High-strength galvanized steel sheet and method of manufacturing the same

Abstract

A high-strength galvanized steel sheet includes a base steel sheet having a specific chemical composition and a zinc coating layer disposed on the surface of the base steel sheet in a coating weight per side of 20 g/m.sup.2 to 120 g/m.sup.2, wherein the amount of hydrogen measured by a specific method is 0.05 mass ppm to 0.40 mass ppm; and I.sub.Si.sup.sur/I.sub.Si.sup.bulk and I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk calculated by a specific method are not more than 2.0 and not more than 3.0, respectively.

Claims

1. A high-strength galvanized steel sheet comprising: a base steel sheet having a chemical composition, by mass %, C.:0.06% to 0.30%, Si: 0. 01% to 1.5%, Mn: 0.1% to 3.0%, P: 0.003% to 0.1%, S: not more than 0.01%, Al: 0.001% to 0.20%, and the balance being Fe and inevitable impurities, and a zinc coating layer disposed on a surface of the base steel sheet at a coating weight per side of 20 g/m.sup.2 to 120 g/m.sup.2, wherein an amount of hydrogen atoms measured by Method (1) in the base steel sheet is 0.05 mass parts per million to 0.40 mass parts per million, and I.sub.Si.sup.sur/I.sub.Si.sup.bulk and I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk as calculated by Method (2) is not more than 2.0(I.sub.Si.sup.sur /I.sub.Si.sup.bulk2.0) and not more than 3.0(I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk3.0), respectively, and such that Method (1) requires cooling a section of the high-strength galvanized steel sheet in liquid nitrogen, removing the zinc coating layer from the high-strength galvanized steel sheet, then heating the base steel sheet from room temperature to 250 C. at a heating rate of 200 C./hr and holding at 250 C. for 30 minutes, and measuring the amount of hydrogen released from the base steel sheet during the heating, such that Method (2) requires that, after the zinc coating layer is removed from the high-strength galvanized steel sheet, the base steel sheet is analyzed by glow discharge spectrometry (GDS) and sputtered at an Ar gas flow rate of 250 cc and a current of 20 mA for 500 seconds to determine the maximum intensity of silicon (I.sub.Si.sup.sur) and the maximum intensity of manganese (I.sub.Mn.sup.sur) in the surface portion of the base steel sheet and the average intensity of silicon (I.sub.Si.sup.bulk) and the average intensity of manganese (I.sub.Mn.sup.bulk) inside the base steel sheet, wherein the surface portion extends to a depth where the Si and Mn concentrations are constant, and I.sub.Si.sup.sur/I.sub.Si.sup.bulk and I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk are calculated.

2. A method of manufacturing a high-strength galvanized steel sheet according to claim 1 comprising: annealing a base steel sheet having the chemical composition described in claim 1 under conditions in which hydrogen partial pressure (P.sub.H2) is 0.10 to 0.50 relative to a total pressure in a furnace atmosphere taken as 1 and a ratio log(P.sub.H2/P.sub.H2O) is 2.5 to 4.0 wherein P.sub.H2O is vapor partial pressure and P.sub.H2 is hydrogen partial pressure relative to the total pressure in the furnace atmosphere taken as 1, cooling the annealed base steel sheet and holding the cooled base steel sheet under conditions in which the hydrogen partial pressure (P.sub.H2) is 0.10 to 0.30 relative to the total pressure in atmosphere taken as 1, the steel sheet temperature is 400 C. to 600 C. and the holding time is 30 seconds or more, and galvanizing the base steel sheet in a galvanizing bath having an amount of Al of not less than 0.15%.

Description

DETAILED DESCRIPTION

(1) We discovered the following:

(2) First, improving the wettability between the molten zinc and the base steel sheet surface requires that the ratio of Si concentrations and that of Mn concentrations between the surface portion of the base steel sheet and the inside portion of the base steel sheet be appropriately controlled so that silicon oxides and manganese oxides that are factors detrimental to wettability are prevented from forming on the surface portion of the base steel sheet.

(3) Secondly, the remedy of blistering requires appropriate control of the amount of hydrogen accumulated inside the base steel sheet, in particular, the amount of hydrogen released when the steel sheet is heated to a temperature of 250 C.

