CN108754319B

CN108754319B - Hot forming steel with tensile strength of more than or equal to 1800MPa produced by ESP production line and method

Info

Publication number: CN108754319B
Application number: CN201810587725.3A
Authority: CN
Inventors: 葛锐; 毛新平; 胡宽辉; 薛欢; 潘利波; 汪水泽
Original assignee: Wuhan Iron and Steel Co Ltd
Current assignee: Wuhan Iron and Steel Co Ltd
Priority date: 2018-06-08
Filing date: 2018-06-08
Publication date: 2020-08-04
Anticipated expiration: 2038-06-08
Also published as: CN108754319A

Abstract

The tensile strength of the hot forming steel produced by an ESP production line is more than or equal to 1800MPa, and the components and the weight percentage are as follows: c: 0.28-0.40%, Si: 0.15 to 0.40%, Mn: 1.40-1.60%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, Als: 0.015-0.050%, Cr is less than or equal to 0.80%, N is less than or equal to 0.005%, B: 0.002-0.005%, Mo is less than or equal to 0.50%, Nb + Ti: 0.025 to 0.090%; the production method comprises the following steps: smelting and continuously casting into a plate blank; rough rolling; soaking; performing finish rolling after conventional high-pressure descaling; laminar cooling; heating; and carrying out punch forming after acid washing. According to the invention, Nb and Ti are added compositely, elements such as Cr, B and Mo in the components are controlled, and the ESP short-flow process is adopted to produce the hot stamping forming steel with the tensile strength of 1800MPa, so that the mechanical property of the hot stamping forming steel can be ensured, the processes of repeatedly heating and uncoiling a plate coil in the production process can be reduced, the cold rolling and annealing heat treatment processes can be cancelled, and the production cost is reduced.

Description

Hot forming steel with tensile strength of more than or equal to 1800MPa produced by ESP production line and method

Technical Field

The invention relates to steel for automobile parts and a production method thereof, and particularly belongs to hot forming steel with tensile strength of more than or equal to 1800MPa produced by an ESP production line and a method thereof.

Background

With the increasing environmental problem, the light weight of the automobile becomes the core problem in the automobile manufacturing field nowadays. The high-strength steel is adopted for the light-weight design of the automobile body, and the method is the best way for meeting the light-weight requirement of the automobile and improving the collision safety. At present, however, for producing ultra-high strength steel above 1000MPa level, iron and steel enterprises must adopt specialized ultra-high strength steel equipment for production and manufacturing; meanwhile, the technical problems of difficult control of stamping and rebounding of parts, poor dimensional accuracy and the like exist in the manufacturing of automobile enterprises.

The advent of hot stamping techniques has well solved the above-mentioned difficulties. The hot stamping forming technology is that a steel plate blank with high hardenability is heated to austenitizing temperature and is kept warm for one time to be austenitized uniformly; and then, the plate material in a high-temperature state is sent into a die with a cooling system by a conveying device for stamping and forming, and meanwhile, the part is subjected to pressure maintaining quenching by the die, and finally the ultrahigh-strength stamped part is obtained. The technology can utilize the existing equipment of steel enterprises to produce the raw material of the hot stamping forming sheet steel, and then the sheet steel is processed and manufactured into the ultrahigh-strength automobile parts with the strength of more than 1500MPa grade by the hot stamping forming specialized equipment. The method has the advantages of low production organization cost, high part forming precision, good part forming performance and the like, and is a hot technology in the field of automobile part manufacturing.

At present, the production process route of 1800 MPa-level hot stamping sheet steel raw materials at home and abroad is generally as follows: molten steel smelting → continuous casting → cooling → casting blank heating → hot continuous rolling → cooling → coiling → uncoiling → acid pickling → cold rolling → coiling → uncoiling → heating → annealing → cooling → coiling. In the above production process, the raw material is repeatedly subjected to uncoiling and coiling, heating and cooling treatments in the processes of hot rolling, cold rolling, annealing and the like, and the production cost of the repeated processes inevitably leads to high price of the produced final thin steel sheet product.

