TW202012649A - Steel sheet - Google Patents

Abstract

Provided is a steel sheet that has a high Mn concentration, and has excellent local ductility and high strength. This steel sheet is characterized by containing predetermined amounts of C, Si, and acid-soluble Al and in mass%, more than 4% but less than 9% of Mn, limited amounts of P, S, N, and O, and an arbitrarily selected element, the balance being iron and impurities, wherein the metal structure at a 1/4t portion in an L cross-section thereof contains, in an area proportion, 25-90% of tempered martensite, 3% or less of ferrite, 10-50% of residual austenite, and 25% or less of fresh martensite, and has an interface density of 2.7 [mu]m-1 or more determined by dividing, by the total area of the first and second regions, the total sum of the lengths of boundaries between first regions that contain either residual austenite or fresh martensite, and second regions which are regions obtained by removing the first regions from the metal structure at the 1/4 position.

Description

Steel plate

The present disclosure relates to a steel sheet having excellent formability and ultra-high strength characteristics, and specifically to a steel sheet having a high Mn concentration and excellent local ductility and high tensile strength.

Background of the invention In order to achieve both light weight and safety of automobile bodies and parts, the steel plates belonging to these blanks are continuing to develop toward high strength. In general, if the steel sheet is increased in strength, the elongation will decrease and the formability of the steel sheet will be impaired. Automobile body parts that use steel plates as blanks are mostly formed by pressing. Therefore, high-strength steel plates used as body parts are required to have excellent press formability. In particular, for a vehicle body member mainly composed of stretch flange forming and reaming, the mechanical properties of the steel sheet are required to have high local ductility in addition to high strength.

In order to improve the workability and formability, a steel that utilizes the deformation-induced plasticity of the residual austenitic iron, so-called TRIP steel (for example: Patent Document 1) has been proposed.

The residual Vostian iron system is made by concentrating C in Vostian iron, thereby making Vostian iron not metamorphose into other phases even at room temperature. As a technology for stabilizing Vostian iron, the following has been proposed: the steel sheet contains elements such as Si and Al that can suppress the precipitation of carbides, and during the manufacturing stage of the steel sheet, the steel is made to be C during the period when the steel sheet is toughened and the iron is deformed. It is concentrated in the Vostian iron. In this technique, the greater the C content in the steel sheet, the more stable Vostian iron can be, and the amount of residual Vostian iron can be increased. As a result, a steel sheet excellent in both strength and elongation can be produced. However, when a steel sheet is used as a structural member, it is often welded to the steel sheet. However, if the C content in the steel sheet is large, it is difficult to sufficiently ensure the weldability, and there is a limit in using it as a structural member. Therefore, it is desirable to increase both the strength and elongation of the steel sheet with a small amount of C content.

As a steel plate with less C content than the above-mentioned TRIP steel, more residual iron in the field than the above-mentioned TRIP steel, and strength and ductility greater than the above-mentioned TRIP steel, a steel added with more than 4.0% by mass of Mn has been proposed (for example: Non-Patent Document 1 ). However, the above-mentioned steels require a long-term heating process such as box annealing. Therefore, the material design of short-time heating processes such as continuous annealing suitable for manufacturing high-strength steel plates for automotive components has not been fully studied, and the requirements for improving the local ductility in this case are not clear.

Patent Document 2 discloses a steel sheet in which 3.5% by mass or more of Mn is added, and by controlling the amount of ferrite to 30% to 80%, it is excellent in tensile strength and elongation.

Patent Document 3 discloses a steel sheet for steel added with 3 to 7 mass% of Mn, which improves local ductility by suppressing the formation of residual Vostian iron to less than 20% by volume. Specifically, the The local elongation of the steel plate is more than 8%.

Patent Document 4 discloses a high-strength steel sheet containing 2.0 to 6.0% by mass of Mn and having a residual amount of iron in the field of 20% by volume or more.

Patent Literature 5 discloses a high-strength steel sheet that contains 1.68 to 3.8% by mass of Mn, has excellent local ductility, and has a local elongation of 4% or more.

Prior technical literature Patent Literature Patent Document 1: Japanese Patent Laid-Open No. 5-59429 Patent Document 2: Japanese Patent Laid-Open No. 2012-237054 Patent Document 3: Japanese Patent Laid-Open No. 2003-138345 Patent Document 4: Japanese Patent Laid-Open No. 7-1888834 Patent Literature 5: Japanese Patent Application Publication No. 2017-53001

Non-patent literature Non-Patent Document 1: Furukawa Kei, Matsumura Ryo, Heat Treatment, Japan, Japan Heat Treatment Association, 2009, Volume 37, Number 4, p.204

Summary of the invention Problems to be solved by invention In order to use a high-strength steel sheet as a member for automobiles, it is desirable to increase the strength and formability of the opposite characteristics before reducing the weldability. Specifically, it is expected to have excellent elongation characteristics and high strength.

However, for example, steel sheets with a high Mn concentration such as those disclosed in Patent Document 2 and Non-Patent Document 1 described above are rich in unrecrystallized fertile iron, and therefore will not be work hardened, and the workability and formability are low. That is, the above-mentioned steel plate with a high Mn concentration that has a structure rich in unrecrystallized fertile iron cannot have both the tensile strength required for automotive steel plates and the workability and formability.

The steel sheet of the invention example described in Patent Document 3 has a low C content of less than 0.2% by mass, so the tensile strength is 1090 MPa or less. Regarding the C content of less than 0.2% by mass, the tensile strength is made 1200 MPa or more Those with high strength show a comparative example in which the tensile strength is 1233 MPa and the local elongation is 1.3%, and the local ductility is reduced.

The steel plate described in Patent Document 4 is a hot-rolled or cold-rolled plate subjected to preliminary heat treatment at 800°C or higher to make the Vostian iron and cooled, and then annealed twice at an annealing temperature of 650 to 750°C. This promotes the concentration of alloying elements in the Vostian iron, and generates residual Vostian iron of more than 20% by volume. However, the steel sheet of the invention described in Patent Document 4 does not obtain sufficient local ductility.

For the steel sheet described in Patent Document 5 with an increased Mn-containing concentration, a comparative example showing a Mn content of 6.0% by mass and a local elongation of 0.9% shows that the local ductility is not high.

Therefore, it is desirable to have a steel sheet with a high Mn concentration that has excellent local ductility and high tensile strength.

Means for solving the problem In order to ensure excellent local ductility and high strength in a steel sheet containing a high Mn concentration, the inventors found that the following is effective: the metal structure of the steel sheet is 25% in area ratio (area %) More than 90% of tempered hemp iron and less than 3% of fertile iron as the parent phase, and contains more than 10% and less than 50% of residual Vostian iron and less than 25% of fresh hemp iron as other phases And, the interface density of the residual Vostian iron and fresh Ma Tian scattered iron relative to the mother phase mainly composed of tempered Ma Tian scattered iron is 2.7 μm ^-1 or more.

Local ductility is the property that the material can withstand the crack propagation generated in the local deformation area of the extension flange forming and bending process, and its index is the local elongation obtained by the uniaxial tensile test.

In the uniaxial tensile test, when there is fresh hemp scattered iron or residual Vostian iron with high hardness in the microstructure, voids may occur in the necking area after the local deformation starts. If voids occur, the voids will connect and expand with each other during the subsequent deformation process, resulting in the generation and expansion of cracks and eventually fracture.

