CN113801984B

CN113801984B - Plastic deformation processing technology for synchronously improving strength and plasticity of low-carbon steel

Info

Publication number: CN113801984B
Application number: CN202010541461.5A
Authority: CN
Inventors: 薛鹏; 王志伟; 倪丁瑞; 张昊; 吴利辉; 马宗义; 肖伯律
Original assignee: Institute of Metal Research of CAS
Current assignee: Institute of Metal Research of CAS
Priority date: 2020-06-15
Filing date: 2020-06-15
Publication date: 2023-03-21
Anticipated expiration: 2040-06-15
Also published as: CN113801984A

Abstract

The invention discloses a plastic deformation processing technology for synchronously improving the strength and plasticity of low-carbon steel, and belongs to the technical field of preparation of high-strength and high-toughness steel materials. Firstly, carrying out underwater friction stir processing on low-carbon steel under low heat input parameters to obtain a ferrite/martensite dual-phase structure with an ultrafine grain size in a processing area; and then carrying out two-phase short-time low-temperature annealing treatment and subsequent water quenching cooling treatment on the two-phase structure of the processing area to finally obtain a ferrite/martensite/austenite/carbide multi-phase structure with excellent performance. The method can improve the strength and the plasticity of the common low-carbon steel material, and the prepared low-carbon steel has the structural characteristics of superfine, equiaxial and multiphase composition. The plastic deformation processed material has strong plastic property superior to the material produced by ATMP in industry at present, and the related method has the advantages of simplicity, convenience, high efficiency and low cost.

Description

Plastic deformation processing technology for synchronously improving strength and plasticity of low-carbon steel

Technical Field

The invention relates to the technical field of preparation of high-toughness steel materials, in particular to a plastic deformation processing technology for synchronously improving the strength and plasticity of low-carbon steel.

Background

In order to meet the application requirements of light weight and high strength, the preparation of steel materials is developing towards the direction of continuously improving the strength, but the strength of the materials is improved while the plasticity of the materials is usually lost, so that the subsequent processability, formability and service performance of the materials are reduced, and the popularization and application of the high-strength materials are severely restricted. Meanwhile, the aims of energy conservation, emission reduction, resource conservation and sustainable development require that the material is continuously subjected to element treatment, namely, the alloy content of the material is reduced as far as possible while the strength of the material is ensured. Therefore, advanced high-strength high-plasticity low-carbon steel (including low-carbon low-alloy steel with a small amount of cheap alloy elements) becomes a research hotspot of domestic and foreign material workers.

At present, fine grain strengthening and multi-phase structure coupling are generally adopted in the industry to improve the strength and the plasticity, and the method mainly comprises two processes of advanced thermomechanical processing (ATMP) and large plastic deformation (SPD) + annealing treatment. ATMP is convenient for batch production, the industrial application background is wide, but the plastic deformation degree is limited, and the grain refining effect is generally inferior to that of the SPD + annealing process; although the SPD technology has higher grain refinement capability, the prepared material has low stability and limited sample size, and is not easy to realize batch production. Although the two processes can be adopted to prepare ferrite, ferrite + carbide, ferrite + martensite and other dual-phase structures with ultra-fine average grain size (< 1 μm), the strength of the low-carbon steel material is improved to more than 1 GPa; however, the plasticity of these superfine materials is not high enough, and the total elongation in the uniaxial static stretching process is below 15%, which severely limits the subsequent processing and service performance. However, the fcc structure austenite phase has a transformation induced plasticity (TRIP) effect during plastic deformation, and is expected to further improve the plasticity of the ultra-fine grained material. However, austenite can exist at room temperature only by increasing the stability of the austenite with alloying elements such as carbon, manganese, chromium and the like, so that the austenite is difficult to be introduced into the low-carbon low-alloy steel obtained by the conventional preparation process.

