CN118119724A

CN118119724A - Steel plate

Info

Publication number: CN118119724A
Application number: CN202280070216.8A
Authority: CN
Inventors: 大贺光阳; 竹田健悟; 中野克哉
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2021-10-21
Filing date: 2022-10-21
Publication date: 2024-05-31
Also published as: JPWO2023068368A1; MX2024004628A; WO2023068368A1; EP4421198A1; KR20240068702A

Abstract

The present invention provides a steel sheet having a predetermined chemical composition, wherein the microstructure comprises ferrite in terms of area ratio: less than 5.0%, martensite and tempered martensite: aggregate over 90.0%, remainder: 1 or 2 or more of bainite, pearlite, and retained austenite; in the cross section in the plate thickness direction, when the average Mn content in the whole plate thickness direction is [ Mnave ], there is a region with Mn content of 1.1 x [ Mnave ] or more, the number density of the region is 5.0 x 10 ^‑4 pieces/μm ² or more, and the interval between the region and the nearest region with other Mn content of 1.1 x [ Mnave ] or more is 10.0 μm or less on average; the tensile strength is more than 1500 MPa.

Description

Steel plate

Technical Field

The present invention relates to a steel sheet.

The present application claims priority based on japanese patent application publication No. 2021-172425, filed on 21, 10, 2021, the contents of which are incorporated herein by reference.

Background

Today, which is highly specialized in industrial technology, special and high performance is required for materials used in each technical field. In particular, in regard to steel sheets for automobiles, there is a significant increase in demand for high-strength steel sheets in order to improve fuel efficiency by reducing the weight of the automobile body in view of global environment. However, most metal materials are associated with a high strength, and these materials deteriorate various characteristics, particularly an increase in hydrogen embrittlement sensitivity. In steel members, if the tensile strength is 1200MPa or more, particularly, the hydrogen embrittlement sensitivity is improved, and it is known that in bolt steels which have been advanced to have higher strength in the automotive field, there are cases of hydrogen embrittlement cracking. Therefore, a high-strength steel sheet having a tensile strength of 1500MPa or more is strongly demanded to fundamentally solve the problem of hydrogen embrittlement.

In a high-strength steel sheet having a tensile strength of 1500MPa or more, the microstructure mainly includes martensite and tempered martensite, but in such a high-strength steel sheet, hydrogen that has entered the steel is segregated into the grain boundaries of martensite, and the grain boundaries are embrittled (the grain boundary strength is lowered), so that cracks (hydrogen embrittlement) occur. Since hydrogen intrusion occurs even at room temperature, there is no method of perfectly suppressing hydrogen intrusion, and in order to fundamentally solve this problem, improvement of the steel internal structure is necessary.

Conventionally, various techniques for improving the hydrogen embrittlement resistance (also referred to as hydrogen embrittlement resistance) of a high-strength steel sheet have been proposed (for example, see patent documents 1 to 6).

Patent document 1 discloses an ultra-high strength steel sheet excellent in hydrogen embrittlement resistance, which is characterized in that: as an ultra-high strength steel sheet excellent in hydrogen embrittlement resistance and workability, C: more than 0.25 to 0.60 percent of Si:1.0 to 3.0 percent of Mn:1.0 to 3.5 percent of P:0.15% or less, S: less than 0.02%, al: less than 1.5% (excluding 0%), mo: less than 1.0% (excluding 0%), nb:0.1% or less (excluding 0%), the remainder including iron and unavoidable impurities, and the metal structure after the drawing work having a working rate of 3% satisfying the retained austenite structure in terms of area ratio relative to the whole structure: more than 1 percent of bainitic ferrite and martensite: total 80% or more ferrite and pearlite: the total of the retained austenite grains is 9% or less (including 0%), and the average axial ratio (major axis/minor axis) of the retained austenite grains is satisfied: 5 or more, and the tensile strength is 1180MPa or more.

Patent document 2 discloses a high-strength steel sheet having a tensile strength of 1500MPa or more, which is obtained by including si+mn in the steel component: 1.0% or more, forming a layer of ferrite and carbide in a main phase structure, and further forming a layer structure of carbide having an aspect ratio of 10 or more and a spacing of 50nm or less between the layers to a volume ratio of 65% or more relative to the entire structure, and setting a fraction of carbide having an aspect ratio of 10 or more and an angle of 25 ° or less relative to a rolling direction to 75% or more in terms of an area ratio in the carbide forming the layer, thereby making the bending property in the rolling direction and the delayed fracture resistance excellent.

Patent document 3 discloses an ultra-high strength cold-rolled steel sheet excellent in bendability, which is characterized in that: a thin ultra-high strength cold-rolled steel sheet excellent in bending properties and delayed fracture resistance comprises, in mass%, C:0.15 to 0.30 percent of Si:0.01 to 1.8 percent of Mn:1.5 to 3.0 percent of P: less than 0.05%, S: less than 0.005%, al: 0.005-0.05%, N:0.005% or less, the remainder comprising Fe and unavoidable impurities, the steel sheet having a steel sheet surface soft portion satisfying the relationship of "hardness of the steel sheet surface soft portion/hardness of the steel sheet central portion is 0.8 or less", the steel sheet surface soft portion having a ratio of 0.10 or more to 0.30 or less in sheet thickness, and tempered martensite in the steel sheet surface soft portion being 90% or more by volume, the steel sheet central portion having a structure of tempered martensite, and tensile strength of 1270MPa or more.

Patent document 4 discloses a cold-rolled steel sheet having a tensile strength of 1470MPa or more and excellent bending workability and delayed fracture resistance, which contains, in mass%, C:0.15 to 0.20 percent of Si:1.0 to 2.0 percent of Mn:1.5 to 2.5 percent of P: less than 0.020%, S: less than 0.005%, al:0.01 to 0.05 percent of N: less than 0.005%, ti: less than 0.1%, nb:0.1% or less, B:5 to 30ppm, the remainder comprising Fe and unavoidable impurities, and the tempered martensite phase being 97% or more by volume and the retained austenite phase being less than 3% by volume.

Patent document 5 discloses an ultra-high strength steel sheet having a tensile strength of 1470MPa or more, which exhibits excellent delayed fracture resistance even at the cut end, and which has a composition containing, in mass%, C:0.15 to 0.4 percent of Mn:0.5 to 3.0 percent of Al: 0.001-0.10%, the remainder comprising iron and unavoidable impurities, wherein P, S, N is limited to P:0.1% or less, S: less than 0.01%, N:0.01% or less, and a structure including martensite in an area ratio relative to the entire structure: 90% or more of retained austenite: 0.5% or more, and the area ratio of the region where the local Mn concentration is 1.1 times or more of the Mn content of the whole steel sheet is 2% or more, and the tensile strength is 1470MPa or more.

Patent document 6 discloses an ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and a tensile strength of 1300MPa or more, which comprises C:0.150 to 0.300 percent of Si:0.001 to 2.0 percent of Mn:2.10 to 4.0 percent of P: less than 0.05%, S: less than 0.01%, N: less than 0.01%, al:0.001% -1.0%, ti:0.001% -0.10%, B: the values of the solid solution B content solB [ mass% ] and the prior austenite grain diameter Dgamma [ mu ] m satisfy the relation of solB.Dgamma.gtoreq.0.0010, wherein the polygonal ferrite in the steel structure is 10% or less, the bainite is 30% or less, the retained austenite is 6% or less, the tempered martensite is 60% or more, the number density of Fe carbide in the tempered martensite is 1 x 10 ⁶/mm² or more, the average dislocation density of the whole steel is 1.0 x 10 ¹⁵～2.0×10¹⁶/m², and the crystal grain diameter is 7.0 [ mu ] m or less.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2006-207019

Patent document 2: japanese patent application laid-open No. 2010-138489

Patent document 3: japanese patent laid-open publication No. 2011-179030

Patent document 4: japanese patent application laid-open No. 2010-215958

Patent document 5: japanese patent laid-open publication 2016-153524

Patent document 6: japanese patent laid-open publication 2016-050343

Disclosure of Invention

Problems to be solved by the invention

As described above, several techniques for improving the hydrogen embrittlement resistance (hydrogen embrittlement resistance) of high-strength steel sheet have been proposed. However, patent document 1 discloses only hydrogen embrittlement resistance when a stress of 1000MPa is applied, and therefore does not show any technical solution for hydrogen embrittlement resistance when a higher stress is applied.

As described above, hydrogen embrittlement occurs because hydrogen is accumulated in grain boundaries and the bonding strength of the grain boundaries is reduced. Therefore, in order to prevent hydrogen embrittlement, it is effective to uniformly and finely disperse a region having higher interaction with the attractive force of H (hydrogen) than the conventional austenite grain boundaries in steel and to prevent H from aggregating on the conventional γ grain boundaries. However, patent documents 1 to 6do not disclose a method for improving the hydrogen embrittlement resistance from such a viewpoint. In recent years, the requirements for hydrogen embrittlement resistance have become more stringent, and in patent documents 1 to 6, such stringent requirements may not be satisfied.

That is, conventionally, there is room for improvement in hydrogen embrittlement resistance of a high-strength steel sheet having a microstructure mainly composed of martensite and tempered martensite.

Accordingly, an object of the present invention is to provide a steel sheet having high strength and excellent hydrogen embrittlement resistance.

Means for solving the problems

As described above, hydrogen embrittlement is considered to be cracking that occurs when hydrogen in steel is segregated to grain boundaries (mainly to prior austenite grain boundaries when the microstructure is mainly composed of martensite and tempered martensite) to reduce the bonding strength of the grain boundaries and the grain boundaries become starting points.