The inventors believe that for crack expansion from any direction, making a microstructure with small unevenness can effectively improve local ductility, and conducted intensive research. As a result, the inventors found that by making the following microstructure, High strength steel sheet with excellent local ductility.

The mother phase of the metal structure of the steel plate is made into tempered Ma Tian scattered iron. Matian scattered iron is a low-temperature metamorphic phase rich in differential phases in this phase, which is a useful phase in increasing the strength of the steel plate. In addition, by setting the tempered hemp scattered iron, the balance between strength and local ductility can be improved.

The ductility is improved by making residual microstructure in the microstructure. In this way, not only the ductility effect brought by the soft Vostian iron is exhibited, but also the TRIP effect is exhibited due to the processing-induced metamorphosis during processing, which becomes the Matian loose iron, which can further enhance the strength-ductility-part Ductile balance.

During the quenching, the surrounding Vostian iron will be adjacent to a part of the newly-born meta-scattered scattered iron. The new-born Ma Tian loose iron and the Ma Tian loose iron produced by the process-induced metamorphosis are harder phases than the tempered Ma Tian loose iron of the parent phase, which not only increases the strength of the steel, but also prevents expansion from the soft phase at the interface. The role of crack expansion.

Specifically, in the L cross section, the metal structure at 1/4 of the thickness from the surface contains 25% or more and 90% or less of tempered hemp iron, 3% or less of fertile iron, and 10% or more in terms of area ratio And less than 50% of the remaining Vostian iron and less than 25% of the new Matian scattered iron. In addition, the L cross section here refers to a plane cut parallel to the thickness direction and the rolling direction and passing through the central axis of the rolling direction of the steel sheet.

However, it was also found that when there is a coarse structure form in which the residual Vostian iron and the fresh Ma Tian scattered iron are interconnected, the interface between the residual Vostian iron and the fresh Ma Tian scattered iron relative to the mother phase of the tempered Ma Tian scattered iron becomes less Therefore, the obstacles to crack expansion are reduced, leading to fracture early, that is, local ductility will deteriorate.

Therefore, the present inventors further discovered that by making the residual Vostian iron and the fresh Ma Tian scattered iron into fine and complex shapes, the local ductility is significantly improved. Specifically, as shown in FIGS. 1 and 2, by making the structure composed of the residual Vostian iron and the newly-born Ma Tian scattered iron into a small and complex interlaced structure with respect to the tempered Ma Tian scattered iron, the residual The interface between the parent phase of Vostian iron and fresh Ma Tian loose iron relative to the tempered Ma Tian loose iron prevents crack expansion from the expansion of the soft layer. FIG. 1 is a scanning electron microscope (SEM) image obtained by observing the L-section of a steel plate according to an embodiment of the present invention after mirror polishing and nitrate etchant treatment. FIG. 2 is a 2-gradation image obtained by subjecting the SEM image of FIG. 1 to binarization. The metal structure of the steel plate is controlled so that the interface density of the residual Vostian iron and the fresh Ma Tian scattered iron relative to the mother phase mainly composed of tempered Ma Tian scattered iron becomes 2.7 μm ^-1 or more. In this metal structure, the interface between the residual Vostian iron and the fresh Ma Tian scattered iron relative to the parent phase exists at a certain ratio and the interface will hinder any crack expansion direction, so the interface strength will be improved, and excellent Local ductility.

The steel plate of the present disclosure was created based on the above knowledge and knowledge, and its gist is as follows. (1) A steel plate, characterized in that it contains in mass %: C: more than 0.20% and less than 0.55%, Si: 0.001% or more and less than 3.50%, Mn: more than 4.00% and less than 9.00%, sol.Al : 0.001% or more and less than 3.00%, P: 0.100% or less, S: 0.010% or less, N: less than 0.050%, O: less than 0.020%, B: 0.0000% or more and less than 0.010%, Cr: 0.00% or more and less than 2.00%, Mo: 0.00% or more and 2.00% or less, W: 0.00% or more and 2.00% or less, Cu: 0.00% or more and 2.00% or less, Ni: 0.00% or more and 2.00% or less, Ti: 0.00% or more and 0.300 % Or less, Nb: 0.00% or more and 0.300% or less, V: 0.00% or more and 0.300% or less, Ca: 0.00% or more and 0.010% or less, Mg: 0.00% or more and 0.010% or less, Zr: 0.00% or more and 0.010 % Or less, REM: 0.00% or more and 0.010% or less, Sb: 0.00% or more and 0.050% or less, Sn: 0.00% or more and 0.050% or less, and Bi: 0.00% or more and 0.050% or less, and the remainder is iron and impurities ; In the L section of the steel plate, the metal structure at the 1/4th position of the thickness from the surface contains 25% or more and 90% or less of tempered hemp scattered iron, and 10% or more and less than 50% of residual Vos Tiantie and Xintian Matian scattered iron below 25%; and the total area length of the first area and the second area is divided by the total area of the first area and the second area, that is, the interface density is 2.7 μm ^-1 or more, the first area is the area where any one of the 1/4 position metal structure is left, and the second area is the metal from the 1/4 position The organization excludes areas other than the aforementioned first area. (2) The steel sheet as described in (1) above, which contains B in mass %: 0.0003% or more and less than 0.010%. (3) The steel sheet as described in (1) or (2) above, which contains one or more of the following elements: in mass %, Cr: 0.01% or more and less than 2.00%, Mo: 0.01% or more and 2.00 % Or less, W: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less. (4) The steel sheet according to any one of (1) to (3) above, which contains one or more of the following elements: in mass %, Ti: 0.005% or more and 0.300% or less, Nb: 0.005 % Or more and 0.300% or less and V: 0.005% or more and 0.300% or less. (5) The steel sheet according to any one of (1) to (4) above, which contains one or more of the following elements: in mass %, Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001 % Or more and 0.0100% or less, Zr: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.0100% or less. (6) The steel sheet according to any one of (1) to (5) above, which further contains one or more of the following elements: in mass %, Sb: 0.0005% or more and 0.0500% or less, Sn: 0.0005% or more and 0.0500% or less and Bi: 0.0005% or more and 0.0500% or less. (7) The steel sheet according to any one of (1) to (6) above, wherein the surface of the steel sheet has a hot-dip galvanized layer. (8) The steel sheet according to any one of (1) to (6) above, wherein the surface of the steel sheet has an alloyed galvanized layer.

Invention effect According to the present disclosure, it is possible to provide a steel sheet with a high Mn concentration that has excellent local ductility and high strength.

Forms for carrying out the invention Hereinafter, an example of the embodiment of the steel sheet of the present disclosure will be described. 1. Chemical composition

The reason why the chemical composition of the steel sheet of the present disclosure is specified in the above manner will be explained. In the following description, the symbol "%" indicating the content of each element refers to mass% unless otherwise specified.

(C: more than 0.20% and less than 0.55%) C is an element that is extremely important for improving the tensile strength of steel. In order to obtain sufficient residual iron content in the field, it must have a C content greater than 0.20%. On the other hand, if too much C is contained, the weldability of the steel sheet is impaired, so the upper limit of the C content is set to less than 0.55%. From the viewpoint of improving tensile strength and total elongation, the lower limit of the C content is preferably 0.24% or more and more preferably 0.28% or more. The upper limit of the C content is preferably 0.40% or less and more preferably 0.35% or less. By making the upper limit of the C content within the above range, the weldability of the steel sheet can be further improved.

