (10) A method of manufacturing a high strength galvanized steel sheet having excellent formability and stability of mechanical properties, the method including hot-rolling a steel slab having the composition described in any one of (1), (3), (4), and (5), performing pickling, optionally performing cold-rolling, subsequently performing holding in a temperature range of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more, performing cooling, and performing a hot-dip galvanizing treatment.
        (11) A method of manufacturing a high strength galvanized steel sheet having excellent formability and stability of mechanical properties, the method including hot-rolling a steel slab having the composition described in any one of (1), (3), (4), and (5), performing pickling, optionally performing cold-rolling, subsequently performing holding in a temperature range of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more, performing cooling, performing a hot-dip galvanizing treatment, and then performing an alloying treatment of a galvanized coating in a temperature range of 470° C. to 600° C.
        (12) A method of manufacturing a high strength aluminium-coated steel sheet having excellent formability and stability of mechanical properties, the method including hot-rolling a steel slab having the composition described in any one of (1), (3), (4), and (5), performing pickling, optionally performing cold-rolling, subsequently performing holding in a temperature range of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more, performing cooling, and performing a hot-dip aluminium-plating treatment.
    
    


Note that, in the description, when referring to a steel composition, “%” always denotes “% by mass.” “High strength steel sheet” refers to a steel sheet having a tensile strength TS of 780 MPa or more.
A high strength steel sheet having a steel composition including a low C content, a TS of 780 MPa or more, a TS×EL of 22000 MPa·% or more in addition to excellent hole expandability and stability of mechanical properties can be produced. Our high strength steel sheet may be applied, for example, for automobile structural members. In this case, fuel economy is improved due to the reduction in the weight of car bodies. Therefore, the high strength steel sheet is highly valuable for industrial applications.



BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagram showing the relationship between annealing temperature (T1) and TS.



