Steel sheet and method for manufacturing the same

Abstract

This invention provides a steel sheet that ensures excellent strength and excellent workability. The steel sheet has a specified composition and a steel microstructure with the following characteristics: ferrite: 45-90%, martensite: 5-30%, bainite: 1-25%, retained austenite: 3% or more, and the average Mn concentration of the steel is set as [Mn], and the average Mn concentration of the martensite is set as [Mn]. M The average Mn concentration of the above ferrite is set as [Mn]. F At that time, [Mn] M / [Mn]:1.00~1.15,[Mn] M / [Mn] F Steel microstructure of 1.00 to 1.30, TS×El: 16000 MPa·% or higher.

Description

Technical Field

[0001] This invention relates to a steel sheet suitable for automotive parts and a method for manufacturing the same. More specifically, this invention relates to a steel sheet with excellent machinability and a method for manufacturing the same. Background Technology

[0002] In recent years, in the automotive industry, one method of giving vehicles a distinctive appearance is to form steel sheets for automobile bodies into complex shapes. In order to use steel sheets formed into complex shapes well in automobile bodies, it is required that the steel sheets have excellent machinability while ensuring a certain level of strength and preventing cracking during forming and processing.

[0003] For such requirements, for example, Patent Document 1 discloses a method for manufacturing steel plates with excellent elongation, which involves controlling the heating rate at a temperature range of 660 to 730°C (lower than the annealing temperature) and controlling the cooling conditions at a low temperature range after annealing, thereby enriching carbon into austenite and stabilizing the retained austenite.

[0004] In addition, Patent Document 2 discloses a method for manufacturing steel plates with excellent elongation, which involves stabilizing the retained austenite by extending the holding time after annealing and cooling to enrich carbon into austenite.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2012-31505

[0008] Patent Document 2: Japanese Patent Application Publication No. 2011-168816 Summary of the Invention

[0009] As mentioned above, especially for steel sheets used in automobile bodies, there is a requirement to balance excellent strength and processability, but the technologies disclosed in Patent Documents 1 and 2 leave room for further research into improving processability.

[0010] The present invention was made in view of the above circumstances, and its object is to provide a steel plate that ensures excellent strength and excellent workability, and a method for manufacturing the same.

[0011] The inventors, in order to achieve the above objectives, conducted further in-depth research and obtained the following insights.

[0012] (a) In order to obtain good workability, it is necessary to increase the elongation (E1), which is an indicator of the ductility of steel, and decrease the yield ratio (YR = YS / TS), which is obtained by dividing the yield strength (YS) by the tensile strength (TS).

[0013] (b) The application of ferrite is effective in obtaining excellent elongation, but excessive ferrite formation significantly degrades strength. Therefore, in order to improve elongation while ensuring strength, it is effective to also utilize retained austenite.

[0014] (c) As a general method for generating retained austenite, one can cite the method of generating bainite during cooling after annealing or during holding after cooling, thereby enriching carbon into the austenite. However, in this method, the problem of increased yield ratio due to excessive bainite formation has been found. On the other hand, as a method to reduce the yield ratio, the inventors focused on more uniformly controlling the concentration distribution of Mn present in the steel. Specifically, the microstructure consisting of two phases of ferrite and austenite or a single phase of austenite during annealing and holding is transformed from austenite into ferrite, martensite, bainite, or other metallic phases during cooling after annealing or during holding after cooling, and the untransformed phase becomes retained austenite. During this cooling or during holding after cooling, Mn moves from the ferrite present during annealing and the ferrite generated by phase transformation to austenite, and ultimately there is a tendency for Mn to enrich into martensite from the austenite. Here, by optimizing the annealing conditions and pre-suppressing the enrichment of Mn into austenite, ferrite is preferably formed compared to bainite. Furthermore, by suppressing the enrichment of Mn into martensite, the ratio of Mn concentration in martensite can be relatively reduced. Thus, it has been found that suppressing the excessive formation of bainite while also keeping the Mn concentration in martensite within a specified range is effective in reducing the yield ratio while maintaining excellent strength and workability.

[0015] (d) Furthermore, the inventors have discovered that, as annealing conditions that can reduce the Mn concentration in martensite to a specified range, annealing needs to be performed more rapidly than before. Specifically, the heating rate in the high-temperature range up to the annealing temperature needs to be increased and the soaking time (annealing holding time) during annealing needs to be kept short.

[0016] This invention was completed based on the above insights and further research. Specifically, the main structure of this invention is as follows.

[0017] 1. A steel plate having, by mass percent, C: 0.08% to 0.16%, Si: 0.5% to 1.5%, Mn: 1.7% to 2.5%, P: less than 0.10%, S: less than 0.050%, Al: 0.01% to 0.20%, N: less than 0.10%, and the remainder consisting of Fe and unavoidable impurities.

[0018] Furthermore, based on the area fraction relative to the overall steel microstructure, ferrite comprises 45%–90%, martensite 5%–30%, bainite 1%–25%, retained austenite ≥3%, and other metallic phases ≤5%.

[0019] Let the average Mn concentration of the steel be [Mn], and let the average Mn concentration of the martensite be [Mn]. M The average Mn concentration of the above ferrite is set as [Mn]. F At that time, [Mn] M / [Mn] is 1.00~

[0020] 1.15, and, [Mn] M / [Mn] F The range is 1.00 to 1.30.

[0021] The product of tensile strength and elongation (TS×E1) is 16000 MPa·% or higher.

[0022] It should be noted that in the present invention described above, "the average Mn concentration of steel ([Mn])" refers to the Mn content in the above-mentioned composition. Therefore, [Mn] in the present invention is in the range of 1.7% to 2.5% by mass. Furthermore, "the average Mn concentration of martensite ([Mn])" in the present invention... M "The average Mn concentration of ferrite ([Mn])" F The tensile strength (TS) and elongation (E1) can be determined according to the methods described later.

[0023] 2. The steel plate according to claim 1 above, wherein the above composition further contains, by mass %, one or more of the following: Nb: less than 0.040%, Ti: less than 0.030%, B: less than 0.0030%, Cr: less than 0.3%, Mo: less than 0.2%, and V: less than 0.065%.

[0024] 3. The steel plate according to 1 or 2 above, wherein the above composition further contains, by mass % less than, one or more of one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr.

[0025] 4. The steel plate according to any one of 1 to 3 above, wherein the yield ratio (YR) calculated from the ratio of yield strength to tensile strength (YS / TS) is 0.80 or less.

[0026] Here, the "yield strength (YS)" can be determined according to the method described later.

