Thin steel sheets and methods for manufacturing them.

Abstract

DEPCT64 Thin steel sheets with a tensile strength of 590 megapascals or higher and in the following condition: A good molding process was established, and methods for producing it were provided. Thin steel sheets contain specific chemical alloys, and their microstructure is characterized by... Ferrite is present in a proportion of 5 percent or more of the area and 60 percent or less. Martensite, as hardened, appears in a proportion of 10 percent or less of the area (including 0). (Percentage) Residual austenite is present in amounts of 5 percent or more and 20 percent. A percentage or less, and upper binite, lower binite, and temper martensite are present in the amount. Total more than 15 percent and less than 85 percent; BCC steel which has a 1-degree misalignment or Less and more surrounding austenite residues with a characteristic ratio of 2.5 or higher are present. The proportion of area is 5 percent or more and 70 percent or less; and the maximum value is 10 percent. The austenite residue is represented by the diameter of the equivalent circle of the austenite residue. All objects enclosed by BCC that have a misalignment of 1 degree or less have an average characteristic ratio of 2.5. or higher -----------------------------------------------------------

Description

Thin Steel Sheet and Method for Producing the Same

[0001] The present invention relates to a thin steel sheet and a method for producing the same. The thin steel sheet of the present invention has a strength with a tensile strength (TS) of 590 MPa or more and excellent flange formability and bendability. Therefore, the thin steel sheet of the present invention is suitable as a material for automotive skeletal members.

[0002] In recent years, from the perspective of global environmental conservation, the entire automotive industry has been aiming to improve the fuel efficiency of automobiles for the purpose of regulating CO ２ emissions. To improve the fuel efficiency of automobiles, weight reduction of automobiles by thinning the used parts is most effective. Therefore, in recent years, the amount of use of high-strength steel sheets as materials for automotive parts has been increasing.

[0003] Generally, as the strength of steel sheets increases, the formability tends to deteriorate. Therefore, in order to further expand the popularization of high-strength steel sheets, it is essential to improve the formability. Various techniques have been known so far for TRIP steel sheets that utilize retained austenite as a method for improving formability.

[0004] For example, in Patent Document 1, ferrite has an average crystal grain size of 3 μm or less and a volume fraction of 5% or less, retained austenite has a volume fraction of 10 to 20%, martensite has an average crystal grain size of 4 μm or less and a volume fraction of 20% or less, and the balance contains bainite and / or tempered martensite. By precipitating cementite particles with an average number of 30 or more cementite particles having a particle size of 0.1 μm or more per 100 μm in the plate thickness cross-section parallel to the rolling direction of the steel sheet, a steel sheet having a high elongation, excellent elongation flangeability, and a high yield ratio and having a strength of 1180 MPa or more can be obtained. ２ In the plate thickness cross-section parallel to the rolling direction of the steel sheet, an elongation is obtained by precipitating cementite particles with an average number of 30 or more cementite particles having a particle size of 0.1 μm or more per 100 μm, and a steel sheet excellent in elongation flangeability and having a high yield ratio and a strength of 1180 MPa or more can be obtained.

[0005] In Patent Documents 2 and 3, the ferrite fraction is 5% or less or the ferrite fraction is more than 5% and 50% or less, respectively, and the amount of retained austenite is 10% or more. Then, by refining MA, which is a composite structure of retained austenite and martensite, and increasing the retained austenite having a size of 1.5 μm or more, a steel sheet excellent in elongation, hole expansion property, and deep drawing property can be obtained.

[0006] Japanese Patent Publication No. WO2015-115059, Japanese Patent Publication No. 2017-214648, Japanese Patent Publication No. 2017-214647

[0007] The technology proposed in Patent Document 1 states that if cementite is not precipitated, the hardness of tempered martensite and bainite increases, and the ductility of the flange deteriorates. In other words, the strength and formability of the steel sheet inevitably change depending on the cementite precipitation state, making it impossible to obtain a steel sheet with stable mechanical properties.

[0008] The technologies proposed in Patent Documents 2 and 3 state that if the carbon-enriched region is too large, the MA becomes coarse, which worsens hole-expanding properties and reduces the hole-expanding ratio. However, while the ductility of TRIP steel increases with increasing carbon concentration in the retained austenite, there is a problem in that the TRIP effect cannot be maximized from the viewpoint of achieving both ductility and ductility.

[0009] In light of the fact that no high-strength steel sheet with the excellent formability proposed in this invention can be found in any patent documents, the present invention aims to provide a thin steel sheet having a tensile strength of 590 MPa or more and good formability, and a method for manufacturing the same.

[0010] To solve the above problems, the inventors diligently studied the requirements for achieving both stretch formability and bendability in TRIP steel sheets. The thickness of the thin steel sheet targeted in this invention is 0.4 mm to 2.6 mm. In TRIP steel sheets, elongation is improved by retained austenite, but on the surface of bending work in which large strain is applied, retained austenite transforms into martensite due to the TRIP effect, and it was difficult to obtain excellent bendability with this martensite.

