Hot rolled steel sheet with a high strength and a distinguished formability

Abstract: A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being in a ratio (V PF /d PF ) of polygonal ferrite volume fraction V PF (%) to polygonal ferrite average grain size d PF (µm) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases and can be produced with a high productivity and without requiring special alloy elements.

Description: This invention relates to a hot rolled steel sheet with a high ductility, a high strength and a distinguished formability applicable to automobiles, industrial machinery, etc., and a process for producing the same. The term "sheet" means "sheet" or "plate" in the present specification and claims. In order to make the automobile steel sheet lighter and ensure the safety at collisions, steel sheets with a higher strength have been in a keen demand. Steel sheets even with a high strength have been required to have a good formability. That is, a steel sheet must have a high strength and a good formability at the same time. A dual phase steel composed of a ferrite phase and a martensite phase, which will be hereinafter referred to as "DP steel", has been so far proposed as a hot rolled steel sheet applicable to the fields requiring a high ductility. It is known that the DP steel has a more distinguished strength-ductility balance than a solid solution-intensified steel sheet with a high strength and a precipitation-intensified steel sheet with a high strength. However, there is such a limit to the strength-ductility balance as TS x T.EI ≦ 2,000, where TS represents a tensile strength (kgf/mm 2 ) and T.EI represents a total elongation (%), and thus the DP steel cannot meet more strict requirements JP-A-60-181 230 discloses a steel composition comprising (by weight) 0.15% < C ≦ 0.20%, ≦ 1.5% Si, 0.3 to 1.5% Mn, ≦ 0.02% P and ≦ 0.01% S. The steel composition is subjected to continuous hot finish rolling at ≧ 40% draft in the entire finish rolling. The rolling is finished at a temperature between (Ar 3 + 50°C) to (Ar 3 -50°C) in the final rolling pass, is cooled at a cooling rate of ≧ 45°C/s after the end of said rolling and is taken up at 300 to 500°C. The temperature of the final pass is made (Ar 3 +50°C) or more for a thin and broad material which is difficult to manufacture. JP-A-60 181 230 aims at improving strength, strength-ductility balance and resistance to fatigue by increasing the content of C. The steel sheet has a fine composite ferrite and bainite structure consisting of a component resembling a general C-Si-Mn system. JP-A-60 43 425 discloses a steel composition consisting of (by weight) 0.30 to 0.65% C, 0.7 to 2.0% Si, 0.5 to 2.0% Mn, the balance being Fe and inevitable impurities. The steel composition is hot-rolled at a finish temperature between Ar 3 and Ar 3 +50°C. The steel sheet is held for 4 to 20 s in a temperature range of 450 to 650°C and is coiled at ≦ 350°C. The final structure consists, by volume fraction, of ≧ 10% ferrite, ≧ 10% austenite, the balance being bainite or martensite. JP-A-60 43 425 aims at obtaining a hot-rolled composite structure steel sheet having high strength, excellent ductility and high workability without requiring any special alloy element. In order to overcome the limit to the strength-ductility balance, that is, to obtain TS x T.EI > 2,000, it has been proposed to utilize a retained austenite phase. For example, the following processes have been proposed: a process for producing a steel sheet having a retained austenite phase, which comprises hot rolling a steel sheet at a finish temperature of Ar 3 to Ar 3 + 50°C, then maintaining the steel sheet at a temperature of 450°C to 650°C for 4 to 20 s, and then coiling the steel sheet at a temperature of not more than 350°C [Japanese Patent Application Kokai (Laid-open) No. 60-43425], a process for producing a steel sheet having a retained austenite phase, which comprising rolling a steel sheet at a finish temperature of 850°C or more with a total draft of 80% or more and under a high reduction with a draft of 60% or more for the last total three passes and a draft of 20% or more for the last pass, and successively cooling the steel sheet down to 300 °C or less at a cooling rate of 50°C/s or more [Japanese Paten Application Kokai (Laid-open) No. 60-165,320], etc. However, the conventional processes requiring the maintenance of a steel sheet at 450° to 650°C for 4 to 20 s during the cooling, the coiling at a low temperature such as not more