The Friction Stir Processing (FSP) is a novel material modification process based on Friction Stir Welding (FSW), a stirring tool consisting of a shaft shoulder and a stirring needle is rotationally pressed into a workpiece to generate thermoplastic deformation, a processing area is formed along with the movement of the stirring tool, and the FSP has the characteristics of obvious grain refining effect, large-area processing, uniform and stable texture of the processing area and the like. In recent years, a low heat input friction stir processing technology is rapidly developed, and the heat input is further reduced by controlling processing parameters and adding a cooling medium, so that the purposes of improving the grain refining effect, controlling phase change and the like are achieved. FSW studies of different carbon steels have shown that by reducing tool speed, increasing tool travel speed, or applying liquid CO ₂ The cooling can successfully reduce the structure of the nugget to an ultra-fine grain size and simultaneously improve the hardness and the strength of the nugget, (Materials Sci)A, 2006.429; script Materialia, 2007.56; journal of Materials Processing Technology, 2016.230. In recent years, researchers have obtained an ultrafine ferrite/martensite dual-phase structure in a common low-carbon steel friction stir processing area by using a TiC ceramic needleless tool and a forced water cooling process (the tool rotation speed is 400 rpm, and the advancing speed is 50 mm/min). Due to the existence of martensite, the structural strength of a processing area can reach more than twice of that of a parent metal; however, the elongation loss is serious due to poor martensite plasticity and less ferrite content in the product, and is only 50% of the parent material (Materials Science and Engineering: A,2013.575 p.30-34.

Compared with the conventional friction stir processing, the low heat input friction stir processing has higher requirements on material selection and design of the stirring tool, and the main problems of high tool cost, easy damage and difficult forming of a processing area are present. In the aspect of tool material selection, the tungsten-based alloy and the boron nitride ceramic which are frequently used at present have enough strength, but the cost is high; the WC ceramic has the problem of low-temperature brittleness although the cost is low. In the design of the tool, the existence of the stirring pin enables the plastic flow nonuniformity of the material to be more prominent in the low-heat input process, and the defects of holes, tunnels and the like caused by insufficient material backfilling are easily caused; the needle-free tool is pressed and molded only by a shaft shoulder, the defects caused by the stirring needle can be eliminated, but the depth of a processing layer is limited, and large-area processing and preparation are difficult to realize. In the aspect of structure control, the structures prepared by the plastic processing method mainly comprise bcc structural phases (ferrite/martensite/pearlite), and the plastic deformation capability of the bcc structural phases in the ultra-fine grain scale range is very limited, so that the common characteristics of high strength and low plasticity of the existing ultra-fine low-carbon steel material are caused.

Object of the Invention

The invention aims to provide a plastic deformation processing technology for synchronously improving the strength and the plasticity of low-carbon steel, which is characterized in that the low-alloy low-carbon steel material with high strength and high plasticity is finally prepared in a processing area by optimizing and improving the existing friction stir processing parameters and tools and combining with a reasonable subsequent heat treatment process to regulate and control the composition of a structure phase.

A plastic deformation processing technique for synchronously improving the strength and the plasticity of low-carbon steel is characterized in that the low-carbon steel base metal is subjected to underwater stirring friction processing to obtain an ultrafine ferrite/martensite dual-phase structure in a processing area; and then carrying out two-phase region short-time low-temperature annealing treatment and subsequent water quenching cooling treatment on the processing region, and finally obtaining the low-carbon steel with high strength and high plasticity in the processing region.

The low-carbon steel comprises the following chemical components in percentage by weight: 0.1 to 0.2 percent of C, 1.0 to 2.0 percent of Si, 2.5 to 3.0 percent of Mn, less than 0.006 percent of S, less than 0.007 percent of P, less than 0.04 percent of Al, less than 0.013 percent of Cr, and the balance of Fe and other trace impurities.

