High-strength and high-ductility low-carbon steel with trimodal ferrite grain size distribution and production process of high-strength and high-ductility low-carbon steel

Abstract

The invention relates to the technical field of metallurgical materials, in particular to high-strength and high-ductility low-carbon steel with trimodal ferrite grain size distribution and a production process of the high-strength and high-ductility low-carbon steel. After a steel billet is prepared through smelting, casting, forging and hot rolling, the steel billet is subjected to follow-up treatment, and the follow-up treatment comprises the following procedures of critical zone annealing, cold rolling and low-temperature annealing. By means of the process, ferrite grains have the characteristic of three-peak size distribution, and meanwhile fine carbides exist. And the tensile strength, the yield strength and the product of strength and elongation are greatly improved when the steel is pulled at room temperature.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgical materials, and in particular to a high-strength and high-ductility low-carbon steel with a trimodal ferrite grain size distribution and a production process thereof. Background Art

[0002] With the rapid development of global industrialization, higher performance requirements are being placed on structural materials, especially in terms of strength and ductility. Although traditional low-carbon steel is widely used in industries such as construction, transportation, and machinery manufacturing, its comprehensive performance often fails to meet the modern industry's pursuit of lightweight, high strength, and high safety.

[0003] At present, the research on improving the performance of ordinary low-carbon steel at home and abroad mainly focuses on the optimization of material composition, the improvement of heat treatment process and the regulation of microstructure. Although certain progress has been made, the existing technology still has shortcomings. For example, the optimization of material composition has not yet achieved the ideal effect, and the use of existing alloy elements may bring about the problem of increased cost or increased processing difficulty. In terms of processing technology, the method of grain refinement is mainly adopted. For example, the Chinese invention patent CN112251687A adopts cyclic quenching and adds elements such as chromium, molybdenum, titanium, tungsten, vanadium and aluminum to develop a high-performance fine-grained steel with uniform grains. Patent CN1373230A proposes a technical solution for refining the ferrite grains of low-carbon steel based on the deformation strengthening phase transformation and dynamic recrystallization of ferrite during the deformation of supercooled austenite, and realizes the ferrite grain size below 4μm. Although the fine grain treatment improves the strength and toughness of low-carbon steel, the fine-grained low-carbon steel has problems such as low work hardening rate and low elongation. By introducing an appropriate amount of relatively coarse grains into the fine-grained structure, that is, creating a grain structure with a bimodal distribution of grain size, the elongation can be greatly improved with little strength loss. Chinese invention patent CNC1632138A discloses a preparation technology for 20CrMnTi steel, which describes a unique process for obtaining an ultrafine grain structure with a bimodal grain size distribution. This method allows the original martensite to transform into ultrafine grains, while the original ferrite forms coarser grains. Ultimately, the technology successfully achieved a bimodal grain size distribution structure consisting of grains with diameters between 50 and 200 nanometers and 1 to 2 microns. Invention patent CN107177783B discloses an ultrafine-grained martensitic-ferrite dual-phase steel with a bimodal ferrite grain distribution and its production process, which is characterized by a bimodal size distribution of ferrite grains, and its elongation is greatly improved when pulled at room temperature. Although these methods have improved the performance of low-carbon steel, its tensile strength and yield strength are still low.

[0004] Therefore, how to further improve the strength and ductility of ordinary low-carbon steel is a technical problem that technical personnel in this field currently need to solve. Summary of the invention

[0005] The present invention is proposed on the basis of in-depth research on the kinetics of ferrite recrystallization. The purpose is to provide a high-strength and high-ductility low-carbon steel with a trimodal ferrite grain size distribution and a production process thereof, so as to obtain an ordinary low-carbon steel with a trimodal ferrite grain size distribution. Its tensile strength, yield strength and strength-ductility product are all greatly improved compared with ordinary low-carbon steel.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows:

[0007] On the one hand, the present invention provides a production process for high-strength and high-ductility low-carbon steel with a trimodal ferrite grain size distribution, wherein a steel billet is obtained by smelting, casting, forging and hot rolling, and the steel billet is subjected to subsequent treatment, wherein the subsequent treatment comprises the following steps:

[0008] (1) Critical annealing: The critical annealing temperature is 750-850°C, and the temperature is kept at this temperature for 8-15 minutes. After the temperature is kept at this temperature, the temperature is cooled to room temperature at a cooling rate of 20-35°C / s.

[0009] (2) Cold rolling: cold rolling the cooled slab;

[0010] (3) Low temperature annealing: The cold rolled slab is subjected to low temperature annealing at a temperature of 500-650°C for 30-60 min, followed by air cooling to room temperature.