DETAILED DESCRIPTION
Hereinafter, our steel sheets and methods will be described in detail.
1) The reasons why the composition of steel is limited to the above-described ranges will be described.
C: 0.03% or More and 0.25% or Less
C is an element necessary to maintain a retained austenite phase. When the content of C is less than 0.03%, it is difficult to form the desired retained austenite. On the other hand, when an excessive amount of C exceeding 0.25% is added, the area ratio of martensite increases beyond 17%, which deteriorates hole expandability and stability of mechanical properties. In addition, spot weldability, arc weldability and the like are deteriorated because a welded zone and a heat-affected zone are considerably hardened and thus the mechanical characteristics of the welded zone are deteriorated. In this respect, the content of C is 0.03% or more and 0.25% or less and preferably 0.05% or more and 0.20% or less.
Si: 0.4% or More and 2.5% or Less
Si is effective in maintaining high ductility because it allows retained austenite to be formed and improves the work hardenability of ferrite. The minimum content of Si is 0.4% because a poor effect is produced when the content of added Si is less than 0.4%. However, adding an excessive amount of Si exceeding 2.5% causes deterioration of surface quality due to the occurrence of red scale or the like. Thus, the content of Si is 0.4% or more and 2.5% or less and preferably 0.5% or more and 2.0% or less.
Mn: 3.5% or More and 10.0% or Less
Mn is an element effective to stabilize austenite and promote formation of retained austenite. Such an effect is confirmed at a Mn content of 3.5% or more. However, adding an excessive amount of Mn exceeding 10.0% suppresses formation of ferrite, which deteriorates formability. In this respect, the content of Mn is 3.5% or more and 10.0% or less and preferably 4.0% or more and 7.0% or less.
P: 0.1% or Less
P is an element effective to increase the strength of a steel. However, adding an excessive amount of P exceeding 0.1% causes embrittlement due to grain boundary segregation, which deteriorates crashworthiness. Thus, the content of P is 0.1% or less.
S: 0.01% or Less
S forms inclusions such as MnS and thereby becomes a sources of deterioration of hole expandability and crashworthiness and formation of cracks along the metal flow in the welded zone. Thus, the content of S is desirably as low as possible. However, in terms of production cost, the content of S is 0.01% or less.
Al: 0.01% or More and 2.5% or Less
Al is an element that serves as a deoxidizer and is effective for cleanliness of steel. Therefore, Al is preferably added in a deoxidation step. The minimum content of Al is 0.01% because a poor effect is produced when the content of added Al is less than 0.01%. However, adding an excessive amount of Al increases the risk of the occurrence of cracks in a steel slab in continuous casting, thus leading to a reduction in productivity. In this respect, the content of Al is 0.01% or more and 2.5% or less and preferably 0.5% or more and 1.5% or less.
N: 0.008% or Less
N is an element that deteriorates the aging resistance of a steel most. Thus, the content of N is preferably as low as possible. However, an N content exceeding 0.008% causes significant deterioration of aging resistance. Thus, the content of N is 0.008% or less.
Si+Al: 1.0% or More
Si and Al are effective to reduce the variation of material quality because they allow the austenite-ferrite dual-phase region to be widened and consequently changes in the phase volume fractions of austenite and ferrite with a change in annealing temperature become small. In this respect, the content of Si+Al is 1.0% or more and preferably 1.5% or more.
The high strength steel sheet has a composition including the above-described fundamental components and the balance being Fe and inevitable impurities. Optionally, the components described below may be included depending on the desired characteristics.
At least one selected from Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less
Cr, V, and Mo may be optionally added as needed because they improve the balance between strength and ductility. Such an effect can be produced at Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. However, when an excessive amount of Cr, V, or Mo exceeding Cr: 1.0%, V: 0.5%, or Mo: 0.5% is added, the volume fraction of the second phase becomes excessively large. This raises the concern that, for example, deterioration of ductility due to a significant increase in strength may occur. In addition, this becomes a factor that increases cost. Thus, when these elements are added, the contents of Cr, V, and Mo are Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, and Mo: 0.005% or more and 0.5% or less.
Ni and Cu are elements effective to strengthen a steel and may be used to strengthen a steel without problems as long as used within our range. The contents of Ni and Cu needed are each 0.05% or more to produce such an effect. On the other hand, when an excessive amount of Ni or Cu exceeding 1.0% is added, the ductility of a steel sheet is deteriorated. In addition, this becomes a factor that increases cost. Thus, when Ni or Cu is added, the contents of added Ni and Cu are each 0.05% or more and 1.0% or less.
At least one selected from Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less
Ti and Nb are effective in precipitation strengthening of a steel. Such an effect is produced when the content of Ti or Nb is 0.01% or more. Ti and Nb may be used to strengthen a steel without problems as long as used within our range. However, when the content of Ti or Nb exceeds 0.1%, ductility and shape fixability are deteriorated. In addition, this becomes a factor that increases cost. Thus, when Ti or Nb is added, the contents of added Ti and Nb are Ti: 0.01% or more and 0.1% or less and Nb: 0.01% or more and 0.1% or less.
B may be optionally added as needed because it suppresses formation and growth of ferrite from the austenite grain boundary. Such an effect can be produced at a B content of 0.0003% or more. However, when the content of B exceeds 0.0050%, formability is deteriorated. In addition, this becomes a factor that increases cost. Thus, when B is added, the content of B is 0.0003% or more and 0.0050% or less.
At least one selected from Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less
Ca and REM are elements effective to reduce the adverse effect of sulfides on hole expandability by forming the sulfides into a spherical shape. The content of Ca or REM needed is 0.001% or more to produce such an effect. However, when an excessive amount of Ca or REM exceeding 0.005% is added, the amount of inclusions or the like increases, which leads to, for example, the occurrence of surface and inner defects. Thus, when Ca or REM is added, the contents of added Ca and REM are each 0.001% or more and 0.005% or less.
