Hot stamping steel and producing method thereof

Abstract

Disclosed is a steel composition for hot stamping that comprises carbon (C) in an amount of about 0.22 to about 0.25 wt %, silicon (Si) in an amount of about 0.2 to about 0.3 wt %, manganese (Mn) in an amount of about 1.2 to about 1.4 wt %, titanium (Ti) in an amount of about 0.02 to about 0.05 wt %, chromium (Cr) in an amount of about 0.11 to about 0.2 wt %, boron (B) in an amount of about 0.005 to about 0.01 wt %, zinc (Zr) in an amount of about 0.005 to about 0.02 wt %, niobium (Nb) in an amount of about 0.01 to about 0.05 wt %, tungsten (W) in an amount of about 0.1 to about 0.5 wt %, iron (Fe) constituting the remaining balance of the steel composition, all the wt % based on the total amount of the steel composition.

Claims

1. A steel composition for hot stamping, comprising: carbon (C) in an amount of about 0.22 to about 0.25 wt %; silicon (Si) in an amount of about 0.2 to about 0.3 wt %; manganese (Mn) in an amount of about 1.2 to about 1.4 wt %; titanium (Ti) in an amount of about 0.02 to about 0.05 wt %; chromium (Cr) in an amount of about 0.11 to about 0.2 wt %; boron (B) in an amount of about 0.005 to about 0.01 wt %; zirconium (Zr) in an amount of about 0.005 to about 0.02 wt %; niobium (Nb) in an amount of about 0.01 to about 0.05 wt %; tungsten (W) in an amount of about 0.1 to about 0.5 wt %; and iron (Fe) constituting the remaining balance of the steel composition, wherein all the wt % based on the total amount of the steel composition.

2. The steel composition for hot stamping of claim 1, consisting essentially of: carbon (C) in an amount of about 0.22 to about 0.25 wt %; silicon (Si) in an amount of about 0.2 to about 0.3 wt %; manganese (Mn) in an amount of about 1.2 to about 1.4 wt %; titanium (Ti) in an amount of about 0.02 to about 0.05 wt %; chromium (Cr) in an amount of about 0.11 to about 0.2 wt %; boron (B) in an amount of about 0.005 to about 0.01 wt %; zirconium (Zr) in an amount of about 0.005 to about 0.02 wt %; niobium (Nb) in an amount of about 0.01 to about 0.05 wt %; tungsten (W) in an amount of about 0.1 to about 0.5 wt %; and iron (Fe) constituting the remaining balance of the steel composition, wherein all the wt % based on the total amount of the steel composition.

3. A hot stamping steel, comprising: a parental metal comprising a steel composition of claim 1; a Zn plating layer coated on the parental metal; and a ZnFe alloy layer formed, by hot stamping, between the parental metal and the Zn plating layer.

4. The hot stamping steel of claim 3, wherein the hot stamping steel has a tensile strength of about 1470 MPa or greater.

5. The hot stamping steel of claim 3, wherein the ZnFe alloy layer has a Zn content of about 90% by weight or greater, based on the total weight of the ZnFe alloy layer.

6. A method for manufacturing a hot stamping steel, comprising steps of: producing a steel plate comprising carbon (C) in an amount of about 0.22 to about 0.25 wt %, silicon (Si) in an amount of about 0.2 to about 0.3 wt %, manganese (Mn) in an amount of about 1.2 to about 1.4 wt %, titanium (Ti) in an amount of about 0.02 to about 0.05 wt %, chromium (Cr) in an amount of about 0.11 to about 0.2 wt %, boron (B) in an amount of about 0.005 to about 0.01 wt %, zirconium (Zr) in an amount of about 0.005 to about 0.02 wt %, niobium (Nb) in an amount of about 0.01 to about 0.05 wt %, tungsten (W) in an amount of about 0.1 to about 0.5 wt %, iron (Fe) constituting the remaining balance of the steel composition, all the wt % based on the total amount of the steel composition; plating the steel plate with zinc (Zn); austenitizing the steel plate; hot stamping the Zn-plated steel plate at a temperature of about 750 to 850 C.; and inducing martensitic transformation in the steel plate.

7. The method of claim 6, wherein the austenitizing step is performed by heating the steel plate to a temperature of about 900 C. or greater.

8. The method of claim 7, further comprising steps of: cooling the heated steel plate at a cooling rate of about 600 C./min or greater to a temperature of about 750 to 850 C. between the austenitizing step and the hot stamping step.

