CN113943896B - Profiled bar steel with tensile strength of more than or equal to 1200MPa and production method thereof - Google Patents

Abstract

The steel for the profiled bar with the tensile strength of more than or equal to 1200MPa comprises the following components in percentage by weight: 0.17 to 0.25 percent of C, 1.5 to 2.0 percent of Si, 2.4 to 2.8 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, 0.01 to 0.10 percent of Als, less than or equal to 0.005 percent of N, less than or equal to 0.05 percent of Nb and less than or equal to 0.05 percent of Ti; the process comprises the following steps: smelting and casting into a plate blank; heating a casting blank; finish rolling after rough rolling; cold rolling after acid washing; continuous annealing; quenching treatment; aging treatment; coiling and air-cooling to room temperature; and flattening, slitting and roll forming after uncoiling again. According to the invention, nb and V are added to form Nb/V (C, N) in a matrix structure, the phase plays a pinning role in a soaking stage of quenching and tempering heat treatment, growth of austenite is hindered, and formation of a refined austenite phase is promoted, so that the ultrahigh-strength steel for the section bar with excellent mechanical properties is obtained, namely the yield strength is not less than 1050MPa, the tensile strength is not less than 1200MPa, and the elongation is not less than 12%.

Description

Profiled bar steel with tensile strength of more than or equal to 1200MPa and production method thereof

Technical Field

The invention relates to ultrahigh-strength steel for solar photovoltaic supports, building profiles, automobiles and electric power facilities and a production method thereof, and particularly belongs to steel for manufacturing profiled bars with the tensile strength of more than or equal to 1200MPa and a production method thereof.

Background

In recent years, carbon peaking and carbon neutralization are actively arranged in China, and green low-carbon development is practically promoted. The steel is a base material for modern production and manufacturing, and is also the industry with the largest carbon emission in the existing manufacturing industry; how to lead the green development of the steel industry through the upstream and downstream industry cooperation has important social value. The strength and the application performance of the steel material are optimized and improved, the light weight of the member is realized, the usage amount of the steel material is effectively reduced, and the method is the best way for promoting the carbon peak reaching of the steel material.

The steel profiled bar is widely used for bearing structures of solar photovoltaic brackets, buildings, automobiles, electric power facilities and the like. Because the service working condition and the application environment of the steel are very complex, the steel material is generally required to have good strength and toughness. The steel profile can be classified into rolled (cold-rolled) profile steel, hot-rolled profile steel, welded profile steel, and the like according to the difference of the processing technology. The rolled deformed steel is one of the most widely used deformed steel processing technologies at present, and has the advantages of simple preparation technology, low continuous production cost and the like; however, the currently manufactured steel profiled bars are generally processed by Q235 or Q345 (the tensile strength of the material is less than or equal to 600 MPa), and the problems of low material strength, heavy self weight and the like exist, so that the application range of the steel profiled bars is limited.

The Chinese patent publication No. CN 111519091 discloses a processing technology of high-strength deformed steel, which is completed by the steps of pretreatment of a steel billet with specific chemical components, multiple cold-drawing forming and annealing treatment; the tensile strength of the produced section steel is 621MPa, the yield strength is 523MPa, and the elongation is not expressed.

The Chinese patent publication No. CN110343965 discloses a high-strength deformed steel and a preparation method thereof, which are processed and finished by the processes of pretreatment of a steel billet with specific chemical components, isothermal spheroidizing annealing, compression, molding, sizing, quenching and repeated tempering; the tensile strength of the produced section steel is 966-985 MPa, the yield strength is 966-985 MPa, and the elongation is not expressed.

The production raw material of the high-strength steel is a thick steel billet, repeated forming and heat treatment are needed, the process is complex, the strength is still low, and the high-strength steel is still to be researched and improved.

Disclosure of Invention

The invention aims to overcome the defects of high strength and elongation not more than 7 percent in the prior art, and provides the steel for the section and the production method thereof, wherein the steel has the tensile strength not less than 1200MPa, the yield strength not less than 1050MPa and the elongation not less than 12 percent, and meets the manufacturing requirements of the section.