(Si: 0.001% or more and less than 3.50%) Si is an element that can strengthen the tempered hemp scattered iron and homogenize the structure, which can effectively improve the local ductility. In addition, Si also has the effect of suppressing the precipitation and coarsening of Xueming carbon iron and making it easier to control the Vostian iron produced during annealing. In order to obtain the above effect, it is necessary to have a Si content of 0.001% or more. The lower limit of Si content is preferably 0.01% or more, more preferably 0.30% or more, and more preferably 0.50% or more. By setting the lower limit of the Si content to the above range, the local ductility of the steel sheet can be further improved. On the other hand, if too much Si is contained, the plating properties and chemical conversion treatability of the steel sheet will be impaired, so the upper limit of the Si content is set to less than 3.50%. In addition, the upper limit of the Si content is preferably 3.00% or less and more preferably 2.50% or less.

(Mn: greater than 4.00% and less than 9.00%) Mn is an element that stabilizes Vostian iron and improves hardenability. In addition, in the disclosed steel sheet system, Mn is distributed to the Vostian iron to make the Vostian iron more stable. In order to stabilize the Vostian iron at room temperature, it must have more than 4.00% Mn. On the other hand, if the steel sheet contains too much Mn, ductility and local ductility will be impaired, so the upper limit of the Mn content is set to less than 9.00%. The lower limit of Mn content should be above 4.30%, and more preferably above 4.80%. The upper limit of Mn content should be below 8.00%, and more preferably below 7.50%. By setting the lower limit value and the upper limit value of the Mn content to the above-mentioned range, the Vostian iron can be more stabilized.

(sol.Al: 0.001% or more and less than 1.00%) Al is a deoxidizer and must contain 0.001% or more. In addition, Al expands the two-phase temperature range during annealing, so it also has the effect of improving the stability of the material. The greater the Al content, the greater the effect. However, if too much Al is contained, the surface properties, paintability, weldability, etc. will be deteriorated. Therefore, the upper limit of sol.Al is set to less than 1.00%. The lower limit of the sol.Al content should be above 0.005%, more preferably above 0.010% and even more preferably above 0.020%. The upper limit of sol.Al content should be 0.80% or less, and more preferably 0.60% or less. By setting the lower limit and upper limit of the sol.Al content to the above range, the balance between the deoxidizing effect and the effect of improving the stability of the material and the surface properties, paintability and weldability can be made better. In addition, in this specification, "sol.Al" means "acid soluble Al".

(P: below 0.100%) P is an impurity. If the steel sheet contains too much P, the toughness and weldability will be impaired. Therefore, the upper limit of the P content is set to 0.100% or less. The upper limit of P content should be 0.050% or less, 0.030% or less and 0.020% or less. The steel plate of this embodiment does not necessarily have P, so it may not contain P substantially, and the lower limit of P content is 0.000%. The lower limit of P content can also be greater than 0.000% or more than 0.001%, and the smaller the P content, the better.

(S: 0.010% or less) S is an impure substance. If the steel sheet contains excessive S, elongation of MnS will be generated due to hot rolling, resulting in deterioration of toughness. Therefore, the upper limit of the S content is set to 0.010% or less. The upper limit of S content should be 0.007% or less, and more preferably 0.003% or less. The steel plate of this embodiment does not necessarily have S, so it may not contain S substantially, and the lower limit of S content is 0.000%. The lower limit of the S content can also be set to more than 0.000% or set to 0.001% or more, and the smaller the S content, the better.

(N: less than 0.050%) N is an impurity. If the steel plate contains more than 0.050% of N, the toughness will be impaired. Therefore, the upper limit of the N content is set to less than 0.050%. The upper limit of N content should be 0.010% or less, and more preferably 0.006% or less. The steel plate of this embodiment does not necessarily have to be N, so it may be substantially free of N, and the lower limit of the N content is 0.000%. The lower limit of N content can also be set to more than 0.000% or set to 0.001% or more, and the smaller the N content, the better.

(O: less than 0.020%) O is an impurity. If the steel sheet contains 0.020% or more of O, ductility will deteriorate. Therefore, the upper limit of the O content is set to less than 0.020%. The upper limit of the O content should be 0.010% or less, more preferably 0.005% or less, and more preferably 0.003% or less. The steel plate of this embodiment does not necessarily have O, so it may be substantially free of O, and the lower limit of the O content is 0.000%. The lower limit of the O content can also be set to more than 0.000% or set to 0.001% or more, and the smaller the O content, the better.

(B: 0.0000% or more and 0.010% or less) B is not an essential element of the steel plate of this embodiment, so it may not be contained, and its content is 0.0000% or more. However, B strengthens the interface between the residual Vostian iron and fresh Ma Tian scattered iron compared to the mother phase with tempered Ma Tian scattered iron as the main phase, and it has a greater effect of improving local ductility. Therefore, the steel plate of this embodiment should contain B. In order to obtain the effect of improving local ductility by adding B, it must have a B content greater than 0.0003%. On the other hand, if excessive B is contained, the toughness is impaired, so the upper limit of the B content is set to 0.010% or less. The lower limit of B content should be above 0.0005% and more preferably 0.0008%, and the upper limit of B content should be below 0.0050% and more preferably below 0.0030%.

The steel plate of this embodiment may further contain one or more elements selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi. However, the steel plate of this embodiment does not necessarily require Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn, and Bi, so it may not contain Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, the lower limit of the content can also be 0%. In this specification, REM refers to a total of 17 elements of Sc, Y, and lanthanides, and REM content refers to its content when REM is one type, and refers to their total content when it is two or more types. In addition, REM is generally supplied in the form of multiple types of REM alloys, that is, rare earth metal alloys. Therefore, one or more individual elements may be added so that the REM content is within the above range, or may be added, for example, in the form of a rare earth metal alloy, and the REM content is included in the above range.

(Cr: 0.00% or more and less than 2.00%) (Mo: 0.00% or more and 2.00% or less) (W: 0.00% or more and 2.00% or less) (Cu: 0.00% or more and 2.00% or less) (Ni: 0.00% or more and 2.00% or less) Since Cr, Mo, W, Cu, and Ni are not essential elements of the steel plate of this embodiment, they may not be included, and their respective contents are 0.00% or more. However, since Cr, Mo, W, Cu, and Ni-based elements enhance the strength of the steel sheet, they may also be included. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may also contain 0.01% or more of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. However, if the steel sheet contains excessive amounts of these elements, surface scratches during hot rolling are likely to occur, and the strength of the hot rolled steel sheet becomes too high and the cold rolling ductility decreases. Therefore, the content upper limit of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni is set to 2.00% or less.

(Ti: 0.000% or more and 0.300% or less) (Nb: 0.000% or more and 0.300% or less) (V: 0.000% or more and 0.300% or less) Ti, Nb, and V are not essential elements of the steel sheet of this embodiment, so they may not be contained, and their respective contents are 0.000% or more. However, Ti, Nb, and V systems generate fine carbide, nitride, or carbonitride elements, so they can effectively increase the strength of the steel sheet. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V. In order to obtain the effect of improving the strength of the steel sheet, the lower limit of the content of one or more elements selected from the group consisting of Ti, Nb, and V is preferably 0.005% or more. On the other hand, if too much of these elements are contained, the strength of the hot-rolled steel sheet may increase excessively and the cold-rolling ductility may decrease. Therefore, the content upper limit of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300% or less.