2) Next, a steel microstructure will be described.
Ferrite Area Ratio: 30% or More and 80% or Less
Ductility is deteriorated when the area ratio of ferrite is less than 30%. It is difficult to maintain a TS of 780 MPa or more when the area ratio of ferrite exceeds 80%. Thus, the area ratio of ferrite is 30% or more and 80% or less.
Martensite Area Ratio: 0% or More and 17% or Less
It is difficult to maintain good hole expandability and stability of mechanical properties when the area ratio of martensite exceeds 17%. Thus, the area ratio of martensite is 0% or more and 17% or less.
Retained Austenite Volume Fraction: 8% or More
Retained austenite effectively improves the ductility of a steel. A targeted ductility cannot be achieved when the volume fraction of retained austenite is less than 8%. Thus, the volume fraction of retained austenite is 8% or more. The maximum volume fraction of retained austenite is not particularly specified, but is about 40% with consideration of the composition described above.
Retained Austenite Average Grain Size: 2 μm or Less
Propagation of cracks during hole-expanding cannot be suppressed when the average grain size of retained austenite exceeds 2 μm. As a result, it is difficult to maintain good hole expandability. Thus, the average grain size of retained austenite is 2 μm or less.
Tempered Martensite Area Ratio: 5% or More
Tempered martensite improves hole expandability by reducing the hardness difference among soft ferrite, hard martensite, and hard retained austenite. It is difficult to produce good hole expandability when the volume fraction of tempered martensite is less than 5%. Tempered martensite is a softened microstructure formed by tempering martensite, which significantly affects a TS. Thus, to maintain high stability of mechanical properties, the area ratio of tempered martensite is preferably 5% or more.
In addition to ferrite, retained austenite, tempered martensite, and martensite, carbides such as cementite, bainite, or pearlite may be formed. However, our desired conditions can be achieved when the above-described ratios of ferrite, retained austenite, and martensite are satisfied.
Optionally, the high strength steel sheet, on the surface of the steel sheet, a hot-dip galvanized coating formed by performing a hot-dip galvanizing treatment; a zinc-based coating such as an alloyed hot-dip galvanized coating formed by performing a hot-dip galvanizing treatment and subsequently performing an alloying treatment of the resulting galvanized coating; or an aluminium-based coating such as a hot-dip aluminium-coating formed by performing a hot-dip aluminium-plating.
3) Next, manufacturing conditions will be described.
A steel having the above-described composition is melted using a converter or the like and then cast into a slab by a continuous casting method or the like. The resulting steel is hot-rolled or hot-rolled followed by cold-rolling and subsequently subjected to an annealing treatment. Although there are no particular limitations on the manufacturing conditions for casting, hot-rolling, and cold-rolling, the preferred manufacturing conditions will be described below.
Casting Conditions
A steel slab used is preferably cast by a continuous casting method to prevent macro segregation of the components. However, an ingot-making method and a thin slab casting method may alternatively be employed. Moreover, in addition to a conventional method in which a steel slab, after being cast, is temporarily cooled to room temperature and subsequently heated again, an energy-saving process such as a direct rolling in which a hot steel slab, without being cooled to room temperature, is inserted into a heating furnace or heat-insulated for a short time and then immediately rolled can be applied without problems.
Hot-Rolling Conditions
Slab Heating Temperature: 1100° C. or More
Regarding the slab heating temperature, low-temperature heating is preferable from an energetic viewpoint. However, when the heating temperature is less than 1100° C., it is difficult to sufficiently dissolve carbides. This causes problems such as an increased risk of the occurrence of trouble caused in hot-rolling due to an increased rolling load. With consideration of an increase in scale loss with an increase in oxidized weight, the slab heating temperature is desirably 1300° C. or less.
A sheet-bar heater that heats sheet bars may be employed to prevent the trouble caused in hot-rolling even when the slab heating temperature is lowered.
Finish-Rolling End Temperature: Ar3 Transformation Point or More
When a finish-rolling end temperature is less than an Ar3 transformation point, the anisotropy of material characteristic may occur, which results in deterioration of formability. Thus, the finish-rolling temperature is desirably the Ar3 transformation point or more.
Coiling Temperature: 450° C. or More and 700° C. or Less
When a coiling temperature is less than 450° C., it is difficult to control the coiling temperature, which is likely to result in temperature irregularities. As a result, problems such as deterioration of cold-rolling characteristics may occur. When the coiling temperature exceeds 700° C., problems such as the occurrence of decarbonization in the surface layer of a base iron may occur. Thus, the coiling temperature is desirably 450° C. or more and 700° C. or less.
In the hot-rolling step, a part or all of finish rolling may be a rolling with lubrication to reduce the rolling load in hot-rolling. Performing the rolling with lubrication is also effective to produce a steel sheet having a uniform shape and a uniform material quality. The coefficient of friction in the rolling with lubrication is preferably 0.25 to 0.10. In addition, a continuous-rolling process in which each sheet bar is joined with the previous and next sheet bars and continuously subjected to finish-rolling is preferably employed. It is desirable to employ the continuous-rolling process in terms of operational stability in hot-rolling. In the case where cold-rolling is further performed after pickling, a cold-rolled steel sheet having a predetermined thickness is formed. Cold-rolling conditions are not particularly limited and may be set in accordance with the conventional method.
Annealing Conditions
Holding in the Temperature Range of Ac1 Transformation Point+10° C. to Ac1 Transformation Point+(Ac3 Transformation Point−Ac1 Transformation Point)/2 for 30 Seconds or More
