CN109487063B - Controlled rolling and controlled cooling process for Ti-V-N composite microalloyed nano-particle reinforced low-carbon steel - Google Patents

Abstract

The invention relates to a controlled rolling and controlled cooling process for Ti-V-N composite microalloyed nano-particles reinforced low-carbon steel, and belongs to the technical field of steel rolling. The Ti-V-N composite microalloyed low carbon steel is heated to 1230°C to 1280°C and kept warm, and then cooled to 1160°C to 1050°C to start rolling in the recrystallized area of the austenite. The prepared Ti-V-N composite microalloyed low carbon steel is rolled in the non-recrystallized region, and then rapidly cooled after rolling: cooled to 550±30°C at a cooling rate greater than 50°C/s, and air-cooled to a temperature of 30 minutes after holding. room temperature. Through the two-stage rolling process and ultra-rapid cooling in the recrystallization and non-recrystallization regions, the invention not only promotes the repeated recrystallization of austenite and refines the grains, but also stores a large amount of deformation in the steel. The energy storage causes a large number of nano-scale second-phase particles to be precipitated in the steel, which increases the grain refinement strengthening increment and precipitation strengthening increment of the steel, and greatly increases the yield strength of the steel.

Description

Controlled rolling and controlled cooling of a Ti-V-N composite microalloyed nanoparticle reinforced low carbon steel craft

technical field

The invention relates to a controlled rolling and controlled cooling process for Ti-V-N composite microalloyed nano-particles reinforced low carbon steel, and belongs to the technical field of steel rolling.

Background technique

In recent years, with the increasing pressure on resources and the environment, environmental protection and energy saving have been paid more and more attention by the steel industry. In order to meet the needs of weight reduction and consumption reduction in all walks of life, the development of high-strength steel has received extensive attention at home and abroad. At present, microalloying technology combined with TMCP process is one of the effective ways to develop low-cost high-strength steel. Under suitable composition and TMCP process conditions, Ti can not only refine the grains by dispersing and precipitation of TiC particles to pin the grain boundaries, but also hinder the movement of dislocations through deformation-induced precipitation, resulting in a larger precipitation strengthening increment. Increase the yield strength of steel. However, the application of single titanium microalloying technology is seldom used at present, mainly because Ti is active and easy to combine with O, S and other elements in steel to form larger-sized inclusions, reducing the effective Ti content in steel; at the same time, due to Ti The solid solubility of (C, N) in steel is small, and it is extremely sensitive to the precipitation temperature and cooling rate. It often precipitates in the higher temperature range, and is prone to coarsening and growth in the subsequent process, losing the precipitation strengthening effect; In addition, the larger-sized Ti precipitates are easily distributed continuously at the grain boundaries, resulting in a decrease in the strength of the grain boundaries and a serious decrease in the plastic toughness of the steel.

In order to improve the comprehensive mechanical properties of Ti microalloyed low carbon steel and improve the stability of steel structure, Chinese invention patent CN108374131 discloses a controlled rolling and controlled cooling process method for ultra-fine austenite grains of Ti-Mo composite microalloyed steel , through three passes of rolling with different reductions and different strain rates, multiple austenite recrystallizations are promoted, and a uniform ultra-fine austenite grain structure is obtained. However, the number of nano-scale precipitation phases in the steel structure is limited, and the incremental increase in precipitation strengthening is not obvious. Chinese invention patent CN108374131 discloses a controlled rolling and controlled cooling process for H-beams with a yield strength of 500MPa and H-beams. Its internal structure is polygonal ferrite, pearlite, acicular ferrite and a small amount of granular bainite, and the yield strength reaches 500MPa. However, the steel described above does not contain Ti element, and the precipitation strengthening increment is limited, which provides limited reference for the design of the controlled rolling and controlled cooling process for the Ti microalloyed low carbon steel. Chinese invention patent CN102500625 discloses a new type of TMCP process, the process of which can effectively control the grain growth after the complete recrystallization zone is rolled, and obtain a finer grain structure. However, the process is mainly designed for refining the grain structure in the steel, and the precipitation and distribution of the precipitation phase in the steel are basically not considered and controlled, and the precipitation strengthening increment is not obvious.