(Ca: 0.000% or more and 0.010% or less) (Mg: 0.000% or more and 0.010% or less) (Zr: 0.000% or more and 0.010% or less) (REM: 0.000% or more and 0.010% or less) Ca, Mg, Zr, and REM (rare earth metals) are not essential elements of the steel sheet of the present disclosure, so they may not be contained, and their respective contents are 0.000% or more. However, Ca, Mg, Zr and REM can improve the toughness of the steel plate. In order to obtain this effect, the lower limit of the content of one or more elements selected from the group consisting of Ca, Mg, Zr, and REM is preferably 0.0001% or more, and more preferably 0.001% or more . However, excessive amounts of these elements will deteriorate the workability of the steel plate, so the upper limit of the content of these elements should be set to 0.010% or less, and is selected from the group consisting of Ca, Mg, Zr and REM Or the total content of two or more elements should be set to 0.030% or less.

(Sb: 0.000% or more and 0.050% or less) (Sn: 0.000% or more and 0.050% or less) (Bi: 0.000% or more and 0.050% or less) Sb, Sn, and Bi are not essential elements of the steel sheet of the present disclosure, so they may not be contained, and their respective contents are 0.000% or more. However, Sb, Sn, and Bi suppress the diffusion of easily oxidizable elements such as Mn, Si, and/or Al in the steel sheet to the surface of the steel sheet to form oxides, thereby improving the surface properties and plating properties of the steel sheet. In order to obtain this effect, the lower limit of the content of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, and more preferably 0.001% or more. On the other hand, if the content of each of these elements is greater than 0.050%, the effect will be saturated, so the upper limit of the content of these elements is set to 0.050% or less.

In the chemical composition of the steel plate of this embodiment, the balance is Fe and impurities.

The so-called "impurity" refers to components that are mixed from raw materials such as ore and scrap or various factors in the manufacturing process when manufacturing steel in the industry, and refers to what is allowed within the range that does not adversely affect the present invention.

2. Metal structure Next, the metal structure of the steel plate of this embodiment will be described. In the following description, the symbol "%" indicating the fraction of each phase refers to the area ratio (%) unless otherwise specified.

In the L section of the steel plate of this embodiment, the metal structure at the 1/4 position (also called 1/4t part) of the thickness from the surface contains 25% or more and 90% or less of tempered hemp iron and 3% or less of fertilizer Grained iron is used as the parent phase, and it contains more than 10% and less than 50% of residual Vostian iron and less than 25% of fresh Ma Tian scattered iron as other phases. In addition, the interface density of the residual Vostian iron and fresh Matian loose iron relative to the mother phase mainly composed of tempered Matian loose iron is 2.7 μm ^-1 or more. Here, the fraction of each phase will change with the annealing conditions, which will affect the strength, local ductility and other materials. The L cross section refers to a plane cut parallel to the thickness direction and the rolling direction and passing through the central axis of the rolling direction of the steel sheet.

The area ratio of tempered Ma Tian scattered iron and fertile iron is calculated based on the structure observation using a scanning electron microscope (SEM). After mirror-polishing the L section of the steel plate, it is corroded with 3% nitrate etchant (3% nitric acid-ethanol solution), and then the metal structure at the 1/4 position from the surface is observed with a scanning electron microscope at a magnification of 5000 times. Measure the area ratio of tempered Ma Tian scattered iron and fertilized iron.

The area ratio of residual Vostian iron was measured by X-ray diffraction method. In the observation of the scanning electron microscope, it is difficult to distinguish between the residual Vostian iron and the new-born Ma Tian scattered iron (that is, the untempered Ma Tian scattered iron), so the area ratio of the tempered Ma Tian scattered iron and the new Ma Tian scattered iron is used It is determined by the following method. After mirror-polishing the L section of the steel plate, it is corroded with a 3% nitrate etchant (3% nitric acid-ethanol solution), and then observed with a scanning electron microscope at a magnification of 5000 times to observe the microscopic position at 1/4 of the thickness from the surface of the steel plate Organization to measure the total area ratio of residual Vossian iron and Shinsei Matian scattered iron. Next, the area ratio of the residual Vostian iron measured by the X-ray diffraction method was subtracted from the total area ratio of the residual Vostian iron and the fresh Ma Tian scattered iron to calculate the area ratio of the fresh Ma Tian scattered iron.

(The area ratio of tempered hemp scattered iron in the metal structure of the 1/4t part of the steel plate: 25% or more and 90% or less) In the L cross section of the steel plate of the present embodiment, the metal structure of the 1/4t portion of the thickness from the surface contains 25% or more and 90% or less of tempered hemp iron in terms of area ratio. The tempered Ma Tian loose iron is the parent phase of the steel plate of this embodiment in which local ductility is improved. Therefore, in order to improve local ductility, more than 25% of tempered hemp iron must be tempered. On the other hand, if there is an excessive amount of tempered hemp iron, the residual Vostian iron and fresh hemp iron will be too little, and not only the tensile strength will be lower, but the ductility will also deteriorate, so the area of tempered hemp iron The upper limit of the rate is set to 90% or less. The lower limit of the area ratio of tempered Ma Tian scattered iron should be above 40%. In addition, the upper limit of the area ratio of tempered hemp scattered iron should be 80% or less.

(The area ratio of residual austenitic iron in the metal structure of the 1/4t part of the steel plate: 10% or more and 50% or less) In the L section of the steel plate of the present embodiment, the metal structure of the 1/4t portion of the thickness from the surface contains 10% or more and 50% or less residual austenitic iron in terms of area ratio.

The residual Vostian iron system will improve the phases of tensile strength and ductility of the steel plate by deforming induced plasticity. Residual Vostian iron will be deformed into Matian loose iron due to the press processing, drawing processing, stretch flange processing or bending processing accompanying tensile deformation, so processing the steel plate also helps to increase the strength of the resulting steel. In order to obtain these effects, the area ratio of the residual austenitic iron must be set to 10% or more. The lower limit of the area ratio of the residual Vostian iron should be above 15%, more preferably above 20% and even more preferably above 30%.

The higher the area ratio of residual Vossian iron, the better. However, when manufacturing an alloy having the above-mentioned chemical composition by the following method, the upper limit of the area ratio of residual austenitic iron is 50%. If it contains more than 9.0% of Mn, the residual Vostian iron can be made more than 50%, but at this time, hot workability and castability will be impaired. Considering the hydrogen embrittlement point of view, the area ratio of the residual Vostian iron is preferably 45% or less, and more preferably 40% or less.

(The area ratio of fresh iron and iron in the metal structure of the 1/4t part of the steel plate: 25% or less) In the L cross section of the steel plate of the present embodiment, the metal structure of the 1/4t portion of the thickness from the surface contains 25% or less of fresh hemp iron in terms of area ratio.

In the steel sheet of the present embodiment, the fresh hemp scattered iron in the metal structure is untempered hemp iron scattered, and it is a phase that can strengthen the steel sheet of the present embodiment and increase the tensile strength of the steel sheet. However, the new-born Ma Tian scattered iron itself is a hard phase, so it also has a function of deteriorating local ductility. In order not to reduce the local ductility, the area ratio of fresh hemp scattered iron in the metal structure is set to 25% or less, preferably 15% or less, and more preferably 10% or less.