When the annealing treatment is performed in the austenite-ferrite dual-phase region, Mn concentrates on the austenite side and consequently the Mn concentration in austenite reaches a concentration equal to or greater than the Mn concentration of a steel composition. When an annealing temperature is less than Ac1 transformation point+10° C., little austenite is formed. Thus, the desired retained austenite cannot be formed after annealing. When the annealing temperature exceeds Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2, an insufficient concentration of Mn in austenite occurs in annealing. Thus, the desired amount of retained austenite cannot be formed. When a holding time is less than 30 seconds, an insufficient concentration of Mn in austenite occurs in annealing. Thus, the desired amount of retained austenite cannot be formed. Furthermore, a large amount of unrecrystallized microstructure remains. Thus, the desired ductility and hole expandability cannot be achieved. As a result, the annealing conditions are as follows: holding in the temperature range of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more.
Any annealing method such as continuous annealing or batch annealing may be employed. When cooling to room temperature is performed after annealing, the cooling method and cooling rate are not particularly specified and any cooling conditions such as furnace cooling or air cooling in batch annealing or gas jet cooling, mist cooling, or water cooling in continuous annealing may be employed.
Optionally, a tempering treatment described below may be performed after cooling to room temperature.
By performing the tempering treatment, martensite, which constitutes a part of the microstructure formed after cooling, is caused to transform into tempered martensite, and the hardness difference between soft ferrite (which transforms into hard martensite in hole-expanding) and retained austenite is reduced. Consequently, hole expandability can be improved. In addition, the area ratio of martensite can be controlled to be stably low by causing martensite to transform into tempered martensite. Furthermore, the stability of mechanical properties may be improved because the area ratio of the hard martensite relates to the variation of material quality.
Tempering Treatment Under the Conditions of the Temperature Range of Room Temperature to 500° C. and exp[160/(580−T)]×ln(t)≦20
Where T: Average holding temperature (° C.) in the temperature range of room temperature to 500° C. and t: Holding time (s) in the temperature range of room temperature to 500° C. When the tempering treatment is performed in the temperature range exceeding 500° C., or when exp[160/(580−T)]×ln(t) exceeds 20, the desired amount of retained austenite cannot be formed and thus it is difficult to maintain TS×EL≧22000 MPa·%. Thus, a tempering condition is exp[160/(580−T)]×ln(t)≦20. T is preferably 150° C. or more to maintain better hole expandability.
After the annealing treatment and the tempering treatment, optionally, a steel sheet may be subjected to temper-rolling to, for example, straighten its shape and adjust its surface roughness. Moreover, coating with a resin or fats and oil, various types of plating, and painting can be performed without problems.
The hot-dip galvanizing treatment may be optionally performed as follows. The steel sheet subjected to the annealing treatment is dipped in a galvanizing bath kept at 440° C. to 500° C. to perform the hot-dip galvanizing treatment. Subsequently, the amount of coating deposited is adjusted by gas wiping or the like. The content of Al in the galvanizing bath used in the hot-dip galvanizing is preferably 0.08% to 0.18%.
Performing the Alloying Treatment of the Galvanized Coating at 470° C. to 600° C.
The alloying treatment of the galvanized coating may be optionally performed by performing an alloying treatment of the galvanized coating at 470° C. to 600° C. after the hot-dip galvanizing treatment. When the alloying treatment is performed at a temperature exceeding 600° C., untransformed austenite transforms into pearlite. Thus, the desired amount of retained austenite cannot be maintained. As a result, ductility is deteriorated. Thus, when the alloying treatment of the galvanized coating is performed, the alloying treatment of the galvanized coating is performed at 470° C. to 600° C.
The hot-dip aluminium-plating treatment may be optionally performed as follows. The steel sheet subjected to the annealing treatment is dipped in an aluminium-plating bath kept at 660° C. to 730° C. to perform the hot-dip aluminium-plating treatment. Subsequently, the amount of coating deposited is adjusted by gas wiping or the like. When a steel is subjected to the hot-dip aluminium-plating treatment using the aluminium-plating bath having a temperature at Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2, fine and stable retained austenite is formed and thereby further improvement of ductility, that is, an increase in TS×EL, is possible.
EXAMPLES
Steels having certain compositions shown in Table 1 with the balance being Fe and inevitable impurities were each melted using a converter and cast into a slab by a continuous casting method. The resulting slabs were heated to 1250° C. and hot-rolled to a sheet thickness of 3.2 mm. Hot-rolling conditions were as follows: finish temperature of 900° C., cooling rate after rolling: 10° C./s, and coiling temperature: 600° C. After pickling, the hot-rolled steel sheets were cold-rolled to a sheet thickness of 1.4 mm and annealed under the manufacturing conditions shown in Table 2. In some Examples, a hot-dip galvanizing treatment, a hot-dip galvanizing treatment followed by an alloying treatment of a galvanized coating, or a hot-dip aluminium-plating was further performed. Nos. 35, 38, and 44 were hot-dip galvanized steel sheets. Nos. 36, 39, 41 to 43, and 47 were galvannealed steel sheets. Nos. 37, 40, and 46 were hot-dip aluminium-coated steel sheets.
A hot-dip galvanizing bath used to hot-dip galvanize steel sheets was a zinc bath containing Al: 0.19% with a bath temperature of 460° C. A hot-dip galvanizing bath used to galvanneal steel sheets was a zinc bath containing Al: 0.14% with a bath temperature of 460° C. The amount of galvanized coating was adjusted to 45 g/m2 per side (double-side coating). In the alloying treatment, the Fe concentration in the coating was adjusted to 9% to 12% by mass. The temperature of a hot-dip aluminium-plating bath used for hot-dip aluminium-coated steel sheets was 700° C. In some Examples, the steel sheets, after being hot-rolled to a sheet thickness of 2.0 mm and pickled, were directly annealed under the manufacturing conditions shown in Table 2.
The resulting steel sheets were examined in terms of sectional microstructure, tensile characteristics, and hole expandability. Table 3 shows the results.




























