CN114941110A - Medium manganese steel for integrally forming automobile body structural part and method - Google Patents

Abstract

The medium manganese steel for integrally forming the automobile body structural part and the method thereof have the advantages that the microstructure of the medium manganese steel is quenched martensite and retained austenite, and in the forming process of the medium manganese steel, a dislocation strengthening mechanism in crystal grains promotes work hardening to inhibit recrystallization and softening, and promotes metal in an undeformed region to flow to a deformed region, so that a necking process in the processing process is delayed to realize integrally forming the automobile body structural part.

Description

Medium manganese steel for integrally forming automobile body structural part and method

Technical Field

The invention relates to the technology of automobile body structure materials, in particular to medium manganese steel for integrally forming an automobile body structural part and a method.

Background

In recent years, with the trend toward multi-material bodies, the overall proportion of ferrous materials has decreased, but the amount of high-strength steel used has increased significantly, and steel remains the primary material for automotive body manufacture. The automobile body is generally formed by spot welding and connecting a plurality of steel plate stamping parts, and the process flow is long and the production efficiency is low. Integrated, modular, and integrated parts have become a new trend, especially in the manufacture of new energy vehicles. Enterprises such as Tesla realize the manufacturing of integrated parts through aluminum alloy casting, the number of parts of a rear frame assembly is changed from 70 parts to 2 parts, and the integrated cast parts are widely applied to a new energy automobile body structure, but the problems of low strength, high cost, casting defects and the like exist in aluminum alloy casting.

The steel parts are also developed in the direction of 'all-in-one' under the promotion of a new material new process, for example, spot welding forming of the door ring piece by a stamping part is gradually developed into laser tailor welding integrated forming, and then the door ring piece is integrally formed without a welding seam, the number of the parts is combined into one, the strength of the parts reaches 1500MPa, weight reduction can be realized by 20%, and collision safety is obviously improved. The Bao steel BCB project is the largest steel vehicle body project in recent years, and aims to explore the application of advanced high-strength steel on a vehicle body, and the hot forming and the ultrahigh-strength high-plasticity steel are utilized to realize the synthesis of a plurality of parts into one part, so that the number of the parts is reduced, and the overall strength and rigidity level of the parts are increased. Ultra-high strength and integrated structures have become a trend in the development of automotive parts.

Under the development requirements of light weight and high safety of automobiles, no matter light materials such as aluminum alloy and the like or ultrahigh-strength steel, the automobile is developed towards integrated parts. The number of integrated parts is reduced, and the number of corresponding dies and production lines is also reduced; the number of the connecting joints of the integrated part is reduced, so that the production cost is reduced due to the reduction of workload and equipment, and the overall strength, rigidity and sealing level of the part are improved; the integral part cancels the lap joint structure of the original tailor-welded part, thereby reducing the total weight of the part and improving the material utilization rate. And the high strength can realize the reduction of the material thickness under the condition of ensuring that the parts have the same function. The integrated ultrahigh-strength steel part has the integration characteristic and structural innovation of parts, realizes the reduction of the quality and the weight, the connection quantity and the processing procedure of the parts, and is a great innovation of a vehicle body structure and steel materials.

The size and strength level of ultra-high strength steel integral automotive parts are limited due to the influence of material properties. For example, when the axle housing is produced by cold stamping of high-strength steel for automobile parts, the forming capability of the axle housing cannot meet the forming requirement of an integrated structure due to limited plasticity level, and the common ultrahigh-strength steel has lower steel plasticity and large rebound quantity, so that the production of integrated cold stamping parts cannot be realized; the hot stamping forming of superhigh strength auto parts mainly adopts MnB steel production at present, receives hardenability and work hardening ability's restriction, and the maximum dimension of producing at present is the door ring part, takes place local attenuate easily and leads to production failure in the thermoforming process of large-scale complicated part.

In conclusion, the integrated ultrahigh-strength automobile structural part is the development direction in the future, but the traditional automobile steel has difficulty in producing integrated cold-stamped and hot-stamped parts due to insufficient plasticity or poor hardenability and work hardening capacity. Therefore, the development of the steel for the integrated automobile part and the integrated part forming technology has important significance and practical value.

