JPWO2020138343A1 - Steel plate - Google Patents

Abstract

In the present invention, in mass%, C: more than 0.10 to 0.45%, Si: 0.001 to 2.50%, Mn: more than 4.00 to 8.00%, sol. Al: 0.001 to 1.50%, the metal structure at the position 1/4 of the thickness from the surface is the area%, tempered martensite: 25 to 90%, and retained austenite: 10 to 50%. The present invention relates to a steel sheet having a standard deviation of Mn concentration of 0.30% by mass or more in the range of 20 μm in the rolling direction and 20 μm in the plate thickness method at a position 1/4 of the thickness from the surface.

Description

The present disclosure relates to a steel sheet having excellent formability, and specifically to a steel sheet having a high work hardening property and high strength and a high Mn concentration.

In order to reduce the weight of automobile bodies and parts, it is required to reduce the thickness of steel plates, which are these materials, and the strength of steel plates is being increased accordingly. In general, when the strength of a steel sheet is increased, the elongation characteristics are lowered, the work hardening characteristics of the steel sheet are impaired, and the formability is lowered. Therefore, in order to use a high-strength steel sheet as a member for an automobile, it is necessary to enhance both strength and formability (particularly work hardening characteristics), which are contradictory characteristics.

In order to improve the elongation characteristics, so-called TRIP (Transformation Induced Plasticity) steel utilizing the transformation-induced plasticity of retained austenite (residual γ) has been proposed (for example, Patent Document 1).

Retained austenite is obtained by concentrating C in austenite to prevent austenite from transforming into other tissues at room temperature. As a technique for stabilizing austenite, it has been proposed that a carbide precipitation inhibitoring element such as Si and Al is contained in the steel sheet to concentrate C in the austenite during the bainite transformation occurring in the steel sheet at the steel sheet manufacturing stage. There is. In this technique, if the C content contained in the steel sheet is high, the austenite can be further stabilized and the amount of retained austenite can be increased, and as a result, a steel sheet having excellent strength and elongation characteristics can be produced. However, when a steel sheet is used for a structural member of an automobile or the like, welding is often performed on the steel sheet, but if the C content in the steel sheet is large, the workability of welding is lowered. Therefore, it is desired to improve both the elongation characteristics and strength of the steel sheet, that is, the work hardening characteristics and strength of the steel sheet with a smaller C content.

Further, as a steel sheet having a larger amount of retained austenite than the above-mentioned TRIP steel and having a ductility exceeding the above-mentioned TRIP steel, a steel to which Mn of more than 4.0% is added has been proposed (for example, Non-Patent Document 1). Since the steel contains a large amount of Mn, the weight reduction effect on the members used is also remarkable. However, the steel requires a long heating process such as box annealing. Therefore, the material design in a short-time heating process such as continuous annealing suitable for the production of high-strength steel sheets used for automobile members has not been sufficiently studied, and the requirement for enhancing the elongation characteristics in that case has not been clarified.

Further, the elongation characteristics are obtained by cold-rolling the steel to which Mn of more than 4.0% is added, heating it for a short time of 300 seconds to 1200 seconds, and controlling the ferrite to 30% to 80% in area%. (For example, Patent Document 2) discloses a steel sheet in which is significantly improved. However, such a steel sheet has a high Mn concentration and contains a large amount of unrecrystallized ferrite, so that the work hardening characteristics are inferior. That is, a steel sheet having such a structure containing ferrite and having a high Mn concentration cannot have both the strength (for example, tensile strength) required for a steel sheet for automobiles and work hardening characteristics.

In connection with this, a method for producing a steel sheet and a plated steel sheet including a step of performing various heat treatments on a steel sheet containing a relatively large amount of Mn in order to obtain desired properties suitable for use as an automobile member has been proposed. (For example, Patent Documents 3 to 5). Further, a steel sheet having a high Mn concentration of more than 4.00% and less than 9.00%, having excellent uniform elongation characteristics and high strength has been proposed (Patent Document 6).

Japanese Unexamined Patent Publication No. 5-59429 Japanese Unexamined Patent Publication No. 2012-237054 Japanese Unexamined Patent Publication No. 2018-21233 Japanese Unexamined Patent Publication No. 2017-53001 JP-A-2007-70660 International Publication No. 2018/131722

Takashi Furukawa, Osamu Matsumura, Heat Treatment, Japan, Japan Heat Treatment Association, 1997, Vol. 37, No. 4, p. 204

Therefore, a steel sheet having excellent work hardening characteristics and high strength and a high Mn concentration is desired.

In order to ensure excellent work hardening characteristics and high strength in a steel sheet having a high Mn concentration, the present inventors control the chemical composition and add 25 to 25 tempered martensite in the steel sheet in an area%. 90% and 10 to 50% of retained austenite are contained, and the rolling direction is 20 μm and the plate thickness method is 20 μm at a position 1/4 of the thickness from the surface of the steel sheet so that the Mn distribution in the steel sheet becomes extremely non-uniform. It was found that it is effective to set the standard deviation of the Mn concentration in the range of 0.30% by mass or more.

The steel sheet of the present disclosure was made based on the above findings, and the gist thereof is as follows.
(1)
The chemical composition is mass%,
C: Over 0.10 to 0.45%,
Si: 0.001-2.50%,
Mn: Over 4.00 to 8.00%,
sol. Al: 0.001 to 1.50%,
P: 0.100% or less,
S: 0.010% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0-2.00%,
W: 0-2.00%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
Zr: 0-0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and the balance: iron and impurities.
The metallographic structure at 1/4 of the thickness from the surface contains tempered martensite: 25-90% and retained austenite: 10-50% in area%.
A steel sheet having a standard deviation of Mn concentration of 0.30% by mass or more in a rolling direction of 20 μm and a plate thickness method of 20 μm at a position 1/4 of the thickness from the surface.
(2)
When the chemical composition is mass%,
Cr: 0.01-0.50%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%,
V: 0.005 to 0.300%, and B: 0.0001 to 0.010%
The steel sheet according to (1), which contains one or more selected from the group consisting of.
(3)
The steel sheet according to (1) or (2), which has a hot-dip galvanized layer on the surface of the steel sheet.
(4)
The steel sheet according to (1) or (2), which has an alloyed hot-dip galvanized layer on the surface of the steel sheet.

According to the present disclosure, it is possible to provide a steel sheet having excellent work hardening characteristics and high strength and having a high Mn concentration.

FIG. 1 is a graph showing a plot of the standard deviation of the Mn concentration with respect to the holding temperature before hot rolling. FIG. 2 is a graph showing a plot of work hardening characteristics (n value) with respect to the holding temperature before hot rolling.

