JP7576167B2 - Highly plastic hot-formed steels and hot-formed processes for automobiles with oxidation resistance - Google Patents

Description

The present invention relates to the technical field of automotive steel, and more specifically to a hot-formed, highly plastic, oxidation-resistant steel for automobiles and a hot-forming process.

In recent years, new materials for car bodies have been continuously developed and applied to car bodies, but ultra-high strength steel plates for cold press processing exceeding 1000 MPa are usually used to manufacture parts with simple shapes due to problems such as being prone to cracking and having a large rebound. In contrast, hot-formed steel is formed in the austest region by the hot forming process, so the rebound amount is small and the assembly precision requirements can be met, and ultra-high strength parts of 1500 MPa or more can be obtained by quenching under pressure, which effectively simplifies the design of car body structures and parts and significantly reduces the weight of vehicles.

At present, hot-formed steel on the market is divided into painted hot-formed steel and unpainted hot-formed steel according to its surface condition, among which, unpainted steel has a tendency to form mill scale on the steel surface during heating in a heating furnace, which easily causes decarbonization and affects the performance of the steel, so it is necessary to use a protective atmosphere during heating and to perform shot peening after hot forming, which increases costs and processes. On the other hand, painted steel is a steel plate with an aluminum silicon-based or zinc-based coating on the surface, which can effectively prevent decarbonization and oxidation of the steel surface during heating and can omit shot peening after hot forming, but the cost of painted hot-formed steel is higher than that of unpainted steel. Currently, hot-formed steel mass-produced and used by conventional technology has a strength level of 1500 MPa, but its elongation rate after hot forming is only 6-9%, which does not meet the development demand of the automotive field. There is still no good technology that can reduce the cost of hot forming steel, solve the problems of surface oxidation and decarbonization, avoid shot peening treatment, and at the same time ensure that the steel after hot forming has good plasticity.

Patent document 1 discloses a method for producing ultra-high strength hot-formed steel for automobiles, the composition of which is, in mass%, C: 0.5%-0.6%, Mn: 0.5%-2.0%, Si: 1.5%-2.5%, Cr: 1.0%-3.0%, Al: 1.0%-2.0%, Nb: 0.01%-0.03%, B: 0.001%-0.005%, and the steel sheet after hot forming has a strength of 1500-2000 MPa and an elongation rate of 10%-20%. The steel sheet disclosed in Patent document 1 has good strength and plasticity matching, but its composition contains high contents of Cr and Al elements, which increases the cost and difficulty of smelting. In addition, the production process is complicated, existing production equipment cannot meet the production requirements, and protective atmosphere and shot peening treatment are also required during production.

Patent Document 2 discloses a high-strength, high-toughness hot-formed steel sheet for automobiles and its manufacturing method. The chemical components of the steel sheet are, in mass %, C: 0.1-0.5%, Si: 0.5-1.5%, Mn: 1.2-2.4%, Ti: 0.01-0.05%, B: 0.001-0.005%, S: 0.01% or less, and P: 0.01% or less. After hot forming, the steel sheet has a tensile strength of 1600 MPa and an elongation rate of 16%, with good overall performance and low alloy cost. However, in order to obtain the final structure and mechanical properties, the steel sheet needs to be deformed in a heating process and quenched twice, and the hot forming process is complicated and cannot be realized with existing equipment. In addition, a protective atmosphere during heating and shot peening treatment after hot forming are also required.

Based on the above, the development of highly plastic hot-formed steel with excellent oxidation resistance for automobiles and the hot-forming process are expected to be applied.

Chinese Patent No. 107354385 Chinese Patent No. 103255340

The present invention was made in consideration of the above technical problems, and provides a hot-formed steel with high plasticity and oxidation resistance for automobiles, and a hot forming process.

The technical means of the present invention are as follows:
The highly plastic hot-formed steel for automobiles having oxidation resistance of the present invention has chemical components, in mass %, of the hot-formed steel: C: 0.18% to 0.28%, Si: 0.20% or less, Mn: 1.20% to 2.0%, P: 0.030% to 0.080%, S: 0.004% or less, Als: 0.02% to 0.06%, Nb: 0.02% to 0.06%, Ti: 0.025% to 0.045%, V: 0.05% to 0.15%, Cr: 0.5% to 2.50%, Mo: 0.10% to 0.30%, B: 0.0015% to 0.0035%, N: 0.005% or less, with the balance being Fe and unavoidable impurities.

