High-strength non-quenched and tempered cold heading steel and production method thereof

Abstract

The application provides a high-strength non-quenched and tempered cold upsetting steel and a production method thereof, and the composition is as follows: C 0.15%-0.28%, Si 0.60%-0.75%, Mn 1.6%-1.9%, Cr 0.2%-0.30%, V 0.08-0.15%, Nb 0.01-0.03%, Alt 0.015%-0.035%, P≤0.015%, S≤0.015%, T.O≤0.0020%, N≤0.0060%, and the rest is Fe and other inevitable impurities. Compared with the prior art, the application adopts high C-high Mn, adds micro-alloy grain elements such as Cr, V and Nb, improves the strength and toughness, obtains a ferrite+martensite+pearlite+residual austenite composite microstructure, and the produced non-quenched and tempered cold upsetting steel is used to manufacture high-strength fasteners with a tensile strength of 1100 MPa or more, and has good strength, plasticity and toughness and delay fracture resistance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of non-quenched and tempered cold upsetting steel, and particularly relates to a high-strength non-quenched and tempered cold upsetting steel and a production method thereof. BACKGROUND

[0002] Fasteners are the most common general-purpose basic parts with a large quantity and wide range, and are widely used in the industries of mechanical manufacturing, engineering structure, railway, automobile and tractor, building, etc. About 70% of the coupled parts and combined devices are coupled by fasteners. The fasteners are mainly formed by cold upsetting, and the material needs to bear a total deformation of up to 70-80% in the production process. Therefore, the material needs to have good plasticity and low hardness before cold upsetting, so that the cold upsetting steel wire produced by the traditional process needs to be subjected to the time-consuming and energy-consuming spheroidizing annealing and quenching and tempering heat treatment processes before cold upsetting and drawing.

[0003] In recent years, under the pressure of energy saving and cost reduction, fastener manufacturers urgently require steel plants to develop new energy-saving cold upsetting steel wire that can save quenching and tempering treatment and spheroidizing annealing treatment before drawing, to replace quenched and tempered steel. The new energy-saving cold upsetting steel wire can not only save energy, but also can avoid problems such as quenching cracks, workpiece deformation, surface oxidation and decarburization caused by heat treatment. In addition, the new energy-saving cold upsetting steel wire can simplify the process, improve production efficiency, and has special significance and necessity for energy saving, cost reduction and environmental pollution reduction, and has significant economic and social benefits. Therefore, the new energy-saving cold upsetting steel wire has been widely used at home and abroad.

[0004] At present, the main method at home and abroad is to achieve the same level of quenched and tempered steel by adopting micro-alloying, controlled rolling and controlled cooling, and cold work hardening. In the 1980s of the 20th century, the research and development of cold work hardening non-quenched and tempered steel began internationally, and non-quenched and tempered cold upsetting steel has also been successfully developed in China. The non-quenched and tempered cold upsetting steel can be used to produce screws, studs and other fasteners with small deformation, and mainly faces the mechanical and building industries.

[0005] However, with the development of technology, the requirements for the strength and performance of fasteners are becoming higher and higher, which brings new challenges to the production of non-quenched and tempered cold upsetting steel.

[0006] The patent CN111206190A published on May 29, 2020 discloses a kind of non-quenched cold upsetting steel for weather-resistant fastener, weather-resistant fastener and the production method of both, its component and its percentage by weight are as follows: C 0.10%~0.20%, Si 0.30%~0.50%, Mn 1.30%~1.50%, Cr 0.60%~0.80%, Ni 0.20%~0.30%, Cu 0.20%~0.30%, V 0.05%~0.10%, Alt 0.040%~0.060%, Ti 0.010%~0.020%, N 0.010%~0.015%, O≤0.0015%, P≤0.030%, S≤0.020%, the rest is Fe and other inevitable impurities, guarantee weathering index I>6.5 on chemical composition; and Ni / Cu≥0.8, (Ti+V) / N≥5.5;The non-quenched cold upsetting steel for weather-resistant fastener has good weather resistance, which can be directly drawn into fastener with strength grade above 9.8 level without annealing, but the highest strength is only 1000MPa, which cannot meet the requirement of high strength of product.