For example, in chinese patent application No. CN 201310258918.1, a hot-forming steel with tensile strength of 1700MPa grade is developed, which comprises the following components by weight percent: c: 0.27 to 0.50%, Si: 0.28 to 0.50%, Mn: 1.20-1.60%, Cr: 0.32-0.60%, Ti: 0.025-0.055%, B: 0.002-0.005%, Als: 0.010-0.06%, P is less than or equal to 0.012%, Nb: 0.010-0.035% or Mo: 0.30-0.45% or less than 0.50% of the mixture of the two, less than or equal to 0.008% of S, less than or equal to 0.005% of N and less than or equal to 0.005% of O; the production method comprises the steps of molten iron desulphurization, converter smelting and casting blank; heating a casting blank, rough rolling, finish rolling, laminar cooling, coiling, acid pickling and cold rolling, annealing, leveling, finishing and shearing, heating under a protective atmosphere, forming, quenching, and low-temperature tempering for later use. The document combines the traditional production flow to produce 1700 MPa-grade hot forming steel, and has complex production process and high energy consumption.

The Chinese patent application No. CN 201510790931.0 discloses a thin steel plate for hot stamping forming with tensile strength of 1500MPa and a CSP production method thereof, and the components and the weight percentage content are as follows: c: 0.20 to 0.25%, Si: 0.25 to 0.40%, Mn: 1.00-1.30%, P is less than or equal to 0.025%, S is less than or equal to 0.01%, Als: 0.015-0.035%, N is less than or equal to 0.005%, B is less than or equal to 0.005%, Nb: 0.020 to 0.060%, Ti: 0.010-0.040%, and the balance of iron and inevitable trace elements. The production process route comprises the following steps: smelting molten steel, continuously casting vertical bending type CSP sheet billet, descaling, soaking in a roller hearth type tunnel furnace, hot continuous rolling with a TMCP six-stand, laminar cooling, reeling, uncoiling, pickling, leveling and reeling. The method of the invention cancels the working procedures of plate cold rolling and annealing heat treatment, and replaces the traditional cold-rolled sheet product with the hot-rolled sheet product, thereby reducing the comprehensive production cost. Also a document with Chinese patent application No. CN 201610713629.X discloses a thin hot-formed steel with tensile strength more than or equal to 1700MPa directly rolled by a thin slab and a production method thereof, and the components and the weight percentage are as follows: 0.26-0.30% of C, Si: 0.31 to 0.35%, Mn: 1.3-1.5%, P is less than or equal to 0.008%, S is less than or equal to 0.005%, Als: 0.015 to 0.060%, Cr: 0.31 to 0.35%, Ti: 0.031-0.035% or Nb: 0.031-0.035% or V: 0.031-0.035%, or a mixture of two or more thereof at any ratio, B: 0.003-0.004%, Mo: 0.20 to 0.25%, Ni: 0.06-0.10% and N is less than or equal to 0.005%. The production steps are as follows: desulfurizing molten iron; smelting and refining in an electric furnace or a converter; continuous casting; descaling before entering a soaking pit; soaking; heating; descaling by high-pressure water before entering a rolling mill; rolling; cooling; coiling; austenitizing; stamping and forming by using a die; and (6) quenching. The two documents adopt a CSP short-flow production process to produce the hot-formed steel, and have the advantages of low energy consumption and low production cost compared with the traditional production flow. However, only thin steel sheets having a thickness of 1.2mm or more can be produced, and the manufacturing demand for automobile parts requiring further weight reduction cannot be satisfied. The steel plate with the thickness of more than 1.2mm can be produced, and the thin steel plate with the thickness of 0.7 to not more than 1.2mm can be produced, and the steel plate is stable in production. Can effectively supplement the deficiency of the prior art in the aspect of product thickness specification; in addition, the ESP thin slab continuous casting and rolling technology is adopted to fully utilize the high-temperature energy of the continuous casting slab and timely perform production and rolling on the high-temperature casting slab on the basis of the existing short-flow production technology, so that a lower rolling force is realized to generate a larger reduction, the production consumption can be further reduced, and the material production cost can be reduced.