[0011] Many previous inventions have improved bendability by using hard tempered martensite or bainite as the matrix structure and reducing the hardness difference with the martensite transformed by the TRIP effect. However, the ductility of the hard phase is poor, and as mentioned above, the TRIP effect was not fully utilized due to the emphasis on bendability. Therefore, we broke away from the conventional approach of prioritizing tempered martensite or bainite in TRIP steel sheets and conducted a thorough investigation into the optimal microstructure and appearance, and diligently considered heat treatment methods to obtain that structure.

[0012] First, in order to improve bendability without impairing the TRIP effect, the shape is changed to retained austenite with a high aspect ratio, and then the retained austenite is surrounded by BCC iron with a small disorder in the crystal structure. It was found that this suppresses damage associated with martensitic transformation on the bent surface and improves bendability. It was found that in order to surround retained austenite with a high aspect ratio with BCC iron with a small disorder in the crystal structure, it is important to promote the uneven distribution of carbon from heating to the subsequent cooling process and to keep the material in a temperature range of 400°C to 520°C. The present invention was completed based on the above findings, and its gist is as follows. [1] A composition comprising, by mass%, C: 0.08% to 0.24%, Si: 0.70% to 2.20%, Mn: 0.8% to 3.4%, P: 0.05% or less, S: 0.005% or less, Al: 0.005% to 0.70%, N: 0.0060% or less, with the remainder being Fe and unavoidable impurities, with a ferrite area ratio of 5% to 60%, an as-quenched martensite area ratio of 10% or less (including 0%), and residual os A thin steel sheet having a steel structure in which tenite accounts for 5% to 20%, upper bainite, lower bainite, and tempered martensite totaling more than 15% but less than 85%, the area ratio of BCC iron with an orientation difference of 1° or less surrounding retained austenite with an aspect ratio of 2.5 or more is 5% to 70%, and the average aspect ratio of retained austenite in the top 10% of the equivalent circular diameter among the retained austenite surrounded by BCC iron with an orientation difference of 1° or less is 2.5 or more. [2] The thin steel sheet according to [1], wherein the component composition further contains, by mass%, one or more of the following: Ti: 0.001% or more and 0.2% or less, Nb: 0.001% or more and 0.2% or less, V: 0.001% or more and 0.5% or less, Cu: 0.001% or more and 0.5% or less, Ni: 0.01% or more and 0.5% or less, Cr: 0.001% or more and 1.0% or less, B: 0.0002% or more and 0.0050% or less.[3] The thin steel sheet according to [1] or [2], wherein the component composition further contains, by mass%, one or more of the following: Mo: 0.001% or more and 1.0% or less, Sb: 0.001% or more and 0.050% or less, REM: 0.0002% or more and 0.050% or less, Mg: 0.0002% or more and 0.050% or less, and Ca: 0.0002% or more and 0.050% or less. A method for manufacturing thin steel sheets, comprising: a cold rolling step in which a hot-rolled steel sheet having the component composition described in any of [4], [1], to [3] is subjected to cold rolling with a reduction ratio of 46% or more; and an annealing step in which, after the cold rolling step, the sheet is heated to 780°C or more and 845°C or less, then cooled to a temperature range of 400°C or more and 520°C or less with an average cooling rate of 8°C / s or more and 25°C or less from 740°C to 600°C, and kept in that temperature range for 10 seconds or more and 80 seconds or less, then cooled to a cooling stop temperature of 150°C or more and 300°C or less with an average cooling rate of 8°C / s or more in the temperature interval from 400°C to 300°C, kept in a temperature range of ±50°C from the cooling stop temperature for 2 seconds or more and 25 seconds or less, then heated to a temperature of 300°C or more and 500°C or more, and kept in that temperature range for 480 seconds or more and 1400 seconds or less.

[0013] According to the present invention, the thin steel sheet of the present invention has high strength with a tensile strength (TS) of 590 MPa or more and excellent formability. If the thin steel sheet of the present invention is applied to automobile parts, further weight reduction of automobile parts can be achieved.

[0014] Figures 1(a) to 1(c) are schematic diagrams illustrating the definition of BCC iron surrounding retained austenite with an aspect ratio of 2.5 or more in the present invention.

[0015] Embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below.

[0016] The thin steel sheet of the present invention will be described in the order of its component composition and steel structure.

[0017] The composition of the thin steel sheet shall contain the following components in mass percent: C: 0.08% to 0.24%, Si: 0.70% to 2.20%, Mn: 0.8% to 3.4%, P: 0.05% or less, S: 0.005% or less, Al: 0.005% to 0.70%, and N: 0.0060% or less. Each component is described below. In the following description, "%" representing the content of a component means "mass percent".

