Hot stamped component, precoated steel sheet used for hot stamping and hot stamping process

Abstract

The present invention relates to a hot stamped component, a precoated steel sheet used for hot stamping, and a hot stamping process. The hot stamped component of the present invention is provided with a coating of aluminium or an aluminium alloy on at least one surface of the base steel, the coating is produced by interdiffusion between the base steel and a precoating of aluminium or aluminium alloy, and the coating has a thickness of 6 to 26 μm.

Claims

1. A hot stamping process, comprising the following steps: a steel sheet austenization step including multi-stage heating a precoated steel sheet for hot stamping or a preformed component thereof to 840° C. or more, wherein the precoated steel sheet for hot stamping is provided with a precoating of aluminium or aluminium alloy on at least one surface of the base steel, the total thickness of the precoated steel sheet for hot stamping is 0.5 mm to 3.0 mm, and the thickness of the precoating is 3 to 19 μm; a steel sheet transfer step including transferring the heated precoated steel sheet to a hot stamping die, wherein it is ensured that the temperature of the precoated steel sheet is more than 550° C. when transferred to the die; and a hot stamping step including stamping the transferred precoated steel sheet in the die to obtain a hot stamped component, wherein, in the steel sheet austenization step, the multi-stage heating is performed in the following manner: for a precoated steel sheet having a thickness of 0.5 to 1.5 mm, the heating temperature is set to 700-875° C. in a preheating stage for coating alloying, the heating temperature in the first stage of austenization heating is set to 920-940° C., the heating temperature in the second stage is set to 940-960° C., the heating temperature in the third stage is set to 920-940° C., the fourth stage is a heat preservation stage, and the temperature is set to 900 to 920° C.; for the precoated steel sheet with a thickness of 0.5 to 1.0 mm, the total heating time for the first to the fourth stage of austenization heating is set to 90 s to 140 s; for the precoated steel sheet with a thickness greater than 1.0 mm and less than or equal to 1.5 mm, the total heating time for the first to the fourth stage of austenization heating is set to 120 s to 180 s; and for the precoated steel sheet having a thickness greater than 1.5 mm and less than or equal to 3.0 mm, the heating temperature in the preheating stage for coating alloying is set to 700 to 895° C.; in the austenization heating process, the heating temperature in the first stage is set to 940-960° C., the heating temperature in the second stage is set to 950-970° C., the heating temperature in the third stage is set to 940-960° C., the fourth stage is a heat preservation stage, and the temperature is set to 920 to 940° C., and wherein: for the precoated steel sheet with a thickness greater than 1.5 mm and less than or equal to 2.0 mm, the total heating time for the first to the fourth stage of austenization heating is set to 180 s-220 s; for the precoated steel sheet having a thickness greater than 2.0 mm and less than or equal to 2.5 mm, the total heating time for the first to the fourth stage of austenization heating is set to 180 s-260 s; and for the precoated steel sheet having a thickness greater than 2.5 mm and less than or equal to 3.0 mm, the total heating time for the first to the fourth stage of austenization heating is set to 200 s-300 s.

2. The hot stamping process according to claim 1, further comprising a paint baking step after the hot stamping step, wherein in the paint baking step, the hot stamped component is heated to 130 to 200° C. and is held for 5 to 60 minutes and then is cooled in any way.

3. The hot stamping process according to claim 1, wherein the base steel comprises, by weight percentage, the following components: 0.28-0.4% of C; 0.6-3.5% of Mn; 0-0.004% of B; 0-0.4% of Nb+Ti+V, 0.05 to 1% of Si, 0.01 to 1% Al; total content of less than 5% of Cr, Mo, Ni, Cu; and unavoidable impurity elements.

4. The hot stamping process according to claim 1, wherein the base steel comprises, by weight percentage, the following components: 0.19 to 0.29% of C; 0.6 to 3.5% of Mn; 0 to 0.004% of B; 0 to 0.4% of Nb+Ti+V; 0 to 2% of Si, 0 to 2% of Al; total content of less than 5% of Cr, Mo, Ni, Cu; and unavoidable impurity elements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view of a coating structure prior to hot stamping of a steel sheet having different precoating thicknesses.

(2) FIG. 2 is a view of a coating structure after hot stamping of a steel sheet having different precoating thicknesses.

(3) FIG. 3 is a schematic view showing the microhardness test position of a steel sheet having different precoating thicknesses after hot stamping.