Hereinafter, an example of the embodiment of the steel sheet of the present disclosure will be described.

It is generally known that Mn is microsegregated in a metallographic structure. More specifically, Mn tends to segregate parallel to the plate thickness direction during melting, and as a result, this segregated portion has a band-like structure (Mn band) parallel to the rolled surface after being rolled. May become. The band-shaped structure brings about remarkable anisotropy in the mechanical properties of the obtained steel sheet, and is not preferable from the viewpoint of bending properties and hole expandability. Therefore, in order to achieve uniform mechanical properties of the steel sheet by homogenizing the metal structure of the steel sheet, it is a usual technical idea to suppress microsegregation of Mn in the metal structure as much as possible. As an effective means for effectively suppressing the microsegregation of Mn, setting a high slab heating temperature (holding temperature before hot rolling) can be mentioned. Further, since the above-mentioned microsegregation of Mn occurs more prominently as the Mn content is higher, the slab heating temperature when manufacturing a steel sheet having a high Mn content is set higher than usual in order to make the metal structure uniform. There is a need to. Therefore, when manufacturing a steel sheet containing a high concentration Mn of more than 4.00% by mass, such as the steel sheet of the present disclosure, the slab heating temperature is set high (for example, at 1200 ° C. or higher). For example, all the steel sheets specifically disclosed in Patent Document 6 are subjected to slab heating (holding before hot rolling) at 1250 ° C.

The present inventors have conducted various studies on a steel sheet having a high Mn content in order to improve work hardening characteristics (n value) while maintaining high strength. As a result, the above-mentioned conventional technical idea It was found that it is important to control the concentration distribution of Mn contained in the steel sheet non-uniformly by setting the slab heating temperature (holding temperature before hot rolling) low. More specifically, the present inventors performed hot rolling, cold rolling, annealing, cooling, and final annealing after slab heating at a low temperature. Cementite is formed in the metal structure by the final annealing, and Mn is distributed to this cementite. Then, the Mn-distributed cementite dissolves to produce austenite. By promoting the distribution of Mn to the austenite, the Mn concentration distribution in the steel sheet becomes non-uniform, and the Mn distribution is microseed. In this way, the present inventors have found that stable austenite is produced and work hardening characteristics are improved. Further, in order for the steel sheet of the present disclosure containing Mn of more than 4.00% by mass to sufficiently promote microsegregation of Mn, the present inventors typically have 1200 at such a high Mn content. It has been found that it is important that the slab heating temperature (holding temperature before hot rolling) set to ℃ or higher is less than 1100 ℃. As described above, according to the present invention, in a steel sheet having a high Mn content, unlike the conventional technical idea, the slab heating temperature is set to less than 1100 ° C. in order to promote Mn segregation, and further, a predetermined heat is obtained. By giving a history, it becomes possible to obtain a steel sheet having excellent work hardening characteristics and high strength as compared with the prior art and having a high Mn concentration.

1. 1. Chemical Composition The reason why the chemical composition of the steel sheet of the present disclosure is defined as described above will be described. In the following description, "%" representing the content of each element means mass% unless otherwise specified. In the chemical composition of steel sheets, the numerical range represented by "~" uses the numerical values before and after "~" as the lower and upper limits unless "super" or "less than" is used. Means the range to be included.

(C: Over 0.10 to 0.45%)
C is an extremely important element for increasing the strength of steel and ensuring retained austenite. In order to obtain a sufficient amount of retained austenite, a C content of more than 0.10% is required. On the other hand, if C is excessively contained, it becomes difficult to weld the steel sheet, so the upper limit of the C content is set to 0.45%.

The lower limit of the C content is preferably 0.15%, more preferably 0.20%. By setting the C content to 0.15% or more and further controlling the area ratio of tempered martensite described later to 30 to 87%, the tensile strength (TS) is said to be 1180 MPa or more without impairing the work hardening characteristics. It becomes possible to obtain a high-strength steel plate. The upper limit of the C content is preferably 0.40%, more preferably 0.35%.

(Si: 0.001-2.50%)
Si is an element effective for strengthening tempered martensite, homogenizing the structure, and improving workability. Si also has the effect of suppressing the precipitation of cementite and promoting the retention of austenite. In order to obtain the above effect, a Si content of 0.001% or more is required. On the other hand, if Si is excessively contained, the plating property and chemical conversion treatment property of the steel sheet are impaired. Therefore, the upper limit of the Si content is set to 2.50%.

The lower limit of the Si content is preferably 0.01%, more preferably 0.30%, still more preferably 0.50%. By setting the lower limit of the Si content to the above range, the residue of austenite can be promoted and the work hardening characteristics of the steel sheet can be further improved. The upper limit of the Si content is preferably 2.10%, more preferably 1.70%.

(Mn: Over 4.00 to 8.00%)
Mn is an element that stabilizes austenite and enhances hardenability. Further, in the steel sheet of the present disclosure, Mn is distributed in austenite to further stabilize austenite. More than 4.00% Mn is required to stabilize austenite at room temperature. On the other hand, if the steel sheet contains Mn excessively, the toughness is impaired, so the upper limit of the Mn content is set to 8.00%.

The lower limit of the Mn content is preferably 4.30%, more preferably 4.80%. The upper limit of the Mn content is preferably 7.50%, more preferably 7.20%. When the Si content is 0.30% or more, controlling the Mn content to a preferable range significantly improves the effect of promoting the residual austenite.

(Sol.Al: 0.001 to 1.50%)
Since Al is an antacid, sol. It is necessary to contain 0.001% or more of Al. In addition, Al also has an effect of improving material stability in order to widen the two-phase temperature range during annealing. The greater the content of Al, the greater the effect. However, if the content of Al is excessive, deterioration of surface properties, paintability, weldability, etc. will occur. The upper limit of Al was set to 1.50%.

sol. The lower limit of the Al content is preferably 0.005%, more preferably 0.01%, and even more preferably 0.02%. sol. The upper limit of the Al content is preferably 1.20%, more preferably 1.00%. sol. By setting the lower limit value and the upper limit value of the Al content in the above range, the balance between the deoxidizing effect and the material stability improving effect and the surface texture, paintability, and weldability becomes better.

(P: 0.100% or less)
P is an impurity, and if the steel sheet contains P in excess, toughness and weldability are impaired. Therefore, the upper limit of the P content is set to 0.100%. The upper limit of the P content is preferably 0.050%, more preferably 0.030%, and even more preferably 0.020%. Since the steel sheet according to this embodiment does not require P, the lower limit of the P content is 0%. The P content may be more than 0% or 0.001% or more, but the smaller the P content, the more preferable.