[0027] 5. The steel plate according to any one of 1 to 4 above, wherein at least one surface further has a coating.

[0028] 6. A method for manufacturing a steel plate, comprising:

[0029] In the hot rolling process, a steel billet having the composition described in any one of the above 1 to 3 is heated at a billet heating temperature of 1200°C or higher, rolled at a final rolling temperature of 840°C to 900°C, and then cooled to a winding temperature of 450°C to 650°C for winding to obtain a hot-rolled plate.

[0030] The cold rolling process involves cold rolling the aforementioned hot-rolled sheet to obtain a cold-rolled sheet; and

[0031] The annealing process involves annealing the aforementioned cold-rolled sheet to obtain a steel sheet.

[0032] In the aforementioned annealing process,

[0033] Heating was performed from 600°C to the annealing temperature at an average heating rate of 1°C / second to 7°C / second.

[0034] After heating, at the above annealing temperature: (A) C1 (point +50℃)~(A) C3 Annealing temperature: +20℃; annealing holding time: 1 second or more but less than 35 seconds.

[0035] After annealing, a first cooling process is performed with an average cooling rate of 10°C / sec to 50°C / sec within the temperature range from the annealing temperature to the first cooling stop temperature, and a first cooling stop temperature of 450°C to 600°C.

[0036] After the first cooling, a second cooling is performed, with a dwell time of 20 to 100 seconds between the first cooling stop temperature and the second cooling stop temperature, and a second cooling stop temperature of 400°C to 500°C.

[0037] 7. The method for manufacturing a steel plate according to 6 above, wherein, after the secondary cooling in the annealing process, a plating process is further performed to plate at least one surface of the steel plate.

[0038] According to the present invention, a steel sheet that ensures excellent strength and excellent machinability can be provided. If the steel sheet of the present invention is applied to automotive parts such as automobile bodies, various shapes can be achieved for the automotive parts while ensuring excellent strength, thus enabling high-performance automobiles. Furthermore, according to the present invention, a manufacturing method can be provided that can produce a steel sheet that ensures such excellent strength and excellent machinability. Detailed Implementation

[0039] The embodiments of the present invention will now be described in detail.

[0040] The following embodiments represent a preferred example of the present invention and are not limited to these examples.

[0041] (steel plate)

[0042] The steel sheet of the present invention has a specified composition and a microstructure with ferrite, martensite, bainite, and retained austenite having specified area ratios. Furthermore, the Mn concentration in the martensite is suppressed to below a specified level, resulting in high TS×E1. The steel sheet of the present invention possesses the above characteristics, enabling it to exhibit excellent workability while ensuring strength.

[0043] The steel plate of the present invention can be readily obtained, for example, by the manufacturing method according to the present invention.

[0044] Furthermore, the steel sheet of the present invention can be used in applications requiring strength and processability, such as automotive parts.

[0045] [Ingredients]

[0046] First, the composition of the steel plate of the present invention will be described. In the following description of the composition, "%" as the unit of element content means "mass %".

[0047] C: 0.08%~0.16%

[0048] Carbon (C) is an element that improves the hardenability of steel and is essential for ensuring the desired strength. In this invention, from the viewpoint of increasing the strength of ferrite through C and ensuring a tensile strength TS ≥ 750 MPa, a C content of 0.08% or more is required. If the C content is less than 0.08%, the desired strength cannot be obtained. The C content is preferably 0.09% or more, more preferably 0.10% or more. On the other hand, if the C content exceeds 0.16%, not only is bainite easily formed, but the yield strength YS in ferrite is relatively increased relative to the tensile strength TS, thus increasing the yield ratio YR. Therefore, the C content is 0.16% or less. The C content is preferably 0.15% or less, more preferably 0.14% or less.

[0049] Si: 0.5%–1.5%

[0050] Si is not only a strengthening element obtained through solid solution strengthening, but it also has the effect of suppressing bainite formation. To obtain this effect, the Si content must be 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more. On the other hand, Si is an element that deteriorates surface properties. In addition, Si increases the yield ratio YR by relatively increasing the yield strength YS relative to the tensile strength TS in ferrite. Therefore, the Si content is 1.5% or less, preferably 1.4% or less, and more preferably 1.2% or less.

[0051] Mn: 1.7%–2.5%

[0052] Mn is included to improve the hardenability of steel and ensure the desired strength. If the Mn content is less than 1.7%, the desired strength cannot be obtained. Therefore, the Mn content is 1.7% or more, preferably 1.8% or more, and more preferably 1.9% or more. On the other hand, if excessive Mn is added, oxides form on the surface of the steel sheet, significantly deteriorating the surface properties. Furthermore, since it promotes the enrichment of Mn into austenite, bainite is formed instead of ferrite during cooling after annealing or during holding after cooling, increasing the yield ratio YR. Therefore, the Mn content is 2.5% or less, preferably 2.4% or less, and more preferably 2.3% or less.

[0053] P: below 0.10%

[0054] Phosphorus (P) is an element that strengthens steel, but if its content is too high, segregation occurs at grain boundaries, deteriorating elongation. Therefore, the P content is 0.10% or less, preferably 0.05% or less, and more preferably 0.03% or less. It should be noted that there is no particular limitation on the lower limit of the P content, but currently, the industrially feasible lower limit is around 0.001%. Therefore, the P content is preferably 0.001%, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0055] S: below 0.050%

[0056] Sulfide (S) degrades elongation by forming MnS and the like. Furthermore, when Ti is present along with S, elongation may degrade by forming TiS, Ti(C, S), etc. Therefore, the S content is 0.050% or less, preferably 0.030% or less, more preferably 0.020% or less, and even more preferably 0.010% or less. It should be noted that the lower limit of the S content is not particularly limited, but the currently industrially feasible lower limit is approximately 0.0002%. Therefore, the S content is preferably 0.0002% or more. The S content is more preferably 0.0005% or more.

[0057] Al: 0.01%–0.20%

[0058] Al is added to achieve sufficient deoxidation, reduce coarse inclusions in the steel, and improve elongation. If the Al content is less than 0.01%, these effects are not achieved. Therefore, the Al content is 0.01% or more, preferably 0.02% or more. On the other hand, if the Al content exceeds 0.20%, nitride precipitates such as AlN are formed coarsely, thus reducing elongation. Therefore, the Al content is 0.20% or less, preferably 0.17% or less, and more preferably 0.15% or less.