than 350°C, or the rolling under a high reduction are not operationally preferable with respect to the energy saving and productivity increase. The formability of the steel sheets obtained according to these processes is, for example, TS x T.EI ≦ 2,416 and thus does not always fully satisfy the level required by users. A steel sheet with a higher TS x T.EI value (desirably more than 2,416) and a process for producing the same with a higher productivity have been in a keen demand. As a result of extensive tests and researches for obtaining a steel sheet with TS x T.EI ≧ 2,000, which is over the limit of the prior art, the present inventors have found that at least 5% by volume of an austenite phase must be contained, as shown in Fig. 1, directed to steel species A in Example that follows, and the TS x T.EI value can be assuredly made to exceed the level of the aforementioned DP steel, i.e. TS x T.EI = 2,000, thereby. The increase in TS x T.EI is based on an increase in uniform elongation, and a uniform elongation of 20% or more can be obtained. The present invention is based on this finding and an object of the present invention is to provide a hot rolled steel sheet with a high strength and a distinguished formability, which contains 5% by volume or more of a retained austenite phase, and also a process for stably, assuredly and economically producing such a steel sheet as above. The foregoing object of the present invention can be attained by the steel sheet and process according to the claims. The term "rare earth metal" or "REM" hereinafter means at least one of the fifteen metallic metals (elements) (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) following lanthanum through lutetium with atomic numbers 57 through 71. The rare earth metal (REM) is added frequently in the form of a mischmetal which is an alloy of REM and that has a composition comprising 50% of lanthanum, neodymium and the other metal in the same series and 50% of cerium. The invention will be described in detail in connection with the drawings in which Fig. 1 is a diagram showing a relationship between the volume fraction of the retained austenite phase and the TS x T.EI value, Fig. 2 is a diagram showing a relationship between the ratio of polygonal ferrite volume fraction V PF (%) to polygonal ferrite average grain size d PF (µm) and the TS x T.EI value, Fig. 3 is a diagram showing a relationship between the coiling temperature and the volume fraction of the retained austenite phase, Fig. 4 is a diagram showing a relationship between the coiling temperature and the hole expansion ratio, Fig. 5 is a diagram showing a relationship between TS and T.EI, Fig. 6 is a temperature pattern diagram showing a relationship among the finish rolling end temperature, the cooling rate 1 ○, T and the cooling rate 2 ○, and Fig. 7 is a temperature pattern diagram showing a relationship among the finish rolling end temperature, the cooling rate 1 ○', T 1 , the cooling rate 2 ○', T 2 and the cooling rate 3 ○'. The means (requisite for constitution) of the present invention will be explained below. First, the contents of the chemical components of the present steel sheet will be described in detail below: C is an indispensable element for the intensification of the steel and below 0.15% by weight of C the retained austenite phase that acts to increase the ductility of the present steel cannot be fully obtained, whereas above 0.4% by weight of C the weldability is deteriorated and the steel is embrittled. According to the invention 0.15 to 0.21% or 0.15 to 0.4% respectively by weight of C must be added. Si is effective for the formation and purification of the ferrite phase that contributes to an increase in the ductility with increasing Si content, and is also effective for the enrichment of C into the untransformed austenite phase to obtain a retained austenite phase. Below 0.5% by weight of Si this effect is not fully obtained, whereas above 2% by weight of Si this effect is saturated and the scale properties and the weldability are deteriorated to the contrary. Thus, 0.5 to 2.0% by weight of Si must be added. Mn contributes, as is well known, to the retaining of the austenite phase as an austenite-stabilizing element. Below 0.5% by weight of Mn the effect is not fully obtained, whereas above 2% by weight of Mn the effect is saturated, resulting in adverse effects, such as deterioration of the weldability, etc. Thus, 0.5 to 2.0% by weight of Mn must be added. S is a detrimental element to the