Then, the present inventors studied a method of preventing aggregation of H to the original γ grain boundary by uniformly finely dispersing a region having higher interaction with the attractive force of H (hydrogen) than the original austenite grain boundary in steel, and focused on the application of the attractive force of Mn and H.

As a result, the following findings were obtained.

A) By dispersing regions having a Mn content higher than the average Mn content at predetermined intervals, aggregation of H into the prior austenite grain boundaries can be suppressed.

B) In a high-strength steel sheet having a microstructure mainly composed of martensite and tempered martensite, it is important to control the annealing condition and the cooling condition after annealing after controlling the dispersion state of cementite by optimizing the rolling condition and coiling condition in the hot rolling step in order to disperse the region having a Mn content higher than the average content.

The present invention has been made in view of the above-described findings. The gist of the present invention is as follows.

[1] One embodiment of the present invention relates to a steel sheet having a chemical composition comprising C：0.150～0.400％、Si：0.01～2.00％、Mn：0.8～2.0％、P：0.0001～0.0200％、S：0.0001～0.0200％、Al：0.001～1.000％、N：0.0001～0.0200％、O：0.0001～0.0200％、Co：0～0.500％、Ni：0～1.000％、Mo：0～1.000％、Cr：0～2.000％、Ti：0～0.500％、B：0～0.0100％、Nb：0～0.500％、V：0～0.500％、Cu：0～0.500％、W：0～0.100％、Ta：0～0.100％、Mg：0～0.050％、Ca：0～0.050％、Y：0～0.050％、Zr：0～0.050％、La：0～0.050％、Ce：0～0.050％、Sn：0～0.050％、Sb：0～0.050％、As：0～0.050％、 in mass% and the remainder: fe and impurities; the microstructure comprises ferrite in terms of area ratio: less than 5.0%, martensite and tempered martensite: aggregate over 90.0%, remainder: 1 or 2 or more of bainite, pearlite, and retained austenite; in the cross section in the plate thickness direction, when the average Mn content in the whole plate thickness direction is [ Mnave ], there is a region with Mn content of 1.1 x [ Mnave ] or more, the number density of the region is 5.0 x 10 ^-4 pieces/μm ² or more, and the interval between the region and the nearest region with other Mn content of 1.1 x [ Mnave ] or more is 10.0 μm or less on average; the tensile strength is more than 1500 MPa.

[2] The steel sheet according to the above [1], wherein the chemical composition may contain a composition selected from Co：0.01～0.500％、Ni：0.01～1.000％、Mo：0.01～1.000％、Cr：0.001～2.000％、Ti：0.001～0.500％、B：0.0001～0.0100％、Nb：0.001～0.500％、V：0.001～0.500％、Cu：0.001～0.500％、W：0.001～0.100％、Ta：0.001～0.100％、Mg：0.0001～0.050％、Ca：0.001～0.050％、Y：0.001～0.050％、Zr：0.001～0.050％、La：0.001～0.050％、Ce：0.001～0.050％、Sn：0.001～0.050％、Sb：0.001～0.050％、 and As:0.001 to 0.050% of 1 or more than 2 kinds.

[3] The steel sheet according to the above [1] or [2], wherein a coating layer containing zinc, aluminum, magnesium or an alloy thereof may be provided on the surface.

Effects of the invention

According to the above aspect of the present invention, a steel sheet having high strength and excellent hydrogen embrittlement resistance can be provided.

Drawings

FIG. 1 is a graph showing the effects of the average interval of the regions of 1.1× [ Mnave ] and the number density of the regions of 1.1× [ Mnave ] on hydrogen embrittlement resistance.

Detailed Description

A steel sheet according to an embodiment of the present invention (steel sheet according to the present embodiment) will be described below.

The steel sheet according to the present embodiment has a predetermined chemical composition, and includes ferrite in terms of area ratio in a microstructure: less than 5.0%, martensite and tempered martensite: aggregate over 90.0%, remainder: when the average Mn content in the whole plate thickness direction is [ Mnave ] in a cross section in the plate thickness direction of 1 or 2 or more of bainite, pearlite and retained austenite, there are regions having Mn contents of 1.1 x [ Mnave ] or more, the number density of the regions is 5.0 x 10 x ^-4 pieces/μm ² or more, and the interval between the regions and the nearest other regions having Mn contents of 1.1 x [ Mnave ] or more is 10.0 μm or less on average, and the tensile strength is 1500MPa or more.

< Chemical composition >

First, the ranges of the contents of the elements constituting the chemical composition of the steel sheet according to the present embodiment will be described. Hereinafter, "%" related to the content of an element means "% by mass". The range indicated by the "to" includes the values at both ends as the lower limit or the upper limit.

C：0.150～0.400％

C is an element effective for inexpensively increasing the tensile strength. If the C content is less than 0.150%, the targeted tensile strength is not obtained, and the fatigue properties of the weld zone are deteriorated. Therefore, the C content is set to 0.150% or more. The C content may be 0.160% or more, 0.180% or more, or 0.200% or more.

On the other hand, if the C content exceeds 0.400%, hydrogen embrittlement resistance and weldability are reduced. Therefore, the C content is set to 0.400% or less. The C content may be 0.350% or less, 0.300% or less, or 0.250% or less.

Si：0.01～2.00％

Si functions as a deoxidizer and is an element that affects the morphology of carbides and residual austenite after heat treatment. If the Si content is less than 0.01%, it becomes difficult to suppress the formation of coarse oxides. The coarse oxide becomes a starting point of cracks, and the cracks propagate in the steel material, thereby deteriorating hydrogen embrittlement resistance. Therefore, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, or 0.30% or more.

On the other hand, if the Si content exceeds 2.00%, the local ductility may be reduced, and the hydrogen embrittlement resistance may be deteriorated. Therefore, the Si content is set to 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, or 1.40% or less.

Mn：0.8～2.0％

Mn is an element effective for improving the strength of the steel sheet. If the Mn content is less than 0.8%, a sufficient effect cannot be obtained. Therefore, the Mn content is set to 0.8% or more. The Mn content may be 1.0% or more, or 1.2% or more.

On the other hand, if the Mn content exceeds 2.0%, mn not only promotes co-segregation with P, S but also sometimes deteriorates corrosion resistance and hydrogen embrittlement resistance. Therefore, the Mn content is set to 2.0% or less. The Mn content may be 1.9% or less, or 1.8% or less.

P：0.0001～0.0200％

P is an element that strongly segregates to ferrite grain boundaries and promotes grain boundary embrittlement. If the P content exceeds 0.0200%, the hydrogen embrittlement resistance is significantly reduced by grain boundary embrittlement. Therefore, the P content is set to 0.0200% or less. The P content may be 0.0180% or less, 0.0150% or less, or 0.0120% or less.

The smaller the P content, the more preferable. However, when the P content is set to less than 0.0001%, a significant increase in cost is incurred due to an increase in time required for refining. Therefore, the P content is set to 0.0001% or more. The P content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

S：0.0001～0.0200％

S is an element that generates nonmetallic inclusions such as MnS in steel. If the S content exceeds 0.0200%, the generation of nonmetallic inclusions that become crack initiation points during cold working becomes remarkable. In this case, cracks from nonmetallic inclusions occur, and the cracks propagate in the steel material, thereby deteriorating hydrogen embrittlement resistance. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0180% or less, 0.0150% or less, or 0.0120% or less.

The smaller the S content, the more preferable. However, when the S content is set to less than 0.0001%, the time required for refining increases, which results in a significant increase in cost. Therefore, the S content is set to 0.0001% or more. The S content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

Al：0.001～1.000％

Al acts as a deoxidizer for steel, and stabilizes ferrite. If the Al content is less than 0.001%, a sufficient effect cannot be obtained. Therefore, the Al content is set to 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, or 0.020% or more.

On the other hand, if the Al content exceeds 1.000%, coarse Al oxide is formed. The coarse oxide becomes a crack initiation point. Therefore, if coarse Al oxide is formed, cracks occur in the coarse Al oxide, and the cracks propagate in the steel material, thereby deteriorating the hydrogen embrittlement resistance. Therefore, the Al content is set to 1.000% or less. The Al content may be 0.950% or less, 0.900% or less, or 0.800% or less.

N：0.0001～0.0200％

N is an element that forms coarse nitrides in the steel sheet and reduces the hydrogen embrittlement resistance of the steel sheet. N is an element that causes blowholes during welding.

If the N content exceeds 0.0200%, the occurrence of pores becomes remarkable while deteriorating the hydrogen embrittlement resistance. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0180% or less, 0.0160% or less, or 0.0120% or less.

On the other hand, when the N content is set to less than 0.0001%, the manufacturing cost increases greatly. Therefore, the N content is set to 0.0001% or more. The N content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

O：0.0001～0.0200％

O is an element that forms an oxide and deteriorates hydrogen embrittlement resistance. In particular, since oxides are often present in the form of inclusions, if they are present in the punched end face or the cut face, scratches in the form of notches and coarse dimples (small) are formed in the end face, stress concentration is caused during the working, and the oxides become the origin of cracks, which greatly deteriorate the workability. If the O content exceeds 0.0200%, the above-mentioned workability tends to be remarkable. Therefore, the O content is set to 0.0200% or less. The O content may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.

Preferably, the O content is relatively low. However, setting the O content to less than 0.0001% is economically undesirable because it will incur excessive increase in cost. Therefore, the O content is set to 0.0001% or more. The O content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.

The basic components of the chemical composition of the steel sheet according to the embodiment of the present invention are as described above. That is, the chemical composition of the steel sheet according to the present embodiment contains the above elements, and the remainder may include Fe and impurities. On the other hand, in order to improve various properties, the chemical composition of the steel sheet according to the present embodiment may contain 1 or more kinds of Co, ni, mo, cr, ti, B, nb, V, cu, W, ta, mg, ca, Y, zr, la, ce, sn, sb, as as an optional component instead of part of Fe in the remaining part.