[0059] N: below 0.10%

[0060] Nitrogen (N) is an element that forms precipitates of nitride systems such as AlN that can pin grain boundaries, and is added to improve elongation. However, if the N content exceeds 0.10%, precipitates of nitride systems such as AlN are formed coarsely, thus reducing elongation. Therefore, the N content is 0.10% or less, preferably 0.05% or less, and more preferably 0.01% or less. It should be noted that there is no particular limitation on the lower limit of the N content, but the lower limit currently feasible in industry is about 0.0006%. Therefore, the N content is preferably 0.0006% or more, and more preferably 0.0010% or more.

[0061] The steel plate of the present invention has a composition containing the aforementioned elements and including a remainder of Fe (iron) and unavoidable impurities. In particular, the steel plate of one embodiment of the present invention preferably has a composition containing the aforementioned elements and a remainder consisting of Fe and unavoidable impurities.

[0062] The steel plate of one embodiment of the present invention may further contain the following elements as optional additives.

[0063] One or more of the following: Nb: less than 0.040%, Ti: less than 0.030%, B: less than 0.0030%, Cr: less than 0.3%, Mo: less than 0.2%, and V: less than 0.065%.

[0064] Furthermore, as an optional additive element, it may contain one or more elements selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr, in a total of less than 0.1%. It should be noted that in this invention, when these optional additive elements are included at a value less than the preferred lower limits described below, the element is included as an unavoidable impurity.

[0065] Nb: below 0.040%

[0066] Nb contributes to increased strength by refining old γ-particles and generating fine precipitates. To achieve this effect, when Nb is actively contained, it is preferable that the Nb content is 0.001% or more, more preferably 0.0015% or more, and even more preferably 0.0020% or more. On the other hand, if a large amount of Nb is contained, the amount of carbonitride precipitates becomes excessive, thus reducing the elongation. In addition, due to the increase in the amount of carbonitride precipitates, the yield strength YS in ferrite is relatively increased relative to the tensile strength TS, thus increasing the yield ratio YR. Therefore, when Nb is contained, the Nb content is preferably 0.040% or less, more preferably 0.035% or less, and even more preferably 0.030% or less.

[0067] Ti: below 0.030%

[0068] Like Nb, Ti contributes to increased strength through the refinement of old γ-particles and the generation of fine precipitates. To achieve this effect, when Ti is actively contained, it is preferable that the Ti content is 0.001% or more, more preferably 0.0015% or more, and even more preferably 0.0020% or more. On the other hand, if Ti is contained in large quantities, the amount of carbonitriding precipitates becomes excessive, thus reducing elongation. In addition, due to the increase in the amount of carbonitriding precipitates, the yield strength YS in ferrite is relatively increased relative to the tensile strength TS, thus increasing the yield ratio YR. Therefore, when Ti is contained, the Ti content is preferably 0.030% or less, more preferably 0.025% or less, and even more preferably 0.020% or less.

[0069] B: Below 0.0030%

[0070] Boron (B) is an element that improves the hardenability of steel. By including B, the desired strength can be easily obtained even with low Mn content. To achieve this effect, when B is actively included, it is preferable that the B content is 0.0001% or more, more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0030%, the amount of nitride precipitates such as BN becomes excessive, thus reducing elongation. Therefore, the B content is preferably 0.0030% or less, more preferably 0.0025% or less, and even more preferably 0.0020% or less.

[0071] Cr: less than 0.3%

[0072] Cr can be included to improve the hardenability of steel. There is no particular limitation on the lower limit of the Cr content, but from the viewpoint of achieving the aforementioned effect, the Cr content is preferably 0.005% or more. However, if the Cr content becomes excessive, the amount of precipitates such as carbides becomes excessive, thus reducing elongation. Furthermore, when the steel sheet surface is further coated, oxide formation reactions sometimes occur, accompanied by the generation of hydrogen ions. This poses the risk of hindering the rise of pH on the base iron surface, hindering the precipitation of zinc phosphate crystals, causing poor formation, and deteriorating the surface properties of the coating. Therefore, when Cr is included, the Cr content is preferably 0.3% or less, more preferably 0.2% or less, and even more preferably 0.1% or less.

[0073] Mo: 0.2% or less

[0074] Like Cr, Mo can be included to improve the hardenability of steel. There is no particular limitation on the lower limit of the Mo content, but from the viewpoint of achieving the aforementioned effect, the Mo content is preferably 0.005% or more. However, if the Mo content becomes excessive, the amount of precipitates such as carbides becomes excessive, thus reducing elongation. Furthermore, if the steel sheet surface is further coated, the surface properties of the coating may deteriorate due to the same mechanism as with Cr. Therefore, when Mo is included, the Mo content is preferably 0.2% or less, more preferably 0.1% or less, and even more preferably 0.04% or less.

[0075] V: below 0.065%

[0076] Like Cr and Mo, V can be included to improve the hardenability of steel. There is no particular limitation on the lower limit of the V content, but from the viewpoint of achieving the aforementioned effect, the V content is preferably 0.005% or more. However, if the V content becomes excessive, the amount of precipitates such as carbides becomes excessive, thus reducing elongation. Furthermore, if the steel sheet surface is further coated, the surface properties of the coating may deteriorate due to the same mechanism as with Cr and Mo. Therefore, when V is included, the V content is preferably 0.065% or less, more preferably 0.050% or less, and even more preferably 0.035% or less.

[0077] Selected from one or more of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr: total less than 0.1%.

[0078] Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr are elements that enhance strength without degrading the plating quality. To achieve this effect, the content of these elements, individually or collectively, is preferably 0.0010% or more. However, if the total content of these elements exceeds 0.1%, the aforementioned effect saturates. Therefore, when containing one or more of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr, the total content of these elements is preferably 0.1% or less.

[0079] [organize]

[0080] Next, the structure of the steel plate of the present invention will be described.

[0081] The microstructure of the steel sheet of the present invention, by area ratio, comprises 45% to 90% ferrite, 5% to 30% martensite, 1% to 25% bainite, and 3% or more retained austenite. Furthermore, in the microstructure of the steel sheet of the present invention, the average Mn concentration in the steel is defined as [Mn], and the average Mn concentration in the martensite is defined as [Mn]. M The average Mn concentration in ferrite is set as [Mn].F At that time, [Mn] M / [Mn] is 1.00~1.15, [Mn] M / [Mn] F It ranges from 1.00 to 1.30.

[0082] It should be noted that the area ratio refers to the proportion of the area of ​​each metal phase relative to the total area of ​​the steel structure.