(4) FIG. 4 is a microhardness trend diagram of a steel sheet having different precoating thicknesses after hot stamping.

(5) FIG. 5 is a three-point bending performance curve of a steel sheet having different precoating thicknesses after hot stamping of a 1.2 mm thick 22MnB5 steel, wherein the picture on the right side is a partial enlarged view of the picture on the left side.

(6) FIG. 6 is a view showing a C enrichment at the boundary between the coating and the substrate after hot stamping of 22MnB5 steel having Al—Si precoating.

(7) FIG. 7 is a view showing hot stamping process parameters of a sheet of 0.5 to 1.5 mm.

(8) FIG. 8 is a view showing hot stamping process parameters of a sheet of 1.5 to 3.0 mm.

(9) FIG. 9 is a view showing a structure of a coating after hot stamping of a steel sheet in prior art.

(10) FIG. 10 is a view showing a hot stamping process in prior art.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(11) The present invention will be described in more detail below with reference to exemplary embodiments. The following embodiments or experimental data are intended to exemplify the present invention, and it should be apparent to those skilled in the art that the present invention is not limited to these embodiments or experimental data.

(12) Firstly, the precoated steel sheet for hot stamping of the present invention has a thickness of 0.5 to 3.0 mm, wherein the precoating has a thickness of 3 to 19 μm, and in the present embodiment, the precoated steel sheet for hot stamping respectively having a precoating of a thickness of 5 μm and 14 μm is provided, wherein the precoating is formed on the upper and lower surfaces of the steel sheet. Further, a precoated steel sheet for hot stamping having a precoating of a thickness of 25 μm is provided for comparison.

(13) Wherein, the base steel of the steel sheet comprises, by weight percentage, the following components: 0.28 to 0.4% C; 0.6 to 3.5% Mn; 0 to 0.004% B; 0 to 0.4% Nb+Ti+V, 0.05-1% Si, 0.01-1% Al; total content less than 5% Cr, Mo, Ni, Cu; and unavoidable impurity elements, wherein V content is preferred 0.1˜0.4%. Alternatively, the base steel comprises, by weight percentage, the following components: 0.19 to 0.28% C; 0.6 to 3.5% Mn; 0 to 0.004% B; 0 to 0.4% Nb+Ti+V; 0 to 2% Si, 0 to 2% Al; total content of less than 5% of Cr, Mo, Ni, Cu; and unavoidable impurity elements.

(14) For example, as a preferred embodiment, the base steel comprises, by weight percentage, the following components: 0.35% C; 1.35% Mn; 0.002% B; 0.04% Ti; 0.2% V; 0.2% Si, 0.05% Al; 0.2% Cr.

(15) The precoating of the precoated steel sheet of the present invention can be formed, for example, by hot-dip coating, and the basic composition of a typical hot-dip coating solution usually includes (by weight): 8 to 11% Si, 2% to 4% Fe, and the balance is Al or an Al alloy and unavoidable impurities. However, the present invention is not limited to this composition, and various aluminium or aluminium alloy coatings may be employed. Among them, Si is mainly for suppressing the formation of intermetallic compounds. If the silicon content is low, the aluminium-rich intermetallic compound Al.sub.2Fe phase is easily formed within 2 minutes of the austenization process, which is easy to transform into Fe.sub.2Al.sub.5 phase in 2 to 6 minutes after austenitization; this layer is a brittle phase and thus is unfavorable to the hot stamping die and to the welding electrode. Therefore, Si content is generally controlled at 8-11%. The high temperature stability and oxidation resistance of the coating are mainly provided by Al, namely a thin and dense Al.sub.2O.sub.3 is used as a protective film. The aluminium content in the precoating formed in this way is usually equal to or more than 60%.

(16) As an example, the base steel and the precoating have the compositions shown in Table 1.

(17) TABLE-US-00001 TABLE 1 Chemical compositions of the base steel and the precoating of the precoated steel sheet according to the present invention Chemical compositions (weight %) C Mn Ti Cr B Si Al Fe Base steel 0.23 1.18 0.04 0.16 0.0025  0.22 0.034 Bal. (22MnB5) Precoating           10    Bal. 3 (AlSi.sub.10Fe.sub.3)

(18) Wherein Bal. represents the balance other than other elements.

