Production method of quenching and tempering-free spring square steel

Abstract

The invention belongs to the technical field of metallurgy, and discloses a production method of quenching and tempering-free spring square steel which comprises the following components in percentage by weight: 0.20%-0.30% of C, 0.30%-0.80% of Si, 1.5%-2.0% of Mn, 0.05%-0.10% of Cr, 0.01%-0.05% of Mo, 0.04%-0.10% of V, 0.020%-0.050% of Al, less than or equal to 0.020% of P, less than or equal to 0.020% of S, less than or equal to 0.20% of Cu, 0.0060%-0.0100% of N and the balance of Fe and inevitable impurities. A series of new rolling technologies such as smelting component design, large reduction, staggered temperature control rolling and slow cooling are adopted, a production structure adopts pearlite and ferrite as a matrix, the hot rolling tensile strength is 600 MPa or above, the austenite grain size of the steel is larger than or equal to the grade 8, and the steel has high strength, high toughness and excellent fatigue performance.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a production method of spring square steel free of quenching and tempering. Background Art

[0002] At present, agricultural machinery is developing in the direction of large-scale and lightweight. The domestic steel industry and engineering machinery industry are facing the challenges and opportunities of "adjusting structure and changing mode". The development and application of scientific research and technology work on energy conservation, emission reduction and pollution reduction have become very important. According to statistics, the application of quenched and tempered structural steel accounts for more than 35% of the total amount of special steel. Quenched and tempered steel usually requires a time-consuming, energy-consuming and environmentally polluting quenching and tempering process to ensure its mechanical properties. However, due to the omission of the quenching and tempering process, quench-free steel can shorten the construction period and reduce energy consumption costs by 25% to 38%. Therefore, if this new type of steel is used instead of quenched and tempered structural steel to manufacture mechanical structural parts, it will have significant economic and social benefits.

[0003] The quenching and tempering-free technology has been applied on a large scale in the fields of automobiles, machinery, etc. by innovating the traditional process by "rolling instead of forging, cooling instead of heating". With the progress of material design and intelligent manufacturing, the proportion of quenching and tempering steel it replaces is expected to increase from the current 30% to more than 50%.

[0004] In recent years, the agricultural vehicle industry has developed rapidly, and the competition for manufacturing costs among enterprises has become increasingly fierce. At present, traditional quenched and tempered structural materials with complex production processes, high energy consumption and serious heat treatment defects are gradually being replaced by quenched and tempered steel with simple production processes, low energy consumption and few defects. The production of structural steel for agricultural vehicles using quenched and tempered steel has become an important breakthrough in improving the market competitiveness of agricultural vehicle manufacturers. The chassis axle beam is one of the core components of the chassis of agricultural vehicles, and its performance largely determines the reliability and life of the vehicle. When working, it bears impact loads and inertial forces for a long time, and its various parts produce alternating stresses such as torsion, bending, tension, compression and shear. Therefore, the chassis axle beam must have good appearance quality and microstructure to ensure that the axle beam has sufficient strength, stiffness, toughness, wear resistance and good balance. For quenched and tempered steel axle beams, a lot of energy is consumed during the heat treatment process, and it will also cause problems such as oxidation, deformation and even cracking during the heating process of parts. During heat treatment and quenching, the axle beam often bends and twists, increasing energy waste and material loss. The non-quenching and tempering steel is widely favored by axle beam manufacturers for its low cost and energy saving and environmental protection. Based on the above requirements, it is imperative to develop high-strength non-quenching and tempering spring square steel. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the present invention provides a production method of non-quenching and tempering spring square steel. The present invention can avoid the problems of high energy consumption, easy oxidation and cracking in the tempering process, and solves the problems of poor hardenability and low strength of non-quenching and tempering steel.

[0006] The technical scheme adopted by the present invention is a production method of spring square steel for non-quenching and tempering, the chemical composition of the steel is as follows by weight percentage: C: 0.20%-0.30%, Si: 0.30%-0.80%, Mn: 1.5%-2.0%, Cr: 0.05%-0.10%, Mo: 0.01%-0.05%, V: 0.04%-0.10%, Al: 0.020%-0.050%, P≤0.020%, S≤0.020%, Cu≤0.20%, N: 0.0060%-0.0100%, and the rest is Fe and unavoidable impurities. The key process steps include:

[0007] (1) Smelting: The converter controls the weight of molten iron entering the furnace, controls the target composition of the final molten steel to be C ≥ 0.08%, P ≤ 0.012%, retains the steel for tapping, prohibits the addition of oxidized slag, and adds a composite deoxidizer for slag formation and deoxidation during the tapping process;

[0008] (2) Refining: LF basicity is controlled at 6.0-8.0. VN alloy is added in the early stage of refining to adjust the V in the steel to the target value. Before the end of LF furnace refining, aluminum wire is fed to adjust the aluminum in the molten steel to 0.050%-0.070%. Before leaving the station, calcium wire is fed to treat the molten steel with calcium, and then a covering agent is added to protect the molten steel. The RH is vacuumed to below 67Pa and the billet is opened for circulation nitrogen addition. After the vacuum is maintained for 15 minutes, the molten steel is broken for hydrogen determination to control the hydrogen content ≤1.5ppm. The RH is opened for circulation nitrogen addition to control the N content in the steel before leaving the station within the target range. The soft blowing time before leaving the station is 25-35 minutes.