[0007] CN101619414A published on January 6, 2010 discloses a kind of 10.9 grade non-quenched cold upsetting steel containing niobium and the rolling method of its hot rolled wire rod, component percentage by weight (%): C 0.08-0.14%, Si 0.03-0.35%, Mn 1.80-2.30%, P≤0.025%, S≤0.015%, B 0.0005-0.003%, Ti 0.01-0.03%, Als 0.010-0.050%, Nb 0.02%-0.04%, the rest is iron and trace impurities. The rolling method of its hot rolled wire rod is as follows: heating temperature 1050-1250℃, rough, medium rolling temperature 1000-1050℃, finish rolling temperature 750-830℃, deformation amount is 50-60%, deformation rate is 20 / s, wire drawing temperature is 730-850℃, after wire drawing, two-stage controlled cooling is controlled, fast cooling is greater than or equal to 5℃ / s above 550℃, slow cooling is 0.1-3℃ / s below 550℃, but the tensile strength is only 770-840MPa, which cannot meet the requirement of high strength of product. SUMMARY

[0008] The purpose of the present application is to provide a kind of high-strength non-quenched cold upsetting steel and its production method, by component design and process optimization, the high-strength non-quenched cold upsetting steel obtained is used to make high-strength fastener with tensile strength above 1100MPa, can save spheroidizing annealing and quenching and tempering treatment, and after drawing and stabilization treatment, the tensile strength R m ≥1100MPa, the yield ratio R P0.2 / R m≥0.93, elongation A ≥12%, area reduction Z ≥44%, good strength and plasticity, and good resistance to delayed fracture.

[0009] The specific technical scheme of the application is as follows:

[0010] A high-strength non-quenched and tempered cold-upsetting steel comprises the following components in percentage by mass:

[0011] C 0.15%~0.28%, Si 0.60%~0.75%, Mn 1.6%~1.9%, Cr 0.2%~0.30%, V 0.08~0.15%, Nb 0.01~0.03%, Alt 0.015%~0.035%, P ≤0.015%, S ≤0.015%, T.O ≤0.0020%, N ≤0.0060%, and the rest is Fe and other inevitable impurities.

[0012] The application provides a production method of the high-strength non-quenched and tempered cold-upsetting steel, which comprises the following technological process: component distribution according to the formula, electric furnace smelting, LF+RH vacuum refining, large round billet continuous casting, heating, initial rolling and blooming, six-continuous rolling, square billet rolling, flaw detection and grinding, high-speed wire rolling, controlled rolling and controlled cooling, wire product, packaging and warehousing.

[0013] In order to ensure the surface quality of the wire product, the large round billet is adopted for continuous casting, the surface quality of the casting billet can be effectively ensured, the wire is rolled after being rolled into a 150mm square billet, and the defect depth of the final product surface scabbing, pit and the like is less than or equal to 0.1mm.

[0014] The high-speed wire rolling is realized by adopting low-temperature rolling to achieve the purpose of online softening, the temperature of the entry reducing and sizing machine is controlled to be 740~760 DEG C, and the wire drawing temperature is 750~770 DEG C.

[0015] In order to obtain the ferrite+martensite+pearlite+residual austenite complex structure required by the application, the application adopts the mode of fast cooling first and slow cooling later, the front five heat preservation covers are opened, the fan is opened to 88% for fast cooling, the cooling rate is 6~8 DEG C / s, and the temperature is cooled to 470~520 DEG C, the rear six to eleven fans are all closed, the heat preservation covers are all closed, the cooling rate is 0.7~1.0 DEG C / s, the generation of the martensite structure is avoided, the temperature is to 400~440 DEG C, the winding is collected, the hook is hooked, and then the air cooling is carried out to room temperature, the package is packed, and the weight is weighed.