(4) The manufacturing of such a steel sheet entails controlling the atmosphere and the temperature during from the annealing step until the galvanizing step. Specifically, the annealing step should be performed such that the hydrogen partial pressure (P.sub.H2) in the furnace atmosphere is 0.10 to 0.50 and the ratio of the hydrogen partial pressure (P.sub.H2) to the vapor partial pressure (P.sub.H2O) in the furnace atmosphere, log(P.sub.H2/P.sub.H2O), is 2.5 to 4.0. These controls make it possible to decrease the oxygen potential without causing an excessive decrease in the dew point in the annealing furnace, thus suppressing the selective oxidation of silicon and manganese on the steel surface. In the holding step between the cooling step and the galvanizing step, it is necessary that the hydrogen partial pressure (P.sub.H2) in the atmosphere be 0.10 to 0.30 and the holding time at a steel sheet temperature of 400 C. to 600 C. be at least 30 seconds. These controls ensure that the hydrogen accumulated inside the base steel sheet during the annealing step is released out of the base steel sheet and the steel sheet may be galvanized without the occurrence of blistering.

(5) Hereinbelow, our steel sheets and methods will be described in detail without limiting the examples described below. In the following description, the unit of the content of each element in the chemical composition of steel is mass % and is written simply as % unless otherwise specified.

(6) A high-strength galvanized steel sheet includes a base steel sheet and a zinc coating layer disposed on the surface of the base steel sheet.

(7) The base steel sheet includes, by mass %, C: 0.01% to 0.30%, Si: 0.01% to 1.5%, Mn: 0.1% to 3.0%, P: 0.003% to 0.1%, S: not more than 0.01%, Al: 0.001% to 0.20%, and the balance being Fe and inevitable impurities.

(8) C: 0.01% to 0.30%

(9) Carbon is an element necessary to increase the strength of base steel sheet. The C content needs to be 0.01% or more to realize the increase in strength of base steel sheet. If, on the other hand, the C content exceeds 0.30%, weldability is deteriorated. Thus, the upper limit is 0.30%. The C content is preferably 0.06% to 0.12%.

(10) Si: 0.01% to 1.5%

(11) Silicon is a solid solution strengthening element. 0.01% or more silicon needs to be added to obtain the strengthening effect. On the other hand, adding more than 1.5% silicon results in a marked increase in the amount of silicon oxides formed on the surface of the base steel sheet during annealing treatment, thereby causing bare-spot defects. Thus, the upper limit is 1.5%.

(12) Mn: 0.1% to 3.0%

(13) Manganese is added to increase the strength. 0.1% or more manganese needs to be added to obtain the strengthening effect. On the other hand, adding more than 3.0% manganese results in a marked increase in the amount of manganese oxides formed on the surface of the base steel sheet during annealing treatment, thereby causing bare-spot defects. Thus, the upper limit is 3.0%. The Mn content is preferably 1.1% to 2.9%.

(14) P: 0.003% to 0.1%

(15) Phosphorus is one of the inevitable elements. Decreasing its content to below 0.003% gives rise to a concern that the cost is increased. Thus, the P content is limited to not less than 0.003%. If, on the other hand, the P content exceeds 0.1%, weldability is deteriorated. Thus, the P content is limited to not more than 0.1%. The P content is preferably not more than 0.015%.

(16) S: not more than 0.01%

(17) Sulfur causes a decrease in toughness by being segregated in grain boundaries or by forming a large amount of MnS. To avoid this, the S content needs to be 0.01% or less. The lower limit of the S content is not particularly limited and the content may be around an impurity level.

(18) Al: 0.001% to 0.20%

(19) Aluminum is added for the purpose of deoxidizing molten steel. This purpose is not fulfilled if the content is less than 0.001%. On the other hand, adding more than 0.20% aluminum results in formation of large amounts of inclusions and consequent defects in the base steel sheet. Thus, the Al content is limited to 0.001% to 0.20%.

(20) The base steel sheet includes the aforementioned essential elements, iron and inevitable impurities. Examples of the inevitable impurities include oxygen and nitrogen.

(21) The zinc coating layer is disposed on the surface of the base steel sheet. The coating weight per side may be a usual amount providing excellent properties such as corrosion resistance and adhesion of the zinc coating layer, and is 20 g/m.sup.2 to 120 g/m.sup.2.

(22) Next, properties of the high-strength galvanized steel sheet will be described.

