Double annealed zinc-coated press-hardenable steels

Abstract

Double-annealed zinc-coated press-hardenable steel with controlled composition and microstructure is provided that achieves galvanic protection and addresses challenges related to liquid metal embrittlement. The double-annealed zinc-coated steel exhibits favorable properties when press hardened. Direct hot press forming may be performed at reduced temperatures where the zinc-based coating is fully solidified. The resultant DHPF zinc-coated sheet products have been found to possess favorable mechanical properties and corrosion resistance.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 704,385 filed Oct. 7, 2024, which is incorporated herein by reference.FIELD OF THE INVENTION

[0002] The present invention relates to zinc-coated press-hardenable steels and to two-step annealing process for making such products.BACKGROUND INFORMATION

[0003] The most widely used press-hardenable steel (PHS) grade is 22MnB5 with a typical composition of about 0.22 weight percent C, 1.23 weight percent Mn, 0.23 weight percent Si, and 0.003 weight percent B. At present, the most common coating applied to such PHS is an Al—Si alloy with 7-11 weight percent Si. This layer provides superior barrier protection against corrosion but does not offer cathodic protection. During direct hot press forming (DHPF), when the steel is austenitized, the Al—Si coating transforms into brittle intermetallic phases that are prone to cracking during forming, which leaves gaps that expose the bare steel to corrosion. As a result, PHS cannot be reliably used in parts where cathodic protection is necessary. A need exists for coatings that can deliver both barrier and cathodic protection.

[0004] Traditionally, Zn-based coatings have been preferred for sheet steel because of their cathodic protection. However, these coatings pose challenges during DHPF. The low melting point of zinc makes it susceptible to liquid metal embrittlement (LME). In addition, during austenitization, Zn coatings develop into a Zn-rich ferrite (α-Fe(Zn)) layer at the interface, often with some liquid Zn phase and oxides at the surface. For improved cathodic protection, the intermetallic Γ-Fe3Zn10 phase is beneficial. However, with longer heating and transfer times, the coating tends to contain more α-Fe(Zn) and less Γ-Fe3Zn10. When the fraction of Γ-Fe3Zn10 drops below about 15 volume percent, cathodic protection declines considerably.

[0005] The present invention overcomes these problems by controlling the steel composition, annealing process and zinc coating, allowing reduced austenitization time and temperature during subsequent press hardening operations.SUMMARY OF THE INVENTION

[0006] Double-annealed zinc-coated press-hardenable steel with controlled composition and microstructure is provided that achieves galvanic protection and addresses challenges related to liquid metal embrittlement. The double-annealed zinc-coated steel exhibits favorable properties when press hardened. Direct hot press forming may be performed at reduced temperatures where the zinc-based coating is fully solidified. The resultant DHPF zinc-coated sheet products have been found to possess favorable mechanical properties and corrosion resistance compared with Zn-coated and Al—Si coated 22MnB5 which are currently available in the market.

[0007] An aspect of the present invention is to provide a method of producing a coated press hardenable steel sheet product comprising from 0.16 to 0.33 weight percent C and from 1.2 to 3.5 weight percent Mn. The method comprises subjecting the steel sheet product to a first step annealing process to achieve a predominantly martensite microstructure, subjecting the steel sheet product to a second step annealing process to achieve a predominantly tempered martensite microstructure, and applying a coating comprising zinc on the steel sheet product to form a coated steel sheet product.

[0008] Another aspect of the present invention is to hot press form the double-annealed zinc-coated steel product produced by the method described above.

[0009] A further aspect of the present invention is to provide a double-annealed coated press hardenable steel sheet product comprising from 0.16 to 0.33 weight percent C and from 1.2 to 3.5 weight percent Mn having a predominantly tempered martensite microstructure and a coating comprising zinc, wherein the steel sheet product has an ultimate tensile strength of at least 700 MPa.

[0010] Another aspect of the present invention is to provide a press hardened zinc-coated steel product produced from the double-annealed zinc-coated press hardenable steel sheet product described above.

