High-strength steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide: a steel sheet that has excellent LME resistance characteristics, high strength, and good moldability and is obtained by a method differing from those of the prior art; and a method for producing the steel sheet.　This high-strength steel sheet contains specific components, has a specific structure, includes Nb in a manner satisfying the relationship between the solid solution Nb amount (Nb sol), the Nb amount (Nbpre) in Nb precipitates having a particle size of less than 20 nm, and the total Nb amount (Nb) included in the steel sheet expressed in formula (1) below, and includes 0.50 mass ppm or less of diffusible hydrogen in steel. Formula 1: (Nbsol / Nb) + (Nbpre / Nb) ≥ 0.40 (in formula (1), Nbsol represents the solid solution Nb amount (mass%) and Nbpre represents the Nb amount (mass%) in Nb precipitates having a particle size of less than 20 nm).

Description

High strength steel plate and method for manufacturing the same

[0001] The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly to a high-strength steel sheet having excellent LME resistance and suitable for use as components formed by cold pressing in industrial fields such as automobiles and electrical machinery, and a manufacturing method thereof.

[0002] In recent years, from the viewpoint of global environmental conservation, there has been an increasing need to reduce the weight of automobile bodies in order to improve fuel efficiency. Therefore, the application of high-strength steel sheets to automobile parts is desired. On the other hand, when cold-rolled steel sheets are used as automobile parts, they are generally formed by welding two or more steel sheets together to form the desired shape.

[0003] Spot welding is primarily used to join steel sheets in automotive parts. However, it is known that when welding a sheet assembly including a high-strength steel sheet with a welded zinc-based plating layer (high-strength zinc-based plated steel sheet), cracks are likely to occur in the weld. This is thought to be because zinc-based plating layers generally have a low melting point, and the plating layer becomes liquid during welding, penetrating the grain boundaries of the base steel sheet and reducing the grain boundary strength. This reduction in strength due to the liquid metal is generally called liquid metal embrittlement (LME), and cracks caused by liquid metal embrittlement are called liquid metal embrittlement (LME) cracking.

[0004] It is also known that even if a high-strength steel sheet is not plated, cracks will still occur in the welded portion of the high-strength steel sheet if the mating material to be welded to the high-strength steel sheet has a zinc-based plated layer. LME cracks that occur in high-strength steel sheets that do not have a plated layer are called transferred LME cracks.

[0005] The aforementioned LME cracking is particularly noticeable when spot welding is performed with an electrode at an angle, where the axis of the welding electrode is not perpendicular to the surface of the steel sheet.

[0006] Therefore, various studies have been conducted to prevent LME cracking of zinc-based plated steel sheets when spot-welded, and to prevent LME cracking of unplated steel sheets.

[0007] For example, Patent Document 1 discloses a technology for realizing a high-strength steel sheet having a tensile strength of 980 MPa or more, a total elongation of 20% or more, and excellent LME resistance by controlling the frequency of coincidence grain boundaries in the surface layer of the steel sheet after a high-temperature tensile test and the thickness of the softened surface layer.

[0008] In addition, in Patent Document 2, oxygen is introduced into the surface layer of a steel slab during continuous casting to form iron oxide, which is a site for the formation of titanium nitride, and this inhibits B in the steel from bonding with dissolved nitrogen during the subsequent annealing process, thereby preventing (Fe, Mn) ２ A technology has been disclosed that promotes the formation of B to realize a steel sheet that has LME resistance, high strength, and excellent ductility.

[0009] Furthermore, Patent Document 3 proposes a technique for preventing LME cracking by removing the plating layer from the portion to be welded prior to spot welding.

[0010] International Publication No. 2020 / 184154 International Publication No. 2020 / 225936 International Publication No. 2016 / 159169

[0011] However, in the technology described in Patent Document 1, it is necessary to reduce Si in order to reduce the frequency of coincidence grain boundaries in the surface layer of the steel sheet, and it is presumed that it is difficult to impart good formability to steel sheets having strength levels higher than 980 MPa class.

[0012] In addition, in the technology described in Patent Document 2, oxygen is introduced into the surface layer of the steel billet during continuous casting to improve the LME resistance of the steel sheet that is finally obtained, but the introduction of oxygen into the surface layer is expected to cause scale to form, resulting in a decrease in yield. ２ Since B is contained in the surface layer, it is expected to cause surface cracks during casting.

[0013] Furthermore, the technology described in Patent Document 3 requires a step of removing the plating layer in advance, which increases manufacturing costs. Furthermore, since the plating layer is removed, it is thought that the corrosion resistance of the welded portion is reduced.

[0014] Therefore, an object of the present invention is to provide a steel sheet having excellent LME resistance, high strength, and good formability, and a manufacturing method thereof, which is different from the conventional techniques. The steel sheet includes a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet such as GA or GI.

[0015] In the present invention, "high strength" means that TS is 980 MPa or more, and "good formability" means that the relationship between tensile strength TS and elongation El shown in the following formula (7) is satisfied. (Formula 7) TS １．５ ×El≧390000

[0016] In order to solve the above problems, the inventors have conducted extensive research into the chemical composition and microstructure of steel sheets, and as a result, have discovered the following by carefully controlling the slab heating conditions, temperature management from hot rolling to coiling, and annealing conditions, and by controlling the state of elements contained in the steel.

[0017] That is, by dissolving Nb in solid solution, or by controlling the particle radius of precipitates to be within a certain range so that they do not cause cracks on the steel surface during casting, the solute elements present in the steel during welding or the solute elements formed by dissolving the precipitates prevent the intrusion of zinc, thereby improving LME resistance. On the other hand, if the precipitate radius of a specific element is larger than a predetermined value, the precipitates will not dissolve sufficiently during welding, and not only will the solute elements not be able to be expected to improve LME resistance, but new cracks may occur originating from the coarse precipitates not only during manufacturing but also after press working and during welding.

[0018] The present invention has been made based on the above findings, and its gist is as follows: [1] A steel sheet having a chemical composition containing, by mass%, C: 0.030% or more and 0.500% or less, Si: more than 0.01% and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.010% or more and 0.200% or less, Nb: 0.005% or more and 0.500% or less, with the balance consisting of Fe and unavoidable impurities, wherein the microstructure of the steel sheet at the 1 / 4 position of the sheet thickness has a total area ratio of tempered martensite and bainite of 40% or more and 85% or less, and an area ratio of fresh martensite of 0% or more and 25% or less, The area ratio of the retained austenite is 5% or more and 20% or less, and the remainder is at least one of ferrite and pearlite in area ratio of 0% or more and 20% or less, and the Nb is the amount of solute Nb (Nb ｓｏｌ ) and the Nb content in Nb precipitates with a grain size of less than 20 nm (Nb ｐｒｅ ) and the total amount of Nb (Nb) contained in the steel sheet satisfy the following (Formula 1), and the diffusible hydrogen in the steel is 0.50 mass ppm or less. ｓｏｌ / Nb) + (Nb ｐｒｅ / Nb) ≧ 0.40 (Formula 1) ｓｏｌ : Solid solution Nb amount (mass%), Nb ｐｒｅ: represents the amount of Nb (mass%) in Nb precipitates with a grain size of less than 20 nm. [2] The high-strength steel plate according to [1], further containing, as a chemical composition, one or more selected from, by mass%, V: 0.500% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less. [3] The high-strength steel sheet according to [1] or [2], which has a plating layer on the surface of the steel sheet. [4] The high-strength steel sheet according to [3], wherein the plating layer is an alloyed plating layer. [5] A method for producing the high-strength steel sheet according to [1] or [2], which comprises heating a steel material having the above-mentioned composition at a temperature T ｓｏｌ a slab heating step in which the slab is heated at a temperature of 1.0°C or higher for 1.0 hour or more; a hot rolling step; and a coiling temperature T ＣＴ a winding process in which the temperature is 650°C or less; and a soaking temperature T ＡＴ The soaking temperature T is 750°C or higher and 950°C or lower. ＡＴ and an annealing step of holding the steel sheet at a temperature of 1000° C. and cooling the steel sheet after the holding step, ＦＥＴ °C, finish rolling end temperature T ＦＤＴ °C, effective time t from the start of finish rolling to the end of finish rolling ＨＲ Q defined by (Equation 3) ＨＲ In the winding step, T ＦＤＴ Residence time t from ° C to 650 ° C ＣＴ , winding temperature T ＣＴ Q defined by (Equation 4) in °C ＣＴ In the annealing step, the soaking temperature T ＡＴ ℃, 650℃ to soaking temperature T ＡＴ Temperature rise time t to °C, soaking temperature T ＡＴ Holding time t in °C ＡＴ Q defined by (Equation 5) ＡＴA method for manufacturing a high strength steel plate where T satisfies (Equation 6). ｓｏｌ = 7900 / (3.42 - (log ([Nb%][C%]))) - 273) where [Nb%] and [C%] are the amount of Nb (mass%) and the amount of C (mass%) contained in the steel, respectively. (Equation 3) Q ＨＲ = 0.5 (T ＦＥＴ +T ＦＤＴ ) × log １０ (t ＨＲ ) (Formula 4)Q ＣＴ = 0.5 (T ＣＴ +650) × log １０ (t ＣＴ ) (Formula 5)Q ＡＴ =0.5(650+T ＡＴ ) × log １０ (t) + T ＡＴ ×log １０ (t ＡＴ ) (Formula 6)Q ＨＲ +Q ＣＴ +Q ＡＴ ≦6000 [6] Q ＡＴ [7] The method for producing a high-strength steel sheet according to [5], wherein the value of is 4700 or less. [7] The method for producing a high-strength steel sheet according to [5] or [6], wherein a plating treatment is performed after the annealing step. [8] The method for producing a high-strength steel sheet according to [7], wherein the plating treatment is an alloying plating treatment.