9. The method of claim 6, wherein the martensitic transformation is achieved by quenching the steel plate at a rate of about 3000 C./min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 illustrates corrosion generation in a Zn plating layer and an AlSi plating layer upon cracking in the related arts,

(3) FIG. 2 is a graph in which the melting points of conventional ZnFe alloys are plotted as a function of the composition thereof,

(4) FIG. 3 is a graph illustrating processes of a conventional hot stamping process; and

(5) FIG. 4 is a graph illustrating an exemplary a hot stamping process according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

(6) The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular forms a, an and the are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms comprises and/or comprising, or includes and/or including, when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components and/or groups thereof.

(7) Unless otherwise defined, the meaning of all terms including technical and scientific terms used herein is the same as that commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning which is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

(8) Reference will now be made in greater detail to various exemplary embodiments of the present invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts.

(9) In accordance with an aspect thereof, the present invention provides a steel composition which may be used for a parental metal. The steel composition may comprise: carbon (C) in an amount of about 0.22 to about 0.25 wt %, silicon (Si) in an amount of about 0.2 to about 0.3 wt %, manganese (Mn) in an amount of about 1.2 to about 1.4 wt %, titanium (Ti) in an amount of about 0.02 to about 0.05 wt %, chromium (Cr) in an amount of about 0.11 to about 0.2 wt %, boron (B) in an amount of about 0.005 to about 0.01 wt %, zirconium (Zr) in an amount of about 0.005 to about 0.02 wt %, niobium (Nb) in an amount of about 0.01 to about 0.05 wt %, tungsten (W) in an amount of about 0.1 to about 0.5 wt %, iron (Fe) constituting the remaining balance of the steel composition, all the wt % based on the total amount of the steel composition.

(10) Further, the present invention provides a hot stamping steel. The hot stamping steel may comprise the parental metal comprising the steel composition as described above, a Zn plating layer coated on the parental metal, and a ZnFe alloy layer formed, by hot stamping, between the parental metal and the Zn plating layer.

(11) The parental metal may be prepared using a typical steel manufacturing process. For example, the parental metal may be a steel plate manufactured by hot rolling, cold rolling, and various thermal treatments.

(12) As referenced herein the term parental metal includes a base metal comprising the components of the composition as described above, and those components may be welded and processed from the molten components of metals to form a suitable metal substrate (e.g. steel sheet, block and the like). In particular, the parental metal may serve as a base substrate metal on which further treatment, e.g. plating, coating or austenitizing, can be performed to produce suitable hot stamping steels.

(13) A Zn plating layer may be formed on the surface of the parental metal, and the Zn-plated parental metal may be subjected to hot stamping, such that the ZnFe alloy layer may be formed between the parental metal and the Zn plating layer.

(14) The Zn plating layer, although susceptible to cracking, may serve as a sacrificial electrode to suppress the corrosion of the parental metal, thus guaranteeing high corrosion resistance of the parental metal.

(15) After completion of the processes, the material metal may have a tensile strength of about 1470 MPa or greater. Particularly, the ZnFe alloy layer formed at the bottom of the Zn plating layer, that is, between the parental metal and the Zn plating layer, may have a Zn content of about 90% by weight, based on the total weight of the ZnFe alloy layer. Due to the composition of the parental metal composition, physical properties and effects of the hot stamping steel may be obtained, which will be further described later in a comparison manner in the following Examples and Comparative Examples.

(16) Further, the present invention provides a method for manufacturing hot stamping steel. The method may comprise steps of: preparing a steel plate comprising carbon (C) in an amount of about 0.22 to about 0.25 wt %, silicon (Si) in an amount of about 0.2 to about 0.3 wt %, manganese (Mn) in an amount of about 1.2 to about 1.4 wt %, titanium (Ti) in an amount of about 0.02 to about 0.05 wt %, chromium (Cr) in an amount of about 0.11 to about 0.2 wt %, boron (B) in an amount of about 0.005 to about 0.01 wt %, zirconium (Zr) in an amount of about 0.005 to about 0.02 wt %, niobium (Nb) in an amount of about 0.01 to about 0.05 wt %, tungsten (W) in an amount of about 0.1 to about 0.5 wt %, iron (Fe) constituting the remaining balance of the steel composition, all the wt % based on the total amount of the steel composition, plating the steel plate with Zn, austenitizing the steel plate at a temperature of about 900 C. or greater, cooling the heated steel plate at a rate of about 600 C./min or greater to a temperature of about 750 to 850 C., hot stamping the steel plate at temperature of about 750 to 850 C., and quenching the steel plate at a rate 3000 C./min or greater to induce martensitic transformation.

