JP7111281B1 - Steel plate, member and manufacturing method thereof - Google Patents

Abstract

To provide a high-strength steel sheet excellent in formability, delayed fracture resistance and LME resistance, and a method for producing the same. % by mass, C: 0.24% or more and 0.40% or less, Si: 0.2% or more and 1.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.50% or less (not including 0%), N: 0.0006% or more and 0.01% or less, and the balance is Fe and inevitable impurities. , martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, retained austenite: 2% or more, and the average grain size of carbide in tempered martensite among martensite is 0.40 μm or less, the average C content in retained austenite is 0.5% by mass or more, the Si concentration from the steel sheet surface to 100 μm in the thickness direction is 1.3% by mass or less, and the tensile strength is 1470 MPa or more. A steel plate.

Description

TECHNICAL FIELD The present invention relates to a high-strength steel sheet suitable for cold press-forming used in automobiles, home electric appliances, etc. through a cold-press-forming process, members made from the high-strength steel sheet, and methods for producing the same.

In recent years, due to the further increase in the need for weight reduction of automobile bodies, tensile strength TS (hereinafter simply referred to as TS) for body frame parts such as center pillar R / F (reinforcement), bumpers, impact beam parts, etc. There is also): The application of high-strength steel sheets of 1320-1470 MPa class is progressing. In addition, from the viewpoint of further weight reduction, investigations of increasing the strength to 1.8 GPa class or higher are being started. In the past, hot pressing was vigorously studied to increase strength, but recently, from the viewpoint of cost and productivity, the application of high-strength steel in cold pressing is being reconsidered.

However, when trying to form a high-strength steel sheet of TS: 1470 MPa class or higher by cold pressing, press cracking is likely to occur due to the decrease in ductility, and it is limited to parts with relatively simple shapes. . Therefore, high-strength steel sheets are required to have better formability than conventional ones. In addition, when a high-strength steel sheet of TS: 1470 MPa class or higher is formed into a part by cold pressing, delayed fracture becomes apparent due to an increase in residual stress in the part and deterioration of delayed fracture resistance due to the material itself. Delayed fracture means that when a part is placed under a hydrogen penetration environment with high stress applied to the part, hydrogen penetrates into the steel sheet, reducing the interatomic bonding strength and causing local deformation. This is a phenomenon in which microcracks are generated as a result, and breakage occurs as the cracks grow.

More recently, when assembling automobile bodies and parts, high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets are spot-welded, and high-strength cold-rolled steel sheets and galvanized steel sheets are spot-welded. It has been confirmed that when welding is performed, liquid metal embrittlement cracking (LMEC: Liquid Metal Embrittlement Cracking, hereinafter also referred to as LME cracking) occurs at the welded portion. LME cracks are cracks that occur when the zinc in the galvanized layer melts during spot welding, the molten zinc penetrates the grain boundaries of the steel structure of the weld, and the stress that occurs when the welding electrode is released acts. be. Even with high-strength cold-rolled steel sheets that are not galvanized, LME cracks may occur when spot welding is performed with galvanized steel sheets due to contact of molten zinc in the galvanized steel sheets with the high-strength cold-rolled steel sheets. be. Since high-strength steel sheets with a TS of 780 MPa or more have a high Si content, there is concern about the risk of LME cracking.

In addition, various high-strength steel sheets have been developed for the purpose of application to automobile parts. For example, Patent Literature 1 discloses a high-strength steel sheet having an excellent steel shape by specifying alloy components and having bainite and tempered martensite as the main phases in the steel structure.

Patent Document 2 discloses a high-strength steel sheet having excellent formability.

Patent Literature 3 discloses a high-strength steel sheet that has both high formability and hydrogen embrittlement resistance at high levels by using ferrite and bainite as main phases in the surface layer structure.

Japanese Patent No. 6291289 Japanese Patent No. 6288394 Japanese Patent No. 6635236

However, in the technique of Patent Document 1, the amount of C is low, and there is a possibility that the strength of the TS: 1470 MPa class cannot be obtained. In addition, since the amount of retained austenite is small and the average amount of C in the retained austenite is low, ductility may also deteriorate.
In the technique of Patent Document 2, the cooling rate is low and contains self-tempered martensite that is tempered at high temperature, so there is a risk that the carbides in the self-tempered martensite will coarsen and deteriorate the delayed fracture resistance. Furthermore, the tempering temperature of martensite is high, and there is a possibility that the strength of TS: 1470 MPa class cannot be obtained. In addition, the carbides in the tempered martensite may coarsen and degrade the delayed fracture resistance.
In the technique of Patent Document 3, the martensite in the surface layer is as small as 30% or less, and this martensite is presumed to be hard martensite in which carbon is concentrated to γ during ferrite transformation. Therefore, this martensite tends to become a starting point of cracks, and there is a possibility that the delayed fracture resistance deteriorates.

The present invention has been made to solve such problems, and has a tensile strength of 1470 MPa or more (TS ≥ 1470 MPa), excellent formability, excellent delayed fracture resistance, and LME resistance. An object of the present invention is to provide an excellent steel plate, member, and manufacturing method thereof.

The tensile strength is the tensile strength TS obtained by cutting out a JIS No. 5 tensile test piece so that the rolling direction is the longitudinal direction and conducting a tensile test based on JIS Z2241.

Further, excellent formability means that the elongation El obtained by cutting out a JIS No. 5 tensile test piece so that the rolling direction is the longitudinal direction and performing a tensile test based on JIS Z2241 is 11% or more.

In addition, "excellent in delayed fracture resistance" means that there is no fracture according to the following evaluation.
(1) A strip test piece of 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction is taken from the 1/4 position of the coil width of the steel plate.
(2) Cutting out the end face of 100 mm length is shearing, and in the state of shearing (without machining to remove burrs), bend so that burrs are on the outer peripheral side of the bend, and Fix the test piece with bolts while maintaining the shape of the test piece when it is bent.
The shearing clearance is 15% and the rake angle is 0 degrees.
Bending is performed with a bending radius where R/t=4 when the tip bending radius is R and the plate thickness is t (for example, if the plate thickness is t: 2.0 mm, the tip is bent with a punch with a punch tip radius of 8.0 mm). Apply so that the internal angle is 90 degrees (V bend).
The punch used has a U-shaped tip with the radius described above (the tip R is semicircular and the thickness of the punch body is 2R), and the corner radius of the die is 30 mm.
Adjust the depth of the punch to press the steel plate, and form so that the bending angle of the tip is 90 degrees (V shape).
For bolt tightening, the test piece is held with a hydraulic jack so that the distance between the flange ends of the straight piece during bending is the same as when bending (to cancel out the opening of the straight piece due to springback). and tighten the bolts in that state.
The bolt is passed through an elliptical hole (minor axis: 10 mm, major axis: 15 mm) previously provided 10 mm inward from the short side edge of the strip test piece and fixed.
(3) Immerse the obtained bolted test piece in 1 L or more of hydrochloric acid (hydrogen chloride aqueous solution) with a pH of 3 per piece, and conduct the test while the pH is constantly controlled at an aqueous solution temperature of 25 ° C. .
Visually or with a camera, always check for the presence or absence of microcracks (initial state of delayed fracture) at a level that can be visually confirmed (1 mm or more in length), and the time from the start of immersion to the start of microcracks is defined as the delayed fracture time. Measure.
(4) Destruction after 10 ^{(-0.0055 x (TS-1760) + 0.3)} hours (10 (-0.0055 x (TS-1760) + 0.3) time) after the start of immersion If not, it is determined that there is no destruction.