(23) Amount of Hydrogen Released from the Base Steel Sheet

(24) In the high-strength galvanized steel sheet, the amount of hydrogen released from the base steel sheet during heating from room temperature to 250 C. after removal of the zinc coating layer is 0.05 mass ppm to 0.40 mass ppm. The hydrogen stored in the base steel sheet is mainly hydrogen taken into the steel from the atmosphere during the annealing treatment. To ensure the effect of the hydrogen in suppressing the selective oxidation of the steel surface due to silicon and manganese, the lower limit of the amount of hydrogen absorbed in the steel is 0.05 mass ppm. If, on the other hand, the above amount of hydrogen exceeds 0.40 mass ppm, the amount of hydrogen accumulated in the steel is so large that blistering will be caused. Thus, the upper limit is 0.40 mass ppm. The amount of hydrogen is preferably 0.10 mass ppm to 0.38 mass ppm. The amount of hydrogen is measured as described in the Examples.

(25) Ratio of Si Concentrations and that of Mn Concentrations Between the Surface Portion and the Inside Portion of the Base Steel Sheet

(26) After removal of the zinc coating layer from the high-strength galvanized steel sheet, the steel was analyzed by glow discharge spectrometry (GDS) in the depth direction from the surface. The analysis confirmed that the Si and Mn concentrations were high near the surface portion, the Si and Mn concentrations decreased with increasing depth and became constant. These concentrations should satisfy I.sub.Si.sup.sur/I.sub.Si.sup.bulk2.0 and I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk3.0 wherein I.sub.Si.sup.sur and I.sub.Mn.sup.sur are the maximum intensities of silicon and manganese in the surface portion, and I.sub.Si.sup.bulk and I.sub.Mn.sup.bulk are the average intensities of silicon and manganese inside the base steel sheet where the Si and Mn concentrations are constant as defined above. Controlling these intensity ratios I.sub.Si.sup.sur/I.sub.Si.sup.bulk and I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk to the above ranges ensures that the amounts of silicon oxides and manganese oxides formed in the surface portion of the base steel sheet during the annealing step are appropriate and the base steel sheet is allowed to exhibit good wettability with respect to the zinc coating layer and is prevented from bare-spot defects. The intensity ratios correspond to the concentration ratios. I.sub.Si.sup.sur/I.sub.Si.sup.bulk is preferably 1.0 to 1.5, and I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk is preferably 1.1 to 2.6.

(27) Next, the manufacturing method will be described.

(28) The manufacturing method includes an annealing step of annealing the aforementioned base steel sheet, a cooling holding step of cooling the annealed base steel sheet and holding the cooled base steel sheet, and a galvanizing step of galvanizing the base steel sheet.

(29) In the annealing step, the base steel sheet is annealed under conditions in which the hydrogen partial pressure (P.sub.H2) is 0.10 to 0.50 relative to the total pressure in furnace atmosphere taken as 1 and the ratio log(P.sub.H2/P.sub.H2O) is 2.5 to 4.0 wherein P.sub.H2O is the vapor partial pressure and P.sub.H2 is the hydrogen partial pressure relative to the total pressure in furnace atmosphere taken as 1.

(30) In the annealing atmosphere, both the increase in hydrogen partial pressure and the decrease in vapor partial pressure lower the oxygen potential in the atmosphere and are thus effective in suppressing selective oxidation of the steel surface due to silicon and manganese. The lower limit of the hydrogen partial pressure is 0.10 because the hydrogen partial pressure of less than 0.10 causes an insufficient reducing ability on the base steel sheet. On the other hand, the upper limit of the hydrogen partial pressure is 0.50 because the hydrogen partial pressure exceeding 0.50 causes heavy accumulation of hydrogen in the steel, resulting in blistering.

(31) Further, if log(P.sub.H2/P.sub.H2O) is less than 2.5, the oxygen potential in the atmosphere is not sufficiently low and selective oxidation of the steel surface due to silicon and manganese is not effectively suppressed. Therefore, the lower limit of log(P.sub.H2/P.sub.H2O) is 2.5. On the other hand, the upper limit of log(P.sub.H2/P.sub.H2O) exceeding 4.0 involves supplying an excessive amount of hydrogen to the atmosphere or lowering the dew point. Such approaches are accompanied by blistering and unstable operation. Therefore, the upper limit of log(P.sub.H2/P.sub.H2O) is 4.0.

(32) In the cooling holding step, the annealed base steel sheet is cooled and the cooled base steel sheet is held under conditions in which the hydrogen partial pressure (P.sub.H2) is 0.10 to 0.30 relative to the total pressure in atmosphere taken as 1, the steel sheet temperature is 400 C. to 600 C. and the holding time is 30 seconds or more.

