Patent Description:
With the urgent need for global energy conservation and emission reduction and an environmentally friendly economy, the automotive industry is developing in the direction of light weight, but the light weight of an automobile is not at the expense of safety. On the contrary, the collision safety requirements for automobiles are getting higher and higher. At present, high-strength and ultra-high-strength steel materials for vehicles are attracting more and more attention in the automotive industry due to their high strength and light weight. For a high strength, a cold stamping method is adopted, such that the formed properties are reduced, large stamping force is required, and cracking easily occurs. In addition, after being formed, the part rebounds greatly, so that its shape and dimensional accuracy can hardly be guaranteed.

The hot stamping technology arising in Europe is a new forming technology that solves the above problems. The technology is a forming technology that heats the blank to a completely austenitized state, rapidly transfers it to a die having a uniform cooling system to be rapidly stamp formed, and meanwhile performs cooling quenching treatment to obtain a super-high-strength steel part with a uniform martensite structure. At high temperatures, the material has good stamping formability, can be stamp formed into complex members, and at the same time eliminates the rebound impact, such that the part has high precision and good quality. At the present time, major automobile manufacturers in Europe and the United States have successfully applied the high-strength steel material hot stamping technology to the production of members such as an A-pillar, a B-pillar, a bumper, a roof frame, an underbody frame and a door anti-collision bar of an automobile. Due to a high strength and the presence of a martensite structure, the performance of the steel for hot stamping in automotive crash safety depends on its toughness, cold bending property and resistance to delayed cracking. Currently, the steel for hot stamping widely used in the automotive industry is structural alloy steel represented by 22MnB5, which has such problems as high austenitizing temperature (AC3 of about <NUM>), low hardenability, poor toughness after forming, limited cold bending performance, and delayed cracking.

<CIT> discloses a high-strength steel material for hot stamping coated with an aluminum alloy. The patent document requires a strength of more than <NUM> MPa, wherein the strength is <NUM> MPa when the carbon content is <NUM>%, and the strength reaches <NUM> to <NUM> MPa or more when the carbon content is above <NUM>, but the document does not mention the elongation and toughness thereof. In fact, the material with an alloy design requires tempering heat treatment to achieve the strength value, and has poor toughness, which cannot meet the elongation and toughness requirement of above <NUM> MPa of hot stamp formed steel and members, and the high carbon content is detrimental to the welding performance.

<CIT> discloses a method of preparing a coated strip and a hot stamped product thereof. A preferred embodiment of this document mentions that heat treatment is required after the hot stamping so that its mechanical properties can reach a yield strength of <NUM> MPa and a tensile strength of above <NUM> MPa, but it does not quantitatively expound the ductility. It only proposes to control the sulfur content (requiring a sulfur content of less than <NUM>. 002wt%) to ensure the ductility and avoid crack propagation caused by sulfide inclusions, but it is difficult and costly in the industry to control the sulfur content below <NUM> ppm. Hence, by controlling the sulfur element content, the problem of low ductility cannot be completely solved. <CIT> discloses a steel sheet for a formed member having enhanced ductility and a method for manufacturing the formed member. The material of the steel sheet includes C, Mn, V, Si+Al, Cr, Ti, Nb, B components in various ranges. A formed member is manufactured using a steel sheet by performing a hot forming process on the steel sheet and then quenching the hot-formed steel sheet to a temperature lower than Mf and subsequently heating the quenched steel sheet to a predetermined temperature range and maintaining the steel sheet within this temperature range.

Therefore, in view of the problems in the prior art, one of the objects of the present invention is to improve the deficiencies of the conventional steel material for hot stamping, hot stamping process and formed member having high toughness and delayed crack resistance after the hot stamping without the need of heat treatment such as tempering.

According to an embodiment of the present invention, there is provided a steel material for hot stamping comprising the following components by weight: <NUM> to <NUM>% of C; <NUM> to <NUM>% of Mn; <NUM> to <NUM>% of V; <NUM> to <NUM> % of Si; <NUM> to <NUM>% of Al; <NUM> to <NUM>% of Cr; <NUM> to <NUM>% of Ti; <NUM> to <NUM>% of Nb; <NUM> to <NUM>% B; a total of less than <NUM>% of Mo, Ni, Cu and other alloying elements that are beneficial to improving the hardenability, other impurity elements, balance Fe.