[0011] These and other aspects of the present invention will be more apparent from the following description.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a temperature vs. time graph sequentially illustrating heat treatment of direct hot press forming processes typically being used for Zn-coated and Al—Si-coated PHS and problems associated with such hot press forming processes.

[0013] FIG. 2 is a plot of temperature versus time schematically illustrating process windows of austenitization heat treatment of direct hot press forming process typically being used for Zn-coated and Al—Si-coated PHS, in comparison with a target window of the present invention having reduced heating temperatures and times.

[0014] FIG. 3 is a schematic of temperature versus time plot illustrating a two-step annealing process that may be conducted on a single production line including a continuous galvanizing line in accordance with embodiment of the present invention.

[0015] FIG. 4 is a photomicrograph illustrating the microstructure of a double-annealed steel substrate in accordance with an embodiment of the present invention prior to a stamping process.

[0016] FIG. 5 is a photomicrograph of a conventional 22MnB5 press hardenable steel alloy.

[0017] FIG. 6 is a photomicrograph illustrating a galvanized coating applied on a double-annealed steel substrate in accordance with an embodiment of the present invention prior to a stamping process.

[0018] FIG. 7 is a plot of temperature versus time illustrating a direct hot press forming process that may be conducted on a galvanized double-anneal steel sheet product to produce Zn-coated press hardening steel in accordance with an embodiment of the present invention.

[0019] FIG. 8 is a photograph illustrating a double-annealed galvanized steel panel of the present invention that has been subjected to a direct hot press forming process.

[0020] FIG. 9 is a photomicrograph illustrating the microstructure of a substrate of Zn-coated direct hot stamped product in accordance with an embodiment of the present invention that has undergone a DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C. showing relatively fine prior austenite grain boundaries.

[0021] FIG. 10 is a photomicrograph illustrating a conventional 22MnB5 press hardened steel alloy that has undergone a conventional DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C. showing relatively coarse prior austenite grain boundaries.

[0022] FIG. 11 is a photomicrograph illustrating the microstructure of the substrate of a Zn-coated direct hot stamped product in accordance with an embodiment of the present invention (developed composition processed under optimized two-step annealing in CGL and, went through DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C.).

[0023] FIG. 12 is a photomicrograph of a conventional 22MnB5 press hardened steel alloy processed under regular CGL process and, went through DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C. As can be seen, while the product of the present invention (FIG. 11) exhibits fine-grain microstructure, the conventionally produced PHS (FIG. 12) exhibits coarse martensite laths as a result of the prior austenite grain size difference illustrated in FIGS. 9 and 10.

[0024] FIG. 13 is a photomicrograph illustrating the coating microstructure of a Zn-coated direct hot stamped product in accordance with an embodiment of the present invention (developed composition processed under optimized two-step annealing in CGL and, went through DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C.).

[0025] FIG. 14 is a photomicrograph of a conventional 22MnB5 press hardened steel alloy processed under regular CGL process and, went through DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C. As can be seen, the coating of the present invention (FIG. 13) contains Zn gamma phase (r-Fe3Zn10) as well as α-Fe(Zn), while the coating of the conventionally produced PHS (FIG. 14) only contains α-Fe(Zn) covered with a significantly thick Zn and Al oxide layer.

[0026] FIG. 15 is a graph showing the results of low-angle x-ray diffraction analysis obtained from a coating of the Zn-coated direct hot stamped product in accordance with an embodiment of the present invention (developed composition processed under optimized two-step annealing in CGL and, went through DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C.).

[0027] FIG. 16 is a graph showing the results of a conventionally produced Zn-coated PHS 22MnB5 alloy processed under regular CGL process and, went through DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C. Results confirm the presence of greater than 15 volume percent of Zn gamma phase (r-Fe3Zn10) in the coating of the Zn-coated direct hot stamped product in accordance with an embodiment of the present invention (FIG. 15) and lack thereof in the coating of the conventionally produced Zn-coated PHS (FIG. 16).

