CN110592326B

CN110592326B - Ultra-fine grain steel and industrial preparation method thereof

Info

Publication number: CN110592326B
Application number: CN201910988035.3A
Authority: CN
Inventors: 蒋虽合; 吕昭平; 高军恒; 马克·瑞福斯
Original assignee: University of Science and Technology Beijing USTB; University of Sheffield
Current assignee: University of Science and Technology Beijing USTB; University of Sheffield
Priority date: 2019-10-17
Filing date: 2019-10-17
Publication date: 2021-05-07
Anticipated expiration: 2039-10-17
Also published as: CN110592326A

Abstract

The invention belongs to the technical field of metallurgy, and relates to ultrafine grain steel for preparing block nano-crystals by utilizing disordered nano precipitated phase regulation and an industrial preparation method, and the industrial preparation method comprises the following components in percentage by weight: cu: 2-8%, Mn 3-30%, Nb: less than or equal to 0.3 percent, V: less than or equal to 0.8 percent, Ti: less than or equal to 0.3 percent, Al: less than or equal to 10 percent, Si: less than or equal to 6 percent, and the balance of Fe and inevitable impurities. The steel is subjected to cold rolling with deformation of more than 30% after solid solution, disordered Cu particles which are rapidly dispersed are separated out in the subsequent recrystallization process, the growth of submicron austenite grains is increased in time and continuously, and the superfine grain steel is prepared by high-temperature long-time heat preservation. The yield strength is increased to over 700MPa in multiples, the tensile strength reaches 2000MPa, and the elongation is more than 50%. The application of such steel in the automotive industry will be greatly advanced.

Description

Ultra-fine grain steel and industrial preparation method thereof

Technical Field

The invention belongs to the technical field of metallurgy, and particularly relates to ultrafine grained steel for preparing block nanocrystals by utilizing disordered nano precipitated phase regulation and an industrial preparation method.

Background

The ultra-fine grain steel often has yield strength and tensile strength far higher than those of conventional steel materials, lower ductile-brittle transition temperature and the like, and has wide application prospect in modern manufacturing industry, particularly on transportation tools such as automobiles and the like. But the industrial production of the ultra-fine grain material is not realized, which is mainly because the grain refinement to submicron scale can generate high-density grain boundaries, and the huge driving force generated by the high-density grain boundaries in the post heat treatment process can quickly coarsen the grains, so that the industrial production process is not easy to control, and the large-scale production of the ultra-fine grain material is limited. At present, the superfine crystal material is mainly prepared by a high-pressure knob, equal-neck-angle extrusion and other large-plastic deformation methods, and the production process is complex, the cost is high and the size is small. In addition, the refinement of the crystal grains to a submicron size causes a rapid decrease in work hardening capacity of the metallic material, thereby causing a decrease in uniform deformation properties of the material, thus limiting industrial application of the ultra-fine grained material. Researches find that the nanocrystalline and ultrafine grain with the crystal grains distributed in a gradient manner, the nano twin crystal with the crystal grains distributed in a gradient manner and the ultrafine grain material with the double peaks or the multiple peaks can effectively improve the work hardening capacity of the fine grain material, thereby improving the uniform deformation capacity of the fine grain material, but the materials also reduce the yield strength of the material to a certain extent, have complex and harsh preparation process, have special requirements on the shape of the material, and do not solve the size problem of the material, so that the industrial production cannot be realized.

Modern manufacturing industry requires ultra-fine grain steel to have high strength and plasticity to achieve the purpose of light weight of products, and also requires low manufacturing cost to realize wide application. However, in order to realize the industrial production of the ultra-fine grain steel under the conventional production process, an effective medium must be sought for stabilizing the grain boundaries, and the generation of the medium cannot conflict with the conventional production process, such as rolling and annealing processes, and cannot damage the grain boundary structure, generate grain boundary precipitates and other defects which are not good for the physicochemical and mechanical properties of the material. At present, the grain boundary of ultrafine crystal and nano crystal is effectively stabilized mainly by the method of alloying element grain boundary segregation and grain boundary precipitation, but the grain boundary segregation and grain boundary precipitation inevitably weaken the grain boundary and reduce the mechanical and corrosion resistance of the grain boundary segregation and grain boundary precipitation. Therefore, the alloy system, the production process (such as solution and annealing temperature) and the medium are reasonably selected to realize uniform intragranular uniform precipitation, so that the aim of stabilizing the grain boundary is fulfilled, and the important significance is achieved without reducing the mechanical and corrosion resistance properties.

Disclosure of Invention

In order to solve the problems, the invention aims to provide a preparation method for preparing bulk nanocrystalline materials including steel materials, which can be applied industrially, and simultaneously obtain great improvement of mechanical properties, and improve the yield strength of TRIP and TWIP steel in multiples without sacrificing plasticity and work hardening of the TRIP and TWIP steel.