Therefore, in view of the above problems in the TMCP process, it is necessary to design a matching TMCP process for the Ti-V-N microalloyed low carbon steel to control the precipitation of nano-scale precipitates in the steel, to increase the precipitation strengthening increment of the steel, and to obtain the microstructure. Ti-V-N composite microalloyed low-carbon high-strength steel with stable performance and large precipitation strengthening increment.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a controlled rolling and controlled cooling process for Ti-V-N composite microalloyed nano-particles reinforced low carbon steel. First, rolling in the recrystallization region can make the austenite grains recrystallize many times, and the fine Austenite grains; then rolling in the non-recrystallized region can increase the deformation energy storage in the steel, resulting in deformation-induced precipitation; by ultra-rapid cooling after rolling and isothermal treatment after cooling to a certain temperature, the steel can be increased. The degree of undercooling increases the driving force of the second phase precipitation transformation, and increases the interphase precipitation phase and the ferrite intragranular dispersed precipitation phase. The technology of the present invention specifically comprises the following steps:

(1) Rolling in the recrystallized area: The Ti-VN composite microalloyed low carbon steel is heated to 1230℃~1280℃, and kept for 600s, and then cooled to 1160℃~1050℃ to start the rolling of the austenite recrystallization area , the cooling rate is 10℃/s; the strain rate of the first pass rolling is 10~15s ^-1 , the deformation is 40%, and the second pass rolling is carried out after the interval of 1~5s, rolling parameters: strain rate 5 ~10s ^-1 , the deformation amount is 20%, and the final rolling temperature is controlled above 980℃;

(2) Rolling in the non-recrystallized region: the Ti-VN composite microalloyed low carbon steel obtained in step (1) is cooled to 880-920°C at a rate of 10°C/s, and the rolling temperature is 850°C-800°C, The strain rate of the first pass rolling is 1~5s ^-1 , the deformation amount is 30%; the second pass rolling is carried out after the interval of 1~5s, rolling parameters: the strain rate is 1~5s ^-1 , the deformation amount is 20% , the final rolling temperature is 780~720℃;

(3) Cooling to 550±30°C at a cooling rate of 50°C/s or more, and air-cooling to room temperature after holding for 30 minutes to obtain a Ti-V-N composite microalloyed nanoparticle reinforced low-carbon steel.

The chemical composition mass percentage of the Ti-V-N composite microalloyed low carbon steel according to the present invention is C: 0.05%-0.2%, Si: 0.1%-0.3%, Mn: 0.5%-2%, Cr: 0.4%- 1.1%, Ti: 0.08%~0.19%, V: 0.3%~0.6%, Ni: 0.1%~0.3%, N: 0.1%~0.3%, P<0.03%, S<0.03%, the rest are Fe and residual of trace impurities.

The rolling equipment of the present invention may be a four-high double-stand rolling mill, and the heating equipment may be a heating furnace.

The microalloyed low carbon steel described in the present invention can be slab, square billet, round billet and the like.