[0019] According to the present invention, a high-strength steel sheet having good formability and excellent LME resistance can be obtained. Therefore, the present invention is highly useful in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts.

[0020] The present invention will be specifically described below. Note that "%" representing the content of component elements means "% by mass" unless otherwise specified.

[0021] (1) The reason why the composition of the steel in the present invention is limited to the above range will be explained.

[0022] C: 0.030% or more and 0.500% or less C is an element necessary for increasing the strength of tempered martensite, bainite, and fresh martensite. To fully obtain this effect, the C content must be at least 0.030% or more. Therefore, the C content is set to 0.030% or more. Preferably, it is 0.050% or more. More preferably, it is 0.070% or more. Even more preferably, it is 0.090% or more, and most preferably, it is 0.100% or more. On the other hand, if the C content exceeds 0.500%, the weldability and LME resistance characteristics, which are important when joining automotive parts, deteriorate. Therefore, the C content is set to 0.500% or less. Preferably, it is 0.400% or less. More preferably, it is 0.300% or less. Even more preferably, it is 0.270% or less, and most preferably, it is 0.250% or less.

[0023] Si: more than 0.01% and not more than 2.50% Si is an element that suppresses the excessive formation and growth of carbides in steel, increases the retained austenite fraction, and improves ductility. If the Si content is 0.01% or less, this effect is poor and good formability is not achieved. Therefore, the lower limit is set to 0.01%. The Si content is set to more than 0.01%. Preferably, it is 0.05% or more. More preferably, it is 0.10% or more. Even more preferably, it is 0.50% or more, and most preferably, it is 0.90% or more. However, if the Si content exceeds 2.50%, the melting point of zinc is lowered, which facilitates zinc penetration into the steel sheet during welding, resulting in a decrease in the LME resistance of the steel sheet. Therefore, the Si content is set to 2.50% or less. Preferably, it is 2.30% or less. More preferably, it is 2.00% or less. Even more preferably, it is 1.80% or less. Most preferably, it is 1.60% or less.

[0024] Mn: 0.10% or more and 5.00% or less Mn is an element that affects the area fractions of tempered martensite, bainite, and fresh martensite by improving hardenability. If the Mn content is less than 0.10%, soft phases such as ferrite are excessively formed, making it impossible to obtain the desired area fractions of tempered martensite, bainite, and fresh martensite, resulting in insufficient steel sheet strength. Therefore, the Mn content is set to 0.10% or more. Preferably, it is set to 0.50% or more. More preferably, it is set to 0.80% or more. Even more preferably, it is set to 1.00% or more, and most preferably, it is set to 2.00% or more. On the other hand, if the Mn content exceeds 5.00%, the area fractions of tempered martensite, bainite, and fresh martensite increase, resulting in a decrease in ductility. Therefore, the Mn content is set to 5.00% or less. Preferably, it is set to 4.50% or less. More preferably, it is set to 4.00% or less. Even more preferably, it is set to 3.70% or less, and most preferably, it is set to 3.50% or less.

[0025] P: 0.100% or less P may adversely affect LME resistance characteristics by segregating at grain boundaries and causing embrittlement, so its amount must be 0.100% or less. Therefore, the P content is set to 0.100% or less. Preferably, it is set to 0.080% or less. More preferably, it is set to 0.070% or less. Even more preferably, it is set to 0.050% or less. Most preferably, it is set to 0.040% or less. There is no particular lower limit, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably set to 0.001% or more. It is more preferably set to 0.003% or more, and even more preferably to 0.005% or more.

[0026] S: 0.0200% or less S segregates at grain boundaries, embrittling steel during hot working, and may adversely affect LME resistance by forming sulfides, so its amount must be 0.0200% or less. Therefore, the S content is 0.0200% or less. Preferably, it is 0.0180% or less. More preferably, it is 0.0150% or less. Even more preferably, it is 0.0100% or less, and most preferably, it is 0.0050% or less. There is no particular lower limit, but due to constraints on production technology, it is preferably 0.0001% or more. It is more preferably 0.0005% or more, and even more preferably, it is 0.0010% or more.

[0027] Al: 0.100% or less Al acts as a deoxidizer and is an effective element for reducing inclusions in steel, and is preferably added during the deoxidation process. However, Al raises the austenitization transformation point and causes ferrite to be included in the microstructure, making it difficult to achieve the desired TS when the Al content exceeds 0.100%. Therefore, the Al content is set to 0.100% or less. Preferably, it is set to 0.080% or less. More preferably, it is set to 0.070% or less, even more preferably, it is set to 0.060% or less, and most preferably, it is set to 0.050% or less. Although there is no particular lower limit, the Al content is preferably set to 0.001% or more. More preferably, it is set to 0.010% or more, and even more preferably, it is set to 0.020% or more.

[0028] N: 0.0100% or less N has an adverse effect on LME resistance characteristics by forming coarse nitrides. If the N content exceeds 0.0100%, a large amount of coarse nitrides is formed, resulting in a significant deterioration of LME resistance characteristics. Since the smaller the amount, the better, the N content is set to 0.0100% or less. Preferably, it is 0.0090% or less. More preferably, it is 0.0080% or less. Even more preferably, it is 0.0070% or less, and most preferably, it is 0.0060% or less. There is no particular lower limit, but due to constraints on production technology, it is preferably 0.0001% or more. It is more preferably 0.0010% or more, and even more preferably, it is 0.0020% or more.

[0029] O: 0.0100% or less O exists as an oxide and reduces the ductility of the steel sheet. Therefore, the O content needs to be 0.0100% or less. Therefore, the O content is set to 0.0100% or less. Preferably, it is 0.0075% or less. More preferably, it is 0.0060% or less. Even more preferably, it is 0.0050% or less, and most preferably, it is 0.0045% or less. Note that there is no particular lower limit for the O content, but due to constraints on production technology, the O content is preferably 0.0001% or more. It is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0030] Ti: 0.010% or more and 0.200% or less Ti contributes to precipitation strengthening and further refines the prior austenite grain size, resulting in refinement of tempered martensite and bainite, thereby effectively improving steel strength. To fully achieve this effect, the Ti content is set to 0.010% or more. Preferably, it is set to 0.012% or more. More preferably, it is set to 0.015% or more. Even more preferably, it is set to 0.020% or more. Most preferably, it is set to 0.025% or more. However, if the Ti content exceeds 0.200%, Ti may remain in an undissolved state during heating of the steel material before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, the Ti content is set to 0.200% or less. Preferably, it is set to 0.180% or less. More preferably, it is set to 0.150% or less, even more preferably, it is set to 0.100% or less, and most preferably, it is set to 0.050% or less.

