High-strength galvanized steel sheet, and a method for manufacturing a hot-rolled steel sheet, a cold-rolled steel sheet, and a high-strength galvanized steel sheet

Abstract

The present invention provides plated steel sheets that have higher strength and superior galvanizability than conventional steel sheets, have good press workability, and are excellent in shape, and a method for manufacturing the same. The microstructure at 1 / 4 of the plate thickness has an area ratio of ferrite of 5.0 to 40.0%, an area ratio of bainite of 40.0% or less, and an area ratio of martensite of 30.0 to 70.0%, and the martensite grain size (d surface ) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) is 0.70 to 2.00, and the martensite fraction (M surface ) and the martensite fraction at 1 / 4 of the plate thickness (M 1 / 4 ) ratio (M surface / M 1 / 4 ) is 0.80 or more, the amount of diffusible hydrogen in the steel is less than 0.75 ppm by mass, and the steel sheet has a zinc plating layer on the surface thereof.

Description

[Technical Field]

[0001] The present invention relates to a high-strength galvanized steel sheet having excellent galvanizability and press formability, and a TS of 980 MPa or more, which is suitable for use as components formed by cold press in industrial fields such as automobiles and electrical machinery, and which also has a good steel sheet shape.The present invention also relates to a hot-rolled steel sheet and a cold-rolled steel sheet for producing the high-strength galvanized steel sheet, and a method for producing the high-strength galvanized steel sheet. [Background technology]

[0002] In recent years, there has been a growing need to reduce the weight of automobile bodies in order to improve fuel efficiency and protect the global environment. This has led to a desire for high-strength steel sheets to be used in automobile parts, and there has been an increasing number of cases in which high-strength steel sheets, even plated steel sheets with high corrosion resistance, are being used.

[0003] However, increasing the amount of alloying elements added to produce a high-strength steel sheet can reduce the wettability of the zinc plating to the steel sheet or reduce the reactivity of the zinc plating layer in alloying, which can result in a decrease in galvanizability. Therefore, there is a demand for a steel sheet that has both high strength and excellent galvanizability.

[0004] Furthermore, as the strength of steel sheets increases, forming parts into more complex shapes becomes more difficult, and good elongation and good stretch flangeability are sometimes required as indicators for this part forming.

[0005] Furthermore, in high-strength steel sheets that utilize martensitic or bainite transformation, the volume expansion during transformation can cause deterioration in the flatness of the steel sheet, making it necessary to straighten it when press-forming it into complex shapes. For this reason, there is a demand for steel sheets that have excellent shapes at the time of manufacturing.

[0006] Various studies have been carried out to solve the problems described above.

[0007] For example, Patent Document 1 discloses a steel sheet having a steel sheet composition in which the ratio of Mn to Si contained in the steel sheet, [%Mn] / [%Si], is 2.9 to 11.7. The steel sheet is characterized in that the ratio of the amount of concentrated Si to the amount of concentrated Mn in its surface layer is 0.7 to 1.3. The steel sheet is a high-strength hot-dip galvanized steel sheet that has excellent dimensional accuracy (YR), punchability, stretch flangeability, bendability, and galvanizability during forming, and achieves a tensile strength of 780 MPa or more, making it possible to manufacture parts with high dimensional accuracy.

[0008] Patent Document 2 also discloses that the area ratio of tempered martensite having a hardness of 330 Hv or more and 450 Hv or less is specified, the grain size of the remaining ferrite is specified, and the ratio between the upper limit and the lower limit of the Mn concentration in the cross section of the steel sheet in the thickness direction is specified. This document discloses a technology for realizing a high-strength steel sheet having a tensile strength of 980 MPa or more, an elongation of 13% or more, and reduced variation in stretch flangeability. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2020 / 170542 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-65307 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the technology described in Patent Document 1 does not take into consideration the shape of the steel sheet.

[0011] Furthermore, the technology described in Patent Document 2 does not take into consideration the galvanizability and shape of the steel sheet. However, if a shape defect occurs after the plating bath and alloying treatment, the shape of the final product will deteriorate, and correction will be necessary before the steel sheet is press-formed, which is disadvantageous in terms of cost.

[0012] Therefore, an object of the present invention is to provide a plated steel sheet that has higher strength and better plateability, good press workability, and excellent shape than conventional techniques, and a method for manufacturing the same. Regarding the method for manufacturing the plated steel sheet, an object is also to provide a method for manufacturing a hot-rolled steel sheet and a cold-rolled steel sheet, which are semi-finished products for manufacturing the plated steel sheet.

[0013] In the present invention, "high strength" means that the TS in a tensile test is 980 MPa or more. "Excellent galvanizability" means that no unplated defects are present on the surface of the steel sheet when the surface of the produced plated steel sheet is visually observed. "Good press formability" means that the butt elongation in a tensile test is 10% or more and the hole expansion ratio is 25% or more. "Excellent shape" means that the steel sheet has a flat appearance with no visible waviness. [Means for solving the problem]

[0014] In order to solve the above-mentioned problems, the inventors have conducted extensive research into the chemical composition and microstructure of steel sheets. As a result, they have found that it is possible to control the microstructural structure of steel sheets by managing the strain rate and reduction rate in hot rolling and the finish rolling completion temperature, and by performing two-stage holding in the cooling process before plating during annealing. This makes it possible to obtain plated steel sheets that are excellent in strength and plateability, have good press formability, and have excellent shapes. The present invention was made based on the above-mentioned findings, and its gist is as follows. [1] The composition, in mass%, is C: 0.020% to 0.300%, Si: 0.01% to 1.50%, Mn: 1.00% to 5.00%, P: 0.100% to 0.0200%, S: 0.0200%, Al: 0.100% to 0.100%, N: 0.0100% to 0.0100%, O: 0.0100%, and Sb: 0.001% to 0.200%, and Sn: 0.001%. % or more and 0.200% or less, with the balance being Fe and unavoidable impurities. The microstructure at 1 / 4 of the plate thickness has an area ratio of ferrite of 5.0% or more and 40.0% or less, an area ratio of bainite of 40.0% or less, and an area ratio of martensite of 30.0% or more and 70.0% or less. The martensite grain size (d surface ) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) is 0.70 or more and 2.00 or less, and the martensite fraction (M surface ) and the martensite fraction at 1 / 4 of the plate thickness (M 1 / 4 ) ratio (M surface / M 1 / 4 ) is 0.80 or more, the amount of diffusible hydrogen in the steel is less than 0.75 ppm by mass, and the steel sheet has a zinc plating layer on the surface. [2] The high-strength galvanized steel sheet according to [1], further containing, as a chemical composition, in mass%, at least one element selected from Ti: 0.500% or less, Nb: 0.500% or less, V: 0.200% 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, Ca: 0.0100% or less, Mg: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, and REM: 0.0100% or less. [3] The high-strength galvanized steel sheet according to [1] or [2], wherein the galvanized layer is a galvannealed layer. [4] For a steel material having the composition described in [1] or [2], the strain rate in each stand of consecutive rolling stands is 5 s -1 More than 200s -1