(S: 0.010% or less)
S is an impurity, and if the steel sheet contains S in excess, MnS stretched by hot rolling is generated, which causes deterioration of moldability such as bendability and hole expandability. Therefore, the upper limit of the S content is set to 0.010%. The upper limit of the S content is preferably 0.007%, more preferably 0.003%. Since the steel sheet according to this embodiment does not require S, the lower limit of the S content is 0%. The S content may be more than 0% or 0.001% or more, but the smaller the S content is, the more preferable.

(N: less than 0.050%)
N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness deteriorates. Therefore, the N content is set to less than 0.050%. The upper limit of the N content is preferably 0.010%, more preferably 0.006%. Since the steel sheet according to this embodiment does not require N, the lower limit of the N content is 0%. The N content may be more than 0% or 0.003% or more, but the smaller the N content, the more preferable.

(O: less than 0.020%)
O is an impurity, and if the steel sheet contains 0.020% or more of O, ductility deteriorates. Therefore, the O content is set to less than 0.020%. The upper limit of the O content is preferably 0.010%, more preferably 0.005%, and even more preferably 0.003%. Since the steel sheet according to this embodiment does not require O, the lower limit of the O content is 0%. The O content may be more than 0% or 0.001% or more, but the smaller the O content, the more preferable.

The steel sheet of the present embodiment is further selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, or one of them. Two or more kinds may be contained. However, since the steel plate according to this embodiment does not necessarily require Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, Cr, Mo , W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and Bi may not be contained, that is, the lower limit of the content may be 0%. ..

(Cr: 0 to 0.50%)
(Mo: 0-2.00%)
(W: 0-2.00%)
(Cu: 0-2.00%)
(Ni: 0-2.00%)
Cr, Mo, W, Cu, and Ni are not essential elements for the steel sheet according to this embodiment, respectively. However, Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet and may be contained. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. , 0.05% or more or 0.10% or more may be contained. However, if the steel sheet contains these elements in excess, surface scratches are likely to occur during hot rolling, and the strength of the hot rolled steel sheet becomes too high, which may reduce the cold rollability. Therefore, among the contents of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni, the upper limit of the Cr content is set to 0.50%, and Mo. The upper limit of the content of each of W, Cu, and Ni is 2.00%. The upper limit of the content of Cr may be 0.40% or 0.30%, and the upper limits of the contents of Mo, W, Cu, and Ni are 1.50% and 1.20%, respectively. Alternatively, it may be 1.00%.

(Ti: 0 to 0.300%)
(Nb: 0 to 0.300%)
(V: 0 to 0.300%)
Ti, Nb, and V are not essential elements for the steel sheet according to this embodiment. However, Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, and are therefore effective in improving 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, it is preferable that the lower limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is 0.005%. It is more preferably 0.010%, and even more preferably 0.030%. On the other hand, if these elements are excessively contained, the strength of the hot-rolled steel sheet may increase too much and the cold rollability may decrease. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300%, preferably 0.250%, and more preferably 0.200. %, More preferably 0.150%.

(B: 0 to 0.010%)
(Ca: 0 to 0.010%)
(Mg: 0 to 0.010%)
(Zr: 0 to 0.010%)
(REM: 0 to 0.010%)
B, Ca, Mg, Zr, and REM are not essential elements for the steel sheet of the present disclosure. However, B, Ca, Mg, Zr, and REM improve the hole expandability of the steel sheet. In order to obtain this effect, the lower limit of each of one or more elements selected from the group consisting of B, Ca, Mg, Zr, and REM is preferably 0.0001%, more preferably 0. It is set to 001%. However, since an excess amount of these elements deteriorates the workability of the steel sheet, the upper limit of the content of each of these elements is set to 0.010%, preferably 0.005%, and B, Ca, Mg, Zr, and REM. The total content of one or more elements selected from the group consisting of is preferably 0.030% or less, preferably 0.020% or less. In addition, in this specification, REM means one kind or two or more kinds of elements selected from the elements contained in Sc, Y, Te, Se, Ag and lanthanoid.

(Sb: 0 to 0.050%)
(Sn: 0 to 0.050%)
(Bi: 0 to 0.050%)
Sb, Sn, and Bi are not essential elements for the steel sheet of the present disclosure. However, Sb, Sn, and Bi suppress that easily oxidizing elements such as Mn, Si, and / or Al in the steel sheet are diffused on the surface of the steel sheet to form an oxide, and improve the surface texture and plating property of the steel sheet. Increase. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005%, more preferably 0.001%. And. On the other hand, if the content of each of these elements exceeds 0.050%, the effect is saturated. Therefore, the upper limit of the content of each of these elements is set to 0.050%, preferably 0.040%.

The steel sheet of the present disclosure has, for example, Cr: 0.01 to 0.50%, Ti: 0.005 to 0.300%, Nb: 0.005 to 0.300%, among the optional elements described above. It may contain one or more selected from the group consisting of V: 0.005 to 0.300% and B: 0.0001 to 0.010%.

In the steel sheet of the present embodiment, the balance other than the above elements consists of iron and impurities. Here, the "impurity" means an element that is mixed due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the steel sheet is industrially manufactured. Such impurities are not limited to those that are not intentionally added.

2. 2. Metal structure Next, the metal structure of the steel sheet according to the present embodiment will be described.

The metal structure in the L cross section at the 1/4 position (also referred to as 1 / 4t portion) of the thickness from the surface of the steel sheet according to the present embodiment is 25 to 90% tempered martensite and 10 to 50% in area%. Includes retained austenite. Here, the L cross section means a surface obtained by cutting a steel sheet parallel to the rolling direction and perpendicular to the surface of the steel sheet. The L cross section in the present embodiment is a surface cut so as to pass through the center in the width direction of the steel sheet.

In the steel sheet of the present disclosure, the residual structure of the metal structure other than the tempered martensite and retained austenite described above is not particularly limited. Residual structures include, for example, ferrite, bainite, fresh martensite, tempered bainite and the like.

The surface integral of each metal structure changes depending on the annealing conditions, and affects the material such as strength, work hardening characteristics, and hole expandability. Since the required material varies depending on, for example, automobile parts, the annealing conditions may be selected as necessary and the tissue fraction may be controlled within the above range.