The stirring friction processing tool is a needleless stirring head made of TiC-based metal ceramic or Ti (C, N) -based metal ceramic, the free end of the stirring head is of a hemispherical convex structure, the sphere radius corresponding to the hemispherical convex structure is 5-10 mm, and the processing depth is 0.5-5 mm.

The parameters of the stirring friction processing technology are as follows: the rotating speed of the tool is 200-300 r/min, and the advancing speed is 25-200 mm/min.

In the stirring and rubbing process, flowing water is used for cooling the processing area, if common water pipes are used for injecting water for cooling the processing area, the diameter of water outlets of the water pipes is 5-15 mm, the water flow rate is 2-5 liters/min, and the water temperature at the water outlets is 15-25 ℃.

In order to obtain an ultrafine grain structure and generate an austenite phase in a processing region, a two-phase region low-temperature short-time annealing treatment different from a conventional heat treatment process is adopted, the annealing temperature is 710-730 ℃, and the heat preservation time is 5-15 minutes; in order to keep the austenite to the room temperature, a low-temperature quenching process is adopted, the temperature of cooling water in quenching treatment is 10-20 ℃, and the transfer speed is less than or equal to 7 seconds.

After the low-carbon steel base material is subjected to stirring friction processing, two-phase region low-temperature short-time annealing treatment and water quenching cooling treatment, the obtained processing region structure consists of ferrite, martensite, austenite and carbide multiphase structures, and the average grain size of the multiphase structures is 0.2-1 mu m. Wherein the average size of the martensite islands is 0.5-1 μm, and the phase proportion is 20-40% (volume fraction); the average grain size of austenite is 0.2-1 μm, and the phase proportion is 5-15% (volume fraction); the average size of the carbide is 0.05-0.1 μm, and the phase proportion is 1-3% (volume fraction).

The invention has the beneficial effects that:

1. the invention provides a plastic processing technology for synchronously improving the strength and the plasticity of low-carbon steel, which adopts a low-heat-input stirring friction processing technology, and can obtain an ultra-fine grain ferrite/martensite two-phase structure with obviously refined grains by cooperatively controlling the heat input in the processing process through two modes of reducing the tool rotating speed and applying cooling water. Then, the ferrite/martensite/austenite/carbide multiphase structure can be obtained in the processing area through the short-time (10 minutes) low-temperature annealing treatment in the two-phase area (710-730 ℃) and the quenching treatment accompanied by cooling water (15 ℃). The average grain size of each phase of the structure is less than 1 mu m, wherein the proportion of martensite islands is 20-40%, the proportion of austenite is 5-15%, and the proportion of carbide is 1-3% (volume fraction); through fine-grain strengthening, multiphase coupling and TRIP effect, the strength and plasticity of the structure in the processing area are greatly improved compared with those of the base material.

2. In the low heat input stirring friction processing technology adopted by the invention, the stirring head is made of the cermet material with low cost and good low-temperature toughness and wear resistance, so that the production cost is greatly reduced. The convex spherical stirring tool with improved design can eliminate the forming defects of holes, tunnels and the like caused by stirring needles, improve the formability of a processing area and enlarge the area of the processing area; the design of no edges and corners can reduce the tool wear by a wide margin, promote the tool life, can promote the stability of course of working equipment simultaneously.

3. Compared with the existing preparation method of the high-strength low-carbon steel material, the method can obviously improve the comprehensive mechanical property of the material in the processing area, has simple and easy-to-use process, low production cost and high efficiency, and has wide application prospect in the field of low-carbon and low-carbon low-alloy steel material preparation.

Drawings

FIG. 1 is a schematic view of the low heat input friction stir processing technique employed in the present invention.

FIG. 2 is a typical scanning electron microscope morphology of a high-strength and high-plasticity processed area tissue prepared in example 1 by low-heat input friction stir processing (processing parameters are 275 rpm and 50 mm/min, and a cooling medium is room-temperature flowing water) and assisted by short-time low-temperature annealing (at 720 ℃ for 10 min) and water quenching (at 15 ℃ for 3 s).