The technical scheme of the invention is as follows: according to the invention, through reasonable thermodynamic calculation, a proper amount of Cu element is added into steel, so that the Cu element can be rapidly dispersed and precipitated at the recrystallization temperature. Because Cu in the steel is precipitated into an fcc structure, when the steel is recrystallized in an austenite phase region, disordered Cu is completely coherent with a matrix, and the formation of the disordered Cu can be completed only by partial aggregation of a small amount of Cu, so that the disordered Cu can be rapidly dispersed and uniformly precipitated during recrystallization to increase a submicron crystal boundary with high mobility. Although the disordered coherent Cu interacts very weakly with dislocations, i.e. does not significantly affect dislocation movement. But the large difference between the chemical components and the matrix ensures that the growth of the crystal grains inevitably needs to overcome the pinning of Cu grains, and the rapidly precipitated and thermally stable Cu nano-particles can pin the growth of the recrystallized grains with submicron sizes in the whole annealing process, thereby still maintaining the superfine crystal structure of the recrystallized grains during long-time recrystallization. Meanwhile, the disordered Cu particles which are uniformly distributed do not damage the mechanical property of the material, so that the grains are refined and the strength is improved by times without damaging other properties. Namely, the aim of preparing the nanocrystalline material by an industrial method is achieved by regulating and controlling the disordered precipitation and the recrystallization behavior.

The invention relates to ultrafine grained steel, which comprises the following components in percentage by weight: cu: 2-8%, Mn 3-30%, Nb: less than or equal to 0.3 percent, V: less than or equal to 0.8 percent, Ti: less than or equal to 0.3 percent, Al: less than or equal to 10 percent, Si: less than or equal to 6 percent, and the balance of Fe and inevitable impurities.

Further, the ultra-fine grained steel further comprises: c: 0-0.8% and rare earth elements.

Further, the sum of the atomic weight of Mn and the atomic weights of Al, Si, V and Nb in the ultra-fine grained steel needs to satisfy the following relational expression: mn is more than or equal to 2(Al + Si + V + Nb).

Another object of the present invention is to provide a method for preparing the above ultra-fine grained steel, specifically comprising the steps of:

s1) weighing the raw materials according to the design components, smelting in sequence, and casting to obtain a steel ingot;

s2) hot rolling the steel ingot obtained in S1), and then carrying out solution treatment to obtain a hot rolled plate;

s3) cold rolling the hot rolled plate processed by the S2), and then carrying out heat preservation treatment to obtain the ultra-fine grained steel.

Further, the grain size of the ultra-fine grained steel is below 1 micron.

Further, the deformation amount of the hot rolling in the S2) is 40-90%; the process of the solution treatment comprises the following steps: the solid solution temperature is 900-1200 ℃, and the solid solution time is 1-24 h.

Further, the cold rolling in S3) has a deformation amount of more than 30%.

Further, the heat preservation processing in S3) is: the temperature is kept between 650 ℃ and 900 ℃ for 3 to 60 minutes.

Further, Cu in the ultra-fine grained steel is precipitated as a disordered fcc structure.

Further, the yield strength of the ultra-fine grained steel is more than 700MPa, the tensile strength reaches 2000MPa, and the elongation is more than 50%.

The invention belongs to the field of preparation of bulk nano-crystals and TRIP and TWIP steel, and particularly relates to a method for preparing superfine crystal TRIP and TWIP steel with crystal grain size below 1 micrometer and excellent mechanical property by utilizing disordered nano precipitated phase regulation and an industrial method. The reason for limiting the content range of each chemical component in the novel industrial bulk nanocrystalline steel and the preparation process thereof will be described below.

Cu: the Cu element is a key element for preparing the ultrafine grained material, and is a key component element for inhibiting the growth of crystal grains and a Cu precipitation phase. As can be seen from the Fe-Cu phase diagram, the maximum solubility of Cu in austenite is about 7%, the excessive Cu content causes problems such as overburning of grain boundaries, and the excessive Cu content causes the solid solution of Cu in high-temperature austenite to be difficult to precipitate, and the pinning capability for grain boundary movement is insufficient, so the Cu content is between 2 and 8%.

Mn: for materials with predominantly dislocation deformation, a grain size reduction below 1 micron results in insufficient work hardening and thus reduced tensile strength and uniform plasticity. But for TRIP and TWIP type alloys, the transformation and twinning may coordinate the plastic deformation so that excellent plasticity is still obtained at the time of ultra-fine grain. Therefore, the Mn content cannot be too low, the transformation from TRIP to TWIP of the alloy occurs sequentially as the Mn content increases, and the Mn content should be between 3 and 30% in order to obtain metastable or stable austenite at room temperature.