These elements may not be necessarily contained, and therefore the lower limit thereof is 0%. Further, even if the following elements are contained as impurities, the effects of the steel sheet according to the present embodiment are not impaired.

Co：0～0.500％

Co is an element effective for controlling the morphology of carbide and improving the strength of steel sheet. Therefore, co may be contained. In order to obtain a sufficient effect, the Co content is preferably set to 0.010% or more. The Co content may be 0.020% or more, 0.050% or more, or 0.100% or more.

On the other hand, if the Co content exceeds 0.500%, coarse Co carbide precipitates. In this case, hydrogen embrittlement resistance may be deteriorated. Therefore, the Co content is set to 0.500% or less. The Co content may be 0.450% or less, 0.400% or less, or 0.300% or less.

Ni：0～1.000％

Ni is an element effective for improving the strength of the steel sheet. Ni is also an element effective in improving wettability and promoting alloying reaction. Therefore, ni may be contained. In order to obtain the above-described effects, the Ni content is preferably set to 0.010% or more. The Ni content may be 0.020% or more, 0.050% or more, or 0.100% or more.

On the other hand, if the Ni content exceeds 1.000%, hydrogen embrittlement resistance may be lowered. Therefore, the Ni content is set to 1.000% or less. The Ni content may be 0.900% or less, 0.800% or less, or 0.600% or less.

Mo：0～1.000％

Mo is an element effective for improving the strength of the steel sheet. Further, mo is an element effective for suppressing ferrite transformation generated when heat treatment is performed by a continuous annealing apparatus or a continuous hot dip galvanization apparatus. Therefore, mo may be contained. In order to obtain the above-described effects, the Mo content is preferably set to 0.010% or more. The Mo content may be 0.020% or more, 0.050% or more, or 0.080% or more.

On the other hand, if the Mo content exceeds 1.000%, the effect of suppressing ferrite transformation is saturated. Therefore, the Mo content is set to 1.000% or less. The Mo content may be 0.900% or less, 0.800% or less, or 0.600% or less.

Cr：0～2.000％

Cr is an element effective for increasing the strength of steel, as well as Mn, and suppresses pearlite transformation. Therefore, cr may be contained. In order to obtain the above-described effects, the Cr content is preferably set to 0.001% or more. The Cr content may be 0.005% or more, 0.010% or more, or 0.050% or more.

On the other hand, if the Cr content exceeds 2.000%, coarse Cr carbide is formed in the center segregation portion, and the hydrogen embrittlement resistance may be lowered. Therefore, the Cr content is set to 2.000% or less. The Cr content may be 1.800% or less, 1.500% or less, or 1.000% or less.

Ti：0～0.500％

Ti is an element that contributes to the improvement of the strength of a steel sheet by precipitate strengthening, grain strengthening by suppressing ferrite grain growth, and dislocation strengthening by suppressing recrystallization. Therefore, ti may be contained. In order to obtain the above-described effects, the Ti content is preferably set to 0.001% or more. The Ti content may be 0.003% or more, 0.010% or more, or 0.050% or more.

On the other hand, if the Ti content exceeds 0.500%, precipitation of carbonitrides increases, and hydrogen embrittlement resistance may be deteriorated. Therefore, the Ti content is set to 0.500% or less. The Ti content may be 0.450% or less, 0.400% or less, or 0.300% or less.

B：0～0.0100％

B is an element that suppresses the formation of ferrite and pearlite and promotes the formation of a low-temperature transformation structure such as bainite or martensite during cooling from the austenite temperature region. In addition, B is an element that is beneficial for increasing the strength of steel. Thus, B may be contained. In order to obtain the above-mentioned effects, the B content is preferably set to 0.0001% or more. The B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more.

On the other hand, if the B content exceeds 0.0100%, coarse B oxides are formed in the steel. Since this oxide becomes a starting point of void generation during cold working, the hydrogen embrittlement resistance may be deteriorated by the generation of coarse B oxide. Therefore, the B content is set to 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

Nb：0～0.500％

Nb is an element effective for controlling the form of carbide like Ti, and is also an element effective for improving toughness by refining the structure. Therefore, nb may be contained. In order to obtain the above-described effects, the Nb content is preferably set to 0.001% or more. The Nb content may be 0.002% or more, 0.010% or more, or 0.020% or more.

On the other hand, if the Nb content exceeds 0.500%, the formation of coarse Nb carbides becomes remarkable. Since cracks are likely to occur in the coarse Nb carbide, the hydrogen embrittlement resistance may be deteriorated by the formation of the coarse Nb carbide. Therefore, the Nb content is set to 0.500% or less. The Nb content may be 0.450% or less, 0.400% or less, or 0.300% or less.

V：0～0.500％

V is an element that contributes to the improvement of the strength of the steel sheet by precipitate strengthening, grain strengthening based on the suppression of ferrite grain growth, and dislocation strengthening via the suppression of recrystallization. And thus may also contain V. In order to obtain the above-mentioned effects, the V content is preferably set to 0.001% or more. The V content may be 0.002% or more, 0.010% or more, or 0.020% or more.

On the other hand, if the V content exceeds 0.500%, precipitation of carbonitrides increases, and hydrogen embrittlement resistance may be deteriorated. Therefore, the V content is set to 0.500% or less. The V content may be 0.450% or less, 0.400% or less, or 0.300% or less.

Cu：0～0.500％

Cu is an element effective for improving the strength of the steel sheet. If the Cu content is less than 0.001%, a sufficient effect cannot be obtained. Therefore, in order to obtain the above-described effects, the Cu content is preferably set to 0.001% or more. The Cu content may be 0.002% or more, 0.010% or more, or 0.030% or more.

On the other hand, if the Cu content exceeds 0.500%, hydrogen embrittlement resistance may be deteriorated. In addition, if the Cu content is high, the steel material is embrittled during hot rolling, and hot rolling may not be performed. Therefore, the Cu content is set to 0.500% or less. The Cu content may be 0.450% or less, 0.400% or less, or 0.300% or less.

W：0～0.100％

W is an element effective for improving the strength of the steel sheet. W is an element that forms precipitates and crystals. Since precipitates and crystals containing W serve as hydrogen trapping sites, W is an element effective for improving hydrogen embrittlement resistance. Therefore, W may be contained. In order to obtain the above-mentioned effects, the W content is preferably set to 0.001% or more. The W content may be 0.002% or more, 0.005% or more, or 0.010% or more.

On the other hand, when the W content exceeds 0.100%, the formation of coarse W precipitates or crystals becomes remarkable. The coarse W precipitates or crystals are prone to cracking, and the cracks propagate in the steel under low load stress. Therefore, if coarse W precipitates or crystals are formed, hydrogen embrittlement resistance may be deteriorated. Therefore, the W content is set to 0.100% or less. The W content may be 0.080% or less, 0.060% or less, or 0.050% or less.

Ta：0～0.100％

Ta is an element effective for controlling the form of carbide and improving the strength of steel sheet, similarly to Nb, V and W. Thus, ta may also be present. In order to obtain the above-described effects, the Ta content is preferably set to 0.001% or more. The Ta content may be 0.002% or more, 0.005% or more, or 0.010% or more.

On the other hand, if the Ta content exceeds 0.100%, a large amount of fine Ta carbide precipitates, and as the strength of the steel sheet increases, ductility may decrease or bending resistance and hydrogen embrittlement resistance may decrease. Therefore, the Ta content is set to 0.100% or less. The Ta content may be 0.080% or less, 0.060% or less, or 0.050% or less.

Mg：0～0.050％

Mg is an element capable of controlling the form of sulfide in a trace amount. Mg may also be contained. In order to obtain the above-described effect, the Mg content is preferably set to 0.001% or more. The Mg content may be 0.005% or more, 0.010% or more, or 0.020% or more.

On the other hand, if the Mg content exceeds 0.050%, coarse inclusions are formed, and hydrogen embrittlement resistance may be lowered. Therefore, the Mg content is set to 0.050% or less. The Mg content may be 0.040% or less, 0.030% or less, or 0.020% or less.

Ca：0～0.050％

In addition to being useful as a deoxidizing element, ca is also an effective element for controlling the morphology of sulfides. Therefore, ca may be contained. In order to obtain the above-mentioned effects, the Ca content is preferably set to 0.001% or more. The Ca content may be 0.002% or more, 0.004% or more, or 0.006% or more.

On the other hand, if the Ca content exceeds 0.050%, coarse inclusions are formed, and hydrogen embrittlement resistance may be lowered. Therefore, the Ca content was set to 0.050% or less. The Ca content may be 0.040% or less, 0.030% or less, or 0.020% or less.

Y：0～0.050％

Y is an element that can control the form of sulfide in a trace amount, similarly to Mg and Ca. And thus may also contain Y. In order to obtain the above-mentioned effects, the Y content is preferably set to 0.001% or more. The Y content may be 0.002% or more, 0.004% or more, or 0.006% or more.

On the other hand, if the Y content exceeds 0.050%, coarse Y oxides are formed, and hydrogen embrittlement resistance may be lowered. Therefore, the Y content is set to 0.050 or less. The Y content may be 0.040% or less, 0.030% or less, or 0.020% or less.

Zr：0～0.050％

Zr is an element that can control the form of sulfide in a trace amount, similarly to Mg, ca, and Y. Therefore, zr may be contained. In order to obtain the above-mentioned effects, the Zr content is preferably set to 0.001% or more. The Zr content may be 0.002% or more, 0.004% or more, or 0.006% or more.