[0011] The purpose of intercritical zone annealing is to obtain ferrite, martensite and pearlite structures, providing the original structure for subsequent processes; the purpose of cold rolling the steel plate after intercritical annealing is to provide sufficient driving force for ferrite recrystallization during the final low-temperature annealing. The cold-rolled structure includes ferrite, martensite and pearlite regions, providing different driving forces for subsequent ferrite recrystallization; the cold-rolled steel plate is annealed at low temperature, and air-cooled to room temperature after heat preservation. At this time, due to the different recrystallization dynamics of ferrite, martensite and pearlite regions in the cold-rolled structure, the ferrite in the structure has the characteristics of a three-peak distribution. The above stages cooperate with each other to achieve the characteristics of a three-peak distribution of ferrite grain size, and the obtained low-carbon steel strip has higher strength and elongation.

[0012] Preferably, in the critical annealing process, after keeping warm, the material is cooled to room temperature at a cooling rate of 20 to 35° C. / s.

[0013] Preferably, the deformation amount of the cold rolling is 60% to 80%.

[0014] Preferably, the final rolling temperature of the hot rolling is 800-900° C., and the hot rolling is followed by air cooling to room temperature.

[0015] Preferably, the composition of the steel billet, by weight percentage, is C 0.10% to 0.20%, Mn 1.50% to 250%, Si 0.10% to 0.20%, and the balance is Fe.

[0016] On the other hand, the present invention provides a low carbon steel obtained by the above production process, wherein the ferrite grain size in the low carbon steel has the characteristics of a trimodal distribution, the ferrite grain size of the ultrafine grain part is <1μm, the ferrite grain size of the fine grain part is 1-5μm, and the ferrite grain size of the coarse grain part is >5μm.

[0017] Preferably, in terms of volume percentage, there are 2-5% of fine carbide structures in the structure.

[0018] Preferably, at room temperature, the low carbon steel has a tensile strength greater than 700 MPa, a yield strength greater than 500 MPa, and an elongation greater than 20%.

[0019] Beneficial effects of the present invention:

[0020] The low carbon steel production process provided by the present invention is simple, does not add special alloy components, and has low production costs. At the same time, the ferrite grain size in the microstructure has the characteristics of a three-peak distribution, and there are 2 to 5% (volume percentage) of fine carbides. Under room temperature tension, its yield strength, tensile strength and strength-ductility product are greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A temperature-time curve diagram of heat treatment in the production process of high-strength and high-ductility low-carbon steel with trimodal ferrite grain size distribution of the present invention;

[0022] Figure 2 This is a scanning electron microscope photograph of the high-strength and high-ductility low-carbon steel structure with a trimodal ferrite grain size distribution produced in Example 1;

[0023] Figure 3 This is a ferrite grain size distribution diagram of the high-strength and high-ductility low-carbon steel with a trimodal ferrite grain size distribution produced in Example 1;

[0024] Figure 4 This is a scanning electron microscope photograph of the high-strength and high-ductility low-carbon steel structure with a trimodal ferrite grain size distribution produced in Example 2;

[0025] Figure 5 This is a ferrite grain size distribution diagram of the high-strength and high-ductility low-carbon steel with a trimodal ferrite grain size distribution produced in Example 2;

[0026] Figure 6This is a scanning electron microscope photograph of the high-strength and high-ductility low-carbon steel structure with a trimodal ferrite grain size distribution produced in Example 3;

[0027] Figure 7 This is a ferrite grain size distribution diagram of the high-strength and high-ductility low-carbon steel with a trimodal ferrite grain size distribution produced in Example 3;

[0028] Figure 8 The tensile stress-strain curves of the high-strength and high-ductility low-carbon steels with trimodal ferrite grain size distribution produced in Examples 1-3 and the ordinary low-carbon steel of Comparative Example 1. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0030] Figure 1 A specific implementation method of the production process of high-strength and high-ductility low-carbon steel with trimodal ferrite grain size distribution provided by the present invention, after the steel billet is obtained by smelting, casting, forging and hot rolling, the steel billet is subsequently treated, including the following steps: intercritical annealing, cold rolling and low-temperature annealing;

[0031] (1) performing intercritical annealing of the hot-rolled steel at 750-850°C for 8-15 min, and cooling the steel to room temperature at a cooling rate of 20-35°C / s after isothermal annealing;

[0032] (2) performing a 60% to 80% cold rolling treatment;

[0033] (3) performing low-temperature annealing treatment in a temperature range of 500 to 650° C. for 30 to 60 min, and cooling to room temperature by air cooling after annealing.