[0016] The hot-rolled microstructure of the high-strength non-quenched and tempered cold-upsetting steel produced according to the above method is: ferrite+martensite+pearlite+residual austenite, the area content of the ferrite is greater than or equal to 30%, the area content of the martensite+pearlite is greater than or equal to 60%, the tensile strength is R m ≥930MPa.

[0017] The high-strength non-quenched cold heading steel produced by the above method is processed into fasteners, and the processing process is: cold drawing→cold heading forming→processing thread→low-temperature stabilization treatment→surface treatment processing fasteners. The low-temperature stabilization treatment process is: heating at 200±10°C for 90±5 minutes and air cooling, which can be combined with galvanizing, Dacromet and other surface treatment processes, and the quenching + tempering treatment can be omitted.

[0018] After the high-strength non-quenched cold heading steel is drawn and stabilized according to the above method, the austenite grain size of the steel is ≥10.0 grade, the tensile strength R m ≥1100MPa, the yield ratio R P0.2 / m ≥0.93, the elongation A after fracture ≥12%, the reduction of area Z ≥44%, the delayed fracture performance R value ≥0.82, and the steel has good strength, plasticity and toughness, and good resistance to delayed fracture.

[0019] The design idea of the present application is as follows:

[0020] C: In order to ensure the 1100MPa high-strength level, the content of C element is appropriately increased, and the content of C must be ≥0.15% to obtain a pearlitic cold strengthening non-quenched steel. With the increase of the content of C, the proportion of pearlite increases, the strength increases, and at the same time, the proportion of ferrite decreases, the plasticity decreases, and the cold workability becomes poor. Therefore, in order to ensure the plasticity and realize the annealing-free treatment, the content of C is controlled to be below 0.28%. The content of C is preferably controlled to be 0.15%-0.28%.

[0021] Si: In order to ensure that the content of ferrite is ≥30%, the content of silicon is appropriately increased, which can increase the content of ferrite in the structure and make the grain finer, thus being beneficial to the improvement of toughness. At the same time, silicon element has a strong solid solution strengthening effect, which can significantly improve the work hardening rate of the steel. Therefore, the content of Si is controlled to be 0.60%-0.75%.

[0022] Mn: The content of Mn can improve the hardness and strength of ferrite and pearlite in the steel, realize the strength of 1100MPa, and at the same time, the content of Mn can improve the hardenability, which is beneficial to obtain a certain amount of martensite and increase the strength. However, excessive Mn will reduce the plasticity of the steel. Therefore, the content of Mn is controlled to be 1.6%-1.9%.

[0023] Cr: Cr can effectively improve the hardenability of the steel, and in combination with Mn element, a certain amount of martensite is obtained to obtain the required high strength, and through solid solution strengthening, the hardness of pearlite can also be significantly improved; at the same time, Cr can also reduce the activity of C, which can reduce the tendency of decarburization on the surface of the steel during heating, rolling and forging, and can obtain high fatigue resistance. However, excessive content will deteriorate the toughness and cold workability of the steel, and therefore the content of Cr is controlled to be 0.2%-0.3%.

[0024] V, Nb: Both V and Nb can refine the grain size and improve the strength and toughness of the steel. V and Nb form V(C,N) and Nb(C,N) precipitates in the steel, which can pin the original austenite grain boundaries, refine the austenite, and improve the nucleation rate of ferrite and pearlite. The steel produced has ultra-fine grains, excellent strength and plasticity, and can be directly drawn and cold headed without annealing. In addition, V(C,N) and Nb(C,N) precipitates have strong hydrogen trapping ability, which can trap hydrogen and make it uniformly dispersed in the grains, thereby improving the resistance to delayed fracture of the steel. The effect of V and Nb is better, so the content of V should be controlled at 0.08% to 0.15%, and the content of Nb should be controlled at 0.01 to 0.03%.