Chinese patent application No. CN 201610766506.2 discloses a method for producing DP600 steel based on an ESP thin slab continuous casting and rolling process, which comprises the following raw materials, by mass, 0.045-0.10% of C, 0.20-0.6% of Si, 1.2-2.0% of Mn, 0.1-1.0% of Cr, less than or equal to 0.05% of Nb, less than or equal to 0.05% of S, less than or equal to 0.015% of P, and the balance of iron, wherein the raw materials are subjected to converter smelting and L F furnace smelting in sequence, molten steel smelted from a L F furnace is subjected to ESP production line to generate hot rolled strip steel with different thicknesses, wherein the temperature of a finish rolling outlet in an ESP production line is not lower than 820 ℃, the proportion of ferrite and martensite in the ESP hot rolled strip steel is determined in sequence by secondary cooling of the hot rolled strip steel, and the tensile strength produced by the document is 600MPa bidirectional steel which is the highest-strength product produced based on the existing production line, and the ESP production line has lower tensile strength and cannot meet the requirements of automobiles on high-end of automobiles.

Disclosure of Invention

The invention aims to overcome the defects of complex process, high energy consumption and low production efficiency in the prior art, and provides the hot forming steel with the tensile strength of more than or equal to 1800MPa produced by adopting an ESP production line and the method, which can reduce production links, reduce energy consumption and shorten production period on the premise of ensuring the strength level.

The measures for realizing the aim are as follows:

the hot forming steel with the tensile strength of more than or equal to 1800MPa produced by an ESP production line comprises the following components in percentage by weight: c: 0.28-0.40%, Si: 0.15 to 0.40%, Mn: 1.40-1.60%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, Als: 0.015-0.050%, Cr is less than or equal to 0.80%, N is less than or equal to 0.005%, B: 0.002-0.005%, Mo is less than or equal to 0.50%, Nb + Ti: 0.025 to 0.090 percent, and the balance being Fe and inevitable impurities; neither Nb nor Ti is zero.

Preferably: the weight percentage content of Nb and Ti is 0.025-0.090%; the two elements are added arbitrarily within the control range.

The method for producing the hot forming steel with the tensile strength of more than or equal to 1800MPa by adopting an ESP production line comprises the following steps:

1) smelting and continuously casting into a plate blank;

2) rough rolling is carried out, and the single-pass reduction rate is controlled to be not lower than 60%;

3) carrying out soaking treatment, and controlling the soaking temperature to be 1200-1250 ℃;

4) carrying out finish rolling after conventional high-pressure descaling, wherein the finish rolling temperature is 815-865 ℃;

5) carrying out laminar cooling to 575-650 ℃;

6) heating at 875-950 ℃, and preserving heat for 3-10 min at the temperature;

7) and (3) carrying out acid washing and punch forming, controlling the quenching speed in the die to be not lower than 20 ℃/S, and cooling to be below 180 ℃.

It is characterized in that: when the punch forming is not needed in time, coiling is carried out after laminar cooling, and then natural cooling is carried out to the room temperature; if the stamping molding is needed, the stamping molding is carried out according to the steps 6) and 7).

Preferably: the heating temperature is 900-930 ℃.

The action and mechanism of each element and the main process in the invention are as follows:

c: carbon is an essential element in steel and is also the most economical and effective strengthening element. The carbon content is designed to be lower, and the strength is reduced after hot stamping forming; however, too high a carbon content lowers the plasticity of the steel and is disadvantageous in weldability. Therefore, in the consideration of economy and comprehensive performance, the control range of the percentage content of carbon is 0.28-0.40%.

Si: silicon is a solid-solution strengthening element, and is solid-dissolved in ferrite, which is advantageous for improving the strength of the base material and the hot-stamped material. With the increase of the silicon content, the strength of the steel is obviously improved, the plasticity is obviously reduced, and the welding performance is reduced. Therefore, the silicon content is controlled within the range of 0.15 to 0.40%.

Mn: manganese has a solid solution strengthening effect and is one of important elements for improving the strength of the material; however, too high manganese content tends to adversely affect weldability. Therefore, the upper limit of manganese is set to 1.60%, and the content of manganese added in the invention is 1.40-1.60%.

P: phosphorus is a harmful element in steel and is easy to cause center segregation of a casting blank. The steel is easy to be deviated to a grain boundary in the subsequent hot continuous rolling heating process, so that the brittleness of the steel is obviously increased. Meanwhile, the content is controlled to be below 0.01 percent based on the consideration of cost and without influencing the performance of steel.