The metallographic structure of the steel sheet is measured as follows. After mirror polishing the L cross section of the steel sheet, the polished surface is corroded with 3% nital (3% nitric acid-ethanol solution), and the microstructure at 1/4 of the thickness is observed from the surface of the steel sheet with a scanning electron microscope. To do. Then, by analyzing the observed image, the area% of each structure of tempered martensite, ferrite, retained austenite, bainite, and fresh martensite is measured. For retained austenite and fresh martensite, first, using a scanning electron microscope, 0.1 mm in length (length in the plate thickness direction) x 0.3 mm in width (length in the rolling direction) at a position 1/4 of the thickness from the surface. ), The microstructure image in the range of) is acquired at a magnification of 5000 times, and the acquired tissue image is analyzed to measure the total area% of retained austenite and fresh martensite, and further X at the plate thickness 1/4 position. The area% of retained austenite is measured by the linear diffraction method. Specifically, MoKα rays are used as the incident X-rays, and the integral intensities of the peaks of the {111}, {200}, {220}, and {311} planes of retained austenite are ferrite {110} and {200. }, {211} The volume ratio of retained austenite is obtained from the intensity ratio of all 12 combinations to the integrated intensity of the peaks, the volume ratio is regarded as the same as the area ratio, and the average value of these is regarded as the area of retained austenite. Let it be a rate. Further, the area% of fresh martensite is calculated by subtracting the area% of retained austenite from the total area% of retained austenite and fresh martensite. Further, the ferrite phase is discriminated as a gray underlying structure, and the austenite phase and the martensite phase are discriminated as a white structure. The tempered martensite phase looks white like the fresh martensite phase, but the one in which the substructure is confirmed in the crystal grains is discriminated as the tempered martensite phase. To distinguish between martensite and bainite, the above-mentioned microstructure image (magnification of 5000 times) is observed using a scanning electron microscope, and cementite present at the interface of the lath or inside the lath is discriminated as bainite.

(Area% of tempered martensite in the metal structure of 1 / 4t part of the steel sheet: 25-90%)
Tempering martensite is a structure that increases the strength and ductility of steel sheets. Within the range of the desired strength level, the area ratio of tempered martensite is set to 25-90% in order to maintain favorable both strength and ductility. The lower limit of the area ratio of tempered martensite is preferably 30%, more preferably 35%, and even more preferably 40%. The upper limit of the area ratio of tempered martensite is preferably 87%, more preferably 80%. As described above, by setting the C content to 0.15% or more and further controlling the area ratio of tempered martensite to 30 to 87% as described above, the work hardening characteristics are not impaired. It is possible to obtain a high-strength steel sheet having a tensile strength (TS) of 1180 MPa or more.

(Area% of retained austenite in the metal structure of 1 / 4t part of the steel sheet: 10 to 50%)
In the steel sheet according to the present embodiment, it is important that the amount of retained austenite in the metal structure is within a predetermined range. Residual austenite is a structure that enhances the ductility and formability of steel sheets, especially the work hardening characteristics of steel sheets, by transformation-induced plasticity. Residual austenite can be transformed into martensite by overhanging, drawing, stretch flangeing, or bending accompanied by tensile deformation, which also contributes to the improvement of the strength of the steel sheet. In order to obtain these effects, the steel sheet according to the present embodiment needs to contain retained austenite in an area ratio of 10% or more in the metal structure. The lower limit of the area ratio of retained austenite is preferably 15%, more preferably 20%.

The higher the area ratio of retained austenite in the metal structure of the 1 / 4t portion of the steel sheet, the more preferable. However, in the steel sheet having the above-mentioned chemical composition, 50% in area ratio is the upper limit of the content of retained austenite. If Mn of more than 8.0% is contained, the retained austenite can be made more than 50% in area ratio, but in this case, casting of a steel sheet becomes difficult. From the viewpoint of improving toughness, the area ratio of retained austenite is preferably 40% or less.

In the steel sheet according to the present embodiment, it is preferable that the amount of ferrite in the metal structure is small. Toughness can be improved by reducing the ferrite content in the metallographic structure. In order to improve the toughness, it is preferable that the area ratio of ferrite in the metal structure is 3% or less. The area ratio of the ferrite is more preferably 1% or less, still more preferably 0%. Therefore, in the steel sheet according to the present embodiment, for example, the area ratio of ferrite may be 0 to 3%, 0 to 2%, or 0 to 1%.

In the steel sheet according to the present embodiment, when bainite is present in the metal structure, island-shaped martensite, which is a hard structure, is inherent in the bainite. Intrinsic island-like martensite during bainite reduces toughness. In order to improve the toughness, the area ratio of bainite in the metal structure is preferably 5% or less, more preferably 3% or less. The area ratio of bainite is more preferably 1% or less, still more preferably 0%. Therefore, in the steel sheet according to the present embodiment, for example, the area ratio of bainite may be 0 to 5%, 0 to 3%, or 0 to 1%.

In the steel sheet according to the present embodiment, it is preferable that the amount of fresh martensite in the metal structure is small. Fresh martensite is martensite that has not been tempered. Fresh martensite has a hard structure and is effective in ensuring the strength of steel sheets. However, the smaller the content of fresh martensite, the higher the hole expandability of the steel sheet. Therefore, the area ratio of fresh martensite may be 0%, but from the viewpoint of increasing the strength of the steel sheet while maintaining the hole-expandability, the metal structure of the steel sheet is preferably 1% or more in area ratio. It contains 2% or more, more preferably 3% or more of fresh martensite. The upper limit of the content of fresh martensite is preferably 65%, more preferably 55%, still more preferably 45%, and most preferably 20% in terms of area ratio from the viewpoint of ensuring hole expandability. Therefore, in the steel sheet according to the present embodiment, for example, the area ratio of fresh martensite is 0 to 65%, 0 to 20%, 1 to 65%, 1 to 20%, 2 to 65%, 2 to 20%, 3 It may be ~ 65%, or 3-20%.

Remaining structures other than tempered martensite, ferrite, retained austenite, bainite, and fresh martensite may include tempered bainite. The area ratio of tempered bainite can be obtained from the observation image obtained by the scanning electron microscope in the same manner as the measurement of the area ratio of tempered martensite, ferrite, retained austenite, bainite, and fresh martensite. The area ratio of tempered bainite in the steel sheet is preferably small, for example, 10% or less, 7% or less, or 5% or less. Since the tempered bainite may not be contained in the steel sheet according to the present embodiment, the lower limit of the area ratio of the tempered bainite may be 0%. Therefore, in the steel sheet according to the present embodiment, for example, the area ratio of tempered bainite may be 0 to 10%, 0 to 7%, or 0 to 5%. The determination between tempered bainite and bainite is performed in the same manner as the above-mentioned discrimination between tempered martensite and tempered martensite.