(Area ratio of fat iron in metal structure of 1/4t part of steel plate: 3% or less) In the L section of the steel plate of this embodiment, the metal structure of the 1/4t portion of the thickness from the surface contains 3% or less of ferrite iron in terms of area ratio.

In the steel sheet of the embodiment, it is important that the iron content of the ferrite particles in the metal structure is small. This is because if the iron content of the ferrite in the metal structure increases, the ductility will decrease. In order not to reduce ductility, the area ratio of the ferrite grains in the metal structure is set to 3% or less, preferably 1% or less, and more preferably 0%.

(In the metal structure of the 1/4t part of the steel plate, the interface density of the remaining bulk iron and fresh hemp scattered iron relative to the mother phase with tempered hemp scattered iron as the main body is 2.7 μm ^-1 or more. In the L cross section, in the metal structure of 1/4t thickness from the surface, the sum of the boundary lengths of the first region and the second region is divided by the total area of the first region and the second region, that is The interface density is 2.7 μm ^-1 or more, the first area is an area where either the Vostian iron or the freshly made Ma Tian scattered iron remains, and the second area is excluded from the first area from the metal structure at the 1/4 position Area. The interface density is more preferably 3.15 μm ^-1 or more.

In the steel plate of the present embodiment, the residual Vostian iron and the fresh Ma Tian scattered iron will form an interface between the mother phase mainly composed of tempered Ma Tian scattered iron. The total area length of the first area and the second area is divided by the total area of the first area and the second area, that is, the interface density is 2.7 μm ^-1 or more, and the first area is the residual Vostian iron Or the area of any one of the newly-born Ma Tian loose iron, the second area is the mother phase with the tempered Ma Tian loose iron as the main body outside the first area. As described above, the steel sheet of the present embodiment has the following structure: the density of the interface between the residual Vostian iron and the freshly made hemp scattered iron relative to the mother phase mainly composed of tempered hemp scattered iron (that is, the interface density) is 2.7 μm ^{− 1} or more organizations. The measurement of the interfacial density of the residual Vostian iron and fresh Ma Tian loose iron relative to the mother phase with tempered Ma Tian loose iron as the main body can be performed based on the microstructure image obtained by the above scanning electron microscope.

The interface density of the residual Vostian iron and fresh Matian scattered iron relative to the mother phase with tempered Matian scattered iron as the main body has a great influence on the local ductility. When the density is above 2.7 μm ^-1 , the residual Vostian iron and the Xinshengmatian scattered iron has a fine and independent structure, so it will become a homogeneous interface distribution, and good local ductility can be obtained.

When the interface density of the residual Vostian iron and fresh Ma Tian scattered iron relative to the mother phase with tempered Ma Tian scattered iron as the main body is less than 2.7μm ^-1 , the residual Vostian iron and fresh Ma Tian scattered iron will appear thick and interconnected The structure, therefore, does not become a homogeneous interface distribution, and the effect of suppressing crack propagation is impaired.

From the microstructure image obtained by the scanning electron microscope as follows, the interface density between the residual Vostian iron and the fresh Ma Tian scattered iron relative to the mother phase mainly composed of tempered Ma Tian scattered iron is derived. First of all, in the microstructure image, the area where either the Vostian iron or the freshly grown field is scattered is regarded as the first area, and the area other than the first area is excluded from the entire area of the aforementioned microstructure image It is the second area. The difference between the first area and the second area is made by the following: the residual microstructure image obtained by the scanning electron microscope of the remaining Vostian iron and the freshly made Matian scattered iron is compared with the area with relatively high brightness of other phase systems. Next, derive the sum of the boundary lengths of the first and second regions, and divide by the total area of the first and second regions, and set the resulting value as the residual Vostian iron and Xintianmatian scattered iron. The interface density of Huomatian scattered iron as the main phase of the main body. The above-mentioned microstructure image may be a metal structure in the 1/4t portion of the thickness from the surface of the L cross section of the steel plate, for example, a scanning electron microscope (SEM) photograph is observed at a magnification of 5,000 times of 24 μm×18 μm. area.

More specifically, the interface density is derived as follows. The interfacial density of the residual Vostian iron and fresh Ma Tian scattered iron relative to the mother phase with tempered Ma Tian scattered iron as the main body was measured using image analysis software ImageJ. The SEM image (24 μm×18 μm) was obtained by using SEM to observe the target tissue at a magnification of 5000 times. Next, 1280×960 divided regions were formed on the SEM image using ImageJ. For each divided area, a binarization process is performed so that the area of any of the remaining Vostian iron or Shinsengmatian scattered iron is black, and the other areas are white, to produce a second-order image. The binarized threshold value is determined using the following method: using the brightness described in "Glasbey, CA (1993), "An analysis of histogram-based thresholding algorithms", CVGIP: Graphical Models and Image Processing 55: 532-537" The average value is used as the limit value. This algorithm is installed in ImageJ, and by using the Auto threshold function and setting the method of determining the limit value as Method=Mean, binarization can be performed automatically. That is, the limit value of the binarization is set by ImageJ Method=Mean, radius=15, centering on the pixel of interest, replacing each pixel value with the average of the pixel values within a radius of 15 pixels, after smoothing The histogram is automatically determined. The interface density of the residual Vostian iron and fresh Ma Tian scattered iron relative to the tempered Ma Tian scattered iron was measured by the following: In the obtained 2nd-degree image, sequence analysis of all isolated residual Vostian iron and The length of the interface between the fresh-born Ma Tian scattered iron and the tempered Ma Tian scattered iron, and the total value is divided by the area of the entire image area (24 μm×18 μm). When the residual Vostian iron is adjacent to Shinseng Matian scattered iron, the two will become an integrated structure during the above-mentioned binarization process. Therefore, the interface length between the residual Vostian iron and Xinsong matian scattered iron will be eliminated. The so-called sequence analysis refers to measuring the interfacial density without repeating the interface for the residual Vostian iron and the new Matian scattered iron.

3. Mechanical properties Next, the mechanical characteristics of the steel sheet of this embodiment will be described.

The tensile strength (TS) of the steel plate of this embodiment is preferably 1200 MPa or more, and more preferably 1320 MPa or more. This is to reduce the thickness of the plate by increasing the strength when using the steel plate as an automobile blank, thereby contributing to weight reduction. The upper limit of the tensile strength of the steel plate is not specifically defined, and may be, for example, 1600 MPa. In addition, in order to subject the steel sheet of the present embodiment to press forming, it is preferable to be excellent in ductility and local ductility. Regarding ductility, the total elongation in the tensile test is preferably 15% or more. The upper limit of the total elongation is not specified, and it may be, for example, 35% or less. Regarding local ductility, the local elongation should be more than 2%, more preferably more than 3%, more preferably more than 4% and more preferably more than 5%. The upper limit of the local elongation is not specified, and it may be, for example, 6% or less.

The steel sheet of the present disclosure has high strength, good local ductility, and excellent formability as described above, and therefore it is most suitable for structural members such as beams. In addition, the steel sheet of the present disclosure has a high concentration of Mn and also contributes to the weight reduction of automobiles, so its contribution in the industry is extremely significant.

