Galvanized steel sheet for hot forming

Abstract

A galvanized steel sheet for hot forming, the galvanized steel sheet including a galvanized film provided on a surface of a steel sheet, wherein the steel sheet has a chemical composition consisting of, in mass %, C: 0.02% to 0.58%, Mn: 0.5% to 3.0%, sol. Al: 0.005% to 1.0%, a predetermined element(s) as necessary; Si: less than or equal to 2.0%, P: less than or equal to 0.03%, S: less than or equal to 0.004%, N: less than or equal to 0.01%, and the balance: Fe and impurities, wherein a content of Mn in a portion from an interface of the steel sheet and the galvanized film to a depth of 5 m is more than or equal to 0.3 mass %, wherein the galvanized film has a plating coverage of 40 g/m.sup.2 to 110 g/m.sup.2, an Al content of more than or equal to 150 mg/m.sup.2 within the galvanized film, and an Al concentration of less than or equal to 0.5 mass %, and wherein the galvanized steel sheet is used for an application in which the galvanized steel sheet is heated to 700 C. or above and is then subjected to hot forming.

Claims

1. A galvanized steel sheet for hot forming, the galvanized steel sheet comprising a galvanized film provided on a surface of a steel sheet, wherein the steel sheet has a chemical composition consisting of, in mass %, C: 0.02% to 0.58%, Mn: 0.5% to 3.0%, sol. Al: 0.005% to 1.0%, Ti: 0% to 0.20%, Nb: 0% to 0.20%, V: 0% to 1.0%, W: 0% to 1.0%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, B: 0% to 0.010%, Mg: 0% to 0.05%, Ca: 0% to 0.05%, REM: 0% to 0.05%, Bi: 0% to 0.05%, Si: less than or equal to 2.0%, P: less than or equal to 0.03%, S: less than or equal to 0.004%, N: less than or equal to 0.01%, and the balance: Fe and impurities, wherein a content of Mn in a portion from an interface of the steel sheet and the galvanized film to a depth of 5 um is more than or equal to 0.3 mass %, wherein the galvanized film has a plating coverage of 40 g/m.sup.2 to 110 g/m.sup.2, an Al content of more than or equal to 150 mg/m.sup.2 within the galvanized film, and an Al concentration of less than or equal to 0.5 mass %, and wherein a metallographic structure of the steel sheet in the portion from the interface of the steel sheet and the galvanized film to the depth of 5 um has a ferrite area percentage of more than or equal to 60%.

2. The galvanized steel sheet for hot forming according to claim 1, wherein the steel sheet comprises one or more selected from the group consisting of, in mass %, Ti: 0.01% to 0.20%, Nb: 0.01% to 0.20%, V: 0.1% to 1.0%, and W: 0.1% to 1.0%.

3. The galvanized steel sheet for hot forming according to claim 1, wherein the steel sheet comprises one or more selected from the group consisting of, in mass %, Cr: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 1.0%, Ni: 0.1% to 1.0%, and B: 0.0010% to 0.010%.

4. The galvanized steel sheet for hot forming according to claim 2, wherein the steel sheet comprises one or more selected from the group consisting of, in mass %, Cr: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 1.0%, Ni: 0.1% to 1.0%, and B: 0.0010% to 0.010%.

5. The galvanized steel sheet for hot forming according to claim 1, wherein the steel sheet comprises one or more selected from the group consisting of, in mass %, Mg: 0.0005% to 0.05%, Ca: 0.0005% to 0.05%, and REM: 0.0005% to 0.05%.

6. The galvanized steel sheet for hot forming according to claim 2, wherein the steel sheet comprises one or more selected from the group consisting of, in mass %, Mg: 0.0005% to 0.05%, Ca: 0.0005% to 0.05%, and REM: 0.0005% to 0.05%.

7. The galvanized steel sheet for hot forming according to claim 3, wherein the steel sheet comprises one or more selected from the group consisting of, in mass %, Mg: 0.0005% to 0.05%, Ca: 0.0005% to 0.05%, and REM: 0.0005% to 0.05%.

8. The galvanized steel sheet for hot forming according to claim 4, wherein the steel sheet comprises one or more selected from the group consisting of, in mass %, Mg: 0.0005% to 0.05%, Ca: 0.0005% to 0.05%, and REM: 0.0005% to 0.05%.

9. The galvanized steel sheet for hot forming according to claim 1, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

10. The galvanized steel sheet for hot forming according to claim 2, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

11. The galvanized steel sheet for hot forming according to claim 3, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

12. The galvanized steel sheet for hot forming according to claim 4, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

13. The galvanized steel sheet for hot forming according to claim 5, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

14. The galvanized steel sheet for hot forming according to claim 6, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

15. The galvanized steel sheet for hot forming according to claim 7, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

16. The galvanized steel sheet for hot forming according to claim 8, wherein the steel sheet comprises, in mass %, Bi: 0.0002% to less than or equal to 0.05%.