The structure of hot-formed steel consists of ferrite, martensite and retained austenite.

The volume fraction of ferrite is 5% to 12%, the volume fraction of martensite is 78% to 89%, and the volume fraction of retained austenite is 6% to 10%.

The hot-formed steel has a tensile strength of 1400 MPa to 1700 MPa, an oxidation resistance rate of 0.1 g/( ^m2 ·h) or less, a yield strength of 900 MPa to 1450 MPa, an elongation rate of 18.0% or more, the surface of the steel is not completely decarburized, the thickness of the decarburized layer is 15 μm or less, and the thickness of the hot-formed steel plate is 0.8 mm to 12.0 mm.

The components of the hot forming steel according to the present invention have the following effects:
C: C ensures the strength of steel and is advantageous for improving the hardenability of steel. If the carbon content is too low, the strength of the steel after hot stamping does not reach the desired target, while if the carbon content is too high, the strength of the hot-formed steel is too high and the plasticity is reduced. In addition, increasing the content of C can lower the phase transition temperature of the steel and lower the austenitizing temperature, which is advantageous for obtaining a surface that does not require shot peening, and the increase in the content of C allows the steel to generate a sufficient amount of supercooled austenite during the heating pressure holding process, thereby improving the plasticity. Therefore, the optimal range of C in the present invention is 0.18% to 0.28%.

Si: Si is an element that does not precipitate carbides in steel, and it has the effect of suppressing the precipitation of carbides during the cooling and pressure holding process of hot forming, and ensuring the amount and stability of retained austenite. However, if the Si content is too high, defects such as a large amount of mill scale and color difference will occur on the surface of the hot formed substrate, affecting the surface quality of the hot formed parts, and if the Si element is too high, the two-phase region will expand, the austenitization temperature will rise, and the steel will be kept at a high temperature, making the steel surface more susceptible to deterioration. Therefore, the Si content in the steel of the present invention is set to 0.20% or less.

Mn: The main role of Mn in this invention is to improve the hardenability of the steel, lower the phase transition temperature, and enable the steel to austenitize at a lower temperature. If the Mn content is too high, it will deteriorate the uniformity of the steel structure, making it more likely that serious band-shaped structural defects will occur in the structure. Therefore, in this invention, the Mn content is selected to be 1.20% to 2.0%.

P: The role of P in this invention is similar to that of Si, in that it can suppress the formation of cementite and improve the stability of retained austenite. In addition, P refines and uniformly distributes lath martensite slabs, improving toughness. The P content in this invention is 0.030% to 0.080%.

S: In the present invention, S is a harmful element that forms MnS impurities and deteriorates the microstructure and mechanical properties of the steel sheet. Therefore, in the present invention, S is limited to 0.004% or less.

Als: Als (acid-soluble aluminum) plays a role in deoxidizing and fixing nitrogen during the smelting process, but if there is too much Als, the amount of aluminum-based impurities increases. Therefore, in this invention, the range of Als is 0.020% to 0.060%.

Cr: Cr is an element that improves the hardenability of steel. In the present invention, the main role of the Cr element is to improve the oxidation resistance of steel at high temperatures, improve the tempering stability of steel, and ensure that tempered martensite does not occur within the holding temperature range of steel. The optimal Cr content is 0.50% to 2.50%.

Mo: Mo is a medium-strength carbide-forming element, which improves the strength and toughness of steel. In the present invention, Mo can lower the martensite transition temperature and significantly improve the stability of retained austenite, and at the same time, the addition of Mo element improves the oxidation resistance of steel. In the present invention, the content of Mo is 0.10% to 0.30%.

Nb, V: Nb and V mainly play a role in fine grain strengthening and precipitation strengthening of steel. In the present invention, both elements are dispersed and precipitated as nano-scale fine carbides, which effectively pin the original austenite grain boundaries and further refine the structure of each phase of the hot-formed steel sheet, thereby improving the overall performance. At the same time, the dispersed and precipitated carbides function as hydrogen traps, pinning diffusible hydrogen in the steel and improving delayed fracture resistance. In addition, the precipitation of VN formed from V and N suppresses the precipitation of BN and can prevent the decrease in strength due to the precipitation of B. In the present invention, the Nb content is 0.020% to 0.060%, and the V content is 0.050% to 0.15%.