[0025] Alt: Alt is a strong deoxidizing element that can improve the oxidation resistance of the steel. Al and C, N form carbide particles that can refine the grain size and improve the resistance to delayed fracture. Too low Al content will result in insufficient AlN precipitation, which cannot inhibit grain growth. Too high Al content will easily form coarse carbonitride, increase the inclusion content, and reduce the purity and resistance to delayed fracture of the steel. The content of Alt should be controlled at 0.015% to 0.035%.

[0026] S and P: S and P impurities will segregate at the grain boundaries, which will greatly reduce the resistance to delayed fracture. P can form micro-segregation during solidification of the steel, and then segregate at the grain boundaries during austenitizing temperature heating, which will significantly increase the brittleness of the steel and increase the susceptibility to delayed fracture. S forms MnS inclusions and segregates at the grain boundaries, which will increase the susceptibility to delayed fracture. Therefore, the content of P and S should be controlled at P ≤ 0.010% and S ≤ 0.010%.

[0027] T.O and N: Oxygen in the steel forms various oxide inclusions. Under the action of stress, stress concentration occurs at these oxide inclusions, which leads to the initiation of micro-cracks, thereby deteriorating the mechanical properties of the steel, especially the toughness and fatigue resistance. Therefore, measures should be taken to reduce the content of T.O as much as possible, and the content of T.O should be controlled at ≤ 0.0020%. N in the steel precipitates Fe4N, which has a slow diffusion rate, which leads to the aging of the steel. N also reduces the cold working performance of the steel, so the content of N should be controlled at ≤ 0.0060%.

[0028] The present application adopts high C-high Mn component design in steel component design to obtain high strength, and adds micro-alloy grain elements such as Cr, V, Nb, etc. to improve strength and toughness, and finally obtains the compound microstructure of ferrite+martensite+pearlite+residual austenite. The high-strength non-quenched and tempered cold heading steel produced by the present application can be used to manufacture high-strength fasteners with tensile strength of 1100 MPa or more, and can save spheroidizing annealing and quenching and tempering treatment. The hot-rolled microstructure is ferrite+martensite+pearlite+residual austenite, the tensile strength is R m ≥930 MPa, the austenite grain size of the steel after drawing and stabilization treatment is ≥10.0 grade, the tensile strength R m ≥1100 MPa, the yield strength R P0.2 / t m ≥0.93, the elongation A is ≥12%, the reduction of area Z is ≥44%, the delayed fracture performance R value is ≥0.82, and the steel has good strength, plasticity and toughness, and good resistance to delayed fracture performance. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The hot-rolled microstructure of the non-quenched and tempered cold heading steel with excellent corrosion resistance is ferrite+martensite+pearlite+residual austenite. DETAILED DESCRIPTION

[0030] The present application is further illustrated by the following examples and comparative examples.

[0031] Examples 1-5

[0032] A non-quenched and tempered cold heading steel with excellent corrosion resistance comprises the following mass percentage components: as shown in Table 1, the balance of Table 1 is Fe and other unavoidable impurities.

[0033] Table 1 Chemical composition of the present application (wt%)

[0034] Case C Si Mn Cr V Nb Alt P S T.O N Example 1 0.18 0.62 1.9 0.20 0.08 0.01 0.018 0.007 0.005 0.0016 0.0040 Example 2 0.15 0.70 1.6 0.25 0.15 0.02 0.020 0.006 0.003 0.0015 0.0048 Example 3 0.28 0.60 1.7 0.30 0.10 0.03 0.023 0.007 0.004 0.0015 0.0043 Example 4 0.25 0.75 1.8 0.28 0.09 0.02 0.015 0.008 0.004 0.0013 0.0037 Example 5 0.20 0.68 1.9 0.26 0.12 0.01 0.016 0.009 0.006 0.0016 0.0054 Comparative Example 1 0.10 ]]> ​ 0.55 ]]> ​ 1.50 ]]> ​ 0.20 0.10 0.02 0.018 0.010 0.008 0.0015 0.0050

[0035] A non-quenched and tempered cold heading steel with excellent corrosion resistance comprises the following mass percentage components: produced according to the composition of Example 1 in Table 1.