TABLE 1


























Ac1
Ac3






















trans-
trans-






















forma-
forma-






















tion
tion












Steel
Chemical composition (mass %)
point 
point 





























type
C
Si
Mn
Al
P
S
N
Ni
Cu
Cr
V
Mo
Nb
Ti
B
Ca
REM
Si + Al
(° C.)
(° C.)
Remarks































A
0.125
1.21
4.22
0.790 
0.023
0.0024
0.0038
—
—
—
—
—
—
—
—
—
—
2.00
644
924
Example 























steel


B
0.089
1.05 
5.92
0.740 
0.021
0.0020
0.0034
—
—
—
—
—
—
—
—
—
—
1.79
595
867
Example 























steel


C
0.370
0.72
2.85
0.028 
0.018 
0.0021
0.0030
—
—
—
—
—
—
—
—
—
—
0.75
673
739
Comparative























Example 























steel


D
0.112
0.48 
4.42
0.026
0.023
0.0024
0.0036
—
—
—
—
—
—
—
—
—
—
0.51
631
736
Comparative























Example 























steel


E
0.092
1.05
5.03
0.920
0.024
0.0026
0.0032
0.29
0.22
—
—
—
—
—
—
—
—
1.97
614
920
Example 























steel


F
0.124
1.29 
4.98 
0.880
0.028 
0.0028
0.0030
—
—
0.32 
0.064
—
—
—
—
—
—
2.17
628
933
Example























steel


G
0.089
1.51
4.55
0.732
0.025
0.0024
0.0031
—
—
—
—
0.21
—
—
—
—
—
2.24
640
934
Example 























steel


H
0.085
1.14
5.56
0.791
0.024
0.0019
0.0033
—
—
—
—
—
0.032
—
—
—
—
1.93
607
894
Example 























steel


I
0.151
1.32
5.22
1.040 
0.029
0.0020
0.0034
—
—
—
—
—
—
0.018
0.0021
—
—
2.36
617
949
Example 























steel


J
0.121
1.18 
4.55
0.720 
0.024
0.0018
0.0029
—
—
—
—
—
—
—
—
0.0014
0.0020
1.90
635
900
Example 























steel


K
0.096
1.01
4.43
0.041
0.024
0.0019
0.0030
—
—
—
—
—
—
—
—
—
—
1.05
637
768
Example 























steel


L
0.320
1.48 
1.77
0.039
0.023 
0.0018
0.0031
—
—
—
—
—
—
—
—
—
—
1.52
713
816
Comparative























Example 























steel









An Ac1 transformation point and an Ac3 transformation point can be calculated by the following expressions:

Ac1 transformation point(° C.)=751−16×(%C)+11×(%Si)−28×(%Mn)−5.5×(%Cu)−16×(%Ni)+13×(%Cr)+3.4×(%Mo)

Ac3 transformation point(° C.)=910−203√(%C)+45×(%Si)−30×(%Mn)−20×(%Cu)−15×(%Ni)+11×(%Cr)+32×(%Mo)+104×(%V)+400×(%Ti)+200×(%Al).

















TABLE 2










Average









Annealing

holding
Holding








temperature:
Annealing
temperature:
time:

Alloying




Steel
Cold
T1
time
T2
t
exp[160/(580 −
temperature



No.
Type
rolling
° C.
s
° C.
s
T2)] × In(t)
° C.
Remarks
























1
A
Yes
680
500
—
—
—
—
Example


2
A
Yes
700
500
—
—
—
—
Example


3
A
Yes
720
500
—
—
—
—
Example


4
A
Yes
680
500
360
500
12.9
—
Example


5
A
Yes
700
500
360
500
12.9
—
Example


6
A
Yes
720
500
360
500
12.9
—
Example


7
A
No
700
500
—
—
—
—
Example


8
A
Yes
510
500
—
—
—
—
Comparative


9
A
Yes
830
500
—
—
—
—
Example











Comparative


10
A
Yes
700
10
—
—
—
—
Example











Comparative


11
A
Yes
700
500
700
1800
2.0
—
Example











Comparative


12
A
Yes
700
500
450
3600
28.0
—
Example











Comparative


13
B
Yes
660
300
—
—
—
—
Example


14
B
Yes
680
300
—
—
—
—
Example


15
B
Yes
700
300
—
—
—
—
Example


16
B
No
680
300
—
—
—
—
Example


17
C
Yes
680
300
—
—
—
—
Example











Comparative


18
C
Yes
700
300
—
—
—
—
Example











Comparative


19
C
Yes
720
300
—
—
—
—
Example











Comparative


20
D
Yes
645
500
—
—
—
—
Example











Comparative


21
D
Yes
665
500
—
—
—
—
Example











Comparative


22
D
Yes
685
500
—
—
—
—
Example











Comparative


23
E
Yes
700
300
—
—
—
—
Example


24
E
Yes
740
300
—
—
—
—
Example


25
F
Yes
680
300
—
—
—
—
Example


26
F
Yes
720
300
—
—
—
—
Example


27
G
Yes
680
400
—
—
—
—
Example


28
G
Yes
720
400
—
—
—
—
Example


29
H
Yes
680
300
—
—
—
—
Example


30
H
Yes
720
300
—
—
—
—
Example


31
I
Yes
700
300
—
—
—
—
Example


32
I
Yes
740
300
—
—
—
—
Example


33
J
Yes
700
500
—
—
—
—
Example


34
J
Yes
740
500
—
—
—
—
Example


35
A
Yes
670
900
—
—
—
—
Example


36
A
Yes
670
900
—
—
—
560
Example


37
A
Yes
670
900
—
—
—
—
Example


38
B
Yes
650
1200
—
—
—
—
Example


39
B
Yes
650
1200
—
—
—
560
Example


40
B
Yes
650
1200
—
—
—
—
Example


41
K
Yes
650
300
—
—
—
550
Example


42
K
Yes
670
300
—
—
—
550
Example


43
K
Yes
690
300
—
—
—
550
Example


44
K
Yes
650
300
—
—
—
—
Example


45
K
Yes
650
300
360
500
12.9
—
Example


46
K
Yes
650
300
—
—
—
—
Example


47
L
Yes
760
300
—
—
—
550
Comparative











Example





Underlined value: Out of the scope of our steel sheets and methods



























TABLE 3












RA












F
RA
M
TM
average











Sheet
area
volume
area
area
grain










Steel
thickness
ratio
fraction
ratio
ratio
size
TS
EL
λ
TS × EL
ΔTS/




No.
type
(mm)
(%)
(%)
(%)
(%)
(μm)
(MPa)
(%)
(%)
(MPa · %)
Δ20° C.
Type
Remarks





























1
A
1.4
68
14
8
8
1.2
1200
23.1
42
27720
55
CR
Example


2
A
1.4
60
19
9
10
1.0
1240
22.0
38
27280

CR
Example


3
A
1.4
55
22
10
12
0.9
1310
20.6
36
26986

CR
Example


4
A
1.4
70
13
2
14
0.9
1000
28.1
52
28100
35
CR
Example


5
A
1.4
62
17
3
17
1.1
1040
27.4
54
28496

CR
Example


6
A
1.4
58
20
3
18
1.0
1070
26.9
58
28783

CR
Example


7
A
2.0
61
18
8
11
1.4
1210
23.4
46
28314
65
HR
Example


8
A
1.4
79
2
4
4
0.3
1270
13.9
21
17653
65
CR
Comparative
















Example


9
A
1.4
26
6
46
18
0.2
1480
11.8
10
17464
65
CR
Comparative
















Example


10
A
1.4
70
4
4
4
0.3
1280
13.9
22
17792
60
CR
Comparative
















Example


11
A
1.4
63
2
8
11
0.3
1090
15.4
31
16786
60
CR
Comparative
















Example


12
A
1.4
64
3
10
10
0.2
1120
14.8
32
16576
65
CR
Comparative
















Example


13
B
1.4
52
25
9
12
1.0
1260
34.5
38
43470
55
CR
Example


14
B
1.4
45
29
10
14
0.9
1320
32.4
36
42768

CR
Example


15
B
1.4
39
32
11
16
0.8
1370
30.2
33
41374

CR
Example


16
B
2.0
48
26
9
14
1.1
1230
32.2
36
39608
50
HR
Example


17
C
1.4
71
6
18
3
3.3
1140
18.4
22
20976
120
CR
Comparative
















Example


18
C
1.4
67
5
23
3
2.9
1260
16.6
21
20916

CR
Comparative
















Example


19
C
1.4
57
6
32
3
3.2
1380
14.9
20
20562

CR
Comparative
















Example


20
D
1.4
63
16
18
2
2.8
970
24.8
16
24056
165
CR
Comparative
















Example


21
D
1.4
51
19
26
3
2.9
1130
21.7
14
24521

CR
Comparative
















Example


22
D
1.4
36
21
38
4
2.7
1300
18.9
13
24570

CR
Comparative
















Example


23
E
1.4
53
24
9
12
1.1
1190
23.2
43
27608
55
CR
Example


24
E
1.4
40
31
11
16
0.9
1300
21.5
39
27950

CR
Example


25
F
1.4
52
22
10
14
1.0
1200
23.3
40
27960
60
CR
Example


26
F
1.4
39
28
11
19
1.2
1320
21.2
36
27984

CR
Example


27
G
1.4
56
22
9
11
1.1
1210
22.9
42
27709
55
CR
Example


28
G
1.4
43
29
10
15
0.8
1320
21.1
40
27852

CR
Example


29
H
1.4
58
20
9
11
0.9
1240
22.2
37
27528
60
CR
Example


30
H
1.4
45
27
10
14
1.1
1360
20.4
35
27744

CR
Example


31
I
1.4
58
19
10
11
1.1
1190
22.9
41
27251
55
CR
Example


32
I
1.4
44
25
9
19
1.2
1300
21.5
38
27950

CR
Example


33
J
1.4
53
18
10
16
1.1
1210
22.9
42
27709
55
CR
Example


34
J
1.4
40
24
10
22
0.8
1320
21.1
40
27852

CR
Example


35
A
1.4
70
17
6
6
1.1
1090
27.8
32
30302
45
GI
Example


36
A
1.4
71
16
6
6
1.0
1080
28.2
33
30456
50
GA
Example


37
A
1.4
70
20
5
5
0.8
1120
30.1
33
33712
55
Al
Example


38
B
1.4
52
26
12
9
0.9
1240
35.2
32
43648
65
GI
Example


39
B
1.4
53
27
11
8
1.0
1250
34.9
31
43625
70
GA
Example


40
B
1.4
54
31
7
7
1.2
1320
35.2
32
46464
65
Al
Example


41
K
1.4
70
12
10
6
1.0
990
27.8
33
27522
45
GA
Example


42
K
1.4
69
14
11
6
1.0
1030
26.5
31
27295

GA
Example


43
K
1.4
70
13
11
6
1.0
1080
24.8
31
26784

GA
Example


44
K
1.4
71
11
10
6
1.1
1000
27.6
31
27600
50
GI
Example


45
K
1.4
70
11
6
12
1.0
980
28.4
42
27832
50
CR
Example


46
K
1.4
69
16
9
6
1.1
1040
29.1
32
30264
55
Al
Example


47
L
1.4
68
3
6
2
0.4
920
21.8
30
20056
70
GA
Comparative
















Example





Underlined value: Out of the scope of our steel sheets and methods


F: Ferrite,


RA: Retained austenite,


M: Martensite,


TM: Tempered martensite


HR: Hot-rolled steel sheet,


CR: Cold-rolled steel sheet,


GI: Hot-dip galvanized steel sheet,


GA: Galvannealed steel sheet,


Al: Hot-dip aluminium-coated steel sheet