Ti: Ti is mainly used in boron steel to fix nitrogen and ensure the hardening effect of boron. In addition, Ti precipitates as fine carbides with C element, reducing the hardness and strength of the martensite structure after hot forming, which is advantageous for improving the plasticity and toughness of the steel. In the present invention, the Ti content is 0.025% to 0.045%.

B: Adding boron to steel significantly improves the hardenability of the steel and ensures the stability of the strength of the steel after hardening. If the B content is too low, the effect is not significant, and if the B content is too high, B compounds tend to form with N in the steel, reducing the performance of the steel plate. Therefore, in the present invention, the B content is 0.0015% to 0.0035%.

N: The lower the N content, the better, but if it is too low, manufacturing becomes difficult and costs increase. Therefore, in the present invention, the N content is 0.005% or less.

In the present invention, by adding alloy elements such as C, Mn, Cr, and Mo, the austenitization temperature is lowered, the hardenability of the steel is improved, the oxidation of the steel is suppressed, and the critical cooling rate of the hot forming of the steel is lowered, so that hot forming steel of a thickness standard can be manufactured. In addition, by combining the chemical composition and the hot forming process, a certain amount of ferrite is obtained in the air cooling stage, and a certain amount of stable retained austenite is obtained in the pressure holding stage after cooling, thereby improving the plasticity of the steel. By adding the elements Si and P to the composition, the precipitation of carbides is suppressed, the content of retained austenite in the steel is ensured, and the mechanical properties of the steel are improved. Furthermore, since the elements Cr and Mo in the composition of the steel have an antioxidant effect, the steel can be heated and kept warm under unprotected atmosphere conditions, and subsequent processes can be carried out directly after hot forming without shot peening.

The present invention further provides a hot forming process for hot forming high plasticity automotive steel with oxidation resistance, comprising the steps of:
Step (1): The hot-formed substrate containing the above components is heated in a heating furnace at a temperature of A _C3 -A _C3 +15°C, and kept at that temperature for 180s-300s.
Step (2): The heated hot-formed substrate is removed from the heating furnace and air-cooled to the _Ar3 temperature, and then after 5 s to 8 s, it is placed in a hot-formed mold to be deformed, cooled to 180 ° C. to 250 ° C. at a cooling rate of 18 ° C./s or more, and then kept under pressure for 40 s to 80 s. Then, the formed part is removed and air-cooled to room temperature to obtain a hot-formed steel.

The steel of the present invention does not require a protective atmosphere during hot forming, does not require shot peening treatment after hot forming, and subsequent processes can be carried out directly, so the overall process cost is lower than that of conventional hot formed products.

The hot-formed substrate is obtained through smelting, hot rolling and cold rolling. The smelting components and their mass percentages are the components and their mass percentages of the oxidation-resistant, high-plasticity hot-formed steel for automobiles described above.

Compared with the prior art, the present invention has the following advantages:
(1) By combining chemical components and a hot forming process to introduce a certain amount of ferrite and retained austenite into the conventional fully martensite structure, the plasticity of the steel can be improved, and an elongation rate of 18% or more can be achieved under conditions where a tensile strength of 1,400 MPa or more can be ensured.
(2) By adding Cr elements, etc., the oxidation resistance performance of the steel at high temperatures is improved. The oxidation resistance rate of the steel is 0.1 g/( ^m2 ·h) or less, and the oxidation resistance level reaches level 1. Therefore, no protective atmosphere is required during hot forming of the steel, and no shot peening treatment is required after hot forming, and the subsequent processes can be carried out directly.
(3) The manufacturing method and hot forming process of the provided hot-formed steel can be carried out using existing equipment, so no equipment modification is required and it can be realized at low cost.
For the above reasons, the present invention can be widely used in fields such as automotive steel.

Where there is no conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other. The described embodiments are only some of the embodiments of the present invention, and not all of the embodiments. The following description of at least one exemplary embodiment is illustrative in nature and does not impose any limitations on the present invention and its applications or uses. All other embodiments that can be obtained based on the embodiments of the present invention without the need for creative labor by those skilled in the art should fall within the scope of the claims of the present invention.