Disclosure of Invention

Aiming at the defects or shortcomings in the prior art, the invention provides medium manganese steel for integrally forming an automobile body structural part and a method.

The technical solution of the invention is as follows:

the medium manganese steel for integrally forming the automobile body structural part is characterized by comprising the following chemical components in percentage by weight: 0.05-0.20 percent of C, 3-6 percent of Mn, less than or equal to 1 percent of Si, 0-3 percent of other alloying metal elements in total, and the balance of Fe and inevitable impurities, wherein the inevitable impurities comprise N, P and S, P is less than or equal to 0.010 percent, S is less than or equal to 0.005 percent, N is less than or equal to 0.007 percent, the microstructure of the medium manganese steel structural member is quenched martensite plus a small amount of residual austenite, and in the forming process of the medium manganese steel, a dislocation strengthening mechanism in crystal grains promotes processing hardening, inhibits recrystallization and softening, promotes metal in an undeformed region to flow to a deformed region, so that the necking process in the processing process is delayed to realize integral forming of the automobile body structural member.

The other alloying metal elements include one or a combination of: cr is 0.1 to 1.2, Mo is 0.01 to 0.5, V is 0.05 to 0.2, Ti is 0.01 to 0.15, Nb is 0.01 to 0.05, and Al is 0.02 to 2.

The medium manganese steel is a steel plate, the steel plate is subjected to austenitizing and heating, then is subjected to stamping forming in a die and is quenched at the same time, the mechanical properties of the tensile strength of more than or equal to 1000MPa and the elongation after fracture of more than or equal to 10 percent are obtained, and the microstructure of the steel plate is lath martensite.

The automotive body structural member includes a base portion and a reinforcement portion integrally integrated with the base portion.

The method for integrally forming the automobile body structural part adopts the medium manganese steel for integrally forming the automobile body structural part and is characterized by comprising the following steps of: step 1, smelting; step 2, casting; step 3, rolling; step 4, annealing; step 5, opening the flat; step 6, blanking; step 7, heating the blank; and 8, stamping and forming.

The smelting adopts converter or electric furnace smelting, LF furnace refining, and VD furnace or RH furnace degassing, and the casting is continuous casting.

The rolling is hot rolling or hot rolling plus cold rolling.

The annealing is used for reducing the hardness of the steel plate and improving the flatness of the steel plate, and the flattening adopts transverse tangent flattening.

The blank heating includes heating the blank above a temperature at which complete austenitization occurs.

The press forming includes forming a steel plate in a die having a rapid cooling function while performing a quenching process of the material to form a lath martensite structure.

The invention has the following technical effects: the invention relates to medium manganese steel for integrally forming an automobile body structural part and a method, which are different from the prior art disclosed in the field in that: 1. the invention adopts a low-carbon medium-Mn alloy system, and ensures that the material has good hardenability and better weldability. 2. The microstructure is a fine martensite structure, and the ultrafine grain size ensures that the steel has higher strength and plastic cooperation, and the bending performance of the steel is improved, namely the collision safety is improved. 3. The material can meet the production requirements of the traditional single ultra-high-strength part, more importantly, the material can be used for producing ultra-large and ultra-thin automobile parts, and is especially suitable for automobile structural parts with complex shapes. 4. The invention belongs to the manufacturing technology of steel for integrated ultrahigh-strength automobile parts and parts, the yield strength of the parts is more than or equal to 900MPa, the tensile strength is more than or equal to 1200MPa, the elongation is more than or equal to 7 percent, and according to the performance requirements of the parts, the strength level can be reduced by means of tempering and the like, so that the plasticity is further improved.

Drawings

FIG. 1 is a schematic view of a steel one-piece part according to example A.

FIG. 2 high temperature tensile curve for example A steel.

FIG. 3 is a transmission electron micrograph of the microstructure of example A steel.

FIG. 4 photographs of resistance spot weld joints of example A steel and DP590 steel.

Detailed Description

The invention is explained below with reference to the figures (fig. 1-4) and the examples.