[0028] FIG. 17 is a graph showing elemental depth profile obtained from an area on a Zn-coated direct hot stamped product in accordance with an embodiment of the present invention (developed composition processed under optimized two-step annealing in CGL and, went through DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C.).

[0029] FIG. 18 is a graph showing element depth profile of a conventionally produced Zn-coated PHS (22MnB5 alloy processed under regular CGL process and, went through DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C.). Enrichment of oxygen only at the surface of the Zn-coated direct hot stamped product of the present invention implies the presence of a significantly thinner oxide layer.

[0030] FIG. 19 includes cross-sectional scanning electron microscopy and energy dispersive spectroscopy thermal maps, showing distribution of iron and zinc, conducted on formed areas of a Zn-coated direct hot stamped product in accordance with an embodiment of the present invention. Numbered areas except area 2 and area 7 were exposed to either minimal strain or compressive forces which left the coating crack-free. Areas 2 and 7 experienced the highest strain, where tensile forces led the formation of cracks through the thickness of the coating along α-Fe(Zn) grain boundaries. No Zn infiltration, indicative of LME, was observed in the substrate.DETAILED DESCRIPTION

[0031] The present invention double-annealed zinc-coated press-hardenable steel (PHS) with controlled compositions that provides robust cathodic protection while addressing challenges related to liquid metal embrittlement. The controlled composition in combination with a controlled two-step annealing process produces desirable microstructures and favorable mechanical properties.

[0032] The steel composition may include carbon and manganese, along with any other suitable alloying additions. Examples of steel compositions including ranges of C, Mn, Mo, V, Ti, Al, P, Nb, and B are listed in Table 1.TABLE 1Steel Composition (weight percent)ExampleCMnMoVTiAlPNbBA0.16-1.2-0-0-0-0-0-0-0-0.333.50.200.20.10.150.050.10.01B0.18-1.3-0.03-0.03-0.01-0.01-0.01-0.01-0.001-0.323.00.180.180.080.120.040.080.008C0.18-1.5-0.05-0.05-0.02-0.02-0.015-0.01-0.0015-0.322.50.150.150.060.100.0350.050.005D0.20-1.5-0.05-0.05-0.02-0.02-0.015-0.01-0.0015-0.302.50.150.150.060.100.0350.050.005E0.20-1.8-0.05-0.05-0.02-0.02-0.015-0.01-0.0015-0.252.20.150.150.060.100.0350.050.005F0.25-1.8-0.05-0.05-0.02-0.02-0.015-0.01-0.0015-0.302.20.150.150.060.100.0350.050.005G0.212.020.090.040.040.050.020.020.003

[0033] In addition to the amounts of C, Mn, Mo, V, Ti, Al, Nb, P and B listed in Table 1, the steel compositions may include minor or impurity amounts of other elements, such as 0.05 max. S, 0.15 max. Cu, 0.06 max. Ni, 0.3 max. Cr, and 0.008 max. N. In certain embodiments, Si is limited to amounts of less than 0.30 weight percent, or less than 0.20 weight percent, or less than 0.10 weight percent, or less than 0.05 weight percent, which may reduce austenitization temperature during subsequent hot press forming operations. However, in certain embodiments, the amount of Si can be above such limits for certain compositions as long as the austenitization temperature does not increase to an undesirable level.

[0034] As used herein, the term “substantially free,” when referring to the composition of the steel sheet product, means that a particular element or material is not purposefully added to the composition, and is only present as an impurity or in trace amounts. The present steel alloys may be substantially free of Si, Cr, Cu, S, N or other elements not listed in Table 1.

[0035] The controlled two-step annealing process includes a supercritical annealing heat treatment, i.e., above Ac3, to achieve fully austenitic microstructure, followed by quenching to achieve a fully martensitic microstructure in the first step, and subcritical annealing heat treatment, i.e., below Ac1, to temper the martensite in the second step followed by immersion into the galvanizing bath. The double-annealing process may be conducted on separate lines or may be conducted on a single production line comprising a continuous galvanizing line.