S sulfur is a very harmful element. Sulfur in steel often exists in the form of sulfide of manganese, and this sulfide inclusion deteriorates toughness of steel and causes anisotropy of properties, so that the lower the sulfur content in steel, the better. The sulfur content in steel is controlled to 0.01% or less in consideration of the manufacturing cost.

And Als: aluminum is added for deoxidation, and when the content of Als is less than 0.015%, the effect thereof cannot be exerted; on the other hand, since addition of a large amount of aluminum easily forms alumina agglomerates, the aluminum content is controlled within a range of 0.015 to 0.050%.

Cr is an important element for improving the hardenability of steel, is dissolved into austenite to improve the stability of the austenite, and is beneficial to improving the hardenability of the steel to obtain a martensite structure; meanwhile, the chromium can improve the tempering stability of the steel. And the chromium content is more than 0.80%, so that the hardenability is improved. Therefore, the chromium content in the present invention is controlled to 0.80% or less.

N can improve the strength of the steel; however, the bonding force of nitrogen with niobium and titanium is strong, and coarse niobium nitride and titanium nitride particles can be formed in the steel at high temperature, so that the plasticity and toughness of the steel are seriously damaged; in addition, higher nitrogen content increases the amount of micro-alloying elements needed to stabilize the nitrogen element, thereby increasing costs. Therefore, the content of nitrogen should be reduced as much as possible, and nitrogen is controlled to 0.005% or less in the present invention.

B, boron is an element for strongly improving the hardenability, and the hardenability can be improved by times only by adding trace amount of boron, so that other expensive metal elements are saved. The hardenability of the steel can be obviously improved by adding trace boron element into the steel. However, the boron content is less than 0.002% or more than 0.005%, and the effect of improving hardenability is not obvious. Therefore, the boron content in the invention is controlled within the range of 0.002-0.005%.

Mo: molybdenum is an element for improving hardenability, is dissolved in austenite in a solid manner to improve the stability of the austenite, and is beneficial to improving the hardenability of steel to obtain a martensite structure; and the hardenability is more than surplus after the content of the molybdenum exceeds 0.5 percent, and the cost is higher. Therefore, the content of molybdenum in the invention is controlled below 0.5%.

Nb + Ti: : niobium and titanium are strong C, N compound-forming elements. A certain amount of niobium carbon and nitride can be formed by adding a small amount of niobium into the steel, so that austenite grains are prevented from growing and being refined, and the strength and the toughness of the steel after hot forming and quenching are greatly improved; the purpose of adding a small amount of titanium to the steel is to fix the N element in the steel and avoid the combination of B and N. However, excessive amounts of niobium and titanium combine with C to form coarse carbonitrides, thereby reducing the hardness and strength of the martensite after quenching of the test steel. Therefore, the total content is controlled to be in the range of 0.025 to 0.090%.

The controlled rough rolling single-pass reduction rate is not lower than 60 percent, and the process can effectively roll a continuous casting blank with the thickness of 80-120 mm into an intermediate plate blank with the thickness of 8-20 mm through a rough rolling mill set, so as to provide a medium-thickness blank raw material with the required thickness for subsequent finish rolling; in addition, the large rough rolling reduction can convert the austenite structure of a coarse continuous casting billet into a flat austenite structure, and the flat austenite structure is converted into a fine austenite structure through subsequent heating recrystallization, so that the grain size of austenite is refined.

The heating temperature is controlled to be 875-950 ℃, and the high-strength steel developed by the invention is kept at the temperature for 3-10 min, because the austenite transformation final temperature Ac3 of all tissues of the high-strength steel is 875 ℃. The heating temperature is not lower than 875 ℃ to realize that the material is 100% austenite before hot stamping forming; however, too high temperature easily causes coarse austenite grains, and simultaneously causes the strength of the heated sheet to be low and easy to deform, thus being inconvenient for sheet transmission and accurate positioning in the hot stamping forming process. The heat preservation time is less than 3min, and complete austenitization of the plate cannot be realized; however, the heat preservation time is too long, which easily causes the austenite grains to be coarse and the strength to be reduced. Therefore, the heating temperature is controlled to be 875-950 ℃, and the temperature is kept for 3-10 min at the temperature.