(33) In the cooling holding step, the cooled base steel sheet is held until the galvanizing step under conditions in which the hydrogen partial pressure (P.sub.H2) is 0.10 to 0.30 relative to the total pressure in atmosphere taken as 1, the steel sheet temperature is 400 C. to 600 C. and the holding time is 30 seconds or more.

(34) At a steel sheet temperature of 600 C. or less, the amount of hydrogen released from the base steel sheet to the atmosphere surpasses the amount of hydrogen absorbed into the base steel sheet from atmosphere. Thus, the hydrogen accumulated in the steel during the annealing step is released during the holding in this temperature range for at least 30 seconds and, consequently, blistering is prevented. The upper limit of the holding time is not particularly limited. The holding time is preferably 32 seconds to 50 seconds. The lower limit of the steel temperature is 400 C. If the steel sheet temperature falls below 400 C., immersion of the base steel sheet into the galvanizing bath takes place while the sheet temperature is lower than the solidifying point of zinc. Thus, the galvanizing treatment encounters difficulties in controlling the coating weight. From the point of view of suppressing selective oxidation of the steel surface due to silicon and manganese, the lower limit of the hydrogen partial pressure during the holding is 0.10 relative to the total pressure in atmosphere taken as 1. The upper limit is 0.30 to ensure a small amount of hydrogen absorbed into the base steel sheet from atmosphere. The hydrogen partial pressure is preferably 0.13 to 0.30.

(35) In the galvanizing step, the base steel sheet after the cooling holding step is galvanized. The galvanizing bath used in this step may be a conventional bath, for example, a galvanizing bath containing a small amount of aluminum. A small amount of aluminum suppresses formation of FeZn alloy layer at the interface between the zinc coating layer and the steel (the base steel sheet) and effectively increases adhesion of the zinc coating layer. A galvanizing bath having an amount of aluminum of not less than 0.15% is preferably used.

(36) In this step, the coating weight of the zinc coating layer may be adjusted to a desired range by a method such as gas wiping.

EXAMPLES

(37) Slabs having chemical compositions described in Table 1 were heated at 1250 C., hot rolled to a thickness of 3.0 mm, and coiled at 550 C. to produce hot-rolled steel sheets. Thereafter, scales on the hot-rolled steel sheets were removed by pickling and the hot-rolled steel sheets were cold rolled to a thickness of 1.4 mm.

(38) Using a continuous galvanizing line CGL, the cold-rolled steel sheets were continuously annealed under conditions described in Table 2, and thereafter galvanized by being immersed into an Al-containing Zn bath. The coating weight was adjusted to 70 g/m.sup.2 per side by gas wiping.

(39) TABLE-US-00001 TABLE 1 Chemical composition (mass %) Steel No. C Si Mn P S Al Remarks A 0.06 0.01 1.4 0.008 0.003 0.02 Ex. B 0.07 0.01 2.1 0.009 0.006 0.01 Ex. C 0.10 0.01 2.4 0.010 0.005 0.01 Ex. D 0.12 0.01 2.9 0.009 0.007 0.01 Ex. E 0.06 0.10 1.1 0.011 0.005 0.02 Ex. F 0.08 0.25 1.4 0.010 0.006 0.01 Ex. G 0.10 0.70 1.9 0.013 0.008 0.01 Ex. H 0.12 1.1 2.3 0.009 0.006 0.01 Ex. I 0.12 1.5 2.6 0.015 0.007 0.01 Ex. J 0.15 1.6 2.6 0.012 0.006 0.02 Comp. Ex.

(40) The galvanized steel sheets (GI) obtained above were subjected to the following evaluations.

(41) Evaluation of Bare Spots

(42) The surface appearance of the galvanized steel sheet was visually inspected for bare spots. The bare spot was evaluated as excellent () when bare spots were completely absent, as good () when slight bare spots were present but did not deteriorate the surface quality, and as poor () when bare spots were present and the surface quality was deteriorated. Those rated as and were acceptable.

(43) Evaluation of Blistering

(44) A 300 mm300 mm sample cut from the galvanized steel sheet was heat treated in a hot air baking furnace. The heat treatment conditions were such that after the steel sheet temperature reached 250 C., the sample was held at the temperature for 30 minutes, air cooled to room temperature and visually inspected for blistering. The blistering was evaluated as good () when blistering was completely absent, and as poor () when blistering was present. Those rated as were acceptable.