[0023] The method for producing a hot-rolled steel sheet for producing a high-strength galvanized steel sheet according to [1] or [2], wherein the hot-rolling is performed as follows: the finishing rolling is performed with a reduction rate of 5% to 50% in each rolling stand, and the temperature on the delivery side of the finishing rolling is 800°C or higher. [5] A method for producing a cold-rolled steel sheet, comprising pickling a hot-rolled steel sheet obtained by the production method described in [4] and cold-rolling the hot-rolled steel sheet. [6] A method for producing a high-strength galvanized steel sheet, comprising: heating a cold-rolled steel sheet obtained by the method described in [5] to 720 to 880°C, holding the steel sheet at the heating temperature for 5 to 600 seconds, performing a first cooling step at an average cooling rate of 3.0°C / s or more to 680°C, performing a first holding step at a holding temperature of 680 to 600°C for t1 s, cooling the steel sheet, performing a second holding step at a holding temperature of 570 to 480°C for t2 s, and setting the total holding time (t1 + t2) s of the first and second holds to be 40 s or more, and then performing a second cooling step at an average cooling rate of 2.0°C / s or more to 470°C, followed by galvanizing the steel sheet, and then cooling the steel sheet. [7] The method for producing a high-strength galvanized steel sheet according to [6], wherein the galvanizing treatment is a galvannealed plating treatment. [Effects of the Invention]

[0015] According to the present invention, it is possible to obtain a plated steel sheet having a high strength of 980 MPa or more, excellent plateability, good press formability with a butt elongation of 10% or more and a hole expansion ratio of 25% or more, and excellent shape. Therefore, the present invention is of great utility in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing a method for calculating the steepness. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in detail below. However, the present invention is not limited to the following embodiments. First, the appropriate range of the chemical composition of the base steel sheet of the high-strength galvanized steel sheet of the present invention and the reasons for limiting it will be described. Unless otherwise specified, "%" representing the content of the chemical elements means "% by mass."

[0018] C: 0.020% or more and 0.300% or less C is an element that affects the martensite fraction and strength. If the C content is less than 0.020%, the area fraction of martensite will be less than 30.0%, and the TS will be less than 980 MPa. On the other hand, if the C content exceeds 0.300%, the martensite will harden, causing an excessive difference in hardness with the surrounding structure, resulting in poor stretch flangeability. Therefore, the C content is set to 0.020% or more and 0.300% or less. The lower limit of the C content is preferably 0.030% or more, more preferably 0.050% or more. The upper limit of the C content is preferably 0.290% or less, more preferably 0.250% or less.

[0019] Si: 0.01% or more and 1.50% or less Si is an element that contributes to solid solution strengthening and improves the strength of steel sheet. If the Si content is less than 0.01%, this effect is insufficient, so the lower limit is set to 0.01%. On the other hand, Si forms oxides on the surface of the base steel sheet during the annealing process, significantly impairing the adhesion of the coating, so the upper limit is set to 1.50%. Therefore, the Si content is set to 0.01% or more and 1.50% or less. The lower limit of the Si content is preferably 0.02% or more, more preferably 0.04% or more. The upper limit of the Si content is preferably 1.40% or less, more preferably 1.20% or less.

[0020] Mn: 1.00% or more and 5.00% or less Mn is an element that affects the area fraction of martensite by improving hardenability. If the Mn content is less than 1.00%, the bainite fraction increases significantly, and the martensite area fraction becomes less than 30.0%, resulting in a TS of less than 980 MPa. On the other hand, if the Mn content exceeds 5.00%, adhesion of the coating layer is hindered, resulting in poor coating properties. Therefore, the Mn content is set to 1.00% or more and 5.00% or less. The lower limit of the Mn content is preferably 1.70% or more, more preferably 2.00% or more, and even more preferably 2.20% or more. The upper limit of the Mn content is preferably 4.50% or less, more preferably 4.30% or less, and even more preferably 4.00% or less.

[0021] P:0.100% or less P segregates at grain boundaries, causing embrittlement and adversely affecting ductility, so its amount must be 0.100% or less. Therefore, the P content is set to 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. A more preferable lower limit for the P content is 0.003% or more.

[0022] S: 0.0200% or less S segregates at grain boundaries, embrittling steel during hot working, and may also adversely affect ductility through the formation of sulfides, so its content must be 0.0200% or less. Therefore, the S content is 0.0200% or less. A preferred upper limit for the S content is 0.0180% or less, and more preferably 0.0100% or less. There is no particular lower limit for the S content, but due to production technology constraints, a content of 0.0001% or more is preferred. A more preferred lower limit for the S content is 0.0003% or more.

[0023] Al: 0.100% or less Al acts as a deoxidizer and is an effective element for reducing inclusions in steel, so it is preferable to add it in the deoxidation process. However, adding a large amount of Al exceeding 0.100% increases Al oxide inclusions and deteriorates stretch flangeability. Therefore, the Al content is set to 0.100% or less. The upper limit of the Al content is preferably 0.080% or less, more preferably 0.070% or less. There is no particular lower limit for the Al content, but it is preferably 0.001% or more, more preferably 0.005% or more.

[0024] N: 0.0100% or less N has a negative effect on stretch flangeability due to the formation of coarse nitrides. If the N content exceeds 0.0100%, a large amount of coarse nitrides is formed, resulting in significant deterioration of stretch flangeability. The smaller the N content, the better, so the N content is 0.0100% or less, preferably 0.0090% or less. A more preferred upper limit for the N content is 0.0080% or less. There is no particular lower limit for the N content, but due to production technology constraints, 0.0005% or more is preferred. A more preferred lower limit for the N content is 0.0010% or more, and even more preferably 0.0020% or more.

[0025] O: 0.0100% or less O exists as an oxide and reduces the ultimate deformability of the steel sheet, resulting in a decrease in stretch flangeability. Therefore, the O content must be 0.0100% or less. Although there is no particular lower limit for the O content, it is preferably 0.0001% or more due to constraints on production technology. Therefore, the O content is set to 0.0100% or less. A preferred lower limit is 0.0001% or more. A preferred upper limit is 0.0050% or less.

[0026] Sb: 0.001% or more and 0.200% or less, Sn: 0.001% or more and 0.200% or less, one or two selected from the group consisting of: Sb is an effective element for suppressing decarburization and oxidation in a region of several tens of micrometers in the surface layer of a steel sheet, which occurs due to nitriding and oxidation of the steel sheet surface, and for achieving good galvanizability on the steel sheet surface. To fully achieve this effect, an Sb content of 0.001% or more is required. However, excessive Sb content exceeding 0.200% embrittles the steel sheet, reducing its ductility. Therefore, when Sb is contained, the content is set to 0.001% or more and 0.200% or less. The lower limit of Sb is preferably 0.003% or more, more preferably 0.005% or more. The upper limit of Sb is preferably 0.180% or less, more preferably 0.150% or less.