(17) Hereinbelow, the components in the steel composition according to the present invention will be described. Unless described otherwise, the wt % given in the following description is based on the total weight of the steel composition.

(18) C (carbon), as used herein, may be added to provide the steel with strength, and further, may influence on the formation of the martensitic phase. When the carbon content is added in an amount less than about 0.22 wt %, the steel may be deteriorated in strength. When the carbon content is greater than about 0.25 wt %, hardness may be increased excessively thereby causing substantial brittleness.

(19) Si (silicon), as used herein, may be added as a deoxidizer and may function to strengthen the solid solution and to increase carbon activity. When silicon is used in an amount of about 0.1 wt %, its deoxidizing effect may be negligible. On the other hand, when the silicon content is greater than about 0.3 wt %, hardenability (degree of easiness to form martensitic structures) may deteriorate.

(20) Mn (manganese), as used herein, may be added to guarantee strength by improving hardenability. When Mn is used in an amount less than about 1.2 wt %, the strength may be reduced. When the Mn content is greater than about 1.4 wt %, grain boundary oxidation may occur, thus deteriorating physical properties.

(21) Ti (titanium), as used herein, may be added to participate in forming carbon nitrides. Ti may increase the stability of the steel at high temperature and improve the steel in strength and toughness. The effect of Ti on strength and toughness may be negligible when it is used in an amount less than about 0.2 wt %. When the Ti content is greater than about 0.5 wt %, deposits coarse may be generated and low-temperature impact resistance may be reduced.

(22) Cr (chromium), as used herein, may be added to improve the formation of carbide deposits and cementite and to increase high-temperature stability and hardenability. Cr may further harden the steel by microstructural refinement. At the Cr content less than about 0.11 wt %, only negligible improvement in hardenability may be obtained. When the Cr content is greater than about 0.20 wt %, grain boundary oxidation may be generated, thereby deteriorating the Zn plating layer, with the consequent deterioration of corrosion resistance and toughness.

(23) B (boron), as used herein, may be added to improve hardenability and strength. The steel may have decreased strength at the B content less than about 0.005 wt % while microcracks are caused in the Zn plating layer at a B content greater than about 0.01 wt %. Particularly, when the B content is less than about 0.005 wt %, transformation from austenitic phase to ferritic phase may not be blocked sufficiently, which may further deteriorate hot stamping processability as well as the formation of martensite, thereby reducing the tensile strength.

(24) Zr (zirconium), as used herein, may be added to form a deposit, remove N, O, and S, elements harmful to the physical properties, prolong the longevity of the steel, and reduce the size of non-metallic inclusions. At the Zr content less than about 0.005 wt %, the non-metallic inclusions may increase in size, thus increasing brittleness and decreasing tensile strength. When the Zr content is greater than about 0.02 wt %, ZrO.sub.2 may be excessively formed, physical properties of the steel, may deteriorate, and may increase the production cost because Zr is an expensive element.

(25) Nb (niobium), as used herein, may be added to greatly induce hardening through carbide formation, improve toughness through the microstructural refinement of grains, and increase recrystallization temperatures. When Nb is used in an amount less than about 0.01 wt %, its hardening effect may be negligible so that the tensile strength may not be improved. When the Nb content is greater than about 0.05 wt %, the recrystallization temperature may increase excessively to decrease hardenability, with the consequent deterioration of processability, productivity, and toughness.

(26) W (tungsten), as used herein, may be added to increase wear resistance at high temperatures and toughness, and to prevent the excessive growth of martensitic structures. Only negligible improvement in tensile strength may be obtained at the W content less than about 0.1 wt %. When the W content is greater than about 0.5 wt %, excessive WC may be formed thereby deteriorating the toughness, and hardenability due to the consumption of carbon contained in the lattice.

(27) The preparation of the parental steel plate may be a typical steel manufacturing process in the related arts without limitations to particular processes.

(28) The plating step may be performed in the same manner as a zinc plating method generally used for steel plates used in automobiles. For example, a hot dip galvanizing method or other various plating methods may be used without limitations.

(29) The austenitizing step may transform the steel structure from perlite and ferrite to austenite so as to increase the processability. For example, the steel plate may be heated to a temperature of about 900 C. or greater such that all the structures of steel may be transformed into an austenitic phase. Typically, carbon steel is transformed into austenite at a temperature of about 723 to 906 C. Accordingly, a temperature of about 900 C. or greater may be sufficient for the austenitic transformation of most steel structures.