[0031] Nb: 0.005% or more and 0.500% or less Nb is an element that improves LME resistance. In particular, the presence of solute Nb in steel during welding has a significant impact on improving LME resistance. Specifically, by dissolving Nb or controlling Nb precipitation so that it falls within a certain particle radius range, the solute elements present in the steel during welding or the solute elements formed by dissolution of precipitates prevent the penetration of zinc, thereby improving LME resistance. To fully achieve this effect, the Nb content must be 0.005% or more. Preferably, it is 0.007% or more. More preferably, it is 0.008% or more. Even more preferably, it is 0.010% or more, and most preferably, it is 0.012% or more. On the other hand, if the Nb content exceeds 0.500%, coarse precipitates such as Nb carbides and nitrides are formed, and the precipitates do not dissolve sufficiently during welding, preventing the improvement in LME resistance due to the solute elements. Furthermore, it is quite conceivable that a lack of El relative to the TS of the steel sheet will cause new cracks to originate from coarse precipitates not only during manufacturing but also after press working and during welding. Therefore, the Nb content is set to 0.500% or less, preferably 0.400% or less, more preferably 0.350% or less, even more preferably 0.200% or less, and most preferably 0.100% or less.

[0032] A high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components with the balance including Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components with the balance consisting of Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, and As. A total content of 0.100% or less of these impurities is permitted.

[0033] In addition to the above components, one or more selected from, by mass%, V: 0.500% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less may be contained.

[0034] V: 0.500% or less V contributes to precipitation strengthening and further refines the prior austenite grain size, thereby refining tempered martensite and bainite, thereby effectively improving steel strength. Therefore, V can be added as needed. While there is no particular lower limit, to achieve the above-mentioned effect, the V content is preferably 0.001% or more. A V content of 0.005% or more is more preferable, and a V content of 0.010% or more is even more preferable. However, if the V content exceeds 0.500%, V may remain in an undissolved state during heating of the steel material before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, when V is contained, the V content is set to 0.500% or less. Preferably, the V content is set to 0.400% or less. More preferably, the V content is set to 0.300% or less, even more preferably, 0.200% or less, and most preferably, 0.100% or less.

[0035] Ta: 0.10% or less Like Ti, Ta contributes to high strength by forming alloy carbides and alloy carbonitrides. Additionally, Ta can be added as needed to partially dissolve in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta)(C, N), significantly suppressing precipitate coarsening and stabilizing the contribution of precipitation strengthening to strength. While there is no particular lower limit, to achieve the above-mentioned effects, the Ta content is preferably 0.01% or more. It is more preferably 0.02% or more, and even more preferably 0.03% or more. However, excessive Ta content saturates the precipitate stabilization effect and increases alloy costs. Therefore, when Ta is added, the Ta content is set to 0.10% or less. It is preferably 0.08% or less. It is more preferably 0.07% or less. It is even more preferably 0.06% or less, and most preferably 0.05% or less.

[0036] W: 0.10% or less W can be added as needed to improve the hardenability of steel and further improve steel strength by refining tempered martensite and bainite. While there is no particular lower limit, to achieve the above-mentioned effects, the W content is preferably 0.01% or more. It is more preferably 0.02% or more, and even more preferably 0.03% or more. However, if the W content exceeds 0.10%, the amount of coarse precipitates such as WN and WS remaining in an undissolved state during slab heating in hot rolling may increase, resulting in reduced ductility. Therefore, when W is contained, the W content is set to 0.10% or less. It is preferably set to 0.08% or less. It is more preferably set to 0.07% or less. It is even more preferably set to 0.06% or less, and most preferably set to 0.05% or less.

[0037] B: 0.0100% or less B is an element that can improve hardenability by segregating at austenite grain boundaries. It forms a microstructure primarily composed of tempered martensite and bainite, improving the strength of the steel sheet and contributing to improved LME resistance. Therefore, B can be added as needed. While there is no particular lower limit, to achieve the above-mentioned effects, the B content is preferably 0.0003% or more. It is more preferably 0.0005% or more, and even more preferably 0.0007% or more. However, if the B content exceeds 0.0100%, coarse precipitates are formed, resulting in reduced ductility. Therefore, when B is added, the B content is set to 0.0090% or less. It is preferably set to 0.0080% or less. It is more preferably set to 0.0070% or less. It is even more preferably set to 0.0050% or less, and most preferably set to 0.0030% or less.

[0038] Cr: 1.00% or less Cr has the effect of improving the balance between strength and ductility, so it can be contained as necessary. Although there is no particular lower limit, in order to obtain the above effect, the Cr content is preferably 0.01% or more. 0.05% or more is more preferable, and 0.07% or more is even more preferable. However, if it is contained in excess of 1.00%, the area ratio of fresh martensite becomes excessive, and dimensional accuracy and ductility during forming decrease. Therefore, when Cr is contained, the Cr content is 1.00% or less. Preferably, it is 0.80% or less. More preferably, it is 0.60% or less. Even more preferably, it is 0.50% or less. Most preferably, it is 0.30% or less.

[0039] Mo: 1.00% or less Mo has the effect of improving the balance between strength and ductility, so it can be added as needed. While there is no particular lower limit, to obtain the above-mentioned effect, the Mo content is preferably 0.01% or more. It is more preferably 0.05% or more, and even more preferably 0.07% or more. However, if it is added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Mo is added, the Mo content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.50% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0040] Co: 1.00% or less Co is an element effective in improving hardenability and strengthening steel, so it can be added as needed. While there is no particular lower limit, to achieve the above effects, the Co content is preferably 0.01% or more. A content of 0.05% or more is more preferable, and a content of 0.07% or more is even more preferable. However, a content exceeding 1.00% results in an excessively large area fraction of fresh martensite, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Co is contained, the Co content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.60% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0041] Ni: 1.00% or less Ni increases the strength of steel through solid solution strengthening, so it can be contained as needed. While there is no particular lower limit, to obtain the above-mentioned effect, the Ni content is preferably 0.01% or more. It is more preferably 0.05% or more, and even more preferably 0.07% or more. However, if Ni is contained in an amount exceeding 1.00%, the area ratio of fresh martensite becomes excessive, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Ni is contained, the Ni content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.60% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0042] Cu: 1.00% or less Cu is an element effective in strengthening steel and can be contained as needed. While there is no particular lower limit, to obtain the above-mentioned effect, the Cu content is preferably 0.01% or more. A Cu content of 0.05% or more is more preferable, and a Cu content of 0.07% or more is even more preferable. However, if the Cu content exceeds 1.00%, the area ratio of tempered martensite, bainite, and fresh martensite becomes excessive, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. It is preferably set to 0.80% or less. It is more preferably set to 0.60% or less. It is even more preferably set to 0.30% or less, and most preferably set to 0.20% or less.

[0043] Sn: 0.200% or less, Sb: 0.200% or less. Sn and Sb suppress decarburization in a region of several tens of micrometers deep in the steel sheet surface, which occurs due to nitriding or oxidation of the steel sheet surface, and prevent a decrease in the area ratio of tempered martensite on the steel sheet surface. In addition to these effects, they can be added as needed to ensure strength and material stability. While there are no particular lower limits, to achieve the above effects, their contents are preferably 0.001% or more. 0.003% or more is more preferable, and 0.005% or more is even more preferable. However, excessive content of either of these elements exceeding 0.200% may cause the steel sheet to become embrittled, resulting in a decrease in ductility. Therefore, when Sn and Sb are contained, the Sn and Sb contents are set to 0.200% or less. Preferably, they are set to 0.100% or less. More preferably, they are set to 0.070% or less. Even more preferably, they are set to 0.050% or less, and most preferably, they are set to 0.030% or less.