Moreover, excellent LME resistance means that no cracks of 0.1 mm or more are found in the following resistance weld cracking test.
(1) Using one test piece cut to 30 mm × 100 mm with the direction perpendicular to the rolling direction of the steel plate as the longitudinal direction, the other is a 980 MPa class hot-dip galvanized steel plate and resistance welding (spot welding) is performed. do.
The welding machine uses a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type attached to a welding gun for a plate assembly in which two steel plates are stacked, with the plate assembly tilted at 5 °. Carry out welding.
Welding conditions are an applied pressure of 3.8 kN and a hold time of 0.2 seconds. Also, the welding current is 5.7 to 6.2 kA, the welding time is 21 cycles, and the hold time is 5 cycles.
(2) After welding, cut the test piece in half and observe the cross section with an optical microscope.
(3) If no cracks of 0.1 mm or more are observed, the LME resistance is considered to be excellent.

The present inventors have made sincere studies to solve the above problems, and have obtained the following findings.
i) In addition to using martensite to increase the strength, the C content is increased to 0.24% by mass or more, and the bainite transformation is used to ensure retained austenite and have excellent formability. It was found that a high-strength steel sheet having a tensile strength of 1470 MPa or more (TS≧1470 MPa) can be obtained.
ii) Among martensite, it was found that by suppressing the coarsening of carbides in the tempered martensite, the delayed fracture resistance of a high-strength steel sheet with TS≧1470 MPa can be improved.
iii) In order to appropriately control the grain size of carbides, it was found necessary to optimize the contents of C, Si, and Mn, the cooling rate after bainite transformation, and the tempering temperature.
iv) By suppressing the concentration of Si in the vicinity of the surface layer, liquid metal embrittlement is less likely to occur, and as a result, excellent resistance weldability (cracks are less likely to occur during resistance welding).

The present invention has been made based on the above findings, and specifically provides the following.
[1] % by mass,
C: 0.24% or more and 0.40% or less,
Si: 0.2% or more and 1.0% or less,
Mn: 1.5% or more and 3.5% or less,
P: 0.002% or more and 0.010% or less,
S: 0.0002% or more and 0.0020% or less,
sol. Al: 0.50% or less (not including 0%),
N: 0.0006% or more and 0.01% or less, and the balance has a component composition consisting of Fe and inevitable impurities,
Having a steel structure containing, in terms of area ratio, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and retained austenite: 2% or more,
Among the martensite, the average grain size of carbide in the tempered martensite is 0.40 μm or less,
The average amount of C in the retained austenite is 0.5% by mass or more,
The Si concentration from the steel plate surface to 100 μm in the plate thickness direction is 1.3% by mass or less,
A steel plate having a tensile strength of 1470 MPa or more.
[2] The steel sheet according to [1], wherein a difference in microhardness between the bainite and the martensite is 1.5 GPa or more.
[3] As the component composition, further in mass%,
Nb: 0.1% or less,
Ti: 0.10% or less,
B: 0.0050% or less,
The steel sheet according to [1] or [2] above, containing one or more selected from Cu: 1% or less and Ni: 1% or less.
[4] As the component composition, further in mass%,
Cr: 1.0% or less,
Mo: less than 0.3%,
V: 0.45% or less,
The steel sheet according to any one of [1] to [3] above, containing one or more selected from Zr: 0.2% or less and W: 0.2% or less.
[5] As the component composition, further by mass%,
The steel sheet according to any one of [1] to [4] above, containing one or two selected from Sb: 0.1% or less and Sn: 0.1% or less.
[6] As the component composition, further in mass%,
Ca: 0.0050% or less,
Mg: 0.01% or less,
REM: The steel sheet according to any one of [1] to [5], containing one or more selected from 0.01% or less.
[7] The steel sheet according to any one of [1] to [6], which has a plating layer on the surface of the steel sheet.
[8] A member obtained by subjecting the steel plate according to any one of [1] to [7] to at least one of forming and joining.
[9] A steel plate manufacturing method for manufacturing the steel plate according to any one of [1] to [6],
A hot rolling step of hot rolling a steel slab to obtain a hot rolled steel sheet;
After the hot-rolling step, a cold-rolling step of cold-rolling the hot-rolled steel plate to obtain a cold-rolled steel plate;
After the cold rolling step, the cold-rolled steel sheet is annealed at an annealing temperature of Ac ₃ or higher for a soaking time of 15 seconds or longer in an atmosphere with a dew point of -60 ° C or higher and -20 ° C or lower,
Cooling at a first average cooling rate of 5 ° C./sec or more to a holding temperature of Ms point or higher (Ms point + 200 ° C.) or lower, holding at the holding temperature for a holding time of 1 second or more and 1000 seconds or less,
A continuous annealing step of cooling at a second average cooling rate of 5 ° C./sec or more to a cooling stop temperature of 250 ° C. or less;
and an overaging treatment step of holding the temperature range of 150° C. or higher and 250° C. or lower for 30 seconds or longer and 1500 seconds or shorter after the continuous annealing step.
[10] The method for producing a steel sheet according to [9], which includes a plating step of plating the surface of the steel sheet before or after the overaging treatment step.
[11] A method for manufacturing a member, comprising a step of subjecting the steel plate manufactured by the method for manufacturing a steel plate according to [9] or [10] to at least one of forming and joining.

According to the present invention, a high-strength steel sheet having excellent formability, excellent delayed fracture resistance, and excellent LME resistance can be obtained. This improvement in properties makes it possible to apply high-strength steel sheets to parts that are difficult to form in cold press forming applications, contributing to improved part strength and weight reduction.

Embodiments of the present invention will be described below. In addition, this invention is not limited to the following embodiment.

The steel sheet of the present invention is, in mass%, C: 0.24% or more and 0.40% or less, Si: 0.2% or more and 1.0% or less, Mn: 1.5% or more and 3.5% or less, P : 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.50% or less (not including 0%), N: 0.0006% or more and 0.01% or less, and the balance is Fe and inevitable impurities. , martensite: 40% to 78%, bainite: 20% to 58%, retained austenite: 2% or more,
Among the above martensites, the grain size of carbides in tempered martensite is 0.40 μm or less, the average amount of C in retained austenite is 0.5% by mass or more, and the Si concentration from the surface of the steel sheet to 100 μm in the thickness direction (Concentration of Si-enriched portion of surface layer) is 1.3% by mass or less, and tensile strength is 1470 MPa or more.

Ingredient Composition First, the content of each ingredient will be described. "%" representing the content of a component means "% by mass" unless otherwise specified.

C: 0.24% or more and 0.40% or less C is contained from the viewpoint of increasing the strength of martensite or bainite and ensuring TS≧1470 MPa. In addition, C is contained from the viewpoint of generating fine carbides that serve as hydrogen trap sites inside tempered martensite and bainite. If the C content is less than 0.24%, it becomes impossible to obtain a desired strength while maintaining excellent delayed fracture resistance. From the viewpoint of maintaining excellent delayed fracture resistance and obtaining TS≧1470 MPa, the C content is made 0.24% or more. If the C content exceeds 0.40%, the strength becomes too high, making it difficult to obtain sufficient delayed fracture resistance. Therefore, C should be 0.24% or more and 0.40% or less. The C content is preferably 0.25% or more, more preferably 0.26% or more, still more preferably 0.28% or more. Also, the C content is preferably 0.37% or less, more preferably 0.35% or less, and still more preferably 0.33% or less.