(30) In the cooling step, the steel plate heated to the temperature 900 C. or greater may be cooled at a rate of about 600 C./min or greater to a temperature of about 750 to 850 C. For example, air cooling or water cooling may be performed. Even after being cooled to the temperature, the structure of the steel may remain austenitic because of the presence of B, Zr, Nb, and W. For example, when B is used in the amount as described herein, the position of the nose in the TTT curve may be shifted in a clockwise direction. The use of B, Zr, Nb, and W in the amounts as describe herein may delay the time at which transformation from austenite to ferrite or pearlite starts, which makes it possible to conduct a hot stamping process at low temperatures. At a cooling rate less than about 600 C./min, sufficient processability may not be guaranteed because transformation from austenite to ferrite or pearlite starts. In addition, the austenite to be transformed into martensite may be deficient, resulting in a decrease in the tensile strength of the final product.

(31) For the hot stamping, a temperature of about 750 to 850 C. may be employed according to the present invention, whereas conventional hot stamping is conducted at a temperature of about 900 C. Since the Zn plating layer is prevented from cracking at such a low temperature, the steel plate's high corrosion resistance may be ensured. At a hot stamping temperature is greater than about 850 C., the Zn plating layer may be cracked, thus deteriorating the corrosion resistance of the steel plate. When hot stamping is conducted at less than about 750 C., transformation from austenite to ferrite or pearlite may occur, thereby decreasing the tensile strength.

(32) In the quenching step, the temperature of the steel plate may be rapidly decreased at a rate of about 3000 C./min down to room temperature to induce transformation of martensite from austenite. For example, a water cooling method may be used. Experimental data show that the quenching may not have a significant influence on the Zn plating layer.

EXAMPLE

(33) The effects of the present invention will be explained in conjunction with Examples and Comparative Examples.

(34) Data of stamping temperatures, cooling rates, and composition of parental steel and comparative steel are given in the following Table 1. The data in Table indicates that, when contents of B, Zr, Nb, and W are not in the ranges described, the tensile strength of the parental steel does not satisfy the necessary standard.

(35) TABLE-US-00001 TABLE 1 Stamping Cooling Tensile Ex. Temp. Rate Content strength No ( C.) ( C./min) B Cr Zr Nb W (MPa) Remark 1 750 650 0.006 0.15 0.019 0.015 0.3 1470 2 800 650 0.006 0.15 0.019 0.015 0.3 1510 3 850 650 0.006 0.15 0.019 0.015 0.3 1550 4 750 650 0.01 0.15 0.005 0.015 0.5 1505 5 800 650 0.01 0.15 0.005 0.015 0.5 1530 6 850 650 0.01 0.15 0.005 0.015 0.5 1570 C. 1 750 650 0.002 0.15 1210 Component B C. 2 800 650 0.002 0.15 1190 Comparative C. 3 850 650 0.002 0.15 1305 Examples C. 4 750 610 0.011 0.15 1375 C. 5 800 650 0.015 0.15 1405 C. 6 850 630 0.013 0.15 1415 C. 7 750 650 0.010 0.15 0.001 0.01 0.42 1210 Component Zr C. 8 800 610 0.009 0.15 0.000 0.021 0.41 1270 Comparative C. 9 850 630 0.009 0.15 0.03 0.01 0.42 1310 Examples C. 10 800 630 0.009 0.010 0.017 0.000 0.42 1190 Component Nb C. 11 810 650 0.008 0.010 0.018 0.005 0.41 1290 Comparative C. 12 800 610 0.009 0.010 0.017 0.060 0.42 1350 Examples C. 13 810 630 0.008 0.010 0.018 0.055 0.41 1310 C. 14 800 630 0.010 0.010 0.017 0.01 0.05 1150 Component W C. 15 810 650 0.009 0.010 0.018 0.02 0.08 1185 Comparative C. 16 800 610 0.010 0.010 0.017 0.01 0.52 1350 Examples C. 17 810 630 0.009 0.010 0.018 0.02 0.60 1285

(36) Physical properties according to hot stamping temperatures are given in the following Table 2. The steel plates were observed to have poor tensile strength at a hot stamping temperature less than about 750 C., and microcracks appeared at a hot stamping temperature greater than about 850 C.