FIG. 3 shows the results of X-ray diffraction (XRD) measurements of the processed area of example 1, with the source of radiation being a Cu target and the scanning speed being 4 degrees/min.

FIG. 4 is a graph comparing the typical tensile curves of the worked section structure and the raw parent material in example 1, and the initial tensile strain rates are all 1X 10 ^-3 s ^-1 。

Detailed Description

The invention is described in detail below with reference to the figures and examples.

According to the invention, the low-carbon steel material with high plasticity is prepared by using a low-heat input stirring friction processing technology and a short-time annealing process to greatly improve the plasticity of the low-carbon steel material while improving the strength of the low-carbon steel material.

The low heat input friction stir processing used in the present invention is illustrated in figure 1. The friction stir processing tool used was a pin-less stir head, the tool material was TiC-based cermet or Ti (C, N) -based cermet, such as 4wt.% Mo content, 6 wt.% TiN content,% Ti (C, N)/NiCrMoAlTi cermet disclosed in document 1 (document 1: wang holomega, high performance Ti (C, N)/NiCrMoAlTi cermet preparation and connection study with steel [ D ]. Institute of metals, china academy of sciences 2007 ]. The free end of the stirring head is designed into a hemispherical convex structure, the radius of a sphere corresponding to the hemispherical convex structure is 5-10 mm, and the processing depth is 0.5-5 mm. The friction stir processing is carried out in the water tank, the bottom surface of the water tank is used as a processing platform, and the workpiece is arranged on the processing platform. The processing area is cooled by flowing water output by a common water pipe, the water temperature is 15-25 ℃, the diameter of the water outlet of the water pipe is 5-15 mm, and the water flow rate is 2-5 liters/minute.

Example 1

The common low-carbon low-alloy steel plate with the thickness of 3 mm is used, and the chemical components of the steel plate are as follows in percentage by weight: 0.16% of C, 1.58% of Si, 2.75% of Mn, 0.005% of S, 0.006% of P, 0.039% of Al, 0.012% of Cr and the balance of Fe. As shown by the elongation curve in FIG. 4, the base material had a tensile strength of 744MPa, a uniform elongation of 17.5%, and a total elongation of 31%. Adopting a convex spherical metal ceramic stirring head with the radius of 7 mm to carry out stirring friction processing, wherein the tool rotating speed is 275 rpm, and the advancing speed is 50 mm/min; in the processing process, flowing water is adopted to cool a processing area, the diameter of a water outlet of a water pipe for applying the flowing water is 8 mm, the flow rate is 2.2 liters/minute, the water temperature of the water outlet is 18 ℃, and the depth of a processing layer is 1.5 mm. After processing, the processing area is subjected to two-phase area low-temperature annealing treatment, the temperature is 720 ℃, and the heat preservation time is 10 minutes. Subsequently, the annealed structure was quenched with 15 ℃ tap water at a transfer rate of 3 seconds. The structure characterization showed that the worked zone structure consisted of ferrite/martensite/polygonal austenite/carbide (fig. 2), with an average grain size of 0.6 μm, a martensite phase content of about 30%, a carbide phase content of about 1.8%, and XRD analysis showed an austenite phase content of about 10.6% (fig. 3); due to the adoption of the low-heat input stirring friction processing and low-temperature short-time annealing process, the tensile strength of the structure of the processing area is 1016MPa, the tensile strength is improved by 36 percent relative to the parent metal, the uniform elongation and the total elongation are respectively 30.3 percent and 39.1 percent, and the tensile strength is respectively improved by 73 percent and 31 percent relative to the parent metal (figure 4).

