Method for producing hot-pressed member

Abstract

A method is provided for producing a hot-pressed member including heating a Ni-based coated steel sheet, which includes, on a surface thereof, a ZnNi alloy coating layer containing 13% by mass or more of Ni, in a temperature region of an Ac3 transformation point to 1200 C.; and then hot-pressing the steel sheet.

Claims

1. A method for producing a hot-pressed member configured to suppress hydrogen entry associated with corrosion, the method comprising: a single heating step consisting of heating a steel sheet that has been plated, the steel sheet that has been plated having a ZnNi alloy coating layer containing 13% by mass or more of Ni, directly from room temperature to a temperature region of 850 C. to 1200 C., thereby forming a Ni-diffusion region in which Ni is present in a solid-solution state over a range of 1 m or more in the depth direction of the steel sheet and an intermetallic compound layer corresponding to a -phase present in a phase equilibrium diagram of ZnNi alloy on the Ni-diffusion region; and then a hot-pressing step comprising hot-pressing the coated steel sheet.

2. A method for producing a hot-pressed member comprising heating a coated steel sheet, which includes, on a surface thereof, a ZnNi alloy coating layer containing 13% by mass or more of Ni, in a temperature region of an Ac.sub.3 transformation point to 1200 C.; and then hot-pressing the steel sheet, wherein the heating in the temperature range of the Ac.sub.3 transformation point to 1200 C. is performed at an average heating rate of 85 C./second or more.

3. The method for producing a hot-pressed member according to claim 1 or 2, wherein the coated steel sheet further includes at least one compound layer selected from a Si-containing compound layer, a Ti-containing compound layer, an Al-containing compound layer, and a Zr-containing compound layer, which is provided on the ZnNi alloy coating layer.

4. The method for producing a hot-pressed member according to claim 3, wherein a base steel sheet of the coated steel sheet has a composition containing, by % by mass, C: 0.15 to 0.5%, Si: 0.05 to 2.0%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, and the balance including Fe and unavoidable impurities.

5. The method for producing a hot-pressed member according to claim 4, wherein the base steel sheet of the coated steel sheet further contains, by % by mass, at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%.

6. The method for producing a hot-pressed member according to claim 5, wherein the base steel sheet of the coated steel sheet further contains, by % by mass, Sb: 0.003 to 0.03%.

7. The method for producing a hot-pressed member according to claim 4, wherein the base steel sheet of the coated steel sheet further contains, by % by mass, Sb: 0.003 to 0.03%.

8. The method for producing a hot-pressed member according to claim 1 or 2, wherein a base steel sheet of the coated steel sheet has a composition containing, by % by mass, C: 0.15 to 0.5%, Si: 0.05 to 2.0%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, and the balance including Fe and unavoidable impurities.

9. The method for producing a hot-pressed member according to claim 8, wherein the base steel sheet of the coated steel sheet further contains, by % by mass, at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%.

10. The method for producing a hot-pressed member according to claim 9, wherein the base steel sheet of the coated steel sheet further contains, by % by mass, Sb: 0.003 to 0.03%.

11. The method for producing a hot-pressed member according to claim 8, wherein the base steel sheet of the coated steel sheet further contains, by % by mass, Sb: 0.003 to 0.03%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a drawing schematically showing a structure of a steel sheet which constitutes a hot-pressed member in a cross-sectional direction along a thickness of the steel sheet.

(2) FIG. 2 is a drawing schematically showing a pressing method used in an example of the invention.

(3) FIG. 3 is a drawing schematically showing an electrochemical cell used in an example of the invention.

DETAILED DESCRIPTION

(4) 1) Hot-Pressed Member

(5) 1-1) Ni-Diffusion Region of Steel Sheet Constituting Member

(6) As described above, the presence of a Ni-diffusion region in a surface layer of a steel sheet constituting a member prevents hydrogen entry into steel associated with corrosion. Although the reason for this is not necessarily known, it is considered as follows: The hydrogen entry into a steel sheet due to corrosion is related to oxidation-reduction reaction of Fe rust in a wet environment, and Fe rust is required to be stable rust which is little converted in order to suppress hydrogen entry. A Ni-diffusion region is effective in stabilizing Fe rust, and hydrogen entry into steel associated with corrosion is suppressed by the presence of the Ni-diffusion region.