The standard deviation of the Mn concentration at the position of 1/4 of the thickness from the surface of the steel sheet according to the present embodiment is 0.30% by mass or more. After the L cross section of the steel sheet is mirror-polished, the standard deviation of the Mn concentration is measured by measuring the position of 1/4 of the thickness from the surface of the steel sheet with an electron probe microanalyzer (EPMA). The measurement conditions are that the acceleration voltage is 15 kV, the magnification is 5000 times, and the distribution image in the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction is measured. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration at 40401 points is measured. Next, the standard deviation of the Mn concentration at the 1/4 position of the thickness from the surface of the steel sheet is calculated based on the Mn concentration obtained from all the measurement points. In the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction, not only the metal structure of a specific phase but also the metal structures of a plurality of phases exist. Therefore, the standard deviation of the Mn concentration in the steel sheet of the present disclosure is measured in a region where such a plurality of metal structures coexist.

(Standard deviation of Mn concentration at 1/4 of the thickness from the surface of the steel sheet: 0.30% by mass or more)
When the standard deviation of the Mn concentration is large, stable austenite is generated and the work hardening characteristics are improved. In order to obtain this effect, the steel sheet according to the present embodiment needs to be controlled to have a standard deviation of Mn concentration of 0.30% by mass or more. The lower limit of the standard deviation of the Mn concentration is preferably 0.35% by mass. The standard deviation of the Mn concentration is an index showing how much Mn is segregated and present in the steel sheet when the steel sheet is observed microscopically. Therefore, by setting the standard deviation of the Mn concentration to 0.30% by mass or more as in the present invention, the distribution of Mn in the steel sheet can be microscopically segregated (distributed), and as a result, stable austenite can be obtained. Is generated, and the work hardening characteristics (n value) are improved.

The higher the standard deviation of the Mn concentration, the more preferable. However, in the steel sheet having the above-mentioned chemical composition, 0.45% by mass is the upper limit of the standard deviation of the Mn concentration.

Next, the mechanical properties of the steel sheet according to this embodiment will be described.

The tensile strength (TS) of the steel sheet according to this embodiment is preferably 780 MPa or more, more preferably 1000 MPa or more, still more preferably 1180 MPa or more. The higher the TS of the steel sheet, the more the thickness can be reduced and the weight can be reduced by increasing the strength when the steel sheet is used as a material for automobiles. The upper limit of TS of the steel sheet according to the present embodiment is not particularly limited, but is, for example, 2500 MPa or 2000 MPa. The tensile test is carried out by the method specified in JIS-Z2241: 2011 using a JIS No. 5 tensile test piece, and the crosshead test speed of the tensile test is 30 mm / min.

Further, in order to use the steel sheet according to the present embodiment for press molding, it is desirable that the work hardening characteristics are excellent. In that case, the n value is preferably 0.10 or more, more preferably 0.15 or more, still more preferably 0.18 or more. The upper limit of the n value is not particularly limited, but is, for example, 0.30, 0.25, or 0.20. In the present specification, the n value means the true strain interval of 4 to 7%, the true stress at the true strains of 4% and 7%, respectively, and the difference between the logarithms of both true stresses is the difference between the logarithms of both true strains. The value divided by. Preferably, when the standard deviation of the Mn concentration is 0.35% by mass or more and the area ratio of the retained austenite is 15% or more, the n value becomes 0.15 or more. Further, more preferably, when the standard deviation of the Mn concentration is 0.35% by mass or more and the area ratio of the retained austenite is 20% or more, the n value becomes 0.18 or more. The uniform elongation test for measuring the n value is performed by the method specified in JIS-Z2241: 2011 using a JIS No. 5 test piece having a parallel portion length of 50 mm, and the crosshead test speed of the uniform elongation test is 30 mm / min. And.

As described above, the steel sheet of the present disclosure has high strength, good work hardening characteristics, and excellent moldability, and is therefore most suitable for use in structural parts of automobiles such as pillars. Further, since the steel sheet of the present disclosure has a high Mn concentration, it also contributes to weight reduction of automobiles, so that the industrial contribution is extremely remarkable. In the steel sheet of the present disclosure, a hot-dip galvanized layer or an alloyed hot-dip galvanized layer can be provided on the surface of the steel sheet depending on the intended use.

3. 3. Manufacturing Method Next, a manufacturing method of the steel sheet according to the present embodiment will be described.

The steel sheet according to the present embodiment is a hot-rolled steel sheet obtained by melting steel having the above-mentioned chemical composition by a conventional method, casting it to produce a slab or an ingot, heating it and hot-rolling it. Is pickled, then cold-rolled and annealed to produce.

Hot rolling may be performed on a normal continuous hot rolling line. In the method for producing a steel sheet according to the present embodiment, annealing can be performed on a continuous annealing line, and this method is excellent in productivity. If the conditions described later are satisfied, either the annealing furnace or the continuous annealing line may be used. Further, skin pass rolling may be performed on the steel sheet after cold rolling.

In order to obtain the metallographic structure of the steel sheet of the present disclosure, the heating conditions of the steel material to be subjected to hot rolling and the heat treatment conditions after cold rolling, particularly the annealing conditions, are carried out within the ranges shown below.

As long as the steel sheet according to the present embodiment has the above-mentioned chemical composition, the molten steel may be melted by a normal blast furnace method, and the raw material is a large amount of scrap like the steel produced by the electric furnace method. It may be included in. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

The above-mentioned slab or steel ingot is heated and hot-rolled. The temperature of the steel material to be subjected to hot rolling shall be 1000 to less than 1100 ° C. The time for holding in the temperature range below 1000 to 1100 ° C. before hot rolling is 900 to 7200 seconds.

(Retention temperature of slab or steel ingot: 1000 to less than 1100 ° C)
The holding temperature of the steel material to be subjected to hot rolling is preferably less than 1000 to 1100 ° C. By setting the temperature of the steel material to be subjected to hot rolling to 1000 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to less than 1100 ° C., the distribution of Mn is controlled non-uniformly, and the work hardening characteristics of the steel are improved.

(Retention time of slab or steel ingot: 900 to 7200 seconds)
The time for holding in the temperature range of 1000 to less than 1100 ° C. before hot rolling is preferably 900 seconds or longer, and more preferably 1800 seconds or longer in order to improve material stability. Further, in order to make the distribution of Mn non-uniform, it is preferably 7200 seconds or less, and more preferably 5400 seconds or less. When direct rolling or direct rolling is performed, it may be subjected to a holding process at 1000 to less than 1100 ° C. for 7200 seconds or less and subjected to hot rolling.