The principle of the present invention: the present invention adopts the Ti-V-N composite microalloying technology combined with the supporting TMCP controlled rolling and cooling technology to disperse and precipitate nano-scale V(C, N) particles in the steel to play a significant role in precipitation strengthening, and through The role of TiN particles in pinning grain boundaries and the two-stage rolling process can significantly refine the grain size in the steel and improve the comprehensive mechanical properties of the microalloyed steel. Since a certain amount of Ti, V, and N alloy elements are added to the steel in the present invention, in order to make as many alloy elements as possible solid-dissolved into the steel, the austenitization temperature is set to 1230°C to 1280°C; The precipitation phase of TiC particles produced in alloy steel cannot refine austenite grains by pinning grain boundaries because it does not have the thermodynamic conditions for precipitation before hot rolling. Compared with the TiC precipitation phase, the TiN particles are easier to precipitate before the heat preservation at 1230°C to 1280°C due to their smaller equilibrium solid solubility product. Therefore, the present invention adds a certain amount of N on the basis of the Ti microalloyed steel to make the steel. A certain amount of TiN is precipitated in the steel, which plays the role of refining austenite grains; however, if the N content in the steel is too high, it may lead to the combination of excessive N and V to form a VN precipitate phase, which is prone to occur due to its high precipitation temperature range. Roughening, so the addition amount of N element in the steel is set to 0.1%~0.3% comprehensively. Further, since Ti is not used as the main precipitation strengthening element in the present invention, and too high Ti content may lead to the generation of larger-sized precipitate particles in the steel, reducing the plasticity and toughness of the steel, so the mass percentage of Ti in the steel is comprehensively considered. Controlled at 0.08%~0.19%. V is used as the main precipitation strengthening element in the present invention. Since V is easy to combine with C, N and other elements in steel to form smaller-sized V (C, N) or VC precipitate particles, which are uniformly distributed and dispersed, can be large The yield strength of the steel is greatly improved, while the plasticity and toughness of the steel are less affected. The present invention controls the content of V at 0.3%-0.6% after comprehensive consideration of various factors such as production cost and performance requirements. The main functions of double-stage rolling, rapid cooling after rolling and cooling to a certain temperature for isothermal treatment in the present invention are as follows: the recrystallization rolling process mainly promotes the repeated recrystallization of austenite grains and refines the austenite grains. The bulk grain structure; the non-recrystallization rolling process mainly produces a large number of crystal defects in the steel through cold deformation, resulting in the precipitation of V (C, N) particles induced by the deformation, and the austenite grains will undergo severe cold deformation, It plays a role in promoting ferrite nucleation; the non-recrystallized area is cooled rapidly and isothermally after rolling, mainly to prevent the precipitation of excessive amounts of V(C, N) and VC in the higher temperature range, which affects the precipitation strengthening effect.

The beneficial effects of the present invention are:

(1) The method of the present invention promotes multiple recrystallization of austenite by rolling in the recrystallized area, refines the austenite grains, and increases the nano-scale precipitation in the steel through rolling in the non-recrystallized area and low temperature isothermal The precipitation of the material increases the precipitation strengthening increment of the steel, thereby greatly increasing the yield strength of the steel.

(2) The method of the present invention utilizes the mechanisms of grain refinement strengthening, precipitation strengthening, and solid solution strengthening to improve the comprehensive mechanical properties of the steel, significantly reduce the amount of alloying elements added, and reduce production costs;

(3) The TMCP process parameter guidance is provided for the production of Ti microalloyed low carbon high strength steel, which improves the performance stability of Ti microalloyed nano-particle reinforced high strength steel.

(4) Produced by the controlled rolling and controlled cooling process, the obtained low-carbon steel structure has small grain size, and at the same time, a large number of nano-scale V(C, N) and VC precipitates are dispersed.

Description of drawings

Fig. 1 is the process flow diagram of the present invention;

Fig. 2 is the metallographic structure diagram of Ti-V-N composite microalloyed low carbon steel after TMCP treatment in Example 1;

FIG. 3 is a SEM image of the nanoscale precipitation phase morphology of the Ti-V-N composite microalloyed low carbon steel in Example 1 after TMCP treatment.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited to the content.

Example 1

A controlled rolling and controlled cooling (TMCP) process of Ti-V-N composite micro-alloyed nano-particles reinforced low-carbon steel, wherein the chemical composition mass percentages of the micro-alloyed low-carbon steel are: C: 0.09%; Mn: 0.9%; Si : 0.15%; Cr: 0.73%; Ti: 0.12%; V: 0.46%; N: 0.25%; Ni: 0.21%; P: 0.002%; Alloy steel is thin slab, and the TMCP process includes:

(1) Rolling in the recrystallized area: The above microalloyed low carbon steel is heated to 1250°C and kept for 600s, and then cooled to 1100°C to start rolling in the austenite recrystallization area. The cooling rate is 10°C/s. The strain rate of the pass rolling is 10s ^-1 , the deformation amount is 40%; the second pass rolling is carried out after an interval of 3s, rolling parameters: the strain rate is 5s ^-1 , the deformation amount is 20%, and the final rolling temperature is controlled at 1000℃ .

(2) Non-recrystallized area rolling: Cool the Ti-VN composite microalloyed low carbon steel rolled in the recrystallized area to 920℃ at a rate of 10℃/s, and the rolling temperature is 830℃. The strain rate of the first rolling was ^{3s -1} , the deformation amount was 30%; the second pass rolling was carried out after an interval of 3s.