On the other hand, if the Zr content exceeds 0.050%, coarse Zr oxide is formed, and hydrogen embrittlement resistance may be lowered. Therefore, the Zr content was set to 0.050 or less. The Zr content may be 0.040% or less, 0.030% or less, or 0.020% or less.

La：0～0.050％

La is an element that can control the morphology of sulfide in a trace amount, as in Mg, ca, Y, zr. Therefore, la may be contained. In order to obtain the above-mentioned effects, the La content is preferably set to 0.001% or more. The La content may be 0.002% or more, 0.004% or more, or 0.006% or more.

On the other hand, if the La content exceeds 0.050%, la oxide is formed, and hydrogen embrittlement resistance may be lowered. Therefore, the La content was set to 0.050 or less. The La content may be 0.040% or less, 0.030% or less, or 0.020% or less.

Ce：0～0.050％

Ce is an element that can control the form of sulfide in a trace amount, similar to La. And thus may also contain Ce. In order to obtain the above-described effects, the Ce content is preferably set to 0.001% or more. The Ce content may be 0.002% or more, 0.004% or more, or 0.006% or more.

On the other hand, if the Ce content exceeds 0.050%, ce oxide is formed, and the hydrogen embrittlement resistance may be lowered. Therefore, the Ce content was set to 0.050 or less. The Ce content may be 0.040% or less, 0.030% or less, or 0.020% or less.

Sn：0～0.050％

Sn is an element contained in steel when scrap is used as a raw material. If the Sn content is high, the hydrogen embrittlement resistance may be lowered due to grain boundary embrittlement. If the Sn content exceeds 0.050, the adverse effect becomes particularly remarkable. Therefore, the Sn content is set to 0.050 or less. The Sn content may be 0.040% or less, 0.030% or less, or 0.020% or less.

The smaller the Sn content, the more preferable the Sn content is, but when the Sn content is set to less than 0.001%, the refining cost increases. Therefore, the Sn content may be set to 0.001% or more. The Sn content may be 0.002% or more, 0.005% or more, or 0.010% or more.

Sb：0～0.050％

Sb is an element contained in the case of using scrap as a steel raw material, similarly to Sn. Sb is an element that strongly segregates to grain boundaries, causing grain boundary embrittlement and a decrease in ductility. If the Sb content exceeds 0.050, the adverse effect thereof becomes particularly remarkable. Therefore, the Sb content was set to 0.050 or less. The Sb content may be 0.040% or less, 0.030% or less, or 0.020% or less.

The smaller the Sb content, the more preferable the Sb content, but when the Sb content is set to less than 0.001%, the refining cost increases. Therefore, the Sb content may be set to 0.001% or more. The Sb content may be 0.002% or more, 0.005% or more, or 0.008% or more.

As：0～0.050％

As is an element which is contained in the case of using scrap As a steel raw material, and which is strongly segregated to grain boundaries, and causes grain boundary embrittlement and a decrease in ductility, like Sn and Sb. If the As content is large, the hydrogen embrittlement resistance may be lowered. If the As content exceeds 0.050, the adverse effect thereof becomes particularly remarkable. Therefore, the As content was set to 0.050 or less. The As content may be 0.040% or less, 0.030% or less, or 0.020% or less.

The smaller the As content, the more preferable the As content, but when the As content is set to less than 0.001%, refining cost increases. Therefore, the As content may be set to 0.001% or more. The As content may be 0.002% or more, 0.003% or more, or 0.005% or more.

As described above, the chemical composition of the steel sheet according to the present embodiment may contain the basic component, and the remainder may contain Fe and impurities, or may contain 1 or more of the basic components, and the remainder may contain Fe and impurities.

The chemical composition of the steel sheet according to the present embodiment may be measured by a general method. For example, as long as it is in accordance with JIS G1201:2014, and measuring the cuttings by adopting ICP-AES (inductively coupled plasma atomic emission spectrometry: inductively Coupled Plasma-Atomic Emission Spectrometry). In this case, the chemical composition is the average content of the total plate thickness. C and S which cannot be measured by ICP-AES can be measured by a combustion-infrared absorption method, N can be measured by an inert gas fusion-thermal conductivity method, and O can be measured by an inert gas fusion-non-dispersive infrared absorption method. The Mn content obtained is referred to as [ Mnave ] hereinafter.

When the steel sheet has a coating layer on the surface, chemical composition analysis may be performed after removing the coating layer by mechanical grinding or the like. In the case where the coating layer is a plating layer, the plating layer may be removed by dissolving the plating layer with an acid solution to which a corrosion inhibitor for inhibiting corrosion of the steel sheet is added.

< Microstructure (Metal Structure) >

Next, the microstructure of the steel sheet according to the present embodiment will be described. In the present embodiment, the microstructure is a microstructure at a position (t/4 portion) of 1/8 to 3/8 of the plate thickness from the surface of the steel plate in the plate thickness direction. The microstructure of the t/4 portion is defined because it is a representative microstructure of the steel sheet and is closely related to the properties of the steel sheet.

The fraction (%) of each phase is the area ratio unless otherwise specified.

Ferrite: 5.0% or less

Ferrite exerts an influence on the deformability of steel having a structure mainly composed of martensite and tempered martensite. As the area ratio of ferrite increases, the local deformability and hydrogen embrittlement resistance decrease. In particular, if the area ratio of ferrite exceeds 5.0%, the hydrogen embrittlement resistance may be lowered due to fracture in elastic deformation at the time of stress loading. Therefore, the area ratio of ferrite is set to 5.0% or less. The area ratio of ferrite may be 4.0% or less, 3.0% or less, or 2.0% or less.

The area ratio of ferrite may be 0%, but if it is set to less than 1.0%, strict control is required in production, resulting in a decrease in yield. Therefore, the area ratio of ferrite may be set to 1.0% or more.

Martensite and tempered martensite: total more than 90.0%

The total area ratio of martensite (so-called primary martensite) and tempered martensite affects the strength of the steel, and the tensile strength increases as the area ratio increases. If the total area ratio of martensite and tempered martensite is 90.0% or less, the targeted tensile strength cannot be achieved, and besides, the fracture and the decrease in hydrogen embrittlement resistance during elastic deformation under a stress load may be caused. Therefore, the total area ratio of martensite and tempered martensite is set to be more than 90.0%. The total area ratio of the martensite and tempered martensite may be 95.0% or more, 97.0% or more, 99.0% or more, or 100.0%.

The remainder: comprises 1 or more than 2 of bainite, pearlite and retained austenite

The area ratio of the structure (the remaining structure) other than the above structure may be 0%, but when the remaining structure is present, the remaining structure contains 1 or 2 or more of bainite, pearlite, and retained austenite.

If the area ratio of the remaining part of the structure exceeds 8.0%, fracture in elastic deformation at the time of stress load may occur, and hydrogen embrittlement resistance may be lowered. Therefore, the area ratio of the remaining tissue is preferably 8.0% or less, more preferably 7.0% or less. Among them, pearlite and retained austenite are particularly preferable because they are structures that deteriorate the local ductility of steel.

On the other hand, if the area ratio of the remaining structure is set to 0%, strict control is required in manufacturing, and thus the yield may be lowered. Therefore, the area ratio of the remaining tissue may be 1.0% or more.

The area ratio of each phase in the microstructure of the steel sheet according to the present embodiment can be obtained by the following method.

(Method for evaluating area ratio of ferrite)

The area ratio of ferrite can be obtained by observing the t/4 portion (the range of 1/8 to 3/8 of the plate thickness centered on a position 1/4 of the plate thickness from the surface in the plate thickness direction) by using an electron channel contrast image of a Field Emission scanning electron microscope (FE-SEM: field Emission-Scanning Electron Microscope). The electron channel contrast image is a method of detecting a difference in crystal orientation within a crystal grain as a difference in contrast of the image, and in the image, a portion of a structure determined to be not pearlite, bainite, martensite, retained austenite, but ferrite is reflected as polygonal ferrite in a uniform contrast. For 8 fields of the 35 μm×25 μm electron channel contrast image, the area ratio of polygonal ferrite in each field was calculated by image analysis, and the average value was used as the area ratio of ferrite.

(Method for evaluating the area ratio of martensite and tempered martensite in total)

The martensite and tempered martensite may be obtained as a total area ratio from the image captured by the electron channel contrast. These structures are less likely to be corroded than ferrite, and therefore exist as projections on the structure observation surface. Tempered martensite is an aggregation of lath-shaped grains, and contains iron-based carbides having a long diameter of 20nm or more, which belong to a plurality of varieties, i.e., a plurality of iron-based carbide groups extending in different directions, inside. The retained austenite also exists as a convex portion on the tissue observation surface. Therefore, the area ratio of the sum of the martensite and tempered martensite can be accurately measured by subtracting the area ratio of the convex portion obtained by the above-described steps from the area ratio of the retained austenite measured by the steps described later.

(Method for evaluating the area ratio of the total of bainite, pearlite, and retained austenite)

The area ratio of the retained austenite can be calculated by measurement using X-rays. That is, the polishing solution was removed from the plate surface of the sample to a position 1/4 of the plate thickness in the plate thickness direction by mechanical polishing and chemical polishing. Then, for the sample after grinding, the structure fraction of the retained austenite was calculated from the integrated intensity ratios of diffraction peaks of (200), (211) of the bcc phase and (200), (220) and (311) of the fcc phase obtained by using mokα rays as characteristic X rays, and was taken as the area fraction of the retained austenite.

The pearlite can be obtained from an image captured by the electron channel contrast as described above. Pearlite is a structure in which plate-like carbide and ferrite are arranged.

The bainite is a group of lath-shaped grains, and contains no iron-based carbide having a length of 20nm or more in the interior, or contains iron-based carbide having a length of 20nm or more in the interior, and the carbide belongs to a single variety, i.e., an iron-based carbide group extending in the same direction. The term "iron-based carbide group extending in the same direction" means that the difference in the extending direction of the iron-based carbide group is within 5 °.

