CN110055466B

CN110055466B - Preparation method of hot-rolled high-strength medium manganese steel with strength-elongation product of more than 30GPa%

Info

Publication number: CN110055466B
Application number: CN201910423071.5A
Authority: CN
Inventors: 潘红波; 魏海莲; 沈晓辉; 闫军; 史娜; 刘永刚; 王会廷; 詹华; 肖洋洋; 章静; 曹杰; 刘伟明
Original assignee: Anhui University of Technology AHUT
Current assignee: Anhui University of Technology AHUT
Priority date: 2019-05-21
Filing date: 2019-05-21
Publication date: 2020-10-20
Anticipated expiration: 2039-05-21
Also published as: CN110055466A

Abstract

The invention discloses a preparation method of hot-rolled high-strength medium manganese steel with a strength-elongation product of more than 30GPa%, belonging to the technical field of production of high-strength steel for automobiles. Firstly, smelting and continuous casting are carried out, then hot rolling is carried out, the hot rolling adopts an ESP thin slab endless rolling technology, a continuous casting slab is subjected to rough rolling by a 3-5 machine frame, the rough rolling is carried out in an austenite recrystallization region, then the rough rolling is carried out by a 5 machine frame, the finish rolling is carried out in a non-recrystallization region and a two-phase region, the rolling is carried out until the thickness is 0.8-2.5mm, and a rolled steel coil is coiled at 600 ℃; then annealing, adopting a bell-type furnace for annealing treatment, wherein the annealing temperature is 600-640 ℃, and preserving heat for 10 hours. The method can obtain fine equiaxed ferrite, lath martensite and austenite composite tissues, the prepared hot-rolled high-strength medium manganese steel has excellent performances of high tensile strength, low yield ratio, continuous yield, high strength-elongation product and the like, the problems of long production flow, high cost, large energy consumption and the like of the traditional thin medium manganese steel cold rolling are solved, and the production efficiency is obviously improved.

Description

Preparation method of hot-rolled high-strength medium manganese steel with strength-elongation product of more than 30GPa%

The technical field is as follows:

the invention belongs to the technical field of production of high-strength steel for automobiles, and particularly relates to a preparation method of hot-rolled high-strength medium manganese steel with a strength-elongation product of more than 30 GPa%.

Background art:

with the development of the automobile industry, the increase of automobile holding capacity and the implementation of environmental regulations, the important directions for the development of automobiles in the future are weight reduction, energy conservation, emission reduction and safety improvement. Practice shows that one of the methods for realizing the most effective lightweight, high reliability and high cost performance of the automobile is to improve the application proportion of the advanced high-strength steel and the ultrahigh-strength steel of the automobile and realize the lightweight of the automobile body by methods of thinning the structure, optimizing the structure of parts and the like. However, as the strength of the traditional high-strength steel is increased, the elongation, formability and weldability are gradually reduced, and the requirement of the development of the automobile industry on raw materials is hardly met. Therefore, the development of the future steel for automobiles will be towards the development of high strength, high plasticity, low cost, easy processing and the like. The third generation of advanced high-strength steel for automobiles draws wide attention in the automobile industry and the metallurgical industry due to the integration of the advantages of low cost and strong manufacturability of the first generation of advanced high-strength steel for automobiles, good formability and high product of strength and elongation of the second generation of advanced high-strength steel for automobiles, and the like.

In recent years, medium manganese steel has gained much attention in the industry as one of the representative steel grades of advanced high strength steel for third generation automobiles. However, due to the high hardenability of the medium manganese steel, a full martensite structure is obtained during hot rolling, so that the cold rolling load and energy consumption of the medium manganese steel are large, intermediate annealing softening treatment is often required in the rolling process (such as Chinese patents CN109680130A, CN105648317A, CN106119493A, CN107858586A, CN108330406A, CN106086640A, CN108330402A, CN104630641A, CN104651734A, CN105648317A, CN103103438A and CN103695765A), and the working procedure, energy consumption and cost are increased. Although the casting and rolling production processes of medium manganese steel (such as Chinese patents CN107794453A, CN108546881A, CN108655354A, CN106756528A and CN107794452A) appear in recent years, the compression ratio is small in the casting and rolling production process, the cast structure cannot be improved well, and the subsequent forming such as stamping, cold bending and the like is not favorable. The ESP can produce extremely thin strip steel (such as 0.8mm) by hot rolling, and can replace cold-rolled plates by hot rolling without cold rolling, thereby not only shortening the working procedures, reducing the cost, improving the yield, improving the uniformity of material structures and the comprehensive performance, but also obviously reducing the energy consumption, the environmental pollution and the construction investment.