[0018] C: 0.08% to 0.24% C contributes to increasing the strength of steel sheets and has the effect of improving the stability of retained austenite and increasing ductility. In order to obtain the properties desired in this invention, it is necessary to include 0.08% or more of C. Preferably, it is 0.09% or more. On the other hand, if the C content exceeds 0.24%, the hardenability becomes too high and it becomes impossible to obtain BCC iron with a small disorder in the crystal structure desired in this invention. For this reason, the range of C content is set to 0.24% or less. Preferably, it is 0.23% or less.

[0019] Si: 0.70% to 2.20% Si is an effective element for increasing the elongation of steel sheets, suppressing cementite precipitation, and obtaining retained austenite. To obtain the desired stretch formability and retained austenite content, the Si content must be at least 0.70%. Preferably, it is 0.80% or more. On the other hand, if the Si content exceeds 2.20%, the chemical conversion treatment properties deteriorate, making it unsuitable as an automotive component, so the Si content is set to 2.20% or less. Preferably, it is 2.10% or less.

[0020] Mn: 0.8% to 3.4% Mn is an austenite-stabilizing element, and it is necessary to include 0.8% or more of it to obtain the desired ferrite area ratio and retained austenite area ratio. Preferably, it is 1.2% or more. On the other hand, if the Mn content is excessive, the hardenability becomes too high, and it becomes impossible to obtain BCC iron with a small disorder in the crystal structure. For the above reasons, the Mn content was set to 3.4% or less. Preferably, it is 3.2% or less.

[0021] P: 0.05% or less. Since phosphorus (P) is a harmful element that causes low-temperature embrittlement and reduces weldability, it is preferable to reduce it as much as possible. In this invention, the P content can be up to 0.05%. Preferably it is 0.02% or less, but for use under more severe welding conditions, it is even preferable to suppress it to 0.01% or less. On the other hand, due to manufacturing processes, 0.002% may inevitably be mixed in.

[0022] S: 0.005% or less. S forms coarse sulfides in steel, which expand during hot rolling to form wedge-shaped inclusions, adversely affecting weldability. Therefore, since S is also a harmful element, it is preferable to reduce it as much as possible. In this invention, since up to 0.005% is acceptable, the S content is set to 0.005% or less. Preferably, it is 0.003% or less, but for use under more severe welding conditions, it is even more preferable to suppress it to 0.001% or less. Due to manufacturing processes, 0.0002% may inevitably be present.

[0023] Al: 0.005% to 0.70% When Al is added as a deoxidizing agent during the steelmaking process, the Al content should be 0.005% or more. Preferably, it should be 0.010% or more. The preferred range for Al is also determined by its relationship with Si. Like Si, Al suppresses cementite precipitation and enhances the stability of retained austenite. It is preferable that the total content of Si and Al be 0.90% or more, and more preferable that the total content of Si and Al be 1.10% or more from the viewpoint of suppressing variations in mechanical properties. On the other hand, Al is an element that reduces castability, so from the viewpoint of manufacturability, it should be 0.70% or less. Preferably, it should be 0.30% or less.

[0024] N: 0.0060% or less. N is a harmful element that negatively affects stretch moldability because it worsens room-temperature aging and causes unexpected cracking. Therefore, it is desirable to reduce the N content as much as possible. In this invention, it is set to 0.0060% or less. Preferably it is 0.0050% or less. Although it is desirable to reduce the N content as much as possible, it may be unavoidable that 0.0005% is mixed in during manufacturing.

[0025] The above constitutes the basic components of the present invention. The thin steel sheet of the present invention contains the above basic components, and the remainder consists of Fe (iron) and unavoidable impurities. Preferably, the thin steel sheet of the present invention contains the above basic components, and the remainder consists of Fe and unavoidable impurities.

[0026] Furthermore, the material may contain one or more of the following elements as optional components, in mass percent: Ti: 0.001% to 0.2%, Nb: 0.001% to 0.2%, V: 0.001% to 0.5%, Cu: 0.001% to 0.5%, Ni: 0.01% to 0.5%, Cr: 0.001% to 1.0%, and B: 0.0002% to 0.0050%. These elements improve bendability by suppressing damage to the bending surface through grain refinement, or they are austenite-stabilizing elements that have an effect on bulge formability through the TRIP effect. On the other hand, excessive inclusion can worsen bulge formability due to inclusion formation, or the hardenability may become too high, making it impossible to obtain the desired steel sheet structure; therefore, the above range was chosen.

[0027] Furthermore, in addition to the above, one or more of the following elements may be included as optional components: Mo: 0.001% to 1.0%, Sb: 0.001% to 0.050%, REM: 0.0002% to 0.050%, Mg: 0.0002% to 0.050%, and Ca: 0.0002% to 0.050%. These elements are used for strength adjustment, inclusion control, etc., but the effects of the present invention are not impaired even if these elements are included within the above ranges.