FIG. 1 is a schematic view of a steel one-piece part according to example A. FIG. 2 high temperature tensile curve for example A steel. FIG. 3 is a TEM micrograph of the microstructure of the steel of example A. FIG. 4 photographs of resistance spot weld joints of example A steel and DP590 steel. Referring to fig. 1 to 4, the medium manganese steel for integrally forming the automobile body structural part comprises the following chemical components in percentage by weight: 0.05-0.20 percent of C, 3-6 percent of Mn, less than or equal to 1 percent of Si, 0-3 percent of other alloying metal elements in total, and the balance of Fe and inevitable impurities, wherein the inevitable impurities comprise N, P and S, P is less than or equal to 0.010 percent, S is less than or equal to 0.005 percent, N is less than or equal to 0.007 percent, the microstructure of the medium manganese steel structural member is quenched martensite plus a small amount of residual austenite, and in the forming process of the medium manganese steel, a dislocation strengthening mechanism in crystal grains promotes processing hardening, inhibits recrystallization and softening, promotes metal in an undeformed region to flow to a deformed region, so that the necking process in the processing process is delayed to realize integral forming of the automobile body structural member. The other alloying metal elements include one or a combination of: 0.1 to 1.2% of Cr, 0.01 to 0.5% of Mo, 0.05 to 0.2% of V, 0.01 to 0.15% of Ti, 0.01 to 0.05% of Nb and 0.02 to 2% of Al. The medium manganese steel is a steel plate, the steel plate is subjected to austenitizing and heating, then is subjected to stamping forming in a die and is quenched at the same time, the mechanical properties of the tensile strength of more than or equal to 1200MPa and the elongation after fracture of more than or equal to 7 percent are obtained, and the microstructure of the steel plate is lath martensite. The automotive body structure includes a base portion and a reinforcement portion integrally integrated with the base portion.

The method for integrally forming the automobile body structural part adopts the medium manganese steel for integrally forming the automobile body structural part and is characterized by comprising the following steps of: step 1, smelting; step 2, casting; step 3, rolling; step 4, annealing; step 5, opening the flat; step 6, blanking; step 7, heating the blank; and 8, stamping and forming. The smelting adopts converter or electric furnace smelting, LF furnace refining, and VD furnace or RH furnace degassing, and the casting is continuous casting. The rolling is hot rolling or hot rolling plus cold rolling. The annealing is used for reducing the hardness of the steel plate and improving the flatness of the steel plate, and the flattening adopts transverse tangent flattening. The blank heating includes heating the blank above a temperature at which complete austenitization occurs. The press forming includes forming a steel plate in a die having a rapid cooling function while performing a quenching process of the material to form a lath martensite structure.

The invention provides medium manganese steel for integrally forming an automobile body structural part and a method. The medium manganese steel for integrally forming the automobile body structural part is characterized by comprising the following components in percentage by mass: 0.08 to 0.20 percent of C, less than or equal to 1 percent of Si, 3 to 6 percent of Mn, and the balance of Fe and inevitable impurities, and 1 or more of the following alloy elements can be added according to the performance requirements of the steel: 0.1-1.2% of Cr, 0.01-0.5% of Mo, 0.05-0.2% of V, 0.01-0.15% of Ti, 0.01-0.05% of Nb, Al: 0.02% -2%. The steel plate is subjected to austenitizing and heating, then is stamped and formed in a die and is quenched, the integration and the integration of parts are realized by utilizing strong work hardening caused by a fine crystal grain dislocation strengthening mechanism of the steel, the mechanical properties of the tensile strength of more than or equal to 1200MPa and the elongation after fracture of more than or equal to 7 percent are obtained, and the microstructure of the steel plate is lath martensite.

The design principle of chemical elements in the medium manganese steel for integrally forming the automobile body structural part provided by the invention is as follows:

carbon: c plays a role in solid solution strengthening when existing in a gap solid solution state, forms carbide in the tempering process to play a role in precipitation strengthening, and stabilizes austenite through partition into an austenite phase in the structure regulation and control of the high-strength high-plasticity steel. In consideration of the strength level and weldability of automobile steel, the carbon content is controlled to be 0.08-0.20% C.

