Method for manufacturing hot press formed part and hot press formed part

Abstract

A method for manufacturing a hot press formed part, includes: preparing a hot press forming object comprising a single-ply portion and a two-ply portion; heating the hot press forming object to a temperature range from an Ac.sub.3 transformation temperature of a base steel sheet of the first coated steel sheet to 1000 C.; press forming the hot press forming object to obtain a formed body, the press forming being started upon temperatures of the single-ply portion and of the two-ply portion being no higher than solidification points of ZnNi coating layers of the first and second coated steel sheets and no lower than an Ar.sub.3 transformation temperature of the base steel sheet of the first coated steel sheet; and quenching the formed body to thereby obtain a hot press formed part, in which the hot press forming object has a thickness ratio from 1.4 to 5.0.

Claims

1. A method for manufacturing a hot press formed part, comprising: preparing a hot press forming object comprising a single-ply portion and a two-ply portion by welding first and second coated steel sheets together in partially overlapping relationship, each of the first and second coated steel sheets having a ZnNi coating layer formed on a surface thereof; heating the hot press forming object to a temperature range from an Ac.sub.3 transformation temperature of a base steel sheet of the first coated steel sheet to 1000 C.; then, cooling the hot press forming object, then, press forming the hot press forming object to obtain a formed body, the press forming being started upon the temperatures of the single-ply portion and of the two-ply portion being no higher than solidification points of the ZnNi coating layers of the first and second coated steel sheets and no lower than the Ara transformation temperature of the base steel sheet of the first coated steel sheet; and quenching the formed body, while squeezing the formed body by a tool of press forming and holding at its press bottom dead center, to thereby obtain a hot press formed part, wherein the hot press forming object has a thickness ratio, expressed as t.sub.2/t.sub.1, from 1.4 to 5.0 where t.sub.1 denotes a thickness in millimeters of the single-ply portion and t.sub.2 denotes a thickness in millimeters of the two-ply portion, and wherein the solidification point of each of the ZnNi coating layers of the first and second coated steel sheets is 800 C. or higher.

2. The method for manufacturing a hot press formed part according to claim 1, wherein each of the ZnNi coating layers of the first and second coated steel sheets has an Ni content from 9 mass % to 25 mass %.

3. The method for manufacturing a hot press formed part according to claim 1, wherein the following relations are satisfied:
0.35[Ni %].sub.1.sup.2+17.1[Ni %].sub.1+72153ln(t.sub.2/t.sub.1)+9.6; and
0.35[Ni %].sub.2.sup.2+17.1[Ni %].sub.2+72153ln(t.sub.2/t.sub.1)+9.6, where [Ni %].sub.1 denotes the Ni content in mass % in the ZnNi coating layer of the first coated steel sheet and [Ni %].sub.2 denotes the Ni content in mass % in the ZnNi coating layer of the second coated steel sheet.

4. A hot press formed part manufactured by the method as recited in claim 1.

5. The method for manufacturing a hot press formed part according to claim 2, wherein the following relations are satisfied:
0.35[Ni %].sub.1.sup.2+17.1[Ni %].sub.1+72153ln(t.sub.2/t.sub.1)+9.6; and
0.35[Ni %].sub.2.sup.2+17.1[Ni %].sub.2+72153ln(t.sub.2/t.sub.1)+9.6, where [Ni %].sub.1 denotes the Ni content in mass % in the ZnNi coating layer of the first coated steel sheet and [Ni %].sub.2 denotes the Ni content in mass % in the ZnNi coating layer of the second coated steel sheet.

6. A hot press formed part manufactured by the method as recited in claim 2.

7. A hot press formed part manufactured by the method as recited in claim 3.

8. A hot press formed part manufactured by the method as recited in claim 5.

9. The method for manufacturing a hot press formed part according to claim 1, wherein the thickness ratio of the hot press forming object is more than 2.0 to 5.0.

10. The method for manufacturing a hot press formed part according to claim 2, wherein the thickness ratio of the hot press forming object is more than 2.0 to 5.0.

11. The method for manufacturing a hot press formed part according to claim 3, wherein the thickness ratio of the hot press forming object is more than 2.0 to 5.0.

12. The method for manufacturing a hot press formed part according to claim 5, wherein the thickness ratio of the hot press forming object is more than 2.0 to 5.0.