[0028] Next, the microstructure of the thin steel sheet of the present invention will be described. The microstructure of the thin steel sheet of the present invention has a ferrite area ratio of 5% or more and 60% or less, an as-quenched martensite area ratio of 10% or less (including 0%), retained austenite of 5% or more and 20% or less, upper bainite, lower bainite, and tempered martensite totaling more than 15% but less than 85%, an area ratio of BCC iron with an orientation difference of 1° or less surrounding retained austenite with an aspect ratio of 2.5 or more, and an average aspect ratio of retained austenite in the top 10% of the equivalent circular diameter among the retained austenite surrounded by BCC iron with an orientation difference of 1° or less is 2.5 or more.

[0029] The ferrite area ratio is 5% or more and 60% or less. Since the ferrite phase is soft, if the area ratio exceeds 60%, the desired steel sheet strength cannot be obtained. Therefore, the area ratio of the ferrite phase is 60% or less. Preferably, it is 50% or less. On the other hand, if the ferrite is less than 5%, the localization effect of carbon is lost, and BCC iron or retained austenite with the desired low disorder in the crystal structure cannot be obtained. Therefore, the area ratio of the ferrite phase is 5% or more. Preferably, it is 12% or more. The ferrite phase of the present invention is polygonal ferrite, and the target structure is one in which corrosion marks or secondary phase structures are not included in the grains.

[0030] Area ratio of as-quenched martensite is 10% or less (including 0%). As-quenched martensite is extremely hard, so during bending, the grain boundaries near the surface become crack initiation points, significantly reducing the bendability. To obtain the bendability required in this invention, the area ratio of as-quenched martensite must be 10% or less. Preferably, it is 8% or less. The lower the area ratio of as-quenched martensite, the better, and it may even be 0%.

[0031] The amount of retained austenite should be between 5% and 20%. Retained austenite improves the formability of the material. To obtain the properties required in this invention, it is necessary to generate 5% or more of retained austenite. Preferably, it should be 8% or more. On the other hand, retained austenite worsens the delayed fracture properties, so it should be 20% or less. Preferably, it should be 17% or less.

[0032] Furthermore, a carbon concentration of 0.6% by mass or higher in the retained austenite is preferable because it increases the stability of the retained austenite after processing and raises El. A more preferable carbon concentration in the retained austenite is 0.7% by mass or higher. The upper limit of the above carbon concentration is not particularly limited, but it is often 1.2% or less.

[0033] Upper bainite, lower bainite, and tempered martensite make up more than 15% but less than 85% of the total. The regions other than those described above are mainly composed of upper bainite, lower bainite, and tempered martensite, and in order to obtain the desired strength and formability, their total must be more than 15% but less than 85%, and preferably more than 30% but less than 80%.

[0034] The area ratio of BCC iron with an orientation difference of 1° or less surrounding retained austenite with an aspect ratio of 2.5 or more is 5% to 70%. The average aspect ratio of retained austenite in the top 10% of the equivalent circle diameter among the retained austenite surrounded by BCC iron with an orientation difference of 1° or less is 2.5 or more. BCC iron with less crystalline disorder is highly ductile, thus improving the stretch formability. Furthermore, one of the features of the present invention is that retained austenite with an aspect ratio of 2.5 or more is surrounded by BCC iron with an orientation difference of 1° or less. Here, "surrounding" means surrounding 70% or more of the outer circumference of retained austenite with an aspect ratio of 2.5 or more when confirmed by the method described in the examples.

[0035] When the aspect ratio of retained austenite falls below 2.5, strain concentration at the interface between BCC iron and retained austenite during punching can lead to void formation or the adverse effects of martensitic transformation of retained austenite becoming apparent, resulting in reduced bendability. Therefore, it is necessary to focus on retained austenite with an aspect ratio of 2.5% or higher.

[0036] Furthermore, the BCC iron surrounding this retained austenite needs to have minimal crystalline disorder. If the disorder is large, the ductility of the BCC iron itself is poor, and the TRIP phenomenon progresses with small strains during stretch forming, resulting in uneven strain distribution and failure to obtain the desired stretch formability. Since the above adverse effects do not become apparent if the orientation difference is within 1°, it is necessary to focus on materials with an orientation difference of 1° or less. Note that the "orientation difference" can be expressed by the KAM value measured by the method described in the examples.

[0037] To obtain the desired bulge formability, the area ratio of BCC iron with an orientation difference of 1° or less surrounding retained austenite with an aspect ratio of 2.5 or more must be 5% or more. Preferably, it is 10% or more. On the other hand, if it exceeds 70%, the desired steel sheet strength cannot be obtained. For this reason, this area ratio should be 70% or less. Preferably, it is 60% or less.

[0038] As mentioned above, the cause of surface damage during bending is strain concentration at the interface between BCC iron and retained austenite, and strain concentration is more likely to occur in coarse retained austenite. Therefore, it is necessary to control the aspect ratio of retained austenite in the top 10% of the equivalent circular diameter among the retained austenite surrounded by BCC iron with an orientation difference of 1° or less. When measuring the aspect ratio, the above-mentioned coarse particles should be targeted. Specifically, the average aspect ratio should be 2.5 or higher. The "retained austenite in the top 10% of the equivalent circular diameter" is confirmed by the method described in the examples.