(19) The precoating structure of the 22MnB5 steel sheet is shown in FIG. 1. Wherein the left side steel sheet (the steel sheet of the present invention) has a precoating having a thickness of 5 μm, the middle steel sheet (the steel sheet of the present invention) has a precoating having a thickness of 14 μm, and the right side steel sheet (conventional steel sheet) has a precoating having a thickness of 25 μm. The substrate for each steel sheet is ferrite and pearlite structure, and the precoating has the structure (from the substrate side) of an intermetallic compound layer (Fe.sub.2Al.sub.5, Fe.sub.2Al.sub.8Si), an aluminium coating.

(20) The analysis on the precoating structures shows that the intermetallic compound layer thicknesses, of the steel sheets for hot stamping having different precoating thicknesses, are approximately equal, all are about 4.5 μm, and the difference lies mainly in the thicknesses of the aluminium coating. The thicknesses of the aluminium coating in the above respective steel sheets are 0.5 μm, 9.5 μm, and 20.5 μm, respectively.

(21) For example, the steel sheet for hot stamping having a precoating of 3 to 19 μm according to the present invention can be produced by the following process.

(22) (1) Steelmaking, according to the above chemical compositions, smelting by vacuum induction furnace, electric furnace or converter; using continuous casting technology to produce slab, or directly using thin slab continuous casting and rolling process.

(23) (2) Uniform heating of the slab, heating the smelted slab to the temperature of 1150 to 1340° C. and keeping it at this temperature for 20 to 2000 minutes.

(24) (3) Hot rolling, the slab is rough-rolled at 1020 to 1280° C., the total press quantity of the steel sheet is 50% or more, and the finishing temperature is controlled at 500° C. or higher to obtain a hot-rolled steel sheet; the hot-rolled product is coiled in a temperature zone below 840° C., and then pickling can be performed to obtain a hot-rolled pickled steel sheet. The final structure is pearlite+ferrite, or pearlite+ferrite+a small amount of bainite, or pearlite+ferrite+a small amount of martensite.

(25) (4) The hot-rolled steel sheet described above may be subjected to cold rolling to obtain a cold-rolled steel strip.

(26) (5) The surface of the cold-rolled steel sheet is subjected to different processes of coating the Al—Si precoating to obtain an Al—Si precoated steel sheet.

(27) Next, the above said precoated steel sheet is subjected to hot stamping, and for example, the following hot stamping process can be employed.

(28) (a) steel sheet austenization: cutting the precoated steel sheet to obtain a steel sheet of predetermined size and shape; using heating devices such as a box furnace, roller hearth furnace or induction heating etc., heating procedure as shown in FIG. 7 and FIG. 8, the steel sheet for hot stamping or a preformed component thereof is rapidly heated to 700 to 970° C. and held for 0.5 to 15 minutes.

(29) (b) Steel sheet transfer: The heated steel sheet is transferred to a hot stamping die and it is ensured that the temperature of the steel sheet is 550° C. or higher when transferred to the die.

(30) (c) Hot stamping: According to the size of the sheet, a reasonable press tonnage is set for stamping; and the holding time is determined according to the sheet thickness, and the die surface temperature can be controlled by the die cooling system so that the steel sheet in the die is cooled to 250° C. or less at an average cooling rate of not less than 10° C./s, and then cooled to room temperature in any manner to obtain a corresponding hot stamped component.

(31) The formed component obtained by the above stamping can further be subjected to the following treatment:

(32) (d) Paint baking: During the component paint baking process, the formed component is heated to 130 to 200° C., kept for 5 to 60 minutes, and then cooled in any manner.