[0009] (3) Continuous casting: weak cooling is adopted, the specific water volume is 0.16L / KG, the target value of the superheat of the tundish is 20-30℃, the temperature entering the straightening machine is 1100-1150℃, and the billet is piled and cooled for 24h;

[0010] (4) Rolling: The heating temperature in the soaking section is controlled at 1180-1220°C, the surface temperature in the intermediate rolling section is controlled at 850-880°C, the core temperature is controlled at 1000-1050°C, and the final rolling temperature is controlled at 750-800°C;

[0011] (5) Slow cooling after rolling: spray cooling after rolling, control the upper cooling bed temperature at 720±20℃, close-packed slow cooling on the cooling bed, and control the temperature of the slow cooling zone below 500℃.

[0012] Furthermore, in the (1) smelting stage, the weight of molten iron entering the furnace is 80t±1 ton, and the weight of scrap steel is 20±1 ton.

[0013] Furthermore, in the (2) refining stage, the first continuous casting furnace is fed with 100 meters of calcium wire, and the continuous casting furnace is fed with 80 meters of calcium wire.

[0014] Furthermore, in the (4) rolling stage, the reduction ratios of the first and second passes are greater than 60%, and the cumulative reduction ratio is greater than 80%.

[0015] Furthermore, the slow cooling rate in the post-rolling slow cooling stage (5) from 700-740°C to 500°C is 0.05°C / S.

[0016] Furthermore, the spring square steel produced has a pearlite + ferrite matrix, a hot rolled tensile strength of more than 600MPa, a surface reduction of more than 50%, a low temperature impact (-20°C) V-type of more than 60J, and an austenite grain size of ≥8 levels.

[0017] Principle of the invention:

[0018] (1) Requirements for oxygen content and inclusions in steel: There have been many experimental studies on the effect of oxygen content on the fatigue life of vehicle axle beams. Japan has conducted experiments on the relationship between oxygen content and fatigue life of carburized alloy steel. When the oxygen content drops from 25ppm to below 10ppm, its fatigue life can be increased several times. The current industry standard has stipulated that the oxygen content of automotive gear steel is ≤20ppm, and many special steel manufacturers can control the oxygen content of gear steel below 15ppm. Non-metallic inclusions of type B and D also have a great impact on the fatigue life of gears. These two types of inclusions are also related to the oxygen content, and are also closely related to the size and distribution of non-metallic inclusions. At present, gear steel customers require that type B inclusions be no more than level 2 and type D inclusions be no more than level 1. Type A inclusions have little effect on the fatigue life of gear steel, but due to customer requirements for gear steel processing performance and precision, requirements are put forward for the upper and lower limits of sulfur content, and requirements are put forward for the number, shape and distribution of type A inclusions. In response to such requirements raised by customers, special refining and continuous casting process measures must be adopted to control them, otherwise it will be difficult to meet the requirements of spring square steel customers. Class C inclusions are silicate inclusions, which can reach a level below Level 1 under existing equipment conditions.

[0019] (2) Grain size: Grain size is another important indicator of spring steel. Small and uniform austenite grain size is of great significance to improving strength. Grain refinement is mainly achieved by adding a certain amount of grain refining elements such as Al, V, N, etc. The steel of the present invention mainly uses AlN and VN to refine the grains. The present invention controls the Al, N, and V contents of spring steel within a reasonable range. By using AlN and VN to refine the grains, a finer grain structure can be obtained, and the austenite grain size can reach above level 8.

[0020] (3) Banded structure: Due to the effect of selective crystallization during the solidification process of steel, the composition segregation will occur in the horizontal and vertical directions of the solidified steel billet. During the cooling process after rolling, the composition segregation will form a layered distribution of the structure (ferrite and pearlite), namely the banded structure. Severe banded structure will cause differences in the microhardness of various parts of the shaft beam, affecting the fatigue life of the shaft beam. Since the equiaxed crystal area of ​​the die-cast material is larger than that of the continuous casting material, its composition uniformity is better than that of the continuous casting material, and the banded structure is lighter. As long as the pouring temperature and speed are controlled well, most furnace numbers can meet the requirement of less than level 3, while it is relatively difficult for the continuous casting material to meet this requirement. The fundamental solution to the banded structure is to reduce the segregation of the composition, combined with the appropriate cooling rate after rolling. The present invention can stably control the banded structure of the rolled square steel within 1.5 levels through the reasonable coordination of the continuous casting and rolling processes.