[0083] Ferrite surface area ratio: 45%–90%

[0084] Ferrite is an essential microstructure from the viewpoint of achieving good elongation and workability of the steel sheet. Therefore, the area fraction of ferrite must be 45% or more, preferably 50% or more, and more preferably 55% or more. That is, a microstructure with the highest ferrite area fraction is preferred. On the other hand, if the area fraction of ferrite becomes too large, the area fraction of martensite, which is used to ensure strength, decreases, making it difficult to ensure the desired strength in the steel sheet. Therefore, the area fraction of ferrite is 90% or less, preferably 85% or less, and more preferably 80% or less.

[0085] It should be noted that the ferrite in this specification is a structure composed of grains of BCC lattice, which is usually generated by austenite phase transformation at higher temperatures.

[0086] Martensite area ratio: 5%–30%

[0087] Martensite contributes to high strength and is therefore an essential microstructure for ensuring adequate strength. Therefore, the martensite area fraction must be 5% or more, preferably 8% or more, and more preferably 10% or more. On the other hand, an increase in the martensite area fraction leads to a decrease in elongation. Therefore, the martensite area fraction is 30% or less, preferably 28% or less, and more preferably 25% or less.

[0088] It should be noted that the martensite in this specification refers to the hard structure formed by austenitic phase transformation below the martensitic transformation point (also referred to as the Ms point), including both so-called fresh martensite in the quenched state and so-called tempered martensite formed by reheating and tempering the fresh martensite.

[0089] Bainite surface area ratio: 1%–25%

[0090] The presence of bainite contributes to an increased yield ratio and therefore needs to be reduced. Therefore, the area fraction of bainite is 25% or less, preferably 20% or less, and more preferably 15% or less. On the other hand, a significant amount of bainite is formed during ferrite formation at the time of cooling after annealing or during holding after cooling; therefore, the lower limit for the area fraction of bainite is 1%. In the prior art, Mn tends to enrich in austenite at high temperatures, thus leading to the formation of a large amount of bainite after cooling following annealing. However, in this invention, by optimizing the annealing conditions as described later, the bainite ratio can be suppressed while ensuring the retained austenite ratio, thereby achieving excellent workability.

[0091] It should be noted that bainite in this specification is a hard structure in which fine carbides are dispersed in acicular or platy ferrite, which is formed by austenitic phase transformation at relatively low temperatures above the Ms point.

[0092] Area ratio of retained austenite: 3% or more

[0093] Retained austenite is an essential microstructure for achieving good elongation. Therefore, the area fraction of retained austenite is 3% or more, preferably 5% or more, and more preferably 7% or more. There is no particular upper limit to the area fraction of retained austenite, but a higher value is preferred from the viewpoint of achieving good elongation. On the other hand, the area fraction of retained austenite is generally preferably 20% or less. From the viewpoint of suppressing bainite formation and readily promoting ferrite formation to reduce the yield ratio, the area fraction of retained austenite is more preferably 15% or less, and even more preferably 10% or less.

[0094] It should be noted that the retained austenite in this specification refers to austenite that has not been transformed into ferrite, martensite, bainite or other metallic phases and remains thereafter.

[0095] Area fraction of other metallic phases: less than 5%

[0096] The microstructure of the steel plate of the present invention may further include other metallic phases besides the aforementioned ferrite, martensite, bainite, and retained austenite. An area fraction of 5% or less of these other metallic phases is permissible, preferably 1% or less. It should be noted that the area fraction of these other metallic phases may also be 0%.

[0097] Other metallic phases include, for example, pearlite. In this specification, pearlite is a microstructure composed of ferrite and acicular cementite.

[0098] Here, the area fraction of each phase can be determined as follows.

[0099] Specifically, for ferrite, martensite, and bainite, test pieces are taken from the base iron region of the steel plate (in the case of a coating described later, this refers to the region excluding the coating) with an L-section parallel to the rolling direction as the test surface. Next, the test surface of the test piece is mirror-polished and the microstructure is revealed using a nitric acid-ethanol etching solution. The test surface of the revealed microstructure is observed using a scanning electron microscope (SEM) at 1500x magnification to obtain SEM images. The area fractions of ferrite, martensite, and bainite at one-quarter of the plate thickness are determined using a point counting method.

[0100] It should be noted that martensite appears as a white structure in SEM images. Furthermore, fine carbides precipitate within tempered martensite. Ferrite appears as a black structure. Bainite precipitates white carbides within a black structure. These criteria are used to identify each phase from the SEM images. However, depending on the orientation of the bulk grains and the degree of etching, sometimes the internal carbides are difficult to visualize; therefore, in such cases, thorough etching and confirmation are necessary.

[0101] In addition, the area ratio of retained austenite can be determined as follows.

[0102] For the surface of the steel plate used as the base iron, which is ground to 1 / 4 of the plate thickness and then further ground by 0.1 mm by chemical grinding, the integrated reflection intensity of the (200), (220), and (311) planes of FCC iron (austenite) and the (200), (211), and (220) planes of BCC iron (ferrite) is measured using Kα rays of Mo in an X-ray diffraction apparatus. The proportion of austenite obtained by the ratio of the integrated reflection intensity from each plane of FCC iron (austenite) to the integrated reflection intensity from each plane of BCC iron (ferrite) is taken as the residual austenite area ratio.

[0103] Then, the area ratios of other metallic phases can be calculated by subtracting the measured area ratios of ferrite, martensite, bainite, and retained austenite from 100%.

[0104] [Mn] M / [Mn]:1.00~1.15

[0105] Average Mn concentration in martensite ([Mn)) M A high average Mn concentration ([Mn]) relative to the steel's density means a large Mn concentration distribution in the final steel sheet product. Generally, as mentioned above, Mn tends to enrich in martensite. A large Mn concentration distribution in martensite indicates a greater expansion of martensite during the austenite-to-martensite phase transformation, which introduces strain into ferrite, contributing to a higher yield ratio. Therefore, from the viewpoint of reducing the yield ratio and improving workability, reducing the Mn concentration distribution in the steel sheet (reducing [Mn]) is beneficial.M / [Mn]) is extremely important.

[0106] Here, the Mn concentration in the austenite of the microstructure during annealing and holding becomes one of the factors determining whether the phase transforming from austenite to ferrite or bainite during cooling or holding after cooling. Furthermore, the Mn concentration in the austenite of the microstructure during annealing and holding is somewhat correlated with the Mn concentration in the martensite of the final steel sheet. For example, the lower the [Mn] concentration... M The higher the [Mn] content, the more effectively the Mn enrichment in austenite is suppressed during annealing, the easier it is for ferrite phase transformation to occur during cooling after annealing. Conversely, the higher the [Mn] content... M [Mn], meaning the enrichment of Mn in austenite during annealing and holding, makes it more prone to bainitic transformation during cooling after annealing, thus increasing the yield ratio. When [Mn]... M When the concentration of [Mn] is more than 1.15 times higher than that of [Mn], the concentration distribution of Mn is high, and in addition, the promotion of bainitic phase transformation is significant. Therefore, [Mn] M / [Mn] must be 1.15 or less, preferably 1.13 or less, and more preferably 1.10 or less.