(7) However, in order to effectively suppress the hydrogen entry, the Ni-diffusion region is preferably present over a range of 1 m or more, more preferably 2 m or more, most preferably 3 m or more, in the depth direction of the steel sheet constituting the member. Although the upper limit of the depth is not particularly specified, the effect is saturated at a depth of about 50 m. The depth of the Ni-diffusion region can be determined by analysis of a section in the thickness direction using EPMA (Electron Probe Micro Analyzer) or analysis in a depth direction using GDS (Glow Discharge Spectroscopy).

(8) As used herein, the term Ni-diffusion region represents a region where Ni diffuses into steel from a Ni-based coating layer during heating before hot-pressing is present in a solid-solution state. In addition, since a hot-pressed member of embodiments of the present invention is produced by hot-pressing a Ni-based coated steel sheet having a ZnNi alloy layer, the Ni-diffusion region may contain Zn as an impurity, but the advantages of the present invention are not impaired.

(9) 1-2) Intermetallic Compound Layer Corresponding to -Phase Present in a Phase Equilibrium Diagram of ZnNi Alloy on the Ni-Diffusion Region

(10) An intermetallic compound layer provided on the Ni-diffusion region has a corrosion potential having a sacrificing anticorrosion effect for steel and is thus effective for improving corrosion resistance after coating. The intermetallic compound layer corresponding to a -phase present in a phase equilibrium diagram of a ZnNi alloy represents a layer composed of an intermetallic compound of any one of Ni.sub.2Zn.sub.11, NiZn.sub.3, and Ni.sub.5Zn.sub.21. Such an intermetallic compound can be detected by direct X-ray diffraction of a surface of the member or electron beam diffraction while observing, with TEM (Transmission Electron Microscope), a slice prepared from a section in the thickness direction by FIB (Focused Ion Beam) processing.

(11) In order to achieve the above-described effect of the intermetallic compound layer, it is desirable to control the abundance of the intermetallic compound layer as described below.

(12) The abundance of the intermetallic compound layer can be measured by an electrochemical method, i.e., a spontaneous immersion potential in a 0.5 M aqueous NaCl air-saturated solution at 25 C.5 C. on the basis of a standard hydrogen electrode. When the spontaneous immersion potential becomes more noble than 360 mV with a small mount of the intermetallic compound layer, the sacrificing anticorrosion effect for steel disappears, and the corrosion resistance after coating is degraded. On the other hand, when the spontaneous immersion potential becomes less noble than 600 mV with a large mount of the intermetallic compound layer, the amount of hydrogen generated increases with corrosion, and hydrogen entry may occur even in the presence of the Ni-diffusion region. Therefore, it is desirable to provide the Ni-diffusion region in such an abundance that the spontaneous immersion potential in a 0.5 M aqueous NaCl air-saturated solution at 25 C.5 C. is 600 to 360 mV based on the standard hydrogen electrode. This abundance is preferably realized by allowing the intermetallic compound layer to be present in an island form. In embodiments of the present invention, the island-like intermetallic compound layer is defined by SEM (Scanning Electron Microscopy) observation of a section as follows:

(13) (1) A specimen of 10 mm10 mmthickness is cut out from the member, buried in a resin mold, and polished.

(14) (2) The specimen buried and polished in (1) is used and a reflection electron composition image is photographed with SEM at a magnification of 500 times and an acceleration voltage of 5 to 25 kV.

(15) (3) The specimen is photographed in any desired 10 fields of view.

(16) (4) In a photograph, as schematically illustrated in FIG. 1, when the intermetallic compound layer is discontinuously present on a surface of a steel sheet, a score is 1, while when the intermetallic compound layer is continuously present or not present in a field of view, a score is 0.

(17) (5) When the total score of the 10 photographs is 7 or more, the intermetallic compound layer is determined to be island-like.