(There are regions having a Mn content of 1.1 x [ Mnave ] or more, a number density of 5.0 x 10 ^-4/μm ² or more, and an average spacing from the nearest other region having a Mn content of 1.1 x [ Mnave ] or more of 10.0 μm or less)

Since Mn and H have attractive interactions, if the dispersion state is controlled, aggregation of H to the prior austenite grain boundaries can be suppressed, and as a result, a steel sheet excellent in hydrogen embrittlement resistance can be obtained.

In the steel sheet according to the present embodiment, as a region having higher interaction with H than the prior austenite grain boundary, when the average Mn content in the whole sheet thickness direction is [ Mnave ] in the section in the sheet thickness direction, a plurality of regions having Mn content of 1.1× [ Mnave ] or more are dispersed.

On the other hand, if the distance between these plural regions, that is, the distance between a region having a Mn content of 1.1× [ Mnave ] or more and the nearest other region having a Mn content of 1.1× [ Mnave ] or more exceeds 10.0 μm on average, the effect of suppressing segregation of H into the prior austenite grain boundaries cannot be sufficiently obtained, and therefore the hydrogen embrittlement resistance is reduced.

Therefore, a plurality of regions having a Mn content of 1.1 x [ Mnave ] or more are present (dispersed) so that the average distance between the regions and the nearest other regions having a Mn content of 1.1 x [ Mnave ] or more is 10.0 [ mu ] m or less.

In addition, even if the region having an Mn content of 1.1× [ Mnave ] or more is dispersed as described above, if the number density is small, sufficient effects cannot be obtained. Therefore, the number density of the regions having an Mn content of 1.1 x [ Mnave ] or more is set to 5.0 x 10 ^-4/μm ² or more.

The identification of the regions having Mn content of 1.1 x Mnave or more, their number density, and average interval between adjacent regions can be obtained by the following method.

The dispersion state of Mn can be measured by an Electron Probe Microanalyzer (EPMA). Specifically, samples were collected so that a cross section parallel to the rolling direction of the steel sheet became a measurement surface, and the element concentration distribution of Mn was obtained at a measurement interval of 0.1 μm with a 50 μm×50 μm region as 1 field of view in the t/4 section of the observation surface (in the range of 1/8 to 3/8 of the plate thickness centered on a position 1/4 of the plate thickness from the surface in the plate thickness direction). This measurement was performed for 10 fields, and data of element distribution of about 10 fields was collected as numerical data, and 2-valued images were created in which the Mn concentration was 1.1× [ Mnave ] or more and the Mn concentration was less than 1.1× [ Mnave ] in terms of color. From the 2-valued image, a region in which 1.1× [ Mnave ] or more regions are connected across a plurality of pixels is regarded as one region of 1.1× [ Mnave ] or more, and the number of regions of 1.1× [ Mnave ] or more is obtained. The number density of the regions of 1.1× [ Mnave ] or more can be obtained by dividing the obtained number by the area of 1 field of view. The value obtained by dividing the area of the 1 field of view by the number obtained as described above is squared, and the obtained value is defined as the average interval between the areas of 1.1× [ Mnave ] or more.

(Mechanical Properties)

In the steel sheet according to the present embodiment, the Tensile Strength (TS) is set to 1500MPa or more as a strength contributing to weight saving of the automobile body.

The upper limit is not necessarily limited, but if the tensile strength is increased, the formability may be lowered, and therefore the tensile strength may be set to 2500MPa or less or 2000MPa or less.

(Plate thickness)

The thickness of the steel sheet according to the present embodiment is not limited, but is preferably 1.0mm or more and 2.2mm or less. The thickness is more preferably 1.05mm or more or 1.1mm or more. The thickness is more preferably 2.1mm or less or 2.0mm or less.

(Coating layer)

The steel sheet according to the present embodiment may have a coating layer containing zinc, aluminum, magnesium, or an alloy thereof on one or both surfaces. The coating layer may be formed of zinc, aluminum, magnesium, or an alloy and impurities thereof.

The surface is provided with a coating layer to improve corrosion resistance. If there is a concern about pits caused by corrosion, the steel sheet for automobiles may not be thinned to a certain fixed plate thickness or less even if the steel sheet is made stronger. One of the purposes of increasing the strength of steel sheets is to reduce the weight due to the reduction in thickness, and therefore, even if high-strength steel sheets are developed, the application sites thereof are limited if the corrosion resistance is low. As a method for solving these problems, forming a coating layer on both front and back surfaces can be considered in order to improve corrosion resistance.

Even if a coating layer is formed, the hydrogen embrittlement resistance of the steel sheet according to the present embodiment is not impaired.

The coating layer is, for example, a hot dip zinc coating layer, an alloyed hot dip zinc coating layer, an electro zinc coating layer, an aluminum plating layer, a Zn-Al alloy coating layer, an Al-Mg alloy coating layer, or a Zn-Al-Mg alloy coating layer.

When the coating layer is provided on the surface, the surface to be the reference of the t/4 section is the surface from which the base metal of the coating layer is removed.

< Manufacturing method >

A method for manufacturing a steel sheet according to the present embodiment (a method for manufacturing a steel sheet according to the present embodiment) described above will be described.

The method for manufacturing a steel sheet according to the present embodiment includes:

(I) A heating step of heating a billet having a predetermined chemical composition;

(II) a hot rolling step of hot-rolling the heated slab to obtain a hot-rolled steel sheet;

(III) a cooling step of cooling the hot-rolled steel sheet to a coiling temperature of 400 to 550 ℃ inclusive at an average cooling rate of 20 to 50 ℃/sec inclusive, starting cooling in less than 1.0 sec from the end of the hot rolling step;

(IV) a coiling step of coiling the hot-rolled steel sheet after the post-hot rolling cooling step at the coiling temperature;

(V) a cold rolling step of pickling and cold-rolling the hot-rolled steel sheet after the coiling step to obtain a cold-rolled steel sheet;

(VI) an annealing step of heating the cold-rolled steel sheet after the cold-rolling step to an annealing temperature such that an average heating rate of from room temperature to 700 ℃ is 15 ℃/sec or more and 100 ℃/sec or less and an average heating rate of from 700 ℃ to 830 ℃ or more and less than 900 ℃ is 5 ℃/sec or more and less than 15 ℃/sec, and holding the temperature at the annealing temperature for 25 to 100 seconds to perform annealing;

(VII) an annealing post-cooling step of cooling the cold-rolled steel sheet after the annealing step to 25-300 ℃ at an average cooling rate of 4-100 ℃/sec.

Hereinafter, preferable conditions in each step will be described.

[ Heating Process ]

In the heating step, a slab or the like having a predetermined chemical composition (the chemical composition does not substantially change in the manufacturing step, and the same chemical composition as the steel sheet according to the present embodiment) is heated before the hot rolling step.

The heating temperature is not limited as long as the rolling temperature in the next step can be ensured. For example, 1000 to 1300 ℃.

The billet used is preferably cast by continuous casting from the viewpoint of productivity, but may be produced by ingot casting or thin slab casting.

In the case where a billet obtained by continuous casting can be supplied to the hot rolling step in an original state at a sufficiently high temperature, the heating step may be omitted.

[ Hot Rolling Process ]

In the hot rolling step, the heated slab is hot-rolled to obtain a hot-rolled steel sheet.

The hot rolling step includes rough rolling and finish rolling, wherein the finish rolling is performed in multiple passes, 4 or more passes among the multiple passes are set as large reduction passes with a reduction ratio of 20% or more, and the inter-pass time of each of the large reduction passes is set to 5.0 seconds. The rolling start temperature is set to 950 to 1100 ℃ and the rolling end temperature is set to 800 to 950 ℃.

( In finish rolling, a large reduction pass with a reduction ratio of 20% or more: more than 4 times )

(Inter-pass time of large reduction pass: within 5.0 seconds)

By controlling the reduction rate, the number of rolling passes, and the time between passes in finish rolling, the morphology of austenite grains can be controlled to be equiaxed and fine. If austenite grains are equiaxed and fine, a pearlite structure can be uniformly and finely formed in the subsequent post-hot rolling cooling step. Since cementite is contained in the pearlite structure, a structure in which cementite is uniformly and finely precipitated (dispersed precipitation) can be obtained. If the pass of the reduction ratio of 20% or more (large reduction pass) is less than 4 passes, sufficient effect cannot be obtained due to the austenite remaining without recrystallization. Therefore, the reduction ratio is set to 20% or more in 4 or more passes (4 or more passes of reduction is performed at a reduction ratio of 20% or more). The reduction ratio is preferably set to 20% or more in 5 or more passes. On the other hand, the upper limit of the number of passes of the reduction ratio of 20% or more is not particularly limited, but if the number of passes exceeds 10 passes, a plurality of rolling stands are required, which may lead to an increase in the size of the equipment and an increase in the manufacturing cost. Therefore, the number of passes (number of passes) of the reduction ratio of 20% or more may be 10 passes or less, 9 passes or less, or 7 passes or less.

In addition, the inter-pass time in finish rolling exerts a large influence on recrystallization and grain growth of austenite grains after rolling. Even when the number of large reduction passes is set to 4 or more, if the inter-pass time of each large reduction pass exceeds 5.0 seconds, grain growth tends to occur, and austenite grains are coarsened. Therefore, the inter-turn time is preferably 3.0 seconds or less or 1.0 seconds or less.