[0107] On the other hand, because Mn is expelled from ferrite and enriched in austenite, [Mn] M The lower limit of / [Mn] is 1.00.

[0108] [Mn] M / [Mn] F 1.00~1.30

[0109] Average Mn concentration in martensite ([Mn)) M The average Mn concentration in ferrite ([Mn)) F The ratio of Mn to ferrite (austenite) becomes one of the factors determining the yield ratio. Generally, as mentioned above, Mn tends to enrich in martensite. A large concentration distribution of Mn enriched in martensite indicates a greater expansion of martensite during the austenite-to-martensite phase transformation, thereby introducing strain into the ferrite and contributing to the increased yield ratio. To obtain a low yield ratio, [Mn] should be reduced. M / [Mn] F It is extremely important. Therefore, [Mn] M / [Mn] F It must be 1.30 or less, preferably 1.25 or less, and more preferably 1.20 or less.

[0110] On the other hand, because Mn is expelled from ferrite and enriched in austenite, [Mn] M / [Mn] F The lower limit is 1.00.

[0111] It should be noted that [Mn] in the steel microstructure M and [Mn] F The following measurements can be taken.

[0112] Specifically, at a position 1 / 4 of the thickness along the thickness direction of the steel plate on any surface of the base iron region, EPMA was used to measure the Mn concentration distribution over a range of 20 μm in both the rolling direction and the thickness direction of the sample at 0.1 μm intervals, using a lattice-like pattern. SEM images of the same locations were also obtained. By comparing the obtained distribution images with the SEM images, the average Mn concentration at all measurement points for both martensite and ferrite was set as [Mn]. M and [Mn] F (Unit: mass%)

[0113] It should be noted that [Mn] refers to the Mn content (unit: mass %) in the composition of the steel plate.

[0114] [Mechanical characteristics]

[0115] (TS×El): 16000MPa·% or more

[0116] In the steel sheet of the present invention, the product of tensile strength TS (MPa) and elongation E1 (%), TS×E1, must be 16000 MPa·% or more. The steel sheet of the present invention needs to improve elongation without reducing strength, thus exhibiting excellent workability. If TS×E1 is less than 16000 MPa·%, at least one of the tensile strength TS or elongation E1 is deficient. TS×E1 is preferably 16500 MPa·% or more, more preferably 17000 MPa·% or more. On the other hand, an excessive increase in TS will reduce the ferrite content, thus becoming a reason to reduce E1 to more than the amount of increase in TS. Conversely, an excessive increase in E1 will reduce TS, becoming a reason to fail to obtain the desired TS. Therefore, TS×E1 is preferably 18000 MPa·% or less.

[0117] YR

[0118] Furthermore, in the steel sheet of the present invention, the yield ratio YR = YS / TS, calculated from the ratio of yield strength YS (MPa) to tensile strength TS (MPa), is preferably 0.80 or less, more preferably 0.75 or less, and even more preferably 0.70 or less. A low yield ratio means, for example, that the steel sheet will not break even when formed into complex shapes, and its workability is excellent. On the other hand, from the viewpoint of obtaining the minimum crash characteristics required for automobiles, the yield ratio YR can be 0.50 or more.

[0119] TS

[0120] From the viewpoint of achieving the desired strength, the tensile strength TS of the steel plate of the present invention is preferably 750 MPa or more, more preferably 780 MPa or more. While there is no particular upper limit to the tensile strength, from the viewpoint of easily achieving a balance with other properties such as elongation E1, the tensile strength is preferably less than 980 MPa.

[0121] Here, tensile strength TS, yield strength YS, and elongation E1 can be determined as follows.

[0122] That is, with the rolling direction as the long side direction, a JIS 5 test piece with a mark spacing of 50 mm and a mark width of 25 mm is taken from the center of the base iron region of the steel plate. Then, using the taken JIS 5 test piece, a tensile test is performed according to JIS Z2241 (2011) to determine TS, YS, and E1. It should be noted that the tensile speed is 10 mm / min.

[0123] [Coating]

[0124] From the viewpoint of imparting desired properties to the steel sheet, such as rust and corrosion resistance, the steel sheet of one embodiment of the present invention may further have a coating on at least one surface, or on two surfaces. Examples of coatings include Zn-based coatings and Al-based coatings. The coating can be formed by a dry method or a wet method, and can be carried out according to known methods; however, from the viewpoint of efficiently and cost-effectively coating large areas, wet methods such as hot-dip galvanizing and electroplating are preferred. Furthermore, from the viewpoint of easily adjusting various coating characteristics such as coating adhesion and coating composition, hot-dip galvanizing is more preferred, and hot-dip zinc plating is even more preferred.

[0125] (Methods for manufacturing steel plates)

[0126] Next, the method for manufacturing the steel plate of the present invention will be described.

[0127] The manufacturing method of the present invention is characterized by using a steel billet with a specified composition and performing hot rolling, cold rolling, annealing, and any other processes under specified conditions, particularly the rapid annealing process. By rapidly performing the annealing process in the manufacturing method of the present invention under specified conditions, it is possible to suppress the excessive formation of bainite while also suppressing the Mn concentration in martensite, thereby enabling the obtained steel sheet to exhibit excellent strength and workability.

[0128] The manufacturing method of the present invention can be applied, for example, to obtaining the steel sheet of the present invention. Furthermore, the manufacturing method of the present invention can be appropriately used for the purpose of obtaining steel sheets requiring strength and processability, such as those for automotive parts.

[0129] It should be noted that, unless otherwise specified, the temperature in the following description refers to the surface temperature of the steel plate (base iron). The surface temperature of the steel plate can be measured, for example, using a radiation thermometer.

[0130] [Hot rolling process]

[0131] In the hot rolling process, a steel billet with the above-mentioned composition is heated to a billet heating temperature of 1200°C or higher, rolled at a final rolling temperature of 840°C to 900°C, and then cooled to a winding temperature of 450°C to 650°C for winding to obtain a hot-rolled plate. The method for preparing the steel billet is not particularly limited; it can be prepared by agglomeration, thin-slab casting, or continuous casting. However, from the viewpoint of preventing macroscopic segregation of constituent elements, continuous casting is preferred.