The invention content is as follows:

the invention aims to provide a preparation method of hot-rolled high-strength medium manganese steel with the strength-plasticity product of more than 30GPa%, which prepares the medium manganese steel with thin specification by continuous casting and rolling of a thin slab, and solves the problem that the medium manganese steel with thin specification cannot be produced by the traditional hot rolling; the manganese steel in cold rolling is difficult to deform in cold rolling, needs intermediate annealing softening treatment and is easy to have the problem of wrinkling of the surface of a part caused by a Luders belt; and the problem of coarse structure under the thin strip casting rolling reduction ratio. And solves the problems of long production flow, high cost, large energy consumption and the like of the cold-rolled medium manganese steel. The method finally obtains fine equiaxed ferrite, lath martensite and austenite composite tissues, and the prepared hot-rolled high-strength medium manganese steel has the yield strength of more than or equal to 450MPa, the tensile strength of more than or equal to 980MPa, the elongation of more than or equal to 30 percent and the yield ratio of less than or equal to 0.50; the tensile stress-strain curve of the prepared high-strength medium manganese steel is continuous yield without the phenomenon of a Luders band.

The preparation method of the hot-rolled high-strength medium manganese steel with the strength-elongation product of more than 30GPa% provided by the invention comprises the following specific steps:

(1) smelting molten steel, namely smelting the molten steel by adopting a conventional converter or an electric arc furnace, then refining by adopting LF and RH, wherein the refining mainly removes S, O, H and other gas elements in the steel, and the components in percentage by mass are as follows: c: 0.18 to 0.22%, Si: 0: 10-0.30%, Mn: 4.80-5.20%, Al: 0.02-0.05%, less than or equal to 0.020% of P, less than or equal to 0.0030% of S, and the balance of Fe and inevitable residual impurity elements.

(2) Continuous casting, wherein full-protection casting and electromagnetic stirring are adopted in the continuous casting, the thickness of a continuous casting blank is 70-90 mm, and a continuously cast slab is directly subjected to hot rolling;

(3) hot rolling, namely ESP rolling is adopted in hot rolling, rough rolling is carried out in an austenite recrystallization region, 3-5 times of high reduction rolling is adopted in the rough rolling stage, the thickness of an intermediate blank is 8-20 mm, finish rolling is carried out in an austenite non-recrystallization region and a two-phase region, 5 times of rolling is adopted in the finish rolling stage, the finish rolling temperature is 620-680 ℃, the thickness is 0.8-2.5mm, and a rolled steel coil is coiled at 600 ℃;

(4) acid washing, namely using push-pull acid washing, and carrying out acid washing on hot-rolled strip steel by a hydrochloric acid tank to remove surface iron oxide scales;

(5) annealing, namely adopting an all-hydrogen hood-type annealing furnace, heating along with the furnace and cooling along with the furnace in the annealing process, wherein the annealing soaking temperature is 600-640 ℃, and the temperature equalizing time is 10 hours.