Si: 0.2% or more and 1.0% or less Si is contained as a strengthening element by solid solution strengthening and from the viewpoint of suppressing carbide precipitation during bainite transformation and obtaining retained austenite. In addition, since the melting point of zinc increases when the Si content is reduced, grain boundary corrosion of zinc during spot welding can be suppressed, and LME resistance can be improved. If the Si content is less than 0.2%, the amount of carbides precipitated during bainite transformation increases, the amount of retained austenite decreases, and formability deteriorates. The Si content is preferably 0.3% or more, more preferably 0.4% or more. On the other hand, the Si content is 1.0% or less, preferably 0.8% or less, more preferably 0.7%, from the viewpoint of welding safety and controlling the surface layer Si concentration within a predetermined range. % or less.

Mn: 1.5% or more and 3.5% or less Mn is contained in order to improve the hardenability of steel and keep the total area ratio of martensite and bainite within a predetermined range. In addition, Mn is contained in order to fix S in the steel as MnS and reduce hot shortness. Mn is an element that particularly promotes the formation and coarsening of MnS at the center of the plate thickness, and is composited with inclusion particles such as Al ₂ O ₃ , (Nb, Ti) (C, N), TiN, TiS, etc. However, these can be avoided by controlling the segregation state of Mn. Moreover, it contains 1.5% or more in order to stabilize retained austenite and obtain retained austenite. However, the upper limit of the Mn content is set to 3.5% from the viewpoint of weldability stability. Moreover, the upper limit of the Mn content is set to 3.5% in order to obtain sufficient delayed fracture resistance. From the above, the Mn content is set to 1.5% or more and 3.5% or less. The Mn content is preferably 1.8% or more, more preferably 2.1% or more, still more preferably 2.3% or more. Also, the Mn content is preferably 3.3% or less, more preferably 3.1% or less, and still more preferably 3.0% or less.

P: 0.002% or more and 0.010% or less P is an element that strengthens steel. Therefore, the P content should be 0.010% or less. From the above point of view, the P content is preferably 0.006% or less. On the other hand, the lower limit currently industrially practicable is 0.002%. From the above, the P content is set to 0.002% or more and 0.010% or less.

S: 0.0002% or more and 0.0020% or less S has a great influence on the delayed fracture resistance through the formation of MnS, TiS, Ti(C,S), etc., so it must be precisely controlled. The S content should be at least 0.0020% or less in order to reduce the harmful effects of inclusion groups. From the viewpoint of improving delayed fracture resistance, S is preferably 0.0010% or less. On the other hand, the lower limit currently industrially practicable is 0.0002%. From the above, the S content should be 0.0002% or more and 0.0020% or less.

sol. Al: 0.50% or less (not including 0%)
Al is contained in order to sufficiently deoxidize and reduce inclusions in the steel. In order to stably deoxidize, sol. Al is preferably 0.005% or more, more preferably 0.01% or more. On the other hand, sol. If Al exceeds 0.50%, the cementite generated during winding becomes less likely to form a solid solution during the annealing process, resulting in deterioration in delayed fracture resistance. Therefore, sol. The Al content is 0.50% or less, preferably 0.45% or less.

N: 0.0006% to 0.01% Through their formation, the delayed fracture resistance is deteriorated. These inclusions prevent obtaining the steel structure specified in the present invention and adversely affect the delayed fracture resistance. In order to reduce such adverse effects, N should be at least 0.01% or less. Preferably, the N content is 0.0055% or less, more preferably 0.0050% or less. At present, the industrially practicable lower limit is 0.0006%.

The balance other than the above has a component composition containing Fe (iron) and unavoidable impurities. Here, the steel sheet of the present invention preferably has a chemical composition containing the above-described basic components, with the balance being Fe and unavoidable impurities. In addition, the following optional elements may be contained. When the following arbitrary elements are contained below the preferred lower limit, the arbitrary elements may be contained as unavoidable impurities.

Nb: 0.1% or less Nb contributes to high strength through refinement of the internal structure of martensite and bainite, and also improves delayed fracture resistance as described above. From this point of view, the Nb content is preferably 0.001% or more, more preferably 0.005% or more. If the Nb content exceeds 0.1%, a large amount of Nb-based inclusions distributed in a dotted pattern in the rolling direction are generated, which is thought to adversely affect the delayed fracture resistance. In order to reduce such adverse effects, when Nb is contained, the content of Nb is set to 0.1% or less. Preferably, the Nb content is 0.08% or less, more preferably 0.06% or less.

Ti: 0.10% or less Ti contributes to high strength through refinement of the internal structure of martensite and bainite. In addition, the delayed fracture resistance is improved through the formation of fine Ti-based carbides/carbonitrides that serve as hydrogen trap sites. It also improves castability. From this point of view, the Ti content is preferably 0.002% or more, more preferably 0.005% or more. If the Ti content becomes excessive, a large amount of Ti-based inclusion particle groups distributed in a dotted pattern in the rolling direction are generated, which is considered to adversely affect the delayed fracture resistance. In order to reduce such adverse effects, when Ti is contained, the content of Ti should be 0.10% or less. Preferably, the Ti content is 0.07% or less, more preferably 0.05% or less.

B: 0.0050% or less B is an element that improves the hardenability of steel, and has the advantage of forming martensite and bainite with a predetermined area ratio even with a small Mn content. In order to obtain such effects of B, the B content is preferably 0.0001% or more, more preferably 0.0005% or more. From the viewpoint of fixing N, B is desirably contained in combination with 0.002% or more of Ti. On the other hand, when B is contained in an amount exceeding 0.0050%, not only does the effect saturate, but the rate of solid solution of cementite during annealing is delayed, and undissolved cementite remains, resulting in deterioration of delayed fracture resistance. do. Therefore, when B is contained, the B content is 0.0050% or less, preferably less than 0.0035%.

Cu: 1% or less Cu improves corrosion resistance in the use environment of automobiles. In addition, the inclusion of Cu has the effect of suppressing the penetration of hydrogen into the steel sheet by coating the surface of the steel sheet with corrosion products. Moreover, Cu is an element that is mixed when scrap is used as a raw material, and by allowing Cu to be mixed, recycled materials can be used as raw materials, and manufacturing costs can be reduced. From the above point of view, the Cu content is preferably 0.01% or more, and from the point of view of improving the delayed fracture resistance, the Cu content is preferably 0.05% or more. However, if the Cu content exceeds 1%, it causes surface defects. Therefore, when Cu is contained, the Cu content is set to 1% or less. Preferably, the Cu content is 0.40% or less, more preferably 0.30% or less.

Ni: 1% or less Ni is also an element that acts to improve corrosion resistance. In addition, Ni has the effect of reducing surface defects that tend to occur when Cu is contained. Therefore, from the above point of view, the Ni content is preferably 0.01% or more, more preferably 0.02% or more. However, when the Ni content exceeds 1%, scale formation in the heating furnace becomes uneven, causing surface defects and a significant increase in cost. Therefore, when Ni is contained, the Ni content is set to 1% or less. Preferably, the Ni content is 0.20% or less, more preferably 0.10% or less.

Cr: 1.0% or less Cr can be added to obtain the effect of improving the hardenability of steel. In order to obtain the effect, the Cr content is preferably 0.01% or more, more preferably 0.02% or more. However, when the Cr content exceeds 1.0%, the cementite dissolution rate during annealing is delayed, and undissolved cementite remains, thereby deteriorating the delayed fracture resistance. Moreover, pitting corrosion resistance is also deteriorated. Furthermore, it also degrades chemical convertibility. Therefore, when Cr is contained, the Cr content is made 1.0% or less. All of the delayed fracture resistance, pitting corrosion resistance, and chemical conversion treatability tend to start to deteriorate when the Cr content exceeds 0.8%. Preferably. More preferably, the Cr content is 0.6% or less, and even more preferably 0.4% or less.