The finish rolling start temperature is preferably 700 to 1000 ° C. By setting the finish rolling start temperature to 700 ° C. or higher, the deformation resistance during rolling can be reduced. The finish rolling start temperature is more preferably 750 ° C. or higher, still more preferably 800 ° C. or higher. By setting the finish rolling start temperature to 1000 ° C. or lower, deterioration of the surface texture of the steel sheet due to intergranular oxidation can be suppressed. The finish rolling start temperature is more preferably 950 ° C. or lower.

The hot-rolled steel sheet obtained by finish rolling can be cooled, wound up, and made into a coil. The winding temperature after cooling is preferably 700 ° C. or lower. By setting the winding temperature to 700 ° C. or lower, internal oxidation is suppressed and subsequent pickling becomes easy. The winding temperature is more preferably 650 ° C. or lower, still more preferably 600 ° C. or lower. In order to suppress breakage during cold rolling, the hot-rolled sheet may be tempered at 300 to 600 ° C. after being cooled to room temperature and before cold rolling.

The hot-rolled steel sheet is pickled by a conventional method and then cold-rolled to obtain a cold-rolled steel sheet.

It is preferable to perform light rolling of about 0 to 5% before or after cold rolling before or after pickling to correct the shape because it is advantageous in terms of ensuring flatness. Further, by performing light rolling before pickling, the pickling property is improved, the removal of surface-concentrating elements is promoted, and there is an effect of improving the chemical conversion treatment property and the plating processability.

From the viewpoint of miniaturizing the structure of the steel sheet after annealing, the reduction ratio of cold rolling is preferably 20% or more. From the viewpoint of suppressing fracture during cold rolling, the rolling reduction ratio of cold rolling is preferably 70% or less.

The cold-rolled steel sheet obtained through the hot rolling step and the cold rolling step is heated and held in a temperature range of 680 ° C. or higher for 10 seconds or longer, and then from a temperature held in a temperature range of 680 ° C. or higher. The temperature range up to 500 ° C. is cooled at an average cooling rate of 2 ° C./sec or more, cooled to room temperature, then heated again, and held in a temperature range of 600 ° C. to less than ₃ points of Ac for 5 to 300 seconds. The heat treatment of the cold-rolled steel sheet is preferably carried out in a reducing atmosphere, more preferably in a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere containing 98% nitrogen and 2% hydrogen. By heat-treating in a reducing atmosphere, it is possible to prevent scale from adhering to the surface of the steel sheet, and it can be sent to the plating process as it is without requiring acid cleaning. It is preferable to hold it in a temperature range of 100 to 500 ° C. for 10 to 1000 seconds, then cool it to room temperature, and then heat it again to hold it in a temperature range of 600 ° C. to less than ₃ points for 5 to 300 seconds.

(Annealing conditions after cold rolling: Hold for 10 seconds or more in a temperature range of 680 ° C or higher)
After cold rolling, it is held in a temperature range of 680 ° C. or higher for 10 seconds or longer to perform the first annealing. By setting the annealing temperature after cold rolling to 680 ° C. or higher, the standard deviation of the Mn concentration of the steel sheet can be increased, and the work hardening characteristics can be improved. The annealing temperature after cold rolling is preferably 740 ° C. or higher. By setting the annealing temperature after cold rolling to 740 ° C. or higher, recrystallization can be remarkably promoted, and the ferrite content in the steel sheet can be reduced to 3% or less. Here, results of investigation at a heating rate of 0.5 to 50 ° C. / sec, the following equation as a _3-point Ac:
Ac ₃ = 910-200√C + 44Si-25Mn + 44Al
Is obtained, and Ac ₃ points can be calculated using this formula.

On the other hand, the upper limit of the annealing temperature after cold rolling is preferably 950 ° C. By setting the annealing temperature to 950 ° C. or lower, damage to the annealing furnace can be suppressed and productivity can be improved. The annealing temperature after cold rolling is preferably 800 ° C. or lower. By setting the annealing temperature after cold rolling to 800 ° C. or lower, the structure in the steel sheet after annealing can be miniaturized.

The annealing time is set to 10 seconds or longer, preferably 40 seconds or longer, in order to completely remove unrecrystallized crystals and stably secure good toughness. From the viewpoint of productivity, the annealing time is preferably 300 seconds or less.

(Cooling conditions after annealing: Cool the temperature range from 680 ° C to 500 ° C at an average cooling rate of 2 ° C / sec or more)
In cooling after annealing, the temperature range from 680 ° C. to 500 ° C. is cooled at an average cooling rate of 2 ° C./sec or more. Grain boundary segregation of P can be suppressed by setting the average cooling rate in the temperature range from 680 ° C. to 500 ° C. after annealing (hereinafter, also referred to as the average cooling rate after annealing) to 2 ° C./sec or more.

The average cooling rate after annealing is preferably 20 ° C./sec or higher, more preferably 50 ° C./sec or higher, still more preferably 200 ° C./sec or higher, and even more preferably 250 ° C./sec or higher. By setting the average cooling rate after annealing to 200 ° C./sec or higher, it is cooled at a critical cooling rate or higher and the formation of bainite and ferrite can be suppressed, so it is easy to control the structure after the final heat treatment and the material stability. Can be enhanced.

The upper limit of the average cooling rate after annealing is not particularly limited, but even if the water quenching cooling method or the mist injection cooling method is used, it is difficult to control the temperature above 2000 ° C. The upper limit is 2000 ° C / sec.

In the cooling after annealing, the stop temperature of cooling performed at the average cooling rate in the above range is preferably 450 ° C. or lower, more preferably 350 ° C. or lower, and further preferably 300 ° C. or lower. By cooling at an average cooling rate in the above range and setting the cooling stop temperature in the above temperature range, the entire steel material after cooling can have a martensite-based structure.

After cooling after the annealing, it may be held in a temperature range of 100 to 500 ° C. for 10 to 1000 seconds.