hole expansibility. Above 0.010% by weight of S the hole expansibility is deteriorated. Thus, the S content must be decreased to not more than 0.010% by weight and not more than 0.001% by weight of S is preferable. In order to improve the hole expansibility, it is effective to reduce the S content, thereby reducing the content of sulfide-based inclusions and also to spheroidize the inclusions. For the spheroidization it is effective to add Ca or rare earth metal, which will be hereinafter referred to "REM". Below 0.0005% by weight of Ca and 0.0050% by weight of REM, the spheroidization effect is not remarkable, whereas above 0.0100% by weight of Ca and 0.050% by weight of REM the spheroidization effect is saturated and the content of the inclusions are rather increased as an adverse effect. Thus, 0.0005 to 0.0100% by weight of Ca and 0.005 to 0.050% by weight of REM must be added. The microstructure of the present steel sheet will be described in detail below. On the basis of steel species A in Example that follows, steel sheets were produced according to the present processes described as the means for attaining the object of the present invention and also under the conditions approximate to those of the present processes and investigated. As a result, the present inventors have found the following facts. In order to improve the ductility of steel sheets, it is necessary to form 5% by volume or more of a retained austenite phase in the present invention and it is desirable to stabilize the austenite phase through the enrichment of such elements as C, etc. To this effect, it is necessary (1) to form a ferrite phase, thereby promoting the enrichment of such elements as C, etc. into the austenite phase and contributing to the retaining of the austenite phase and (2) to promote the enrichment of such elements as C, etc. into the austenite phase with the progress of bainite phase transformation, thereby contributing to the retaining of the austenite phase. In order to promote the enrichment of such elements as C, etc. into the austenite phase through the formation of the ferrite phase, thereby contributing to the retaining of the austenite phase, it is necessary to increase the ferrite volume fraction, and to make the ferrite grains finer, because the sites at which the C concentration is highest and the austenite phase is liable to be retained are the boundaries between the ferrite phase and the untransformed austenite phase, and the boundaries can be increased with increasing ferrite volume fraction and decreasing ferrite grain size. In order to obtain at least TS x T.EI > 2,000 assuredly, it has been found that the ratio V PF /d PF , i.e. a ratio of polygonal ferrite volume fraction V PF (%) to polygonal ferrite grain size d PF (µm), must be 7 or more, as obvious from Fig. 2 showing the test results obtained under the same conditions as in Fig. 1. Polygonal ferrite volume fraction and polygonal ferrite average grain size are determined on optical microscope pictures. Ferrite grain whose axis ratio (long axis/short axis) = 1 to 3, is defined as polygonal ferrite. Besides the ferrite phase and the retained austenite phase, the remainder must be a bainite phase that contributes to the concentration of such elements as C, etc. into the austenite phase, because C is enriched into the untransformed austenite phase with the progress of the bainite phase transformation, thereby stabilizing the austenite phase, that is, the bainite phase has a good effect upon the retaining of the austenite phase. It is necessary not to form any pearlite phase or martensite phase that reduce the retained austenite phase. The process of the present invention will be described in detail below: In order to increase the ferrite volume fraction V PF ,low temperature rolling, rolling under a high pressure, and isothermal holding or slow cooling at a temperature around the nose temperature for the ferrite phase transformation (from Ar 1 to Ar 3 ) on a cooling table after the finish rolling, where the nose temperature for the ferrite phase transformation means a temperature at which the isothermal ferrite phase transformation starts and ends within a minimum time, are effective steps. In order to make the ferrite grains finer, that is, the reduce d PF , low temperature rolling, rolling under a high reduction, rapid cooling around the Ar 3 transformation point and rapid cooling after the ferrite phase transformation to avoid grain growth are effective steps. Thus, processes based on