[0039] In addition, if the area ratio of BCC iron within an orientation difference of 1° surrounding retained austenite with an aspect ratio of 2.5 or more is within the above range, the area ratios of the following (i) to (iii) can be suppressed to 80% or less, and the effects of the present invention can be achieved. It is more preferable that the total of the structures of the following (i) to (iii) is 65% or less. (i) The area ratio of BCC iron within an orientation difference of 1° surrounding retained austenite with an aspect ratio of less than 2.5. (ii) The area ratio of BCC iron with an orientation difference of more than 1° surrounding retained austenite with an aspect ratio of 2.5 or more. (iii) The area ratio of BCC iron with an orientation difference of more than 1° surrounding retained austenite with an aspect ratio of less than 2.5. The remaining structure is not particularly defined, but as long as the above-described steel structure is achieved, the effects of the invention will not be impaired even if other structures are mixed.

[0040] Next, a method for manufacturing the thin steel sheet of the present invention will be described. The method for manufacturing the thin steel sheet of the present invention is a method for manufacturing a thin steel sheet having the above-described component composition, and includes a hot rolling step, a cold rolling step, and an annealing step. In addition, in the manufacturing method of the present invention, it is preferable to have a heat treatment step after the cold rolling step and before the annealing step. Hereinafter, each step will be described.

[0041] The hot rolling step is a step of hot rolling a steel material having the above-described component composition.

[0042] The melting method for manufacturing the above-described steel material is not particularly limited, and known melting methods such as a converter and an electric furnace can be adopted. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, from the viewpoints of productivity and quality, it is preferable to use a continuous casting method to obtain a slab (steel material). Also, a known casting method such as an ingot-slab rolling method or a thin slab continuous casting method may be used to obtain a slab.

[0043] The hot rolling conditions for hot rolling the above-described steel material are not particularly limited and may be set as appropriate. For example, the coiling temperature after hot rolling is preferably 580°C or lower, and more preferably 530°C or lower from the viewpoint of the shape of the coil for cold rolling.

[0044] The cold rolling process is a process that involves pickling and cold rolling after the hot rolling process described above. In cold rolling, it is necessary to promote recrystallization during annealing, refine the austenite, promote the localization of carbon, and increase the number of transformation nucleation sites to promote the formation of BCC iron with low crystal strain. The cold rolling ratio should be 46% or more. Preferably, it should be 50% or more. There is no upper limit, but due to the cold rolling load, it is practically 75% or less. The pickling conditions are not particularly limited, and any known manufacturing method may be used.

[0045] From the perspective of carbon localization, which will be discussed later, a heat treatment process may be performed after the cold rolling process and before the annealing process, in which the material is heated to 480°C or higher and then cooled to room temperature. In the case of a box-type annealing furnace, this heating is performed at 480°C to 680°C for 3 hours or more, and in the case of a continuous annealing furnace, it is performed at 760°C to 820°C. In the case of a box-type annealing furnace, if the temperature is not above 480°C, carbon localization will be insufficient, and the value of box annealing will be reduced. On the other hand, if the temperature exceeds 680°C, austenite will be formed, and the effect of carbon localization will be reduced, so the temperature was set to 480°C to 680°C. In the case of continuous annealing, the heating time is shorter than in a box-type annealing furnace. Therefore, although austenite is formed at 760°C, the effect of carbon localization can be obtained. On the other hand, if the temperature exceeds 820°C, similar to a box-type annealing furnace, the amount of austenite produced increases, and the concentration of carbon in the austenite becomes dilute. Therefore, the preferred temperature range for continuous annealing is set to 760°C to 820°C. After heating, it is more preferable to cool with water to freeze the uneven distribution of carbon.

[0046] The annealing process is a process in which, after the cold rolling process or the heat treatment process, it is heated to 780°C or higher and 845°C or lower, then cooled from 740°C to 600°C at an average cooling rate of 8°C / s or higher and 25°C / s or lower to 400°C or higher and 520°C or lower, held at 400°C or higher and 520°C or lower for 10 seconds or longer and 80 seconds or shorter, cooled from 400°C to 300°C at an average cooling rate of 8°C / s or higher to a cooling stop temperature of 150°C or higher and 300°C or lower, held in a temperature range of ±50°C from the cooling stop temperature for 2 seconds or longer and 25 seconds or shorter, then heated to a temperature of 300°C or higher and 500°C or higher, and held in this temperature range for 480 seconds or longer and 1400 seconds or shorter. The annealing process is preferably carried out on a continuous annealing line.

[0047] Heating to 780°C or higher and 845°C or lower: During heating, it is necessary to generate a ferrite phase by recrystallization and austenitize to an appropriate fraction. In order to obtain a desired ferrite area ratio, the heating temperature needs to be 780°C or higher. Preferably, it is 790°C or higher. On the other hand, when the heating temperature exceeds 845°C, the C localization effect is lost, and in particular, retained austenite with a large aspect ratio cannot be obtained. Therefore, the heating temperature is set to 845°C or lower. Preferably, it is 840°C or lower.