[0044] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. When Ca, Mg, and REM are each 0.0100% or less, coarse precipitates and inclusions do not increase and Nb precipitation is not affected, so LME resistance does not deteriorate. Therefore, when Ca, Mg, and REM are contained, the Ca, Mg, and REM contents are set to 0.0100% or less. Preferably, they are set to 0.0050% or less. More preferably, they are set to 0.0040% or less, even more preferably, 0.0035% or less, and most preferably, 0.0030% or less. Note that there are no particular lower limits for the Ca, Mg, and REM contents. However, because these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, it is preferable that the Ca, Mg, and REM contents be set to 0.0001% or more. The content is more preferably 0.0005% or more, even more preferably 0.0007% or more, and most preferably 0.0010% or more.

[0045] Zr: 0.100% or less, Te: 0.100% or less. Zr and Te, when each is 0.100% or less, do not increase coarse precipitates or inclusions and do not affect Nb precipitation, so LME resistance does not deteriorate. Therefore, when Zr and Te are contained, their contents are set to 0.100% or less. They are preferably 0.080% or less. They are more preferably 0.070% or less, even more preferably 0.060% or less, and most preferably 0.050% or less. While there are no particular lower limits for the Zr and Te contents, they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the Zr and Te contents are preferably 0.001% or more. They are more preferably 0.010% or more, and even more preferably 0.020% or more.

[0046] Hf: 0.10% or less If Hf is 0.10% or less, coarse precipitates and inclusions do not increase and Hf does not affect the precipitation of Nb, so LME resistance does not deteriorate. Therefore, when Hf is contained, the Hf content is set to 0.10% or less. Preferably, it is set to 0.080% or less. More preferably, it is set to 0.070% or less, even more preferably, it is set to 0.060% or less, and most preferably, it is set to 0.050% or less. Note that there is no particular lower limit for the Hf content. However, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, when Hf is contained, the Hf content is preferably set to 0.003% or more. More preferably, it is set to 0.010% or more. More preferably, it is set to 0.020% or more, and even more preferably, it is set to 0.030% or more.

[0047] Bi: 0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase and Bi does not affect the precipitation of Nb, so LME resistance does not deteriorate. Therefore, when Bi is contained, the Bi content is set to 0.200% or less. Preferably, it is 0.100% or less. More preferably, it is 0.050% or less, even more preferably, it is 0.030% or less, and most preferably, it is 0.020% or less. Note that there is no particular lower limit for the Bi content, but since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. More preferably, it is 0.005% or more, and even more preferably, it is 0.010% or more.

[0048] In addition, when the content of each of the above-mentioned V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferred lower limit, the effect of the present invention is not impaired, and therefore, these elements are included as unavoidable impurities.

[0049] (2) Next, the microstructure will be explained.

[0050] Area Fraction of Tempered Martensite and Bainite: 40% to 85% Tempered martensite and bainite contribute to the strength of the steel sheet. Specifically, having a steel sheet mainly comprised of tempered martensite and bainite is effective in maintaining high strength. To fully achieve this effect, the total area fraction of bainite and tempered martensite must be at least 40% or more. Preferably, it is 42% or more. More preferably, it is 45% or more, even more preferably, it is 47% or more, and most preferably, it is 50% or more. On the other hand, if the sum of the area fractions of bainite and tempered martensite exceeds 85%, the steel structure will be dominated by hard phases such as tempered martensite, bainite, and fresh martensite. Furthermore, since these hard phases constrain the retained austenite, the TRIP effect will not be effectively manifested, resulting in poor ductility and making it difficult to achieve good formability. Therefore, the total area fraction of bainite and tempered martensite must be 85% or less, preferably, it is 83% or less. It is more preferably 82% or less, even more preferably 81% or less, and most preferably 80% or less.

[0051] Area fraction of fresh martensite: 0% or more and 25% or less Fresh martensite is a very hard phase, so it improves steel strength. Fresh martensite is not necessarily required if steel sheet strength is ensured, but if fresh martensite is included in the steel sheet structure, the steel sheet strength is further improved and even higher strength can be achieved. Therefore, the area fraction of fresh martensite must be 0% or more. Preferably, it is 2% or more. More preferably, it is 3% or more. Even more preferably, it is 4% or more, and most preferably, it is 5% or more. On the other hand, fresh martensite reduces the ductility of steel, making it difficult to achieve good formability. Therefore, the area fraction of fresh martensite must be 25% or less. Preferably, it is 23% or less. More preferably, it is 22% or less. Even more preferably, it is 21% or less, and most preferably, it is 20% or less.

[0052] Area Fraction of Retained Austenite: 5% or More and 20% or Less. Retained austenite transforms into martensite due to the TRIP effect during processing, increasing strength while also enhancing strain dispersion and improving ductility. Therefore, to ensure good formability, the area fraction of retained austenite must be 5% or more. Preferably, it is 7% or more, more preferably 8% or more, even more preferably 9% or more, and most preferably 10% or more. On the other hand, if the area fraction of retained austenite exceeds 20%, elements concentrated in the retained austenite, particularly Si, diffuse during welding, causing a decrease in the melting point of zinc. This may facilitate zinc penetration into the steel sheet during welding and result in a deterioration of the LME resistance of the steel sheet. Therefore, to ensure sufficient ductility while preventing deterioration of LME resistance, the area fraction of retained austenite must be 20% or less. Preferably, it is 18% or less, more preferably 17% or less, even more preferably 16% or less, and most preferably 15% or less.

[0053] In the present invention, even if a residual structure such as ferrite or pearlite is contained, the effects of the present invention are not impaired. When a residual structure is present, the residual structure includes at least one of ferrite and pearlite, and specifically, the area ratio of at least one of ferrite and pearlite is 0% or more. The area ratio of the residual structure is preferably 20% or less, more preferably 18% or less, even more preferably 15% or less, and most preferably 13% or less.

[0054] The amount of dissolved Nb in the steel sheet (Nb ｓｏｌ ) and the Nb content in Nb precipitates with a grain size of less than 20 nm (Nb ｐｒｅ ) and the total amount of Nb (Nb) contained in the steel sheet satisfies the following (Formula 1): (Formula 1) (Nb ｓｏｌ / Nb) + (Nb ｐｒｅ / Nb) ≧ 0.40 (Formula 1) ｓｏｌ : Solid solution Nb amount (mass%), Nb ｐｒｅ : represents the amount of Nb (mass%) in Nb precipitates with a grain size of less than 20 nm. ｓｏｌ, unit is mass %, and the Nb content in Nb precipitates with a grain size of less than 20 nm (Nb ｐｒｅ , unit: mass %, and the total amount of Nb (Nb) contained in the steel is one of the important components of the present invention. In the present invention, by making the Nb in the steel sheet solid solution or making it possible to make it solid solution during welding, LME due to transfer LME is prevented and LME resistance properties are improved. The inventors varied the state of Nb contained in the steel sheet by producing various steels, and found that LME resistance properties are improved when (Equation 1) is satisfied. The mechanism by which the solid solution state of Nb in steel during welding improves LME resistance properties is unclear, but it is thought that when Nb is present in a solid solution state, it prevents zinc penetration into the steel sheet, which is the direct cause of LME cracking. From the above, it is necessary to satisfy (Equation 1). (Nb ｓｏｌ / Nb) + (Nb ｐｒｅ / Nb) is preferably 0.41 or more, and more preferably 0.42 or more. There is no particular upper limit to the value, but (Nb ｓｏｌ / Nb) + (Nb ｐｒｅ / Nb) is preferably 0.95 or less, and more preferably 0.90 or less. The lower limit of the Nb precipitates is not particularly limited, but the particle size may be 0.1 nm or more.