(45) Evaluation of Amount of Hydrogen Stored in Steel

(46) A 5 mm100 mm sample cut from the galvanized steel sheet was immersed in liquid nitrogen and cooled at approximately 196 C. to give a test piece for the quantitative determination of amount of hydrogen in steel. While keeping the test piece at 100 C. or below, the zinc coating layer on the steel surface was removed by grinding. After the steel surface was cleaned with alcohol, the test piece was set on a gas chromatograph and the amount of hydrogen was determined. The measurement conditions were such that the temperature was raised to 250 C. at a heating rate of 200 C./hr and the test piece was held at 250 C. for 30 minutes. The amount of hydrogen released from the heating step to the holding step was measured. Three test pieces for each sheet were tested, and the results were averaged.

(47) Evaluation of Surface to Bulk Concentration Ratios in Base Steel Sheet

(48) A 30 mm30 mm sample cut from the galvanized steel sheet was immersed in a mixture liquid of 195 cc of an aqueous 20 mass % NaOH-10 mass % triethanolamine solution and 7 cc of a 35 mass % hydrogen peroxide solution, and the coating layer was dissolved, thereby preparing a test piece. The test piece was set on GDS and sputtered at an Ar gas flow rate of 250 cc and a current of 20 mA for 500 seconds. The average intensity from 450 seconds to 500 seconds of sputtering time was defined as the intensity inside the base steel sheet. Based on the intensity profiles of silicon and manganese, the ratio of the maximum intensity to the average intensity inside the base steel sheet was obtained for silicon and manganese. Three test pieces for each sheet were tested, and the results were averaged.

(49) The results obtained and the manufacturing conditions are described in Table 2.

(50) From Table 2, the surface appearance was good in all of the Examples. In contrast, the Comparative Examples falling outside our scope were evaluated as poor in any of Evaluation of bare spots and Evaluation of blistering.

(51) TABLE-US-00002 TABLE 2 Manufacturing conditions Steel sheet properties Cooling holding step Amount of Holding time at steel hydrogen in Test Annealing step sheet temperatures of steel Evaluations No. Steel No. P.sub.H2 log(P.sub.H2/P.sub.H2O) P.sub.H2 400 to 600 C. (sec) (mass ppm) I.sub.Si.sup.sur/I.sub.Si.sup.bulk I.sub.Mn.sup.sur/I.sub.Mn.sup.bulk Bare spots Blistering Remarks 1 A 0.20 3.7 0.20 45 0.18 1.0 1.3 Ex. 2 B 0.10 2.7 0.14 45 0.11 1.0 2.6 Ex. 3 B 0.15 2.9 0.17 45 0.13 1.0 1.9 Ex. 4 B 0.22 3.1 0.15 45 0.17 1.0 1.4 Ex. 5 B 0.35 3.3 0.20 45 0.30 1.0 1.3 Ex. 6 B 0.48 3.6 0.25 45 0.38 1.0 1.1 Ex. 7 B 0.52 3.6 0.25 45 0.42 1.0 1.1 X Comp. Ex. 8 B 0.15 2.4 0.12 45 0.10 1.0 3.5 X Comp. Ex. 9 B 0.12 2.7 0.13 45 0.13 1.0 1.9 Ex. 10 B 0.15 3.4 0.16 45 0.27 1.0 1.3 Ex. 11 B 0.20 3.2 0.28 45 0.35 1.0 1.5 Ex. 12 B 0.25 3.0 0.31 45 0.41 1.0 1.3 X Comp. Ex. 13 B 0.19 2.8 0.22 32 0.37 1.0 1.4 Ex. 14 B 0.20 2.7 0.24 24 0.44 1.0 1.5 X Comp. Ex. 15 C 0.19 3.2 0.20 45 0.20 1.0 2.0 Ex. 16 D 0.30 3.5 0.25 45 0.34 1.0 2.4 Ex. 17 E 0.15 3.0 0.19 50 0.19 1.2 1.6 Ex. 18 F 0.20 3.3 0.19 50 0.23 1.2 1.7 Ex. 19 G 0.25 3.4 0.25 50 0.29 1.4 1.9 Ex. 20 H 0.25 3.4 0.25 50 0.30 1.5 1.8 Ex. 21 I 0.25 3.6 0.30 50 0.34 1.5 2.0 Ex. 22 J 0.30 3.7 0.30 50 0.39 2.4 2.2 X Comp. Ex.