[0036] FIG. 2 includes plots of temperature versus time illustrating a process window of austenitization heat treatment of direct hot press forming process typically being used for Zn and Al—Si coated PHS along with the target window of the present invention.

[0037] FIG. 3 includes a schematic of temperature versus time plot illustrating a two-step annealing process conducted on a single production line comprising a continuous galvanizing line in accordance with embodiment of the present invention. The resultant tempered martensitic microstructure of the coated product, such as shown in FIG. 4, is distinct from conventional ferritic-pearlitic microstructures available in the market, such as shown in FIG. 5.

[0038] The resultant steel alloys have a tempered martensite microstructure in which the tempered martensite comprises at least 90 volume percent, or at least 95 volume percent, or at least 98 volume percent, or at least 99 volume percent. Other than tempered martensite, the steel alloys may be substantially free of other phases such as pearlite, ferrite and austenite. For example, such phases, if present, are less than 10 volume percent, or less than 5 volume percent, or less than 2 volume percent, or less than 1 volume percent, or less than 0.5 volume percent, or less than 0.1 volume percent, or zero volume percent. The amounts of martensite, pearlite, ferrite and austenite may be determined by standard EBSD techniques.

[0039] The steel sheet products may subsequently be press hardened, e.g., during stamping operations. The press-hardened microstructure may be predominantly fresh martensite. The fresh martensite may comprise more than 50 volume percent, for example, at least 80 volume percent, or at least 90 volume percent, or at least 95 volume percent, or at least 98 volume percent, or at least 99 volume percent, or 100 percent. Other than fresh martensite, the press-hardened steel products may comprise ferrite, bainite and / or retained austenite in a total amount up to 20 volume percent, or up to 10 volume percent, or up to 5 volume percent, or up to 2 volume percent, or up to 1 volume percent, or may be substantially free thereof.

[0040] The double-annealed steel substrate may be uniformly coated with a zinc coating, typically on both sides, prior to direct hot press forming (DHPF). Due to the improved austenitic transformation thermodynamic and kinetics provided by the composition and the optimized two-step annealing process, a processing window, e.g., of <=785° C. austenitization temperature and 180 s austenitization time, has been found to effectively meet press hardenable steel production criteria. In addition, applying lower austenitization temperature and time results in the formation of a significantly thin oxide layer during DHPF process (FIG. 11 and FIG. 13), which would eliminate the need for post form operations including shot blasting the parts. Also, austenitization at lower temperatures and times ensures formation of >=15% volume fraction of Zn gamma phase (r-Fe3Zn10), which is required for robust coating cathodic protection (FIG. 11 and FIG. 13). The volume fraction of Γ-Fe3Zn10 may be greater than 15 volume percent of the total coating volume, or greater than 20 volume percent, or greater than 30 volume percent, or greater than 40 volume percent. The composition exhibits significantly improved hardenability compared to conventional PHS grades, which allows stamping temperatures of <=700° C. to prevent liquid metal embrittlement and still meet automotive requirements.

[0041] As an embodiment of the present invention, it is believed that optimizing the heat treatment window of the direct hot press forming process, e.g., peak annealing temperature of 720-800° C., annealing time of 120-300 s, stamping temperature of <=700° C., through the composition in combination with the optimized two-step annealing process enables producing Zn-coated press hardened steels with TS>=1000 MPa with robust cathodic protection with no indication of LME. Table 2 summarizes exemplary mechanical properties of the present alloy pre-DHPF and post-DHPF process.TABLE 2OYS (MPa)UTS (MPa)TE (%)Pre-DHPF103210397Post-DHPF90513937