combinations of the former steps with the latter steps can be utilized. Rolling temperature: In order to increase the ferrite volume fraction and make the ferrite grains finer, low temperature rolling is effective. At a temperature lower than (Ar 3 - 50°C), the deformed ferrite is increased, deteriorating the ductility, whereas at a temperature higher than (Ar 3 + 50°C) the ferrite phase is not thoroughly formed. Thus, the effective finish rolling end temperature is any temperature within a range between (Ar 3 + 50°C) and (Ar 3 - 50°C). Furthermore, the ferrite formation and the refinement of ferrite grains can be promoted by setting the finish rolling start temperature to a temperature not higher than (Ar 3 + 100°C). However, the low temperature rolling has operational drawbacks such as an increase in the rolling load, a difficulty in controlling shapes of sheet, etc. when a thin steel sheet (sheet thickness ≦ 2 mm) is rolled, and particularly when a high carbon equivalent material or a high alloy material with a high deformation resistance is rolled. Thus, it is also effective to form the ferrite phase and make the ferrite grains finer by controlling the cooling on a cooling table after the hot finish rolling, as will be described later. In that case, a hot finish rolling end temperature exceeding Ar 3 + 50°C will not increase the afore-mentioned effect, but must be often employed on operational grounds. Draft: The formation of the ferrite phase and the refinement of finer ferrite grains can be promoted by making the total draft 80% or more in the hot finish rolling and steel sheet with a good formability can be obtained thereby. Thus, the lower limit to the total draft is 80% Cooling: Necessary ferrite formation and C enrichment for the retaining of austenite phase are not fully carried out by cooling between Ar 3 and Ar 1 at a cooling rate of 40°C/s or more after the hot rolling, and thus it is carried out to cool or hold isothermally the steel down to T (Ar 1 < T ≦ lower temperature of Ar 3 or the rolling end temperature) at a cooling rate of less than 40°C/s along the temperature pattern, as shown in Fig. 6, after the hot rolling. More preferably, it is necessary that it is carried out for 3 to 25 s to cool the steel within a temperature range from the lower one of the Ar 3 or the rolling end temperature to the temperature T or to hold the steel isothermally within said temperature range. When the cooling or the isothermal holding is carried out for 3 s or more, the ferrite formation and C enrichment are more sufficiently carried out. When the time of the cooling or isothermal holding exceeds 25 s, a length of a line of from a finish rolling mill to a coiling machine becomes remarkably long. Thus, the upper limit to the time is 25 s. Incidentally, as means for conducting the cooling at a cooling rate of less than 40°C/s or the isothermal holding, there are a heat-holding equipment using electric power, gas, oil and the like, a heat-insulating cover using heat-insulating material and the like, etc. A more desirable cooling pattern is as given in Fig. 7: the ferrite grains formed through the ferrite transformation can be made finer and the growth of grains including the ferrite grains, formed during the hot rolling, can be suppressed by carrying out the cooling down to T 1 (Ar 1 < T < lower one of Ar 3 or the rolling end temperature) at a cooling rate of 40°C/s or more after the hot rolling; and after that, the ferrite volume fraction can be increased around the ferrite transformation nose by carrying out the cooling down to T 2 (Ar 1 < T 2 ≦ T 1 ) at a cooling rate of less than 40°C/s or the isothermal holding, more preferably by carrying out the cooling or the isothermal holding within a temperature range from the temperature T 1 to the temperature T 2 for 3 to 25 second. In this manner, steel sheet with a better formability can be obtained. At a temperature above Ar 3 , no ferrite phase is formed even with cooling at a cooling rate of less than 40°C/s or conducting the isothermal holding, and a parlite phase is formed by cooling down to a temperature below Ar 1 at a cooling rate of less than 40°C/s or by conducting the isothermal holding at a temperature below Ar 1 . Thus, Ar 1 < T 2 ≦ T 1 < (the lower one of Ar 3 or the finish rolling end temperature) is determined. The successive cooling rate down to the coiling temperature is 40°C/s or more from the viewpoint of avoiding formation of a pearlite phase and suppressing the growth of grain. In