[0027] Sn suppresses decarburization in a region of several tens of micrometers in the surface layer of the steel sheet, which occurs due to nitriding or oxidation of the steel sheet surface, and prevents an excessive increase in the area ratio of ferrite on the steel sheet surface, thereby ensuring strength. To fully achieve these effects, an Sn content of 0.001% or more is required. However, excessive Sn content exceeding 0.200% embrittles the steel sheet, reducing its ductility. Therefore, when Sn is contained, the content is set to 0.001% or more and 0.200% or less. The lower limit of Sn is preferably 0.003% or more, more preferably 0.005% or more. The upper limit of Sn is preferably 0.180% or less, more preferably 0.150% or less.

[0028] Therefore, by containing one or two elements selected from Sb and Sn, plating properties can be improved.

[0029] The base steel sheet of a high-strength galvanized 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. Furthermore, it is preferable that the steel sheet according to one embodiment of the present invention contains only the above-mentioned basic components and the balance, with the balance being Fe (iron) and unavoidable impurities.

[0030] In addition to the above components, the alloy may contain, by mass%, at least one element selected from Ti: 0.500% or less, Nb: 0.500% or less, V: 0.200% 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, Ca: 0.0100% or less, Mg: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, and REM: 0.0100% or less.

[0031] Ti: 0.500% or less Ti contributes to precipitation strengthening and further refines the prior austenite grain size, which in turn refines martensite and bainite, thereby effectively improving steel strength. Therefore, when Ti is contained, the content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.015% or more. However, if Ti is added in an amount exceeding 0.500%, 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, when Ti is contained, the content is set to 0.500% or less. The preferred upper limit is 0.100% or less, and more preferably 0.070% or less.

[0032] Nb: 0.500% or less Nb is an element that improves the strength of steel sheet by precipitation strengthening through the formation of fine precipitates. Therefore, when Nb is contained, the content is preferably 0.001% or more. On the other hand, when the Nb content exceeds 0.500%, precipitates such as Nb carbides and nitrides become coarse, resulting in poor ductility. Therefore, when Nb is contained, the content is set to 0.500% or less. The lower limit of the Nb content is more preferably 0.005% or more, and even more preferably 0.010% or more. The upper limit of the Nb content is preferably 0.100% or less, and even more preferably 0.070% or less.

[0033] V:0.200% or less V contributes to precipitation strengthening and further refines the prior austenite grain size, which in turn refines martensite and bainite, thereby effectively improving steel strength. Therefore, when V is contained, the V content is preferably 0.001% or more. However, if V is added in an amount exceeding 0.200%, V may remain in an undissolved state when the steel material is heated before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, when V is contained, the V content is set to 0.200% or less. The lower limit of the V content is more preferably 0.005% or more, and even more preferably 0.010% or more. The upper limit of the V content is preferably 0.180% or less, and even more preferably 0.150% or less.

[0034] Ta: 0.10% or less Like Ti, Ta contributes to high strength by forming alloy carbides and alloy carbonitrides. Furthermore, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb,Ta)(C,N). Therefore, Ta can be added as needed to significantly suppress precipitate coarsening and stabilize the contribution of precipitation strengthening to strength. Therefore, when Ta is added, its content is preferably 0.01% or more. However, excessive Ta addition saturates the precipitate stabilization effect and increases alloy costs. Therefore, when Ta is added, its content is set to 0.10% or less. A more preferred lower limit for the Ta content is 0.02% or more, and an even more preferred lower limit is 0.05% or more. A preferred upper limit for the Ta content is 0.08% or less, and an even more preferred upper limit is 0.07% or less.

[0035] W: 0.10% or less W can be added as needed to improve the hardenability of steel and further improve steel strength by refining martensite and bainite. Therefore, when W is contained, the W content is preferably 0.01% or more. However, if the W content exceeds 0.10%, the amount of coarse precipitates such as WN and WS that remain 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. The lower limit of the W content is more preferably 0.02% or more, and even more preferably 0.05% or more. The upper limit of the W content is preferably 0.08% or less, and even more preferably 0.05% or less.

[0036] B: 0.0100% or less B is an element that can improve hardenability by segregating at austenite grain boundaries, and can form a microstructure mainly composed of martensite and bainite, thereby improving the strength of the steel sheet, so it can be added as needed. Therefore, when B is contained, the content is preferably 0.0001% or more. However, if the B content exceeds 0.0100%, coarse precipitates are formed and ductility decreases. Therefore, when B is contained, the content is set to 0.0100% or less. The lower limit of the B content is more preferably 0.0002% or more, and even more preferably 0.0005% or more. The upper limit of the B content is preferably 0.0080% or less, and even more preferably 0.0070%.

[0037] Cr:1.00% or less Cr has the effect of improving the balance between strength and ductility, so it can be added as needed. Therefore, when Cr is contained, the Cr content is preferably 0.01% or more. However, if added in excess of 1.00%, the area fraction of martensite becomes excessive, and ductility decreases. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. A more preferable lower limit of the Cr content is 0.03% or more, and an even more preferable lower limit is 0.05% or more. A preferable upper limit of the Cr content is 0.80% or less, and an even more preferable upper limit is 0.50% or less.

[0038] Mo: 1.00% or less Mo has the effect of improving the balance between strength and ductility, so it can be added as needed. Therefore, when Mo is contained, the content is preferably 0.01% or more. However, if Mo is added in excess of 1.00%, the area fraction of martensite becomes excessive, and ductility decreases. Therefore, when Mo is contained, the content is set to 1.00% or less. The lower limit of the Mo content is more preferably 0.03% or more, and even more preferably 0.05% or more. The upper limit of the Mo content is preferably 0.80% or less, and even more preferably 0.50% or less.

[0039] Co: 1.00% or less Co is an element effective in improving hardenability and strengthening steel, so it can be added as needed. Therefore, when Co is contained, the content is preferably 0.01% or more. However, adding Co in excess of 1.00% results in an excessively large area fraction of martensite, reducing ductility. Therefore, when Co is contained, the content is set to 1.00% or less. A more preferred lower limit is 0.03% or more, and an even more preferred lower limit for the Cr content is 0.05% or more. A preferred upper limit for the Cr content is 0.80% or less, and an even more preferred upper limit is 0.50% or less.

[0040] Ni: 1.00% or less Ni increases the strength of steel through solid solution strengthening, so it can be added as needed. Therefore, when Ni is contained, the content is preferably 0.01% or more. However, when Ni is added in an amount exceeding 1.00%, the area fraction of martensite becomes excessively large, and ductility decreases. Therefore, when Ni is contained, the content is set to 1.00% or less. A more preferable lower limit of the Ni content is 0.03% or more, and an even more preferable lower limit is 0.05% or more. A preferable upper limit of the Ni content is 0.80% or less, and an even more preferable upper limit is 0.50% or less.