(Final annealing conditions after cooling: Hold for 5 to 300 seconds in a temperature range of 600 ° C to less than ₃ points of Ac)
After cooling the above annealing, after cooling to room temperature, it is heated again and held in a temperature range of 600 ° C. to less than ₃ points of Ac (that is, ₁ point to less than ₃ points of Ac) for 5 to 300 seconds to perform the final annealing. Do. Cementite is produced during the heating of the main annealing, and Mn is distributed to this cementite. The Mn-distributed cementite melts at a temperature of 600 ° C. to less than ₃ points of Ac, and Mn-enriched austenite is produced. By setting the final annealing temperature to 600 ° C. to less than ₃ points of Ac, the formation of austenite is promoted and the work hardening characteristics are improved. In order to promote the distribution of Mn to austenite and make the distribution of Mn non-uniform, the final annealing time is 5 seconds or longer, preferably 30 seconds or longer, more preferably 60 seconds or longer. Further, in order to leave tempered martensite, the final annealing time is set to 300 seconds or less. The heating rate at the time of final annealing is not particularly limited, but preferably, when heating to a temperature range of 600 ° C. to less than ₃ points of Ac, the temperature range from 500 ° C. to 600 ° C. is 3 to 6 ° C./sec. The temperature is raised at the average heating rate. By setting the average heating rate to 3 ° C./sec or higher in the temperature range of 500 ° C. to 600 ° C., nucleation of cementite in the metal structure is unlikely to be excessive, and Mn distribution to cementite can be sufficiently realized. .. Further, by setting the average heating rate to 6 ° C./sec or less, a sufficient time for Mn distribution to cementite can be secured. From this, since Mn distribution to cementite in the metal structure can be sufficiently performed, the distribution of Mn of austenite obtained by dissolving cementite can be made more non-uniform.

If the steel sheet is not plated, the cooling after the final annealing may be carried out as it is to room temperature. When plating a steel sheet, it is manufactured as follows.

When hot-dip galvanized steel sheet is manufactured by hot-dip galvanizing the surface of the steel sheet, cooling after the final annealing is stopped in a temperature range of 430 to 500 ° C., and then the cold-rolled steel sheet is subjected to a hot-dip galvanized bath. Is immersed in hot-dip galvanized. The conditions of the plating bath may be within the normal range. After the plating treatment, it may be cooled to room temperature.

When the surface of a steel sheet is hot-dip galvanized to produce an alloyed hot-dip galvanized steel sheet, the temperature is 450 to 580 ° C. after the steel sheet is hot-dip galvanized and before the steel sheet is cooled to room temperature. The hot dip galvanizing process is alloyed at temperature. The alloying treatment conditions may be within the usual range.

By manufacturing the steel sheet as described above, a high-strength steel sheet having a tensile strength (TS) of preferably 780 MPa or more, more preferably 1180 MPa or more can be obtained. As a result, when the steel sheet is used as a material for automobiles, the plate thickness can be reduced by increasing the strength, which can contribute to weight reduction. Further, the work hardening characteristics can be improved, and a steel sheet having high strength and excellent work hardening characteristics having an n value of preferably 0.10 or more, more preferably 0.15 or more can be obtained.

As described above, the steel sheet manufactured by the manufacturing method of the present disclosure has high strength, good work hardening characteristics, and excellent moldability, and is therefore suitable for use in structural parts of automobiles such as pillars. Can be used. Further, since the steel sheet of the present disclosure has a high Mn concentration, it also contributes to weight reduction of automobiles, so that the industrial contribution is extremely remarkable.

The steel sheet of the present disclosure will be described more specifically with reference to an example. However, the following examples are examples of the steel sheets of the present disclosure, and the steel sheets of the present disclosure are not limited to the aspects of the following examples.

1. 1. Production of Steel Sheet for Evaluation The steel having the chemical composition shown in Table 1 was melted in a converter, and a slab having a thickness of 245 mm was obtained by continuous casting.

The obtained slab is hot-rolled under the conditions shown in Table 2 to produce a 2.6 mm-thick hot-rolled steel sheet, and then the obtained hot-rolled steel sheet is pickled and cold-rolled. A cold-rolled steel plate with a thickness of .2 mm was produced. In the production of the hot-rolled steel sheet according to all the examples, the start temperature of the finish rolling was 920 ° C., the winding temperature was 550 ° C., and the hot-rolled steel sheet according to some examples was tempered at 350 to 500 ° C. Further, in the production of the cold-rolled steel sheet according to all the examples, the cold rolling ratio was set to 40%.

The obtained cold-rolled steel sheet was subjected to heat treatment under the conditions shown in Table 3 to prepare an annealed cold-rolled steel sheet. The heat treatment of the cold-rolled steel sheet was carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen. In the annealing immediately after cold rolling, the average cooling rate after annealing is 50 ° C./sec, and when the steel sheet temperature is maintained in the temperature range of 100 to 500 ° C. after the cooling is stopped, the holding time is 30. It was set to seconds. Example No. For annealing after cold spreading of 43, it was held at 740 ° C. for 40 seconds and then further held at 800 ° C. for 100 seconds.

For some examples of hot-dip cold-rolled steel sheets, after the final annealing, cooling after annealing is stopped at 460 ° C., and the cold-dip steel sheet is immersed in a hot-dip galvanizing bath at 460 ° C for 2 seconds to melt. It was galvanized. The conditions of the plating bath are the same as those of the conventional one. When the alloying treatment described later was not performed, the mixture was cooled to room temperature at an average cooling rate of 10 ° C./sec after holding at 460 ° C. Examples of hot dip galvanizing are shown as "plating" in Table 3.

For some examples of annealed cold-rolled steel sheets, after hot-dip galvanizing treatment, they were subsequently alloyed without being cooled to room temperature. It was heated to 520 ° C. and held at 520 ° C. for 5 seconds for alloying treatment, and then cooled to room temperature at an average cooling rate of 10 ° C./sec. An example in which the alloying treatment was performed after the hot-dip galvanizing treatment is shown as “alloying” in Table 3. Example No. For 45, the cooling of annealing after cold spreading was stopped at 460 ° C., and hot dip galvanizing treatment and alloying treatment were performed as described above.

The annealed cold-rolled steel sheet thus obtained was temper-rolled at an elongation rate of 0.1% to prepare various evaluation steel sheets.

2. 2. Evaluation method The tempered cold-rolled steel sheets obtained in each example were subjected to microstructure observation, tensile test, and uniform elongation test to obtain tempered martensite, ferrite, retained austenite, bainite, fresh martensite, and tempered bainite. The area ratio, standard deviation of Mn concentration, tensile strength (TS), and work hardening characteristics (n value) were evaluated. The method of each evaluation is as follows.