[0034] In a specific embodiment, the above-mentioned steel billet is smelted in a 50kg induction furnace, and its chemical composition, by weight percentage, is C 0.10%-0.20%, Mn 1.50%-2.50%, Si 0.10%-0.20%, and the balance is Fe; after the ingot is smelted and cast, it is forged into a square billet for hot rolling; the hot rolling final rolling temperature is 800-900°C, and after rolling, it is air-cooled to room temperature to obtain a steel billet with a thickness of 3.0-4.0 mm.

[0035] The following non-limiting embodiments may enable a person skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0036] In the embodiment of the present invention:

[0037] 1. The microstructure of the annealed samples was observed using a Quanta 600 scanning electron microscope. The grain size was statistically analyzed using the average intercept length method. About 1,000 grains were counted for each sample to determine the grain size and their distribution.

[0038] 3. According to GB / T228-2002, standard tensile specimens with rectangular cross-sections were made and tensile tests were carried out on a CMT5105-SANS microcomputer-controlled electronic universal testing machine.

[0039] Example 1

[0040] The weight percentage of the ingot composition is: C 0.11%, Mn 1.83%, Si 0.12%, and the balance is Fe. The hot rolling final rolling temperature is 850°C, and the thickness of the rolled steel billet is 3.0 mm.

[0041] The hot rolled steel billet is subsequently processed, including the following steps:

[0042] (1) performing intercritical annealing on the hot-rolled steel billet at a temperature of 770°C for 10 min, and then cooling to room temperature at a cooling rate of 25°C / s;

[0043] (2) cold rolling the cooled slab by 70%, and the thickness of the slab after rolling is 0.9 mm;

[0044] (3) The cold-rolled slab is subjected to low-temperature annealing, wherein the annealing temperature is 600°C, the temperature is kept isothermally for 50 minutes, and then air-cooled to room temperature.

[0045] The high-strength and high-ductility low-carbon steel plate with a trimodal ferrite grain size distribution obtained in Example 1 was measured by scanning electron microscopy to have an ultrafine grain size of <1 μm, a fine grain size of 1-5 μm, and a coarse grain size of >5 μm. At room temperature, its tensile strength was 730.64 MPa, its yield strength was 557.15 MPa, its elongation was 25.87.3%, and its strength-ductility product was 18.9 GPa%. The scanning electron microscopy structure is shown in FIG. Figure 2 As shown, the ferrite grain size distribution is Figure 3 The stress-strain curve is shown in Figure 8 The mechanical properties are shown in Table 1.

[0046] Example 2

[0047] The weight percentage of the ingot composition is: C 0.15%, Mn 1.76%, Si 0.10%, and the balance is Fe. The hot rolling final rolling temperature is 830°C, and the thickness of the rolled steel billet is 3.2 mm.

[0048] The hot rolled steel billet is subsequently processed, including the following steps:

[0049] (1) annealing the hot-rolled steel billet in the critical region, wherein the annealing temperature is 780°C, the temperature is kept isothermally for 12 min, and then the steel billet is cooled to room temperature at a cooling rate of 30°C / s;

[0050] (2) cold rolling the cooled slab by 75%, and the thickness of the slab after rolling is 0.8 mm;

[0051] (3) The cold-rolled slab is subjected to low-temperature annealing, wherein the annealing temperature is 550°C, the temperature is kept constant for 40 minutes, and then air-cooled to room temperature.

[0052] The high-strength and high-ductility low-carbon steel plate with a trimodal ferrite grain size distribution obtained in Example 2 was measured by a scanning electron microscope to have a ferrite grain size of <1 μm in the ultrafine grain part, a ferrite grain size of 1 to 5 μm in the fine grain part, and a coarse grain size of >5 μm. At room temperature, its tensile strength was 775.50 MPa, its yield strength was 588.57 MPa, and its elongation was 22.60%. The strength-ductility product was 17.53 GPa% The scanning electron microscope structure is as follows Figure 4 As shown, the ferrite grain size distribution is Figure 5 The stress-strain curve is shown in Figure 8 The mechanical properties are shown in Table 1.

[0053] Example 3

[0054] The weight percentage of the ingot composition is: C 0.10%, Mn 1.50%, Si 0.13%, and the balance is Fe. The hot rolling final rolling temperature is 800°C, and the thickness of the rolled steel billet is 3.0 mm.