The design idea of the invention is as follows: the method adopts the chemical components of the traditional medium manganese steel, does not need to add other alloy elements, is easy to carry out smelting production, and has low alloy cost and economy. In the hot rolling process, low-temperature rolling is adopted, and a certain amount of fine equiaxed ferrite is generated in the structure in a mode of inducing ferrite transformation through deformation, so that on one hand, the ferrite improves the plasticity of the steel, and on the other hand, alloy elements in the ferrite are diffused into austenite, the stability of the untransformed austenite is improved, the content of the austenite is increased, and the plasticity of the steel is further improved. In the invention, the alloy elements for stabilizing the austenite are diffused twice, and one of the two effects is that the alloy elements in the ferrite are favorably diffused to the unconverted austenite by adopting higher-temperature coiling after hot rolling, so that the stability of the austenite is improved; the other is that the alloy elements in the lath martensite are transformed to austenite and part of the martensite is decomposed to be transformed to austenite in the critical cover annealing process of the two-phase region.

The influence of the preparation process of the invention on the product of the invention is as follows:

smelting and refining treatment: the method aims to ensure the basic component requirements of molten steel, remove harmful gases such as oxygen and hydrogen and harmful impurities such as P, S in the steel, reduce inclusions in the steel, prevent cold brittleness and hydrogen induced crack sensitivity, and add necessary alloy elements such as carbon, manganese, silicon and the like to adjust the alloy elements.

Continuous casting: the continuous casting adopts full-protection pouring and electromagnetic stirring to reduce inclusions in steel, prevent Mn segregation and improve the quality of casting blanks; and directly carrying out hot rolling on the continuously cast slab.

Hot rolling: the hot rolling adopts ESP rolling, the rough rolling is carried out in an austenite recrystallization region, the rough rolling stage adopts 3-5 times of high reduction rolling to fully recrystallize and refine an austenite structure, the thickness of an intermediate blank is 8-20 mm, the finish rolling is carried out in an austenite non-recrystallization region and a two-phase region, the finish rolling stage adopts 5 times of rolling, the finish rolling temperature is 620-680 ℃, and the deformation induction phase transformation effect is generated in the deformation process through low-temperature deformation, so that a certain amount of fine isometric ferrite is formed in the structure; rolling to the thickness of 0.8-2.5mm, and coiling the rolled steel coil at 600 ℃ to ensure that alloy elements in ferrite are fully diffused into untransformed austenite to form fine equiaxed ferrite and lath martensite structures.

Acid washing: and (3) pickling, namely adopting push-pull type pickling, and pickling the hot-rolled strip steel by a hydrochloric acid tank to remove surface iron scales and improve the surface quality of the strip steel.

Annealing: the all-hydrogen hood-type annealing furnace is mainly used for improving the surface quality and the heat transfer efficiency of the strip steel and enabling the annealing to be more uniform and sufficient. Heating along with the furnace and cooling along with the furnace in the annealing process, wherein the annealing soaking temperature is 600-640 ℃, and the temperature equalizing time is 10 hours, so that the purposes of tempering lath martensite and improving plasticity are achieved; on the other hand, alloy elements in lath martensite are diffused into untransformed austenite, so that the plasticity is further improved; at the same time, partial martensite is decomposed to obtain a small amount of fine austenite structure.

The invention has the following technical characteristics:

1. the invention adopts the thin slab continuous casting and rolling ESP process technology to produce the medium manganese steel with the thickness of 0.8-2.5mm, and the limit specification of the medium manganese steel can not be realized by the traditional hot rolling and thin strip casting and rolling at present. The method can replace the traditional cold rolling process, shorten the process flow, reduce the energy consumption, improve the performance stability, the yield and the production efficiency and obviously reduce the production cost.

2. The invention adjusts and controls the structure by optimizing the hot rolling and annealing process to obtain the fine ferrite, lath martensite and austenite composite structure, ensures that the annealed steel plate has high strength and high plasticity, has the characteristics of lower yield ratio, continuous yield and the like, and effectively eliminates the phenomenon of long-length yield platform of the cold-rolled medium manganese steel, thereby obviously improving the formability of the product.

3. The invention adopts the traditional low-carbon, low-silicon and manganese alloying design, and other alloy elements are not added in the steel, thereby reducing the smelting production difficulty and the alloy cost and saving resources.