[0048] Cooling from 740°C to 600°C at an average cooling rate of 8°C / s or higher and 25°C / s or lower: In the cooling after heating, it is necessary to suppress the formation of polygonal ferrite. If polygonal ferrite forms during this period, the crystal structure disorder becomes large, and retained austenite with a large aspect ratio cannot be obtained, resulting in a decrease in bendability. From this perspective, the average cooling rate from 740°C to 600°C, which is the polygonal ferrite formation range, is set to 8°C / s or higher. Preferably, it is 10°C / s or higher. On the other hand, when the cooling rate is high, the C localization cannot be effectively carried out. Therefore, the upper limit of the cooling rate is set to 25°C / s or lower. Preferably, it is 22°C / s or lower.

[0049] Cooling to a temperature range of 400°C to 520°C is necessary to suppress the formation of polygonal ferrite and produce BCC iron with less disorder in the crystal structure surrounding the retained austenite with a large aspect ratio. Below 400°C, martensitic transformation proceeds, increasing the disorder in the crystal structure and preventing the acquisition of the desired steel microstructure. Therefore, the cooling stop temperature is set to 400°C or higher. Above 520°C, the aspect ratio of the retained austenite decreases due to the influence of polygonal ferrite formation. Therefore, the cooling stop temperature is set to 520°C or lower. Preferably, it is between 420°C and 510°C.

[0050] Retention in a temperature range of 400°C to 520°C for 10 to 80 seconds Following the above cooling process, it is necessary to remain in a temperature range of 400°C to 520°C for a certain period of time. This retention effectively promotes the formation of BCC iron, which has a small disorder in the crystal structure surrounding retained austenite with a large aspect ratio. If the retention temperature falls below 400°C or exceeds 520°C, the desired steel structure cannot be obtained due to the formation of martensite and ferrite. Also, if the residence time in this temperature range is less than 10 seconds, the area ratio of BCC iron with a small disorder in the crystal structure surrounding retained austenite with a large aspect ratio cannot be obtained, and the desired stretch formability cannot be obtained. Therefore, the above residence time should be 10 seconds or more. Preferably, it should be 20 seconds or more. On the other hand, if it exceeds 80 seconds, an excessive amount of BCC iron with a small disorder in the crystal structure will be formed, and the desired steel sheet strength cannot be obtained. Therefore, the above residence time should be 80 seconds or less. Preferably, it should be 60 seconds or less. In this retention process, the temperature only needs to remain in the range of 400°C to 520°C for a predetermined time, and there are no constraints on the thermal history, such as heating, cooling, or isothermal holding.

[0051] Cooling is performed at an average cooling rate of 8°C / s or higher in the temperature range from 400°C to 300°C, down to a cooling stop temperature of 150°C to 300°C. To freeze the steel structure which changes above 400°C, it is necessary to cool from 400°C to 300°C at an average cooling rate of 8°C / s or higher. If it is below 8°C / s, there will be an excess of BCC iron with less disorder in the crystal structure. Preferably it is 10°C / s or higher. After cooling, cooling is stopped at 150°C to 300°C. If the cooling stop temperature is below 150°C, the austenite present in the steel sheet will undergo martensitic transformation, making it impossible to obtain the desired amount of retained austenite. A preferred cooling stop temperature is 180°C or higher. More preferably it is 200°C or higher. On the other hand, if the cooling stop temperature exceeds 300°C, the amount of as-quenched martensite increases, and the bendability deteriorates. Preferably it is 280°C or lower, more preferably 240°C or lower.

[0052] The material should remain in the temperature range of ±50°C from the cooling stop temperature for 2 to 25 seconds. In addition, lower bainite transformation progresses in the temperature range of ±50°C from the cooling stop temperature. This lower bainite transformation increases the carbon concentration in the retained austenite, further improving the formability of the material. To obtain this effect, it is necessary to remain in the temperature range of ±50°C from the cooling stop temperature for 2 to 25 seconds, from the time the material is cooled to the cooling stop temperature of 150°C to 300°C until reheating. If the period is less than 2 seconds, the lower bainite transformation will not progress sufficiently and the desired effect will not be obtained. On the other hand, if the period exceeds 25 seconds, the effect will not only saturate, but variations will occur in the reheating effect in the next step at 300°C to 500°C or higher, resulting in large variations in material properties, especially strength. Preferably, the period is 3 to 20 seconds.