(18) 1-3) ZnO Layer on Intermetallic Compound Layer Corresponding to -Phase Present in a Phase Equilibrium Diagram of ZnNi Alloy

(19) A ZnO layer provided in the outermost layer is excellent not only in adhesion to the intermetallic compound layer but also in adhesion to a chemical conversion-treated film formed in pretreatment for coating, thereby significantly increasing coating adhesion. With a thickness of 0.1 m or more, the adhesion to the conversion-treated film become satisfactory, while with a thickness of 5 m or less, the coating adhesion is not impaired by cohesive failure of the ZnO layer. Therefore, the thickness of the ZnO layer is preferably 0.1 to 5 m.

(20) Like the intermetallic compound layer, the ZnO layer can be observed by X-ray diffraction or electron beam diffraction through TEM observation, and the thickness thereof can be measured.

(21) The ZnO layer has excellent adhesion to the intermetallic compound layer provided below the ZnO layer, but the adhesion is further improved by providing, directly below the ZnO layer, at least one compound layer selected from a Si-containing compound layer, a Ti-containing compound layer, an Al-containing compound layer, and a Zr-containing compound layer, resulting in more excellent coating adhesion.

(22) 2) Production Method

(23) The hot-pressed member of an exemplary embodiment of the present invention can be produced by heating the Ni-based coated steel sheet including the ZnNi alloy coating layer containing 13% by mass or more of Ni on a surface of the steel sheet in a temperature range of an Ac.sub.3 transformation point to 1200 C., and then hot-pressing the steel sheet.

(24) As described above, when the Ni-based coated steel sheet is heated in the temperature range of the Ac.sub.3 transformation point to 1200 C., Ni in the coating layer diffuses into the steel sheet, forming the Ni-diffusion region. In addition, the intermetallic compound layer described above is formed from the ZnNi alloy coating layer provided on the surface and containing 13% by mass or more of Ni, and at the same time, Zn partially diffuses to the surface, forming the ZnO layer in the outermost layer.

(25) Even when the Ni content in the ZnNi alloy coating layer is less than 13% by mass, the Ni content is 10% by mass or more, and the coating weight of the ZnNi alloy coating layer exceeds 50 g/m.sup.2 per side of the steel sheet, so that the hot-pressed member of this embodiment of the present invention can be produced by hot-pressing after heating in the temperature range of the Ac.sub.3 transformation point to 1200 C. at an average hating rate of 12 C./second or more.

(26) When the Ni content in the ZnNi alloy coating layer is less than 10% by mass or the average heating rate is less than 12 C./second, not only the Ni-diffusion region is not sufficiently formed, but also Zn evaporation becomes excessively active, thereby failing to form the above-described intermetallic compound layer. In addition, when the coating weight of the ZnNi alloy coating layer is 50 g/m.sup.2 or less per side of the steel sheet, the Ni-diffusion region is not sufficiently formed. Here, the average heating rate of heating in the temperature range of the Ac.sub.3 transformation point to 1200 C. is defined as a value obtained by dividing a temperature difference from room temperature to the highest ultimate sheet temperature by a time required from room temperature to the highest ultimate sheet temperature.

(27) Since the surface of the steel sheet is coated with the ZnNi coating layer regardless of the Ni content, scales are not produced during heating before hot-pressing.

(28) The average heating rate of heating in the temperature range of the Ac.sub.3 transformation point to 1200 C. is preferably 85 C./second or more. Since the time of retention of the steel sheet at a high temperature is shortened by increasing the heating rate, austenite grains in the steel sheet can be made fine during heating, thereby improving the toughness of the member after hot-pressing. In addition, Zn evaporation can be significantly suppressed, and thus corrosion resistance after coating can be improved by forming the above-described intermetallic compound layer. Further, the excessive formation of the ZnO layer can be prevented, and thus coating adhesion can be stably secured. Such a heating rate can be realized by electric heating or high-frequency heating.

(29) The Ni-based coating layer of the Ni-based coated steel sheet may be a ZnNi alloy coating single layer or multiple layers including the ZnNi alloy coating layer provided on a Ni layer or a Ni-based alloy layer not containing Zn. As the Ni-based alloy, an alloy containing Ni and a total of 20% by mass or less of at least one element selected from Fe, Co, Cr, Mn, Cu, and Mo can be used.