13. A hot press formed part manufactured by the method as recited in claim 9.

14. A hot press formed part manufactured by the method as recited in claim 10.

15. A hot press formed part manufactured by the method as recited in claim 11.

16. A hot press formed part manufactured by the method as recited in claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) In the accompanying drawings:

(2) FIG. 1 illustrates the relationship between the thickness ratio and the Ni content in the ZnNi coating layer;

(3) FIG. 2 illustrates the relationship between the thickness ratio and the temperature difference between a two-ply portion and a single-ply portion;

(4) FIG. 3 illustrates the relationship between the Ni content in each ZnNi coating layer and the solidification point of the coating layer;

(5) FIG. 4 is a schematic view of a hot press forming object according to an embodiment of the present disclosure;

(6) FIG. 5 is a schematic view of a tool of press forming according to an embodiment of the present disclosure;

(7) FIG. 6 is a schematic view of a hot press formed part manufactured in an example;

(8) FIG. 7 is a micrograph for ascertaining the presence or absence of liquid metal embrittlement cracking in a hot press formed part; and

(9) FIG. 8 illustrates the relationship between the thickness ratio and the temperature difference between two-ply and single-ply portions.

DETAILED DESCRIPTION

(10) In an embodiment of the present disclosure, a method for manufacturing a hot press formed part comprises: (i) preparing a hot press forming object comprising a single-ply portion and a two-ply portion by welding first and second coated steel sheets together in partially overlapping relationship, each of the first and second coated steel sheets having a ZnNi coating layer formed on a surface thereof; (ii) heating the hot press forming object to a temperature range from an Ac.sub.3 transformation temperature of a base steel sheet of the first coated steel sheet to 1000 C.; (iii) press forming the hot press forming object to obtain a formed body, the press forming being started upon temperatures of the single-ply portion and of the two-ply portion being no higher than solidification points of the ZnNi coating layers of the first and second coated steel sheets and no lower than an Ar.sub.3 transformation temperature of the base steel sheet of the first coated steel sheet; and (iv) quenching the formed body, while squeezing the formed body by a tool of press forming and holding at its press bottom dead center, to thereby obtain a hot press formed part.

(11) The following provides details of the hot press forming object prepared in (i), and of (ii), (iii), and (iv).

(12) <Hot Press Forming Object>

(13) The hot press forming object prepared in (i) uses a coated steel sheet having a ZnNi coating layer formed on the surface of a base steel sheet. First, the coated steel sheet is described below.

(14) A ZnNi alloy has a very high solidification point compared to ordinary Zn or Zn alloy coating layers, such as pure Zn coating layers or ZnFe alloy coating layers, as can be seen from the -phase, which appears in the ZnNi alloy phase equilibrium diagram and improves corrosion resistance, having a solidification point of 800 C. or higher. For this reason, a ZnNi coated steel sheet was used as the material of the hot press forming object. It is also possible to use two steel sheets, each having ZnNi coating applied only on one side.

(15) The base steel sheet is not particularly limited, and, for example, a hot-rolled steel sheet (pickled steel sheet) having a predetermined chemical composition or a cold-rolled steel sheet obtainable by cold rolling a hot-rolled steel sheet (pickled steel sheet) may be used. There is also no particular restriction on the manufacturing conditions on the base steel sheet.

(16) The method for forming a ZnNi coating layer on a surface of the base steel sheet includes, for example, after degreasing and pickling a base steel sheet, subjecting the base steel sheet to electrogalvanizing in a plating bath containing nickel sulfate hexahydrate at a concentration of 100 g/L to 400 g/L and zinc sulfate heptahydrate at a concentration of 10 g/L to 400 g/L, at a pH of 1.0 to 3.0 and a bath temperature of 30 C. to 70 C., with a current density of 10 A/dm.sup.2 to 150 A/dm.sup.2. When a cold-rolled steel sheet is used as the base steel sheet, the cold-rolled steel sheet may be subjected to annealing treatment before subjection to the degreasing and pickling.

(17) The Ni content in the ZnNi coating layer is preferably 9 mass % or more. The Ni content in the ZnNi coating layer is preferably 25 mass % or less. For example, by appropriately adjusting the concentration of zinc sulfate heptahydrate and the current density within the above-identified ranges, it is possible to obtain a desired Ni content (ranging from 9 mass % to 25 mass %).

(18) In the case of forming a ZnNi coating layer on a surface of the base steel sheet by electrogalvanizing, a -phase having a crystal structure of either Ni.sub.2Zn.sub.11, NiZn.sub.3, or Ni.sub.5Zn.sub.21 is formed when the Ni content in the coating layer is set in a range from 9 mass % to 25 mass %. This -phase has a high melting point, and is thus advantageous in suppressing the evaporation of the coating layer, which is a concern during (ii). The phase is also advantageous in suppressing liquid metal embrittlement cracking, which is problematic if it occurs during high-temperature hot press forming. In addition, the -phase has a sacrificial protection effect on steel and is also effective for improving corrosion resistance.