The measures for realizing the aim are as follows:

the steel for the profiled bar with the tensile strength of more than or equal to 1200MPa comprises the following components in percentage by weight: 0.17 to 0.25% of C, si:1.5 to 2.0%, mn: 2.4-2.8%, P is less than or equal to 0.02%, S is less than or equal to 0.01%, als:0.01 to 0.10 percent of the total weight of the alloy, less than or equal to 0.005 percent of N, less than or equal to 0.05 percent of Nb, less than or equal to 0.05 percent of Ti, and the balance of Fe and inevitable impurities.

Preferably: the components and the weight percentage content are as follows: c:0.19 to 0.23%, si:1.7 to 1.9%, mn: 2.5-2.7%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, als:0.01 to 0.07 percent, nb is not more than 0.038 percent, and Ti is not more than 0.035 percent.

The method for producing the profiled bar steel with the tensile strength of more than or equal to 1200MPa comprises the following steps:

1) Smelting and continuously casting into a plate blank;

2) Heating a casting blank, wherein the tapping temperature is 1150-1250 ℃;

3) Carrying out finish rolling after conventional rough rolling, and controlling the finish rolling temperature to be 850-900 ℃;

4) Cold rolling to the thickness of a finished product after conventional pickling, and controlling the total cold rolling reduction to be not less than 50%;

5) Carrying out continuous annealing, controlling the annealing temperature at 880-930 ℃, and preserving the heat for 120-180 s at the temperature;

6) Quenching treatment is carried out by adopting one of nitrogen, hydrogen, water and quenching liquid, and the plate blank is rapidly cooled to 180-230 ℃ at a cooling speed of not less than 45 ℃/s;

7) Carrying out aging treatment, rapidly heating to 330-380 ℃, and carrying out aging treatment for 180-300 s;

8) Coiling and slowly cooling to room temperature;

9) And uncoiling again, and then sequentially flattening, slitting and rolling for forming.

Preferably: the annealing temperature is controlled to be 888-923 ℃.

Preferably: the cooling temperature of the quenching treatment stage is controlled to be 193-220 ℃.

Preferably: the aging treatment temperature is controlled to be 343-372 ℃.

The action and mechanism of each element and main process in the invention

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 low, and the strength of the material is obviously reduced; however, too high a carbon content lowers the plasticity of the steel and is disadvantageous in weldability. Therefore, in view of economy and comprehensive performance, the carbon percentage in the present invention is controlled within a range of 0.17 to 0.25%, and preferably within a range of 0.19 to 0.22%.

Si: silicon is the most basic element in steel and is one of the most important elements in the steel of the present invention. Si can suppress the precipitation of cementite within a certain temperature range, but has a relatively limited suppression effect on epsilon carbide. Si inhibits cementite precipitation so that carbon atoms diffuse from the martensite into the residual austenite to stabilize the residual austenite. The content of Si is not lower than 1.5 percent generally, otherwise, the function of inhibiting cementite precipitation cannot be realized; the Si content is not generally more than 2.0%, otherwise the steel sheet is easily thermally cracked during welding, which makes the application of the steel sheet difficult, so the Si content in the steel is usually controlled to 1.5-2.0%, preferably 1.7-1.9%.

Mn: manganese has a solid solution strengthening effect and is one of the most important elements in the steel of the present invention. It is known that Mn is an important element for expanding the austenite phase region, and can reduce the critical quenching rate of steel, stabilize austenite, refine grains, and delay transformation of austenite to pearlite. In the invention, in order to ensure the strength of the steel plate, the Mn content is generally controlled to be more than 2.4 percent, the Mn content is too low, and when the steel plate is subjected to air cooling in the first stage of sectional cooling, the super-cooled austenite is unstable and is easily transformed into a pearlite type structure such as sorbite and the like; meanwhile, the Mn content is generally not more than 2.8%, mn segregation is likely to occur during steel making, and heat cracking is likely to occur during slab continuous casting. Therefore, the Mn content in the steel is generally controlled to 2.4 to 2.8%, preferably in the range of 2.5 to 2.7%.