(30) The depth of the Ni-diffusion region and the thickness of the ZnO layer can be adjusted by adjusting the heating conditions (temperature and time), and the abundance of the intermetallic compound layer can be adjusted by the coating weight of the Ni-based coating. The ZnO layer can be spontaneously formed by usual heating in air or heating in an atmosphere at an oxygen concentration of 0.1% by volume or more.

(31) The Ni-based coating layer described above can be formed by an electroplating method or the like.

(32) When at least one compound layer selected from a Si-containing compound layer, a Ti-containing compound layer, an Al-containing compound layer, and a Zr-containing compound layer is further provided on the ZnNi alloy coating layer formed on the surface of the steel sheet and is heated in the temperature range of the Ac.sub.3 transformation point to 1200 C., Zn partially passes through the compound layer and diffuses to the surface, forming the ZnO layer in the outermost layer. Therefore, at least one compound layer selected from a Si-containing compound layer, a Ti-containing compound layer, an Al-containing compound layer, and a Zr-containing compound layer can be provided immediately below the ZnO. In this case, when the thickness of the compound layer provided on the ZnNi alloy layer is 0.1 m or more, coating adhesion can be sufficiently improved, while when the thickness of the compound layer is 3.0 m or less, the Si-containing compound layer is not embrittled, and coating adhesion is not degraded. Therefore, the thickness is preferably 0.1 to 3.0 m and more preferably 0.4 to 2.0 m.

(33) Examples which can be applied as a Si-containing compound include silicone resins, lithium silicate, silicate soda, colloidal silica, a silane coupling agent, and the like. Examples which can be applied as a Ti-containing compound include titanates such as lithium titanate, calcium titanate, and the like, a titanium coupling agent containing titanium alkoxide or a chelate-type titanium compound as a main component, and the like. Examples which can be applied as an Al-containing compound include aluminates such as sodium aluminate, calcium aluminate, and the like, an aluminum coupling agent containing aluminum alkoxide or a chelate-type aluminum compound as a main component, and the like. Examples which can be applied as a Zr-containing compound include zirconates such as lithium zirconate, calcium zirconate, and the like, a zirconium coupling agent containing zirconium alkoxide or a chelate-type zirconium compound as a main component, and the like.

(34) The compound layer may be formed on the ZnNi alloy coating layer by depositing on the ZnNi alloy coating layer at least one compound selected from the Si-containing compound, the Ti-containing compound, the Al-containing compound, and the Zr-containing compound and then heat-drying the deposited compound without water washing. This compound may be deposited by any one of a coating method, a dipping method, and a spray method using a roll coater, a squeeze coater, or a die coater. In this case, after coating, dipping, or spraying using a squeeze coater or the like, an air knife method or roll squeeze method may be performed for adjusting the coating amount and achieving uniformity in appearance and uniformity in thickness. In addition, heat-drying is preferably performed so that the highest ultimate temperature of the steel sheet is 40 C. to 200 C., more preferably 50 C. to 160 C.

(35) The compound layer can also be formed on the ZnNi alloy coating layer by reactive treatment in which the Ni-based coated steel sheet including the ZnNi alloy coating layer is dipped in an acid aqueous solution containing at least one cation selected from Si, Ti, Al, and Zr and at least one anion selected from phosphate ion, hydrofluoric ion, and fluoride ion, and then heat-drying the steel sheet with or without water washing.

(36) In order to produce the hot-pressed member having a strength of 980 MPa or more, it is preferred to use, as a base steel sheet of the Ni-based coated steel sheet, for example, a steel sheet having a composition containing, by % by mass, C: 0.15 to 0.5%, Si: 0.05 to 2.0%, Mn: 0.5 to 3%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, and the balance including Fe and unavoidable impurities, or a steel sheet further containing, by % by mass, at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%, and Sb: 0.003 to 0.03% either alone or in combination.

(37) The reason for limiting each of the component elements is described below. Here, % representing the content of each component is % by mass unless otherwise specified.

(38) C: 0.15 to 0.5%

(39) C is an element which improves strength of steel, and a C content of 0.15% or more is required for producing a hot-pressed member having a TS of 980 MPa or more. On the other hand, with a C content exceeding 0.5%, blanking workability of the steel sheet used as a material is significantly decreased. Therefore, the C content is 0.15% to 0.5%.