Comparative example 1

The difference from the embodiment 1 is that the embodiment does not adopt a convex spherical stirring tool for processing, but adopts a conventional tool with a stirring pin, and the specific method is as follows:

a low-carbon low-alloy steel sheet having the same thickness, composition and mechanical properties as those of example 1 was used. The metal ceramic stirring head with the diameter of a shaft shoulder being 12 mm and the diameter and the length of the root part of the stirring needle being 5 mm and 2 mm respectively is adopted for stirring friction processing, the rotating speed of a tool is 275 r/min, and the advancing speed is 50 mm/min; the processing process adopts flowing water to cool the processing area, the diameter of the water outlet of a water pipe for applying the flowing water is 8 mm, the flow rate is 2.2 liters/min, and the water temperature at the water outlet is 18 ℃. Because the stirring tool does not adopt a convex spherical design, the stirring pin is broken from the root part and fails in the processing process, and the stirring pin cannot be continuously processed; the bottom of the processed area has a large number of holes and obvious tunnels, and the width of the processed area is less than 5 mm.

Comparative example 2

The difference from the embodiment 1 is that the embodiment does not adopt a water cooling process to perform auxiliary cooling on the processing area, and the specific method is as follows:

the low carbon low alloy steel sheet having the same thickness, composition and mechanical properties as those of example 1 was used, and the tool material, dimensions and processing parameters were also the same as those of example 1. In the processing process, forced water cooling is not adopted, but the processing area is naturally cooled in the air. The tissue in the processing zone was then incubated at 720 ℃ for 10 minutes and quenched in tap water at 15 ℃ with a transfer rate of 3 seconds. Because the water cooling process is not adopted for forced cooling, the average grain size of the structure of the final processing area is larger and is about 6 mu m; the content of austenite phase is low, about 2%; the uniform elongation and the total elongation were 20.2% and 37.8%, respectively, and were slightly increased from the parent material, but the tensile strength was only 710MPa, and was not increased from the parent material.

Comparative example 3

The difference from embodiment 1 is that the annealing treatment is not performed on the processed region structure in this embodiment, and the specific method is as follows:

the low carbon low alloy steel sheet having the same thickness, composition and mechanical properties as those of example 1 was used, and the tool material, dimensions, processing parameters and auxiliary cooling process were also the same as those of example 1. The annealing treatment is not carried out on the structure of the processing area after the processing. The average grain size of the structure obtained in the processing area is less than 1 μm, and the structure is a ferrite/martensite dual-phase structure. Because the phase components are not regulated and controlled by subsequent annealing treatment, the austenite phase is not detected in the processing region, the martensite phase content is up to more than 90%, and the tensile test result shows that the tensile strength of the organization of the processing region is 1825MPa which is 1.4 times of that of the parent metal, but the plasticity is greatly reduced relative to the parent metal, the uniform elongation is lower than 3%, and the total elongation is lower than 10%.

Comparative example 4

The difference from embodiment 1 is that the embodiment adopts the annealing treatment at higher temperature and shortens the annealing time, and the specific method is as follows:

the low carbon low alloy steel sheet having the same thickness, composition and mechanical properties as those of example 1 was used, and the tool material, dimensions, processing parameters and auxiliary cooling process were also the same as those of example 1. After processing, the tissue of the processing area is annealed at the medium temperature of 740 ℃ in a two-phase area for 3 minutes. Subsequently, the annealed structure was quenched with 15 ℃ tap water at a transfer rate of 3 seconds. Due to the unreasonable annealing process, martensite and austenite phases are not detected in the final processing region, and the content of carbide phases is up to more than 4%; mechanical tests show that the tensile strength of the texture of the processing area is 1029MPa, the tensile strength is increased by 33% relative to the parent metal, but the plasticity is greatly reduced relative to the parent metal, the uniform elongation is lower than 1%, and the total elongation is lower than 15%.