case that the finish rolling end temperature is between not more than the Ar 3 and above the (Ar 3 - 50°C), some deformed ferrite is formed. On the other hand, it is effective in recovering the ductility of the deformed ferrite that the step of cooling at a rate of less than 40°C/s is performed within a temperature range from the finish rolling end temperature to more than Ar 1 . More preferably, it is effective that the cooling or isothermal holding is conducted for 3 to 25 s. Results of rolling and cooling tests for steel species A that follows while changing the coiling temperature are shown in Fig. 3 and Fig. 4. When the coiling temperature exceeds 500°C, the bainite transformation excessively proceeds after the coiling, or a pearlite phase is formed, and consequently 5% by volume or more of the retained austenite phase cannot be obtained, as shown in Fig. 3. Thus, the upper limit to the coiling temperature is 500°C. When the coiling temperature is less than 350°C or not more than 350°C, martensite is formed to deteriorate the hole expansibility, as shown in Fig. 4. Thus, the lower limit to the coiling temperature is not less than 350°C, preferably over 350°C. In order to avoid excessive bainite transformation and retain a larger amount of the austenite phase, it is more effective to cool the steel sheet down to 200°C or less at a cooling rate of 30°C/h or more by dipping in water, mist spraying, etc. after the coiling as shown in Fig. 3. The present processes based on combinations of the foregoing steps are shown in Fig. 6 and Fig. 7, where the finish rolling end temperature is further classified into two groups, i.e. a lower temperature range (Ar 3 ± 50°C) and a higher temperature range {more than (Ar 3 + 50°C)}. Besides the foregoing 4 processes, a process in which the upper limit to the hot finish rolling start temperature is Ar 3 + 100°C or less and a process in which the cooling step after the coiling is limited or a process based on a combination of these two steps are available. Needless to say, a better effect can be obtained by a multiple combination of these process steps. The present invention will now be described in detail, referring to Examples. Examples Steel sheets having a thickness of 1.4 to 6.0 mm were produced from steel species A to K having chemical components given in Table 1 under the conditions given in Tables 2 and 3 according to the process pattern given in Fig. 6 or Fig. 7, where the steel species C shows those whose C content is below the lower limit of the present invention, and the steel species E and H show those whose Si content is below the lower limit of the present invention and those whose Mn content is below the lower limit of the present invention, respectively. The symbols given in Table 2 and 3 have the following meanings: FT 0 : finish rolling start temperature (°C) FT 7 : finish rolling end temperature (°C) CT: coiling temperature (°C) TS: tensile strength (kgf/mm 2 ) T.EI: total elongation (%) γ R : volume fraction of retained austenite (%) V PF : polygonal ferrite volume fraction (%) d PF : polygonal ferrite grain size (µm) In Table 1, the Ar 1 temperature of steel species A was 650°C and the Ar 3 temperature of that was 800°C. The steel species according to the present invention are Nos. 1, 2, 4, 6, 7, 9, 22, to 39, 41, 44, 45, 46, 48, 49, 51, 52, and 54 to 67. Initially TS x T.EI ≧ 2,000 was aimed at, whereas much better strength-ductility balance such as TS x T.EI > 2,416 was obtained owing to the synergistic effect, as shown in Fig. 5. In comparative Examples, no good ductility was obtained on the following individual grounds; Nos. 3 and 53: the C content was too low. Nos. 6 and 47: the Si content was too low. Nos. 8 and 50: the Mn content was too low. No. 10: the total draft was too low at the finish rolling. No. 11: the finish rolling end temperature was too low. No. 12: the temperature T was too high. Nos. 13, 14 and 15 : the temperatures T and T 2 were too low. Nos. 16 and 40: the cooling rate 1 ○ was too high. Nos. 17 and 42: the cooling rate 2 ○ was too low. No. 18: the cooling rate 2 ○' was too high. No. 19: the cooling rate 3 ○' was too low. Nos 20 and 43: the coiling temperature was too high. No. 21: the coiling temperature was too low. Furthermore, Nos. 25, 28, 32, 36 and 39 are examples of controlling the rolling start temperature and controlling the cooling step after the coiling, and Nos. 62 to 67 are examples of conducting the isothermal holding step in the course of the cooling step.