[0042] A two-step annealing process may be used to produce the steel products. Within each of the first and second annealing steps, multiple methodologies for undertaking the heat treatment may be used. The annealing steps may be performed on separate lines or on a single line. Examples of two-step annealing include a continuous annealing line (CAL) followed by a continuous annealing line (CAL) production that is coated by zinc using electro-galvanizing or other approach such as vapor deposition, a CAL plus continuous galvanizing line (CGL) production route, a specially designed line allowing for both CAL+CAL or CAL+CGL steps to take place in a single facility.Step 1

[0043] The goal of the first step of the annealing process is to achieve a martensitic microstructure in a cold rolled or hot rolled steel sheet product. In the first annealing stage of the first step, an annealing temperature above the A3 (Ac3) temperature may typically be used, for example, an annealing temperature of at least 800° C. may be used, or at least 820° C., or at least 830° C., or at least 840° C., or at least 850° C. In certain embodiments, the first stage annealing temperature may typically range from 800 to 980° C., for example, from 820 to 950° C., or from 840 to 940° C., or from 850 to 930° C. In certain embodiments, the peak annealing temperature may be typically held for at least 20 seconds, for example, from 20 to 500 seconds, or from 30 to 200 seconds. Heating may be accomplished by conventional techniques such as a non-oxidizing or oxidizing direct-fired furnace (DFF), oxygen-enriched DFI, induction, gas radiant tube heating, electric radiant heating, and the like. Examples of heating systems that may be adapted for use in the processes of the present invention are disclosed in U.S. Pat. Nos. 5,798,007; 7,368,689; 8,425,225; and 8,845,324, U.S. Patent Application No. 2009 / 0158975, and Published PCT Application No. WO / 2015083047, assigned to Fives Stein. Additional examples of heating systems that may be adapted for use in the processes of the present invention include U.S. Pat. No. 7,384,489 assigned to Drever International, and U.S. Pat. No. 9,096,918 assigned to Nippon Steel and Sumitomo Metal Corporation. Any other suitable known types of heating systems and processes may be adapted for use in Step 1 and Step 2.

[0044] In the first stage, after the peak annealing temperature is reached and held for the desired period of time, the steel sheet is quenched to room temperature, or to a controlled temperature above room temperature, as more fully described below. The quench temperature may not necessarily be room temperature but should be below the martensite start temperature (Ms), and preferably below the martensite finish temperature (MF), to form a microstructure of predominantly martensite. In certain embodiments, between the first step process and the second step process, the steel sheet product may be cooled to a temperature below 300° C., for example, below 200° C.

[0045] Quenching may be accomplished by conventional techniques such as water quenching, submerged knife / nozzle water quenching, gas cooling, rapid cooling using a combination of cold, warm or hot water and gas, water solution cooling, other liquid or gas fluid cooling, chilled roll quench, water mist spray, wet flash cooling, non-oxidizing wet flash cooling, and the like. A quench rate of from 30 to 2,000° C. / see may typically be used.

[0046] Various types of cooling and quenching systems and processes known to those skilled in the art may be adapted for use in the processes of the present invention. Suitable cooling / quenching systems and processes conventionally used on a commercial basis may include water quench, water mist cooling, dry flash and wet flash, oxidizing and non-oxidizing cooling, alkane fluid to gas phase change cooling, hot water quenching, including two-step water quenching, roll quenching, high percentage hydrogen or helium gas jet cooling, and the like. For example, dry flash and / or wet flash oxidizing and non-oxidizing cooling / quenching such as disclosed in published PCT Application No. WO2015 / 083047 to Fives Stein may be used. Other Fives Stein patent documents describing cooling / quenching systems and processes that may be adapted for use in the processes of the present invention include U.S. Pat. Nos. 6,464,808B2; 6,547,898B2; and 8,918,199B2, and U.S. Patent Application Publication Nos. US2009 / 0158975A1; US2009 / 0315228A1; and US2011 / 0266725A1. Other examples of cooling / quenching systems and processes that may be adapted for use in the processes of the present invention include those disclosed in U.S. Pat. Nos. 8,359,894B2; 8,844,462B2; and 7,384,489B2, and U.S. Patent Application Publication Nos. 2002 / 0017747A1 and 2014 / 0083572A1.Step 2

[0047] The second step of the annealing process may be carried out at a temperature at or below the Ac1 temperature to promote property uniformity and manufacturability. The temperatures are controlled in order to promote the formation of the desired tempered martensite microstructure in the final product.