On the other hand, the lower limit of the inter-pass time is not limited, but if the inter-pass time of each large reduction pass is less than 0.2 seconds, the recrystallization of austenite is not completed, and the proportion of unrecrystallized austenite increases, and a sufficient effect may not be obtained. Therefore, the inter-pass time of the large reduction pass is preferably set to 0.2 seconds or more. The inter-pass time may be 0.3 seconds or more or 0.5 seconds or more.

(Rolling start temperature: 950-1100 ℃ C.)

(Rolling finishing temperature: 800-950 ℃ C.)

The rolling start temperature is an important factor for controlling the recrystallization of austenite. If the rolling start temperature is lower than 950 ℃, ferrite grains are formed along the elongated unrecrystallized austenite grain boundaries due to the temperature decrease during rolling and the unrecrystallized austenite remains, and the unrecrystallized austenite in the grains becomes a pearlite structure. In this case, the size of ferrite grains increases, and when Mn is concentrated in the pearlite structure, the average spacing of Mn concentration portions will exceed 10.0 μm.

On the other hand, if the rolling start temperature exceeds 1100 ℃, the temperature in the middle of rolling reaches a high temperature, and the alloy element that suppresses ferrite transformation becomes easy to concentrate at the austenite grain boundaries. In this case, the transformation of ferrite is delayed during the cooling after finish rolling, the proportion of pearlite structure increases, and the concentration of Mn in pearlite structure to cementite is insufficient. In this case, a region having an Mn content of 1.1× [ Mnave ] or more cannot be obtained sufficiently at the end.

If the rolling end temperature is lower than 800 ℃, unrecrystallized austenite remains, and as a result, the average interval of the regions of 1.1× [ Mnave ] increases.

If the rolling completion temperature exceeds 950 ℃, ferrite transformation is excessively suppressed, and the interval of cementite enriched with Mn is increased or the number density is decreased, so that the average interval and the number density of the region having a final Mn content of 1.1× [ Mnave ] or more do not fall within the preferable ranges.

Therefore, the rolling start temperature is set to 950 to 1100 ℃ and the rolling end temperature is set to 800 to 950 ℃.

[ Cooling step after Hot Rolling ]

[ Winding Process ]

In the post-hot rolling cooling step, the hot-rolled steel sheet obtained in the hot rolling step is cooled from the end of the hot rolling step to a coiling temperature of at least 400 ℃ and less than 550 ℃ at an average cooling rate of at least 20 ℃/sec and at most 50 ℃/sec within less than 1.0 second. In the coiling step, the hot-rolled steel sheet after the cooling step after hot rolling is coiled at the coiling temperature.

By performing these steps under predetermined conditions, the pearlite structure is finely dispersed during cooling, and cementite in the pearlite structure is also finely dispersed. Further, mn is concentrated in cementite after coiling.

When the time from the end of hot rolling to the start of cooling is 1.0 seconds or more, the average cooling rate is less than 20 ℃/sec, or the cooling stop temperature (coiling temperature) is 550 ℃ or more, ferrite grains excessively grow, and the dispersion interval between pearlite structures increases, so that the average interval of cementite enriched with Mn after coiling increases, which is not preferable.

On the other hand, if the average cooling rate exceeds 50 ℃/sec, a hardening phase such as bainite or martensite is easily formed, and the proportion of pearlite structure is reduced, so that cementite enriched in Mn cannot be sufficiently obtained.

Further, if the cooling stop temperature is lower than 400 ℃, the concentration of Mn in the pearlite structure to cementite may not be sufficiently generated.

[ Cold Rolling Process ]

In the cold rolling step, the hot-rolled steel sheet after the coiling step is unwound, and subjected to pickling and cold rolling to obtain a cold-rolled steel sheet.

The pickling removes the scale on the surface of the hot-rolled steel sheet, thereby improving the chemical conversion treatability and the plating property of the cold-rolled steel sheet. The acid washing may be performed under known conditions, and may be performed once or in a plurality of times. The rolling reduction of the cold rolling is not particularly limited, and is, for example, 20 to 80%.

[ Annealing Process ]

[ Post annealing Cooling step ]

In the annealing step, the cold-rolled steel sheet after the cold rolling step is heated to an annealing temperature at which the average temperature rise rate of room temperature (for example, 25 ℃) to 700 ℃ is 15 to 100 ℃/sec, and the average temperature rise rate of the annealing temperature from 700 ℃ to 830 ℃ and lower than 900 ℃ is 5 ℃/sec or higher and lower than 15 ℃/sec, and is kept at the annealing temperature for 25 to 100 seconds, thereby annealing. The average temperature rise rate to 700℃is preferably 17℃per second or more, more preferably 20℃per second or more. The average temperature rise rate of the annealing temperature from 700 ℃ to 830 ℃ and lower than 900 ℃ is preferably 6 ℃ to 14 ℃ per second, more preferably 7 ℃ to 13 ℃ per second.

Then, in the post-annealing cooling step, the cold-rolled steel sheet after the annealing step is cooled to 25 to 300 ℃ at an average cooling rate of 4 to 100 ℃/sec.

In these steps, the martensite-based structure in which the Mn-concentrated portion is dispersed can be obtained by heating to the austenite single-phase region to melt cementite while maintaining the dispersion state of the Mn concentrated in cementite, and then cooling to 300 ℃.

During the temperature rise in the annealing step, cementite coarsens if the average temperature rise rate from room temperature (e.g., 25 ℃) to 700 ℃ is less than 15 ℃/sec. The cementite size may be increased by the same size as the Mn enriched portion, and the average interval between the annealed regions of 1.1× [ Mnave ] or more may be increased, and the number density may be decreased. On the other hand, if the average temperature rise rate exceeds 100 ℃/sec, special equipment is required, and practical costs are increased.

In addition, in a temperature region from 700 ℃ to an annealing temperature (830 ℃ or more and less than 900 ℃), diffusion of Mn can be suppressed by promoting recrystallization and reducing the densities of grain boundaries and dislocations. If the average temperature rise rate in this temperature range is less than 5 ℃/sec, mn diffuses, and the dispersion state of the Mn-rich portion may be released. On the other hand, if the average temperature rise rate is 15 ℃ per second or more, recrystallization does not occur, mn is diffused by grain boundaries and dislocations, and the dispersed state of the Mn-rich portion may be released.

If the annealing temperature is lower than 830 ℃, the austenite reverse transformation is not finished, so that the volume ratio of the ferrite structure is increased, and sometimes the strength does not reach the standard. This is because cementite enriched in Mn has high thermal stability and is likely to remain unmelted, so that a ferrite structure remains. On the other hand, if the annealing temperature is 900 ℃ or higher, mn diffuses, and preferably the dispersed state of the dispersed Mn-concentrated portion is released.

If the annealing time (holding time) is less than 25 seconds, austenitization is insufficient, and sometimes the target structure cannot be obtained. On the other hand, if the annealing time exceeds 100 seconds, mn diffuses, and the dispersion state of the Mn-rich portion may be released.

In the post-annealing cooling step, if the average cooling rate to the cooling stop temperature is less than 4 ℃/sec, a structure such as ferrite or bainite is formed, and the target structure cannot be obtained. On the other hand, if the average cooling rate exceeds 100 ℃/sec, special equipment is required, and practical costs are increased.

If the cooling stop temperature exceeds 300 ℃, a bainite structure is formed, and the target structure cannot be obtained. On the other hand, when the cooling stop temperature is set to less than 25 ℃, a special cooling medium or the like needs to be used, which causes problems in cost and productivity.

[ Tempering step ]

In the method for manufacturing a steel sheet according to the present embodiment, a tempering step of raising the temperature of the cold-rolled steel sheet after the annealing step to 50 ℃ or higher and lower than 500 ℃ and holding the cold-rolled steel sheet for 5 seconds or higher and lower than 1000 seconds may be further performed.

By tempering under the above conditions, the martensite becomes tempered martensite, and the formability can be improved.

If the tempering temperature (holding temperature) is lower than 50 ℃ or the holding time is lower than 5 seconds, the above-mentioned effect is not obtained. On the other hand, if the tempering temperature is 500 ℃ or higher, there is a concern that the Mn diffusion and the dispersion state of the Mn-concentrated portion are released. Further, there is a case where a decrease in tensile strength is caused by a decrease in dislocation density in tempered martensite. Further, if the holding time is 1000 seconds or more, the strength is lowered and the productivity is lowered. Tempering may be performed in a continuous annealing facility or may be performed off-line after continuous annealing by another facility.

In the method for producing a steel sheet according to the present embodiment, a coating layer containing zinc, aluminum, magnesium, or an alloy thereof may be formed on the surface of the steel sheet in any one of the steps from the post-annealing cooling step to the tempering step.

As the coating layer, a coating layer containing zinc, aluminum, magnesium, or an alloy thereof is preferable. The coating layer is, for example, a plating layer.

The coating method is not limited, and for example, when a coating layer mainly composed of zinc is formed by hot dip plating, a condition in which a cold-rolled steel sheet is adjusted (by heating or cooling) until the temperature of the steel sheet reaches (bath temperature-40) to (bath temperature +50) c, and then immersed in a bath at 450 to 490 c to form a coating layer can be exemplified.

This condition is preferable because if the steel sheet temperature at the time of immersion in the plating bath is lower than the hot dip galvanization bath temperature of-40 ℃, the heat release at the time of immersion in the plating bath increases, and a part of the hot dip galvanization solidifies, so that the appearance of the plated layer may be deteriorated, and if the hot dip galvanization bath temperature exceeds +50℃, there is a possibility that an operational problem is induced that the plating bath temperature increases.