[0041] Cu:1.00% or less Cu is an element effective in strengthening steel and can be added as needed. Therefore, when Cu is contained, the content is preferably 0.01% or more. However, when added in an amount exceeding 1.00%, the area fraction of martensite becomes excessive, resulting in reduced ductility. Therefore, when Cu is contained, the content is set to 1.00% or less. A more preferred lower limit of the Cu content is 0.03% or more, and an even more preferred lower limit is 0.05% or more. A preferred upper limit of the Cu content is 0.80% or less, and an even more preferred upper limit is 0.50% or less.

[0042] Ca: 0.0100% or less, Mg: 0.0100% or less Ca and Mg are elements that spheroidize the shape of sulfides. Therefore, when Ca and Mg are contained, their contents are preferably 0.0001% or more, respectively. However, excessive addition of more than 0.0100% each can cause an increase in inclusions, resulting in surface and internal defects and reduced ductility. Therefore, when Ca and Mg are contained, their contents are set to 0.0100% or less, respectively. The lower limits of the Ca and Mg contents are more preferably 0.0003% or more, and even more preferably 0.0005% or more. The upper limits of the Ca and Mg contents are preferably 0.0090% or less, and even more preferably 0.0080% or less.

[0043] Zr: 0.100% or less, Te: 0.100% or less Zr and Te can be added as needed to improve the hardenability of steel, form fine Zr-containing carbides, and refine martensite and bainite to improve steel strength. Therefore, when Zr and Te are contained, their contents are preferably 0.001% or more. However, excessive addition of Zr and Te exceeding 0.100% each can increase inclusions, cause surface and internal defects, and reduce ductility. Therefore, when Zr and Te are contained, their contents are set to 0.100% or less. The lower limits of the Zr and Te contents are more preferably 0.003% or more, and even more preferably 0.005% or more. The upper limits of the Zr and Te contents are preferably 0.080% or less, and even more preferably 0.050% or less.

[0044] Hf: 0.10% or less Hf is an element that affects the distribution of oxides. When Hf is contained, the content is preferably 0.01% or more. However, excessive addition of more than 0.10% increases coarse precipitates and inclusions, impairing the material stability of the steel sheet. Therefore, when Hf is contained, the content is set to 0.10% or less. A more preferable lower limit of the Hf content is 0.03% or more, and an even more preferable lower limit is 0.05% or more. A preferable upper limit of the Hf content is 0.08% or less, and an even more preferable upper limit is 0.05% or less.

[0045] Bi:0.200% or less If Bi is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the material stability of the steel sheet will not be impaired. When Bi is contained, the content is preferably 0.001% or more. Therefore, when Bi is contained, the content is set to 0.200% or less. The lower limit of the Bi content is more preferably 0.003% or more, and even more preferably 0.005% or more. The upper limit is preferably 0.150% or less, and even more preferably 0.100% or less.

[0046] REM: 0.0100% or less REM is an element that spheroidizes the shape of sulfides. When REM is added to achieve this effect, its content is preferably 0.0001% or more. However, excessive addition of REM exceeding 0.0100% can increase inclusions, causing surface and internal defects and reducing ductility. Therefore, when REM is added, its content should be 0.0100% or less. A more preferable lower limit for the REM content is 0.0003% or more, and an even more preferable lower limit is 0.0005% or more. A preferable upper limit for the REM content is 0.0090% or less, and an even more preferable upper limit is 0.0080% or less. REM is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content here refers to the total content of these elements.

[0047] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Ca, Mg, Zr, Te, Hf, Bi and REM is less than the preferable lower limit value, the effect of the present invention is not impaired, and therefore these elements are included as unavoidable impurities.

[0048] Next, the microstructure will be described. The position for defining each structure is the 1 / 4 part of the plate thickness. The method for measuring the microstructure will be as described in the examples.

[0049] Ferrite area ratio: 5.0% to 40.0% Ferrite contributes to the ductility and strength of steel sheets. If the ferrite area fraction is less than 5.0%, the ductility falls below 10%, so the ferrite area fraction must be 5.0% or more. On the other hand, if the ferrite area fraction exceeds 40.0%, the TS falls below 980 MPa, so the ferrite area fraction must be 40.0% or less. The ferrite area fraction is preferably 38.0% or less.

[0050] Bainite area ratio: 40.0% or less Bainite contributes to the strength of the steel plate. If the area fraction of bainite exceeds 40.0%, the TS will be less than 980 MPa, so the area fraction of bainite must be 40.0% or less. The area fraction of bainite is preferably 38.0% or less.

[0051] Although there is no particular lower limit for the area ratio of bainite, it is preferably 1.0% or more, and more preferably 2.0% or more.

[0052] Area ratio of martensite: 30.0% to 70.0% Martensite contributes to the strength of the steel sheet. Furthermore, including martensite in the steel sheet structure is effective in maintaining high strength. When the area fraction of martensite is less than 30.0%, the TS is less than 980 MPa. Therefore, the area fraction of martensite must be at least 30.0% or more. The area fraction of martensite is preferably 32.0% or more. On the other hand, if the area fraction of martensite exceeds 70.0%, the elongation will be less than 10%. Therefore, the upper limit of the area fraction of martensite must be 70.0% or less, and preferably 68% or less.

[0053] The effects of the present invention are not impaired even if the steel sheet structure contains structures other than ferrite, bainite, and martensite. The balance structure is preferably 15.0% or less, more preferably 10.0% or less, and even more preferably 7.5% or less. The balance structure is a known structure other than ferrite, bainite, and martensite, and is not particularly limited.

[0054] The martensite grain size (d surface ) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) is 0.70 or more and 2.00 or less Martensite is a structure formed when steel sheet is cooled, and while it is very hard, it expands when cooled. Since steel sheet is cooled from the surface, it is thought that the surface layer of the steel sheet transforms into martensite first, and then the interior of the steel sheet transforms into martensite. At this time, if the grain size of martensite in the surface layer structure is larger than the grain size of martensite in the interior, the expansion caused by the martensite transformation in the surface layer of the steel sheet will occur locally, causing the steel sheet shape to deteriorate. Therefore, the martensite grain size (d surface ) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) is set to 2.00 or less, and preferably 1.80 or less. Here, the "surface layer" of the steel sheet refers to the range of 10 μm to 30 μm from the surface of the base steel sheet. Here, martensite includes both quenched martensite and recoiled martensite. Furthermore, the grain size refers to the average grain size.

[0055] Furthermore, when drilling and hole expanding processes are performed, if martensite is present in the surface layer of the steel sheet, voids will occur due to the difference in hardness between the martensite and the surrounding structure. The voids that occur will grow during forming, and cracks will be observed on the steel sheet surface. At this time, the martensite grain size (d surface ) is the martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ), the probability of voids occurring in the surface layer increases, resulting in poor stretch flangeability. Therefore, the ratio of martensite grain size between the surface layer and the 1 / 4 thickness part (d surface / d 1 / 4 ) is 0.70 or more, preferably 0.75 or more.