Example 2

The general low-carbon low-alloy steel plate with the thickness of 4 mm is used, and the chemical components of the steel plate comprise the following components in percentage by weight: 0.12% of C, 1.05% of Si, 2.07% of Mn, 0.004% of S, 0.003% of P, 0.021% of Al, 0.007% of Cr and the balance of Fe; the tensile strength, uniform elongation and total elongation were 857MPa, 15% and 22%, respectively. Adopting a convex spherical metal ceramic stirring head with the radius of 10 mm to carry out stirring friction processing, wherein the tool rotating speed is 220 r/min, and the advancing speed is 30 mm/min; the processing process adopts flowing water to cool a processing area, the diameter of a water outlet of a water pipe is 8 mm, the flow rate is 2.5 liters/min, and the water temperature at the water outlet is 19 ℃. After processing, the processing area is subjected to two-phase area low-temperature annealing treatment, the temperature is 710 ℃, and the heat preservation time is 10 minutes. Subsequently, the annealed structure was quenched with 15 ℃ tap water at a transfer rate of 5 seconds. Due to the adoption of an improved processing tool, lower processing parameters and a reasonable annealing process, the finally obtained processed area structure has the average grain size of less than 1 mu m and contains about 6 percent of austenite phase and 38 percent of martensite phase; the tensile strength is 1200MPa, the tensile strength is improved by 40 percent relative to the parent metal, the uniform elongation and the total elongation are respectively 21 percent and 31 percent, and the tensile strength is respectively improved by 40 percent and 41 percent relative to the parent metal.

Example 3

The general low carbon steel plate with the thickness of 4 mm is used, and the chemical components of the low carbon steel plate are as follows in percentage by weight: 0.20% of C, 1.34% of Si, 2.25% of Mn, 0.004% of S, 0.005% of P, 0.030% of Al, 0.006% of Cr and the balance of Fe; the tensile strength, uniform elongation and total elongation were 550MPa, 20% and 34%, respectively. Adopting a convex ball type metal ceramic stirring head with the radius of 5 mm to carry out stirring friction processing, wherein the rotating speed of the stirring head is 300 revolutions per minute, and the advancing speed is 100 mm per minute; the diameter of a water outlet of a water pipe used for cooling a processing area in the processing process is 8 mm, and the flow rate is 2.5 liters/minute. After processing, the processing area is subjected to two-phase area annealing treatment, the temperature is 720 ℃, and the heat preservation time is 15 minutes. Subsequently, the annealed structure was quenched with tap water at 15 ℃ for 5 seconds. Microstructure observation shows that the average grain size of the finally obtained processed region structure is close to 1 μm, the volume fraction of an austenite phase is about 8 percent, the volume fraction of a martensite phase is about 33 percent, and the volume fraction of a carbide phase is about 2.1 percent; the tensile result shows that the tensile strength of the texture of the processing area is 930MPa, the tensile strength is increased by 69 percent relative to the parent metal, the uniform elongation and the total elongation are respectively 35 percent and 44 percent, and the tensile strength is increased by 75 percent and 29 percent relative to the parent metal.

Example 4

A common low-carbon steel plate with the thickness of 3 mm is used, and the chemical components of the low-carbon steel plate are as follows in percentage by weight: 0.11% of C, 1.75% of Si, 2.05% of Mn, 0.004% of S, 0.006% of P, 0.020% of Al, 0.006% of Cr and the balance of Fe; the tensile strength, uniform elongation and total elongation were 600MPa, 25% and 29%, respectively. Adopting a convex ball type metal ceramic stirring head with the radius of 7 mm to carry out stirring friction processing, wherein the rotating speed of the stirring head is 275 rpm, and the advancing speed is 100 mm/min; the diameter of the water outlet of the cooling water pipe adopted in the processing process is 8 mm, and the flow rate is 2.5 liters/minute. After processing, the two-phase region annealing treatment is carried out on the processing region, the temperature is 710 ℃, and the heat preservation time is 5 minutes. Subsequently, the annealed structure was quenched with tap water at 15 ℃ for 5 seconds. Microstructure observation indicates that the final worked region has an average grain size of about 0.7 μm, and contains about 14% by volume of austenite phase, about 39% by volume of martensite phase, and about 1.7% by volume of carbide phase; the tensile result shows that the tensile strength of the texture of the processing area is 1300MPa, the tensile strength is increased by 117 percent relative to the parent metal, the uniform elongation and the total elongation are respectively 28 percent and 33 percent, and the tensile strength is increased by 12 percent and 14 percent relative to the parent metal.