17. The galvanized steel sheet for hot forming according to claim 1, wherein the galvanized steel sheet is an alloyed hot dip galvanized steel sheet.

18. The galvanized steel sheet for hot forming according to claim 1, wherein the galvanized film is a pure zinc plated film or a zinc alloy plated film.

Description

EXAMPLES

(1) Slabs of steel grades A to J having chemical compositions specified in Table 1 were hot-rolled. Finish rolling was carried out at a sheet temperature of 900 C. from the viewpoint of avoiding a reduction in the amount of Mn in the surface layer. After One second from finish rolling, the steel sheets were cooled at a rate of 50 C./sec and coiled at a sheet temperature of 600 C. Thereafter, in order to recover the total amount of Mn in a portion of less than or equal to 5 m from the surface layer that effectively functions in hot forming, the steel sheet was placed in a holding furnace and was gradually cooled from 600 C. to 400 C. at a rate of 5 C./hr to obtain a hot-rolled steel sheet having a thickness of 2.8 mm. The hot-rolled steel sheet was pickled and was then cold-rolled at a rolling reduction of 60% to prepare a 1.2 mm-thick cold-rolled steel sheet. Some hot-rolled steel sheets were not cold-rolled after the pickling.

(2) TABLE-US-00001 TABLE 1 Chemical composition (unit: mass %, balance: Fe and impurities) Steel grade C Si Mn P S N Al Cr Ti B Others A 0.22 1.50 1.6 0.01 0.002 0.004 0.03 B 0.22 0.06 1.5 0.01 0.002 0.004 0.04 Mg: 0.001 Ca: 0.002 C 0.21 0.04 1.2 0.01 0.001 0.004 0.04 0.2 0.02 0.0016 D 0.20 0.30 1.3 0.02 0.002 0.004 0.03 0.2 0.02 0.0014 Bi: 0.002 E 0.21 0.21 1.2 0.01 0.002 0.003 0.04 0.5 0.02 0.0015 F 0.21 0.07 2.1 0.01 0.001 0.004 0.04 0.2 0.03 0.0020 G 0.21 0.07 2.1 0.01 0.001 0.004 0.80 0.2 0.03 0.0020 REM: 0.001 H 0.13 0.07 2.0 0.01 0.001 0.005 0.04 0.2 0.03 0.0023 I 0.10 0.06 2.3 0.02 0.001 0.004 0.04 0.2 0.02 0.0022 Cu0.2, Ni0.2, Mo0.2 J 0.30 0.20 1.7 0.01 0.001 0.003 0.04 0.2 0.03 0.0013 Nb0.08, V0.2, W0.1

(3) The hot-rolled steel sheets and the cold-rolled steel sheets were galvanized in a hot dip galvanizing equipment. Annealing was carried out under conditions of 750 C.200 seconds at a dew point of 40 C., followed by cooling to 540 C. at 6 C./sec. Plating was carried out under varied conditions of bath temperature: 450 to 460 C., Al concentration in bath: 0.10 to 0.15%, and coverage of galvanizing per one surface: 40 to 80 g/m.sup.2. After hot dip galvanizing, alloying was carried out at a sheet temperature of 500 to 600 C. to prepare alloyed hot dip galvanized steel sheets. Some hot dip galvanized steel sheets were not subjected to the alloying treatment.

(4) The amount of Mn in a portion from the interface of the steel sheet and the galvanized film to a depth of 5 m was measured by glow discharge spectroscopy (GDS). For three places for each of the steel sheets, the profile of the amount of Mn in a portion from the interface of the galvanized steel sheet to a depth of 50 m was examined by GDS. The amount of Mn within 5 m from the surface was expressed in terms of an average value of the amount of Mn in a portion from the interface of the galvanized steel sheet to a depth of 5 m. Further, the measurement values of the amount of Mn in a portion from the interface of the galvanized steel sheet to a depth of 50 m for three measurement places were averaged.

(5) The ferrite area percentage was determined as follows. The steel sheet was taken off and was subjected to nital etching. A portion near the surface layer was observed for each ten visual fields at a magnification of 500 times with an optical microscope, and the ferrite area percentage was calculated.

(6) Specimens for hot pressing (specimen size: 250 mm in width200 mm in length) were extracted from the manufactured hot dip galvanized steel sheets and alloyed hot dip galvanized steel sheets. The temperature of the steel sheet specimen within the heating furnace was allowed to reach 900 C. The steel sheet specimen was held at the temperature for 3 minutes and was then taken out, and, immediately after that, hot pressing was carried out with a sheet steel mold, followed by rapid cooling.