(19) The coating weight is preferably 10 g/m.sup.2 or higher per side. The coating weight is preferably 90 g/m.sup.2 or lower per side. The coating weight can be set as desired by adjusting the energizing time.

(20) The method for forming a ZnNi coating layer on a surface of the base steel sheet is not particularly limited, and any methods such as hot-dip galvanizing and electrogalvanizing may be used. When a hot-rolled steel sheet (pickled steel sheet) is used as the base steel sheet, the hot-rolled steel sheet (pickled steel sheet) may be subjected to ZnNi coating treatment to obtain a coated steel sheet. Alternatively, when a cold-rolled steel sheet is used as the base steel sheet, a cold-rolled steel sheet may be subjected to ZnNi coating treatment either directly after subjection to the cold rolling, or after subjection to annealing treatment following the cold rolling, to obtain a coated steel sheet.

(21) The coated steel sheet thus obtained is used to produce a hot press forming object. Specifically, a first coated steel sheet as a base material and a second coated steel sheet as a reinforcing material are blanked with predetermined dimensions, then the second coated steel sheet is partially overlapped on the first coated steel sheet, and these coated steel sheets are joined by spot welding to produce a hot press forming object comprising a two-ply portion and a single-ply portion. The single-ply portion is formed from the first coated steel sheet, and its thickness t.sub.1 (millimeters) is the same as that of the first coated steel sheet. Thickness t.sub.2 (millimeters) of the two-ply portion is the total thickness of the first and second coated steel sheets.

(22) When performing hot press forming on such a hot press forming object, it is necessary to cool the hot press forming object to a predetermined temperature after heating before the start of the press forming. However, the cooling rate varies in the two-ply and single-ply portions of the hot press forming object even under the same cooling condition; the temperature of the single-ply portion is lower. In addition, as the thickness ratio t.sub.2/t.sub.1 becomes large, the temperature difference T between the two-ply portion and the single-ply portion increases.

(23) On the other hand, to prevent a reduction in hardenability or in shape fixability of the single-ply portion formed from the first coated steel sheet, it is necessary to set the press forming start temperature for the hot press forming object at or above an Ar.sub.3 transformation temperature of the base steel sheet of the first coated steel sheet (hereinafter, where reference is made simply to the Ar.sub.3 transformation temperature, this refers to the Ar.sub.3 transformation temperature of the base steel sheet of the first coated steel sheet). However, when the temperature of the single-ply portion is set at or above the Ar.sub.3 transformation temperature, and particularly when the thickness ratio is large, the temperature of the two-ply portion becomes equal to or higher than the solidification point of the ZnNi coating layer, which causes the coating layer of the coated steel sheet to melt and consequently liquid metal embrittlement cracking to occur.

(24) Therefore, the thickness ratio of the hot press forming object needs to be 5.0 or less. The thickness ratio is preferably 4.0 or less, and more preferably 3.0 or less. Further, from the perspective of efficiently reinforcing a portion to be reinforced without a significant increase in weight, the thickness ratio of the hot press forming object needs to be 1.4 or more. The thickness ratio is preferably 1.6 or more, and more preferably 1.8 or more.

(25) Here, the upper limit for the thickness ratio of the hot press forming object is determined by the solidification points of the ZnNi coating layers and the temperature difference T between the two-ply portion and the single-ply portion of the hot press forming object.

(26) As described above, to produce a -phase having a high solidification point and exhibiting excellent corrosion resistance, the upper limit for the Ni content is set to 25 mass %, in which case the solidification point of the ZnNi alloy is about 880 C.

(27) On the other hand, to prevent a reduction in hardenability or in shape fixability during press forming, it is necessary to set the press forming start temperature for the hot press forming object no lower than the Ar.sub.3 transformation temperature (approximately 600 C. or higher).

(28) Accordingly, up to 280 C. may be allowed as the temperature difference between the two-ply portion and the single-ply portion of the hot press forming object. To meet the requirements for this temperature difference, the upper limit for the thickness ratio is set to 5.0.