(33) In the austenization step of the steel sheet, for the precoated steel sheet having a thickness of 0.5 to 1.5 mm, the heating temperature is set to 700 to 875° C. in the preheating stage for coating alloying, and it is heated to 840° C. or more in the austenization stage; specifically, the heating temperature in the first heating stage of austenization is set to 920 to 940° C., in the second stage it is set to 940 to 960° C., in the third stage it is set to 920 to 940° C., the fourth stage is a heat preservation stage, in which the temperature is set to 900 to 920° C. For the precoated steel sheet having a thickness of 0.5 to 1.0 mm, the total heating time (stages 1 to 4) is set to 90 s-140 s, for the precoated steel sheet with thickness greater than 1.0 mm and less than or equal to 1.5 mm, the total heating time is set to 120 s-180 s; for the precoated steel sheet with thickness greater than 1.5 mm and less than or equal to 3.0 mm, the heating temperature is set to 700 to 895° C. in the preheating stage for coating alloying, in the austenization heating process, in the first stage the heating temperature is set to 940 to 960° C., in the second stage it is set to 950 to 970° C., and in the third stage it is set to 940 to 960° C., the fourth stage is a heat preservation stage, and the temperature is set to 920 to 940° C. For the precoated steel sheet having a thickness of more than 1.5 mm and less than or equal to 2.0 mm, the total heating time is set to 180 s to 220 s, and for the precoated steel sheet having a thickness of more than 2.0 mm and less than or equal to 2.5 mm, the total heating time is set to 180 s-260 s, for the precoated steel sheets with a thickness greater than 2.5 mm and less than or equal to 3.0 mm, the total heating time is set to 200 s to 300 s. In the case where the total heating time requirement is satisfied, the heating time in each stage can be distributed according to the thickness of the steel sheet or the like.

(34) For example, the parameters in the austenization step of the steel sheet described above can be set as shown in Table 2.

(35) TABLE-US-00002 TABLE 2 Hot stamping process parameters 4th stage total Preheating Heat heating stage for 1st 2nd 3rd preser- time Sheet coating stage stage stage vation 1st-4th thickness alloying Quick heating stage stage stage   1 mm 875° 935° 950° 930° 910° 140 s C./60 s C./20 s C./20 s C./20 s C./80 s 1.5 mm 875° 935° 960° 940° 920° 180 s C./90 s C./30 s C./30 s C./30 s C./90 s 2.0 mm 895° 945° 970° 950° 930° 220 s C./120 s C./40 s C./40 s C./40 s C./100 s 2.5 mm 895° 945° 970° 955° 930° 260 s C./150 s C./50 s C./50 s C./50 s C./110 s

(36) Then, the coating of the formed component obtained by the above process is analysed.

(37) Table 3 illustrates variations in coating thickness before and after hot stamping of steel sheets having different precoating thicknesses, wherein IS1 and IS2 represent the precoated steel sheets of the present invention, and CS1 represents a conventional precoated steel sheet.

(38) TABLE-US-00003 TABLE 3 Coating thickness before and after stamping of precoated steel sheets Coating Total thickness thickness Precoating after hot of the thickness stamping steel sheet Sample Base steel Precoating (μm) (μm) (mm) IS1 22MnB5 AlSi.sub.10Fe.sub.3 5 10 1.2 IS2 22MnB5 AlSi.sub.10Fe.sub.3 14 21 1.2 CS1 22MnB5 AlSi.sub.10Fe.sub.3 25 34 1.2

(39) In the above-mentioned precoated steel sheets for hot stamping having a precoating having a thickness of 5 μm, 14 μm, and 25 μm, respectively, after hot stamping, the coating thickness is changed to 10 μm, 21 μm, and 34 μm, respectively, and the coating structure is as shown in FIG. 2. The microhardness is measured for each layer, and the measurement position and results are shown in FIGS. 3 and 4.

(40) Specifically, the following layers are sequentially arranged from the base to the outside:

(41) (a) First layer: an interdiffusion layer having a thickness of 6 to 14 μm, and in the illustrated example, 8 to 12 μm. The thickness of the interdiffusion layer needs to be greater than or equal to 6 μm to ensure that a certain transition layer is formed to prevent crack propagation to the substrate; the thickness of 14 μm or less is mainly to avoid C enrichment due to excessive diffusion of C. The interdiffusion layer is α-Fe rich in Al and Si, wherein the Fe content is 70% by weight or more. Further, the interdiffusion layer may contain intermetallic compound phases of Fe and Al, such as Fe.sub.3Al of nanoparticles. Further, the surface of the interdiffusion layer may further be covered by an intermetallic compound FeAl layer of 1 to 2 μm. The intermetallic compound, such as Fe.sub.3Al and FeAl accounts are equal to or less than 20% by volume in the first layer. The hardness value HV10gf of the first layer is 220 to 410.

(42) When the thickness of the precoating is thin, the hardness of the surface layer is reduced greatly after the steel of the present invention is subjected to hot stamping. Specifically, when the coating is composed only of the first layer, the surface hardness is 220 to 410 HV10gf. Even in the case where the interdiffusion layer contains the Fe.sub.3Al phase, since the coating of the present invention is thin and the Fe.sub.3Al phase content is extremely small, the coating hardness is generally speaking not more than 410 HV10gf. In contrast, the surface layer after hot stamping of the conventional steel sheet is a continuously distributed Fe.sub.2Al.sub.5 phase, and the fracture toughness value of the Fe.sub.2Al.sub.5 phase itself is 1 MPa√{square root over (m)}, and the hardness value is as high as 900 to 1150 HV10gf.