(Test method for metallographic structure)
The area ratios of tempered martensite, ferrite, retained austenite, bainite, fresh martensite and tempered bainite were calculated from microstructure observation and X-ray diffraction measurement with a scanning electron microscope. The L cross section of the steel plate cut parallel to the thickness direction and the rolling direction is mirror-polished, and then the microstructure is exposed with 3% nital, and at a position 1/4 from the surface using a scanning electron microscope. The microstructure was observed at a magnification of 5000 times, and tempered martensite, ferrite, retained austenite, bainite, fresh martensite and tempered bainite were performed by image analysis (Photoshop®) for a range of 0.1 mm × 0.3 mm. And the total area ratio of retained austenite and fresh martensite were calculated. Further, a test piece having a width of 25 mm and a length of 25 mm was cut out from the obtained steel plate, and the test piece was chemically polished to reduce the thickness by 1/4, and the surface of the test piece after the chemical polishing was subjected to the chemical polishing. , X-ray diffraction analysis using Co tubes was performed three times, the obtained profiles were analyzed, the area ratio of retained austenite was calculated by averaging each, and the total area of retained austenite and fresh martensite was calculated. The area ratio of fresh martensite was calculated by subtracting the area ratio of retained austenite from the ratio. The ferrite phase was identified as a gray underlying structure, the austenite phase and the fresh martensite phase were identified as a white structure, and the fresh martensite phase and the tempered martensite phase had substructures confirmed in the crystal grains. Was identified as a tempered martensite phase. Furthermore, those in which cementite was present at the interface of the lath or inside the lath were identified as bainite. Moreover, among the bainite, the one in which the substructure was confirmed in the crystal grains was determined to be tempered bainite.

The standard deviation of the Mn concentration is measured by measuring the distribution image in the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction using EPMA at a position 1/4 of the thickness from the surface of the steel sheet at a measurement interval of 0.1 μm. The standard deviation of the Mn concentration was calculated based on each Mn concentration measured at all measurement points.

(Test method for mechanical properties)
A JIS No. 5 tensile test piece was taken from a direction perpendicular to the rolling direction of the steel sheet, and the tensile strength (TS) and work hardening characteristics (n value) were measured. The tensile test was carried out by the method specified in JIS-Z2241: 2011 using a JIS No. 5 tensile test piece. The uniform elongation test was carried out by the method specified in JIS-Z2241: 2011 using a JIS No. 5 test piece having a parallel portion length of 50 mm. The n value shall be a value obtained by determining the true stress at 4% and 7% of the true strain, respectively, and dividing the difference between the logarithms of both true stresses by the difference between the logarithms of both true strains. The crosshead test speed of the tensile test and the uniform elongation test was 30 mm / min.

3. 3. Evaluation Results Table 4 shows the results of the above evaluation. In the examples, an n value of 0.10 or more and a TS of 780 MPa or more were obtained. “Unable to measure” for the n value in Table 4 means that the n value could not be measured because the work hardening characteristics were significantly lowered.

Example No. 1 to 4, 6 to 12, 14 to 17, 19, 22 to 24, 27 to 33 and 36 to 41 have a predetermined chemical composition and are produced according to a predetermined production method, so that a desired metal structure can be obtained. As a result, the standard deviation of the Mn concentration was 0.30% by mass or more, and as a result, it had excellent characteristics (strength (TS) and work hardening characteristics (n value)).

Example No. In No. 5, the holding time before hot rolling was long, and the distribution of Mn could not be made sufficiently non-uniform, so that the work hardening characteristics (n value) were insufficient. Example No. In No. 13, the C content was insufficient and sufficient retained austenite could not be obtained, so that the strength (TS) and work hardening characteristics (n value) were insufficient. Example No. No. 18 had insufficient work hardening characteristics (n value) because the Mn content was insufficient and sufficient retained austenite could not be obtained. Example No. In No. 20, the final annealing temperature was low and a desired metal structure could not be obtained, so that the work hardening characteristics (n value) were insufficient. Example No. In No. 21, the final annealing time was long and sufficient tempered martensite could not be obtained, so that the work hardening characteristics (n value) could not be measured. Example No. In No. 25, the final annealing time was short and the distribution of Mn could not be made sufficiently non-uniform, so that the work hardening characteristics (n value) were insufficient. Example No. In No. 26, the holding temperature before hot rolling was high, and the distribution of Mn could not be made sufficiently non-uniform, so that the work hardening characteristics (n value) were insufficient. Example No. In No. 34, the annealing temperature after cold spreading was low, and the distribution of Mn could not be made sufficiently non-uniform, so that the work hardening characteristics (n value) were insufficient. Example No. In No. 35, the final annealing temperature was high and sufficient tempered martensite could not be obtained, so that the work hardening characteristics (n value) could not be measured. Example No. In No. 42, since the final annealing was not performed, sufficient tempered martensite could not be obtained, and the work hardening characteristics (n value) were insufficient. Example No. In Nos. 43 to 45, since the final annealing was not performed, sufficient retained austenite could not be obtained, and the work hardening characteristics (n value) could not be measured.

Next, the example Nos. of Tables 2 and 3 are shown. Based on the production conditions of 26, only the holding temperature before hot rolling was changed, and the standard deviation of the Mn concentration and the dependence of the work hardening characteristics (n value) on the holding temperature before hot rolling were investigated. FIG. 1 shows a plot of the standard deviation of the Mn concentration with respect to the holding temperature before hot rolling, and FIG. 2 shows a plot of the n value with respect to the holding temperature before hot rolling.

According to FIG. 1, by setting the holding temperature before hot rolling to less than 1100 ° C., the standard deviation of the Mn concentration was set to 0.30% by mass or more, that is, the Mn concentration distribution could be made non-uniform. Further, according to FIG. 2, the work hardening characteristics (n value) could be improved by making the concentration distribution of Mn non-uniform.

Claims (4)

The chemical composition is mass%,
C: Over 0.10 to 0.45%,
Si: 0.001-2.50%,
Mn: Over 4.00 to 8.00%,
sol. Al: 0.001 to 1.50%,
P: 0.100% or less,
S: 0.010% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0-2.00%,
W: 0-2.00%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
Zr: 0-0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and the balance: iron and impurities.
The metallographic structure at 1/4 of the thickness from the surface contains tempered martensite: 25-90% and retained austenite: 10-50% in area%.
A steel sheet having a standard deviation of Mn concentration of 0.30% by mass or more in a rolling direction of 20 μm and a plate thickness method of 20 μm at a position 1/4 of the thickness from the surface. When the chemical composition is mass%,
Cr: 0.01-0.50%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%,
V: 0.005 to 0.300%, and B: 0.0001 to 0.010%
The steel sheet according to claim 1, which contains one kind or two or more kinds selected from the group consisting of. The steel sheet according to claim 1 or 2, which has a hot-dip galvanized layer on the surface of the steel sheet. The steel sheet according to claim 1 or 2, which has an alloyed hot-dip galvanized layer on the surface of the steel sheet.

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