[0132] Billet heating temperature: above 1200℃

[0133] If the billet heating temperature is below 1200°C, precipitates such as AlN will not dissolve in solid solution, thus coarsening during hot rolling and deteriorating elongation. Therefore, the billet heating temperature is 1200°C or higher, preferably 1230°C or higher, and more preferably 1250°C or higher. It should be noted that there is no particular upper limit to the billet heating temperature, but from the viewpoint of manufacturing cost, it is preferably 1400°C or lower, and more preferably 1350°C or lower.

[0134] Final rolling temperature: 840℃~900℃

[0135] If the final rolling temperature is less than 840°C, it takes time to lower the temperature until the final rolling temperature is reached, resulting in the formation of inclusions and coarse carbides, which deteriorates the elongation. Furthermore, the internal quality of the steel sheet may also be reduced. Therefore, the final rolling temperature is 840°C or higher, preferably 860°C or higher. On the other hand, if the holding time at high temperatures during rolling is long, coarse inclusions are formed, further deteriorating the elongation. Therefore, the final rolling temperature is 900°C or lower, preferably 880°C or lower.

[0136] Winding temperature: 450℃~650℃

[0137] If the winding temperature exceeds 650°C, the surface of the steel sheet used as the base metal may decarburize, resulting in a difference in microstructure between the interior and surface of the steel sheet, which could lead to uneven alloy concentration. Furthermore, the formation of coarse carbides and nitrides deteriorates the elongation. Therefore, a winding temperature of 650°C or below, preferably 630°C or below, is preferable. On the other hand, to prevent a decrease in cold rollability in the next process, a winding temperature of 450°C or above, preferably 470°C or above, is preferable.

[0138] Pickling can be performed on the wound hot-rolled sheet. There are no particular limitations on the pickling conditions; conventional methods are sufficient. Alternatively, the wound hot-rolled sheet can be subjected to heat treatment for softening the microstructure.

[0139] [Cold rolling process]

[0140] In the cold rolling process, the hot-rolled sheet obtained in the hot rolling process is cold-rolled to obtain a cold-rolled sheet. The purpose of the cold rolling process is to control the sheet thickness to the target thickness; therefore, as long as the desired thickness can be achieved, the rolling conditions are not particularly limited. However, if the cold rolling ratio is low, recrystallization is difficult to occur in the subsequent annealing process, resulting in the formation of non-recrystallized ferrite, which may lead to a decrease in elongation. Therefore, the cold rolling ratio is preferably 20% or more, more preferably 30% or more. On the other hand, if the cold rolling ratio is high, excessive strain is applied, making recrystallization difficult to occur in the subsequent annealing process, resulting in the formation of non-recrystallized ferrite, which may also lead to a decrease in elongation. Therefore, the cold rolling ratio is preferably 90% or less, more preferably 80% or less.

[0141] [Annealing process]

[0142] In the annealing process, the cold-rolled sheet obtained in the cold rolling process is annealed to obtain a steel sheet. In this invention, it is extremely important to control the annealing conditions as follows: heating is performed at an average heating rate of 1°C / second to 7°C / second from 600°C to the annealing temperature, and after heating, the annealing temperature is maintained at (A... C1 (point +50℃)~(A) C3 Annealing should be performed at a temperature of +20°C and a holding time of 1 second to less than 35 seconds. After annealing, the temperature range from the annealing temperature to the primary cooling stop temperature should be cooled at an average cooling rate of 10°C / second to 50°C / second and a primary cooling stop temperature of 450°C to 600°C. After the primary cooling, a secondary cooling should be performed at a holding time of 20 seconds to 100 seconds from the primary cooling stop temperature to the secondary cooling stop temperature and a secondary cooling stop temperature of 400°C to 500°C. If the annealing conditions are not controlled as described above, the area ratio and Mn concentration distribution of the resulting steel sheet cannot be suppressed within the specified range, and the steel sheet cannot simultaneously achieve excellent strength and workability.

[0143] Average heating rate from 600℃ to annealing temperature: 1~7℃ / second

[0144] Increasing the heating rate in the annealing process is one of the main features of this invention. In particular, if the heating rate in the high temperature range from 600°C to the annealing temperature is too slow, Mn, which has a slow diffusion rate, also enriches in austenite. Then, if the Mn concentration in austenite is high, bainitic transformation is promoted during subsequent phase transformations, and Mn is enriched in martensite, increasing the Mn concentration distribution. This leads to an increase in the yield ratio, and consequently, a deterioration in workability. Therefore, the average heating rate in the temperature range from 600°C to the annealing temperature is 1°C / second or more, preferably 2°C / second or more, and more preferably 3°C / second or more. On the other hand, from the viewpoint of recrystallizing ferrite to ensure the ferrite area ratio and enriching carbon in austenite to ultimately generate retained austenite, a slower heating rate is preferable. Therefore, the average heating rate in the temperature range from 600°C to the annealing temperature is 7°C / second or less, preferably 6°C / second or less, and more preferably 5°C / second or less.

[0145] Annealing temperature: (A) C1 (point +50℃)~(A) C3 (point +20℃)

[0146] If the annealing temperature is less than (A) C1 If the annealing temperature is increased by 50°C, coarse Fe-based precipitates will form, thus reducing strength and elongation. Therefore, the annealing temperature should be (A +50°C). C1 (point +50℃) or above, preferably (A) C1 Above 60°C. On the other hand, if the annealing temperature exceeds (A... C3 If the annealing temperature is increased by 20°C, the area fraction of the ferrite phase decreases, and the elongation decreases. Therefore, the annealing temperature is (A +20°C). C3 Below +20℃, preferably (A) C3 Below +10℃.

[0147] It should be noted that A in this instruction manual C1 Point and A C3 The points are calculated using the following formulas (1) and (2) respectively.

[0148] A C1 =723+22(%Si)-18(%Mn)+17(%Cr)+4.5(%Mo)+16(%V)···(1)

[0149] A C3 =910-203√(%C)+45(%Si)-30(%Mn)-20(%Cu)-15(%Ni)+11(%Cr)+32(%Mo)+104(%V)+400(%Ti)+460(%Al)···(2)

[0150] In formulas (1) and (2), (% element symbol) represents the content (mass%) of each element in the composition, and is 0 when it does not contain any element.

[0151] Annealing holding time: more than 1 second and less than 35 seconds

[0152] Shortening the holding time at the annealing temperature (annealing holding time) is also one of the main features of this invention. Annealing holding time is a crucial factor in controlling the Mn concentration in austenite before and after the phase transformation. From the viewpoint of suppressing Mn enrichment in austenite and further suppressing Mn enrichment from ferrite to martensite during the phase transformation, thereby reducing the yield ratio, a shorter annealing holding time is preferable. Therefore, the annealing holding time is less than 35 seconds, preferably 30 seconds or less, more preferably 25 seconds or less, and even more preferably 20 seconds or less. On the other hand, if the annealing holding time is less than 1 second, the coarse Fe-based precipitates do not dissolve, thus reducing the elongation. Therefore, the annealing holding time is 1 second or more, preferably 5 seconds or more.