(29) Furthermore, the solidification point of each ZnNi coating layer varies with the Ni content in the coating layer, and the thickness ratio allowable in the hot press forming object varies according to the difference in the solidification point. Therefore, it is preferable that the thickness ratio and the Ni content in the ZnNi coating layer satisfy the relation given by:
0.35[Ni %].sup.2+17.1[Ni %]+72153ln(t.sub.2/t.sub.1)+9.6(1)
where [Ni %] denotes the Ni content (mass %) in the ZnNi coating layer, t.sub.2 denotes the thickness (millimeters) of the two-ply portion, and t.sub.1 denotes the thickness (millimeters) of the single-ply portion.

(30) In a situation in which the ZnNi coating layers of the first and second coated steel sheets have different Ni contents, it is preferable that the following relations are satisfied:
0.35[Ni %].sub.1.sup.2+17.1[Ni %].sub.1+72153ln(t.sub.2/t.sub.1)+9.6(1a)
0.35[Ni %].sub.2.sup.2+17.1[Ni %].sub.2+72153ln(t.sub.2/t.sub.1)+9.6(1b)
where [Ni %].sub.1 denotes the Ni content in mass % of the ZnNi coating layer of the first coated steel sheet and [Ni %].sub.2 denotes the Ni content in mass % in the ZnNi coating layer of the second coated steel sheet.

(31) FIG. 1 illustrates the relationship between the thickness ratio t.sub.2/t.sub.1 and the Ni content in the ZnNi coating layer [Ni %]. In the figure, the hatched portion shows a range that satisfies expression (1) when the thickness ratio and the Ni content in the ZnNi coating layer are in a predetermined range.

(32) The derivation of expression (1) follows.

(33) First, we investigated a relationship between the thickness ratio t.sub.2/t.sub.1 and the temperature difference T between two-ply portions and single-ply portions. The results are presented in FIG. 2. It is noted that the temperature difference T between the two-ply portion and the single-ply portion refers to the temperature difference between the two-ply portion and the single-ply portion at a point in time when the temperature of the single-ply portion reached 600 C. after the hot press forming object being heated throughout to the same temperature and air-cooled. It can be seen from FIG. 2 that the temperature difference T increases with increasing thickness ratio t.sub.2/t.sub.1. These results yielded a regression equation given by expression (2) for the thickness ratio t.sub.2/t.sub.1 and the temperature difference T:
T=153ln(t.sub.2/t.sub.1)+9.6(2)

(34) We then investigated a relationship between the Ni content in each ZnNi coating layer, expressed as [Ni %], and the solidification point of the ZnNi coating layer, expressed as T.sub.fp. The relationship is presented in FIG. 3. It can be seen from FIG. 3 that the solidification point of each ZnNi coating layer rises with increasing Ni content. Additionally, these results yielded a regression equation between [Ni %] (the Ni content in a ZnNi coating layer) and T.sub.fp (the solidification point of the ZnNi coating layer) given by:
T.sub.fp=0.35[Ni %].sup.2+17.1[Ni %]+672(3)

(35) One condition required to prevent liquid metal embrittlement cracking during the hot press forming of the hot press forming object as described above is to set the temperature of the two-ply portion at the start of press forming no higher than the solidification point of the ZnNi coating layer. As described above, the temperature difference T between the two-ply portion and the single-ply portion in expression (2) represents the temperature difference between the two-ply portion and the single-ply portion at a point in time when the temperature of the single-ply portion reached 600 C. Accordingly, it suffices for a sum of 600 C.+ the temperature difference T between the two-ply portion and the single-ply portion defined by equation (2) not to exceed the solidification point of the ZnNi coating layer, as presented below:
T.sub.fp600+153ln(t.sub.2/t.sub.1)+9.6(4)

(36) By substitution of the regression equation (3) for the solidification point T.sub.fp of the coating layer into expression (4), equation (1) is derived.

(37) If the relation of expression (1) is satisfied, it is possible to more effectively avoid liquid metal embrittlement cracking at the two-ply portion.

(38) <Heating>

(39) In (ii), the hot press forming object prepared in (i) is heated to a predetermined heating temperature in a heating furnace in air atmosphere, for example, and is retained for a predetermined holding time. At this time, the hot press forming object is heated to a temperature range from the Ac.sub.3 transformation temperature to 1000 C. The holding time is not particularly limited, yet is preferably set in a range from 10 s to 60 s.

(40) When the base steel sheets of the first and second coated steel sheets have different Ac.sub.3 transformation temperatures, it is preferable to set the heating temperature for the hot press forming object no lower than the Ac.sub.3 transformation temperature of the base steel sheet of the first coated steel sheet and no lower than the Ac.sub.3 transformation temperature of the base steel sheet of the second coated steel sheet.