[0055] Diffusible hydrogen content in steel is 0.50 mass ppm or less In order to ensure ductility and have good formability, the diffusible hydrogen content in steel is set to 0.50 mass ppm or less. It is preferably set to 0.30 mass ppm or less. It is more preferably set to 0.25 mass ppm or less. It is even more preferably set to 0.20 mass ppm or less, and most preferably set to 0.15 mass ppm or less. There is no particular lower limit for the diffusible hydrogen content in steel, but due to constraints on production technology, the diffusible hydrogen content in steel is preferably set to 0.01 mass ppm or more. It is more preferably set to 0.02 mass ppm or more. It is even more preferably set to 0.03 mass ppm or more, and most preferably set to 0.05 mass ppm or more.

[0056] (3) Next, the manufacturing method and manufacturing conditions will be explained.

[0057] [Slab (steel material) heating process] Heating conditions of the steel material: temperature T represented by (Equation 2) ｓｏｌ Heating at ℃ or higher for 1.0 hour or more Precipitates present in the heating stage of the steel material exist as coarse precipitates in the finally obtained steel sheet, which not only adversely affects the LME resistance characteristics but also does not contribute to strength and cannot be expected to improve the TS × El balance. Therefore, it is necessary to re-dissolve as many coarse precipitates that precipitated during casting as possible. ｓｏｌ If the heating temperature is below T °C or the heating time is less than 1 hour, the solid solution of Nb becomes insufficient and the formula 1 is not satisfied, which leads to poor LME resistance and an increased risk of trouble occurring during hot rolling due to an increased rolling load. Also, from the viewpoint of scaling off defects such as bubbles and segregation in the surface layer of the steel material, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, the heating temperature of the steel material is T ｓｏｌ ° C. or higher. Therefore, the heating temperature of the steel material is T ｓｏｌ ° C. or higher. Preferably, T ｓｏｌ × 1.1 ° C. or more. More preferably, T ｓｏｌ × 1.2°C or more. More preferably, T ｓｏｌ × 1.3 ° C or more, and most preferably T ｓｏｌ × 1.5°C or more. Furthermore, the heating time at the above temperature is 1.0 hour or more. Preferably, it is 1.1 hours or more. More preferably, it is 1.2 hours or more. Even more preferably, it is 1.3 hours or more, and most preferably, it is 1.5 hours or more. Furthermore, there is no particular upper limit to the heating temperature of the steel material, but if it exceeds 1500°C, the amount of oxidation increases and scale loss increases, so the heating temperature of the steel material is preferably 1500°C or less. More preferably, it is 1450°C or less. Even more preferably, it is 1400°C or less. (Equation 2) T ｓｏｌ =7900 / (3.42-(log([Nb%][C%])))-273

[0058] [Hot rolling process] Finish rolling start temperature of hot rolling: T ＦＥＴThe steel material after heating is hot-rolled to form a hot-rolled steel sheet. At this time, the upper and lower limits of the start temperature of the finish rolling are not particularly defined. ｓｏｌ If the temperature is less than 100°C, the Nb dissolved during heating of the steel material will precipitate, the precipitate grain size will become large, and the formula (1) will no longer be satisfied, resulting in poor LME resistance. Therefore, the finish rolling start temperature should be set to T ｓｏｌ It is preferable to set the temperature at 100°C or higher. ｓｏｌ °C - 70 °C or higher. More preferably, T ｓｏｌ °C - 50 °C or higher. Most preferably, T ｓｏｌ °C -30 °C or higher. On the other hand, if the finish rolling start temperature exceeds 1200 °C, scale loss during slab preheating increases and it may cause the plate to crack during hot rolling. For this reason, the finish rolling start temperature for hot rolling is preferably 1200 °C or lower. More preferably, it is 1170 °C or lower. Even more preferably, it is 1150 °C or lower. Most preferably, it is 1120 °C or lower.

[0059] Finishing temperature of hot rolling: T ＦＤＴ The heated steel material is hot-rolled to obtain a hot-rolled steel sheet. While there are no particular upper or lower limits for the finish rolling temperature, if the finish rolling temperature is below 800°C, Nb dissolved during heating of the steel material precipitates, resulting in an increased amount of Nb precipitation. Furthermore, the rolling load increases, which may hinder cold rolling. Therefore, the finish rolling temperature of hot rolling is preferably 800°C or higher, more preferably 820°C or higher, even more preferably 840°C or higher, and most preferably 850°C or higher. Furthermore, if the finish temperature exceeds 1000°C, the amount of oxide (scale) generated increases rapidly, the interface between the base steel and the oxide becomes rough, and the surface quality after pickling and cold rolling tends to deteriorate. Furthermore, the crystal grains become excessively coarse, which may cause surface roughness of the pressed product during processing. Therefore, the finish rolling temperature is preferably 1000°C or lower, and more preferably 980°C or lower. More preferably, it is 970° C. or less, and most preferably, it is 950° C. or less.

[0060] Effective time: t ＨＲThe time required from the start of finish rolling to the end of finish rolling is called effective time t ＨＲ Since Nb contained in the steel sheet begins to precipitate and grow during finish rolling, the effective time t ＨＲ is one of the parameters for controlling the morphology of Nb to improve the LME resistance characteristics. ＨＲ Although there is no particular upper or lower limit for the effective time t, it is preferably 3 seconds or more, more preferably 4 seconds or more, even more preferably 5 seconds or more, and most preferably 7 seconds or more. ＨＲ is preferably 15 seconds or less, more preferably 12 seconds or less, even more preferably 11 seconds or less, and most preferably 10 seconds or less.

[0061] [Winding process] T ＦＤＴ Residence time from ° C. to 650 ° C.: t ＣＴ The time from the finish rolling completion temperature to 650°C is called the residence time t ＣＴ From the time when finish rolling is completed until the sheet temperature reaches 650°C, the precipitation of Nb and the growth of precipitated Nb are likely to proceed, so the residence time t ＣＴ is one of the parameters for controlling the morphology of Nb to improve the LME resistance. ＣＴ Although there is no particular upper or lower limit for the residence time t, it is preferably 5 seconds or more, more preferably 7 seconds or more, even more preferably 8 seconds or more, and most preferably 9 seconds or more. ＣＴ is 20 seconds or less, more preferably 18 seconds or less, even more preferably 17 seconds or less, and most preferably 15 seconds or less.

[0062] Coiling temperature after hot rolling T ＣＴ°C: 650°C or less If the coiling temperature after hot rolling is higher than 650°C, precipitation of Nb and growth of the precipitated Nb will proceed excessively, and Equation 1 will no longer be satisfied, which may cause deterioration of LME resistance. In addition, an oxide film that is difficult to remove by pickling will form on the surface of the hot-rolled sheet, which may cause a deterioration in the surface appearance after cold rolling. Therefore, the coiling temperature after hot rolling is set to 650°C or less. Preferably, it is set to 630°C or less. More preferably, it is set to 620°C or less. Still more preferably, it is set to 610°C or less, and most preferably, it is set to 600°C or less. Note that there is no particular lower limit for the coiling temperature; however, if the coiling temperature is lower than 300°C, the strength of the hot-rolled sheet will increase, the rolling load in cold rolling will increase, and defects in the sheet shape will occur, resulting in a decrease in productivity. Therefore, the lower limit of the coiling temperature is preferably set to 300°C or more. More preferably, it is set to 320°C or more. Still more preferably, it is set to 350°C or more. Most preferably, the temperature is 400°C or higher.

[0063] The obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to intermediate heat treatment at a temperature of less than 650°C as necessary to prevent an increase in load during subsequent cold rolling. In this case, the temperature is preferably 150°C or higher. For example, heat treatment may be performed in a box annealing furnace at a soaking temperature of 500°C for a soaking time of 4 hours.

[0064] The obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to treatment such as pickling, if necessary. The pickling method for the hot-rolled coil may be a conventional method. Furthermore, the hot-rolled coil may be subjected to skin-pass rolling in order to correct the shape and improve the pickling properties.

