Use of a steel alloy

Abstract

A steel grade for use in a hot forming and press hardening process has the following composition, in weight percent: C 0.15%=C<0.35%, Mn 0.8-2.5%, Si 1.5-2.5%, Cr max. 0.4%, Al max. 0.1%, Ni max. 0.3%, B 0.0008-0.005%, Ti 0.005-0.1%, Nb max. 0.1%, remainder iron and unavoidable impurities as well as a hot formed and press hardened structural part made of this steel grade.

Description

The invention describes the use of a steel alloy.

DE 24 52 486 C2 discloses a process for press forming and hardening a steel sheet of slight material thickness and good dimensional precision, whereby a steel sheet from a boron-alloyed steel is heated to above Ac₃ and then pressed at substantial change in shape in less than 5 seconds into the final shape between two indirectly cooled molds and subjected to a rapid cooling while remaining in the press so as to obtain a martensitic and/or bainitic fine-grained structure. This process is to be understood hereinafter as hot forming and press hardening and has proven effective for producing high-strength, relatively thin parts of complex configuration and high dimensional precision for structural and safety parts such as A and B pillars or bumpers in the automobile industry. Metal sheets of typical thicknesses of 3 mm or less are hereby formed and steels with slight carbon content are used. The mentioned printed publication describes a steel alloy with C<0.4%; silicon of a content dependent on the steel production process but essentially negligible; 0.5 to 2.0% Mn; max. 0.05% P; max. 0.05% S; 0.1 to 0.5% Cr, and/or 0.05 to 0.5% Mo; up to 0.1% Ti; 0.0005 to 0.01% B; up to a total of 0.1% Al, and optional contents of copper and nickel up to 0.2% each.

A typical boron-alloyed steel for hot forming and hardening is disclosed for example in DE 197 43 802 C2. DE 197 43 802 C2 describes a process for the manufacture of a metallic molded structural part for motor vehicle components with regions of higher ductility. For this purpose, a blank is made of a steel alloy, which, expressed in weight percent, includes carbon (C) 0.18% to 0.3%; silicon (Si) 0.1% to 0.7%; manganese (Mn) 1.0% to 2.5%; phosphorous (P) at most 0.025%; chromium (Cr) 0.1% to 0.8%; molybdenum (Mo) 0.1% to 0.5%; sulfur (S) at most 0.01%; titanium (Ti) 0.02% to 0.05%; boron (B) 0.002% to 0.005%; aluminum (Al) 0.01% to 0.06% and remainder iron, including impurities resulting from smelting. The mentioned alloy is eminently suited for hot forming and press hardening. However, the alloying structure is predominantly composed of martensite when hardened. As a consequence, ductility is not always sufficient in the material for the respective load situation at hand.

DE 10 2005 054 847 B3 proposes therefore to use a hot formed and press hardened structural component which has been heat-treated at 320 to 400 degrees Celsius after the hot forming and press hardening process. This heat treatment influences in a desired manner the high-strength properties of the component. The yield strength R_p0.2 and the elongation A₅ remain nearly unchanged. Only the tensile strength values Rm are reduced by 100 to 200 N/m². When the afore-described steel grade comprised, in weight percent, of carbon (C) 0.18% to 0.3%, silicon (Si) 0.1% to 0.7%, manganese (Mn) 1.0% to 2.5%, phosphorus (P) at most 0.025%, chromium (Cr) up to 0.8%, molybdenum (Mo) up to 0.5%, sulfur (S) at most 0.01%, titanium (Ti) 0.02% to 0.05%, boron (B) 0.002% to 0.005%, and aluminum (Al) 0.01% to 0.06%, remainder iron including impurities resulting from smelting, is involved, a heat treatment at 320 to 400° C. results in a tensile strength Rm of 1200 to 1400 N/mm², a yield strength R_p0.2 of 950 to 1250 N/mm², and an elongation A₅ of 6-12%. The material still has the necessary high-strength mechanical properties, but, due to the somewhat lower tensile strength Rm, it has enough ductility that it crumples instead of breaking or rupturing in a collision. The additional tempering process is however again relatively complicated and expensive.

U.S. Pat. No. 6,544,345 also belongs to the prior art and relates to the production of a high-strength steel alloy. The steel alloy has a microstructure comprised of ferrite and/or bainite as well as residual austenite. This steel alloy is especially suitable to absorb high forces when exposed to dynamic stress. The hot formed microstructure is suitable for press forming.

Further belonging to the state of the art are EP 2 003 221 A1 and EP 2 039 791 A1 which relate each to high-strength steel alloys and respective manufacturing methods.