The present invention provides a hot-formed steel with high plasticity and oxidation resistance for automobiles, and the chemical components of the hot-formed steel are, in mass %, C: 0.18% to 0.28%, Si: 0.20% or less, Mn: 1.20% to 2.0%, P: 0.030% to 0.080%, S: 0.004% or less, Als: 0.02% to 0.06%, Nb: 0.02% to 0.06%, Ti: 0.025% to 0.045%, V: 0.05% to 0.15%, Cr: 0.5% to 2.50%, Mo: 0.10% to 0.30%, B: 0.0015% to 0.0035%, N: 0.005% or less, and the balance being Fe and unavoidable impurities.

The structure of hot-formed steel consists of ferrite, martensite and retained austenite.

The volume fraction of ferrite is 5% to 12%, the volume fraction of martensite is 78% to 89%, and the volume fraction of retained austenite is 6% to 10%.

The hot-formed steel has a tensile strength of 1400 MPa to 1700 MPa, an oxidation resistance rate of 0.1 g/( ^m2 ·h) or less, a yield strength of 900 MPa to 1450 MPa, an elongation rate of 18.0% or more, the surface of the steel is not completely decarburized, the thickness of the decarburized layer is 15 μm or less, and the thickness of the hot-formed steel is 0.8 mm to 12.0 mm.

In this embodiment, the hot-formed steel with high strength and excellent oxidation resistance is smelted, hot-rolled, and cold-rolled to obtain a hot-formed substrate with a thickness of 0.8 mm to 12.0 mm. Then, a hot-forming process is performed, which specifically includes the following steps:

Step (1): The hot-formed substrate is placed in a heating furnace at a temperature of A _C3 -A _C3 +15°C and kept at the temperature for 180s-300s. The purpose is to make the hot-formed substrate fully austenitized and retain the original small size of austenite grains. The low austenitizing temperature can also reduce the oxidation of the surface of the hot-formed substrate.

Step (2): The heated hot-formed substrate is removed from the heating furnace and air-cooled to the _Ar3 temperature, and then after 5 s to 8 s, it is placed in a hot-formed mold to be deformed, cooled to 180 ° C. to 250 ° C. at a cooling rate of 18 ° C./s or more, and then kept under pressure for 40 s to 80 s. Then, the formed part is removed and air-cooled to room temperature to obtain a hot-formed steel.

The components, hot forming process parameters, and structure and performance of the steel sheet after hot forming in the examples of the present invention are shown in Tables 1 to 3.

In this embodiment, by combining chemical components and hot forming process, a certain amount of ferrite and residual austenite is introduced into the conventional fully martensite structure to improve the plasticity of the steel, and the elongation rate can be 18% or more under the condition that the tensile strength can be ensured to be 1400 MPa or more. By adding elements such as Cr and Mo, the oxidation resistance performance of the steel is improved, the oxidation resistance rate of the steel is 0.1 g / ( ^m2 · h) or less, the oxidation resistance level reaches level 1, the steel does not need a protective atmosphere during hot forming, does not need shot peening treatment after hot forming, and the subsequent process can be carried out directly. The cost of the provided hot forming steel and the whole process of the hot forming process is lower than the production cost of conventional hot forming parts, and can be realized with existing equipment without equipment modification.

Finally, the following should be explained: The above embodiments are merely for the purpose of explaining the technical means of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the above embodiments, it is possible to modify the technical means described in the above embodiments, or to implement equivalent replacements for part or all of the technical features, and it will be understood by those skilled in the art that such modifications and replacements do not depart from the essence of the corresponding technical means of the embodiments of the present invention.

(Additional Note)
(Appendix 1)
A hot-formed steel having high plasticity and oxidation resistance for automobiles, the chemical components of the hot-formed steel being, in mass%, C: 0.18% to 0.28%, Si: 0.20% or less, Mn: 1.20% to 2.0%, P: 0.030% to 0.080%, S: 0.004% or less, Als: 0.02% to 0.06%, Nb: 0.02% to 0.06%, Ti: 0.025% to 0.045%, V: 0.05% to 0.15%, Cr: 0.5% to 2.50%, Mo: 0.10% to 0.30%, B: 0.0015% to 0.0035%, N: 0.005% or less, the balance being Fe and unavoidable impurities.
A hot-formed, highly plastic, oxidation-resistant steel for automobiles.

(Appendix 2)
The structure of the hot-formed steel is composed of ferrite, martensite and retained austenite.
2. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 1.