Mo: less than 0.3% Mo has the effect of improving the hardenability of steel, the effect of generating fine carbides containing Mo that serve as hydrogen trap sites, and the improvement of delayed fracture resistance by refining martensite. can be added for the purpose of obtaining the effect of If a large amount of Nb or Ti is added, these coarse precipitates are formed and the delayed fracture resistance deteriorates, but the solid solubility limit of Mo is larger than that of Nb and Ti. When added in combination with Nb and Ti, it forms fine precipitates in which these and Mo are combined, and has the effect of refining the structure. Therefore, by adding Mo in addition to small amounts of Nb and Ti, it is possible to refine the structure without leaving coarse precipitates and disperse a large amount of fine carbides, thereby improving delayed fracture resistance. can be improved. In order to obtain the effect, the Mo content is preferably 0.01% or more, more preferably 0.02% or more. However, when Mo is contained in an amount of 0.3% or more, the chemical conversion treatability deteriorates. Therefore, when Mo is contained, Mo should be less than 0.3%. Preferably, the Mo content is 0.2% or less, more preferably 0.1% or less.

V: 0.45% or less V has the effect of improving the hardenability of steel, the effect of forming fine carbides containing V that serve as hydrogen trap sites, and the effect of improving delayed fracture resistance by refining martensite. can be added for the purpose of obtaining In order to obtain the effect, the V content is preferably 0.003% or more, more preferably 0.005% or more. However, if the V content exceeds 0.45%, the castability is remarkably deteriorated. Therefore, when V is contained, the V content should be 0.45% or less. Preferably, the V content is 0.2% or less, more preferably 0.1% or less.

Zr: 0.2% or less Zr contributes to increased strength through the refinement of prior γ grains and the resulting reduction in the block size, which is the internal structural unit of martensite and bainite, and the bain grain size, etc., as well as the resistance to delayed fracture. Improve properties. In addition, through the formation of fine Zr-based carbides/carbonitrides that serve as hydrogen trapping sites, the strength is increased and the delayed fracture resistance is improved. It also improves castability. From this point of view, the Zr content is preferably 0.001% or more, more preferably 0.005% or more. However, if a large amount of Zr is added, coarse precipitates of ZrN and ZrS that remain undissolved when the slab is heated in the hot rolling process increase, degrading the delayed fracture resistance. Therefore, when Zr is contained, the Zr content should be 0.2% or less. Preferably, the Zr content is 0.05% or less, more preferably 0.01% or less.

W: 0.2% or less W contributes to increasing the strength and improving the delayed fracture resistance through the formation of fine W-based carbides/carbonitrides that act as hydrogen trap sites. From such a point of view, the W content is preferably 0.005% or more, more preferably 0.01% or more. However, if the W content exceeds 0.2%, coarse precipitates that remain undissolved when the slab is heated in the hot rolling process increase, and the delayed fracture resistance deteriorates. Therefore, when W is contained, the W content should be 0.2% or less. Preferably, the W content is 0.1% or less, more preferably 0.05% or less.

Sb: 0.1% or less Sb suppresses the oxidation and nitridation of the surface layer, thereby suppressing the reduction of C and B. Suppressing the reduction of C and B suppresses the formation of ferrite in the surface layer, contributing to higher strength and improved delayed fracture resistance. From this point of view, the Sb content is preferably 0.002% or more, more preferably 0.005% or more. However, if the Sb content exceeds 0.1%, the castability deteriorates, and Sb segregates in the prior γ grain boundaries to deteriorate the delayed fracture resistance. Therefore, when Sb is contained, the Sb content is made 0.1% or less. Preferably, the Sb content is 0.06% or less, more preferably 0.04% or less.

Sn: 0.1% or less Sn suppresses oxidation and nitridation of the surface layer, thereby suppressing a decrease in the content of C and B in the surface layer. Suppressing the reduction of C and B suppresses the formation of ferrite in the surface layer, contributing to higher strength and improved delayed fracture resistance. From such a viewpoint, the Sn content is preferably 0.002% or more, more preferably 0.004% or more. However, if the Sn content exceeds 0.1%, the castability deteriorates, and Sn segregates at prior γ grain boundaries, resulting in deterioration of delayed fracture resistance. Therefore, when Sn is contained, the Sn content is set to 0.1% or less. Preferably, the Sn content is 0.04% or less, more preferably 0.02% or less.

Ca: 0.0050% or less Ca fixes S as CaS and improves delayed fracture resistance. In order to obtain this effect, it is preferable to contain 0.0001% or more of Ca. More preferably, the Ca content is 0.0005% or more. However, when the Ca content exceeds 0.0050%, the surface quality and bendability deteriorate. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less.

Mg: 0.01% or less Mg fixes O as MgO and improves delayed fracture resistance. In order to obtain this effect, it is preferable to contain 0.0001% or more of Mg. However, if the Mg content exceeds 0.01%, the surface quality and bendability are degraded. Therefore, when Mg is contained, the Mg content should be 0.01% or less. Preferably, the Mg content is 0.005% or less, more preferably 0.001% or less.

REM: 0.01% or less REM refines inclusions and reduces fracture starting points, thereby improving bendability and delayed fracture resistance. In order to obtain this effect, it is preferable to contain 0.0001% or more of REM. However, if the REM content exceeds 0.01%, the inclusions become coarse and the bendability and delayed fracture resistance deteriorate. Therefore, when REM is contained, the REM content is set to 0.01% or less. Preferably, the REM content is 0.004% or less, more preferably 0.002% or less.

Steel Structure The steel structure of the steel sheet of the present invention has the following structure.
(Configuration 1) In terms of area ratio, martensite is 40% or more and 78% or less, bainite is 20% or more and 58% or less, and retained austenite is 2% or more.
(Configuration 2) The average amount of C in retained austenite is 0.5% by mass or more.
(Constitution 3) Among martensite, the average grain size of carbides in tempered martensite is 0.40 μm or less.
(Structure 4 (preferred requirement)) The difference in microhardness between bainite and martensite is 1.5 GPa or more.

(Configuration 1) In terms of area ratio, martensite is 40% or more and 78% or less, bainite is 20% or more and 58% or less, and retained austenite is 2% or more.
In order to obtain a high strength of TS≧1470 MPa, the area ratio of martensite in the steel structure is set to 40% or more. If the content is less than this, bainite and retained austenite will increase and the strength will decrease. In order to obtain higher TS, it is preferably 50% or more. On the other hand, when the martensite content exceeds 78%, bainite and retained austenite are insufficient, resulting in deterioration of formability. In order to obtain higher moldability, the area ratio of martensite is preferably 70% or less. In the present invention, martensite includes tempered martensite in which carbide is precipitated.

Bainite is a structure with excellent strength and formability. The area ratio of bainite is set to 20% or more in order to obtain high moldability. If the content is less than this, the amount of martensite increases and the moldability decreases. Since the amount of retained austenite increases as bainite is produced, the area ratio of bainite is preferably 30% or more. On the other hand, when the bainite content exceeds 58%, the martensite content decreases and the strength decreases. Therefore, the bainite content is 58% or less, preferably 50% or less in order to obtain better strength.

Retained austenite enhances the balance between strength and ductility. If the retained austenite is less than 2%, a good balance of strength and ductility cannot be obtained. Therefore, the area ratio of retained austenite is 2% or more, and in order to obtain a better balance between strength and ductility, the area ratio of retained austenite is preferably 3% or more, more preferably 4% or more. . Although the upper limit is not specified, if the retained austenite is excessive, it transforms into martensite during forming, increasing the starting points of delayed fracture. The area ratio of retained austenite is preferably 20% or less, more preferably 15% or less. Moreover, in the present invention, the retained austenite satisfies the following condition (structure 2).