Silicon: si plays a role in solid solution strengthening in steel to improve the ferrite strength, but too high content affects the coating performance. In the invention, the silicon content is controlled to be less than or equal to 1 percent;

manganese: mn is an element for enlarging the austenite region, and plays a role in solid solution strengthening and improving the hardenability of steel. In the structure control of high-strength and high-plasticity steel, austenite is stabilized by partitioning into austenite phase. In consideration of weldability of steel, the Mn content should be controlled to 3.0% -6.0%.

Aluminum: al is used as a deoxidizer in the invention, the toughness of steel can be improved by alloying with Al element, the defects of nozzle blockage and the like are easily caused by overhigh Al content, and the Al element content is 0.02-2 percent in the invention.

Chromium: the Cr element is mainly used for improving the hardenability, precipitation strengthening and corrosion resistance of the steel, and the content of the Cr element is 0.1-1.2 percent.

Molybdenum: mo not only effectively improves the hardenability of steel, but also strengthens crystal boundary and reduces temper brittleness, and can generate nano-scale composite precipitation when being added with Ti element, and the content of the Mo element in the invention is controlled at 0.1-0.5%.

Vanadium: v can improve the hardenability of steel, can precipitate MC carbide in the steel to play a role in precipitation strengthening, and can prevent crystal grains from growing to play a role in refining the crystal grains when being reheated. In the invention, the content of the V element is controlled to be 0.05-0.2%.

Niobium: nb is a carbonitride forming element, the fine grain strengthening effect is achieved, and the content of the Nb element is controlled to be 0.01-0.05%.

Titanium: ti is a strong carbonitride forming element, forms fine dispersed carbonitride, forms a nano-scale composite precipitated phase with Mo element and the like, can play a role of refining grains in the heating process, and has the element content controlled between 0.01 and 0.15 percent.

In addition, the steel of the invention inevitably contains N, P, S-based impurity elements, wherein P is less than or equal to 0.010 percent, S is less than or equal to 0.005 percent, and N is less than or equal to 0.007 percent.

The microstructure of the medium manganese steel for integrally forming the automobile body structural part is quenched martensite and a small amount of retained austenite.

The invention relates to an integrated forming method of an automobile body structural part, which sequentially comprises the following steps: (1) smelting; (2) casting; (3) rolling; (4) annealing; (5) flattening, (6) blanking, (7) heating blank, (8) stamping and forming, and (9) post-processing.

In the method, in the step (1), the smelting method adopts converter or electric furnace smelting, LF refining and RH or VD degassing.

In the method, in the step (2), the casting mode is continuous casting or die casting.

In the step (3), the casting blank heating comprises two seed separation modes of hot charging and hot conveying or cold discharging and reheating, wherein the heating temperature is 1100-; the casting blank is hot-rolled to the specified thickness in multiple passes, the final rolling temperature is 800-; the hot rolled coil is subjected to cold rolling processing after softening annealing, the softening annealing temperature is 500-700 ℃, the annealing is carried out in a cover type annealing furnace, the cold rolling deformation is more than or equal to 50%, cold continuous rolling or reciprocating rolling can be adopted, and the thickness specifications of the cold rolled coil and the hot rolled coil are determined according to the product requirements.

In the step (4), the cold-rolled steel coil is subjected to softening annealing, so that the hardness is reduced, and the requirement of the plate shape unevenness in the flattening process is met.

In the method, in the step (5), the steel coil is cut into a set size in a decoiler.

In the method, in the step (6), blank blanking processing is performed according to the size of the part.

In the step (7), the blank for the part is heated in the heating furnace to a temperature higher than the A3 temperature of the steel so as to achieve the effect of complete austenitizing, and the heating temperature ranges from 750 ℃ to 900 ℃ according to the material components.

In the method, in the step (8), the part blank is heated in the step (7) and then is stamped and formed in a die, and a cooling medium can be introduced into the die to ensure that the surface temperature of the die is kept the same under the condition of continuous production.

In the above method, in the step (9), the part press-formed in the step (8) is subjected to surface treatment to remove surface oxidizing substances, including shot blasting, sand blasting, or the like, to obtain a metal surface.