[0056] The martensite fraction in the microstructure of the surface layer (M surface ) and the martensite fraction at 1 / 4 of the plate thickness (M 1 / 4 ) ratio (M surface / M 1 / 4 ) is 0.80 or higher: Martensite is a structure formed when steel sheet is cooled, and although it is very hard, it expands when cooled. Since steel sheet is cooled from the surface layer, it is thought that the surface layer of the steel sheet transforms into martensite first, and then the interior of the steel sheet transforms into martensite. At this time, if the martensite fraction in the surface layer structure is low, the deformation caused by expansion due to martensite transformation inside the steel sheet cannot be restrained, and the deformation propagates to the surface layer and edge of the steel sheet, deteriorating the shape of the steel sheet. Therefore, the martensite fraction (M surface ) is the martensite fraction (M 1 / 4 ), and the ratio of martensite fraction between the surface layer and the 1 / 4 thickness part (M surface / M 1 / 4 If the ratio of martensite fraction in the surface layer to that in the 1 / 4 thickness portion (M surface / M 1 / 4 ) is not particularly limited, but is preferably 2.00 or less, more preferably 1.80 or less.

[0057] The amount of diffusible hydrogen in the steel is less than 0.75 ppm by mass When high-alloy steel sheets are plated, hydrogen in the atmosphere during the plating process is thought to become hydrogen in the steel that remains in the steel sheet product. This hydrogen in the steel is thought to hinder improvement in stretch flangeability. Therefore, the amount of diffusible hydrogen in the steel is set to less than 0.75 ppm by mass.

[0058] Zinc plating layer The high-strength galvanized steel sheet of the present invention has a galvanized layer on its surface. This galvanized layer preferably contains 0.08% to 0.30% Al. The effects of the present invention remain unchanged even if this galvanized layer contains elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn in addition to Zn, Al, Mg, and Si. Furthermore, this galvanized layer may be an alloyed galvanized layer that has been subjected to an alloying treatment.

[0059] Next, the manufacturing conditions will be described.

[0060] The methods for producing a hot-rolled steel sheet and a cold-rolled steel sheet described below are methods for producing intermediate products for obtaining the high-strength galvanized steel sheet of the present invention, and contribute to improving the above-mentioned properties of the high-strength galvanized steel sheet, which is the final product.

[0061] In the case of the one-step method, the process starts with steel material such as a slab, goes through hot-rolled steel sheet and cold-rolled steel sheet, and then becomes plated steel sheet.

[0062] The method for producing a cold-rolled steel sheet is a manufacturing method in which, among the above steps, a hot-rolled steel sheet obtained as a result of the method for producing a hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet.

[0063] The manufacturing method of the high-strength galvanized steel sheet as the final product is a manufacturing method in which a hot-rolled steel sheet obtained as a result of the manufacturing method of the hot-rolled steel sheet or a cold-rolled steel sheet obtained as a result of the manufacturing method of the cold-rolled steel sheet is subjected to a heat treatment and a plating treatment to obtain a high-strength galvanized steel sheet, where each temperature is the surface temperature of the steel sheet.

[0064] <Hot-rolled steel sheet manufacturing method> The method for producing a hot-rolled steel sheet is a manufacturing method that starts with a steel material such as a slab, goes through a hot rolling process, and produces a hot-rolled steel sheet.

[0065] [Steel slab (steel material) manufacturing process] First, a steel material having the above-described composition is melted to produce a steel slab. The method for melting the steel material is not particularly limited, and any known melting method, such as a converter or electric furnace, can be used. Furthermore, the steel slab (steel material) is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method or a thin slab casting method. After the steel slab is produced, it is cooled to room temperature and then reheated, as in the conventional method. Energy-saving processes, such as direct rolling, in which the slab is charged into a heating furnace as a hot slab without cooling, or is immediately rolled after a short heat retention period, can also be applied.

[0066] [Hot rolling process] Precipitates present during the heating stage of the steel material remain as coarse precipitates in the final steel sheet and do not contribute to strength. Therefore, it is necessary to redissolve the coarse precipitates precipitated during casting as much as possible. Furthermore, the increased rolling load increases the risk of problems occurring during hot rolling. Furthermore, from the perspective of scaling off defects such as bubbles and segregations in the surface layer of the steel material, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, it is preferable to heat the steel material to 1100°C or higher for 1 hour or more. While there is no particular upper limit to the heating temperature of the steel material, a heating temperature of 1400°C or lower is preferable because a heating temperature above 1400°C increases the amount of oxidation, resulting in increased scale loss and reduced productivity.

[0067] Strain rate in each stand of successive rolling stands: 5 s -1 More than 200s -1 below The strain rate in each rolling stand is 5 s -1 If the rolling reduction ratio is less than 1 / 2, dynamic recrystallization is likely to occur even when the rolling reduction ratio is small, and the austenite grains in the surface layer of the steel sheet are particularly refined excessively. As a result, the martensite grain size in the surface layer of the final structure is excessively refined. This leads to a decrease in the martensite grain size (d surface ) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) becomes less than 0.70, resulting in poor stretch flangeability. Therefore, the strain rate in each rolling stand is 5 s -1 Above 7.5s, preferably 7.5s -1 The strain rate must be 200 s -1 If the rolling temperature exceeds d, the processing heat increases and the rolling temperature becomes high. As a result, dynamic recrystallization is more likely to occur, especially in the surface layer that has been processed, and the austenite grains in the surface layer of the steel sheet may be excessively refined. This may result in the martensite grain size in the surface layer of the final structure being excessively refined. As a result, the martensite grain size (d surface) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) becomes less than 0.70, which may result in poor stretch flangeability. Therefore, the strain rate is set to 200 s -1 Below 170s, preferably -1 More preferably, 160 seconds or less. -1 The following applies.

[0068] Here, the method for adjusting the strain rate will be explained below. The strain rate is calculated by the following formula 1. (Equation 1) [Strain rate] = -ln(h1 / h2) / t Here, h1 (mm): sheet thickness at the entrance of each rolling stand, h2 (mm): sheet thickness at the exit of each rolling stand, t (s): roll contact time. The roll contact time t is calculated by the following equation 2. (Formula 2) t=L d / V where L d (mm): roll contact arc length, V (mm / s): roll speed. In addition, the roll contact arc length L d is calculated from the above h1, h2 (mm), R (mm): the roll radius of each rolling stand, using the following formula 3. (Formula 3) L d =√(R×(h1-h2)) Therefore, the strain rate can be adjusted to a desired range by adjusting the reduction rate of each rolling stand, the plate thickness at the entry and exit sides of each rolling stand, the roll contact arc length, and the roll speed and roll contact time of each rolling stand.