(40) Si: 0.05 to 2.0%

(41) Like C, Si is an element which improves strength of steel, and a Si content of 0.05% or more is required for producing a hot-pressed member having a TS of 980 MPa or more. On the other hand, with a Si content exceeding 2.0%, the occurrence of surface defects referred to as red scales is significantly increased during hot rolling, the rolling load is increased, and ductility of the hot-rolled steel sheet deteriorates. Further, with a Si content exceeding 2.0% by mass, when a coating film mainly containing Zn or Al is formed on the surface of the steel sheet by plating, plating processability may be adversely affected. Therefore, the Si content is 0.05 to 2.0%.

(42) Mn: 0.5 to 3%

(43) Mn is an effective element for improving hardenability by suppressing ferrite transformation and is also an effective element for decreasing the heating temperature before hot-pressing because the Ac.sub.3 transformation point is decreased. In order to exhibit such an effect, a Mn content of 0.5% or more is required. On the other hand, with a Mn content exceeding 3%, segregation occurs to decrease homogeneity of the characteristics of the steel sheet used as a material and the hot-pressed member. Therefore, the Mn content is 0.5 to 3%.

(44) P: 0.1% or less

(45) When the P content exceeds 0.1%, segregation occurs to decrease homogeneity of the characteristics of the steel sheet used as a material and the hot-pressed member and also decrease toughness. Therefore, the P content is 0.1% or less.

(46) S: 0.05% or less

(47) When the S content exceeds 0.05%, toughness of the hot-pressed member is decreased. Therefore, the S content is 0.05% or less.

(48) Al: 0.1% or less

(49) When the Al content exceeds 0.1%, blanking workability and hardenability of the steel sheet used as a material are decreased. Therefore, the Al content is 0.1% or less.

(50) N: 0.01% or less

(51) When the N content exceeds 0.01%, nitride AlN is formed during hot rolling and heating before hot-pressing, and blanking workability and hardenability of the steel sheet used as a material are decreased. Therefore, the N content is 0.01% or less.

(52) The balance includes Fe and unavoidable impurities, but preferably, at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%, and Sb: 0.003 to 0.03% are either alone or in combination added for the reasons described below.

(53) Cr: 0.01 to 1%

(54) Cr is an effective element for strengthening steel and improving hardenability. In order to exhibit this effect, the Cr content is preferably 0.01% or more. On the other hand, when the Cr content exceeds 1%, the cost is significantly increased. Therefore, the upper limit is preferably 1%.

(55) Ti: 0.2% or less

(56) Ti is an effective element for strengthening steel and improving toughness by forming fine grains. Also, Ti forms a nitride in priority to B described below and is an effective element for exhibiting the effect of improving hardenability by solid-dissolved B. However, when the Ti content exceeds 0.2%, the rolling load during hot rolling is extremely increased, and toughness of the hot-pressed member is decreased. Therefore, the upper limit is preferably 0.2% or less.

(57) B: 0.0005 to 0.08%

(58) B is an effective element for improving hardenability during hot-pressing and toughness after the hot-pressing. In order to exhibit the effect, the B content is preferably 0.0005% or more. On the other hand, when the B content exceeds 0.08%, the rolling load during hot rolling is extremely increased, and a martensite phase and a bainite phase are produced after hot rolling, thereby causing cracks in the steel sheet. Therefore, the upper limit is preferably 0.08%.

(59) Sb: 0.003 to 0.03%

(60) Sb has the effect of suppressing the occurrence of a decarburized layer in the surface layer of the steel sheet during the time from heating of the steel sheet before hot-pressing to cooling of the steel sheet by a series of hot-pressing treatments. In order to exhibit the effect, a Sb content of 0.003% or more is required. On the other hand, when the Sb content exceeds 0.03%, the rolling load is increased, thereby decreasing productivity. Therefore, the Sb content is 0.003 to 0.03%.

(61) Examples of the heating method before hot-pressing include, but are not limited to, heating with an electric furnace or gas furnace, flame heating, electric heating, high-frequency heating, inductive heating, and the like.