As shown in Table 1, the inventive steel is subjected to 80-ton converter smelting, LF furnace refining and RH furnace degassing treatment, a continuous casting slab is heated at a temperature of more than 1200 ℃ for more than 2 hours, then the continuous casting slab is rolled into a 3.5mm-4.0mm coil by a hot continuous rolling method, the coil is subjected to softening annealing at a temperature of 500 ℃ and 680 ℃ in a hood-type annealing furnace, then the steel plate is cold-rolled into a cold-rolled coil with the plate thickness of 1.2-2.0mm by a 20-roll cold rolling mill, and the cold-rolled coil is flattened into the steel plate after the softening annealing.

TABLE 1 chemical composition of inventive steels

Number of	C	Si	Mn	P	S	Others
							A	0.109	0.083	4.94	0.0067	0.0012	Is free of
B	0.17	0.10	5.02	0.009	0.006	Ti：0.1，Mo：0.2

The inventive steels shown in table 1 have good hardenability, and the mechanical properties obtained by heating the test steels at 800 ℃ and cooling the test steels in air for different times by water cooling are shown in table 2, and the results show that the inventive steels can obtain high-strength and high-plasticity mechanical properties even if the quenching process is completed at a lower temperature in the process of manufacturing integrated parts. The characteristic makes the invention steel suitable for the processing of hot stamping forming parts of complex large parts.

TABLE 2 mechanical Properties of the inventive steels

The invention steel A shown in the table 1 is heated at 850 ℃, then is rapidly transferred into a die for stamping and forming, and simultaneously finishes the quenching process to prepare an integrated part (the schematic diagram is shown in figure 1), replaces the tailor-welded structure of parts such as an A column, a B column, a C column, a threshold and the like in the original structure, and has the mechanical property anatomical result that the tensile strength reaches over 1200MPa and the elongation after fracture reaches over 10 percent.

The high-temperature tensile curve of the inventive steel is shown in fig. 2, the inventive steel has larger uniform elongation and continuous work hardening capacity under the high-temperature deformation condition, and during the forming process, a dislocation strengthening mechanism in fine grains promotes work hardening to inhibit recrystallization and softening, promotes the metal in an undeformed region to flow to a deformed region, and thus postpones the necking process in the processing process, namely has complex forming capacity.

The microstructure photograph of the integrated part made of the inventive steel is shown in fig. 3, the inventive steel is subjected to hot stamping forming to obtain a lath martensite structure, and the fine grain structure can ensure that the inventive steel has higher plasticity level and cold bending deformation capability.

The invention steel has good weldability, and the pictures of the nugget of the invention steel and DP590 steel are shown in figure 4, so that the metallurgical quality of the joint is good.

Integrated ultrahigh-strength automobile part steel and part manufacturing technology are used for preparing integrated automobile body parts from steel materials, and integration of the parts is achieved.

The integrated ultrahigh-strength steel for automobile parts comprises the following chemical components in percentage by mass: 0.08 to 0.20 percent of C, less than or equal to 1 percent of Si, 3 to 6 percent of Mn, and the balance of Fe and inevitable impurities, and 1 or more of the following alloy elements can be added according to the performance requirements of the steel: 0.1-1.2% of Cr, 0.01-0.5% of Mo, 0.05-0.2% of V, 0.01-0.15% of Ti, 0.01-0.05% of Nb, Al: 0.02% -2%.

The automobile steel is in the shape of a plate, has a certain width, and is suitable for production of automobile parts.

A plurality of parts can be combined into one part, and the number of parts is reduced.

The structure member and the reinforcing member of the automobile body structure can be combined into one piece, and multiple functions of structure, local reinforcement and the like are realized.

The integral type part is a whole, and inside does not contain the attach fitting of different parts, can include the steel sheet width not enough and the raw materials steel sheet concatenation that leads to.

The anatomical mechanical property of the part can realize that the tensile strength is more than or equal to 1000MPa and the elongation after fracture is more than or equal to 7 percent.