4. Manufacturing method Next, the manufacturing method of the steel plate of this embodiment will be described.

The steel plate of this embodiment is a method of smelting steel with the above chemical composition by a conventional method and casting it. After making a steel blank or steel ingot, it is heated and hot rolled, and then the obtained hot rolled steel plate is pickled and then cooled It is manufactured by rolling and annealing.

Hot rolling can be carried out in a general continuous hot rolling line. As long as the annealing meets the conditions described below, any one of the annealing furnace and the continuous annealing line may be used, and it is preferable that the first annealing and the second annealing described later can be performed using the continuous annealing line, so Improve productivity. The first annealing and the second annealing should be performed under a reducing gas environment, and may also be performed under a reducing gas environment such as nitrogen 98% and hydrogen 2%. By performing heat treatment in a reducing gas environment, the scale can be prevented from adhering to the surface of the steel plate, and the plating step can be directly performed without acid cleaning. Furthermore, the cold-rolled steel sheet may be flat-rolled.

In order to obtain the mechanical properties of the steel sheet of the present disclosure, the hot rolling conditions, especially the annealing conditions, are preferably performed within the range shown below.

As long as the steel plate of the present embodiment has the above-mentioned chemical composition, the molten steel may be smelted by a general blast furnace method, or it may be a material such as steel made by an electric furnace method that contains a large amount of scrap in its raw materials. The steel embryo can be produced by a general continuous casting process or a thin flat steel embryo.

The above-mentioned steel blank or steel block is heated and hot rolled. The temperature of the steel material for hot rolling should be set to 1100°C or higher and 1300°C or lower. By setting the temperature of the steel material for hot rolling to be above 1100°C, the deformation resistance of the hot rolling delay can be made smaller. On the other hand, by setting the temperature of the steel material supplied for hot rolling to 1300° C. or lower, it is possible to suppress a decrease in productivity due to an increase in scale loss. In this specification, the temperature is measured at the center of the surface of the steel plate.

The time to maintain the temperature range from 1100°C or more to 1300°C before hot rolling is not particularly limited, but in order to improve the bendability, it is preferably 30 minutes or more and more preferably 1 hour or more. In addition, in order to suppress excessive scale loss, the time to maintain the temperature range of 1100°C or more and 1300°C or less is preferably 10 hours or less and more preferably 5 hours or less. In addition, the direct rolling or direct rolling delay can also be directly supplied to the hot rolling without heating treatment.

The finishing rolling start temperature is preferably set to 700°C or more and 1000°C or less. By setting the finishing temperature of finishing rolling to 700° C. or higher, the deformation resistance during rolling delay can be reduced. On the other hand, by setting the finishing rolling start temperature to 1000° C. or lower, the deterioration of the surface properties of the steel sheet due to grain boundary oxidation can be suppressed.

The hot rolled steel plate obtained by finishing rolling is cooled and coiled to make a coil. The coiling temperature after cooling should be set below 700℃. By setting the coiling temperature to 700°C or lower, internal oxidation can be suppressed, and subsequent pickling becomes easy. The coiling temperature is preferably below 650°C, more preferably below 600°C. The lower limit of the coiling temperature is not specified, and may be room temperature, for example. In order to suppress the fracture of the cold rolling delay, after cooling to room temperature, the hot rolled sheet can also be tempered at a temperature above 300°C and below 600°C before cold rolling.

After the hot-rolled steel sheet is pickled in a conventional manner, cold rolling is performed to make a cold-rolled steel sheet.

Before cold rolling and before or after pickling, light rolling of more than 0% to about 5% is used to correct the shape, which is advantageous from the viewpoint of ensuring flatness, which is suitable. In addition, by performing mild rolling before pickling, the pickling property can be improved, the removal of surface thickening elements can be promoted, and the effect of improving the chemical conversion treatment property and the plating treatment property can be improved.

From the viewpoint of controlling the interfacial density of the residual Vostian iron and fresh Ma Tian loose iron relative to the mother phase with tempered Ma Tian loose iron as the main body, it is important to reduce the reduction ratio of cold rolling. By suppressing the reduction ratio of cold rolling to a low level, the structure after annealing can be homogenized, that is, the residual Vostian iron and fresh Ma Tian loose iron relative to the mother phase with tempered Ma Tian loose iron as the main phase The interface distribution will be homogenized. As a result, there can be more interfaces between the residual Vostian iron and the new Matian scattered iron in the metal structure. In order to obtain this effect, the upper limit of the reduction ratio of cold rolling is 50% or less, preferably 20% or less, more preferably 18% or less and even more preferably 15% or less. The lower limit of the reduction ratio of cold rolling is 0% or more, preferably 5% or more. Setting the reduction ratio of the cold rolling to 50% or less is an important requirement for satisfying the interface density condition specified in the present invention. By setting the reduction ratio of cold rolling to 18% or less, a large interface density such as 3.15 μm ^-1 or more can be obtained, and a large local elongation of 3.0% or more can be obtained.

The cold rolled steel sheet prepared through the above hot rolling step and cold rolling step is heated to 740° C. or higher and lower than 800° C. to perform the first annealing. Maintain in the temperature range above 740°C and below 800°C for 10 seconds or more, and then cool from the above maintenance temperature above 740°C and below 800°C at an average cooling rate of 2°C/sec and above 2000°C/sec To the temperature zone below 500 ℃, and should be cooled to room temperature. Next, it is heated again to 600° C. or higher and lower than the Ac ₃ point to perform the second annealing. Maintain in the temperature range above 600°C and below Ac ₃ point for 5 seconds or more, and then cool from the above maintenance temperature above 600°C and below Ac ₃ point to an average cooling rate of 10°C/sec or more to 300°C or below Up to the temperature zone. Next, it is maintained in a temperature range of 200°C or higher and 450°C or lower for 30 seconds or longer, and then cooled to room temperature.

The first annealing temperature after cold rolling is 740°C or higher and lower than 800°C. By making the above annealing temperature above 740°C, recrystallization can be significantly promoted and the area ratio of ferrite in the steel sheet can be reduced, and the ductility can be improved. In order to make the area ratio of ferrite iron 0%, the first annealing temperature is preferably Ac ₃ or more. On the other hand, if the annealing temperature is lower than 800°C, the growth of the Vostian iron grains will be inhibited. Due to the refinement effect of the grains of the old Vostian iron grains, the residual Vostian iron and the freshly grown Ma Tian scattered iron are compared to the tempered Ma Tian scattered The interfacial density of iron increases, which helps to improve local ductility. In order to completely remove unrecrystallized to stably ensure good toughness, the time maintained in the temperature range above 740°C and below 800°C is preferably set to 10 seconds or more. From the viewpoint of productivity, the time to maintain the temperature in the range of 740°C or higher and lower than 800°C is preferably set to within 300 seconds. Here, for a variety of cold-rolled steel sheets containing C: 0.05% to 0.5%, Si: 0% to 3.5%, Mn: 0 to 9.0%, and Al: 0 to 2.0%, at a heating rate of 0.5 to 50°C/sec Measure and study the Ac ₃ points. As a result, the following formula can be obtained as the Ac ₃ points: Ac ₃ =910-200√C+44Si-25Mn+44Al; this formula can be used to calculate the Ac ₃ points. The symbol of the element in the above formula is substituted into the content (mass %) of the corresponding element.