[0053] The process involves heating to a temperature between 300°C and 500°C, followed by a residence period of 480 seconds to 1400 seconds within that temperature range. The purpose is to concentrate carbon in the retained austenite, allowing it to remain as retained austenite when cooled to room temperature, while also reducing the disorder of the crystal structure of the microstructure formed at low temperatures. If the residence temperature falls below 300°C or the residence time falls below 480 seconds, the retained austenite will not be concentrated, and the thermally unstable austenite will undergo martensitic transformation when cooled to room temperature. As a result, not only will the desired amount of retained austenite not be obtained, but the amount of BCC iron with less disorder in the crystal structure will also decrease. On the other hand, if the residence temperature exceeds 500°C or the residence time exceeds 1400 seconds, cementite will precipitate in the austenite, so the desired amount of retained austenite will not be obtained. Therefore, the reheating step after cooling from 150°C to 300°C involves a residence period of 480 seconds to 1400 seconds within the range of 300°C to 500°C.

[0054] Steel materials with a thickness of 250 mm having the component composition shown in Table 1 were subjected to hot rolling, pickling, and cold rolling, followed by annealing in a continuous annealing furnace under the conditions shown in Table 2, and then temper rolling to an elongation rate of 0.2% to 0.4% to produce steel sheets for evaluation. Some materials underwent a heat treatment process in a box annealing furnace or continuous annealing furnace before cold rolling or before the final annealing process. The resulting thin steel sheets were then evaluated using the following methods.

[0055] (i) Microstructure observation (area ratio of steel microstructure) A section of steel was cut from the steel plate so that the cross-section parallel to the rolling direction would be the observation surface. The center of the plate thickness was etched with 1% nital, and 10 fields of view were photographed at a depth of 1 / 4 of the plate thickness from the surface of the steel plate (hereinafter simply referred to as the 1 / 4t portion). Ferrite is a microstructure in which no corrosion traces or second phase structures are observed within the grains. As-quenched martensite is a microstructure that is observed as a white contrast in SEM images, but retained austenite and cementite also exhibit similar morphologies. Therefore, cementite was exposed by etching with picral etching, and its area ratio was determined. The sum of the area ratios of as-quenched martensite, cementite, and retained austenite was obtained from the SEM images, and the area ratio of as-quenched martensite was obtained by subtracting the area ratio of cementite and the area ratio of retained austenite described later from this sum. Upper bainite is a structure in which corrosion traces and a second phase structure are observed within the grains, while tempered martensite and lower bainite are structures in which lath structures and a fine second phase structure are observed within the grains. The sum of the structures of upper bainite, lower bainite, and tempered martensite was calculated as the sum of the area percentages of all of the above.

[0056] For measuring BCC iron surrounding retained austenite with an aspect ratio of 2.5 or higher, EBSD (electron beam backscattering diffraction) was used on the same cross-section as observed by SEM. Specifically, 1 × 10 in the 1 / 4t thickness section. ３ μm ２ The above region was analyzed using OIM Analysis 6 from TSL Solutions Co., Ltd. with a measurement step of 0.1 μm. From the results of this analysis, the disorder of the crystal structure was determined by the KAM (Kernel Average Misorientation) method, and BCC iron with a KAM value of 1° or less was identified. The aspect ratio of the retained austenite was determined by measuring the longest and shortest lengths of the retained austenite in question, and the ratio of (longest length) / (shortest length) was used as the aspect ratio of each retained austenite.

[0057] For the measurement of area ratio, the sectioning method was used for both SEM and EBSD images. Twenty horizontal and twenty vertical lines, each 30 μm in length, were drawn in a grid pattern on the obtained images. The tissue at the intersections was identified, and the ratio of the number of intersections for each tissue to the total number of intersections was defined as the area ratio of each tissue. Here, for each measurement point, BCC iron with an aspect ratio of 2.5 or higher that does not cross large-angle grain boundaries with an orientation difference of 15° or more, and does not cross BCC iron with a KAM value greater than 1°, and BCC iron with an aspect ratio of 1° or less that is in contact with retained austenite with an aspect ratio of 2.5 or higher, and retained austenite with an aspect ratio of 2.5 or higher that covers 70% or more of the entire circumference with BCC iron with a KAM value of 1° or lower were identified as BCC iron surrounding retained austenite with an aspect ratio of 2.5 or higher. According to the above definition, BCC iron that matches (a) and (b) below is outside the scope of the above definition, and only BCC iron that matches (c) below is within the scope of the above definition. (a) BCC iron in which retained austenite with an aspect ratio of 2.5 or higher is in contact with two BCC iron crystal grains across a large-angle grain boundary with an orientation difference of 15° or higher, and the length of the boundary between the BCC iron in the two regions and the retained austenite with an aspect ratio of 2.5 or higher is both greater than 30% of the total circumference of the retained austenite with an aspect ratio of 2.5 or higher. (b) BCC iron containing retained austenite with an aspect ratio of 2.5 or higher and adjacent BCC iron crystal grains with a KAM value of 1° or higher. (c) BCC iron in which retained austenite with an aspect ratio of 2.5 or higher is in contact with two BCC iron crystal grains across a large-angle grain boundary with an orientation difference of 15° or higher, but the length of the boundary between the BCC iron in the two regions and the retained austenite with an aspect ratio of 2.5 or higher is not greater than 30% of the total circumference of the retained austenite with an aspect ratio of 2.5 or higher. Schematic diagrams of (a) to (c) above are shown in Figure 1. Furthermore, in calculating the area ratio of BCC iron surrounding retained austenite with an aspect ratio of 2.5 or higher, a crystal grain of BCC iron that satisfies (c) above, where at least one BCC iron surrounding retained austenite with an aspect ratio of 2.5 or higher exists, and which is surrounded by a large-angle grain boundary with an orientation difference of 15° or more, was extracted, and this region was calculated as the area ratio of BCC iron surrounding retained austenite with an aspect ratio of 2.5 or higher.