(41) If the heating temperature for the hot press forming object is below the Ac.sub.3 transformation temperature, an appropriate amount of austenite cannot be obtained during heating and ferrite will form during press forming, which makes it difficult to guarantee adequate strength or favorable shape fixability after the hot press forming. On the other hand, if the heating temperature for the hot press forming object exceeds 1000 C., the coating layer evaporates or excessive oxides form in the surface layer part, leading to a deterioration in oxidation resistance or corrosion resistance of the hot press formed part. Therefore, the heating temperature for the hot press forming object is set in a range from the Ac.sub.3 transformation temperature to 1000 C. The heating temperature is preferably no lower than the temperature [the Ac.sub.3 transformation temperature+30 C.]. The heating temperature is preferably no higher than 950 C.

(42) The method for heating the hot press forming object is not particularly limited, and any methods may be used, such as heating in an electric furnace, induction heating furnace, direct current furnace, gas heating furnace, or infrared heating furnace.

(43) <Press Forming>

(44) After being heated in (ii), the hot press forming object is subjected to press forming to obtain a formed body. The press forming is started upon temperatures of the single-ply portion and of the two-ply portion being no higher than solidification points of the ZnNi coating layers of the first and second coated steel sheets and no lower than the Ar.sub.3 transformation temperature of the base steel sheet of the first coated steel sheet.

(45) Setting the press forming start temperature no lower than the Ar.sub.3 transformation temperature can prevent a deterioration in hardenability or shape fixability. In addition, setting the press forming start temperature no higher than the solidification points of the ZnNi coating layers can prevent occurrence of liquid metal embrittlement cracking.

(46) The lower limit for the press forming start temperature is preferably no lower than [the Ar.sub.3 transformation temperature+30 C.], and the upper limit is preferably no higher than [the solidification points of the ZnNi coating layers of the first and second coated steel sheets30 C.].

(47) Additionally, the press forming is carried out by crash forming which does not use a blank holder or deep drawing which uses a blank holder. The tool of press forming has round portions at the punch shoulder and at the die shoulder, for example, and the clearance between the die and the punch is adjusted in accordance with the position at which the two-ply and single-ply portions of the hot press forming object abut each other in the tool of press forming.

(48) <Quenching>

(49) In the quenching, the formed body obtainable by the above press forming is quenched while being squeezed by the tool of press forming and held at its press bottom dead center, to thereby obtain a hot press formed part. To quench the formed body using the tool of press forming following the press forming, it is preferable to release the heat from the formed body after subjection to the press forming by holding for a predetermined time (3 seconds to 60 seconds) at the press bottom dead center.

(50) Upon completion of the quenching, the hot press formed part thus obtained is released from the tool of press forming.

Examples

(51) Next, the effects of the method for manufacturing a hot press formed part according to the disclosure are described based on examples.

(52) In the disclosed examples, cold-rolled steel sheets, each having a chemical composition containing 0.22 mass % of C, 0.15 mass % of Si, 1.43 mass % of Mn, 0.02 mass % of P, 0.004 mass % of S, 0.03 mass % of Al, and 0.004 mass % of N (and the balance being Fe and incidental impurities), were used as base steel sheets (Ac.sub.3 transformation temperature: 805 C.), and either a ZnNi coating layer, a pure Zn coating layer, or a ZnFe coating layer was formed on a surface of each cold-rolled steel sheet.

(53) In this case, the Ac.sub.3 transformation temperature was calculated by the following expression (see William. Leslie, The Physical Metallurgy of Steels, translated by Hiroshi Kumai and Tatsuhiko Noda, translation supervised by Shigeyasu Koda, Maruzen Co., Ltd., 1985, p. 273):
Ac.sub.3( C.)=910203[C].sup.0.5+44.7[Si]30[Mn]+700[P]+400[Al]
where [C], [Si], [Mn], [P], and [Al] are the contents (mass %) of the respective elements (C, Si, Mn, P, and Al) enclosed in the brackets.

(54) Each coating layer was formed under the following conditions.

(55) <ZnNi Coating Layer>

(56) Some of the cold-rolled steel sheets were passed through a continuous annealing line, heated to a temperature range from 800 C. to 900 C. at a heating rate of 10 C./s, retained in this temperature range for 10 s to 120 s, and then cooled to a temperature range of 500 C. or lower at a cooling rate of 15 C./s. Then, these cold-rolled steel sheets were subjected to electrogalvanizing treatment in a plating bath containing nickel sulfate hexahydrate at a concentration of 100 g/L to 400 g/L and zinc sulfate heptahydrate at a concentration of 10 g/L to 400 g/L, at a pH of 1.0 to 3.0 and a bath temperature of 30 C. to 70 C., with a current density of 10 A/dm.sup.2 to 150 A/dm.sup.2, whereby ZnNi coating layers were formed with predetermined Ni content and coating weight. The Ni content in each ZnNi coating layer was set to a predetermined content by adjusting the concentration of zinc sulfate heptahydrate and the current density. The coating weight of each coating layer was set to a predetermined coating weight by adjusting the energizing time.