The presence of ferrite reduces strength because ferrite is very soft. In addition, since the difference in hardness from martensite is large, strain concentrates at the interface between martensite and ferrite during deformation, which may become the starting point of fracture and deteriorate the delayed fracture resistance. Therefore, the area ratio of ferrite is preferably 3% or less, more preferably 0%. Pearlite is a structure composed of layered ferrite and cementite, and when it is formed, the amount of C in martensite may decrease, resulting in a decrease in strength. The area ratio of pearlite is preferably 3% or less, more preferably 0%. That is, in the present invention, the total area ratio of ferrite and pearlite is preferably 6% or less, more preferably 2%, and still more preferably 0%.

(Configuration 2) The average amount of C in retained austenite is 0.5% by mass or more.
In the present invention, the average amount of C in retained austenite is 0.5% by mass or more. The higher the average C content in the retained austenite, the higher the stability of the retained austenite and the better balance between strength and ductility. If the average C content in retained austenite is less than 0.5% by mass, a good balance between strength and ductility cannot be obtained. Furthermore, since the stability is low, the amount of retained austenite that transforms into martensite during molding increases, and it may become the starting point of delayed fracture, degrading the delayed fracture resistance. Therefore, the average C content in retained austenite is 0.5% by mass or more, preferably 0.7% by mass or more. Although the upper limit of the average C content in the retained austenite is not specified, if the average C content in the retained austenite is excessively increased, the transformation of the retained austenite to martensite due to tensile deformation does not proceed sufficiently, resulting in sufficient work hardening. Noh can't be obtained. The average C content in retained austenite is preferably 2.0% by mass or less.

(Constitution 3) Among martensite, the average grain size of carbides in tempered martensite is 0.40 μm or less.
In the steel structure of the present invention, configuration 2 is important for improving the delayed fracture resistance of the steel plate. The average grain size of carbides in tempered martensite is 0.40 μm or less. By setting the average grain size of the carbide to 0.40 μm or less, the delayed fracture resistance can be improved. If the carbide becomes coarser than this, the delayed fracture resistance may deteriorate. The average particle size is preferably 0.38 μm or less, more preferably 0.36 μm or less. Although the lower limit is not particularly limited, the average grain size of the carbide is preferably 0.001 μm or more, more preferably 0.01 μm or more, in order to improve toughness.

(Structure 4 (preferred requirement)) The difference in microhardness between bainite and martensite is 1.5 GPa or more.
In the steel structure of the present invention, configuration 4 is important for obtaining high formability. It is considered that the greater the hardness difference between bainite and martensite, the greater the plastic deformation gradient formed during plastic deformation, and the greater the GN dislocation density accumulated in bainite. Therefore, the greater the difference in hardness, the greater the amount of work hardening of bainite due to the GN dislocation density, and the higher the elongation. Therefore, the difference in hardness between bainite and martensite is preferably 1.5 GPa or more. The upper limit is not particularly limited, but if the difference in hardness is excessively large, the hole expansibility tends to deteriorate. Therefore, the difference in hardness between bainite and tempered martensite is preferably 15 GPa or less, more preferably 13 GPa or less. preferable.

(Tissue measurement conditions)
Quantification of the metal structure is performed by polishing the L cross section of the steel plate (the cross section parallel to the rolling direction and perpendicular to the steel plate surface), corroding it with nital, and SEM at the 1/4 thickness position from the steel plate surface at a magnification of 2000 times. Observation is carried out in four fields of view, and the photographed tissue photograph is image-analyzed and measured. Here, martensite indicates a gray structure in SEM. Among them, tempered martensite refers to a structure containing fine carbides in martensite. On the other hand, bainite and ferrite are regions exhibiting black contrast in SEM. In addition, although tempered martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, it is difficult to exclude them, so the area ratio of the region including these is the area ratio. . Ferrite is produced by transformation from austenite at a relatively high temperature and has a structure composed of crystal grains of bcc lattice. Bainite is formed from austenite at a relatively low temperature (above the martensite transformation point) and has a structure in which spherical carbides are dispersed in needle-like or plate-like ferrite. Pearlite exhibits a structure in which cementite is precipitated in layers in ferrite.
In addition, the retained austenite is measured by chemically polishing a 200 μm surface layer of the steel sheet with oxalic acid, and using the surface of the steel sheet as a target, by the X-ray diffraction intensity method. It is calculated from the integrated intensities of (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ diffraction plane peaks measured by Mo-K _α rays.

The area ratio of the retained austenite obtained as described above is subtracted from 100%, and the remaining area ratio is the martensite, bainite, ferrite, and pearlite obtained by the point counting method by observing the structure photograph with the above SEM. By distributing the ratio, it is possible to specify the area ratios of martensite, bainite, ferrite, pearlite and retained austenite.

The average amount of C in the retained austenite is obtained by calculating the lattice constant (α _γ ) of γ from the {220} peak angle of γ using a Co—K _α ray source, and substituting the amount of the alloying element contained in the following formula. ask for
αγ = _3.578 + 0.00095 (% Mn) + 0.022 (% N) + 0.0056 (% Al) + 0.033 (% C)
In the formula, (%Mn), (%N), and (%Al) are the contents (% by mass) of Mn, N, and Al, respectively. (%C) is the average amount of C (% by mass) in the retained austenite.

Quantification of the size of carbides inside tempered martensite among martensite is performed by polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, corroding it with nital, and measuring it with SEM at a quarter thickness position from the steel sheet surface. Observe two fields of view at a magnification of 10,000 times. Then, in the observation photograph, finely dispersed white matter in the tempered martensite structure is regarded as carbide, and the length of the major axis of the carbide is measured by appropriately enlarging it. Chars are assumed to be elliptical or acicular in shape and the major axis is specified. Specifically, the major axis lengths of five carbides from each of three martensite blocks are measured and averaged to obtain the average grain size of carbides in the tempered martensite.

Microhardness is measured by nanoindentation with a load of 100 μN on a polished plate surface at a position of ¼ of the plate thickness. Five points are measured for each structure of bainite and martensite, the average value is obtained, and the difference is taken as the hardness difference. The maximum load is 500 μN, and a Berkovich type indenter with a twist angle of 115 degrees is used.

Si concentration from the steel plate surface to 100 μm in the plate thickness direction (surface Si concentration): 1.3% by mass or less In the present invention, the Si concentration (surface layer Si concentration) from the steel plate surface to 100 μm in the plate thickness direction is 1.3% or less. be. In order to obtain excellent resistance weldability, the Si concentration within 100 μm from the surface of the steel sheet must be 1.3% by mass or less. It is believed that when the concentration of Si is within this range, liquid metal embrittlement hardly occurs, resulting in excellent resistance weldability. The Si concentration in the surface layer is preferably 1.2% by mass or less, more preferably 1.1% by mass or less.
In the present invention, the L section is used as an evaluation surface, and the Si concentration in a region within 100 μm in the plate thickness direction from the steel plate surface is measured using a Field Emission-Electron Probe Micro Analyzer (FE-EPMA). , 10,000 points are extracted from an area of 100 μm×100 μm with an electron beam diameter of 1 μm, and the average value of the top 10% of the concentrations is taken.

The steel sheet of the present invention as described above may have a plating layer on its surface. The type of plating layer is not particularly limited, and may be either a Zn plating layer (zinc plating layer) or a plating layer of a metal other than Zn. Also, the plating layer may contain components other than the main component such as Zn. The galvanized layer is, for example, a hot dip galvanized layer or an electrogalvanized layer.