[0065] After hot rolling and / or intermediate heat treatment and / or pickling, the steel may be subjected to an annealing step (heat treatment) as described below, or may be subjected to cold rolling followed by heat treatment. When cold rolling is performed, the cold reduction is preferably 25% or more, more preferably 30% or more, even more preferably 32% or more, and most preferably 35% or more. On the other hand, excessive reduction increases the rolling load and leads to an increase in the load on the cold rolling mill, so the upper limit of the cold rolling reduction is preferably 75% or less, more preferably 70% or less, even more preferably 67% or less, and most preferably 65% ​​or less.

[0066] [Annealing process] From 650 ° C to soaking temperature T ＡＴ Temperature rise time to 650°C: t When the sheet temperature reaches 650°C or higher, precipitation of Nb and growth of the precipitated Nb progress, so the temperature rise time t is one of the parameters for controlling the form of Nb to improve LME resistance. Although no upper or lower limit is particularly set for the temperature rise time t, it is preferably 300 seconds or more. More preferably, it is 400 seconds or more. Even more preferably, it is 450 seconds or more, and most preferably, it is 490 seconds or more. Furthermore, the temperature rise time t is preferably 700 seconds or less. More preferably, it is 650 seconds or less. Even more preferably, it is 600 seconds or less, and most preferably, it is 590 seconds or less.

[0067] Soaking temperature: T ＡＴ When the soaking temperature T is maintained in the temperature range of 750°C or higher and 950°C or lower, the steel sheet is held in the two-phase region, and the area ratio of tempered martensite and bainite in the final structure becomes less than 40%, making it impossible to ensure sufficient strength of the steel sheet. ＡＴ °C is set to 750 °C or higher, preferably 770 °C or higher, more preferably 800 °C or higher, and further preferably 840 °C or higher. On the other hand, when the temperature is maintained in a temperature range exceeding 950 °C, Nb is likely to precipitate, and the growth of Nb precipitates proceeds excessively, so that (Equation 1) is no longer satisfied, and the LME resistance characteristics deteriorate. For this reason, the soaking temperature T ＡＴThe temperature is set to 950°C or less, preferably 940°C or less, more preferably 930°C or less, and further preferably 920°C or less.

[0068] Soaking temperature T ＡＴ Holding time at ° C: t ＡＴ Soaking temperature T ＡＴ Holding time t in °C ＡＴ is one of the control parameters for promoting the austenitization of the steel sheet during soaking, and is also involved in the precipitation of Nb and the growth of Nb precipitates, and is therefore one of the parameters for controlling the LME resistance characteristics. ＡＴ Although there is no particular upper or lower limit for this time, in order to sufficiently promote austenitization, it is preferably 15 seconds or more, more preferably 30 seconds or more, even more preferably 50 seconds or more, and most preferably 100 seconds or more. Furthermore, from the viewpoint of making the grains of Nb precipitates fine and improving the LME resistance characteristics, it is preferably 1000 seconds or less, more preferably 500 seconds or less, even more preferably 400 seconds or less, and most preferably 350 seconds or less.

[0069] Q ＨＲ and Q ＣＴ and Q ＡＴ The sum of the above is 6000 or less. In order to suppress the precipitation of Nb and the growth of Nb precipitates during heating in the hot rolling process, coiling process, and annealing process, the finish rolling start temperature T ＦＥＴ °C, finish rolling end temperature T ＦＤＴ °C, effective time t from the start of finish rolling to the end of finish rolling ＨＲ Q defined by the following (Equation 3) ＨＲ and in the winding step, T ＦＤＴ Residence time t from 650 ° C ＣＴ , winding temperature T ＣＴ Q defined by the following formula (4) in ° C. ＣＴ In the annealing step, the soaking temperature T ＡＴ ℃, 650℃ to soaking temperature T ＡＴ Temperature rise time t to °C, soaking temperature T ＡＴ Holding time t in °C ＡＴ Q defined by the following (Equation 5) ＡＴsatisfies the following (Equation 6), that is, Q ＨＲ and Q ＣＴ and Q ＡＴ It is an important requirement in the present invention that the sum of Q (Equation 3) is 6000 or less, and the reason for this will be explained below. ＨＲ = 0.5 (T ＦＥＴ +T ＦＤＴ ) × log １０ (t ＨＲ ) (Formula 4)Q ＣＴ = 0.5 (T ＣＴ +650) × log １０ (t ＣＴ ) (Formula 5) Q ＡＴ =0.5(650+T ＡＴ ) × log １０ (t) + T ＡＴ ×log １０ (t ＡＴ ) (Formula 6)Q ＨＲ +Q ＣＴ +Q ＡＴ ≦6000 In order to suppress the precipitation and growth of Nb during the hot rolling process, coiling process, and heating process, T ｓｏｌ The effective time t when heat of 650°C or more is applied is as follows: ＨＲ , residence time t ＣＴ , heating time t, holding time t ＡＴ For each of the above, Q is defined by the temperature at the start and end of each time. ＨＲ and Q ＣＴ and Q ＡＴ It is an important requirement in the present invention that the sum of

[0070] Q ＨＲ and Q ＣＴ and Q ＡＴ If the sum of Q exceeds 6000, Nb will precipitate excessively and the precipitates will grow, the amount of Nb that improves the LME resistance will decrease, and the LME resistance will be poor. ＨＲ and Q ＣＴ and Q ＡＴ The sum of these is 6000 or less, preferably 5990 or less, more preferably 5980 or less, even more preferably 5970 or less, and most preferably 5950 or less. ＨＲ and Q ＣＴ and Q ＡＴAlthough the lower limit of the sum is not particularly set, taking into consideration the range that can be implemented in actual operation, it is preferably 3500 or more, more preferably 3600 or more, even more preferably 3700 or more, most preferably 3800 or more, and most preferably 3900 or more.

[0071] Q ＡＴ The value of Q is 4700 or less (preferable condition). By taking into consideration the heat applied during heating in the hot rolling process, coiling process, and annealing process, from the viewpoint of suppressing excessive precipitation and growth of Nb, ＨＲ and Q ＣＴ and Q ＡＴ Therefore, the upper and lower limits of each value are not particularly set, but from the viewpoint of suppressing excessive growth of Nb during heating in the annealing process, which is directly related to the final structure of the steel sheet, Q ＡＴ It is preferable that Q be 4700 or less, more preferably 4650 or less, and further preferably 4630 or less. It is most preferably 4600 or less. From the viewpoint of increasing the strength of steel, in order to more precisely control the austenite fraction during annealing, Q ＡＴ is preferably 3500 or more, more preferably 3800 or more, even more preferably 4100 or more, and most preferably 4120 or more.

[0072] Similarly, in the hot rolling process and the coiling process, from the viewpoint of suppressing excessive growth of Nb, Q ＨＲ It is preferable that Q is 1200 or less, more preferably 1170 or less, and further preferably 1150 or less. ＣＴ It is preferable that Q is 850 or less, more preferably 840 or less, and further preferably 835 or less. On the other hand, there is no particular lower limit, but Q ＨＲ It is preferable that the value is 250 or more. ＨＲ is more preferably 600 or more, and further preferably 700 or more. ＣＴ It is preferable that the value of Q is 150 or more. ＣＴ is more preferably 170 or more, and even more preferably 180 or more.

[0073] In the annealing process, the steel sheet is heated to a soaking temperature T ＡＴ and retention time t ＡＴ After holding the temperature for a certain period, the material may be cooled as is, or may be cooled to an arbitrary holding temperature and then held, or may be cooled to an arbitrary cooling stop temperature, then reheated to the holding temperature, held, and then cooled.

[0074] Reheating may also be performed after the annealing process. Forming martensite in the steel sheet and then tempering it concentrates C in the untransformed austenite, improving the stability of the austenite and thereby increasing the area ratio of retained austenite contained in the steel sheet after cooling, thereby further improving ductility. For this reason, when reheating is performed, it is preferable to cool the steel sheet to a cooling stop temperature of 100°C or higher and 350°C or lower, reheat it to a holding temperature of 200°C or higher and 450°C or lower, hold it, and then cool it to room temperature. The reheating temperature is preferably 200°C or higher, more preferably 210°C or higher, even more preferably 230°C or higher, and most preferably 250°C or higher. The reheating temperature is preferably 450°C or lower, more preferably 430°C or lower, even more preferably 410°C or lower, and most preferably 400°C or lower.