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 of the steel is controlled to be below 0.02 percent based on cost consideration and without influencing the performance of the 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 be below 0.01% in consideration of the manufacturing cost.

And Als: aluminum is added for deoxidation, and when the content of Als is less than 0.01%, 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 in the range of 0.01 to 0.10%, preferably Als:0.01 to 0.07 percent.

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.

Nb, ti: niobium, titanium are strong C, N compound-forming elements. A small amount of niobium and titanium are added into the steel to form a certain amount of niobium and titanium carbon and nitride, so that austenite grains are prevented from growing and refined, the ultrahigh strength is obtained, and the steel-plastic toughness can be improved. However, excessive amounts of niobium and titanium combine with C to form coarse carbonitrides, thereby reducing the hardness and strength of the material. Therefore, nb is defined: not more than 0.05%, preferably Nb: not more than 0.038%; defining a Ti: not more than 0.05%, preferably Ti: not more than 0.035%; and both are defined to be not zero and between 0.025% and 0.080%.

The invention controls the tapping temperature of the casting blank to 1150-1250 ℃ in order to ensure the homogenization of alloy elements of the casting blank, reduce the deformation resistance of the material and facilitate the production and rolling. If the tapping temperature is lower than the lower limit of the invention, the homogenization of alloy elements is not facilitated, the deformation resistance of the material is large, the load of a rolling mill is large, and the energy consumption is high; if the tapping temperature is higher than the limited upper limit, the surplus heat of the casting blank is more, and the manufacturing cost is increased.

The invention controls the finish rolling temperature of finish rolling to be 850-900 ℃, because the finish rolling deformation termination temperature has important influence on the structure performance of steel. The higher the deformation termination temperature is, the stronger the tendency of crystal grain to focus and grow is, the coarser the obtained austenite crystal grains are, and the lower the strength is; in order to ensure that the slab is rolled in a uniform austenite region to obtain a uniform structure and good properties, the finish rolling temperature of the finish rolling is required to be controlled to be equal to or higher than the starting temperature of ferrite transformation.

The continuous annealing temperature is controlled to be 880-930 ℃ in order to ensure that the deformed structure is completely recovered and recrystallized after cold rolling and is fully austenitized. The annealing temperature is too low, the material recovery recrystallization period is longer, and complete austenitization cannot be realized; the annealing temperature is too high, austenite grains are easy to be coarse, and the performance of the material is influenced.

The cooling speed is not lower than 45 ℃/s, and the slab is rapidly cooled to 180-230 ℃ so as to ensure that most of austenite is directly transformed into martensite, thereby obtaining higher material strength. The cooling speed is too low, and the austenite structure is easy to be cooled and transformed into other structures except martensite, thereby directly influencing the strength of the material. The temperature is too high after the cooling is finished, the martensite structure fraction obtained by cooling is lower, and the material strength is lower; the temperature is too low at the end of cooling, the fraction of martensite structure obtained by cooling is higher, and the toughness and the elongation of the material are reduced.

The invention rapidly heats the cooled plate to 330-380 ℃ for isothermal partition aging treatment, and aims to promote C element in supersaturated martensite to diffuse into untransformed austenite, improve the stability of retained austenite, and obtain 5-10% of retained austenite at room temperature, so as to ensure that the material has good toughness and plasticity.

Compared with the prior art, nb and Ti are added to form Nb/Ti (C, N) with a certain size in a matrix structure, the phase plays a pinning role in a soaking stage of an annealing and heat treatment process, growth of austenite is hindered, and a refined austenite phase is promoted to be formed, so that the ultrahigh-strength steel for the section bar with excellent mechanical properties is obtained, namely the yield strength is more than or equal to 1050MPa, the tensile strength is more than or equal to 1200MPa, and the elongation is more than or equal to 12%.

