Galvanized steel sheets and components, and methods for manufacturing them.

Abstract

The present invention provides galvanized steel sheets that combine high yield strength with excellent spot weld durability. The base steel sheet has a predetermined composition, with a total area ratio of tempered martensite and lath-like bainite of 45% to 80%, an area ratio of massive bainite of 1% to 15%, an LMn / HMn ratio of 0.20 or higher, and an Lv / Hv ratio of 0.2 to 2.5, and the diffusible hydrogen content of the base steel sheet is 0.60 ppm by mass or less.

Claims

1. A galvanized steel sheet having a base steel sheet and a zinc plating layer on the surface of the base steel sheet, The base steel plate is, In mass percent, C: 0.150% to 0.450%, Si: 0.50% to 3.00%, Mn: 1.50% to 4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less and O: 0.0100% or less The component composition is such that the remainder is Fe and unavoidable impurities, At the position of the aforementioned base steel plate from the 1 / 8 thickness to the 3 / 8 thickness, Total area ratio of tempered martensite and lath-like bainite: 45% to 80% Area percentage of massive bainite: 1% to 15%, and, LMn / HMn: 0.20 or higher, LMn and HMn are defined as the area percentage (%) of the region where the Mn concentration is 3.0 mass% or less and the area percentage (%) of the region where the Mn concentration is greater than 4.0 mass%, respectively, obtained by measuring in a 100 μm × 100 μm area of ​​the analysis surface, with the analysis surface being at the 1 / 4 thickness position of the base steel plate, and respectively. Lv / Hv: 0.2 to 2.5, Lv and Hv are, respectively, the points in the nanohardness distribution at the 1 / 4 thickness position of the base steel plate where the nanohardness is 3.0 GPa or less and the points where the nanohardness is 4.5 GPa or more. The nanohardness distribution at the 1 / 4 thickness position of the base steel plate was obtained by measuring the nanohardness at 225 points in the rolling direction of the base steel plate at the 1 / 4 thickness position, with a distance of 2 μm between indentations. The diffusible hydrogen content of the aforementioned base steel plate is 0.60 ppm by mass or less. A galvanized steel sheet, wherein the aforementioned zinc plating layer is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

2. The component composition of the aforementioned base steel plate is further expressed in mass %, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less, Mo: 1.00% or less Cr: 1.00% or less, Sb: 0.200% or less, Sn: 0.200% or less, Zr: 0.1000% or less, Te: 0.100% or less, Cu: 1.000% or less, Ni: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Co: 0.500% or less, Ta: 0.10% or less, Hf: 0.10% or less, Bi: 0.200% or less, As: 0.100% or less, Pb: 0.100% or less and Zn: 0.100% or less A galvanized steel sheet according to claim 1, comprising at least one selected from among the following.

3. In the steel structure of the aforementioned base steel plate, Area percentage of retained austenite: 5% to 30% and Previous austenite grain size: 3 μm to 12 μm And, The galvanized steel sheet according to claim 1, wherein the n value at 2% tensile strain is 0.15 or greater.

4. In the steel structure of the aforementioned base steel plate, Area percentage of retained austenite: 5% to 30% and Previous austenite grain size: 3 μm to 12 μm And, The galvanized steel sheet according to claim 2, wherein the n value at 2% tensile strain is 0.15 or more.

5. The aforementioned base steel plate has a soft surface layer, The aforementioned soft surface layer is a region from the surface of the base steel plate to a point where 1 / 4 of the thickness of the base steel plate is present, where the Vickers hardness is 90% or less of the Vickers hardness at the point where 1 / 4 of the thickness of the base steel plate is present. The zinc-plated steel sheet according to claim 1, wherein the thickness of one side of the surface soft layer is 10 μm to 150 μm.

6. The aforementioned base steel plate has a soft surface layer, The aforementioned soft surface layer is a region from the surface of the base steel plate to a point where 1 / 4 of the thickness of the base steel plate is present, where the Vickers hardness is 90% or less of the Vickers hardness at the point where 1 / 4 of the thickness of the base steel plate is present. The zinc-plated steel sheet according to claim 2, wherein the thickness of one side of the surface soft layer is 10 μm to 150 μm.

7. The aforementioned base steel plate has a soft surface layer, The aforementioned soft surface layer is a region from the surface of the base steel plate to a point where 1 / 4 of the thickness of the base steel plate is present, where the Vickers hardness is 90% or less of the Vickers hardness at the point where 1 / 4 of the thickness of the base steel plate is present. The zinc-plated steel sheet according to claim 3, wherein the thickness of one side of the aforementioned soft surface layer is 10 μm to 150 μm.

8. The aforementioned base steel plate has a soft surface layer, The aforementioned soft surface layer is a region from the surface of the base steel plate to a point where 1 / 4 of the thickness of the base steel plate is present, where the Vickers hardness is 90% or less of the Vickers hardness at the point where 1 / 4 of the thickness of the base steel plate is present. The zinc-plated steel sheet according to claim 4, wherein the thickness of one side of the surface soft layer is 10 μm to 150 μm.

9. A member made using a galvanized steel sheet according to any one of claims 1 to 8.

10. A method for manufacturing a galvanized steel sheet according to any one of claims 1 to 8, The aforementioned method, A hot rolling step is performed to obtain a hot-rolled steel sheet by hot rolling a steel slab having the component composition described in claim 1 or 2, A cold rolling process is performed on the hot-rolled steel sheet to obtain a cold-rolled steel sheet. A primary heat treatment step involves heating and cooling the cold-rolled steel sheet, A zinc plating process is performed on the cold-rolled steel sheet to obtain a zinc-plated steel sheet, A secondary heat treatment step involves cooling and reheating the galvanized steel sheet, It has, In the aforementioned primary heat treatment process, Intermediate heating temperature T1: 800℃~Ac ３ Point (°C) Maximum heating temperature T2: Ac ３ Point (°C) ~ 950°C Average heating rate V1 from T1 to T2: 6°C / sec or higher. The thermal influence index E is defined by the following equation (1): 150 to 1500. Primary cooling stop temperature T3: (Ms point + 50) °C to 650 °C and Average cooling rate V2 between T2 and T3: 4°C / sec or higher And, In the aforementioned secondary heat treatment process, Secondary cooling stop temperature T4: 50°C to 300°C, Reheating temperature T5: greater than 300°C and 450°C or less, Holding time t2 in the temperature range of over 300°C and under 450°C during reheating: 1 sec to 150 sec A method for manufacturing galvanized steel sheets. E=T2×(4×log(T2-T1)+10) / V1...(1)

11. The method for manufacturing a galvanized steel sheet according to claim 10, wherein the final reduction ratio in the finish rolling of the hot rolling is 25% to 60%.

12. The method for manufacturing a galvanized steel sheet according to claim 10, wherein the dew point in the primary heat treatment step is -35°C or higher.

13. The method for manufacturing a galvanized steel sheet according to claim 11, wherein the dew point in the primary heat treatment step is -35°C or higher.

14. A method for manufacturing a member, comprising the step of forming or joining a galvanized steel sheet according to any one of claims 1 to 8, wherein the joining process is welding, riveting, or crimping.

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