(43) (b) Second layer: it is mainly intermetallic compound phase, such as continuous brittle Fe.sub.2Al.sub.5, FeAl.sub.3, intermetallic compounds of Fe and Al containing Si. The thickness of the second layer is 0 to 8 μm, and is 5 to 7 μm in the illustrated example, wherein the Fe content is 30% to 47.9% by weight, and the hardness value HV10gf is 800 to 960.

(44) (c) Third layer: it is mainly an intermetallic compound phase such as FeAl or FeAl.sub.2. The thickness of the third layer is about 0 to 10 μm, and is 4 to 7 μm in the illustrated example, wherein the Fe content is 48 to 69% by weight, and the hardness value HV10gf is 400 to 630.

(45) (d) a discontinuously distributed surface layer, comprising a third layer structure, that is, an intermetallic compound phase such as FeAl, FeAl.sub.2, and a second layer structure, that is, an intermetallic compound phase such as Fe.sub.2Al.sub.5, FeAl.sub.3, etc., wherein the content of the third layer structure is at least 30% by volume, and the second layer structure is 70% or less, preferably without the second layer structure. The thickness of the surface layer is about 0 to 4 μm, and in the illustrated example, it is 0 to 3 μm, and the hardness value HV10gf is 650 to 850.

(46) It should be noted that the coating structure of the formed component is also different according to the precoating thickness and the heating procedure of the precoated steel sheet, and the precoated steel sheet and the heating process according to the present invention ensure that the formed component has the coating structure required by the present invention. For example, when the thickness of the precoating is 5 μm, the thickness of the coating after stamping by the process of the present invention becomes 10 μm; as shown in the picture on the left side of FIG. 2, the coating structure includes only the layer a, that is, the interdiffusion layer, CT* in the figure indicates the thickness of the coating after hot stamping. When the precoating thickness is 14 μm, as shown in the middle picture of FIG. 2, the thickness of the coating after the stamping process of the present invention is changed to 21 μm, and the coating structure contains layers a, b, c and the discontinuous layer d.

(47) In addition, it should be emphasized that as the coating is thinned, its microstructure gradually evolves, and the hardness of the surface layer of the coating gradually evolves as well. For example, a sheet having a precoating thickness of 14 μm is hot-stamped, and its coating structure is composed of layers a, b and c. The outermost layer is not mainly composed of brittle Fe.sub.2Al.sub.5 phase, but mainly FeAl or FeAl.sub.2 phase, so the hardness of surface layer is correspondingly reduced greatly, and the hardness value HV10gf is 400-630, even if it contains a small amount of Fe.sub.2Al.sub.5 phase, the hardness value HV10gf does not exceed 630.

(48) On the other hand, after the hot-stamping of the sheet having a precoating thickness of 5 μm, the final coating structure has only an interdiffusion layer with a thickness of about 10 μm, and the interdiffusion layer is mainly composed of α-Fe rich in Al and Si. In this instance, the hardness value HV10gf of the outermost layer is 220 to 410.

(49) In contrast, in the case of a conventional steel having a precoating with a thickness of 25 μm, the final coating thickness after hot stamping is about 34 μm, and the outermost layer is a continuously distributed Fe.sub.2Al.sub.5 phase having a hardness of about 950 HV10gf.

(50) As described above, after the precoated steel sheet of the present invention is hot stamped, the surface hardness gradually decreases as the thickness of the coating is reduced. In this way, not only the degree of wear of the stamping die is reduced, but also the life of the welding electrode is prolonged.

(51) A bending test is performed on a bending test apparatus specified in the VDA238-100 standard. The sample is placed on two rollers, wherein it is ensured as far as possible that the installed rollers are free of friction. The preload threshold is set to 30N. Preloading is carried out at a beam displacement speed of 10 mm/min. After reaching the set value, it is pressed downward at a beam displacement speed of 20 mm/min. The standard for finishing the test is that the pressure has a drop of 30 to 60 N after the pressure reached a maximum value.