In addition, so-called TRIP steels (engl. TRansformation Induced Plasticity, germ.: umwandlungsbewirkte Plastizität) are generally known. Especially high-strength steel alloys are involved here having a multiphase microstructure. TRIP steels are stronger while at the same time more ductile than conventional steel grades. As a result, they allow the manufacture of lighter structural parts at a predefined required strength and elongation. The TRIP effect involves the particular martensitic formation during shaping. This causes a simultaneous increase in hardness and formability at mechanical formation in the product manufacture or product use. The realization of the effect is mainly influenced by the cost-efficient alloying elements aluminum and silicon. In addition, significantly more expensive alloying elements such as nickel can be saved. The material-inherent yield strength is higher than comparable steels because silicon permits the formation of solid solution strengthening. As soon as the deformation reaches the plastic range, the metastable carbon-rich austenite begins a strain-induced transformation into martensite. As a result, the strength of the TRIP steel is tailored after plastic deformation. Cold formed structural parts with high yield strength and tensile strength are however limited as far as complexity of the geometry is concerned. Moreover, cold forming requires consideration of an elastic recovery of the steel, when the tool is designed. In addition, the residual elongation in the formed region is lower than in the region that is not formed. The structural part has therefore uneven properties.

WO 2004/022794 A1 shows a method for the production of a steel with a proportion of residual austenite in the steel microstructure, whereby a respective steel is heated to produce austenite and then quenched to transform at least in part the austenite into martensite. Then, carbon is partitioned to transfer carbon from the martensite to the still present austenite. This partitioning takes place in the range of the martensitic starting temperature. Therefore, the steel is held in this temperature range for a respectively long period or heated again and then cooled down in a desired manner. WO 2004/022794 A1 does not disclose a boron-alloyed steel.

DE 10 2008 010 168 A1 discloses the use of a steel grade for armoring a vehicle, having a composition, expressed in weight percent, of 0.35 to 0.55% of carbon; 0.1 to 2.5% of silicon; 0.3 to 2.5% of manganese; max. 0.05% of phosphorus; max. 0.01% of sulfur; max. 0.08% of aluminum; max. 0.5% of copper; 0.1 to 2.0% of chromium; max. 3.0% of nickel; max. 1.0% of molybdenum; max. 2.0% of cobalt; 0.001 to 0.005% of boron; 0.01 to 0.08 of niobium; max. 0.4% of vanadium; max. 0.02% nitrogen; max. 0.2% titanium; remainder iron including impurities resulting from smelting. Also this steel grade is hot formed. Apart from the application of this alloy for armoring purposes, it has a relatively high carbon content which decreases welding capability.

Starting from this state of the art, the invention is therefore based on the object to provide a hot formed and press hardened structural part with a high yield strength and a high tensile strength but at the same time has a better ductility compared to the state of the art.

This object is solved by the use of a steel alloy which has a composition, expressed in weight percent, of:

C  0.15% ≦ C < 0.35%
Mn 0.8-2.5%
Si 1.5-2.5%
Cr max. 0.4%
Al max. 0.1%
Ni max. 0.3%
B  0.0008-0.005%
Ti 0.005-0.1%
Nb max. 0.1%,

remainder iron and unavoidable impurities in a hot forming and press hardening process. A blank separated from a strip material or a previously pre-formed structural part is heated hereby to a temperature above the Ac₃ point of the alloy so as to transform the microstructure into austenite. Thereafter, the blank or the preformed structural part is placed in a force-cooled mold, shaped and hardened at the same time by cooling it to a temperature below approximately 200° C. Pressing in the closed mold prevents distortion. Subsequently, the finished hot formed and press hardened structural part is removed from the mold. The particular composition of the steel, especially the relatively substantial addition of silicon, not only produces martensite during hardening. Instead, part of austenite is retained as residual austenite which remains stable up to temperatures of minus 100° C. The microstructure may contain also proportions of bainite in addition to residual austenite. The silicon in the steel prevents carbide formation so that carbon is available for stabilizing the residual austenite. The residual austenite provides the steel according to the invention with a higher breaking elongation than classic boron-alloyed, purely martensitic hot formed steel. Moreover, a later deformation, e.g. in the event of a crash when structural or safety components are involved and for which the hot formed and press hardened structural parts are typically used, causes formation of martensite from the still present residual austenite so that the steel is additionally hardened in the event of a crash. As a result, tensile strengths are attained which are comparable to the conventional hot formed steel with comparable carbon content.

The desired microstructure is reached by the invention not in the hot rolling process but during the hot forming process (press hardening). When the microstructure is present already after hot rolling, the steel is suitable for cold forming. When forming the steel, the metastable residual austenite, present in the hot strip, can be transformed into martensite. On the other hand, during hot forming/press hardening, the hot strip, which may have any microstructure in the initial state, is austenitized, hot formed and press hardened so that in combination with a following tempering the desired microstructure of predominantly martensite with proportions of bainite and residual austenite is realized.