EXAMPLES

(62) Both surfaces of a cold-rolled steel sheet having an Ac.sub.3 transformation point of 818 C., a thickness of 1.6 mm, and a composition containing, by % by mass, C: 0.23%, Si: 0.12%, Mn: 1.5%, P: 0.01%, S: 0.01%, Al: 0.03%, N: 0.005%, Cr: 0.4%, B: 0.0022%, and the balance including Fe and unavoidable impurities were electroplated in a plating bath containing 50 g/L (litter) of sodium sulfate, 100 g/L of nickel sulfate hexahydrate, and 50 g/L of zinc sulfate heptahydrate at pH2 and a temperature of 50 C. with a current density changed from 10 to 50 A/dm.sup.2 to form ZnNi alloy coating layers having different Ni contents and coating weights shown in Tables 1 and 2. Then, any one of a Si-containing compound, a Ti-containing compound, an Al-containing compound, and Zr-containing compound shown in Tables 1 and 2 was applied to each of the steel sheets with some exception and then dried under a condition in which the ultimate temperature was 140 C. to form any one of a Si-containing compound layer, a Ti-containing compound layer, an Al-containing compound layer, and Zr-containing compound layer having a thickness of 0.5 m. Then, a blank of 200 mm220 mm collected from each of the resultant steel sheets as a material was heated at an average heating rate of 8 C./sec in an air atmosphere in an electric furnace for 10 minutes at each of the heating temperatures shown in Tables 1 and 2. Then, each of the blanks was taken out from the furnace and immediately drawn by a pressing method schematically shown in FIG. 2 to form hot-pressed member Nos. 1, 4, 7 to 21, 28 to 30, 34, 37, 40, and 41. In addition, some of the steel sheets were heated by direct electric heating at an average heating rate of 12 C./sec or 90 C./sec, taken out from the furnace after each of the heating temperatures shown in Tables 1 and 2 was attained, and immediately drawn by the same pressing method as described above to form hot-pressed member Nos. 2, 3, 5, 6, 22 to 27, 31 to 33, 35, 36, 38, and 39. In drawing, the punch width was 70 mm, and the processing height was 30 mm. In addition, a sample was collected from a flat portion of the top of each member, and the depth of the Ni-diffusion region, the thickness of the ZnO layer, and the spontaneous immersion potential, which was an index for the abundance of the intermetallic compound layer, were measured by the above-described method. At the same time, the state of the intermetallic compound layer was confirmed by SEM observation of the section described above. In addition, scale resistance, coating adhesion, corrosion resistance after coating, and hydrogen entry resistance were examined by the methods described below.

(63) Scale resistance: evaluated by visually observing a punch non-contact surface after hot-pressing on the basis of the following criteria:

(64) Circle: Adhesion of no scale

(65) Cross: Adhesion of scales

(66) Coating adhesion: A sample was collected from a flat portion of the top of the member, and a punch non-contact surface was chemical conversion-treated using PB-SX35 manufactured by Nihon Parkerizing Co., Ltd. under standard conditions, and then electro-deposition paint GT-10HT gray manufactured by Kansai Paint Co., Ltd. was deposited to a thickness of 20 m under the baking conditions of 170 C. and 20 minutes to form a coated test piece. The conversion-treated and electro-deposited surface of the thus-formed test piece was cross-cut to the steel base material in a grid-like pattern (1010 squares, 1 mm spacing) with a cutter knife, and subjected to a cross-cut tape peel test in which an adhesive tape was applied and peeled. Evaluation was performed on the basis of the following criteria, and circle and triangle marks were regarded as satisfying an object of the present invention.

(67) Circle: No peeling

(68) Triangle: Peeling occurred in 1 to 10 squares

(69) Cross: Peeling occurred in 11 or more squares

(70) Corrosion resistance after coating: The conversion-treated and electro-deposited surface of a test piece prepared by the same method as described above for the coating adhesion was cross-cut with a cutter knife, and then subjected to a corrosion test under corrosion test cycle conditions according to SAE-J2334. The maximum coating blistering width on one side after 25 cycles was measured and evaluated on the basis of the following criteria, and circle and triangle marks were regarded as satisfying an object of the present invention.