[0036] The production method of the non-quenched and tempered cold heading steel with excellent corrosion resistance in each of the above examples adopts the following process flow to produce:

[0037] According to the component allocation→electric furnace smelting→LF+RH vacuum refining→large round billet continuous casting→heating→primary rolling and blooming→six continuous rolling→rolled square billet→flaw detection and grinding→high-speed wire rolling→controlled rolling and controlled cooling→wire product→packing and warehousing.

[0038] The process parameters of each embodiment and comparative example are shown in Table 2.

[0039] Table 2 Wire rod rolling process of the embodiments and comparative examples of the application

[0040] Example Reducing mill temperature / °C Spinning temperature / °C Quench cooling rate °C / s Quench cooling rate °C / s Microstructure and content in hot rolled state Tensile strength in hot rolled state / MPa Grain size Average grain size / μm Example 1 760 770 6 0.7 Ferrite (35%) + Martensite + Pearlite (62%) + Retained Austenite 935 10.5 16.0 Example 2 755 760 6.8 1 Ferrite (33%) + Martensite + Pearlite (65%) + Retained Austenite 940 10.5 14.3 Example 3 740 755 8 0.8 Ferrite (33%) + Martensite + Pearlite (66%) + Retained Austenite 950 10.0 15.7 Example 4 750 765 7.3 0.9 Ferrite (32%) + Martensite + Pearlite (63%) + Retained Austenite 945 10.5 12.8 Example 5 743 750 7.5 1 Ferrite (35%) + Martensite + Pearlite (60%) + Retained Austenite 955 10.0 13.6 Comparative Example 1 745 758 7.4 0.8 Ferrite (29%) + Pearlite + Bainite + Martensite Comparative Example 2 ]]> ​ 948 10.5 12.9 Ferrite (18%) + Pearlite + Bainite + Martensite 860 ]]> ​ 849 ]]> ​ 3.5 ]]> ​ 4 ]]> ​ Case Tensile strength (MPa) ]]> ​ 905 ]]> ​ 10.5 13.1

[0041] The high-strength non-quenched and tempered cold heading steel produced by the above embodiment 1-embodiment 5 and comparative example 1 does not need to be quenched and tempered, and is formed by cold drawing→cold heading→processing threads→low-temperature stabilization treatment at 200℃±10℃ for 90min→surface treatment to process fasteners, and the mechanical properties are shown in Table 3.

[0042] Mechanical property test method GB / T 228.1 Tensile test of metallic materials Part 1: room temperature test method

[0043] The delayed fracture performance is evaluated and analyzed by slow strain rate tensile test (SSRT) at room temperature, and the delayed fracture strength ratio R = R BN / R BN0 (the notched tensile strength of the hydrogen-charged sample is R BN ; and the notched tensile strength of the non-hydrogen-charged sample is R BN0 ) (there is no uniform standard for delayed fracture performance at present)

[0044] Table 3 Performance of the final product of the embodiments of the application

[0045] Yield strength (MPa) Yield ratio Elongation (%) Area reduction (%) Delay fracture performance R value Example 1 Example 2 Example 3 1135 1070 0.943 12 45 0.885 Example 4 1140 1083 0.950 13 48 0.912 Example 5 1153 1082 0.938 14 47 0.837 Comparative Example 1 1145 1099 0.960 14 44 0.829 Comparative Example 2 1150 1096 0.953 15 46 0.898 ​ 1065 ]]> ​ 998 0.937 13 42 ]]> ​ 0.647 ]]> ​ ​ 1042 ]]> ​ 966 11 ]]> ​ 11 ]]> ​ 40 ]]> ​ 0.513 ]]> ​

[0046] The above underlined data do not meet the requirements of the present application.

[0047] Each embodiment is processed into a fastener by the above method, and the tensile strength R m ≥1100MPa, the yield ratio R P0.2 / R m ≥0.93, the elongation A≥12%, the reduction of area Z≥44%, and the austenite grain size of the steel is greater than or equal to 10.0 grade, indicating that the embodiments have good strength and toughness, and also have good delayed fracture resistance.