The specific implementation mode is as follows:

the preparation method of the hot-rolled high-strength medium manganese steel with the strength-elongation product of more than 30GPa% comprises the following specific steps: (1) smelting; (2) continuous casting; (3) hot rolling; (4) acid washing; (5) and (6) annealing. The molten steel is poured into continuous casting equipment from a tundish, then the continuously cast plate blank is directly rolled in a full-continuous mode, rolled to the target thickness according to a set process and coiled, and finally acid-washed and annealed in a critical area to obtain a required product.

In the examples, GB/T228-2010 Metal Material tensile test part 1, Room temperature test method, is adopted for mechanical property detection, and the gauge length of a tensile sample is 50 mm.

Example 1: the chemical composition of the steel comprises: c: 0.19%, Si: 0.16%, Mn: 5.14%, Al: 0.042%, P: 0.018%, S: 0.0021%, and the balance of Fe and inevitable residual impurity elements.

The process flow comprises the following steps: smelting → continuous casting → hot rolling → acid washing → cover annealing → finished product.

The technological parameters are as follows: the hot rolling finishing temperature is 672 ℃, and the thickness: 2.4mm, the bell-type annealing temperature is 610 ℃, and the yield strength, the tensile strength and the elongation of the obtained hot-rolled high-strength medium manganese steel product are 482.9MPa, 1092.2MPa, 32.3 percent and 35.3 GPa.

Example 2: the chemical composition of the steel comprises: c: 0.22%, Si: 0.25%, Mn: 4.87%, Al: 0.028%, P: 0.016%, S: 0.0027%, and the balance of Fe and inevitable residual impurity elements.

The technological parameters are as follows: the hot rolling finishing temperature is 653 ℃, and the thickness: 2.0mm, the bell-type annealing temperature is 630 ℃, and the yield strength of the obtained hot-rolled high-strength medium manganese steel product is 473.6MPa, the tensile strength is 1068.3MPa, the elongation is 32.3 percent, and the product of strength and elongation is 34.5 GPa.

Example 3: the chemical composition of the steel comprises: c: 0.22%, Si: 0.25%, Mn: 4.87%, Al: 0.028%, P: 0.016%, S: 0.0027%, and the balance of Fe and inevitable residual impurity elements.

The technological parameters are as follows: the hot rolling finishing temperature is 617 ℃, and the thickness: 1.8mm, the bell-type annealing temperature is 630 ℃, and the yield strength of the obtained hot-rolled high-strength medium manganese steel product is 460.8MPa, the tensile strength is 1045.6MPa, the elongation is 31.9 percent, and the product of strength and elongation is 33.4 GPa.

Example 4: the chemical composition of the steel comprises: c: 0.19%, Si: 0.16%, Mn: 5.14%, Al: 0.042%, P: 0.018%, S: 0.0021%, and the balance of Fe and inevitable residual impurity elements.

The technological parameters are as follows: the hot rolling finishing temperature is 645 ℃, and the thickness: 2.3mm, the bell-type annealing temperature is 610 ℃, and the yield strength, the tensile strength and the elongation of the obtained hot-rolled high-strength medium manganese steel product are 488.6MPa, 1107.3MPa, 30.9 percent and 34.2 GPa.

The embodiment result shows that no alloy element is added on the basis of the traditional medium manganese steel, the ESP process is adopted, the low-temperature final rolling and the full-hydrogen cover type annealing are combined, the production flow is shortened, the energy consumption is reduced, the yield and the production efficiency are improved, and the cost is obviously reduced.

Claims

1. The preparation method of the hot-rolled high-strength medium manganese steel with the strength-elongation product of more than 30GPa% is characterized by comprising the following specific steps:

(1) smelting molten steel, wherein the molten steel is smelted by adopting a conventional converter or an electric arc furnace, and then is refined by adopting LF and RH, and the molten steel comprises the following components in percentage by mass: c: 0.18 to 0.22%, Si: 0: 10-0.30%, Mn: 4.80-5.20%, Al: 0.02-0.05%, P is less than or equal to 0.020%, S is less than or equal to 0.0030%, and the balance is Fe and inevitable residual impurity elements;