Next, the method for manufacturing the steel sheet of the present invention will be described.
The steel sheet manufacturing method of the present invention comprises a hot rolling step of obtaining a hot rolled steel sheet by hot rolling a steel slab having the above chemical composition, and cold rolling of the hot rolled steel sheet after the hot rolling step. After the cold-rolling process to obtain the cold-rolled steel sheet and the cold-rolling process, the cold-rolled steel sheet is soaked at an annealing temperature of Ac ₃ or more in an atmosphere with a dew point of -60 ° C or higher and -20 ° C or lower for 15 hours. After annealing for 2 seconds or longer, the steel is cooled to a holding temperature of Ms point or higher (Ms+200°C) at a first average cooling rate of 5°C/second or higher, and held at the holding temperature for a holding time of 1 second or longer and 1000 seconds or shorter, A continuous annealing step of cooling at a second average cooling rate of 5 ° C./sec or more to a cooling stop temperature of 250 ° C. or less, and after the continuous annealing step, a temperature range of 150 ° C. or higher and 250 ° C. or lower for 30 seconds or more and 1500 seconds or less. and an aging treatment step.

Hot rolling process As a method of hot rolling a steel slab, there are a method of rolling after heating the slab, a method of directly rolling the slab after continuous casting without heating, and a method of subjecting the slab after continuous casting to heat treatment for a short time. There are methods such as rolling. In hot rolling, the average heating rate during slab heating should be 5 to 15°C/min, the finish rolling temperature FT should be 840 to 950°C, and the coiling temperature CT should be 400 to 700°C, as usual.

Descaling may be performed as appropriate to remove primary and secondary scales formed on the surface of the steel sheet. It is preferable to sufficiently pickle the hot-rolled coil before cold-rolling to reduce residual scale. Moreover, from the viewpoint of reducing the cold rolling load, the hot-rolled steel sheet may be annealed as necessary.

Cold Rolling Step In the cold rolling, if the rolling reduction (cold rolling rate) is 40% or more, the recrystallization behavior and texture orientation in subsequent continuous annealing can be stabilized. If the content is less than 40%, some of the austenite grains during annealing may become coarse and the strength may decrease.

Continuous Annealing Process Steel sheets after cold rolling are subjected to annealing and tempering in a continuous annealing line (CAL).
In the present invention, in order to obtain the predetermined martensite and bainite, the annealing temperature must be Ac ₃ points or more and the soaking time must be 15 seconds or more. If the annealing temperature is less than Ac ₃ points, or if the soaking time is less than 15 seconds, sufficient austenite is not generated during annealing, and the desired martensite and/or bainite cannot be obtained in the final product. , a tensile strength of 1470 MPa or more cannot be obtained. The upper limits of the annealing temperature and soaking time are not particularly limited, but if the annealing temperature or soaking time exceeds a certain level, the austenite grain size may become coarse and the delayed fracture resistance may deteriorate, so the annealing temperature is 950 ° C. Hereinafter, the soaking time is preferably 900 seconds or less.

Further, in the present invention, the dew point of the atmosphere in the annealing is set to −60° C. or higher and −20° C. or lower. By setting the dew point to −60° C. or more and −20° C. or less, the concentration of Si in the surface layer of the steel sheet can be made 1.3% by mass or less, and excellent LME resistance can be obtained. If the dew point is less than -60°C, there is a risk of increasing equipment costs and manufacturing costs. In addition, the concentration of Si in the surface layer of the steel sheet cannot be reduced to 1.3% by mass or less, and desired delayed fracture resistance and LME resistance cannot be obtained. On the other hand, if the dew point is higher than −20° C., the decarburization phenomenon in the surface layer is promoted, the strength is reduced, and there is a possibility that the strength of 1470 MPa cannot be obtained. Also, the desired delayed fracture resistance cannot be obtained.
Therefore, in the present invention, annealing is performed in an atmosphere with a dew point of -60°C or higher and -20°C or lower. Preferably, the dew point is -55°C or higher. Also preferably, the dew point is -25° C. or lower.

After that, in order to reduce ferrite, it is necessary to cool at a first average cooling rate of 5° C./s or more to a holding temperature of Ms point or more (Ms point+200° C.) or less. If the first average cooling rate is less than 5°C/sec, a large amount of ferrite is produced. Therefore, the first average cooling rate is 5° C./s or higher, preferably 7° C./s or higher, more preferably 10° C./s or higher.

After that, in order to obtain a predetermined bainite, it is necessary to hold isothermally for 1 second or more and 1000 seconds or less at a holding temperature of (Ms point+200° C.) or higher and Ms point or higher. If the holding temperature is higher than (Ms point+200° C.), retained austenite cannot be obtained, or a large amount of ferrite is generated. On the other hand, if the holding temperature is lower than the Ms point, martensite (self-tempered martensite) that is tempered at a temperature higher than 250° C. during isothermal holding is generated, and carbide coarsens in grains and block grain boundaries. becomes conspicuous, and there is a possibility that the delayed fracture resistance deteriorates.
The holding temperature is preferably (Ms point +20°C) or higher, more preferably (Ms point +30°C) or higher. In addition, the lower the holding temperature, the easier it is to obtain retained austenite.
If the holding time is shorter than 1 second, there is a problem that bainite and retained austenite are less and formability is deteriorated. Therefore, the retention time is 1 second or longer, preferably 15 seconds or longer, and more preferably 30 seconds or longer. On the other hand, if the holding time is longer than 1000 seconds, the amount of martensite decreases, and there is a problem that a tensile strength of 1470 MPa or more cannot be obtained. Therefore, the holding time is 1000 seconds or less, preferably 500 seconds or less, more preferably 300 seconds or less.

After that, in order to obtain tempered martensite with excellent delayed fracture resistance, it is necessary to cool to a cooling stop temperature of 250° C. or less at a second average cooling rate of 5° C./second or more. If the cooling stop temperature exceeds 250 ° C. or the second average cooling rate is less than 5 ° C./sec, self-tempered martensite with coarse intragranular carbides may be generated, and delayed fracture resistance may deteriorate. . Therefore, in the present invention, cooling is performed at a second average cooling rate of 5°C/sec or more to a cooling stop temperature of 250°C or less. The second average cooling rate is preferably 50° C./second or higher, more preferably 100° C./second or higher. Also, the cooling stop temperature is preferably 150° C. or lower, more preferably 50° C. or lower.

Overaging Treatment Process Tempered martensite is a carbide that forms during holding in a low temperature range after quenching, and must be properly controlled in order to ensure delayed fracture resistance and TS≧1470 MPa. That is, after quenching to 250° C. or less, it is necessary to reheat or hold the temperature at 150° C. or higher and 250° C. or lower, and control the holding time to 30 seconds or more and 1500 seconds or less.
If it is less than 150° C. or less than 30 seconds, the carbide distribution density becomes insufficient, and toughness may deteriorate. On the other hand, if it exceeds 250° C. or 1500 seconds, coarsening of carbides in grains and block grain boundaries becomes significant, and there is a possibility that delayed fracture resistance deteriorates.

Plating Treatment Step Before or after the overaging treatment step, the surface of the obtained steel sheet may be subjected to plating treatment. A steel sheet having a plating layer on its surface can be obtained by plating. The type of plating treatment is not particularly limited, and may be either plating using a plating technique using a spray or electroplating. In addition, when performing a plating process and performing the said skin pass rolling, a skin pass rolling is performed after a plating process.