[0048] In the second step, an annealing temperature of at least 500° C. may be used, for example, at least 550° C., or at least 600° C. The second step annealing temperature may be up to 800° C., or up to 775° C., or up to 750° C. The second step annealing temperature may typically range from 500 to 800° C., or from 550 to 775° C., or from 600 to 750° C. The peak second step annealing temperature may be typically held for at least 15 seconds, for example, from 20 to 300 seconds, or from 30 to 150 seconds.

[0049] The first and second annealing steps may be accomplished by any suitable heating system or process, such as using radiant heating, induction heating, direct fired furnace heating and the like.

[0050] In accordance with certain embodiments, one or both of the first-step and second-step annealing processes may be performed on a continuous annealing line (CAL). After going through a CAL+CAL process, the steel may be coated, e.g., electrogalvanized to produce a zinc based coated product.

[0051] In certain embodiments, the double-annealed steel sheet may be hot-dip galvanized. Galvanizing temperatures may typically range from 440 to 480° C., for example, from 450 to 470° C. In certain embodiments, the galvanizing step may be performed as part of the second-step annealing process on a continuous galvanizing line (CGL). This CAL+CGL process can be used to produce both a zinc-based or zinc alloy-based hot-dip galvanized products or reheated after coating to produce an iron-zinc galvanneal type coated product. An optional nickel-based coating step can take place between the CAL and CGL steps in the process to improve zinc coating properties. A galvanized product or zinc-based alloy hot-dip coated product can also be made on a specially designed CGL in which the two-step annealing can take place in a single line. Galvannealing can also be an option in this case.

[0052] The following examples are intended to illustrate various aspects of the present invention, and are not intended to limit the scope of the invention.EXAMPLES

[0053] FIG. 4 is a photomicrograph illustrating the microstructure of a steel substrate comprising, in weight percent, 0.22C-2.23Mn-0.27Si-0.03Cu-0.02Ni-0.25Cr-0.15Mo-0.10V-0.04Ti-0.04Al-0.02Nb-0.004B, which has been processed under a two-step annealing treatment as illustrated in FIG. 3 in a CGL prior to a stamping process.

[0054] FIG. 5 is a photomicrograph of a conventional 22MnB5 press hardenable steel alloy processed under a conventional CGL process.

[0055] FIG. 6 is a photomicrograph illustrating a galvanized coating applied on a steel substrate with a composition corresponding to Example G in Table 1 prior to a stamping process.

[0056] FIG. 7 includes a plot of temperature versus time illustrating a direct hot press forming process conducted on the galvanized product to produce Zn-coated press hardening steel in accordance with an embodiment of the present invention.

[0057] FIG. 8 is a photograph of a galvanized 120 by 200 mm panel with a composition corresponding to Example G in Table 1 that has been heat treated in a two-step annealing process as shown in FIG. 3, coated in a galvanizing bath, and subjected to the direct hot press forming process shown in FIG. 7.

[0058] FIG. 9 is a photomicrograph illustrating the microstructure of a substrate of Zn-coated direct hot stamped product having a composition corresponding to Example G in Table 1 in accordance with an embodiment of the present invention that has undergone a DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C., showing relatively fine prior austenite grain boundaries.

[0059] FIG. 10 is a photomicrograph illustrating a conventional 22MnB5 press hardened steel alloy that has undergone a conventional DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C., showing relatively coarse prior austenite grain boundaries.