[0153] Average cooling rate from annealing temperature to primary cooling stop temperature: 10–50 °C / second

[0154] During the initial cooling process following annealing at the annealing temperature, the cooling rate needs to be controlled to generate ferrite. If the cooling rate during the initial cooling is slow, pearlite will also form in addition to ferrite, resulting in poor elongation. Therefore, to suppress pearlite formation, cooling needs to be accelerated. Thus, the average cooling rate (average primary cooling rate) over the temperature range from the annealing temperature to the primary cooling stop temperature is 10°C / second or more, preferably 12°C / second or more, and more preferably 15°C / second or more. On the other hand, if the cooling rate is too fast, ferrite will not form, and bainite will form during the subsequent secondary cooling, increasing the yield ratio. Therefore, the average primary cooling rate is 50°C / second or less, preferably 45°C / second or less, and more preferably 40°C / second or less.

[0155] First cooling stop temperature: 450℃~600℃

[0156] The temperature range above 600°C is the formation temperature range for ferrite and pearlite. Therefore, if the initial cooling stop temperature exceeds 600°C, the TS×E ratio will decrease during subsequent secondary cooling due to excessive ferrite or pearlite formation, leading to deterioration in workability. Therefore, the initial cooling stop temperature is below 600°C, preferably below 550°C. On the other hand, the temperature range below 450°C is the formation temperature range for bainite. Therefore, if the initial cooling stop temperature is below 450°C, the residence temperature during subsequent secondary cooling becomes too low, resulting in excessive bainite formation. Therefore, the initial cooling stop temperature is above 450°C, preferably above 480°C.

[0157] Residence time from the primary cooling stop temperature to the secondary cooling stop temperature: 20–100 seconds

[0158] During the secondary cooling process from the primary cooling stop temperature to the secondary cooling stop temperature, in order to enrich carbon into austenite and generate retained austenite, it is necessary to control the residence time between the aforementioned temperatures. The longer the residence time from the primary cooling stop temperature to the secondary cooling stop temperature, the more carbon is enriched into austenite, and the elongation increases due to the resulting retained austenite. Therefore, the residence time from the primary cooling stop temperature to the secondary cooling stop temperature is 20 seconds or more, preferably 25 seconds or more, and more preferably 30 seconds or more. On the other hand, if the residence time from the primary cooling stop temperature to the secondary cooling stop temperature is too long, bainite is generated, and the yield ratio increases. Therefore, the residence time from the primary cooling stop temperature to the secondary cooling stop temperature is 100 seconds or less, preferably 90 seconds or less, and more preferably 80 seconds or less.

[0159] Secondary cooling shutdown temperature: 400℃~500℃

[0160] The temperature range exceeding 500°C is the pearlite formation temperature range. Therefore, if the secondary cooling stop temperature exceeds 500°C, the TS×El decreases due to pearlite formation, resulting in deteriorated workability. Therefore, the secondary cooling stop temperature is 500°C or lower, preferably 490°C or lower. On the other hand, if the secondary cooling stop temperature is less than 400°C, excessive carbide formation in bainite results in less carbon enrichment in austenite. Consequently, the TS×El decreases due to the reduction in retained austenite, leading to deteriorated workability. Therefore, the secondary cooling stop temperature is 400°C or higher, preferably 440°C or higher.

[0161] [Plating Process]

[0162] The manufacturing method of the present invention can further include a plating process that applies a plating treatment to at least one surface of the steel sheet after secondary cooling in the above-described annealing process and, depending on the circumstances, other processes described later. However, it is preferable that the plating process does not change the properties of the steel sheet as the base iron. Regarding the coating, as described above, a Zn-based coating, an Al-based coating, etc., can be formed in the plating process. Furthermore, the plating process can be a dry method or a wet method, and can be carried out according to known methods, but from the viewpoint of efficient and low-cost plating of large areas, wet methods such as hot-dip plating and electroplating are preferred. Furthermore, as described above, from the viewpoint of easily adjusting various plating characteristics, hot-dip plating is more preferred, and hot-dip galvanizing is even more preferred. The plating process can be carried out according to known methods.

[0163] [Other processes]

[0164] In addition to the steps described above, the manufacturing method of the present invention may further include, for example, a tempering and rolling step for shape adjustment after the annealing step, or other steps. Alternatively, or in addition to these steps, it may further include, for example, a tempering and rolling step for shape adjustment after the plating step, a heat treatment step for dehydrogenation, or other steps. The conditions for these other steps are not particularly limited, as long as they are performed according to conventional methods. It should be noted that in the heat treatment step for dehydrogenation, if the temperature is high, the properties will change due to tempering; therefore, a temperature of 100°C or below is preferred.

[0165] According to the manufacturing method of the present invention described above, by performing hot rolling, cold rolling, and annealing processes under specified conditions, the phase fraction and Mn concentration distribution in the steel microstructure of the steel sheet can be controlled, resulting in a steel sheet with excellent workability while ensuring strength. Therefore, the obtained steel sheet can be well used in automotive parts such as automobile bodies.

[0166] Example

[0167] The present invention will now be specifically described with reference to embodiments. It should be noted that the following embodiments represent preferred examples of the present invention and are not intended to limit the invention in any way. Furthermore, the following embodiments may be implemented with modifications that remain within the scope of the spirit of the present invention, and such modifications are also included within the technical scope of the present invention.

[0168] Evaluation of steel plate manufacturing

[0169] A steel billet with the composition shown in Table 1, the remainder consisting of Fe and unavoidable impurities, is melted in a vacuum melting furnace and then rolled to obtain a 27 mm thick rolled material.

[0170] The obtained roughed-out rolled material was hot-rolled to a thickness of 4.0 mm under the conditions shown in Table 2 to obtain a hot-rolled sheet. Next, the hot-rolled sheet was ground to a thickness of 3.0 mm, and then cold-rolled to a thickness of 1.8–0.9 mm under the conditions shown in Table 2 to obtain a cold-rolled sheet. The cold-rolled sheet was then annealed under the conditions shown in Table 2, and further plated under the conditions shown in Table 2 in several examples to manufacture steel sheets.