(57) <Pure Zn Coating Layer>

(58) Some of the cold-rolled steel sheets were passed through a continuous hot-dip galvanizing line, heated to a temperature range from 800 C. to 900 C. at a heating rate of 10 C./s, retained in this temperature range for 10 s to 120 s, then cooled to a temperature range from 460 C. to 500 C. at a cooling rate of 15 C./s, and dipped into a galvanizing bath at 450 C., whereby Zn coating layers were formed. The coating weight of each Zn coating layer was adjusted to a predetermined coating weight using a gas wiping method.

(59) <ZnFe Coating Layer>

(60) The other cold-rolled steel sheets were passed through a continuous hot-dip galvanizing line, heated to a temperature range from 800 C. to 900 C. at a heating rate of 10 C./s, retained in this temperature range for 10 s to 120 s, then cooled to a temperature range from 460 C. to 500 C. at a cooling rate of 15 C./s, and dipped into a galvanizing bath at 450 C., whereby Zn coating layers were formed. The coating weight of each Zn coating layer was adjusted to a predetermined coating weight using a gas wiping method. As soon as the Zn coating layer was adjusted to a predetermined coating weight using the gas wiping method, the corresponding cold-rolled steel sheet was heated to a temperature range from 500 C. to 550 C. and retained for 5 s to 60 s in an alloying furnace to form a ZnFe coating layer. The Fe content in each coating layer was set to a predetermined content by changing the heating temperature in the alloying furnace and the holding time at the heating temperature within the above-mentioned ranges.

(61) From each of the coated steel sheets thus obtained (Steel A to Steel I), a first coated steel sheet (200 mm400 mm) as a base material and a second coated steel sheet (120 mm200 mm) as a reinforcing material were punched out. Then, as illustrated in FIG. 4, each second coated steel sheet was partially overlapped on the corresponding first coated steel sheet and joined by spot welding to obtain a hot press forming object 1 comprising a two-ply portion 3 and a single-ply portion 5.

(62) Table 1 presents the types, coating weights, solidification points, Ar.sub.3 transformation temperatures, and thicknesses of the coating layers of the coated steel sheets used in the examples (Steel A to Steel I). In this case, measurement was made of the Ar.sub.3 transformation temperature of Steel A to Steel I as follows. Samples for thermal expansion measurement were collected from the base steel sheets of Steel A to Steel I and heated for austenization to 950 C. The Ar.sub.3 transformation temperature was then measured for each sample. Air cooling was carried out by allowing each sample to cool in the air.

(63) TABLE-US-00001 TABLE 1 Coating layer Ar.sub.3 Coating Solidification transformation Thick- Steel weight point temperature ness ID Type (g/m.sup.2) ( C.) ( C.) (mm) A Zn12% Ni 45 827 610 1.8 B Zn10% Ni 65 808 630 2.3 C Zn15% Ni 30 850 580 1.2 D Zn22% Ni 22 879 660 3.9 E Zn13% Ni 25 835 670 5.0 F Zn 40 419.5 610 1.8 G 40 419.5 630 2.3 H Zn11% Fe 45 665 610 1.8 I 45 665 630 2.3

(64) Then, each hot press forming object 1 was heated in an electric furnace in air atmosphere under the conditions in Table 2. Subsequently, each hot press forming object 1 was set in a tool of press forming 11 (in an open position) as illustrated in FIG. 5, and press forming was performed at the press forming start temperatures listed in Table 2 to obtain formed bodies. The press forming was carried out by crash forming in which the punch 15 was pushed against and into the die 13 without using the blank holder, which was thus lowered. After being quenched in the tool of press forming 11 while being held at the press bottom dead center for 30 s, each formed body was released from the tool of press forming 11, and as a result a hot press formed part with a hat cross section shape as illustrated in FIG. 6 was obtained.

(65) As illustrated in FIG. 5, the tool of press forming 11 has a cross section shape such that point A (a round portion of the punch shoulder) and point B (a round portion of the die shoulder) both have a radius of curvature R of 5 mm. Clearances CR1 and CR2 between the die 13 and the punch 15 were adjusted in the tool of press forming to match the thickness of the two-ply portion of the hot press forming object and the thickness of the single-ply portion, respectively.