The present invention controls the quenching speed in the die to not less than 20 ℃/S because the steel sheet developed by the present invention is converted into a critical cooling speed of 100% martensite during the quenching treatment, otherwise it cannot be sufficiently quenched to achieve the required strength.

Compared with the prior art, the invention adds Nb and Ti compositely and controls elements such as Cr, B, Mo and the like in the components,

and the ESP short-flow process is adopted to produce the hot stamping forming steel with the tensile strength of 1800MPa, so that the mechanical property of the steel can be ensured, the processes of repeatedly heating, uncoiling, coiling and the like of the plate coil in the production process can be reduced, the cold rolling and annealing heat treatment processes can be cancelled, and the production cost is reduced.

Drawings

FIG. 1 shows a typical metallographic structure of a steel sheet according to the invention: ferrite + pearlite;

FIG. 2 is a typical metallographic structure of a steel sheet of the invention after quenching: martensite.

Detailed Description

The present invention is described in detail below:

table 1 is a list of chemical compositions for each example of the present invention and comparative example;

table 2 is a table of the main process parameters of each example of the present invention and comparative example;

table 3 is a table of the results of the performance tests of the examples of the present invention and the comparative examples.

The production of each embodiment of the invention is carried out according to the following steps:

1) smelting and continuously casting into sheet billets;

4) carrying out finish rolling after conventional high-pressure descaling, and controlling the finish rolling temperature to be 815-865 ℃;

5) carrying out laminar cooling, and cooling to 575-650 ℃ at a cooling speed of 10-20 ℃/S;

6) heating at 875-950 ℃, and preserving heat for 3-10 min at the temperature;

Table 1 chemical composition (wt.%) of each embodiment of the present invention

TABLE 2 tabulation of values of main process parameters for each example of the invention and comparative example

TABLE 3 results of testing the properties of the inventive and comparative examples

As can be seen from Table 3, the ESP short-flow rolling process realizes that the tensile strength of the steel reaches more than 1800MPa, and meanwhile, the strength of the steel is far higher than that of the product of the existing ESP production line, so that the ESP short-flow rolling process has important significance for promoting the lightweight horizontal lifting of automobiles.

The present embodiments are merely preferred examples, and are not intended to limit the scope of the present invention.

Claims

1. The hot forming steel with the tensile strength of more than or equal to 1800MPa produced by an ESP production line comprises the following components in percentage by weight: c: 0.36%, Si: 0.28%, Mn: 1.44%, P: 0.005%, S: 0.002%, Als: 0.033%, Cr:0.35%, N: 0.002%, B: 0.0032%, Mo: 0.45%, Nb + Ti: 0.027%, the balance being Fe and unavoidable impurities; nb or Ti are not zero;

the method for producing the hot forming steel with the tensile strength of more than or equal to 1800MPa by adopting the ESP production line comprises the following steps:

1) smelting and continuously casting into sheet billets;

2) carrying out rough rolling, and controlling the single-pass reduction rate to be 64%;

3) carrying out soaking treatment, and controlling the soaking temperature to be 1224-1238 ℃;

4) carrying out finish rolling after conventional high-pressure descaling, and controlling the finish rolling temperature to be 821-835 ℃;

5) carrying out laminar cooling to 603-615 ℃;

6) heating at 930 deg.C for 5 min;

7) and carrying out punch forming after acid cleaning, wherein the quenching speed in the die is controlled to be 22 ℃/S.

2. The hot forming steel with the tensile strength of more than or equal to 1800MPa produced by an ESP production line comprises the following components in percentage by weight: c: 0.29%, Si: 0.33%, Mn: 1.60%, P: 0.002%, S: 0.004%, Als: 0.021%, Cr:0.51%, N: 0.04%, B: 0.0027%, Mo: 0.28%, Nb + Ti: 0.039% and the balance of Fe and inevitable impurities; nb or Ti are not zero;

1) smelting and continuously casting into sheet billets;

2) carrying out rough rolling, and controlling the single-pass reduction rate to be 68%;

3) carrying out soaking treatment, and controlling the soaking temperature to be 1231-1245 ℃;

4) carrying out finish rolling after conventional high-pressure descaling, and controlling the finish rolling temperature to be 818-833 ℃;

5) carrying out laminar cooling to 578-593 ℃;

6) heating at 950 deg.C for 3 min;

7) and carrying out punch forming after acid cleaning, wherein the quenching speed in the die is controlled to be 35 ℃/S.