The above Ac ₃ points and Ms points are calculated from the following formulas (1) and (2) described in "Leslie Steel Materials Science" (Maruzen Co., Ltd., 1985, pp. 273, 231). be. [M%] is the content of each element M (% by mass).
Ac ₃ (°C) = 910 - 203 x [C%] ^1/2 + 44.7 x [Si%] - 30 x [Mn%] + 700 x [P%] + 130 x [Al%] - 15.2 x [ Ni%]−11×[Cr%]−20×[Cu%]+31.5×[Mo%]+104×[V%]+400×[Ti%] (1)
Ms (° C.)=561−474×[C%]−33×[Mn%]−17×[Ni%]−17×[Cr%]−21×[Mo%] (2)

The steel sheet of the present invention obtained by the above manufacturing method preferably has a thickness of 0.5 mm or more. Moreover, it is preferable that the plate|board thickness of a steel plate is 2.5 mm or less.

Next, the member of the present invention and its manufacturing method will be described.

The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Further, the method for manufacturing a member of the present invention includes a step of subjecting the steel plate manufactured by the method for manufacturing a steel plate of the present invention to at least one of forming and joining.

The steel sheet of the present invention has a tensile strength of 1470 MPa or more, and has excellent formability, delayed fracture resistance and LME resistance. Therefore, members obtained using the steel sheet of the present invention also have high strength, and are excellent in formability, delayed fracture resistance, and LME resistance as compared with conventional high-strength members. Moreover, if the member of the present invention is used, the weight can be reduced. Therefore, the member of the present invention can be suitably used for complex-shaped members used in the automobile field, such as body frame parts.

For molding, general processing methods such as press processing can be used without limitation. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, riveting, caulking, and the like can be used. The molding conditions and bonding conditions are not particularly limited, and conventional methods may be followed.

[Example 1]
Examples of the present invention will be described below.

A cold-rolled steel sheet having a sheet thickness of 1.2 mm and having the chemical composition shown in Table 1 was subjected to heat treatment under the annealing conditions and overaging treatment conditions shown in Table 2.
The soaking time at the annealing temperature was 300 seconds. Also, No. For No. 20, the surface of the steel sheet was plated after the overaging treatment. Electrogalvanizing was used as the plating condition.
The cold-rolled steel sheet was hot-rolled (average heating rate during heating: 10°C/min, finish rolling temperature FT: 900°C, coiling temperature CT: 500°C) on a steel slab having the chemical composition shown in Table 1. and then cold-rolled (rolling reduction: 55%).

The obtained steel sheet was subjected to quantification of the metal structure, and then to a tensile test and a delayed fracture resistance evaluation test.

Quantification of the metal structure is performed by polishing the L cross section of the steel plate (the cross section parallel to the rolling direction and perpendicular to the steel plate surface), corroding it with nital, and SEM at the 1/4 thickness position from the steel plate surface at a magnification of 2000 times. 4 visual fields were observed, and the photographed tissue photographs were image-analyzed and measured. Here, martensite indicates a gray structure in SEM. Among them, tempered martensite refers to a structure containing fine carbides in martensite.
On the other hand, bainite and ferrite are regions exhibiting black contrast in SEM. Note that the interior of martensite and bainite contains trace amounts of carbides, nitrides, sulfides, and oxides, but since it is difficult to exclude these, the area ratio of the region including these was used as the area ratio.
Among the metal structures observed above, ferrite is a structure formed by transformation from austenite at a relatively high temperature and composed of crystal grains of bcc lattice. Bainite is formed from austenite at a relatively low temperature (above the martensite transformation point) and has a structure in which spherical carbides are dispersed in needle-like or plate-like ferrite. Pearlite exhibits a structure in which cementite is precipitated in layers in ferrite.
In addition, the residual austenite was measured by chemically polishing the 1/4 thickness of the surface layer of the steel sheet with oxalic acid, and using the sheet surface as a target, it was obtained by the X-ray diffraction intensity method. It was calculated from the integrated intensities of (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ diffraction surface peaks measured by Mo-K _α rays.

The area ratio of the retained austenite obtained as described above is subtracted from 100%, and the remaining area ratio is the martensite, bainite, ferrite, and pearlite obtained by the point counting method by observing the structure photograph with the above SEM. By distributing the ratio, the area ratios of martensite, bainite, ferrite, pearlite, and retained austenite were specified.

The average amount of C in the retained austenite is obtained by calculating the lattice constant (α _γ ) of γ from the {220} peak angle of γ using a Co—K _α ray source, and substituting the amount of the alloying element contained in the following formula. I asked.
αγ = _3.578 + 0.00095 (% Mn) + 0.022 (% N) + 0.0056 (% Al) + 0.033 (% C)
In the formula, (%Mn), (%N), and (%Al) are the contents (% by mass) of Mn, N, and Al, respectively. (%C) is the average amount of C (% by mass) in the retained austenite.

Quantification of the size of carbides inside tempered martensite among martensite is performed by polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, corroding it with nital, and measuring it with SEM at a quarter thickness position from the steel sheet surface. Two fields of view were observed at a magnification of 10000 times. Then, in the observation photograph, the white matter that was finely dispersed in the tempered martensite structure was defined as carbide, and the major axis length of the carbide was measured by appropriately enlarging it. Chars are assumed to be elliptical or acicular in shape and the major axis is specified. Specifically, the major axis lengths of five carbides from each of three martensite blocks were measured and averaged to obtain the average grain size of carbides in the tempered martensite.

For the tensile test, a JIS No. 5 tensile test piece was cut so that the rolling direction was the longitudinal direction, and a tensile test (according to JIS Z2241) was performed to evaluate TS and El.

The microhardness was measured using nanoindentation with a load of 100 μN on a polished plate surface at a position of ¼ of the plate thickness. Five points were measured for each structure of bainite and martensite, the average value was obtained, and the difference was taken as the hardness difference. A Triboindenter manufactured by Hysitron was used as the indentation device, the maximum load was set to 500 μN, and the indenter used was a Berkovich type with a winding angle of 115 degrees.
Regarding the Si concentration (surface layer Si concentration) from the surface of the steel sheet to 100 μm in the sheet thickness direction, the L cross section was used as the evaluation surface. Specifically, using a field emission electron probe microanalyzer (FE-EPMA), a range of 100 μm × 100 μm is analyzed with an electron beam diameter of 1 μm, 10000 points are extracted, and the The average value of the top 10% of concentrations was used.

For the evaluation of the delayed fracture resistance of the steel plate, the delayed fracture of the steel plate base material was evaluated.

The delayed fracture evaluation of the steel plate base material was carried out by extracting a strip test piece of 100 mm in the rolling direction and 30 mm in the rolling direction from the 1/4 position of the coil width of the obtained steel plate. The cut-out processing of the end face of 100 mm length is shear processing, and in the state of shear processing (without mechanical processing to remove burrs), bending is performed so that the burrs are on the outer peripheral side of the bend, and the bending shape is obtained. The test piece was fixed with bolts while maintaining the shape of the test piece. The shearing clearance was 15% and the rake angle was 0 degrees. Bending is performed with a bending radius of R/t=4 where R is the tip bending radius and plate thickness t (plate thickness t: 1.2 mm, punch tip radius: bending with a punch of 5.0 mm). It was applied so that the tip inner angle was 90 degrees (V-bend). The punch used had a U-shaped tip with the above-mentioned radius (the tip R portion was semicircular and the thickness of the punch body was 2R), and the corner radius of the die was 30 mm. The depth to which the punch pushes the steel plate was adjusted, and the steel plate was formed so that the bending angle of the tip was 90 degrees (V shape). For bolt tightening, the test piece is held with a hydraulic jack so that the distance between the flange ends of the straight piece during bending is the same as when bending (to cancel out the opening of the straight piece due to springback). was clamped and tightened, and the bolt was tightened in that state. The bolt was passed through an elliptical hole (minor axis: 10 mm, major axis: 15 mm) previously provided 10 mm inward from the short side edge of the strip test piece and fixed. The obtained test piece after bolting was immersed in 1 L or more of hydrochloric acid (hydrogen chloride aqueous solution) having a pH of 3 per piece, and the test was carried out while the pH was constantly controlled at an aqueous solution temperature of 25°C. Visually or with a camera, always check for the presence or absence of microcracks (initial state of delayed fracture) at a level that can be visually confirmed (1 mm or more in length), and the time from the start of immersion to the start of microcracks is defined as the delayed fracture time. Measurements were taken up to 96 hours.