(66) In each hot press formed part thus prepared, the presence or absence of liquid metal embrittlement cracking was judged by observing a cross section of a sample cut out from the two-ply portion (at a portion contacting the R portion of the punch shoulder) as illustrated in FIG. 6.

(67) As illustrated in FIG. 6, samples were further collected from the surface of a top portion 23 of each hot press formed part 21, which is located at the two-ply portion, and from a wall portion 25, which is located at the single-ply portion, respectively, and the hardness was measured with a Vickers hardness meter. The hardness of each sample was determined by averaging the results obtained by measurement at intervals of 0.1 mm along the thickness direction of each sample under a load of 2.94 N. In this case, the targeted hardness was 400 Hv or more.

(68) FIG. 7 presents micrographs that were taken for observing the presence or absence of liquid metal embrittlement cracking in hot press formed parts prepared by performing hot press forming at different press forming start temperatures on hot press forming objects, whose first and second coated steel sheets were both formed from Steel A (coating layer's solidification point: 827 C.). For hot press formed parts for which the press forming start temperature at the two-ply portion was set to 776 C. ((c) in FIG. 7) or 806 C. ((b) in FIG. 7), liquid metal embrittlement cracking did not occur. In contrast, for the other hot press formed part for which the press forming start temperature at the two-ply portion was set to 830 C. ((a) in FIG. 7), which is higher than the solidification point of the ZnNi coating layer, liquid metal embrittlement cracking occurred from the surface of the hot press formed part toward the inside of the base steel sheet.

(69) Table 2 also presents the types of coated steel sheets used for the hot press forming objects, thickness ratios t.sub.2/t.sub.1, heating conditions for the hot press forming objects, press forming start temperatures, presence or absence of liquid metal embrittlement cracking, and hardness measurements. The thickness ratio t.sub.2/t.sub.1 was calculated as [the thickness of the first coated steel sheet+ the thickness of the second coated steel sheet]/[the thickness of the first coated steel sheet].

(70) TABLE-US-00002 TABLE 2 Heating conditions for hot press forming object Press forming start temp. Evaluation results First Second Thickness Heating Holding ( C.) Liquid metal Hardness (Hv) coated coated ratio temp. time Single-ply Two-ply embrittlement Single-ply Two-ply steel sheet steel sheet t.sub.2/t.sub.1 ( C.) (s) portion portion cracking portion portion Remarks Steel A Steel A 2.00 900 10 680 759 Not occurred 485 473 Example 1 870 20 630 727 Not occurred 475 462 Example 2 890 15 700 772 Not occurred 488 472 Example 3 910 5 780 830 Occurred 485 471 Comparative Example 1 900 30 570 689 Not occurred 365 461 Comparative Example 2 Steel A Steel B 2.28 900 15 670 766 Not occurred 481 475 Example 4 Steel B Steel C 1.52 850 20 640 695 Not occurred 473 468 Example 5 Steel C Steel B 2.92 900 10 630 776 Not occurred 483 471 Example 6 Steel C Steel D 4.25 910 60 620 826 Not occurred 462 443 Example 7 Steel A Steel E 3.78 920 40 620 806 Not occurred 476 451 Example 8 Steel C Steel E 5.17 920 60 620 854 Occurred 469 435 Comparative Example 3 Steel F Steel F 2.00 880 30 620 721 Occurred 462 465 Comparative Example 4 Steel F Steel G 2.28 900 10 650 755 Occurred 478 472 Comparative Example 5 Steel H Steel H 2.00 900 15 675 756 Occurred 481 475 Comparative Example 6 Steel H Steel I 2.28 910 10 640 749 Occurred 469 463 Comparative Example 7 Steel F Steel F 2.00 900 30 450 639 Occurred 255 420 Comparative Example 8 Steel H Steel H 2.00 880 10 530 685 Occurred 278 443 Comparative Example 9

(71) It can be seen from Table 2 that for Examples 1 to 8, the thickness ratio, the type of coating layer (ZnNi coating layer), the heating temperature for the hot press forming object, and the press forming start temperature are all within the appropriate ranges, and liquid metal embrittlement cracking did not occur in the hot press formed parts, which exhibited sufficient hardness.