When forming a zinc-based plating layer, the composition of the plating bath is preferably: the effective Al content (the value obtained by subtracting the total Fe content from the total Al content in the plating bath) is 0.050 to 0.250 mass%, and if necessary, mg is contained, and the balance is Zn and impurities. If the effective Al amount in the plating bath is less than 0.050 mass%, the penetration of Fe into the plating layer is excessively performed, and there is a concern that the plating layer adhesion is lowered. On the other hand, if the effective Al amount in the plating bath exceeds 0.250 mass%, al-based oxides that inhibit movement of Fe atoms and Zn atoms may be generated at the boundary between the steel sheet and the plating layer, and the adhesion of the plating layer may be lowered.

Examples

Example 1

Steels having chemical compositions shown in tables 1-1 to 1-2 were melted and cast into billets. The slab was inserted into a furnace heated to 1200 ℃ and, after being subjected to homogenization treatment for 60 minutes, was taken out in the atmosphere, and hot-rolled to obtain a steel sheet having a sheet thickness of 2.8 mm. In the hot rolling, a rolling mill having 7 rolling stands was used, and all of the 7 passes of finish rolling was continuously performed (in a manner that the inter-pass time was fixed), wherein the rolling passes having a reduction ratio exceeding 20% were performed 4 times. In finish rolling, the inter-pass time between each rolling pass to which a reduction of 20% or more is applied and the rolling pass before 1 pass among the rolling passes is set to 0.6 seconds. The finish rolling was started at 1070℃and ended at 850℃and after 0.8 seconds from the finish rolling, the product was cooled by water cooling to 530℃at an average cooling rate of 29.0℃per second, and then wound. However, with AH-0, the steel sheet is embrittled and cracked during hot rolling, and thus the subsequent steps are not performed.

Further, the scale on the hot-rolled steel sheet was removed by pickling, and then cold rolling was performed to a reduction of 50.0%, thereby finishing the sheet thickness to 1.4mm.

The cold-rolled steel sheet was heated from room temperature to 700 ℃ at an average heating rate of 35.0 ℃/sec, and heated from 700 ℃ to 860 ℃ at an average heating rate of 10 ℃/sec. After 80 seconds of holding at 860 ℃, the cooling was carried out to 190 ℃ at an average cooling rate of 38.0 ℃/sec.

Continuing, reheating to 230 ℃ and tempering is performed for 180 seconds. No plating treatment was performed.

The obtained cold-rolled steel sheet was subjected to microstructure observation in the above-described manner to determine the area ratio of each phase in the t/4 section. In the t/4 section, the number density of the region having an Mn content of 1.1× [ Mnave ] or more and the average distance from the nearest other region were measured.

The results are shown in Table 2.

Further, samples collected from the produced steel sheets were analyzed, and the chemical compositions thereof were the same as those of the steels shown in tables 1-1 to 1-2.

Further, the obtained cold-rolled steel sheet was evaluated for elongation properties and hydrogen embrittlement resistance (hydrogen embrittlement resistance) in the following manner.

(Evaluation method of tensile Property)

The tensile test was performed by collecting a test piece of JIS No. 5 from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the steel strip in accordance with JIS Z2241 (2011), and measuring the Tensile Strength (TS) and the total elongation (El).

(Evaluation method of Hydrogen embrittlement resistance)

The hydrogen embrittlement resistance of the steel sheet produced by the method for producing a steel sheet according to the embodiment of the present invention was evaluated by the following method. Specifically, after the steel sheet was cut at a clearance of 10%, a U-bend test was performed at 10R. A strain gauge was attached to the center of the obtained test piece, and both ends of the test piece were fastened with bolts, thereby applying stress. The applied stress may be calculated from the strain of the strain gauge being monitored. The load stress applies a stress corresponding to 80% of the Tensile Strength (TS) (e.g., at a-0 of table 2, the applied stress=2213 mpa×0.8=1770 MPa). This is because it is considered that the residual stress introduced during forming corresponds to the tensile strength of the steel sheet.

The obtained U-shaped bending test piece was immersed in an aqueous HCl solution at a liquid temperature of 25℃and a pH of 2, and kept for 48 hours, and the presence or absence of cracks was examined. The lower the pH of the aqueous HCl solution and the longer the immersion time, the greater the amount of hydrogen that intrudes into the steel sheet, and therefore the hydrogen embrittlement environment is a severe condition.

After dipping, the U-shaped bending test piece was evaluated, and the case where a crack exceeding 1.00mm in length was observed was evaluated as NG, and the case where a crack exceeding 1.00mm in length was not observed was evaluated as OK.

A steel sheet having a tensile strength of 1500MPa or more and excellent hydrogen embrittlement resistance was evaluated as a steel sheet having high strength and excellent hydrogen embrittlement resistance, when the evaluation was OK.

TABLE 1-1

TABLE 1

TABLE 2

As is clear from tables 1-1 to 2, in Nos. A-0 to O-0, the number density of the regions having a chemical composition, a microstructure area ratio, and a Mn content of 1.1X Mnave or more, and the average distance from the nearest other regions were within the range of the present invention, and the tensile strength and hydrogen embrittlement resistance were excellent.

In contrast, in the case of No. P-0 to AN-0, the chemical composition was outside the range of the present invention, and therefore, 1 or more of the tensile strength and hydrogen embrittlement resistance were poor.

P-0 has a tensile strength of less than 1500MPa due to the low C content.

Q-0 has a high C content, and therefore, decreases hydrogen embrittlement resistance.

R-0 has a high Si content, and therefore, decreases hydrogen embrittlement resistance.

S-0 has a tensile strength of less than 1500MPa due to its low Mn content.

T-0 has a high Mn content, and thus deteriorates hydrogen embrittlement resistance.

U-0 has a high P content, and therefore, the hydrogen embrittlement resistance is reduced by grain boundary embrittlement.

V-0 has a high S content, and thus, decreases hydrogen embrittlement resistance.

W-0 has a high Al content, and thus generates coarse Al oxides, which reduces hydrogen embrittlement resistance.

X-0 has a high N content, and thus generates coarse nitrides, which reduces hydrogen embrittlement resistance.

Y-0 forms an oxide due to its high O content, and decreases hydrogen embrittlement resistance.

Since Z-0 has a high Co content, coarse Co carbides are precipitated, and hydrogen embrittlement resistance is reduced.

AA-0 has a high Ni content, and therefore, decreases hydrogen embrittlement resistance.

AB-0 has a high Mo content, and therefore, coarse Mo carbide crystals are precipitated, and hydrogen embrittlement resistance is reduced.

Since AC-0 has a high Cr content, coarse Cr carbide is precipitated, and hydrogen embrittlement resistance is reduced.

AD-0 has a high Ti content, and therefore, increases the precipitation of carbonitrides and decreases hydrogen embrittlement resistance.

AE-0 has a high B content, and thus coarse B oxides are formed in steel, and hydrogen embrittlement resistance is reduced.

AF-0 has a high Nb content, and thus generates coarse Nb carbides, which reduces hydrogen embrittlement resistance.

AG-0 has a high V content, and therefore, increases the precipitation of carbonitrides and decreases hydrogen embrittlement resistance.

Since AH-0 has a high Cu content, the steel sheet is embrittled during hot rolling to generate cracks, and no subsequent evaluation thereof is performed.

AI-0 has a high W content, and thus coarse W precipitates are formed, which reduces hydrogen embrittlement resistance.

AJ-0 and AK-0 have high Mg and Ca contents, and thus form coarse inclusions, which reduce hydrogen embrittlement resistance.

Since AL-0 has a high Zr content, coarse Zr oxide is formed, and hydrogen embrittlement resistance is reduced.

AM-0 and AN-0 have high contents of Sn and Sb, respectively, and therefore, the hydrogen embrittlement resistance is reduced by grain boundary segregation.

Example 2

In order to examine the influence of the production conditions, steel types a to O having excellent characteristics as shown in table 2 were subjected to cold rolling to form a cold rolled steel sheet having a sheet thickness of 2.3mm under the production conditions shown in table 3-1 in the same equipment as in example 1, and after the cold rolled steel sheet was subjected to cold rolling to a reduction of 55%, annealing and cooling were performed under the conditions shown in tables 3-2 to 3-3, and tempering was performed as needed. Further, a part of the cold-rolled steel sheet was plated, and a zinc plating layer was formed on the surface. Wherein, symbols GI and GA of plating species of tables 3 to 3 represent a method of zinc plating treatment, GI is a steel sheet in which a zinc plating layer is formed on the surface of a steel sheet by immersing the steel sheet in a hot dip galvanizing bath at 465 ℃, and GA is a steel sheet in which an alloy layer of iron and zinc is formed on the surface of a steel sheet by heating the steel sheet to 490 ℃ after immersing the steel sheet in a hot dip galvanizing bath at 465 ℃. In tables 3 to 3, the examples in which tempering was indicated as "-" are examples in which tempering was not performed.

The obtained cold-rolled steel sheet was subjected to microstructure observation in the same manner as in example 1 to determine the area ratio of each phase in the microstructure of the t/4 portion. In the t/4 section, the number density of the regions having a Mn content of 1.1× [ Mnave ] or more and the average distance from the nearest other regions having a Mn content of 1.1× [ Mnave ] or more were measured.

The obtained cold-rolled steel sheet was evaluated for tensile properties in the same manner as in example 1.

The hydrogen embrittlement resistance (hydrogen embrittlement resistance) was evaluated in the following manner.

(Evaluation method of Hydrogen embrittlement resistance)

After the steel sheet was cut at a clearance of 10%, a U-bend test was performed at 10R. A strain gauge was attached to the center of the obtained test piece, and both ends of the test piece were fastened with bolts, thereby applying stress. The applied stress may be calculated from the strain of the strain gauge being monitored. The load stress applies a stress corresponding to 80% of the Tensile Strength (TS) (e.g., at a-1 of table 4, the applied stress=2101 mpa×0.8=1681 MPa). This is because it is considered that the residual stress introduced during forming corresponds to the tensile strength of the steel sheet.