According to a preferred embodiment, the steel according to the invention has the following composition, expressed in weight percent, of:

C  0.22-0.25%
Mn 1.5-1.7%
Si 1.95-2.1%
Cr max. 0.15%
Al 0.03-0.05%
Ni max. 0.2%
B  0.002-0.0035%
P  max. 0.015%
S  max. 0.01%,
Ti 0.005-0.1%
Nb max. 0.1%
N  max. 0.01%,

remainder iron and unavoidable impurities. Preferably is hereby the ratio of titanium to nitrogen 1 Ti to 3.4 N to 5 N. In this way, enough nitrogen is bound by titanium. After heating above Ac₃ and hot forming and press hardening in a hot forming tool which is indirectly cooled with water, this alloy composition reaches a yield strength Rm of >1600 MPa N/mm², a tensile strength R_p0.2 of >1050 MPa, and a breaking elongation A₅ of >10.5%. The hardened microstructure is comprised of martensite and residual austenite.

As a result of the proportions of residual austenite in the finished structural part, the breaking elongation of the structural part is increased. A fastest possible and direct cooling process that is typical for press hardening is sufficient for reaching the desired microstructure. There is no need for a separate carbon partitioning. An elastic recovery of the material is not to be expected as a result of the hot forming and press hardening. Moreover, in view of the high proportion of silicon, the surface oxidizes less than in conventional hot formed steels. As a consequence, it is possible to produce a hot formed and press hardened structural part with a surface which can be coated directly with EDP in the absence of a preceding irradiation. Moreover, the hardened steel is more stable for tempering as a result of the high proportion of silicon. The development of carbides during tempering is suppressed so that the material can be galvanized even at 400 to 450° C., while the tensile strength Rm remains still >1450 MPa. As the high silicon content increases the Ac₃ temperature of the alloy, the heating temperature must be selected respectively higher. It is at least 960° C., when the silicon content is 2%.

Overall, the use in accordance with the invention of the alloy composition according to the invention in a hot forming and press hardening process is well suited to produce a dimensionally precise high-strength structural part with increased ductility.

Claims

1.-8. (canceled)

9. A steel alloy for use in a hot forming and press hardening process, said steel alloy comprising in weight percent: C 0.15% = C < 0.35% Mn 0.8-2.5% Si 1.5-2.5% Cr max. 0.4% Al max. 0.1% Ni max. 0.3% B 0.0008-0.005% Ti 0.005-0.1% Nb max. 0.1%, remainder iron and unavoidable impurities.

10. The steel alloy of claim 9, comprising in weight percent: C 0.22-0.25 % Mn 1.5-1.7% Si 1.95-2.1% Cr max. 0.15% Al 0.03-0.05% Ni max. 0.2 % B 0.002-0.0035% P max. 0.015% S max. 0.01%, Ti 0.005-0.1% Nb max. 0.1% N max. 0.01%, remainder iron and unavoidable impurities.

11. The steel alloy of claim 9, wherein a ratio of Ti to N is 1:3.4 to 5.

12. The steel alloy of claim 9, wherein the steel alloy has a microstructure comprised predominantly of martensite with proportions of residual austenite and bainite.

13. A hot formed and press hardened structural part made from a steel grade comprising, in weight percent: C 0.15% = C < 0.35% Mn 0.8-2.5 % Si 1.5-2.5 % Cr max. 0.4% Al max. 0.1% Ni max. 0.3% B 0.0008-0.005% Ti 0.005-0.1% Nb max. 0.1 %, remainder iron and unavoidable impurities.

14. The hot formed and press hardened structural part of claim 13, wherein the steel grade is made of a composition, in weight percent of: C 0.22-0.25% Mn 1.5-1.7% Si 1.95-2.1% Cr max. 0.15% Al 0.03-0.05% Ni max. 0.2% B 0.002-0.0035% P max. 0.015% S max. 0.01%, Ti 0.005-0.1% Nb max. 0.1% N max. 0.01%, remainder iron and unavoidable impurities

15. The hot formed and press hardened structural part of claim 13, wherein the steel grade has predominantly a martensitic microstructure with proportions of residual austenite and bainite.

16. The hot formed and press hardened structural part of claim 13, for use as a structural and/or safety component.

17. A method of making a steel alloy, comprising the steps of: C 0.15% = C < 0.35% Mn 0.8-2.5% Si 1.5-2.5% Cr max. 0.4% Al max. 0.1% Ni max. 0.3% B 0.0008-0.005% Ti 0.005-0.1% Nb max. 0.1%, remainder iron and unavoidable impurities, to a temperature above Ac3;

heating a blank having a steel composition comprising in weight percent:

placing the blank in a force-cooled mold;

hot forming and press hardening the blank; and

cooling the blank to a temperature below 200° C.

18. The method of claim 17, wherein the hot forming and press hardening step is carried out, while the mold is closed.

19. The method of claim 17, wherein the steel composition comprises, in weight percent: C 0.22-0.25% Mn 1.5-1.7% Si 1.95-2.1% Cr max. 0.15% Al 0.03-0.05% Ni max. 0.2% B 0.002-0.0035% P max. 0.015% S max. 0.01%, Ti 0.005-0.1% Nb max. 0.1% N max. 0.01%, remainder iron and unavoidable impurities.