3. The hot forming steel with the tensile strength of more than or equal to 1800MPa produced by an ESP production line comprises the following components in percentage by weight: c: 0.32%, Si: 0.16%, Mn: 1.55%, P: 0.004%, S: 0.006%, Als: 0.025%, Cr:0.27%, N: 0.002%, B: 0.0035%, Mo: 0.37%, Nb + Ti: 0.039% and the balance of Fe and inevitable impurities; nb or Ti are not zero;

1) smelting and continuously casting into sheet billets;

2) carrying out rough rolling, and controlling the single-pass reduction rate to be 65%;

3) carrying out soaking treatment, and controlling the soaking temperature to be 1225-1241 ℃;

4) carrying out finish rolling after conventional high-pressure descaling, and controlling the finish rolling temperature to be 835-849 ℃;

5) carrying out laminar cooling to 611-623 ℃;

6) heating at 880 deg.C for 10 min;

7) and carrying out punch forming after acid cleaning, wherein the quenching speed in the die is controlled to be 50 ℃/S.

4. The hot forming steel with the tensile strength of more than or equal to 1800MPa produced by an ESP production line comprises the following components in percentage by weight: c: 0.30%, Si: 0.26%, Mn: 1.52%, P: 0.003%, S: 0.005%, Als: 0.028%, Cr:0.38%, N: 0.003%, B: 0.0033%, Mo: 0.19%, Nb + Ti: 0.044 percent, and the balance of Fe and inevitable impurities; nb or Ti are not zero;

1) smelting and continuously casting into sheet billets;

2) carrying out rough rolling, and controlling the single-pass reduction rate to be 62%;

3) carrying out soaking treatment, and controlling the soaking temperature to be 1218-1230 ℃;

4) carrying out finish rolling after conventional high-pressure descaling, and controlling the finish rolling temperature to be 838-850 ℃;

5) carrying out laminar cooling to 625-638 ℃;

6) heating at 920 deg.C for 8 min;

7) and carrying out punch forming after acid cleaning, wherein the quenching speed in the die is controlled to be 40 ℃/S.

5. The hot forming steel with the tensile strength of more than or equal to 1800MPa produced by an ESP production line comprises the following components in percentage by weight: c: 0.35%, Si: 0.35%, Mn: 1.47%, P: 0.005%, S: 0.007%, Als: 0.033%, Cr:0.52%, N: 0.002%, B: 0.041%, Mo: 0.41%, Nb + Ti: 0.053 percent of Fe and inevitable impurities in balance; nb or Ti are not zero;

1) smelting and continuously casting into sheet billets;

2) carrying out rough rolling, and controlling the single-pass reduction rate to be 66%;

3) carrying out soaking treatment, and controlling the soaking temperature to be 1227-1239 ℃;

4) carrying out finish rolling after conventional high-pressure descaling, and controlling the finish rolling temperature to 827-839 ℃;

5) carrying out laminar cooling to 607-620 ℃;

6) heating at 900 deg.C for 7 min;

7) and carrying out punch forming after acid cleaning, wherein the quenching speed in the die is controlled to be 63 ℃/S.

6. The hot forming steel with the tensile strength of more than or equal to 1800MPa produced by an ESP production line comprises the following components in percentage by weight: c: 0.31%, Si: 0.24%, Mn: 1.51%, P: 0.008%, S: 0.003%, Als: 0.019%, Cr:0.77%, N: 0.002%, B: 0.035%, Mo: 0%, Nb + Ti: 0.067 percent, and the balance of Fe and inevitable impurities; nb or Ti are not zero;

1) smelting and continuously casting into sheet billets;

2) carrying out rough rolling, and controlling the single-pass reduction rate to be 63%;

3) carrying out soaking treatment, and controlling the soaking temperature to be 1232-1246 ℃;

4) carrying out finish rolling after conventional high-pressure descaling, and controlling the finish rolling temperature to be 831-844 ℃;

5) carrying out laminar cooling to 633-645 ℃;

6) heating at 935 deg.C for 6 min;

7) and carrying out punch forming after acid cleaning, wherein the quenching speed in the die is controlled to be 45 ℃/S.