(Appendix 3)
The ferrite has a volume fraction of 5% to 12%, the martensite has a volume fraction of 78% to 89%, and the retained austenite has a volume fraction of 6% to 10%.
3. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 2.

(Appendix 4)
The tensile strength of the hot-formed steel is 1400 MPa to 1700 MPa;
2. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 1.

(Appendix 5)
The anti-oxidation rate of the hot-formed steel is 0.1 g/( ^m2 ·h) or less;
2. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 1.

(Appendix 6)
The yield strength of the hot formed steel is 900 MPa to 1450 MPa;
2. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 1.

(Appendix 7)
The elongation rate of the hot-formed steel is 18.0% or more;
2. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 1.

(Appendix 8)
The surface of the hot-formed steel is not completely decarburized, and the thickness of the decarburized layer is 15 μm or less.
2. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 1.

(Appendix 9)
The thickness of the hot-formed steel is 0.8 mm to 12.0 mm;
2. A hot-formed, highly plastic, oxidation-resistant steel for automobiles according to claim 1.

(Appendix 10)
The hot-formed substrate containing the chemical composition according to any one of claims 1 to 9 is heated in a heating furnace at A _C3 -A _C3 +15°C and kept at the same temperature for 180s-300s;
The heated hot-formed substrate is removed from the heating furnace and air-cooled to the _Ar3 temperature, and then after 5 s to 8 s, is placed in a hot-formed mold to be deformed, cooled to 180°C to 250°C at a cooling rate of 18°C/s or more, and then held for 40 s to 80 s, and then the formed part is removed and air-cooled to room temperature to obtain the hot-formed steel.
1. A hot forming process for hot forming high plasticity automotive steel with oxidation resistance, characterized in that

Claims (8)

A hot-formed steel having high plasticity and oxidation resistance for automobiles, the chemical components of the hot-formed steel being, in mass%, C: 0.18% to 0.28%, Si: 0.20% or less, Mn: 1.20% to 2.0%, P: 0.030% to 0.080%, S: 0.004% or less, Als: 0.02% to 0.06%, Nb: 0.02% to 0.06%, Ti: 0.025% to 0.045%, V: 0.05% to 0.15%, Cr: 0.5% to 2.50%, Mo: 0.10% to 0.30%, B: 0.0015% to 0.0035%, N: 0.005% or less, the balance being Fe and unavoidable impurities;
The structure of the hot-formed steel is composed of ferrite, martensite and retained austenite,
The ferrite has a volume fraction of 5% to 12%, the martensite has a volume fraction of 78% to 89%, and the retained austenite has a volume fraction of 6% to 10% .
A hot-formed, highly plastic, oxidation-resistant steel for automobiles. The tensile strength of the hot-formed steel is 1400 MPa to 1700 MPa;
2. Highly plastic hot forming steel for automobiles with oxidation resistance according to claim 1. The anti-oxidation rate of the hot-formed steel is 0.1 g/( ^m2 ·h) or less;
2. Highly plastic hot forming steel for automobiles with oxidation resistance according to claim 1. The yield strength of the hot formed steel is 900 MPa to 1450 MPa;
2. Highly plastic hot forming steel for automobiles with oxidation resistance according to claim 1. The elongation rate of the hot-formed steel is 18.0% or more;
2. Highly plastic hot forming steel for automobiles with oxidation resistance according to claim 1. The surface of the hot-formed steel is not completely decarburized, and the thickness of the decarburized layer is 15 μm or less.
2. Highly plastic hot forming steel for automobiles with oxidation resistance according to claim 1. The thickness of the hot-formed steel is 0.8 mm to 12.0 mm;
2. Highly plastic hot forming steel for automobiles with oxidation resistance according to claim 1. The hot-formed substrate containing the chemical components according to any one of claims 1 to 7 is heated in a heating furnace at A _C3 -A _C3 +15°C, and kept at the same temperature for 180s-300s;
The heated hot-formed substrate is removed from the heating furnace and air-cooled to the _Ar3 temperature, and then after 5 s to 8 s, is placed in a hot-formed mold to be deformed, cooled to 180°C to 250°C at a cooling rate of 18°C/s or more, and then held for 40 s to 80 s, and then the formed part is removed and air-cooled to room temperature to obtain the hot-formed steel.
1. A hot forming process for hot forming high plasticity automotive steel with oxidation resistance, characterized in that