(52) The test result of the three-point bending test shows that the maximum bending angle α.sub.max in the rolling direction is 65±0.3°, 62±0.4°, 58±2.0°, after the 1.2 mm thick 22MnB5 sheet having the precoating thickness of 5 μm, 14 μm and 25 μm respectively is hot stamped, as shown in FIG. 5.

(53) That is to say, for a sheet having a thickness of 1.2 mm, the sheet of the present invention having a precoating of 5 μm has a VDA maximum bending angle of about 65 degrees after hot stamping, whereas the conventional sheet having a precoating of 25 μm only has a VDA maximum bending angle of about 58 degrees after hot stamping, and its data scattering is large. It is apparent that the steel sheet having the precoating of 3 to 19 μm thick of the present invention can increase the maximum three-point bending angle by about 7 degrees as compared with the conventional precoated steel sheet. The reasons why the steel sheet of the present invention improves the VDA bending toughness are as follows.

(54) The inventors analyses the coating and the distribution of elements in the substrate, and detects the C element distribution of the 22MnB5 coated sheet after hot stamping using the electron probe linear scanning function. As shown in FIG. 6, it is found that in the heating and austenization process, C atoms diffused into the substrate and formed a C-rich zone of 1 to 4 μm thick in the vicinity of the boundary between the interdiffusion layer and the substrate, and after quenching the C-rich zone remains in the vicinity of the interface between the substrate and the interdiffusion layer, forming a high C martensite, which is brittle, and the premature failure at this position during cold bending is an important cause of poor cold bending performance.

(55) The inventors further found through research that the amount of C atom segregation can be reduced by reducing the thickness of the precoating, thereby improving the cold bending performance. Through calculation according to the experimental data of the embodiments, the interface of the alloying layer of the conventional steel sheet having 25 μm thick precoating moves towards the side of the base steel by 9 μm after hot stamping; in contrast, the interface of the alloying layer of the steel sheet having 14 μm and 5 μm thick precoating according to the present invention moves towards the side of the base steel by 7 μm and 5 μm respectively after hot stamping. Therefore, the total enrichment amount of C atoms in the base steel near the boundary between the coating and the substrate after hot stamping, in the conventional steel sheet having the precoating of 25 μm is 1.8 and 1.4 times that in the steel sheet having the precoating of 14 μm and 5 μm according to the present invention.

(56) Further, the mechanical properties are tested for the formed component through hot stamping of the precoated steel sheet shown in Table 3. The test results are shown in the following Table 4. Wherein, the formed components ISP1, ISP2, and CSP1 sequentially correspond to the precoated steel sheets IS1, IS2, CS1; TS represents tensile strength, YS represents yield strength, TE represents total elongation, and αmax represents the maximum bending angle (the bending angle under the maximum bending force), TD represents the direction that is perpendicular to the rolling direction, and RD represents the direction that is parallel to the rolling direction.

(57) TABLE-US-00004 TABLE 4 Mechanical Property Results (JIS5 Sample) Sample Amax(TD/RD) number TS/MPa YS/MPa TE/% degree ISP1 1569 ± 12 1128 ± 15 7.2 ± 0.17 62 ± 0.8/65 ± 0.3 ISP2 1151 ± 14 1116 ± 26 7.1 ± 0.21 60 ± 0.7/62 ± 0.4 CSP1 1545 ± 19 1132 ± 27 6.8 ± 0.49 54 ± 1.1/58 ± 2.0

(58) As can be seen from the above table, as compared with the conventional precoated steel sheet, the precoated steel sheet of the present invention can increase the maximum bending angle and improve the cold bending performance; at the same time, it can also achieve tensile strength, yield strength and elongation which are comparable to or even higher than those of the conventional precoated steel sheet.

(59) The hot stamped component of the present invention can be used as a safety structural component, a reinforcing structural component, a wheel component, a high-strength automobile structural component or a chassis structural component of a motor vehicle, including but not limited to A-pillar, B-pillar, automobile bumper, roof frame, chassis frame, and car door anti-collision bar.

(60) The preferred embodiments of the present invention have been described above, but it should be understood by those skilled in the art that any possible changes or substitutions may be made without departing from the spirit and scope of the present invention.

(61) For example, in the above embodiments, the case where the precoated steel sheet has the precoating on both upper and lower surfaces is described, but the precoating may be formed on one surface.

(62) Further, the data and various parameters described in the embodiments are merely exemplary and are not intended to limit the present invention.