(37) TABLE-US-00002 TABLE 2 Stamping Tensile Depth of No. of Corrosion Test Ex. Temp. Content strength microcrack Microcrack (corrosion No ( C.) B Cr Zr Nb W (MPa) (m) (EA/cm.sup.2) free time) 1 750 0.006 0.006 0.015 0.03 0.4 1470 None None 40 days 2 800 0.006 0.006 0.013 0.03 0.1 1470 None None 40 days 3 850 0.005 0.005 0.010 0.04 0.25 1485 None None 40 days 4 750 0.009 0.009 0.015 0.025 0.13 1490 None None 40 days 5 800 0.010 0.010 0.018 0.01 0.05 1510 None None 40 days C. 1 700 0.011 0.010 0.017 0.01 0.44 1210 None None 40 days C. 2 680 0.005 0.010 0.016 0.02 0.11 1190 None None 40 days C. 3 900 0.006 0.010 0.015 0.045 0.40 1505 95 30 23 days C. 4 930 0.010 0.010 0.01 0.01 0.30 1510 103 30 20 days

(38) In Table 3, physical results of the steel plates of Comparative Examples in which the cooling rate was below the standard (600 C./min) after the austenitizing step are given. The steel plates of Comparative Examples were observed to have low tensile strength, and low corrosion resistance due to the generation of microcracks.

(39) TABLE-US-00003 TABLE 3 Stamping Cooling Tensile Depth of No. of Corrosion Test Ex. Temp. Rate Content strength Microcrack Microcrack (corrosion No ( C.) ( C./min) B Cr Zr Nb W (MPa) (m) (EA/cm.sup.2) free time) 1 750 650 0.006 0.006 0.015 0.03 0.4 1470 None None 40 days 2 800 610 0.006 0.006 0.013 0.03 0.1 1470 None None 40 days 3 850 650 0.005 0.005 0.010 0.04 0.25 1485 None None 40 days 4 750 630 0.009 0.009 0.015 0.025 0.13 1490 None None 40 days 5 800 700 0.010 0.010 0.018 0.01 0.05 1510 None None 40 days C. 1 800 550 0.010 0.010 0.018 0.01 0.05 1380 9 9 40 days C. 2 810 500 0.010 0.010 0.018 0.01 0.05 1280 15 16 40 days C. 3 800 580 0.010 0.010 0.018 0.01 0.05 1190 7 10 23 days C. 4 810 520 0.010 0.010 0.018 0.01 0.05 1230 13 20 20 days

(40) The Comparative Examples in which the B content was not in the standard range (0.005-0.010 wt %) are shown in Table 4. The steel plates exhibited inferior tensile strength at the B content less than about 0.005 wt %, and microcracks were generated and thus exhibited low corrosion resistance at a B content greater than about 0.010 wt %.

(41) TABLE-US-00004 TABLE 4 Stamping Tensile Depth of No. of Corrosion Test Ex. Temp. Content strength microcrack Microcrack (corrosion No ( C.) B Cr Zr Nb W (MPa) (m) (EA/cm.sup.2) free time) 1 750 0.006 0.006 0.015 0.03 0.4 1470 None None 40 days 2 800 0.006 0.006 0.013 0.03 0.1 1470 None None 40 days 3 850 0.005 0.005 0.010 0.04 0.25 1485 None None 40 days 4 750 0.009 0.009 0.015 0.025 0.13 1490 None None 40 days 5 800 0.010 0.010 0.018 0.01 0.05 1510 None None 40 days C. 1 800 0.003 0.010 0.011 0.04 0.25 1250 None None 40 days C. 2 800 0.015 0.010 0.01 0.04 0.27 1310 30 7 25 days

(42) The Comparative Examples in which the Cr content was not in the standard range (0.11-0.2 wt %) are shown in Table 5. The steel plates exhibited inferior tensile strength at the Cr content less than about 0.11 wt %, and underwent the generation of microcracks and thus exhibited low corrosion resistance at a B content greater than about 0.2 wt %.

(43) TABLE-US-00005 TABLE 5 Stamping Tensile Depth of No. of Corrosion Test Ex. Temp. Content strength Microcrack Microcrack (corrosion No ( C.) B Cr Zr Nb W (MPa) (m) (EA/cm.sup.2) free time) 1 750 0.006 0.11 0.015 0.03 0.4 1470 None None 40 days 2 800 0.006 0.15 0.013 0.03 0.1 1470 None None 40 days 3 850 0.005 0.15 0.010 0.04 0.25 1485 None None 40 days 4 750 0.009 0.19 0.015 0.025 0.13 1490 None None 40 days 5 800 0.010 0.17 0.018 0.01 0.05 1510 None None 40 days C. 1 850 0.010 0.05 0.017 0.00 0.45 1250 None None 40 days C. 2 850 0.010 0.21 0.017 0.00 0.45 1410 50 10 30 days

(44) The Comparative Examples in which the Zr content was not in the standard range (0.005-0.020 wt %) are shown in Table 6. The steel plates exhibited inferior tensile strength at the Zr content less than about 0.005 wt % or greater than about 0.020 wt %.