(71) Circle: 0 nmblistering width<1.5 mm

(72) Triangle: 1.5 nmblistering width<3.0 mm

(73) Cross: 3.0 nmblistering width

(74) Resistance to hydrogen entry: A sample was collected from a flat portion of the top of the member, and one surface (punch non-contact surface) was mirror-ground to a thickness of 1 mm. Next, the ground surface of the sample was Ni-coated and used as a hydrogen detection surface, and the sample serving as a working electrode and platinum serving as a counter electrode were set in an electrochemical cell schematically shown in FIG. 3 to measure the amount of hydrogen entry into steel by an electrochemical hydrogen permeation method under corrosion of the unground surface at room temperature in air. That is, the hydrogen detection surface side was filled with a 0.1 M aqueous NaOH solution, and a reference electrode (Ag/AgCl) was set through a salt bridge. In addition, a 0.5 M NaCl solution was dropped on the unground surface (evaluation surface: punch non-contact surface), followed by corrosion at room temperature in air. The potential on the hydrogen detection surface side was set to 0 V vs Ag/AgCl, and the hydrogen permeation current value was continuously measured for 5 days by dropping pure water to the corrosion portion one time per day. The resistance to hydrogen entry with corrosion was evaluated from the maximum current value on the basis of the criteria below. Double circle and circle marks were regarded as satisfying an object of the present invention. In addition, the member on which scales significantly occurred during hot-pressing was tested after the scales were removed from the surfaces by shot blasting.

(75) Double circle: The maximum current was 1/10 or less of the cold-rolled steel sheet.

(76) Circle: The maximum current was over 1/10 to or less of the cold-rolled steel sheet.

(77) Cross: The maximum current was over of to the same as the cold-rolled steel sheet.

(78) The results are shown in Tables 3 and 4. It is found that hot-pressed member Nos. 1 to 27 and 30 according to aspects of the present invention are excellent not only in scale resistance, coating adhesion, and corrosion resistance after coating but also in resistance to hydrogen entry.

(79) TABLE-US-00001 TABLE 1 Surface layer structure of steel sheet material ZnNi alloy coating Heating conditions Hot- layer Si/Ti/Al/Zr-containing compound before hot pressing pressed Coating layer Heating member Ni content weight Thickness temperature Heating rate No. (mass %) (g/m2) Type (m) ( C.) ( C./sec) 1 13 30 No 900 8 2 15 40 No 900 12 3 14 50 No 900 90 4 15 30 Silicone resin 0.5 950 8 5 13 50 Silicone resin 0.5 950 12 6 14 4 Silicone resin 0.5 950 90 7 15 30 Lithium silicate 0.5 950 8 8 15 30 Colloidal silica 0.5 950 8 9 15 30 Silane coupling agent 0.5 950 8 10 16 30 Silicone resin 0.5 950 8 11 18 30 Silicone resin 0.5 1000 8 12 13 30 No 950 8 13 13 40 Silicone resin 0.5 1100 8 14 13 50 Silicone resin 0.5 850 8 15 13 20 Silicone resin 0.5 900 8 16 13 30 Lithium titanate 0.5 950 8 17 13 30 Titanium coupling agent 0.5 950 8 18 13 30 Sodium aluminate 0.5 950 8 19 13 30 Aluminum coupling agent 0.5 950 8 20 13 30 Lithium zirconate 0.5 950 8 21 13 30 Zirconium coupling agent 0.5 950 8 Surface layer structure of hot-pressed member Presence of Thickness Hot- Depth if Ni- State of Si/Ti/Al/Zr- of Spontaneous pressed diffusion intermetallic containing ZnO immersion member region compound compound layer potential No. (m) layer layer (m) (mV) Remark 1 5 Island No 2 550 Invention Example 2 3 Island No 2 550 Invention Example 3 1 Island No 2 550 Invention Example 4 20 Island Yes 2 500 Invention Example 5 5 Island Yes 2 500 Invention Example 6 2 Island Yes 2 500 Invention Example 7 20 Island Yes 2 500 Invention Example 8 20 Island Yes 2 500 Invention Example 9 20 Island Yes 2 500 Invention Example 10 25 Island Yes 3 450 Invention Example 11 30 Island Yes 5 400 Invention Example 12 10 Island Yes 3 450 Invention Example 13 30 Island Yes 2 450 Invention Example 14 20 Island Yes 2 450 Invention Example 15 20 Island Yes 2 450 Invention Example 16 20 Island Yes 2 500 Invention Example 17 20 Island Yes 2 500 Invention Example 18 20 Island Yes 2 500 Invention Example 19 20 Island Yes 2 500 Invention Example 20 20 Island Yes 2 500 Invention Example 21 20 Island Yes 2 500 Invention Example