[0048] Comparative example 1 has C, Si and Mn contents not within the range of the application, and the strength produced according to the process of the application is not enough.

[0049] The performance of comparative example 2 in Table 4 is after quenching and tempering treatment, the quenching process is at 890℃ for 100min, and the tempering process is at 500℃ for 120min. Comparative example 2 uses the chemical composition of embodiment 1 and obtains a ferrite+pearlite+behenite+martensite structure by using a conventional slow cooling process, needs to be adjusted, and the strength does not reach the 1100MPa level, and the delayed fracture performance is also poor.

Claims

1. A high-strength non-quenched and tempered cold heading steel, characterized in that, The high-strength non-quenched and tempered cold heading steel comprises the following components by weight percentage: C 0.15%~0.28%, Si 0.60%~0.75%, Mn 1.6%~1.9%, Cr 0.2%~0.30%, V 0.08~0.15%, Nb 0.01~0.03%, Alt 0.015%~0.035%, P ≤0.015%, S ≤0.015%, TO≤0.0020%, N≤0.0060%, with the remainder being Fe and other unavoidable impurities; The hot-rolled microstructure of the high-strength non-quenched and tempered cold heading steel is: ferrite + martensite + pearlite + retained austenite, wherein the ferrite area content is ≥30%, the martensite + pearlite area content is ≥60%, and the tensile strength is R. m ≥930MPa; The production method of the high-strength non-quenched and tempered cold heading steel includes the following process flow: raw material distribution according to composition → electric furnace smelting → LF+RH vacuum refining → continuous casting of large round billets → heating → initial rolling → six-rolling → rolling square billets → flaw detection and grinding → high-speed wire rod rolling → controlled rolling and controlled cooling → finished wire rod → packaging and warehousing; The controlled rolling and controlled cooling process involves first cooling to 470-520°C at a cooling rate of 6-8°C / s, then cooling to 400-440°C at a cooling rate of 0.7-1.0°C / s, and finally air cooling to room temperature.

2. A method for producing high-strength non-quenched and tempered cold heading steel according to claim 1, characterized in that, The production method includes the following process flow: raw material distribution according to composition → electric furnace smelting → LF+RH vacuum refining → continuous casting of large round billet → heating → initial rolling → six-rolling → rolling square billet → flaw detection and grinding → high-speed wire rolling → controlled rolling and controlled cooling → finished wire rod → packaging and warehousing. The hot-rolled microstructure of the high-strength non-quenched and tempered cold heading steel is: ferrite + martensite + pearlite + retained austenite, wherein the ferrite area content is ≥30%, the martensite + pearlite area content is ≥60%, and the tensile strength is R. m ≥930MPa; The controlled rolling and controlled cooling process involves first cooling to 470-520°C at a cooling rate of 6-8°C / s, then cooling to 400-440°C at a cooling rate of 0.7-1.0°C / s, and finally air cooling to room temperature.

3. The production method according to claim 2, characterized in that, The high-speed wire rolling process controls the temperature at the sizing mill to be 740–760°C and the wire exit temperature to be 750–770°C.

4. The production method according to claim 2, characterized in that, The high-strength non-quenched and tempered cold heading steel produced by the aforementioned production method is then processed into fasteners through drawing and stabilization treatment.

5. The production method according to claim 4, characterized in that, The stabilization treatment involves heating to 200±10℃, holding at that temperature for 90±5 minutes, and then air cooling.

6. The production method according to claim 4 or 5, characterized in that, After stabilization treatment, the austenitic grain size of the product is ≥10.0 grade.

7. The production method according to claim 4 or 5, characterized in that, After stabilization treatment, the tensile strength R of the product m ≥1100MPa, yield strength ratio R P0.2 / R m ≥0.93, elongation after fracture A≥12%, reduction of area Z≥44%, delayed fracture performance R value≥0.82.

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