(45) TABLE-US-00006 TABLE 6 Stamping Tensile Depth of No. of Corrosion Test Ex. Temp. Content strength microcrack Microcrack (corrosion No ( C.) B Cr Zr Nb W (MPa) (m) (EA/cm.sup.2) free time) 1 750 0.006 0.006 0.015 0.03 0.4 1470 None None 40 days 2 800 0.006 0.006 0.013 0.03 0.1 1470 None None 40 days 3 850 0.005 0.005 0.010 0.04 0.25 1485 None None 40 days 4 750 0.009 0.009 0.015 0.025 0.13 1490 None None 40 days 5 800 0.010 0.010 0.018 0.01 0.05 1510 None None 40 days C. 1 800 0.010 0.010 0.001 0.01 0.42 1210 None None 40 days C. 2 810 0.009 0.010 0.000 0.021 0.41 1270 None None 40 days C. 3 800 0.010 0.010 0.03 0.01 0.42 1310 None None 40 days C. 4 810 0.009 0.010 0.025 0.02 0.41 1290 None None 40 days

(46) The Comparative Examples in which the Nb content was not in the standard range (0.01-0.05 wt %) are shown in Table 7. The steel plates exhibited inferior tensile strength at the Nb content less than about 0.01 wt % or greater than about 0.05 wt %.

(47) TABLE-US-00007 TABLE 7 Stamping Tensile Depth of No. of Corrosion Test Ex. Temp. Content strength microcrack Microcrack (corrosion No ( C.) B Cr Zr Nb W (MPa) (m) (EA/cm.sup.2) free time) 1 750 0.006 0.006 0.015 0.03 0.4 1470 None None 40 days 2 800 0.006 0.006 0.013 0.03 0.1 1470 None None 40 days 3 850 0.005 0.005 0.010 0.04 0.25 1485 None None 40 days 4 750 0.009 0.009 0.015 0.025 0.13 1490 None None 40 days 5 800 0.010 0.010 0.018 0.01 0.05 1510 None None 40 days C. 1 800 0.009 0.010 0.017 0.000 0.42 1190 None None 40 days C. 2 810 0.008 0.010 0.018 0.005 0.41 1290 None None 40 days C. 3 800 0.009 0.010 0.017 0.060 0.42 1350 None None 40 days C. 4 810 0.008 0.010 0.018 0.055 0.41 1310 None None 40 days

(48) The Comparative Examples in which the W content was not in the standard range (0.1-0.5 wt %) are shown in Table 8. The steel plates exhibited inferior tensile strength at the W content less than about 0.1 wt % or greater than about 0.5 wt %.

(49) TABLE-US-00008 TABLE 8 Stamping Tensile Depth of No. of Corrosion Test Ex. Temp. Content strength microcrack Microcrack (corrosion No ( C.) B Cr Zr Nb W (MPa) (m) (EA/cm.sup.2) free time) 1 750 0.006 0.006 0.015 0.03 0.4 1470 None None 40 days 2 800 0.006 0.006 0.013 0.03 0.1 1470 None None 40 days 3 850 0.005 0.005 0.010 0.04 0.25 1485 None None 40 days 4 750 0.009 0.009 0.015 0.025 0.13 1490 None None 40 days 5 800 0.010 0.010 0.018 0.01 0.05 1510 None None 40 days C. 1 800 0.010 0.010 0.017 0.01 0.05 1150 None None 40 days C. 2 810 0.009 0.010 0.018 0.08 0.41 1185 None None 40 days C. 3 800 0.010 0.010 0.017 0.52 0.42 1350 None None 40 days C. 4 810 0.009 0.010 0.018 0.60 0.41 1285 None None 40 days

(50) As described above, the hot stamping steel and the manufacturing method thereof in accordance with the present invention provide the following effects.

(51) First, substantial improvement may be obtained particularly in corrosion resistance because the corrosion may be prevented even though the plating layer undergoes microcracking.

(52) Second, the hot stamping steel can be manufactured at low temperature ranges, thereby reducing the production cost.

(53) Finally, process conditions may be improved, thereby improving the productivity.

(54) Although the various exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.