(80) TABLE-US-00002 TABLE 2 Surface layer structure of material steel sheet Hot- ZnNi alloy coating Si/Ti/Al/Zr-containing Heating conditions pressed layer compound layer before hot pressing member Ni content Coating weight Thickness Heating temperature Heating rate No. (mass %) (g/m.sup.2) Type (m) ( C.) ( C./sec) 22 10 60 No 950 12 23 12 60 No 950 12 24 12 60 Silicone resin 0.5 950 12 25 10 60 No 950 90 26 12 60 NO 950 90 27 12 60 Silicone resin 0.5 950 90 28 10 60 No 950 8 29 12 60 NO 950 8 30 12 60 Silicone resin 0.5 950 12 31 10 30 No 950 12 32 12 30 No 950 12 33 12 30 Silicone resin 0.5 950 12 34 9 60 No 950 8 35 9 60 No 950 12 36 9 60 No 950 90 37 9 60 Silicone resin 0.5 950 8 38 9 60 Silicone resin 0.5 950 12 39 9 60 Silicone resin 0.5 950 90 40 Galvanized steel sheet 950 8 41 Cold-rolled steel sheet 950 8 Surface layer structure of hot-pressed member Hot- Depth of Ni- State of Presence of Thickness Spontaneous pressed diffusion intermetallic Si/Ti/Al/Zr- of ZnO immersion member region compound containing layer potential No. (m) layer compound layer (m) (mV) Remark 22 3 Island No 3 400 Invention Example 23 3 Island No 3 380 Invention Example 24 5 Island Yes 2 500 Invention Example 25 1 Island No 3 400 Invention Example 26 1 Island No 3 380 Invention Example 27 2 Island Yes 2 500 Invention Example 28 5 No No 5 350 Comparative Example 29 5 No No 3 350 Comparative Example 30 10 Island Yes 2 500 Invention Example 31 3 Island No 3 400 Comparative Example 32 3 Island No 3 380 Comparative Example 33 5 Island Yes 2 500 Comparative Example 34 2 No No 5 350 Comparative Example 35 1 Island No 3 350 Comparative Example 36 1 Island No 3 350 Comparative Example 37 2 No Yes 3 350 Comparative Example 38 1 Island Yes 3 350 Comparative Example 39 1 Island Yes 3 350 Comparative Example 40 0 No No 5 700 Comparative Example 41 0 No No 0 300 Comparative Example

(81) TABLE-US-00003 TABLE 3 Hot- Corrosion pressed Scale Coating resistance Hydrogen member resist- adhe- after entry No. ance sion coating resistance Remarks 1 Invention Example 2 Invention Example 3 Invention Example 4 Invention Example 5 Invention Example 6 Invention Example 7 Invention Example 8 Invention Example 9 Invention Example 10 Invention Example 11 Invention Example 12 Invention Example 13 Invention Example 14 Invention Example 15 Invention Example 16 Invention Example 17 Invention Example 18 Invention Example 19 Invention Example 20 Invention Example 21 Invention Example

(82) TABLE-US-00004 TABLE 4 Hot- Coat- Corrosion pressed Scale ing resistance Hydrogen member resist- adhe- after entry No. ance sion coating resistance Remarks 22 Invention Example 23 Invention Example 24 Invention Example 25 Invention Example 26 Invention Example 27 Invention Example 28 X X Comparative Example 29 X X Comparative Example 30 Invention Example 31 X Comparative Example 32 X Comparative Example 33 X Comparative Example 34 X X X Comparative Example 35 X X X Comparative Example 36 X X X Comparative Example 37 X X X Comparative Example 38 X X X Comparative Example 39 X X X Comparative Example 40 X X Comparative Example 41 X X X X Comparative Example