After the first annealing, the above-mentioned maintenance temperature above 740°C and below 800°C is preferably cooled to a temperature range below 500°C at an average cooling rate of 2°C/sec or more. By setting the average cooling rate at 2°C/sec or more and cooling at or above the critical cooling rate, the entire steel material after cooling can be the structure of the main body of Ma Tian scattered iron. On the other hand, even if the water quench cooling method or the spray cooling method is used, it is difficult to control the average cooling rate to 2000°C/sec or more, so the substantial upper limit of the average cooling rate is 2000°C/sec. In addition, by setting the cooling stop temperature to 500° C. or lower and cooling to the temperature below the temperature at which the malada scattered iron metamorphosis starts, the entire steel material after cooling can be made into the structure of the main body of the material. After the above cooling, the steel plate is preferably cooled to room temperature. Before the second annealing after the above cooling, the tempering treatment may be performed in a temperature range of 100° C. or more and 500° C. or less for 10 seconds to 1000 seconds.

The second annealing temperature after the first annealing is 600° C. or higher and lower than the Ac ₃ point. By setting the second annealing temperature at 600° C. or higher and lower than the Ac ₃ point, the tempered hemp scattered iron can be controlled to a desired area ratio, and tensile strength and local ductility can be improved. From the viewpoint of dissolving Xueming ferrocarbon to stably ensure good toughness, the time to maintain the temperature range above 600°C and below the Ac ₃ point is preferably set to 5 seconds or more. In addition, from the viewpoint of productivity, the time maintained in the temperature range of 600° C. or higher and lower than the Ac ₃ point is preferably 300 seconds or less.

After the second annealing, the above-mentioned maintained temperature below 600°C and below Ac ₃ point is cooled to a temperature range below 300°C at an average cooling rate of 10°C/sec or more. Next, the temperature range of 200°C or higher and 450°C or lower is maintained for 30 seconds or longer.

By cooling to a temperature range below 300°C at an average cooling rate of 10°C/sec or more, the coarsening of the residual austenitic iron structure can be suppressed. The interface distribution of the mother phase of Huomatian scattered iron can be made into the desired interface density. That is, by cooling to a temperature range below 300°C at an average cooling rate of 10°C/sec or more, the interface distribution generated in the first annealing can be maintained and the interface density can be avoided, which helps to improve local Ductility. In addition, as described above, it is difficult to control the average cooling rate to 2000°C/sec or more. Therefore, during cooling after the second annealing, the substantial upper limit of the average cooling rate is also 2000°C/sec. Next, by maintaining the temperature range of 200°C or more and 450°C or less for 30 seconds or more, the supersaturated carbon in the fresh Ma Tian scattered iron can be diffused into the residual Vostian iron, and the formation of the residual Vostian iron can be promoted. The high density promotes the diffusion of C, and the effect of making the C inside the remaining Vostian iron uniform becomes uniform, resulting in improved local ductility.

After cooling for 30 seconds or longer in a temperature range of 200°C or higher and 450°C or lower, the steel plate can be directly cooled to room temperature without plating the steel plate. In addition, when plating a steel plate, it is manufactured as follows.

When the hot-dip galvanized steel sheet is manufactured on the surface of the steel sheet by hot-dip galvanizing, the steel sheet that has been maintained in the temperature range of above 200°C and below 450°C for more than 30 seconds is heated again to the temperature range of 430-500°C, and then The steel sheet is immersed in a molten zinc plating bath and subjected to molten galvanizing treatment. The conditions of the plating bath may be set within a general range. After the plating process, it can be cooled to room temperature.

When alloying hot-dip galvanizing is performed on the surface of the steel sheet to produce alloyed hot-dip galvanizing steel sheet, the steel sheet is melted at a temperature of 450 to 620°C before the steel sheet is subjected to hot-dip galvanizing treatment and before the steel sheet is cooled to room temperature Galvanized alloying treatment. The alloying treatment conditions may be set within a general range.

By manufacturing the steel sheet as described above, the steel sheet of this embodiment can be produced. Examples

The steel plate of the present disclosure will be described more specifically with reference to examples. However, the following example is an example of the disclosed steel plate, and the disclosed steel plate is not limited to the following examples.

1. Manufacturing evaluation steel plate Steel with the chemical composition shown in Table 1 was smelted in a converter, and a steel blank with a thickness of 245 mm was produced by continuous casting.

表1中向下的箭頭意指與其上一欄相同。[Table 1]

The downward arrow in Table 1 means the same as its previous column.

The obtained steel blank was hot rolled under the conditions shown in Table 2 to produce a hot rolled steel plate with a thickness of 2.6 mm. Next, the obtained hot-rolled steel sheet was pickled and cold-rolled at the cold-rolling rate shown in Table 2 to produce cold-rolled steel plates of various thicknesses shown in Table 2.

[Table 2]

The obtained cold-rolled steel sheet was subjected to heat treatment under the conditions shown in Table 3 to produce an annealed cold-rolled steel sheet. The heat treatment of the cold-rolled steel sheet is carried out in a reducing gas environment of 98% nitrogen and 2% hydrogen.

[table 3]

For some examples of annealed cold-rolled steel sheets, after final annealing, cooling was stopped at 460°C, and the cold-rolled steel sheets were immersed in a molten zinc plating bath at 460°C for 2 seconds to perform hot-dip galvanizing treatment. The conditions of the plating bath are the same as before. When the alloying treatment described below is not performed, after maintaining at 460°C, it is cooled to room temperature at an average cooling rate of 10°C/sec.

For some examples of annealed cold-rolled steel sheets, after performing hot-dip galvanizing treatment, alloying treatment is continuously performed without cooling to room temperature. It was heated to 520°C and maintained at 520°C for 5 seconds for alloying treatment, and then cooled to room temperature at an average cooling rate of 10°C/sec.

The annealed cold-rolled steel sheet prepared as described above was temper rolled at an elongation of 0.1%, and various steel sheets for evaluation were prepared.

2. Evaluation method For the annealed cold-rolled steel plates produced in the examples, the following items were evaluated: area ratio of tempered hemp iron, residual Vostian iron, fresh hemp Mafic iron and ferrite iron, residual Vostian iron and Xinma Hetian iron The interface density, tensile strength, total elongation and local elongation of the mother phase with tempered Ma Tian scattered iron as the main body. The evaluation methods are as follows.

The area ratio of tempered Ma Tian scattered iron and fertilized iron is calculated based on the observation of the structure using a scanning electron microscope. The area ratios of the residual Vostian iron and the fresh Matian scattered iron are calculated based on the structure observation using a scanning electron microscope and X-ray diffraction measurement. For the L section obtained by cutting the steel plate parallel to the rolling direction, mirror polishing is performed, and then the microstructure is exposed with 3% nitrate etchant, and then observed with a scanning electron microscope at a magnification of 5000 times from the surface 1 /4 The microstructure at the position of 0.1mm×0.3mm is calculated by image analysis (Photoshоp (registered trademark)) to calculate the area ratio of tempered hemp scattered iron, the area ratio of fertilized iron and the residual Vostian iron and The total area ratio of Xinsheng Matian Powder. In addition, a test piece having a width of 25 mm and a length of 25 mm was cut from the obtained steel plate, and the test piece was chemically polished to reduce the thickness of 1/4 of the thickness of the plate, and the surface of the chemically polished test piece was applied three times with a Co tube X-ray diffraction analysis of the ball, and analysis of the obtained curve, respectively averaged and calculated the area ratio of the residual Vostian iron. The area ratio of the residual Vostian iron calculated by observation with a scanning electron microscope and the total area ratio of the residual Vostian iron calculated by X-ray diffraction measurement are subtracted from the total area ratio of the residual Vostian iron calculated by X-ray diffraction measurement. The area ratio of iron.