Comparative example 4

The difference from embodiment 4 is that in this embodiment, a higher heat input parameter is used in the friction stir processing, and the specific method is as follows:

using a general mild steel sheet having the same thickness, composition and mechanical properties as those of example 4, the tool material, size and traveling speed were also the same as those of example 4, but the tool rotation speed was increased to 400 rpm. The process was the same as example 4 with forced cooling of the process zone. After processing, the surface of the stirring head can be seen with cracks by naked eyes and has obvious abrasion phenomenon. The processed region structure was then subjected to the same annealing and quenching treatment as in example 4. The final processing area is subjected to structure observation, so that obvious abrasion particles exist in the final processing area, and the average grain size is close to 1 mu m; the tensile test result shows that the tensile strength of the texture of the processing area is 900 +/-100 MPa, although the tensile strength is improved to a certain extent relative to the base material, the strength fluctuation is obvious due to the introduction of wear particles; the uniform elongation and the total elongation are respectively 17% and 19%, and are both remarkably reduced relative to the parent material.

Claims

1. A plastic deformation processing technology for synchronously improving the strength and the plasticity of low-carbon steel is characterized in that: the process comprises the steps of carrying out underwater stirring friction processing on a low-carbon steel base material to obtain a superfine ferrite/martensite dual-phase structure in a processing area; then carrying out two-phase region low-temperature short-time annealing treatment and subsequent water quenching cooling treatment on the processing region, and finally obtaining low-carbon steel with high strength and high plasticity in the processing region;

the friction stir processing parameters are as follows: the rotating speed of the tool is 220-300 r/min, and the advancing speed is 25-200 mm/min; in the stirring and friction processing process, flowing water is adopted to cool a processing area, the water temperature is 15-25 ℃, the diameter of a water outlet is 5-15 mm, and the water flow rate is 2-5 liters/minute;

in the low-temperature short-time annealing treatment process of the two-phase region, the annealing temperature is 710-730 ℃, and the heat preservation time is 5-15 minutes; in the water quenching cooling treatment, the temperature of cooling water is 10-20 ℃, and the transfer speed is less than or equal to 7 seconds;

the stirring friction processing tool is a needleless stirring head, the free end of the stirring head is designed into a hemispherical convex structure, the sphere radius corresponding to the hemispherical convex structure is 5-10 mm, the processing depth is 0.5-5 mm, and the tool is made of TiC-based metal ceramic or Ti (C, N) -based metal ceramic;

after the base metal is subjected to stirring friction processing, low-temperature short-time annealing treatment and water quenching cooling treatment in a two-phase region, the obtained processing region tissue consists of ferrite, martensite, austenite and carbide, and the average grain size is 0.2-1 mu m; wherein the average size of the martensite islands is 0.5-1 μm, and the volume ratio of the martensite phase is 20-40%; the average grain size of austenite is 0.2-1 μm, and the volume proportion of austenite phase is 5-15%; the average size of the carbide is 0.05-0.1 μm, and the volume ratio of the carbide phase is 1-3%.

2. The plastic deformation processing technology for synchronously improving the strength and the plasticity of the low-carbon steel according to claim 1, characterized in that: the low-carbon steel comprises the following chemical components in percentage by weight: 0.1 to 0.2 percent of C, 1.0 to 2.0 percent of Si, 2.0 to 3.0 percent of Mn, less than 0.006 percent of S, less than 0.007 percent of P, less than 0.04 percent of Al, less than 0.013 percent of Cr and the balance of Fe.