Drawings

FIG. 1 is a typical metallographic structure of an ultra-high strength steel plate according to the present invention;

description of the drawings: the metallographic structure is martensite + retained austenite.

Detailed Description

The present invention is described in detail below:

table 1 is a list of values of the components of each example and comparative example of the present invention;

table 2 shows the values of the process parameters of the examples and comparative examples of the present invention;

table 3 is a table of the results of testing the performance of each example and comparative example of the present invention.

The preparation method comprises the following steps:

1) Smelting and continuously casting into a plate blank;

2) Heating a casting blank, wherein the tapping temperature is 1150-1250 ℃;

3) Carrying out finish rolling after conventional rough rolling, and controlling the finish rolling temperature to be 850-900 ℃;

4) Cold rolling to the thickness of a finished product after conventional pickling, and controlling the total cold rolling reduction to be not less than 50%;

5) Carrying out continuous annealing, controlling the annealing temperature at 880-930 ℃, and preserving the heat for 120-180 s at the temperature;

6) Quenching treatment is carried out by adopting one of nitrogen, hydrogen, water and quenching liquid, and the plate blank is cooled to 180-230 ℃ at a cooling speed of not less than 45 ℃/s;

7) Carrying out aging treatment, rapidly heating to 330-380 ℃, wherein the aging treatment time is 180-300 s;

8) Coiling and slowly cooling to room temperature;

9) And uncoiling again, and then sequentially flattening, slitting and rolling for forming.

TABLE 1 tabulation (wt%) of chemical constituents of each example of the invention and comparative example

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

TABLE 3 Performance test results of inventive and comparative examples

As can be seen from Table 3, the ultrahigh strength cold rolled steel sheet for the section bar produced by the alloy composition design and the related process has yield strength of not less than 1050MPa, tensile strength of not less than 1200MPa and elongation (A) ₅₀ ) Not less than 12 percent, has good processing formability and weldability, and meets the use requirements of cold-rolled ultrahigh-strength steel for profiles such as photovoltaic brackets, building profiles, logistics brackets and the like.

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

Claims (1)

1. The steel for the profiled bar with the tensile strength of more than or equal to 1200MPa comprises the following components in percentage by weight: c:0.17 to 0.24%, si:1.5 to 2.0%, mn: 2.4-2.8%, P is less than or equal to 0.02%, S is less than or equal to 0.01%, als:0.01 to 0.02 percent, less than or equal to 0.002 percent of N, nb:0.019 to 0.028%, ti:0.014 to 0.019 percent, and the balance of Fe and inevitable impurities; the metallographic structure is as follows: 5-10% of retained austenite, and the balance of martensite; mechanical properties: the yield strength is more than or equal to 1050MPa, the tensile strength is more than or equal to 1200MPa, and the elongation is more than or equal to 12 percent; the production method comprises the following steps:

1) Smelting and continuously casting into a plate blank;

2) Heating a casting blank, wherein the tapping temperature is 1150-1250 ℃;

3) Carrying out finish rolling after conventional rough rolling, and controlling the finish rolling temperature to be 850-900 ℃;

4) Cold rolling to the thickness of a finished product after conventional pickling, and controlling the total cold rolling reduction to be not less than 50%;

5) Carrying out continuous annealing, controlling the annealing temperature at 902-930 ℃, and preserving the heat for 120-180 s at the temperature;

6) Quenching treatment is carried out by adopting one of nitrogen, hydrogen, water and quenching liquid, and the plate blank is rapidly cooled to 195-223 ℃ at a cooling speed of not less than 45 ℃/s;

7) Carrying out aging treatment, rapidly heating the plate to 330-347 ℃, wherein the aging treatment time is 180-300 s;

8) Coiling and slowly cooling to room temperature;

9) And uncoiling again, and then sequentially flattening, slitting and rolling for forming.

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