The interfacial density of the residual Vostian iron and fresh Ma Tian scattered iron relative to the mother phase with tempered Ma Tian scattered iron as the main body was measured using image analysis software ImageJ. First, the SEM image (24 μm×18 μm) was obtained by observing the target tissue using a SEM at a magnification of 5000 times. Next, 1280×960 divided regions were formed on the SEM image using ImageJ. For each divided area, a binarization process is performed so that the area of any of the remaining Vostian iron or Shinsengmatian scattered iron is black, and the other areas are white, to produce a second-order image. The threshold value of binarization is determined by the following method: using the brightness described in "Glasbey, CA (1993), "An analysis of histogram-based thresholding algorithms", CVGIP: Graphical Models and Image Processing 55: 532-537" The average value is used as the limit value. This algorithm is installed in ImageJ, and by using the Auto threshold function and setting the method of determining the limit value as Method=Mean, it is automatically binarized. That is, the limit value of the binarization is set by ImageJ Method=Mean, radius=15, centering on the pixel of interest, replacing each pixel value with the average of the pixel values within a radius of 15 pixels, after smoothing The histogram is automatically determined. The interface density of the residual Vostian iron and fresh Ma Tian scattered iron relative to the tempered Ma Tian scattered iron was measured by the following: In the obtained 2nd-degree image, sequence analysis of all isolated residual Vostian iron and The length of the interface between the fresh-born Ma Tian scattered iron and the tempered Ma Tian scattered iron, and the total value is divided by the area of the entire image area (24 μm×18 μm). However, when the residual Vostian iron is adjacent to the fresh Ma Tian scattered iron, the two will become an integrated structure during the above-mentioned binarization process. Therefore, the interface length between the residual Vostian iron and the fresh Ma Tian scattered iron will be eliminated. The so-called sequence analysis refers to measuring the interfacial density without repeating the interface for the residual Vostian iron and the new Matian scattered iron.

(Mechanical properties) After taking a JIS No. 5 tensile test piece at a right angle to the rolling direction of the steel sheet, the tensile strength (TS), total elongation (EL), and local elongation (LEL) were measured. The tensile test was performed using the JIS No. 5 tensile test piece, and the method was specified in JIS Z2241:2011. The measurement of the total elongation is carried out using a JIS No. 5 test piece in accordance with the method specified in JIS Z2241:2011. The measurement of the local elongation is calculated by subtracting the value of the elongation (uniform elongation) of the maximum load point from the value of the total elongation when the broken test piece is opposed.

3. Evaluation results The above evaluation results are shown in Table 4. A steel sheet showing an interface density of 2.7 μm ^-1 or more, a tensile strength of 1200 MPa or more, and a local elongation of 2.0% or more was evaluated as a steel sheet having excellent local ductility and high strength.

[Table 4]

FIG. 1 shows an SEM image obtained by observing the L section of the steel plate of sample No. 31 after performing the above-mentioned mirror polishing and nitrate etchant treatment. FIG. 2 shows a second-order image obtained by subjecting the SEM image of FIG. 1 to binarization. FIG. 3 shows an SEM image obtained by observing the L-section of the steel plate of Sample No. 4 after the above-mentioned mirror polishing and nitrate etchant treatment. The interface density measured from the second-order image of Fig. 2 was 3.35 μm ^-1 . The interface density measured from the second-order image obtained by binarizing the SEM image of FIG. 3 is 1.50 μm ^-1 .

FIG. 1 is a scanning electron microscope (SEM) image obtained by observing the L section of the steel plate prepared in the examples after mirror polishing and nitrate etchant treatment. FIG. 2 is a second-order image obtained by binarizing the SEM image of FIG. 1. FIG. 3 is a SEM image obtained by observing the L-section of the steel plate prepared in the comparative example after mirror polishing and nitrate etchant treatment.

Claims (8)

A steel plate characterized in that it contains in mass %: C: more than 0.20% and less than 0.55%, Si: 0.001% or more and less than 3.50%, Mn: more than 4.00% and less than 9.00%, sol.Al: 0.001% Above and less than 3.00%, P: 0.100% or less, S: 0.010% or less, N: less than 0.050%, O: less than 0.020%, B: 0.0000% or more and less than 0.010%, Cr: 0.00% or more and less than 2.00%, Mo: 0.00% or more and 2.00% or less, W: 0.00% or more and 2.00% or less, Cu: 0.00% or more and 2.00% or less, Ni: 0.00% or more and 2.00% or less, Ti: 0.000% or more and 0.300% or less, Nb: 0.000% or more and 0.300% or less, V: 0.000% or more and 0.300% or less, Ca: 0.000% or more and 0.010% or less, Mg: 0.000% or more and 0.010% or less, Zr: 0.000% or more and 0.010% or less, REM: 0.000% or more and 0.010% or less, Sb: 0.000% or more and 0.050% or less, Sn: 0.000% or more and 0.050% or less, and Bi: 0.000% or more and 0.050% or less, and the remainder is iron and impurities; the steel plate In the L section, the metal structure at 1/4th of the thickness from the surface contains 25% or more and 90% or less of tempered hemp bulk iron, 3% or less of fertile iron, 10% or more and 50% in area ratio The following residual Wustfield iron and 25% or less of the new Matian scattered iron; and the value obtained by dividing the sum of the boundary lengths of the first area and the second area by the total area of the first area and the second area, That is, the interface density is 2.7 μm ^-1 or more, the first region is a region in which any one of the Vostian iron or the newly-born Matian scattered iron remains in the metal structure at the 1/4 position, and the second region is from the aforementioned 1 The metal structure at the position /4 excludes the area other than the first area. As for the steel plate of claim 1, it contains B by mass %: 0.0003% or more and less than 0.010%. If the steel plate of claim 1 or 2, it contains one or more of the following elements: In terms of mass %, Cr: 0.01% or more and less than 2.00%, Mo: 0.01% or more and 2.00% or less, W: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less and Ni: 0.01% or more and 2.00% or less. The steel sheet according to any one of claims 1 to 3 contains one or more of the following elements: In terms of mass %, Ti: 0.005% or more and 0.300% or less, Nb: above 0.005% and below 0.300% and V: 0.005% or more and 0.300% or less. The steel sheet according to any one of claims 1 to 4 contains one or more of the following elements: In terms of mass %, Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, Zr: 0.0001% or more and 0.0100% or less and REM: 0.0001% or more and 0.0100% or less. The steel sheet according to any one of claims 1 to 5 contains one or more of the following elements: In terms of mass %, Sb: 0.0005% or more and 0.0500% or less, Sn: 0.0005% or more and 0.0500% or less and Bi: 0.0005% or more and 0.0500% or less. As in the steel sheet according to any one of claims 1 to 6, the surface of the foregoing steel sheet has a hot-dip galvanized layer. As in the steel sheet according to any one of claims 1 to 6, the surface of the foregoing steel sheet has an alloyed galvanized layer.

Images