C: c is an important solid solution strengthening element in steel, and in this invention, in addition to solid solution strengthening in solid solution and austenite, C promotes work hardening of the alloy at the time of cold rolling to store a large number of dislocations at a relatively low deformation amount, thereby promoting nucleation of recrystallized grains. However, C is not essential, and dislocations can be obtained by increasing the plastic deformation, so that the C content is limited to 0 to 0.8%

Nb, V, Ti: these elements are strong carbide-forming elements, and too high a content of these elements leads to the formation of carbides at the grain boundary sites during recrystallization, and although these precipitates inhibit grain growth, they inevitably deteriorate the plasticity and toughness of the material to a large extent. Therefore, Nb is defined: less than or equal to 0.3 percent, V: less than or equal to 0.8 percent, Ti: less than or equal to 0.3 percent.

Al, Si: since Al and Si are elements that inhibit carbide formation in steel, addition of Al and Si not only reduces the density of the steel material and causes solid solution strengthening, but also inhibits formation of coarse carbides in austenite to some extent. However, too high not only suppresses the TRIP and TWIP effects, but also reduces austenite stability and induces brittleness. Therefore, Al: less than or equal to 10 percent, Si: less than or equal to 6 percent.

The invention has the beneficial effects that: due to the adoption of the technical scheme, the invention has the following characteristics:

1. without introducing any other processing conditions requiring additional processing while maintaining the ultra-fine grain structure for extended annealing time, which provides an operation window for industrialization because it is difficult to make the structure uniform when a plate of a certain thickness is annealed for a short time in actual industry.

Compared with most elements, Cu has extremely low cost, ultrafine crystal with limit is prepared by Cu alloying, and any crystal defect which is unfavorable for the plasticity and toughness of the material is not introduced, namely the Cu alloying strategy greatly improves the comprehensive performance of the material without increasing the cost, and simultaneously meets the requirement of industrial production.

3. The method for preparing the ultra-fine grain steel needs the precipitated phases to be disordered and completely coherent with the matrix, and can block the growth of ultra-fine grains timely and continuously only if the conditions are met.

4. For some materials mainly based on dislocation deformation, the ultra-fine crystallization of TRIP and TWIP effect materials is emphasized in the invention because the too small grain size can reduce the work hardening behavior of the alloy, but the ultra-fine crystallization can still be prepared by the scheme provided by the invention.

Drawings

FIG. 1 is an EBSD topography of the ultra-fine grained steel prepared by the method of the invention.

FIG. 2 is a schematic view of an electron microscope showing that high-density coherent Cu precipitates are uniformly distributed in ultrafine grained steel according to the present invention.

FIG. 3 is a schematic view of the mechanical properties of the ultra-fine grained steel of the present invention.

FIG. 4 shows EBSD grain size distributions for different annealing temperature treatments.

Detailed Description

The technical solution of the present invention is further described with reference to the following specific embodiments.

Further, the grain size of the ultra-fine grained steel is below 1 micron.

Further, the deformation amount of the hot rolling in the S2) is 40-90%; the process of the solution treatment comprises the following steps: the temperature is 900-1200 ℃, and the solid solution time is 1-24 h.

Further, the cold rolling in S3) has a deformation amount of more than 30%.

Example 1:

firstly, weighing Cu: 3%, Mn: 22%, Nb: 0.3%, C: 0.6% V: 0.8%, Ti: 0.3%, Si: 6%, the balance being Fe and unavoidable impurities:

and smelting and casting the components in a medium-frequency induction furnace to obtain a steel ingot. The steel ingot is subjected to hot rolling for 40% and then is subjected to solid solution for 1h at 1200 ℃.

And (3) carrying out cold rolling with the deformation of 75%, and keeping the temperature for 5 minutes at 760 ℃ to obtain the ultrafine grained steel.

Example 2:

firstly, weighing Cu: 4%, Mn: 22%, C: 0.6%, the balance being Fe and unavoidable impurities:

and smelting and casting the components in a medium-frequency induction furnace to obtain a steel ingot. The steel ingot is subjected to hot rolling for 40% and then is subjected to solid solution at 1100 ℃ for 3 h.

And (3) carrying out cold rolling with the deformation of 75%, and keeping the temperature for 1 hour at 760 ℃ to obtain the ultrafine grained steel.

Example 3:

firstly, weighing Cu: 3%, Mn: 15%, Nb: 0.3%, C: 0.6%, V: 0.02%, the balance being Fe and unavoidable impurities:

and smelting and casting the components in a medium-frequency induction furnace to obtain a steel ingot. The steel ingot is subjected to hot rolling for 65 percent and then is subjected to solid solution for 16 hours at 960 ℃.