The obtained U-shaped bending test piece was immersed in an aqueous HCl solution at a liquid temperature of 25℃and a pH of 2, and kept for 96 hours, and the presence or absence of cracks was examined. The lower the pH of the aqueous HCl solution and the longer the immersion time, the greater the amount of hydrogen that intrudes into the steel sheet, and therefore the hydrogen embrittlement environment becomes a severe condition. After dipping, the total length of the cracks of the U-bend test piece (when a plurality of cracks can be seen, as the sum of the values measured one by one) was measured.

The smaller the total length of the crack, the more excellent the hydrogen embrittlement resistance, and in particular, the case where a crack exceeding 1.00mm in length was observed was evaluated as NG, and the case where no crack or a slight crack having a crack length of 1.00mm or less was observed was evaluated as OK. The case evaluated as OK was regarded as pass, and the case evaluated as NG was regarded as fail. The crack length of 0.50mm or less was considered to be particularly excellent in hydrogen embrittlement resistance.

The results obtained are shown in Table 4.

TABLE 3-

TABLE 33- -2

TABLE 3-3

TABLE 4 Table 4

As is clear from tables 3-1 to 3-3 and table 4, in all the examples according to the present invention, steel sheets having high strength and excellent hydrogen embrittlement resistance can be obtained by appropriately controlling the conditions of hot rolling, coiling, annealing, and cooling after annealing.

On the other hand, A-2 has a low hot rolling start temperature, and therefore, the temperature is lowered during rolling, and austenite which has not been recrystallized remains in a large amount, and as a result, the average interval between the regions of 1.1× [ Mnave ] is increased, and the hydrogen embrittlement resistance is improved.

Since B-2 has a high hot rolling start temperature, the alloying element segregates to austenite grain boundaries during rolling, ferrite transformation is delayed, and as a result, the number density of the region of 1.1 x Mnave is reduced, and the hydrogen embrittlement resistance is deteriorated.

C2 forms unrecrystallized austenite due to a low hot rolling end temperature, and as a result, the average interval of the regions of 1.1 x Mnave increases, and the hydrogen embrittlement resistance is deteriorated.

D-2 excessively suppresses ferrite transformation due to a high hot rolling end temperature, and as a result, the number density of the region of 1.1× [ Mnave ] is reduced, and hydrogen embrittlement resistance is deteriorated.

E-2 has a long inter-pass time of hot rolling, and therefore, the unrecrystallized austenite remains in a large amount, and as a result, the average interval between the regions of 1.1× [ Mnave ] increases, and the hydrogen embrittlement resistance is deteriorated.

F-2 has a small number of passes at a reduction ratio of 20% or more, and as a result, the average interval between the regions of 1.1× [ Mnave ] increases, and the hydrogen embrittlement resistance is improved.

H-2 excessively causes transformation of the pig iron body because of a long time from finish rolling to start of cooling, and the average interval of the region of 1.1× [ Mnave ] increases, and is resistant to deterioration of hydrogen embrittlement.

I-2 causes excessive transformation of the pig iron body due to a low cooling rate after completion of hot rolling, and increases the average interval of the region of 1.1 x [ Mnave ], thereby resisting deterioration of hydrogen embrittlement.

J-2 has a high cooling rate after finish rolling, and therefore, a bainite and martensite structure are formed in a large amount, and as a result, the number density of the region of 1.1 x [ Mnave ] is reduced, and the hydrogen embrittlement resistance is improved.

K-2 has a low coiling temperature, and therefore, a bainite and martensite structure is formed, and as a result, the number density of the region of 1.1 x Mnave is reduced, and the hydrogen embrittlement resistance is deteriorated.

Since the L-2 is high in coiling temperature, the ratio of ferrite structure increases, and as a result, the average interval of the region of 1.1× [ Mnave ] increases, and the hydrogen embrittlement resistance is deteriorated.

Since M-2 and N-3 have a low average temperature rise rate to 700 ℃, cementite coarsens during the temperature rise, and the average interval between the regions of 1.1× [ Mnave ] increases. In N-3, the number density of the region 1.1× [ Mnave ] was reduced. As a result, the hydrogen embrittlement resistance is deteriorated.

Since O-2 and N-4 have a low heating rate up to the highest heating temperature (annealing temperature) in the annealing step, diffusion of Mn occurs, and the number density of the region of 1.1× [ Mnave ] is reduced, and the hydrogen embrittlement resistance is deteriorated.

A-3 and N-5 have a high heating rate up to the highest heating temperature in the annealing step, and therefore, recrystallization is not completed, and the Mn enrichment is released by dislocation and grain boundaries, resulting in a reduction in the number density of the region satisfying 1.1 x [ Mnave ]. In addition, in N-5, the average interval of the regions of 1.1× [ Mnave ] increases. As a result, the hydrogen embrittlement resistance is reduced.

B-3 has a low maximum heating temperature in the annealing step, and therefore the ratio of the ferrite structure is higher than 5%, resulting in deterioration of hydrogen embrittlement resistance. Further, since the ferrite structure is high in proportion, the tensile strength is lower than 1500MPa, and the hydrogen embrittlement resistance is also deteriorated.

C-3, because the highest heating temperature in the annealing step is too high, releases the Mn enrichment, and as a result, the area satisfying 1.1 x Mnave is reduced, and the hydrogen embrittlement resistance is reduced.

D-3 has a short holding time at the highest heating temperature in the annealing step, and therefore, the martensite structure ratio is less than 90% and the tensile strength is less than 1500MPa.

E-3 is a material that has a long holding time at the highest heating temperature in the annealing step, so that Mn diffuses and the dispersion state of Mn-rich portions is released, and the area satisfying 1.1 x [ Mnave ] is reduced, and hydrogen embrittlement resistance is reduced.

F-3 has a tensile strength of less than 1500MPa because ferrite and bainite transformation occurs due to a too slow cooling rate from the highest heating temperature in the annealing step.

I-3 has a high tempering temperature, so that the dislocation density in the martensitic structure is reduced, and the tensile strength is lower than 1500MPa.

K-3 has a higher cooling stop temperature, so that bainite transformation occurs, and the tensile strength is lower than 1500MPa.

FIG. 1 is a graph showing the effects of the average interval between the regions of 1.1× [ Mnave ] and the number density of the regions of 1.1× [ Mnave ] on the hydrogen embrittlement resistance of the steel sheets of examples 1 and 2. In the figure, a steel sheet excellent in hydrogen embrittlement resistance is shown, and x in the figure shows an example of poor hydrogen embrittlement resistance. As is evident from fig. 1: a steel sheet excellent in hydrogen embrittlement resistance can be obtained by controlling the average interval of the regions of 1.1 x [ Mnave ] to 10.0 [ mu ] m or less and the number density of the regions of 1.1 x [ Mnave ] to 5.0 x 10 ^-4/mu ] m ² or more.

Industrial applicability

According to the present invention, a steel sheet having high strength and excellent hydrogen embrittlement resistance can be provided. When used as a steel sheet for automobiles, the steel sheet contributes to improvement of fuel efficiency by reducing the weight of a vehicle body.

Claims

1. A steel sheet, characterized in that,

The chemical composition comprises the following components in mass percent: c:0.150 to 0.400 percent,

Si：0.01～2.00％、

Mn：0.8～2.0％、

P：0.0001～0.0200％、

S：0.0001～0.0200％、

Al：0.001～1.000％、

N：0.0001～0.0200％、

O：0.0001～0.0200％、

Co：0～0.500％、

Ni：0～1.000％、

Mo：0～1.000％、

Cr：0～2.000％、

Ti：0～0.500％、

B：0～0.0100％、

Nb：0～0.500％、

V：0～0.500％、

Cu：0～0.500％、

W：0～0.100％、

Ta：0～0.100％、

Mg：0～0.050％、

Ca：0～0.050％、

Y：0～0.050％、

Zr：0～0.050％、

La：0～0.050％、

Ce：0～0.050％、

Sn：0～0.050％、

Sb：0～0.050％、

As:0 to 0.050%, and

The remainder: fe and impurities;

The microstructure comprises in area ratio:

Ferrite: less than 5.0 percent,

Martensite and tempered martensite: total more than 90.0%, and

The remainder: 1 or 2 or more of bainite, pearlite, and retained austenite;

In the cross section in the plate thickness direction, when the average Mn content in the whole plate thickness direction is [ Mnave ], there is a region with Mn content of 1.1 x [ Mnave ] or more, the number density of the region is 5.0 x 10 ^-4 pieces/μm ² or more, and the interval between the region and the nearest region with other Mn content of 1.1 x [ Mnave ] or more is 10.0 μm or less on average;

The tensile strength is more than 1500 MPa.

2. The steel sheet according to claim 1, wherein the chemical composition contains 1 or 2 or more elements selected from the group consisting of:

Co：0.01～0.500％、

Ni：0.01～1.000％、

Mo：0.01～1.000％、

Cr：0.001～2.000％、

Ti：0.001～0.500％、

B：0.0001～0.0100％、

Nb：0.001～0.500％、

V：0.001～0.500％、

Cu：0.001～0.500％、

W：0.001～0.100％、

Ta：0.001～0.100％、

Mg：0.0001～0.050％、

Ca：0.001～0.050％、

Y：0.001～0.050％、

Zr：0.001～0.050％、

La：0.001～0.050％、

Ce：0.001～0.050％、

Sn：0.001～0.050％、

sb:0.001 to 0.050%, and

As：0.001～0.050％。

3. The steel sheet according to claim 1 or 2, wherein the surface has a coating layer comprising zinc, aluminum, magnesium or an alloy thereof.