[0075] Furthermore, if reheating is not performed, it is preferable to hold the temperature at 200°C or higher and 450°C or lower for 10 seconds or longer and then cool to room temperature. It is more preferable to hold the temperature at 200°C or higher and 450°C or lower for 20 seconds or longer, and even more preferable to hold it for 25 seconds or longer. There is no particular upper limit to the holding time, but it is preferably 500 seconds or shorter, and more preferably 400 seconds or shorter.

[0076] For example, when hot-dip galvanizing is performed, the annealed steel sheet is immersed in a galvanizing bath at 440°C or higher and 500°C or lower to perform the hot-dip galvanizing treatment, and then the coating weight is adjusted by gas wiping or the like. The temperature of the coating bath is preferably 440°C or higher, more preferably 450°C or higher, and even more preferably 455°C or higher. The temperature of the coating bath is also preferably 500°C or lower, more preferably 490°C or lower, and even more preferably 485°C or lower. There are no particular restrictions on the coating treatment conditions, but the coating weight (coating weight per side) is preferably 20 g / m from the viewpoint of corrosion resistance and coating weight control. ２ The plating coverage is preferably 25 g / m or more. ２ More preferably, it is 30 g / m or more. ２ It is even more preferable that the thickness is 32 g / m or more. ２ From the viewpoint of adhesion, it is most preferable that the thickness is 120 g / m or more. ２ The plating weight is preferably 100 g / m or less. ２ More preferably, it is 70 g / m or less. ２ It is even more preferable that the thickness is 65 g / m or less. ２ Most preferably, the following is true:

[0077] It is preferable that the hot-dip galvanizing be performed using a galvanizing bath containing 0.08% or more and 0.30% or less of Al. That is, the Al content in the hot-dip galvanizing is preferably 0.08% or more, more preferably 0.09% or more, even more preferably 0.10% or more, and most preferably 0.12% or more. The Al content in the hot-dip galvanizing is preferably 0.30% or less, more preferably 0.25% or less, even more preferably 0.22% or less, and most preferably 0.20% or less. Furthermore, the effects of the present invention remain unchanged even if the plating bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.

[0078] When hot-dip galvanizing is performed, the hot-dip galvanizing treatment is performed in a temperature range of 450°C or higher and 600°C or lower after the hot-dip galvanizing treatment. If the alloying treatment is performed at a temperature higher than 600°C, untransformed austenite may transform into pearlite, resulting in a retained austenite area ratio of less than 5%, and ductility may be reduced. Therefore, when performing hot-dip galvanizing treatment, the hot-dip galvanizing treatment is preferably performed in a temperature range of 450°C or higher. 460°C or higher is more preferable. 465°C or higher is even more preferable, and 470°C or higher is most preferable. Furthermore, when performing hot-dip galvanizing treatment, the hot-dip galvanizing treatment is preferably performed in a temperature range of 600°C or lower. 570°C or lower is more preferable. 550°C or lower is even more preferable, and 530°C or lower is most preferable. The Fe concentration of the coating layer of the alloyed hot-dip galvanized steel sheet is preferably 8 to 17%. That is, the Fe concentration in the plating layer of the alloyed hot-dip galvanized steel sheet is preferably 8% or more, more preferably 9% or more, and even more preferably 10% or more. The Fe concentration in the plating layer of the alloyed hot-dip galvanized steel sheet is preferably 17% or less, more preferably 16% or less, and even more preferably 15% or less. An alloyed plating layer is formed by performing alloying plating treatment.

[0079] Steels having the chemical compositions shown in Tables 1-1 and 1-2, with the balance being Fe and unavoidable impurities, were melted in a converter and continuously cast into slabs. The resulting slabs were subjected to a heating process, a hot rolling process, a coiling process, and an annealing (heating) process under the conditions shown in Tables 2-1 and 2-2. If reheating was performed, the slabs were cooled to a cooling stop temperature of 100°C to 350°C, then reheated to a holding temperature of 200°C to 450°C, and held thereafter, and then cooled to room temperature. If reheating was not performed, the slabs were held at a holding temperature of 200°C to 450°C for 10 seconds or more, then cooled to room temperature, and cold-rolled at a reduction rate of 50% to obtain high-strength cold-rolled steel sheets (CR). Some steel sheets were subjected to the coiling process, and then the steels were subjected to a heating process without cold rolling to obtain hot-rolled steel sheets (HR). Furthermore, some of the steel sheets were subjected to a hot-dip galvanizing treatment to obtain hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA). The hot-dip galvanizing bath used for the hot-dip galvanized steel sheets (GI) was a zinc bath containing 0.19 mass% Al, and the hot-dip galvanized steel sheets (GA) was a zinc bath containing 0.14 mass% Al, and the bath temperature was 465°C. The coating weight per side was 45 g / m ２ The GA was adjusted so that the Fe concentration in the plating layer was in the range of 9 mass % or more and 12 mass % or less.

[0080] The cross-sectional microstructure, tensile properties, and LME resistance properties of the obtained steel sheets were investigated, and the results are shown in Tables 3-1 and 3-2.

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087] The area ratios of fresh martensite, tempered martensite, and bainite were determined by polishing a thickness cross section (L cross section) of the steel sheet parallel to the rolling direction, etching it with nital, and observing 10 fields of view at 2000x magnification using a scanning electron microscope (SEM) at a 1 / 4 position in the thickness direction (a position corresponding to 1 / 4 of the thickness in the depth direction from the steel sheet surface). Using the obtained structural images, the area ratios of each structure (the total of tempered martensite and bainite, and fresh martensite) were calculated. In the structural images, fresh martensite was defined as the light gray region of the structure, and tempered martensite and bainite were defined as the region where carbides precipitated in the dark gray structure.

[0088] The area ratio of retained austenite was determined by polishing the steel plate from the 1 / 4 position of the plate thickness down to a surface of 0.1 mm, and then polishing the surface by a further 0.1 mm by chemical polishing, measuring the integrated intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron using CoKα radiation with an X-ray diffractometer, and averaging the nine integrated intensity ratios obtained.

[0089] To measure the amount of hydrogen in steel, test pieces of approximately 5 x 30 mm were cut out of hot-rolled, cold-rolled, or galvanized steel sheets, and the galvanized steel sheets were used to remove the plating from the surface of the test pieces. After placing the test pieces in a quartz tube and replacing the atmosphere in the quartz tube with Ar, the temperature was increased at 200°C / hr, and the amount of hydrogen released up to 400°C was measured by temperature-programmed analysis using a gas chromatograph. The cumulative amount of hydrogen detected in the temperature range from room temperature (25°C) to less than 250°C was taken as the amount of diffusible hydrogen.