[0058] In measuring the average aspect ratio, we focused on the top 10% of retained austenite in terms of equivalent circular diameter. Here, the "top 10% of austenite in terms of equivalent circular diameter" was determined by measuring the equivalent circular diameter of retained austenite within the region of KAM value 1° or less, extracting the top 10% of retained austenite in terms of particle size, and then measuring the (long axis) / (short axis) of each extracted retained austenite and taking the average value. The long axis and short axis can be determined by measuring the longest and shortest distances of the retained austenite, respectively.

[0059] (ii) Measurement of retained austenite fraction by XRD After polishing the steel plate to a position 1 / 4 of its thickness, the surface was further polished by 0.1 mm by chemical polishing. Using an X-ray diffractometer with Mo Kα rays, the integrated reflectance intensity of the (200), (220), and (311) surfaces of FCC iron (austenite) and the (200), (211), and (220) surfaces of BCC iron (ferrite) was measured. The proportion of austenite obtained from the intensity ratio of the integrated reflectance intensity from each surface of FCC iron (austenite) to the integrated reflectance intensity from each surface of BCC iron (ferrite) was considered as the retained austenite fraction (area fraction).

[0060] The carbon concentration in retained austenite was determined from the lattice constants obtained from the peak angles of the (200), (220), and (311) planes of the austenite measured by Cu-Ka radiation, and from equation (1). C concentration in retained austenite = 3.5780 + 0.0330 [% C] + 0.00095 [% Mn] + 0.0056 [% Al] + 0.0220 [% N] (1) Here, [% M] (M = C, Mn, Al, N) is the concentration of each alloying element. A C concentration of 0.6% or higher in retained austenite determined by this method was considered a suitable range.

[0061] (iii) Tensile Test A JIS No. 5 tensile test specimen was prepared from the obtained steel sheet perpendicular to the rolling direction, and a tensile test was performed five times in accordance with the provisions of JIS Z 2241 (2011), and the average tensile strength (TS) and total elongation (El) were determined. The crosshead speed for the tensile test was set to 10 mm / min. In Table 3, a tensile strength of 590 MPa or more and a product of TS and El of 17500 MPa·% or more were defined as the mechanical properties of the steel sheet obtained with the steel of the present invention.

[0062] Furthermore, to improve formability, it is effective to suppress necking and cracking by distributing the strain when subjected to severe processing. In the present invention, when processing is applied, BCC iron with less crystalline disorder deforms first, followed by BCC iron with greater crystalline disorder and retained austenite with a large aspect ratio inherent in BCC iron with less crystalline disorder, thus providing excellent stability against deformation of the retained austenite. Therefore, it is also suitable as an automotive component formed into a complex shape. In the present invention, for example, as a condition for suppressing necking and cracking in severe processing such as roll forming where bending and unbending are performed, the stability of retained austenite is expressed as dσ / dε at 80% of ε that satisfies the plastic instability condition (dσ / dε = 0) on the true stress (σ)-true strain (ε) curve, and a value of 1.4 or more obtained by dividing dσ / dε by the tensile strength is considered a suitable range.

[0063] (iv) Evaluation of stretch formability The stretch formability was evaluated by fixing the sample with a 100-ton load using a die with a diameter of 150 mm, deforming the sample using a spherical punch with a radius of 75 mm, and evaluating the forming height at which fracture occurred. The forming height required in this invention is 70 mm or more if the TS is 590 MPa or more and less than 780 MPa, 60 mm or more if it is 780 MPa or more and less than 980 MPa, 50 mm if it is 980 MPa or more and less than 1180 MPa, and 43 mm or more if it is 1180 MPa or more, with a particularly suitable range of 45 mm for steel plates with a tensile strength of 1180 MPa or more.

[0064] (v) Bending Test To investigate the bendability, strip-shaped samples measuring 100 mm in width and 35 mm in length were cut out, and bending tests were conducted using the V-block method with a 90° apex angle in accordance with JIS Z 2248. The minimum die diameter (R) at which cracking did not occur was determined, and the limit bending radius (R / t) was calculated by dividing this by the plate thickness (t). The preferred range was set to 2.0 or less.

[0065] In all of the examples of the present invention, a tensile strength TS of 590 MPa or higher was achieved, demonstrating excellent moldability. On the other hand, the comparative examples that fell outside the scope of the present invention either did not reach a tensile strength of 590 MPa or failed to obtain the stretch moldability and bendability required by the present invention.

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