[0069] The number of rolling stands is preferably four or more. When there are four or more rolling stands, the rolling load of each rolling stand can be reduced. This allows for the downsizing of the rolling equipment, and also makes it easier to control the shape of the steel sheet by rolling in each stand. The number of rolling stands is more preferably seven or more.

[0070] Reduction rate in each rolling stand: 5% to 50% If the reduction ratio in each rolling stand exceeds 50%, excessive rolling strain is applied, and the austenite grains in the surface layer of the steel sheet are excessively refined, resulting in excessive refinement of the martensite grain size in the surface layer of the final structure. As a result, the martensite grain size (d surface ) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) becomes less than 0.70, resulting in poor stretch flangeability. On the other hand, if the reduction rate in each rolling stand is less than 5%, the amount of strain applied to the steel sheet is small, making it difficult for dynamic recrystallization to occur, and furthermore, decarburization occurs during hot rolling, making it easier for grain boundaries to migrate. As a result, the martensite grain size (d surface ) and martensite grain size at 1 / 4 of the plate thickness (d 1 / 4 ) ratio (d surface / d 1 / 4 ) exceeds 2.00, which deteriorates the shape of the steel sheet. Therefore, the reduction ratio in each rolling stand is set to 5% to 50%. The lower limit of the reduction ratio in each rolling stand is preferably 7% or more, more preferably 10% or more. The upper limit of the reduction ratio in each rolling stand is preferably 48% or less, more preferably 45% or less.

[0071] Finishing rolling outlet temperature: 800°C or higher The heated steel material is hot-rolled to produce a hot-rolled steel sheet. If the finish rolling exit temperature is less than 800°C, the steel sheet structure will be non-uniform, resulting in reduced ductility and hole expandability after the annealing process. Therefore, the finish rolling exit temperature in hot rolling is 800°C or higher, preferably 850°C or higher, and more preferably 870°C or higher. There is no particular upper limit for the finish rolling exit temperature, but if the finish rolling exit temperature exceeds 1000°C, the amount of oxide (scale) generated will increase, the interface between the base steel and the oxide will become rough, and the surface quality after pickling and cold rolling may deteriorate. Therefore, the upper limit for the finish rolling exit temperature is preferably 950°C or lower, more preferably 930°C or lower, and even more preferably 900°C or lower.

[0072] Although there are no particular upper or lower limits for the coiling temperature, if the coiling temperature after hot rolling is higher than 750°C, 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 of the final product. Therefore, the coiling temperature after hot rolling is preferably 750°C or lower, more preferably 700°C or lower, and even more preferably 670°C or lower. If the coiling temperature is lower than 350°C, the strength of the hot-rolled steel sheet will increase, which may increase the rolling load in the subsequent cold rolling or cause defects in the sheet shape. Therefore, since productivity will decrease, the lower limit of the coiling temperature is preferably 350°C or higher, more preferably 370°C or higher, and even more preferably 400°C or higher.

[0073] <Method of manufacturing cold-rolled steel sheets> The obtained hot-rolled steel sheet (hot-rolled coil) is subjected to pickling. The pickling method for the hot-rolled coil may be a conventional method. In addition, the hot-rolled coil may be subjected to skin-pass rolling in order to correct the shape and improve the pickling properties.

[0074] After pickling, cold rolling is performed. The cold rolling reduction is preferably 25% or more, and more preferably 30% or more. However, excessive reduction increases the rolling load and increases the load on the cold rolling mill, so the upper limit is preferably 75% or less. The upper limit of the cold rolling reduction is more preferably 70% or less.

[0075] <Method of manufacturing high-strength galvanized steel sheets> [Heat treatment process] Heat to 720℃~880℃ If the heating temperature is lower than 720°C, the austenite fraction generated in the steel sheet during heating and holding will decrease, and the martensite fraction contained in the final structure of the steel sheet will be less than 30.0%, making it impossible to obtain a high-strength steel sheet with a TS of 980 MPa or more. Furthermore, if the heating temperature is higher than 880°C, the martensite fraction will be 70.0% or more, making it impossible to obtain a high-strength steel sheet with an elongation of 10% or more. Therefore, the heating temperature must be between 720°C and 880°C. It is preferably between 730°C and 870°C, and more preferably between 740°C and 860°C.

[0076] Hold at the heating temperature for 5s to 600s If the holding time is shorter than 5 seconds, the nuclei of austenite generated by heating do not grow, and the martensite fraction in the final structure of the steel sheet is less than 30.0%, making it impossible to obtain a high-strength steel sheet with a TS of 980 MPa or more. Furthermore, if the holding time is longer than 600 seconds, the decarburization of the steel sheet proceeds excessively, ferrite is formed in the surface layer of the steel sheet, and the martensite fraction decreases. As a result, the martensite fraction (M surface ) and the martensite fraction at 1 / 4 of the plate thickness (M 1 / 4 ) ratio (M surface / M 1 / 4 ) becomes less than 0.80, which deteriorates the shape of the steel sheet. Therefore, the holding time during heating needs to be 5 to 600 seconds. The lower limit of the holding time is preferably 10 seconds or more, more preferably 15 seconds or more. The upper limit of the holding time is preferably 550 seconds or less, more preferably 500 seconds or less.

[0077] First cooling with an average cooling rate of 3.0°C / s or more to 680°C If the cooling rate to 680°C is less than 3.0°C / s, excessive ferrite will be formed, the area ratio of ferrite in the final structure of the steel sheet will exceed 40%, and the TS will be less than 980MPa. Therefore, the cooling rate to 680°C is set to 3.0°C / s or more, preferably 3.2°C / s or more. There is no particular upper limit to the cooling rate to 680°C, but if the cooling rate exceeds 20.0°C / s, the effect of suppressing ferrite formation relative to the cost required for cooling will saturate. Therefore, the cooling rate to 680°C is preferably 20.0°C / s or less, more preferably 15.0°C / s or less.

[0078] The first and second holds between the end of the first cooling period after heating and the start of the plating process are an important component of the present invention. Bainite transformation occurs in plated steel sheets, particularly during plating bath and alloying treatment. Rapid cooling before the plating bath can easily cause a temperature difference between the surface and interior of the steel sheet. As a result, the amounts of bainite and ferrite transformation between the surface and interior of the steel sheet change, resulting in a difference in the martensite fraction between the surface and the quarter thickness portion in the final structure, resulting in shape defects. Furthermore, differences in the amounts of ferrite and bainite transformation during cooling can cause shape defects before plating, making it difficult to control the coating weight during the plating process and resulting in poor galvanic properties.

[0079] Therefore, the inventors have succeeded in reducing temperature unevenness during cooling of the steel sheet by providing a holding process between heating and cooling, and in making the amount of bainite transformation and the amount of ferrite transformation uniform between the surface layer and the interior of the steel sheet. In other words, they have come up with the idea that shape defects can be suppressed by making the amount of martensite in the remaining portion uniform between the surface layer and the interior of the steel sheet.