[0055] The hot rolled steel billet is subsequently processed, including the following steps:

[0056] (1) performing intercritical annealing on the hot-rolled steel billet at a temperature of 750°C for 10 min, and then cooling to room temperature at a cooling rate of 30°C / s;

[0057] (2) cold rolling the cooled slab by 80%, and the thickness of the slab after rolling is 0.6 mm;

[0058] (3) The cold-rolled slab is subjected to low-temperature annealing, wherein the annealing temperature is 550°C, the temperature is kept constant for 30 minutes, and then air-cooled to room temperature.

[0059] The high-strength and high-ductility low-carbon steel plate with a trimodal ferrite grain size distribution obtained in Example 3 was measured by scanning electron microscopy to have an ultrafine ferrite grain size of <1 μm, a fine ferrite grain size of 1-5 μm, and a coarse ferrite grain size of >5 μm. At room temperature, its tensile strength was 786.53 MPa, its yield strength was 717.47 MPa, its elongation was 20.53%, and its strength-ductility product was 16.15 GPa%. The scanning electron microscopy structure is shown in FIG. Figure 6 As shown, the ferrite grain size distribution is Figure 7 The stress-strain curve is shown in Figure 8 The mechanical properties are shown in Table 1.

[0060] Comparative Example 1

[0061] Comparative Example 1 is a common hot-rolled low-carbon steel plate, which uses the same steel billet as Example 3, and its ingot composition weight percentage is: C 0.10%, Mn 1.50%, Si 0.13%, and the balance is Fe. The hot rolling final rolling temperature is 800°C, and the thickness of the steel billet after rolling is 3.0mm. The steel billet is mainly ferrite and pearlite at room temperature, and its tensile strength, yield strength and strength-ductility product are 395.07MPa, 294.23MPa and 13.20GPa% respectively.

[0062] Comparative Example 2

[0063] According to the literature "A novel technique for developing bimodal grain size distributions in low carbon steels", a low carbon steel with bimodal ferrite distribution was prepared. The weight percentage of the ingot composition was: C0.17%, Mn 0.74%, Al 0.04%, P 0.008%, S 0.009%, N 0.0047%, and the balance was Fe. Its yield strength, tensile strength and strength-ductility product were 460MPa, 550MPa and 5.98GPa%.

[0064] Table 1 Mechanical properties

[0065]

[0066] It can be seen from Table 1 that, compared with Comparative Examples 1 and 2, the yield strength, tensile strength and strength-ductility product of the low carbon steel produced by the present invention under room temperature tensile stress are greatly improved, showing the advantage of the trimodal ferrite grain distribution.

[0067] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A process for producing high-strength and high-ductility low-carbon steel with a trimodal ferrite grain size distribution, wherein a steel billet is obtained by smelting, casting, forging and hot rolling, characterized in that: The steel billet is subjected to subsequent processing, and the subsequent processing includes the following steps: (1) Critical annealing: The critical annealing temperature is 750-850°C, kept at this temperature for 8-15 minutes, and then cooled to room temperature; (2) Cold rolling: cold rolling the cooled slab; (3) Low temperature annealing: The cold rolled slab is subjected to low temperature annealing at a temperature of 500-650°C for 30-60 min, followed by air cooling to room temperature.

2. The process for producing high-strength and high-ductility low-carbon steel with trimodal ferrite grain size distribution according to claim 1, characterized in that: In the intercritical annealing process, the material is cooled to room temperature at a cooling rate of 20 to 35° C. / s after being kept warm.

3. The process for producing high strength and high ductility low carbon steel with trimodal ferrite grain size distribution according to claim 1, characterized in that: The deformation amount of the cold rolling is 60% to 80%.

4. The process for producing high-strength and high-ductility low-carbon steel with trimodal ferrite grain size distribution according to claim 1, characterized in that: The final rolling temperature of the hot rolling is 800-900° C., and the hot rolling is followed by air cooling to room temperature.

5. The process for producing high strength and high ductility low carbon steel with trimodal ferrite grain size distribution according to claim 1, characterized in that: The composition of the steel billet, in terms of weight percentage, is 0.10% to 0.20% C, 1.50% to 250% Mn, 0.10% to 0.20% Si, and the balance is Fe.

6. A low carbon steel obtained by the production process according to any one of claims 1 to 5, characterized in that: The ferrite grain size in the low carbon steel has a trimodal distribution characteristic, the ferrite grain size of the ultrafine grain part is <1 μm, the ferrite grain size of the fine grain part is 1-5 μm, and the ferrite grain size of the coarse grain part is >5 μm.

7. The low carbon steel according to claim 6, characterized in that In terms of volume percentage, 2-5% of fine carbide structures exist in the structure.

8. The low carbon steel according to claim 6, characterized in that At room temperature, the low carbon steel has a tensile strength of >700 MPa, a yield strength of >500 MPa, and an elongation of >20%.

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