As the strength of steel sheets increases, the concern about delayed fracture increases, so even with high strength, there is a difference in delayed ^fracture characteristics. (−0.0055 × (TS-1760) + 0.3) power time) or more is evaluated as “○” (accepted) as being excellent in delayed fracture characteristics, and “×” (failed) when the fracture time is less than that passed).

The LME resistance properties were determined by a resistance weld cracking test. One test piece cut into 30 mm × 100 mm with the longitudinal direction perpendicular to the rolling direction of the obtained steel plate is used, and the other is a 980 MPa class hot-dip galvanized steel plate and resistance welding (spot welding) is performed. did. The welding machine uses a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type attached to a welding gun for a plate assembly in which two steel plates are stacked, with the plate assembly tilted at 5 °. Welding was performed. Welding conditions were an applied pressure of 3.8 kN and a hold time of 0.2 seconds. Also, the welding current was 5.7 to 6.2 kA, the welding time was 21 cycles, and the hold time was 5 cycles. After welding, cut the test piece in half and observe the cross section with an optical microscope. If no cracks of 0.1 mm or more are observed, the LME cracking resistance is good (○), and if cracks of 0.1 mm or more are observed. was considered to be poor (×) for LME cracking.

A steel in which the chemical composition, hot rolling conditions, and annealing conditions are optimized has a TS of 1470 MPa or more. In addition, retained austenite having an average C content of 0.5% by mass or more was obtained in terms of area ratio of 2% or more, and an elongation of 11% or more was obtained.
Regarding the delayed fracture resistance of the steel plate base material, the fracture time is 10 ^{(−0.0055×(TS-1760)+0.3)} hours or more, and excellent delayed fracture characteristics are obtained. The steel sheet of the present invention has a tensile strength of 1470 MPa or more, an elongation of 11% or more, and a fracture time of 10 ^{(−0.0055 × (TS-1760) + 0.3)} hours or more in a delayed fracture resistance evaluation test, which is excellent. It has LME resistant properties. The steel sheets of the comparative examples do not satisfy any one of those conditions.

[Example 2]
Manufacturing condition No. in Table 2 of Example 1. A galvanized steel sheet that had been subjected to galvanizing treatment was press-molded with respect to No. 1 (conforming steel) to manufacture members of the present invention examples. Furthermore, manufacturing condition No. in Table 2 of Example 1. 1 (conforming steel), and the manufacturing conditions No. 1 in Table 2 of Example 1. A galvanized steel sheet that had been galvanized with respect to No. 2 (conforming steel) was spot-welded to manufacture a member of an example of the present invention.
These members of the present invention have a tensile strength TS of 1470 MPa or more, and are excellent in formability, delayed fracture resistance, and LME resistance. .

Similarly, manufacturing conditions No. in Table 2 of Example 1 were used. The steel plate according to Example 1 (suitable example) was press-formed to produce a member of an example of the present invention. Furthermore, manufacturing condition No. in Table 2 of Example 1. 1 (suitable example) and the manufacturing condition No. 1 in Table 2 of Example 1. 2 (suitable example) was joined by spot welding to manufacture a member of the present invention example. These members of the present invention have a tensile strength TS of 1470 MPa or more, and are excellent in formability, delayed fracture resistance, and LME resistance. .

According to the present invention, a high-strength steel sheet having excellent formability, delayed fracture resistance, and LME resistance can be obtained. This improvement in properties makes it possible to apply high-strength steel sheets to parts that are difficult to form in cold press forming applications, contributing to improved part strength and weight reduction.

Claims (11)

in % by mass,
C: 0.24% or more and 0.40% or less,
Si: 0.2% or more and 1.0% or less,
Mn: 1.5% or more and 3.5% or less,
P: 0.002% or more and 0.010% or less,
S: 0.0002% or more and 0.0020% or less,
sol. Al: 0.50% or less (not including 0%),
N: 0.0006% or more and 0.01% or less, and the balance has a component composition consisting of Fe and inevitable impurities,
Having a steel structure containing, in terms of area ratio, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and retained austenite: 2% or more,
Among the martensite, the average grain size of carbide in the tempered martensite is 0.40 μm or less,
The average amount of C in the retained austenite is 0.5% by mass or more,
The Si concentration from the steel plate surface to 100 μm in the plate thickness direction is 1.3% by mass or less,
A steel plate having a tensile strength of 1470 MPa or more. The steel sheet according to claim 1, wherein a difference in microhardness between the bainite and the martensite is 1.5 GPa or more. As the component composition, further by mass%,
Nb: 0.1% or less,
Ti: 0.10% or less,
B: 0.0050% or less,
The steel sheet according to claim 1 or 2, containing one or more selected from Cu: 1% or less and Ni: 1% or less. As the component composition, further by mass%,
Cr: 1.0% or less,
Mo: less than 0.3%,
V: 0.45% or less,
The steel sheet according to any one of claims 1 to 3, containing one or more selected from Zr: 0.2% or less and W: 0.2% or less. As the component composition, further by mass%,
The steel sheet according to any one of claims 1 to 4, containing one or two selected from Sb: 0.1% or less and Sn: 0.1% or less. As the component composition, further by mass%,
Ca: 0.0050% or less,
Mg: 0.01% or less,
REM: The steel sheet according to any one of claims 1 to 5, containing one or more selected from 0.01% or less. The steel sheet according to any one of claims 1 to 6, which has a plating layer on the surface of the steel sheet. A member obtained by subjecting the steel plate according to any one of claims 1 to 7 to at least one of forming and joining. A steel plate manufacturing method for manufacturing the steel plate according to any one of claims 1 to 6,
A hot rolling step of hot rolling a steel slab to obtain a hot rolled steel sheet;
After the hot-rolling step, a cold-rolling step of cold-rolling the hot-rolled steel plate to obtain a cold-rolled steel plate;
After the cold rolling step, the cold-rolled steel sheet is annealed at an annealing temperature of Ac ₃ or higher for a soaking time of 15 seconds or longer in an atmosphere with a dew point of -60 ° C or higher and -20 ° C or lower,
Cooling at a first average cooling rate of 5 ° C./sec or more to a holding temperature of Ms point or higher (Ms point + 200 ° C.) or lower, holding at the holding temperature for a holding time of 1 second or more and 1000 seconds or less,
A continuous annealing step of cooling at a second average cooling rate of 5 ° C./sec or more to a cooling stop temperature of 250 ° C. or less;
and an overaging treatment step of holding the temperature range of 150° C. or higher and 250° C. or lower for 30 seconds or longer and 1500 seconds or shorter after the continuous annealing step. The method for manufacturing a steel sheet according to claim 9, comprising a plating step of plating the steel sheet surface before or after the overaging treatment step. A method for manufacturing a member, comprising a step of subjecting the steel plate manufactured by the steel plate manufacturing method according to claim 9 or 10 to at least one of forming and joining.