[0060] Table 3 summarizes the average and standard deviation values of prior austenite grain size measured on the SEM images shown in FIGS. 9 and 10. Finer prior austenite grains as shown in FIG. 9 lead to finer martensite in press hardened steel in comparison with coarser martensite resulting from coarser prior austenite as shown in FIG. 10.TABLE 3Prior Austenite Grain Size (μm)MaterialAverageStandard DeviationPresent Invention3.811.93Conventional PHS8.183.58

[0061] FIG. 11 is a photomicrograph illustrating the microstructure of a substrate of the Zn-coated direct hot stamped product having a composition corresponding to Example G in Table 1 in accordance with an embodiment of the present invention processed under optimized two-step annealing in CGL, and subjected to a DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C. FIG. 12 is a photomicrograph of a conventional 22MnB5 press hardened steel alloy processed under regular CGL process, and subjected to a DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C. As can be seen, while the product of the present invention exhibits fine-grain microstructure, the conventionally produced PHS exhibits coarse martensite laths as a result of the prior austenite grain size difference comparatively illustrated in FIGS. 9 and 10.

[0062] FIG. 13 is a photomicrograph illustrating the coating microstructure of a Zn-coated direct hot stamped product having a composition corresponding to Example G in Table 1 in accordance with an embodiment of the present invention processed under optimized two-step annealing in CGL, and subjected to a DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C. FIG. 14 is a photomicrograph of a conventional 22MnB5 press hardened steel alloy processed under regular CGL process, and subjected to a DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C.). As can be seen, the coating of the present invention shown in FIG. 13 contains Zn gamma phase (r-Fe3Zn10) as well as α-Fe(Zn), while the coating of the conventionally produced PHS shown in FIG. 14 only contains α-Fe(Zn) covered with a significantly thick Zn and Al oxide layer.

[0063] FIG. 15 is a graph showing the results of low-angel x-ray diffraction analysis obtained from a coating of the Zn-coated direct hot stamped product having a composition corresponding to Example G in Table 1 in accordance with an embodiment of the present invention processed under optimized two-step annealing in CGL, and subjected to a DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C.). FIG. 16 is a graph showing the results of a conventionally produced Zn-coated PHS 22MnB5 alloy processed under regular CGL process, and subjected to a DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C. Results confirm the presence of >15% volume fraction of Zn gamma phase (Γ-Fe3Zn10) in the coating of the Zn-coated direct hot stamped product in accordance with an embodiment of the present invention and lack of it in the coating of the conventionally produced Zn-coated PHS.

[0064] FIG. 17 is a graph showing elemental depth profile obtained from an area on a Zn-coated direct hot stamped product having a composition corresponding to Example G in Table 1 in accordance with an embodiment of the present invention processed under optimized two-step annealing in CGL, and subjected to a DHPF process with austenitization temperature of 785° C., austenitization time of 180 s, and stamping temperature of 650° C.). FIG. 18 is a graph showing element depth profile of a conventionally produced Zn-coated PHS 22MnB5 alloy processed under regular CGL process, and subjected to a DHPF process with austenitization temperature of 900° C., austenitization time of 390 s, and stamping temperature of 700° C. Enrichment of oxygen only at the surface of the Zn-coated direct hot stamped product of the present invention implies the presence of a significantly thinner oxide layer.

[0065] FIG. 19 includes cross-sectional scanning electron microscopy and energy dispersive spectroscopy thermal maps, showing distribution of iron and zinc, conducted on formed areas of a Zn-coated direct hot stamped product having a composition corresponding to Example G in Table 1 in accordance with an embodiment of the present invention. All numbered areas except area 2 and area 7 were exposed to either minimal strain or compressive forces which left the coating crack-free. Areas 2 and 7 experienced the highest strain, where tensile forces led the formation of some cracks through the thickness of the coating along α-Fe(Zn) grain boundaries. No Zn infiltration, indicative of LME, was observed in the substrate.

[0066] As used herein, “including,”“containing” and like terms are understood in the context of this application to be synonymous with “comprising” and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases or method steps. As used herein, “consisting of” is understood in the context of this application to exclude the presence of any unspecified element, material, phase or method step. As used herein, “consisting essentially of” is understood in the context of this application to include the specified elements, materials, phases, or method steps, where applicable, and to also include any unspecified elements, materials, phases, or method steps that do not materially affect the basic or novel characteristics of the invention.