(3) Rapid cooling after rolling: Cool to 550°C at a cooling rate of 55°C/s, hold for 30 minutes and then air-cool to room temperature to obtain Ti-V-N composite microalloyed nanoparticle reinforced low carbon steel.

Example 2

A controlled rolling and controlled cooling (TMCP) process of Ti-V-N composite micro-alloyed nano-particles reinforced low-carbon steel, the chemical composition mass percentage of the micro-alloyed low-carbon steel is: C: 0.11%; Mn: 1.2%; Si : 0.18%; Cr: 0.81%; Ti: 0.14%; V: 0.55%; N: 0.29%; Ni: 0.28%; P: 0.004%; Alloy steel is thin slab, and the TMCP process includes:

(1) Rolling in the recrystallized area: The above-mentioned microalloyed low carbon steel is heated to 1280°C and kept for 600s, and then cooled to 1130°C to start rolling in the austenite recrystallization area. The cooling rate is 10°C/s. The strain rate of the pass rolling is 15s ^-1 , the deformation amount is 40%; the second pass rolling is carried out after an interval of 2s, rolling parameters: the strain rate is 10s ^-1 , the deformation amount is 20%, and the final rolling temperature is controlled at 1020℃ .

(2) Non-recrystallized area rolling: Cool the Ti-VN composite microalloyed low carbon steel rolled in the recrystallized area to 900℃ at a rate of 10℃/s, and the rolling temperature is 850℃. The strain rate of the first rolling was 5s ^-1 , the deformation amount was 30%; the second pass rolling was performed after an interval of 5s, and the rolling parameters were: strain rate 3 s ^-1 , deformation amount 20%, and final rolling temperature of 730℃.

(3) Rapid cooling after rolling: Cool to 520°C at a cooling rate of 54°C/s, hold for 30 minutes and then air-cool to room temperature to obtain a Ti-V-N composite microalloyed nanoparticle reinforced low carbon steel.

Example 3

A controlled rolling and controlled cooling (TMCP) process of Ti-V-N composite micro-alloyed nano-particle reinforced low-carbon steel, the chemical composition mass percentage of the micro-alloyed low-carbon steel is: C: 0.07%; Mn: 1.5%; Si : 0.21%; Cr: 0.85%; Ti: 0.16%; V: 0.58%; N: 0.22%; Ni: 0.21%; P: 0.005%; Alloy steel is thin slab, and the TMCP process includes:

(1) Rolling in the recrystallized area: The above microalloyed low carbon steel is heated to 1240°C and kept for 600s, and then cooled to 1070°C to start rolling in the austenite recrystallization area. The cooling rate is 10°C/s. The strain rate of the pass rolling is 12s ^-1 , the deformation amount is 40%; the second pass rolling is carried out after an interval of 5s, rolling parameters: the strain rate is 8s ^-1 , the deformation amount is 20%, and the final rolling temperature is controlled at 1000 ℃ .

(2) Non-recrystallized area rolling: Cool the Ti-VN composite microalloyed low carbon steel rolled in the recrystallized area to 910℃ at a rate of 10℃/s, and the rolling temperature is 800℃. The strain rate of the first rolling was ^{3s -1} , the deformation amount was 30%; the second pass rolling was carried out after an interval of 2s.

(3) Rapid cooling after rolling: Cool to 530°C at a cooling rate of 58°C/s, hold for 30 minutes and then air-cool to room temperature to obtain a Ti-V-N composite microalloyed nanoparticle reinforced low carbon steel.