And (3) carrying out cold rolling with the deformation of 60 percent, and keeping the temperature for 5 minutes at 860 ℃ to obtain the superfine crystal steel.

Example 4:

firstly, weighing Cu: 5%, Mn: 15%, Al: 1%, the balance being Fe and unavoidable impurities:

and smelting and casting the components in a medium-frequency induction furnace to obtain a steel ingot. The steel ingot is hot rolled for 60 percent and then solid-dissolved for 12 hours at 1000 ℃.

And (3) carrying out cold rolling with the deformation of 45%, and keeping the temperature for 5 minutes at 910 ℃ to obtain the ultrafine grained steel.

Example 5:

firstly, weighing Cu: 3%, Mn: 15%, C: 0.4%, the balance being Fe and unavoidable impurities:

the components are smelted by a medium-frequency induction furnace and a steel ingot is cast. The steel ingot is hot rolled by 50 percent and then solid-soluted for 3 hours at 1040 ℃.

And (3) carrying out cold rolling with the deformation of 75%, and keeping the temperature at 810 ℃ for 5 minutes to obtain the ultrafine grained steel.

Comparative example:

the components are as follows: 22% of Mn, C: 0.6%, the balance being Fe and unavoidable impurities: and smelting and casting the components in a medium-frequency induction furnace to obtain a steel ingot. The steel ingot is hot rolled by 50 percent and then solid dissolved for 3 hours at 1040 ℃.

And (3) carrying out cold rolling with the deformation of 75%, and keeping the temperature for 5 minutes at 860 ℃ to obtain the ultrafine grained steel.

The tissues of example 2 and comparative example were analyzed by EBSD function of scanning electron microscopy at 860 ℃ for different time periods as shown in FIG. 4. After Cu is added, the grain size basically does not grow obviously along with the rise of the holding temperature and is maintained below 1 micron. The steels of comparative examples, which had no disordered precipitated phase formed upon recrystallization, had grain sizes that were significantly larger, exceeding 2 μm, upon low temperature recrystallization for a short time. The grain size exceeds 20 microns with increasing temperature. It can be seen that under the action of disordered nano precipitation (as shown in figure 2), ultrafine crystals can be prepared even if the temperature is kept at high temperature for a long time. Meanwhile, no precipitated phase is formed on the grain boundary, so that the ultrafine grained steel is prepared by an industrialized method.

Quasi-static tensile test pieces were performed on examples 1, 3 and comparative examples, and the tensile results are shown in fig. 3. both examples 1 and 3 have greatly improved mechanical properties compared to the comparative example without Cu. Wherein, the yield strength is improved by 2 times, the uniform elongation rate exceeds 50 percent, and the work hardening is more obvious. TRIP and TWIP are important supports of the future automobile industry, but the low yield strength of the TRIP and the TWIP is difficult to meet the industrial requirements, and the TRIP and the TWIP can improve the strength to 700MPa and improve the tensile strength to 200 MPa. This plays an important role in the practical application of such high work absorption materials in the automotive industry.

While several embodiments of the present invention have been presented herein, it will be appreciated by those skilled in the art that changes may be made to the embodiments herein without departing from the spirit of the invention. The above examples are merely illustrative and should not be taken as limiting the scope of the invention.

Claims

1. The ultrafine grained steel is characterized by comprising the following components in percentage by weight: cu: 2-5%, Mn 3-30%, Nb: less than or equal to 0.3 percent, V: less than or equal to 0.8 percent, Ti: less than or equal to 0.3 percent, Al: less than or equal to 10 percent, Si: less than or equal to 6 percent, and the balance of Fe and inevitable impurities;

the sum of the atomic weight of Mn and the atomic weights of Al, Si, V and Nb in the ultra-fine grained steel needs to satisfy the following relational expression: mn is more than or equal to 2(Al + Si + V + Nb);

cu in the ultra-fine grained steel is precipitated as a disordered fcc structure.

2. A method for producing the ultra fine grained steel according to claim 1, characterized in that the method comprises the following steps:

3. The method as claimed in claim 2, wherein the grain size of the ultra-fine grained steel is below 1 μm.

4. The method according to claim 2, wherein the hot rolling in S2) has a deformation amount of 40-90%; the process of the solution treatment comprises the following steps: the solid solution temperature is 900-1200 ℃, and the solid solution time is 1-24 h.

5. The method as claimed in claim 2, wherein the cold rolling in S3) has a deformation of more than 30%.

6. The method as claimed in claim 2, wherein the heat preservation process in S3) is: the temperature is kept between 650 ℃ and 900 ℃ for 3 to 60 minutes.

7. The method as claimed in claim 2, wherein the ultra-fine grained steel has a yield strength of more than 700MPa, a tensile strength of up to 2000MPa and an elongation of more than 50%.