(72) For Examples 1 to 3, Steel A having an Ni content in the ZnNi coating layer of 12 mass % was used for both the first and second coated steel sheets, and solving expressions (1a) and (1b) both yield t.sub.2/t.sub.14.13. For Examples 1 to 3, the thickness ratio t.sub.2/t.sub.1 is 2.00, which satisfies expressions (1a) and (1b).

(73) For Example 4, Steel A (the Ni content in the ZnNi coating layer=12 mass %) was used for the first coated steel sheet and Steel B (the Ni content in the ZnNi coating layer=10 mass %) for the second coated steel sheet, and solving expressions (1a) and (1b) yield t.sub.2/t.sub.14.13 and t.sub.2/t.sub.13.65, respectively. For Example 4, the thickness ratio t.sub.2/t.sub.1 is 2.28, which satisfies expression (1a) and (1b).

(74) In Example 5, Steel B (the Ni content in the ZnNi coating layer=10 mass %) was used for the first coated steel sheet and Steel C (the Ni content in the ZnNi coating layer=15 mass %) for the second coated steel sheet, and solving expressions (1a) and (1b) yield t.sub.2/t.sub.13.65 and t.sub.2/t.sub.14.80, respectively. For Example 5, the thickness ratio t.sub.2/t.sub.1 is 1.52, which satisfies expressions (1a) and (1b).

(75) For Example 6, Steel C (the Ni content in the ZnNi coating layer=15 mass %) was used for the first coated steel sheet and Steel B (the Ni content in the ZnNi coating layer=10 mass %) for the second coated steel sheet, and solving expressions (1a) and (1b) yield t.sub.2/t.sub.14.80 and t.sub.2/t.sub.13.65, respectively. For Example 6, the thickness ratio t.sub.2/t.sub.1 is 2.92, which satisfies expressions (1a) and (1b).

(76) For Example 7, Steel C (the Ni content in the ZnNi coating layer=15 mass %) was used for the first coated steel sheet and Steel D (the Ni content in the ZnNi coating layer=22 mass %) for the second coated steel sheet, and solving expressions (1a) and (1b) yield t.sub.2/t.sub.14.80 and t.sub.2/t.sub.15.80, respectively. For Example 7, the thickness ratio t.sub.2/t.sub.1 is 4.25, which satisfies expressions (1a) and (1b).

(77) For Example 8, Steel A (the Ni content in the ZnNi coating layer=12 mass %) was used for the first coated steel sheet and Steel E (the Ni content in the ZnNi coating layer=13 mass %), and solving expression (1a) and (1b) yield t.sub.2/t.sub.14.13 and t.sub.2/t.sub.14.36, respectively. In Example 8, the thickness ratio t.sub.2/t.sub.1 is 3.78, which satisfies expressions (1a) and (1b).

(78) In contrast, for Comparative Example 1, the press forming start temperature at the two-ply portion was higher than the solidification point (827 C.) of the ZnNi coating layer (the Ni content in the ZnNi coating layer=12 mass %) of each of the first and second coated steel sheets, and liquid metal embrittlement cracking occurred in the hot press formed part.

(79) For Comparative Example 2, the press forming start temperature at the single-ply portion was lower than the Ar.sub.3 transformation temperature (610 C.), and the hot press formed part suffered a reduction in hardness at the single-ply portion.

(80) For Comparative Example 3, the thickness ratio was outside the appropriate range, the press forming start temperature at the two-ply portion was higher than the solidification point (850 C.) of the ZnNi coating layer of the first coated steel sheet, and liquid metal embrittlement cracking occurred in the hot press formed part.

(81) For Comparative Examples 4 to 9, the coating layers were pure Zn coating layers (Comparative Examples 4, 5 and 8) or ZnFe coating layers (Comparative Examples 6, 7 and 9), which had lower solidification points, and in any of these cases liquid metal embrittlement cracking occurred in the hot press formed part.

(82) Additionally, for Comparative Examples 8 and 9, the press forming start temperature at the single-ply portion was set at or below the Ar.sub.3 transformation temperature of the base steel sheet of the first coated steel sheet, and in either case the hot press formed part suffered a reduction in hardness at the single-ply portion.

(83) As described above, the present disclosure enables manufacture of high-strength, lightweight, and high-fatigue-strength hot press formed parts without causing liquid metal embrittlement cracking even when performing hot press forming on hot press forming objects having a larger thickness ratio than that of conventional ones.

REFERENCE SIGNS LIST

(84) 1 Hot press forming object 3 Two-ply portion 5 Single-ply portion 11 Tool of press forming 13 Die 15 Punch 17 Blank holder 21 Hot press formed part 23 Top portion 25 Wall portion