The steel material for hot stamping of the present invention has a heating temperature range of <NUM> to <NUM> during the hot stamping process, and preferably has a composite carbide of VC and/or V with Ti, Nb at the austenite grain boundary during the austenitizing process. In the austenitizing heating process of the steel material for hot stamping of the present invention, the precipitated particle size of the composite carbide of VC and/or V with Ti, Nb at the austenite grain boundary is preferably from <NUM> to <NUM>. In the hot stamping process, the steel material for hot stamping of the present invention precipitates a certain amount of composite carbide of VC and/or V with Ti, Nb in the austenite crystal including grain boundaries during the cooling after the austenitizing, and a carbide particle size in the austenite crystal is <NUM> to <NUM>. The volume fraction of the composite carbide of VC and/or V with Ti, Nb in the steel material for hot stamping of the present invention is more than <NUM>%.

The hot stamped steel material of the present invention achieves a yield strength of <NUM> MPa to <NUM> MPa, a tensile strength of <NUM> to <NUM> MPa, and an elongation of <NUM> to <NUM>% without tempering; and achieves a yield strength of <NUM> to <NUM> MPa, a tensile strength of <NUM> to <NUM> MPa, and an elongation of <NUM> to <NUM>% after tempering heat treatment.

The hot stamped steel material of the present invention includes a hot-rolled steel sheet, a hot-rolled pickled steel sheet, a cold-rolled steel sheet, or a steel sheet with a coating layer. The steel sheet with a coating layer is a zinc-coated steel sheet which is a hot-rolled steel sheet or a cold-rolled steel sheet on which a metal zinc layer is formed, wherein the zinc-coated steel sheet includes at least one selected from hot dip galvanizing, galvanizing annealing, zinc plating, or zinc-iron plating. The steel sheet with a coating layer is a hot-rolled steel sheet or a cold-rolled steel sheet on which an aluminum-silicon layer is formed, or a steel sheet with an organic coating layer.

In accordance with another embodiment of the present invention, a hot stamping process is provided that includes the following procedures:.

According to still another embodiment of the present invention, there is also provided a tempering process comprising the steps of:.

The hot stamped component formed by the hot stamping process of the present invention can be used for automotive high-strength members including, but not limited to, an A-pillar, a B-pillar, a bumper, a roof frame, an underbody frame, and a door bumper bar of an automobile.

After hot stamping or equivalent heat treatment, the steel material of the present invention achieves a yield strength of <NUM> MPa to <NUM> MPa, a tensile strength of <NUM> to <NUM> MPa, and an elongation of <NUM> to <NUM>% after direct hot stamping quenching (without tempering). After the tempering treatment of the present invention, <NUM> MPa - <NUM> MPa - <NUM>%, and preferably <NUM> MPa - <NUM> MPa - <NUM>% can be reached. This property cannot be achieved by direct quenching (no tempering) of the composition in the prior art.

The present invention will be described in more detail below with reference to exemplary embodiments. The following embodiments or experimental data are intended to illustrate the invention exemplarily, and those skilled in the art should be aware that the present invention is not limited to these embodiments or experimental data.

The steel material for hot stamping comprising the following components by weight: <NUM> to <NUM>% of C; <NUM> to <NUM>% of Mn; <NUM> to <NUM>% of V; <NUM> to <NUM> % of Si; <NUM> to <NUM>% of Al; <NUM> to <NUM>% of Cr; <NUM> to <NUM>% of Ti; <NUM> to <NUM>% of Nb; <NUM> to <NUM>% of B; the content of Mo, Ni, Ca in combination amounts a total of less than <NUM>%, inevitable impurities, and balance Fe.

The martensite strength improves with increasing carbon content, but high carbon content leads to the formation of twinned martensite, which reduces the toughness of the material. The twinned martensite must be tempered to prevent brittle fracture. The steel material of the present invention adds a specific composition of V element to the alloy composition, so that the full austenitizing heating temperature range during the hot stamping process is <NUM> to <NUM>; since over <NUM>% of V and over <NUM>% of C are added to the material, according to the condition of the solubility product of VC precipitation, there will be a certain amount of composite carbide of VC and/or (V, Ti, Nb) C at the austenite grain boundary during the austenitizing process, and the second phase particles effectively pin the austenite grains, which will refine the prior austenite grains. Therefore, the precipitation of VC has an important influence on controlling the size of the prior austenite grains. According to a preferred embodiment of the present invention, the prior austenite grain size is <NUM> to <NUM>, and the grain refinement and strengthening can not only improve the yield strength but also increase the toughness. <NUM> shows a prior austenite grain boundary morphology of the steel material of the present invention after the hot stamping.