The manufacturing technology of the steel and the parts for the integrated ultrahigh-strength automobile parts sequentially comprises the following steps: (1) smelting; (2) casting; (3) rolling; (4) annealing; (5) flattening, (6) blanking, (7) heating blank, (8) stamping and forming, and (9) post-processing.

The smelting process comprises the following steps: converter or electric furnace smelting, LF furnace refining, VD or RH furnace degassing.

The casting mode is continuous casting.

The rolling mode is hot rolling or hot rolling and cold rolling.

The annealing aims to reduce the hardness of the steel plate and improve the flatness of the steel plate, and the open-flat trial production adopts the transverse tangent open-flat.

The blank heating process is to heat the blank to a temperature higher than the temperature of complete austenitization.

The stamping and forming process refers to the process of forming the steel plate in a die capable of realizing rapid cooling and simultaneously realizing the quenching of the material.

The final microstructure of the part is lath martensite.

Those skilled in the art will appreciate that the invention may be practiced without these specific details. It is pointed out here that the above description is helpful for the person skilled in the art to understand the invention, but does not limit the scope of protection of the invention. Any such equivalents, modifications and/or omissions as may be made without departing from the spirit and scope of the invention may be resorted to.

Claims (10)

1. The medium manganese steel for integrally forming the automobile body structural part is characterized by comprising the following chemical components in percentage by weight: the steel is characterized in that C is 0.05-0.20, Mn is 3-6, Si is less than or equal to 1, the total amount of other alloying metal elements is 0-3, and the balance is Fe and inevitable impurities, wherein the inevitable impurities comprise N, P and S, P is less than or equal to 0.010, S is less than or equal to 0.005, N is less than or equal to 0.007, the microstructure of the medium manganese steel structural member is quenched martensite and retained austenite, and in the forming process of the medium manganese steel, a dislocation strengthening mechanism in crystal grains promotes processing hardening, inhibits recrystallization and softening, promotes metal in an undeformed region to flow to a deformed region, so that the necking process in the processing is delayed to realize integral forming of the automobile body structural member.

2. The medium manganese steel for integral forming of automotive body structural parts according to claim 1, wherein the other alloying metal elements comprise one or a combination of the following: 0.1 to 1.2% of Cr, 0.01 to 0.5% of Mo, 0.05 to 0.2% of V, 0.01 to 0.15% of Ti, 0.01 to 0.05% of Nb and 0.02 to 2% of Al.

3. The medium manganese steel for integrally forming the automobile body structural part according to claim 1, wherein the medium manganese steel is a steel plate, the steel plate is subjected to austenitizing and heating, then is subjected to stamping forming in a die and is quenched, mechanical properties of tensile strength of not less than 1000MPa and elongation after fracture of not less than 7% are obtained, and the microstructure of the medium manganese steel is lath martensite.

4. The medium manganese steel for integral forming of automotive body structural member according to claim 1, wherein said automotive body structural member comprises a base portion and a reinforcing portion integrally integrated with said base portion.

5. The method for integrally forming the structural member of the automobile body adopts the medium manganese steel for integrally forming the structural member of the automobile body as claimed in claim 1, and is characterized by comprising the following steps of: step 1, smelting; step 2, casting; step 3, rolling; step 4, annealing; step 5, opening the flat; step 6, blanking; step 7, heating the blank; and 8, stamping and forming.

6. The method for integrally forming the structural member of the automobile body according to claim 5, wherein the smelting is converter or electric furnace smelting, LF furnace refining, and VD furnace or RH furnace degassing, and the casting is continuous casting.

7. The method for integrally forming the structural member of the automobile body according to claim 5, wherein the rolling is hot rolling, or hot rolling plus cold rolling.

8. The method of claim 5, wherein the annealing is performed to reduce the hardness and improve the flatness of the steel sheet, and the flattening is performed by transverse tangent flattening.

9. The method of integrally forming an automotive body structure as described in claim 5 wherein said blank heating comprises heating the blank above a temperature at which it is fully austenitized.

10. The method as claimed in claim 5, wherein the press forming includes forming a steel plate in a die having a rapid cooling function while performing a quenching process of the material to form a lath martensite structure.

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