It can be seen from the analysis of the metallographic structure diagram and the SEM image that the microstructure of the Ti-V-N composite micro-alloyed nanoparticle-reinforced low-carbon steel prepared in Examples 1 to 3 of the present invention is mainly composed of soft and tough ferrite structure. There are a large number of dispersed nano-scale VC and V (C, N) particles inside the ferrite grains, and a very small amount of larger Ti (C, N) precipitate particles exist in the ferrite matrix. The larger Ti(C,N) particles were mainly precipitated during the high-temperature austenitization process and recrystallization rolling process, and coarsened during the subsequent processing, which basically lost the effect of precipitation strengthening. The following is a detailed description of the Ti-V-N composite microalloyed nanoparticle reinforced low carbon steel of Example 1,

The microstructure of Ti-V-N composite microalloyed low carbon steel after TMCP treatment is shown in Figure 2. It can be seen from the figure that its structure is basically dominated by polygonal ferrite and granular ferrite. The size is small, the steel does not contain large-sized precipitate particles, the structure distribution is relatively uniform, and the comprehensive properties of the steel are better; the nano-scale precipitation phase in the steel after the Ti-V-N composite microalloyed low carbon steel is treated by TMCP The SEM image is shown in Figure 3. It can be seen from the figure that there are VC and V (C, N) precipitate particles with a size range of 10-100nm dispersed in the ferrite grains, and basically no size of 100nm. above sediment particles. The shape of the smaller precipitate particles is mainly regular spherical, which is mainly dispersed and precipitated in the low temperature isothermal process after controlled cooling. Because of the large supercooling in the ferrite crystal after rapid cooling and accumulated a certain amount of deformation energy storage, while the larger subcooling degree and deformation energy storage provided the driving force for the phase transition for the precipitation of VC and V(C, N), and at the same time reduced the surface energy to be overcome for nucleation , so that VC and V (C, N) nucleate uniformly in the ferrite crystal; due to the low temperature at this time, the microalloying elements basically do not diffuse, and the precipitate does not coarsen and grow after nucleation; precipitation The interfacial energies along the surface of the solid particles are basically the same in all directions, so the precipitates produced are mostly regular spherical. The spherical precipitate particles can improve the strength of the steel by hindering the movement of dislocations, and the material will not produce stress concentration during the stress process, and has little effect on the plastic toughness.

The matrix structure of the Ti-V-N composite microalloyed low carbon steel prepared by the method is dispersed and distributed with a large number of nano-scale precipitation phase particles, and the grain structure is small at the same time, so that a larger increment of fine-grain strengthening and precipitation strengthening can be obtained. The comprehensive mechanical properties of steel are significantly improved. Its mechanical properties are shown in Table 1:

Table 1

Claims (1)

1. A controlled rolling and controlled cooling process for Ti-V-N composite microalloyed nano-particle reinforced low-carbon steel is characterized by comprising the following steps:

(1) and (3) rolling in a recrystallization area: heating Ti-V-N composite microalloyed low carbon steel to 1230-1280 ℃, preserving the heat for 600s, then cooling to 1160-1050 ℃, and starting the rolling of an austenite recrystallization region, wherein the cooling speed is 10 ℃/s; the strain rate of the first pass rolling is 10-15 s^-1And the deformation is 40%, rolling for the second time is carried out after the interval of 1-5 s, and the rolling parameters are as follows: strain rate of 5-10 s^-1The deformation is 20 percent, and the finishing temperature is controlled to be more than 980 ℃;

(2) rolling in a non-recrystallization area: cooling the Ti-V-N composite microalloyed low-carbon steel obtained in the step (1) to 880-920 ℃ at the speed of 10 ℃/s, the initial rolling temperature of 850-800 ℃, and the first-pass rolling strain rate of 1-5 s^-1The deformation amount is 30%; and (3) carrying out second pass rolling after the interval of 1-5 s, wherein the rolling parameters are as follows: strain rate of 1-5 s^-1The deformation is 20 percent, and the finishing temperature is 780-720 ℃;

(3) cooling to 550 +/-30 ℃ at a cooling speed of more than or equal to 50 ℃/s, preserving heat for 30 minutes, and then air-cooling to room temperature to obtain Ti-V-N composite microalloyed nano particle reinforced low-carbon steel;

the Ti-V-N composite microalloyed low-carbon steel comprises, by mass, 0.05-0.2% of C, 0.1-0.3% of Si, 0.5-2% of Mn, 0.4-1.1% of Cr, 0.08-0.19% of Ti, 0.3-0.6% of V, 0.1-0.3% of Ni and the following chemical components in percentage by mass: 0.1-0.3%, P less than 0.03%, S less than 0.03%, and the balance of Fe and residual trace impurities.

Images