(7) In such a state that two hot pressed steel sheet specimens were superimposed on top of each other, spot welding was carried out under the following conditions, followed by a 1000-point continuous dotting test for the evaluation of weldability. The number of times of spark was counted, and the specimen that caused deposition in the 1000-point continuous dotting test was described as deposited. Applied pressure: 400 kgf Weld time: 15 cycles Holding time: 9 cycles Welding current: Current immediately before dust Shape of electrode tip: DR type, end of tip is 6 mm-40R

(8) The results of the spot weldability test, together with manufacturing conditions (cold rolling and alloying treatment done or not done) and the results of analysis of the galvanized film, are summarized in Table 2.

(9) TABLE-US-00002 TABLE 2 Ferrite area Content of Mn in percentage in portion from portion from Results of analysis of galvanized film Number of times interface of interface of of steel sheet before hot pressing of occurrence Cold Alloying steel sheet and steel sheet and Al Al Fe of spark in rolling treatment galvanized galvanized Cover- amount concen- concen- spot welding Test Steel done or done or film to depth film to depth age (mg/ tration tration after hot Classification No. grade not done not done of 5 m (mass %) of 5 m (%) (g/m.sup.2) m.sup.2) (%) (%) forming(times) Example of 1 A Done Done 1.0 64 43 154 0.37 11.5 0 present 2 B Done Done 0.8 60 64 170 0.27 13.6 0 disclosure 3 C Done Done 0.3 81 61 152 0.26 11.2 0 4 C Done Done 0.7 62 54 194 0.36 11.4 0 5 C Not done Done 0.8 80 55 186 0.34 10.7 0 6 D Done Done 0.3 54 78 152 0.24 10.0 3 7 E Done Done 0.7 58 60 166 0.28 12.7 5 8 E Done Not done 0.6 65 63 174 0.28 <0.5 0 9 E Done Done 0.3 62 65 152 0.30 10.5 0 10 F Done Done 1.1 58 60 162 0.27 12.5 2 11 F Done Done 0.3 67 62 151 0.26 11.5 0 12 G Done Done 1.0 65 48 156 0.33 12.4 0 13 H Done Done 1.3 54 60 192 0.32 13.4 1 14 I Done Done 1.5 75 54 158 0.30 11.9 0 15 J Done Done 1.1 74 64 160 0.25 12.0 0 Comparative 16 A Done Done 0.1 50 43 94 0.22 11.2 303(Test Example stopped due to adhesion on 500th dotting) 17 B Done Done 0.1 65 44 98 0.22 10.6 158 18 C Done Done 0.2 62 45 100 0.22 10.4 102 19 C Done Done 0.2 69 38 94 0.25 10.9 80 20 D Done Done 0.1 68 53 126 0.24 11.7 121 21 D Done Done 0.2 63 53 130 0.25 10.7 20 22 D Done Done 0.2 70 64 178 0.26 10.2 54 23 E Done Done 0.2 65 62 160 0.28 11.5 25 24 E Done Not done 0.2 67 65 189 0.30 10.8 58 25 E Done Done 0.2 62 58 152 0.32 10.6 21 26 F Done Done 0.2 65 52 136 0.27 11.0 56 27 G Done Done 0.1 64 56 142 0.25 11.0 60 28 H Done Done 0.2 63 52 118 0.23 11.6 75 29 I Done Done 0.2 67 46 130 0.29 11.1 80 30 J Done Done 0.2 68 41 128 0.31 11.2 85

(10) In Table 2, test Nos. 1 to 15 are examples of the present disclosure, and test Nos. 16 to 30 are examples where rapid cooling after hot rolling and recovering treatment of the amount of Mn in coiling or continuous annealing were not carried out, that is, comparative examples where the content of Mn in a portion from the interface of the steel sheet and the galvanized film to a depth of 5 m is low and the amount of Al in the galvanized film is excessively small. In test Nos. 1 to 15 that are examples of the present disclosure, the number of times of the occurrence of spark is small in continuous dotting test of spot welding, and, in all of test Nos. 1 to 15, the spot weldability was good. On the other hand, in test Nos. 16 to 30 that are comparative examples, the number of times of the occurrence of spark is so large that, in all of test Nos. 16 to 30, the spot weldability was poor. From the above results, it was found that, when the content of Mn in a portion from the interface of the steel sheet and the galvanized film to a depth of 5 m is more than or equal to 0.3% and when the content of Al in the galvanized film is larger than 150 g/m.sup.2, a deterioration in spot weldability of the hot formed galvanized steel sheet can be prevented.