[0080] The first holding temperature is 680℃~600℃ If the first holding temperature is less than 600°C, the bainite transformation will progress excessively during the subsequent second holding, plating, and alloying processes, resulting in an area ratio of bainite exceeding 40.0% and a TS of less than 980MPa. If the first holding temperature is higher than 680°C, excessive decarburization will occur in the surface layer, reducing the martensite fraction in the surface layer. As a result, the martensite fraction (M surface ) and the martensite fraction at 1 / 4 of the plate thickness (M 1 / 4 ) ratio (M surface / M 1 / 4 ) becomes less than 0.80, which deteriorates the shape of the steel sheet. Therefore, the first holding temperature is set to 680°C or lower and 600°C or higher.

[0081] The second holding temperature is 570℃~480℃ The reason why the second holding temperature is set to 570°C to 480°C is that it is necessary to maintain an appropriate temperature relative to the coating bath temperature. If the second holding temperature exceeds 570°C, a sudden change in the sheet temperature occurs during the coating bath, causing uneven coating in areas where the temperature drops locally, resulting in reduced galvanizability. If the second holding temperature is less than 480°C, zinc solidifies during the coating bath, and this zinc adheres to the steel sheet, resulting in reduced galvanizability. Therefore, the second holding temperature is set to 570°C or lower and 480°C or higher.

[0082] The total holding time of the first and second holds (t1 + t2) is 40 seconds or more. The holding time of the first holding is t1s, the holding time of the second holding is t2s, and the total holding time (t1 + t2)s is 40s or more. If the total holding time (t1 + t2)s is less than 40s, the cooling will proceed without eliminating the temperature difference between the surface layer and the interior of the steel sheet. As a result, the amount of bainite transformation in the surface layer and the interior will change, and the amount of martensite transformation in the final structure will also change. Then, the martensite fraction (M surface ) and the martensite fraction at 1 / 4 of the plate thickness (M 1 / 4 ) ratio (M surface / M 1 / 4 ) becomes less than 0.80, resulting in defective shape. Therefore, the total holding time is set to 40 seconds or more. Although there is no upper limit to the total holding time, if the total holding time exceeds 1000 seconds, the effect of eliminating the temperature difference between the surface layer portion and the interior of the steel sheet becomes saturated, so the total holding time is preferably set to 1000 seconds or less, and more preferably 750 seconds or less.

[0083] Secondary cooling with an average cooling rate of 2.0°C / s or more to 470°C If the cooling rate to 470°C is less than 2.0°C / s, excessive bainite will be formed, the area fraction of bainite in the final structure of the steel sheet will exceed 40.0%, and the TS will be less than 980 MPa. Therefore, the cooling rate to 470°C is set to 2.0°C / s or more, preferably 2.5°C / s or more. There is no particular upper limit to the cooling rate to 470°C, but if the cooling rate to 470°C is set to 15.0°C / s or more, the cost will not be justified by the reduction in the amount of bainite formed. Therefore, the cooling rate to 470°C is preferably set to 15.0°C / s or less, and more preferably 10.0°C / s or less.

[0084] Zinc plating treatment The galvanizing treatment can be carried out by known methods such as hot-dip galvanizing and electrogalvanizing. When hot-dip galvanizing is carried out, the steel sheet that has been subjected to the annealing treatment (heat treatment) is immersed in a galvanizing bath at a temperature of 440°C to 500°C, and then the coating weight is adjusted by gas wiping or the like. 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 It is preferable that the thickness is 120 g / m or more from the viewpoint of adhesion. 2 The coating weight is preferably 25 g / m or less. 2 More preferably, it is 30 g / m or more. 2 It is even more preferable that the plating coverage is 100 g / m or more. 2 It is more preferable that the density is 70 g / m or less. 2 It is even more preferable that the Al content be 0.08% or more and 0.30% or less. 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. Furthermore, the effects of the present invention will not change even if the galvanizing bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.

[0085] Applying alloying treatment When performing a zinc-plating alloying treatment, the zinc-plating alloying treatment is performed after the zinc-plating treatment in a temperature range of 450°C or higher and 600°C or lower. If the alloying treatment is performed at a temperature above 600°C, the alloying process may proceed excessively, resulting in deterioration of plating adhesion and poor plating properties. Therefore, when performing a zinc-plating alloying treatment, it is preferable to perform the zinc-plating alloying treatment in a temperature range of 450°C or higher and 600°C or lower. The Fe concentration in the plating layer of the alloyed zinc-plated steel sheet is preferably 8 to 17%. [Example]

[0086] Steel having the chemical composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and formed into slabs by continuous casting. The obtained slabs were hot-rolled under the conditions shown in Table 2, annealed (heat treated) under the conditions shown in Table 3, reheated, and then subjected to hot-dip galvanizing to obtain hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA). For the hot-dip galvanized steel sheets (GI), a zinc bath containing 0.19% by mass of Al was used, and for the galvannealed steel sheets (GA), a zinc bath containing 0.14% by mass of Al was used, with the bath temperature set at 465°C. The coating weight was 45 g / m per side. 2 The GA was adjusted so that the Fe concentration in the plating layer was within the range of 9 mass % or more and 12 mass % or less.

[0087] The cross-sectional microstructure, tensile properties, delayed fracture resistance, coating appearance, and steel sheet shape of the high-strength galvanized steel sheets obtained as the final products were investigated, and the results are shown in Table 4.

[0088] For invention examples: condition numbers 43 and 44, the manufacturing method of the semi-finished product, cold-rolled steel sheet, is described as invention example (semi-finished product): condition number 53. Also, for invention example: condition number 54, addition 1, the manufacturing method of the semi-finished product, hot-rolled steel sheet, is described as invention example (semi-finished product): condition number addition 2.

[0089] [Table 1]

[0090] [Table 2]

[0091] [Table 3]

[0092] [Table 4]

[0093] The area fractions of ferrite, bainite, and martensite and the martensite grain size were measured as follows. A thickness cross section (L cross section) parallel to the rolling direction of the steel sheet was polished and then etched with 3 vol.% nital. Ten fields of view were then observed at 2000x magnification using a scanning electron microscope (SEM) at the 1 / 4 position (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the steel sheet surface) and the surface layer position (a region 10 μm to 30 μm deep from the steel sheet surface). From the obtained structural images, the area fractions of each structure (ferrite, bainite, martensite) were calculated as the average value of the 10 fields of view using Image-Pro from Media Cybernetics. In the above structural images, ferrite was defined as the black structural region, bainite as the black region containing island-shaped retained austenite or the gray region containing aligned carbides, and martensite as the light gray structural region and the region where carbides were scattered within the black structural region.

[0094] The martensite grain size is determined by image analysis of the obtained structure image using Image-Pro from Media Cybernetics, and the average value of 10 fields of view is taken as the martensite grain size (average grain size).