[0171] The coating adhesion amount was determined by ICP (Inductively Coupled Plasma) method. More specifically, a test piece was prepared by degreasing the surface of a steel plate with a coating, and then a single plating was performed using a high-precision balance. Next, the test piece was placed in 30 cc of a 1:3 HCl solution with 2-3 drops of inhibitor added, ensuring the solution was leak-proof. After the generation of H2 gas on the surface of the test piece ceased, the solution was collected. After the test piece was completely dried, a second plating was performed. The difference between the first and second plating values ​​was divided by the area per unit area to obtain the coating adhesion amount.

[0172] It should be noted that empty columns in Table 1 represent elements added unintentionally and are not necessarily 0% of mass; sometimes they are included as unavoidable impurities.

[0173] In addition, the blank columns in Table 2 indicate that no plating process was performed. In the plating methods in Table 2, “GI”, “GA” and “EG” represent GI: hot-dip galvanizing, GA: alloyed hot-dip galvanizing and EG: electro-galvanizing, respectively.

[0174] In the manufacture of the steel plates used for the above evaluation, in the electroplating of zinc, for pure Zn, the electroplating solution used is an electroplating solution in which 440 g / L of zinc sulfate heptahydrate is added to pure water and the pH is adjusted to 2.0 with sulfuric acid (Table 2, Nos. 13 and 14). For Zn-Ni, an electroplating solution is used in which 150 g / L of zinc sulfate heptahydrate and 350 g / L of nickel sulfate hexahydrate are added to pure water and the pH is adjusted to 1.3 with sulfuric acid (Table 2, No. 15). For Zn-Fe, an electroplating solution is used in which 50 g / L of zinc sulfate heptahydrate and 350 g / L of Fe sulfate are added to pure water and the pH is adjusted to 2.0 with sulfuric acid (Table 2, No. 16).

[0175]

[0176]

[0177]

[0178] For the obtained steel plate, the area fraction and Mn concentration of each phase in the microstructure of the steel plate (base iron), as well as the mechanical properties of the steel plate (base iron), were measured according to the above-described procedure. Specifically, in the microstructure identification (point counting method), 16×15 lattice points were evenly spaced on the SEM-based observation area (82μm×57μm region). Then, the number of points of each phase in the lattice points was counted, and the ratio of the number of lattice points occupied by each phase to the total number of lattice points was taken as the area fraction of each phase. In addition, the area fraction of each phase was the average of the area fractions of each phase obtained from three separate SEM images.

[0179] Processability is evaluated according to the following criteria.

[0180] ◎TS×El: ≥16500MPa·% and YR: ≤0.75

[0181] 〇TS×El: ≥16000MPa·% and YR: ≤0.80 (excluding ◎)

[0182] ×TS×El: less than 16000 MPa·% and / or YR: greater than 0.80

[0183] Then, the steel plates were judged as either qualified or unqualified according to the following criteria. Qualified plates are indicated as invention examples in the remarks of Table 3, and unqualified plates are indicated as comparative examples in the remarks of Table 3. The results are shown in Table 3.

[0184] Qualified TS: 750MPa or above and processability: 0 or ◎

[0185] Non-compliant TS: less than 750MPa and / or processability: ×

[0186]

[0187]

[0188] As shown in Tables 1 and 3, steel plates with specified composition and area ratio of each phase, Mn concentration distribution suppressed below specified limits, and specified high TS×El values ​​can exhibit excellent workability while ensuring strength.

[0189] Furthermore, as shown in Tables 1-3, by using steel billets with a specified composition and appropriately controlling the hot rolling, cold rolling, and annealing processes, the obtained steel plates have the desired microstructure, Mn concentration distribution, and TS×El value, and can exhibit excellent workability.

[0190] Furthermore, such high-processability steel sheets can be well coated. These high-processability steel sheets are suitable for applications requiring the forming of complex shapes, such as automobile bodies.

Claims

1. A steel plate having the following composition by mass %: C: 0.08%–0.16%, Si: 0.5%–1.5%, Mn: 1.7%–2.5%, P: less than 0.10%, S: less than 0.050%, Al: 0.01%–0.20%, N: less than 0.10%, and the balance being Fe and unavoidable impurities. Furthermore, based on the area fraction relative to the overall steel microstructure, ferrite comprises 45%–90%, martensite 5%–30%, bainite 1%–25%, retained austenite ≥3%, and other metallic phases ≤5%. Let the average Mn concentration of the steel be [Mn], the average Mn concentration of the martensite be [Mn] M , and the average Mn concentration of the ferrite be [Mn] F , then [Mn] M / [Mn] is 1.00 to 1.15, and [Mn] M / [Mn] F is 1.00 to 1.30, The product of tensile strength and elongation is above 16000 MPa·%.

2. The steel sheet according to claim 1, wherein, The composition further contains, by mass%, one or more of the following: Nb: less than 0.040%, Ti: less than 0.030%, B: less than 0.0030%, Cr: less than 0.3%, Mo: less than 0.2%, and V: less than 0.065%.

3. The steel sheet according to claim 1 or 2, wherein, The composition further contains, by mass percent, less than 0.1% of one or more of the following: Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr.

4. The steel plate according to any one of claims 1 to 3, wherein, The yield ratio, calculated from the ratio of yield strength to tensile strength, is below 0.

80.

5. The steel plate according to any one of claims 1 to 4, wherein, At least one surface further has a coating.

6. A method for manufacturing a steel plate, comprising the following steps: In the hot rolling process, a steel billet having the composition of any one of claims 1 to 3 is heated at a billet heating temperature of 1200°C or higher, rolled at a final rolling temperature of 840°C to 900°C, and then cooled to a winding temperature of 450°C to 650°C for winding to obtain a hot-rolled plate. The cold rolling process involves cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; The annealing process involves annealing the cold-rolled sheet to obtain a steel sheet; in, In the annealing process, Heating was performed from 600°C to the annealing temperature at an average heating rate of 1°C / second to 7°C / second. After heating, annealing is performed at the annealing temperature: (A C1 Point + 50°C) to (A C3 Point + 20°C), annealing holding time: 1 second or more and less than 35 seconds After annealing, a first cooling process is performed with an average cooling rate of 10°C / sec to 50°C / sec within the temperature range from the annealing temperature to the first cooling stop temperature, and the first cooling stop temperature being 450°C to 600°C. After the first cooling, a second cooling is performed with a residence time of 20 to 100 seconds from the first cooling stop temperature to the second cooling stop temperature, and a second cooling stop temperature of 400°C to 500°C.

7. The method for manufacturing a steel plate according to claim 6, wherein, Following the secondary cooling in the annealing process, a plating process is further performed to plate at least one surface of the steel plate.

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