Claims: A hot rolled steel sheet with a high strength and distinguished formability, having a strength-ductility balance TSxT.EI >2416 (TS = tensile strength in kgf/mm 2 ; T.EI = total elongation in %) comprising (by weight) 0.15 to 0.21% C, 0.5 to 2.0% Si, 0.5 to 2.0% Mn with the balance being iron plus inevitable impurities and having a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being in the ratio (V pf /d pf ) of 7 or more of polygonal ferrite volume fraction V pf (%) to polygonal ferrite average grain size d pf (µm) and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. A hot rolled steel sheet with a high strength and distinguished formability, having a strength-ductility balance TSxT.EI >2416 (TS = tensile strength in kgf/mm 2 ; T.EI = total elongation in %) comprising (by weight) 0.15 to 0.4% C, 0.5 to 2.0% Si, 0.5 to 2.0% Mn and one of 0.0005 to 0.0100% Ca and 0.005 to 0.050% rare earth metal with S being limited to not more than 0.010% and the balance being iron and inevitable impurities and having a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being in the ratio (V pf /d pf ) of 7 or more of polygonal ferrite volume fraction V pf (%) to polygonal ferrite average grain size d pf (µm) and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. A hot rolled steel sheet according to claim 1 or 2, wherein said steel sheet has a uniform elongation of at least 20%. A process for producing a hot rolled steel sheet according to claim 1, comprising the steps of subjecting the steel composition defined in claim 1 to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is at least Ar 3 - 50°C, successively cooling down the steel to a desired temperature T within a temperature range from the lower one of either the Ar 3 temperature or said rolling end temperature to Ar 1 at a cooling rate of less than 40°C/s, successively cooling the steel at a cooling rate of 40°C/s or more and coiling the steel at a temperature of more than 350°C to 500°C. A process for producing a hot rolled steel sheet according to claim 2, comprising the steps of subjecting the steel composition defined in claim 2 to a hot finish rolling with a total draft of at least 80% in such a manner that its rolling end temperature is at least Ar 3 - 50°C, successively cooling down the steel to a desired temperature T within a temperature range from the lower one of either the Ar 3 temperature or said rolling end temperature to Ar 1 at a cooling rate of less than 40°C/s, successively cooling the steel at a cooling rate of 40°C/s or more and coiling the steel at a temperature of more than 350°C to 500°C. A process according to claim 4 or 5, wherein the cooling of said steel within a temperature range from the lower one of either the Ar 3 temperature or said rolling end temperature to said desired temperature T is conducted for 3 to 25 s or said steel is held isothermally within said temperature range. A process according to claim 4 or 5, comprising the steps of setting two desired temperatures T 1 and T 2 , wherein T 1 ≧ T 2 , within a temperature range from the lower one of either the Ar 3 temperature or said rolling end temperature to Ar 1 , successively cooling the steel down to the temperature T 1 at a cooling rate of 40°C/s or more, successively cooling the steel down to the temperature T 2 at a cooling rate of less than 40°C/s, further cooling the steel at a cooling rate of 40°C/s or more, and coiling the steel at a temperature of from more than 350°C to 500°C. A process according to claim 7, wherein the cooling of said steel within a temperature range from said desired temperature T 1 to said desired temperature T 2 is conducted for 3 to 25 s or said steel is held isothermally within said temperature range. A process according to any of claims 4 to 8, wherein the rolling end temperature exceeds Ar 3 +50°C. A process according to any of claims 4 to 8, wherein the rolling end temperature is within a range between Ar 3 +50°C and Ar 3 -50°C. A process according to any of claims 4 to 10, wherein the hot finish rolling starting temperature of the steel is not more than (Ar 3 +100°C). A process according to any of claims 4 to 11, wherein the steel sheet after the coiling is cooled down to not more than 200°C at a cooling rate of 30°C/h or more.

Inventors: ESAKA KAZUYOSHI, KAWANO OSAMU, TAKAHASHI MANABU, WAKITA JUNICHI, ESAKA, KAZUYOSHI, KAWANO, OSAMU, TAKAHASHI, MANABU, WAKITA, JUNICHI, ESAKA, KAZUYOSHI C/O OITA WORKS OF, KAWANO, OSAMU C/O OITA WORKS OF, WAKITA, JUNICHI C/O OITA WORKS OF
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1988
Main classification: C21D8
IPC codes: C21D8, C22C38, C21D1