[0090] The total amount of Nb contained in the steel (Nb ｔｏｔａｌ ), the amount of dissolved Nb in the steel sheet (Nb ｓｏｌ ) and the amount of Nb with a particle size of less than 20 nm (Nb ｐｒｅ) was measured by the following procedure. First, the total amount of Nb (Nb) contained in the steel was measured by wet chemical analysis. Next, the method for measuring the amount of Nb with a particle size of 20 nm or less will be described. For the amount of Nb precipitates with a particle size of 20 nm or less, precipitates in the steel material were captured as residuals, the amount of Nb in the total residuals was determined, and then the amount of Nb present in the residuals with a particle size of 20 nm or more was determined, and the amount of Nb was determined by obtaining the difference between the amount of Nb in the total residuals and the amount of Nb present in the residuals with a particle size of 20 nm or more. The specific procedure is as follows. Multiple test pieces of hot-rolled steel sheet, cold-rolled steel sheet, or galvanized steel sheet cut to approximately 20 x 50 mm were prepared, and for galvanized steel sheet, the plating on the surface of the test piece was removed using a router (precision grinder). The surfaces of the collected test pieces were polished to a depth of approximately 50 μm by preliminary electrolytic polishing to obtain a new surface. The obtained test pieces were electrolyzed using 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol as an electrolyte for deposit extraction. The obtained electrolytic solution was passed through a filter with a pore size of 0.2 μm to capture the residue, and then the residue was acid-decomposed and the Nb concentration was quantified in mass% using ICP atomic emission spectrometry. This was taken as the amount of Nb in the total residue. Subsequently, the remaining test pieces were electrolyzed using the same 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol. At this time, the residues adhering to the remaining metal sample after electrolysis were immersed in separately prepared methanol, and the residues adhering to the remaining metal sample were collected in a container using ultrasonic vibration. Thereafter, the electrolytic solution and the methanol containing the residues adhering to the remaining metal sample were used to capture the residues using an alumina filter with a pore size of 20 nm. These residues were immersed in hexametaphosphoric acid and dispersed in the hexametaphosphoric acid using ultrasonic vibration. Here, the residuals in the hexametaphosphoric acid in which the residuals were dispersed were captured using a new alumina filter with a pore size of 20 nm. Here, the residuals captured on the alumina filter had a particle size of 20 nm or more. These captured residuals were acid-decomposed, and the Nb concentration was quantified in mass% using ICP emission spectrometry. This was taken as the amount of Nb present in the residuals with a particle size of 20 nm or more.Here, the difference between the amount of Nb in all residuals and the amount of Nb present in residuals with a particle size of 20 nm or more was calculated, and this was taken as the amount of Nb present in residuals with a particle size of 20 nm or less (Nb. ｐｒｅ Next, the amount of solute Nb in the steel was calculated by calculating the difference between the total amount of Nb (Nb) contained in the steel and the amount of Nb in the total residual calculated by the above method. ｓｏｌ The Nb precipitates are mainly NbC, but may contain NbN or other Nb precipitates.

[0091] The tensile test was carried out in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece, which was sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet, and TS (tensile strength) and EL (total elongation) were measured. In the examples, steel sheets with a TS of 980 MPa or more were judged to have achieved the target strength and passed. １．５ Steel sheets with an xEl of 390,000 or more were judged to have achieved the target formability and passed the test.

[0092] The LME resistance was evaluated using an evaluation sample obtained by taking a sample from the steel plate with a dimension of 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction, and overlapping it with a 980GA sample cut to the same size.

[0093] Resistance spot welding was performed on the evaluation samples with a stroke angle θ, and the welding pressure was 3.5 kN. Here, the stroke angle in spot welding is defined as the angle θ between a line passing through the major axis of the nugget and a line parallel to the surface of the steel sheet in the cross section of the spot-welded member. The welding current pattern was controlled so that the resulting nugget diameter was in the range of 3.5√t to 5.5√t, where t is the thickness of one steel sheet (1.2 mm). A Dr6-type CuCr electrode was used for resistance spot welding, and the clearance between the overlapping evaluation samples and the electrode was 2.0 mm. The welding stroke angle and hold time for the evaluation of LME resistance characteristics are shown in Tables 3-1 and 3-2.

[0094] For each evaluation, five evaluation samples were spot welded to prepare welded components, and the cross section of the spot-welded components was observed using an optical microscope (magnification 100x) to evaluate the LME resistance. When cracks occurred in two or less components and the average crack depth was less than 100 μm (including cases where no cracks occurred), the evaluation was rated as ◎; when cracks occurred in two or less components and the average crack depth was 100 μm or more but less than 300 μm, the evaluation was ◯; when cracks occurred in two or less components and the average crack depth was 300 μm or more, the evaluation was △; and when cracks occurred in three or more components, the evaluation was ×. In the examples, steel sheets evaluated as ◎, ○, or △ were judged to have excellent LME properties and were designated as inventive examples, while steel sheets evaluated as × were judged to have poor LME properties and were designated as comparative examples.

[0095] The high-strength steel sheets of the invention examples all have high TS, TS×El balance, and excellent LME resistance, while the comparative examples are inferior in at least one of TS, TS×El balance, and LME resistance.

Claims

1. In mass percent, C: 0.030% or more and 0.500% or less, Si: more than 0.01% and less than 2.50%, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less; and Ti: 0.010% or more and 0.200% or less, Nb: 0.005% or more and 0.500% or less, The composition is composed of Fe and unavoidable impurities. The microstructure of the steel plate at the 1 / 4 position of the plate thickness is The total area ratio of tempered martensite and bainite is 40% or more and 85% or less, The area ratio of fresh martensite is 0% or more and 25% or less, The area ratio of retained austenite is 5% or more and 20% or less, The remainder is at least one of ferrite and pearlite in an area ratio of 0% to 20%. The Nb is the amount of solid solution Nb (Nb sol ) and the amount of Nb in Nb precipitates with a grain size of less than 20 nm (Nb pre ) and the total Nb amount (Nb) contained in the steel sheet satisfies the following (Formula 1), A high-strength steel plate having a diffusible hydrogen content of 0.50 mass ppm or less. (Formula 1) (Nb sol / Nb) + (Nb pre / Nb) ≥ 0.40 In formula 1, Nb sol : Solid solution Nb amount (mass%), Nb pre : represents the amount of Nb (mass%) in Nb precipitates having a grain size of less than 20 nm.

2. Furthermore, the composition is as follows: V: 0.500% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, The high strength steel plate according to claim 1 , further comprising one or more selected from the following:

3. 3. The high strength steel sheet according to claim 1, further comprising a plating layer on the surface of the steel sheet.

4. The high strength steel sheet according to claim 3, wherein the plating layer is an alloy plating layer.

5. A method for producing a high strength steel plate according to claim 1 or 2, The steel material having the above-mentioned composition is heated to a temperature T sol A slab heating step of heating at 0.4 ° C. or more for 1.0 hour or more; A hot rolling process; Winding temperature T CT a winding step in which the temperature is 650° C. or less; Soaking temperature T AT The soaking temperature T is 750° C. or more and 950° C. or less. AT and an annealing step of holding the steel at 0.4 ° C. and cooling the steel after holding the steel, In the hot rolling process, the finish rolling start temperature T FET °C, finish rolling end temperature T FDT ° C., effective time t from the start of finish rolling to the end of finish rolling HR Q defined by (Equation 3) HR and, In the winding process, T FDT Residence time t from 650°C to 650°C CT , winding temperature T CT Q defined by (Equation 4) in °C CT and, In the annealing step, the soaking temperature T AT ℃, 650℃ to soaking temperature T AT Heating time t up to °C, soaking temperature T AT Holding time t in °C AT Q defined by (Equation 5) AT A manufacturing method of a high strength steel plate satisfying (Equation 6). (Formula 2) T sol =7900 / (3.42-(log([Nb%][C%])))-273 Here, [Nb%] and [C%] are the amount of Nb (mass%) and the amount of C (mass%) contained in the steel, respectively. (Equation 3) Q HR =0.5(T FET +T FDT )×log 10 (t HR ) (Formula 4) Q CT =0.5(T CT +650)×log 10 (t CT ) (Equation 5) Q AT = 0.5(650 + T AT ) × log 10 (t) + T AT × log 10 (t AT ) (Formula 6)Q HR +Q CT +Q AT ≦6000

6. Q AT The method for producing a high strength steel plate according to claim 5, wherein the value of is 4700 or less.

7. The method for producing a high strength steel sheet according to claim 5, further comprising the step of: carrying out a plating treatment after the annealing step.

8. A method for manufacturing a high-strength steel plate as described in claim 6, wherein a plating treatment is performed after the annealing process.

9. The method for producing a high strength steel sheet according to claim 7, wherein the plating treatment is an alloying plating treatment.

10. A method for manufacturing a high-strength steel plate as described in claim 8, wherein the plating process is an alloying plating process.