[0067] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0068] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0069] In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and / or” unless specifically stated otherwise, even though “and / or” may be explicitly used in certain instances. In this application and the appended claims, the articles “a,”“an,” and “the” include plural referents unless expressly and unequivocally limited to one referent.

[0070] Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

Claims

1. A method of producing a coated press hardenable steel sheet product comprising from 0.16 to 0.33 weight percent C and from 1.2 to 3.5 weight percent Mn, the method comprising:subjecting the steel sheet product to a first step annealing process to achieve a predominantly martensite microstructure;subjecting the steel sheet product to a second step annealing process to achieve a predominantly tempered martensite microstructure; andapplying a coating comprising zinc on the steel sheet product to form a coated steel sheet product.

2. The method of claim 1, wherein the steel sheet product further comprises from 0.001 to 0.008 weight percent B.

3. The method of claim 2, wherein the steel sheet product further comprises at least one of from 0.03 to 0.18 weight percent V, from 0.01 to 0.08 weight percent Ti, and from 0.01 to 0.08 weight percent Nb.

4. The method of claim 2, wherein the steel sheet product further comprises from 0.03 to 0.18 weight percent Mo.

5. The method of claim 2, wherein the steel sheet product further comprises from 0.01 to 0.12 weight percent Al.

6. The method of claim 2, wherein the steel sheet product further comprises from 0.01 to 0.04 weight percent P.

7. The method of claim 1, wherein the first step annealing process is performed at a temperature of at least 800° C.

8. The method of claim 1, wherein the first step annealing process is performed at a temperature of from 820 to 950° C.

9. The method of claim 1, wherein the second step annealing process is performed at a temperature of less than 800° C.

10. The method of claim 1, wherein the second step annealing process is performed at a temperature of from 550 to 775° C.

11. The method of claim 1, wherein the zinc coating is applied on the steel sheet product at a temperature of from 400 to 500° C.

12. The method of claim 1, wherein the zinc coating is applied on the steel sheet product at a temperature of from 440 to 480° C.

13. The method of claim 1, wherein the zinc coating has a thickness of from 1 to 100 microns.

14. The method of claim 1, wherein the zinc coating has a thickness of from 5 to 40 microns.

15. The method of claim 1, further comprising hot press forming the double-annealed zinc-coated steel sheet product to austenitize the coated steel sheet product.

16. The method of claim 15, wherein the hot press forming is performed at a temperature below Ac1.

17. The method of claim 15, wherein the austenitization is performed at a temperature below 820° C. during hot press forming.

18. The method of claim 17, wherein the austenitization is performed for a time of less than 300 seconds during hot press forming.

19. The method of claim 15, wherein after the hot press forming the coating comprises at least 15 volume percent Γ-Fe3Zno based on the volume of the coating.

20. A double-annealed coated press hardenable steel sheet product comprising from 0.16 to 0.33 weight percent C and from 1.2 to 3.5 weight percent Mn having a predominantly tempered martensite microstructure and a coating comprising zinc, wherein the steel sheet product has an ultimate tensile strength of at least 700 MPa.

21. A press hardened coated steel sheet product produced from the double-annealed zinc-coated press hardenable steel sheet product of claim 20, wherein the zinc coating comprises at least 15 volume percent Γ-Fe3Zn10 based on the volume of the coating.

22. The press hardened coated steel sheet product of claim 20, wherein the zinc coating is from 5 to 40 microns thick.

23. The press hardened coated steel sheet product of claim 20, wherein the press hardened zinc-coated steel sheet product has an ultimate tensile strength of at least 900 MPa.

24. The press hardened coated steel sheet product of claim 20, wherein the press hardened zinc-coated steel sheet product has an ultimate tensile strength of up to 2500 MPa.

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