The area ratios of ferrite and tempered martensite were calculated as follows. The thickness-direction cross-section parallel to the rolling direction of a steel sheet was polished and corroded with a 3% nital. Then, 10 fields of view were observed with a SEM (scanning electron microscope) at a 2000-fold magnification. Each phase was colored with a different color on a transparent OHP sheet. The resulting image was captured and the areas of targeted crystal grains were determined using Image-Pro produced by Media Cybernetics, Inc. The total sum of these areas was divided by the total area to calculate the area ratios. The volume fraction of retained austenite was determined by grinding the steel sheet until a plane at 1⁄4 thickness position was exposed and measuring the intensity of X-ray diffraction on the plane at 1⁄4 thickness position. The incident X-ray was MoKα radiation. The intensity ratio was calculated for all combinations of the peak integrated intensities of {111}, {200}, {220}, and {311} planes of the retained austenite phase and {110}, {200}, and {211} planes of ferrite, and their average was considered as the volume fraction of retained austenite. The area ratio of martensite was determined by subtracting, from the whole, the area ratios of ferrite and tempered martensite determined above, the volume fraction of retained austenite determined above, and area ratios of other phases capable of being identified by a SEM (carbides such as cementite, bainite, and pearlite). The average grain size of retained austenite was determined by observing 20 pieces of retained austenite with a TEM (transmission electron microscope), determining the area of each piece using the above-mentioned Image-Pro, calculating their equivalent circle diameters, and averaging the equivalent circle diameters.
A tensile test was conducted in accordance with JIS Z 2241 (1998) using a JIS No. 5 test piece that was a sample taken so that its tensile direction was perpendicular to the rolling direction of the steel sheet, and a TS (tensile strength) and an EL (total elongation) were measured. It was evaluated that high ductility was achieved when TS×EL≧22000 MPa·%.
Hole expandability (stretch flangeability) was examined in accordance with The Japan Iron and Steel Federation Standard JFST 1001 (2008). Each prepared steel sheet was cut into a 100 mm×100 mm piece, and a hole having a diameter of 10 mm was punched with a clearance of 12%±1% for a sheet thickness of 2.0 mm or more and with a clearance of 12%±2% for a sheet thickness of less than 2.0 mm. Then, a 60° conical punch was inserted into the hole while a die having an inside diameter of 75 mm held the steel sheet at a blank-holding force of 9 tons, and the diameter of the hole at the cracking-initiation limit was measured. The critical hole-expansion ratio λ(%) was calculated by the following expression, and the stretch flangeability was evaluated on the basis of the critical hole-expansion ratio:

critical hole-expansion ratio λ(%)={(Df−D0)/D0}×100

where Df is a hole diameter (mm) at the initiation of cracking and D0 is an initial hole-diameter (mm). The hole expandability was evaluated as good when λ≧30(%).

The stability of mechanical properties was evaluated as follows. TS variation was examined for steel sheets prepared under the same conditions except for the annealing temperature T1 and prepared with different annealing temperatures T1. Then, the variation (ΔTS) per 20° C. change in annealing temperature was calculated from the TS variation. The evaluation was conducted on the basis of the TS variation (ΔTS). The stability of mechanical properties was evaluated as good when the TS variation per 20° C. (ΔTS/Δ20° C.) was 80 MPa or less.
FIG. 1 shows the relationship between the annealing temperature (T1) and TS of Nos. 1 to 3 (Examples, without the tempering treatment), Nos. 4 to 6 (Examples, with the tempering treatment), and Nos. 20 to 22 (Comparative Examples) manufactured in Examples. In Nos. 20 to 22, the TS variation was wide; the TS variation per 20° C. was 165 MPa. In contrast, in Nos. 1 to 3, the TS variation was small; the TS variation per 20° C. was 55 MPa, which was half or less of the TS variation of Nos. 20 to 22. In Nos. 4 to 6, where the tempering treatment was performed, the TS variation was further smaller than that of Nos. 1 to 3; the TS variation per 20° C. was 35 MPa.
In the Examples, all the high strength steel sheet had a TS of 780 MPa or more, a TS×EL of 22000 MPa·% or more, a λ of 30% or more, the TS variation per 20° C. of 80 MPa or less. Thus, high strength steel sheets having excellent formability and stability of mechanical properties were produced. In some Examples where the tempering treatment was performed by our method after cooling to room temperature, high strength steel sheets having a higher λ and more excellent stability of mechanical properties were produced. On the other hand, in the Comparative Examples, although the TS was 780 MPa or more, one or more characteristics of TS×EL, λ, or the TS variation per 20° C. was deteriorated.
In Nos. 37, 40, and 46 (hot-dip aluminium-coated steel sheets), steels were prepared using an aluminium-plating bath having a temperature of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2. Thus, a higher TS×EL than those of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet were achieved. In No. 47, (galvannealed steel sheet), the steel composition included a high C content and a low Mn content, which was not within our range. Thus, untransformed austenite transformed into pearlite in alloying. Consequently, the desired amount of retained austenite could not be maintained. As a result, a high TS×EL could not be achieved.
INDUSTRIAL APPLICABILITY
Our high strength steel sheets have a steel composition including a low C content, a TS of 780 MPa or more, and a TS×EL of 22000 MPa·% or more, that is, high ductility, in addition to excellent hole expandability and stability of mechanical properties. Our high strength steel sheets may be applied, for example, for automobile structural members. In this case, fuel economy is improved due to the reduction in the weight of car bodies. Therefore, our high strength steel sheets are highly valuable for industrial applications.