If the achievement of a tensile strength of above <NUM> MPa only relies on a high carbon addition, the martensite formed contains twinned martensite, such that it has poor toughness and can have ductile fracture only after tempering treatment. After tempering at <NUM> for <NUM> minutes (usually <NUM> ~ <NUM>, <NUM> - <NUM> minutes for automotive coating), the yield strength of the material is increased by <NUM> - 100MPa, the tensile strength is reduced by about 50MPa, and the elongation can be increased to more than <NUM>%. In the prior art (for example, the material composition and properties publicized by Nippon Steel Corporation): Fe-<NUM>. 3Mn-Ti-B% has an elongation of about <NUM>% upon the brittle fracture under the strength of <NUM> MPa in the hot stamping state (quenching), and has a strength of <NUM> MPa and an elongation of <NUM>% after tempering at <NUM> for <NUM> minutes. Poor toughness before the tempering increases the risk of delayed cracking of the member; moreover, the automotive member is welded before entering the coating procedure, such that the poor toughness of the member in the hot stamped state (not tempered) tends to cause cracking in the welding assembly process.

According to the present invention, over <NUM>% of V and over <NUM>% of C are added to the steel material alloy composition, and VC or (V, Ti, Nb) C with a volume fraction of above <NUM>% will be further precipitated during the cooling process of <NUM> to <NUM> after the austenitizing treatment and before the rapid cooling of the hot stamping die, wherein the uniform fine second phase particles can increase the tensile strength by over <NUM> MPa, and preferably the precipitated particle size is <NUM>~<NUM>, the average particle size is <NUM>, the volume fraction is about <NUM>% (<NUM>% is calculated from the amount of precipitation in the carbon replica sample by conversion from two-dimension to threedimension, and the calculated volume fraction of Thermal-Cac is <NUM>%), wherein the frequency of occurrence of <NUM>~<NUM> is as high as <NUM>%, and according to the precipitation strengthening mechanism, the precipitation strengthening enhancement thereof can reach <NUM> MPa. The precipitation of the VC or (V, Ti) C will consume the carbon in the austenite and reduce its carbon content, thereby decreasing the fraction of twinned martensite formed in the martensite after phase transformation, and therefore, based on the VC precipitation of the present invention, the toughness of the martensite itself can be improved, the strength of the martensite is lowered due to a decrease of the carbon content therein, but the strength of the material is enhanced by VC precipitation strengthening and fine grain strengthening of the prior austenite grains. <NUM> shows a precipitated particle morphology and size of the steel material of the present invention after hot stamping.

In addition, VC and H have high binding energy, are an irreversible hydrogen trap and can easily fix hydrogen atoms around them, which can improve the hydrogeninduced delayed cracking ability of the material (Reference: <NPL>).

After hot stamping the steel material of the present invention achieves a tensile strength of <NUM> to <NUM> MPa, a yield strength of <NUM> MPa to <NUM> MPa, and an elongation of <NUM> to <NUM>% after direct hot stamping quenching and without tempering. Preferably, it reaches <NUM> MPa-<NUM> MPa-<NUM>%, <NUM> MPa-<NUM> MPa-<NUM>%, and the property cannot be achieved by the alloy composition of the prior art upon direct quenching (no tempering); even if the coating process can realize the function of tempering treatment , in order to meet the welding requirement that brittle fracture of the part does not occur in the welding process, tempering heat treatment must be conducted after the hot stamping. In contrast, a major advantage of the present invention is that the process step of tempering heat treatment is eliminated, thereby simplifying the forming process.

The specific manufacturing process of the steel material for hot stamping of the present invention is disclosed in the appended claims.

<NUM> shows a hot stamping process diagram of a preferred embodiment of the present invention. According to the present invention, the hot stamping process of the invention can include the following procedures:.

The following are experimental data of the steel material. It should be apparent to those skilled in the art that these data are merely exemplary.

Claim 1:
A hot stamped steel material, characterized in that, the hot stamped steel material comprises the following components by weight:
<NUM> to <NUM>% of C;
<NUM> to <NUM>% of Mn;
<NUM> to <NUM>% of V;
<NUM> to <NUM>% of Si;
<NUM> to <NUM>% of Al;
<NUM> to <NUM>% of Cr;
<NUM> to <NUM>% of Ti;
<NUM> to <NUM> of Nb;
<NUM> to <NUM>% of B;
the content of Mo, Ni, Cu in combination amounts a total of less than <NUM>%; and inevitable impurity elements,
wherein the balance consists of Fe,
the hot stamped steel material having a yield strength of <NUM> MPa to <NUM> MPa, a tensile strength of <NUM> to <NUM> MPa, and an elongation of <NUM> to <NUM>% without tempering.