[0095] Tensile tests were conducted in accordance with JIS Z 2241 (2011) using JIS No. 5 test pieces, which were taken so that the tensile direction was perpendicular to the rolling direction of the steel plate, and TS (tensile strength), YS (0.2% yield strength), and EL (butt elongation) were measured.

[0096] The stretch flangeability was evaluated by a hole expansion test. The hole expansion test was performed in accordance with JIS Z 2256. A 100 mm × 100 mm sample was sheared from the obtained steel sheet. A 10 mm diameter hole was punched into the sample with a clearance of 12.5%. Using a die with an inner diameter of 75 mm, a conical punch with an apex angle of 60° was pressed into the hole while holding the periphery of the hole with a blank holder force of 9 ton (88.26 kN), and the hole diameter at the crack initiation limit was measured. The hole expansion ratio: λ (%) was calculated from the following equation 4, and the stretch flangeability was evaluated from this hole expansion ratio value. (Equation 4) Limiting hole expansion ratio: λ (%) = {(D f -D0) / D0}×100 However, in the above formula, D f is the hole diameter (mm) when the crack occurs, and D0 is the initial hole diameter (mm).

[0097] The galvanizability was evaluated by visually inspecting the front and back surfaces of the obtained hot-dip galvanized steel sheets and galvannealed steel sheets for the presence or absence of unplated defects, and rating steels where unplated defects were found as × and steels where no unplated defects were found as ◯, with ◯ being judged to have excellent galvanizability.

[0098] The steel sheet shape was evaluated by visually comparing the size of the waves on the entire steel sheet or on the edge of the steel sheet for the obtained hot-dip galvanized steel sheet and galvannealed steel sheet, and determining the largest wave as the largest. A sheet measuring 500 mm in thickness, width, and rolling direction (L) cut from the steel sheet was placed on a surface plate and δ was measured over a predetermined length (approximately 200 mm). δ and p were determined for the largest wave within that length, as shown in Figure 1. The arrow indicating the wave pitch p indicates the rolling direction. The steepness Λ (%), defined by the following formula 5, was calculated. A steepness value of more than 2.0% was evaluated as "X," a value of 2.0% to 1.0% but not greater than 1.0% was evaluated as "△," a value of 1.0% to 0.5% was evaluated as "〇," and a value of 0.5% or less was evaluated as "◎." The values ​​of "△," "〇," and "◎" were determined to be excellent in shape. (Formula 5) Steepness: Λ(%)=(δ / p)×100 In the above equation, δ is the wave height (mm) and p is the wave pitch (mm).

[0099] To measure the amount of diffusible hydrogen in steel, cold-rolled or galvanized steel sheets were prepared as test pieces measuring approximately 5 x 30 mm. For galvanized steel sheets, the plating on the surface was removed using a router (precision grinder) before the test pieces were placed in a quartz tube. The atmosphere inside the quartz tube was then purged with Ar, and the temperature was raised at 200°C / hr. The amount of hydrogen evolved up to 400°C was measured using a gas chromatograph with temperature-programmed analysis. 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.

[0100] As shown in Table 4, the examples of the present invention have a TS of 980 MPa or more, a butt elongation of 10% or more, a hole expansion ratio of 25% or more, no uncoated defects, and excellent shape and appearance. On the other hand, the comparative examples are inferior in any one or more of these characteristics. The examples of the present invention referred to here are those manufactured by the method for manufacturing a hot steel sheet, which is a semi-finished product for manufacturing the high-strength galvanized steel sheet of the present invention, and the method for manufacturing a cold-rolled steel sheet, which is manufactured by the method for manufacturing a high-strength galvanized steel sheet, which is a final product. If the manufacturing conditions of these semi-finished products and / or the manufacturing conditions of the final product are not met, one or more of these properties will be inferior.

Claims

1. The component composition is in mass%: C: 0.020% or more and 0.300% or less, Si: 0.01% or more and 1.50% or less, Mn: 1.00% 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 Sb: 0.001% or more and 0.200% or less, and Sn: 0.001% or more and 0.200% or less. the balance being Fe and unavoidable impurities; The microstructure at 1 / 4 of the plate thickness is The area ratio of ferrite is 5.0% or more and 40.0% or less, The area ratio of bainite is 40.0% or less, The area ratio of martensite is 30.0% or more and 70.0% or less, The martensite grain size in the microstructure of the surface layer (d ｓｕｒｆａｃｅ ) and martensite grain size at 1 / 4 of the plate thickness (d １／４ ) ratio (d ｓｕｒｆａｃｅ / d １／４ ) is 0.70 or more and 2.00 or less, The martensite fraction (M ｓｕｒｆａｃｅ ) and the martensite fraction at 1 / 4 of the plate thickness (M １／４ ) ratio (M ｓｕｒｆａｃｅ / M １／４ ) is 0.80 or more, The amount of diffusible hydrogen in the steel is less than 0.75 ppm by mass, High-strength galvanized steel sheet with a zinc plating layer on the surface.

2. Furthermore, the component composition is, in mass%, Ti: 0.500% or less, Nb: 0.500% or less, V: 0.200% 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, Ca: 0.0100% or less, Mg: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, and REM: 0.0100% or less, The high-strength galvanized steel sheet according to claim 1, further comprising at least one element selected from the group consisting of:

3. The high-strength galvanized steel sheet according to claim 1 or 2, wherein the zinc plating layer is a zinc alloy plating layer.

4. A steel material having the component composition according to claim 1 or 2, The strain rate in each stand of successive rolling stands is 5 s －１ More than 200s －１ Finish rolling is performed with a reduction rate of 5% to 50% in each rolling stand. The method for producing a hot-rolled steel sheet for producing a high-strength galvanized steel sheet according to claim 1 or 2, wherein hot rolling is performed at a finish rolling delivery temperature of 800°C or higher.

5. A method for producing a cold-rolled steel sheet, comprising pickling the hot-rolled steel sheet obtained by the method according to claim 4 and cold-rolling the hot-rolled steel sheet.

6. The cold-rolled steel sheet obtained by the manufacturing method according to claim 5 is heated to 720 ° C to 880 ° C and held at the heating temperature for 5 s to 600 s, A first cooling step is performed to 680°C at an average cooling rate of 3.0°C / s or more. Then, the first holding temperature is 680°C to 600°C. １ After cooling, a second holding temperature of 570°C to 480°C is performed. ２ s, and the total holding time of the first holding and the second holding (t １ +t ２ ) s is 40s or more, Thereafter, a heat treatment step is performed in which a second cooling step is performed at an average cooling rate of 2.0°C / s or more to 470°C, and then Zinc plating treatment is applied, This is a manufacturing method for high-strength galvanized steel sheets, which are then cooled.

7. The method for producing a high-strength galvanized steel sheet according to claim 6, wherein the galvanizing treatment is a galvannealing treatment.

Images