## Claims

1. A high strength steel sheet having excellent formability and stability of mechanical properties, the steel sheet having a composition including, by mass, C: 0.03% or more and 0.25% or less, Si: 1.01% or more and 2.5% or less, Mn: 4.22% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% or more and 2.5% or less, N: 0.008% or less, Si+Al: 1.0% or more, and the balance being Fe and inevitable impurities, wherein the steel sheet has a steel microstructure comprising an area ratio of ferrite: 30% or more and 80% or less, an area ratio of martensite: 0% or more and 17% or less and a volume fraction of retained austenite: 8% or more, an average grain size of the retained austenite is 2μm or less, and
an area ratio of tempered martensite: 5% or more.

2. The high strength steel sheet according to claim 1, wherein the composition further comprises, by mass, at least one element selected from the group consisting of Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less.

3. The high strength steel sheet according to claim 1, wherein the composition further comprises, by mass, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less.

4. The high strength steel sheet according to claim 1, wherein the composition further comprises, by mass, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.

5. The high strength steel sheet according to claim 1, further comprising a zinc-based coating on a surface of the steel sheet.

6. The high strength steel sheet according to claim 5, wherein the zinc-based coating is a hot-dip galvanized coating or an alloyed hot-dip galvanized coating.

7. The high strength steel sheet according to claim 1, further comprising an aluminium-based coating on a surface of the steel sheet.

8. A method of manufacturing a high strength steel sheet comprising:
hot-rolling a steel slab having the composition according to claim 1;
performing pickling;
optionally performing cold-rolling;
subsequently performing holding at a temperature of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more;
performing cooling to room temperature; and
optionally performing a tempering treatment at a temperature of room temperature to 500° C. under a condition:


exp[160/(580−T)]×ln(t)≦20



wherein,
T: Average holding temperature (° C.) in a range of room temperature to 500° C.
t: Holding time (s) in a range of room temperature to 500° C.
exp(X) and ln(X) are an exponential function of X and a natural logarithm of X, respectively.

9. A method of manufacturing a high strength steel sheet according to claim 1 comprising:
hot-rolling a steel slab having the composition;
perfoll ing pickling;
optionally performing cold-rolling;
subsequently performing holding at a temperature of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more;
performing cooling; and
performing a hot-dip galvanizing treatment.

10. A method of manufacturing a high strength steel sheet according to claim 1 comprising:
hot-rolling a steel slab having the composition;
performing pickling;
optionally performing cold-rolling;
subsequently performing holding at a temperature of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more;
performing cooling;
performing a hot-dip galvanizing treatment; and
performing an alloying treatment of a galvanized coating at 470° C. to 600° C.

11. A method of manufacturing a high strength steel according to claim 1 sheet comprising:
hot-rolling a steel slab having the composition;
performing pickling;
optionally performing cold-rolling;
subsequently performing holding at a temperature of Ac1 transformation point+10° C. to Ac1 transformation point+(Ac3 transformation point−Ac1 transformation point)/2 for 30 seconds or more;
performing cooling; and
performing a hot-dip aluminium-plating treatment.

12. The high strength steel sheet according to claim 2, wherein the composition further comprises, by mass, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less.

13. The high strength steel sheet according to claim 2, wherein the composition further comprises, by mass, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.

14. The high strength steel sheet according to claim 3, wherein the composition further comprises, by mass, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.

15. The high strength steel sheet according to claim 2, further comprising a zinc-based coating on a surface of the steel sheet.

16. The high strength steel sheet according to claim 1, wherein a volume fraction of retained austenite is 11% or more.

17. The high strength steel sheet according to claim 1, wherein TS variation per 20° C. of annealing temperature (ΔTS/Δ20° C.) is 80 MPa or less.

18. The high strength steel sheet according to claim 1, wherein TS variation per 20° C. of annealing temperature (ΔTS/Δ20° C.) is 50 MPa or less.

19. The high strength steel sheet according to claim 1, wherein TS×El is 26784 MPa % or more.

20. The high strength steel sheet according to claim 1, wherein TS is 980 MPa or more.

21. The high strength steel sheet according to claim 1, wherein Al is 0.5% or more and 2.5% or less.

22. The high strength steel sheet according to claim 12, wherein the composition further comprises, by mass, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.