[0021] (4) Low-multiple rating: The high low-multiple rating of square steel is an important reason for the failure of axle beam products. For this reason, the present invention conducts process design based on product requirements and controls the low-multiple rating of square steel within 0.5 levels.

[0022] The beneficial effects of the present invention are:

[0023] (1) The present invention adopts a series of new rolling technologies such as smelting component design, large reduction, staggered temperature controlled rolling and slow cooling, breaking through the existing material and process limits, giving full play to the advantages of equipment, and producing square steel with pearlite + ferrite as the matrix, hot rolling tensile strength above 600MPa, surface shrinkage above 50%, low temperature impact (-20℃) V-type above 60J without quenching and tempering, promoting the research and development of high-quality high-alloy steel. The austenite grain size of the steel is ≥8, suitable for making solid square steel with a side length of 50-120mm, and has high strength and high toughness, and excellent fatigue performance.

[0024] (2) The present invention achieves grain refinement and improves hardenability by increasing the Mn and Cr contents in the steel and reasonably setting the Al and N contents in the steel without significantly increasing the contents of precious alloys such as V and Mo.

[0025] (3) The present invention adopts a large reduction initial rolling technology, with the reduction rate of the first two passes reaching 60% to break the grains; the staggered temperature rolling technology is used to regulate the deformation behavior and microstructure of the material, and the final rolling temperature is controlled at 750-800°C to further refine the grains.

[0026] (4) The present invention performs slow cooling after rolling, and the upper cooling bed temperature is controlled between 700-740°C and 500°C at a slow cooling rate of 0.05°C / S to ensure that the original structure does not contain brittle structures such as bainite.

[0027] (5) The present invention utilizes the existing equipment and process conditions of general steel mills to create a process without significantly increasing investment and production costs, thereby meeting performance and quality requirements and providing experience for the research and development of high-quality steel to special steel products. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 is a metallographic diagram of the square steel of Example 1 of the present invention; DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] A method for producing square spring steel without quenching and tempering, wherein the chemical composition of the steel is as follows by weight: C: 0.20%-0.30%, Si: 0.30%-0.80%, Mn: 1.5%-2.0%, Cr: 0.05%-0.10%, Mo: 0.01%-0.05%, V: 0.04%-0.10%, Al: 0.020%-0.050%, P≤0.020%, S≤0.020%, Cu≤0.20%, N: 0.0060%-0.0100%, and the rest is Fe and unavoidable impurities. The key process steps include:

[0032] (1) Smelting: The weight of molten iron entering the converter is controlled to be 80t±1 ton, and the weight of scrap steel is controlled to be 20±1 ton; the target composition of the molten steel at the end point is controlled to be C≥0.08%, P≤0.012%, steel is retained during tapping, oxidized slag is prohibited, and composite deoxidizer is added during tapping to make slag and deoxidize;

[0033] (2) Refining: LF basicity is controlled at 6.0-8.0. VN alloy is added in the early stage of refining to adjust the V in the steel to the target value. Before the end of LF furnace refining, aluminum wire is fed to adjust the aluminum in the molten steel to 0.050%-0.070%. Before leaving the station, calcium wire is fed to treat the molten steel with calcium. The first continuous casting furnace is fed with 100 meters of calcium wire, and the continuous casting furnace is fed with 80 meters of calcium wire. Then, a covering agent is added to protect the molten steel. The RH is vacuumed to below 67Pa and the billet is opened for circulation nitrogen addition. After the vacuum is maintained for 15 minutes, the molten steel is broken for hydrogen determination to control the hydrogen content ≤1.5ppm. The RH is opened for circulation nitrogen addition to control the N content in the steel before leaving the station within the target range. The soft blowing time before leaving the station is 25-35 minutes.

[0034] (3) Continuous casting: Weak cooling is adopted, the specific water volume is 0.16L / KG, the target value of the superheat of the tundish is 20-30℃, the water distribution in each zone is fine-tuned through the light pressure reduction process, the crystallizer and the end electromagnetic stirring are coordinated to weaken the continuous casting segregation, the temperature of the center point of the inner arc surface of the billet entering the straightening machine is monitored at 1100-1150℃, and the billet is stacked and cooled for 24 hours;

[0035] (4) Rolling: Control the heating temperature of the soaking section to 1180-1220°C, and the first and second pass reduction ratios to be greater than 60%; adopt staggered temperature rolling after the initial rolling, with the intermediate rolling surface temperature at 850-880°C and the core temperature at 1000-1050°C, increase the temperature gradient between the surface and the core, and use the temperature gradient to regulate the deformation behavior and microstructure of the material; the final rolling temperature is controlled at 750-800°C, and the cumulative reduction ratio is greater than 80%;

[0036] (5) Slow cooling after rolling: spray cooling after rolling, control the upper cooling bed temperature at 720±20℃, close-packed slow cooling on the cooling bed, and control the temperature of the slow cooling zone below 500℃.

[0037] The present invention will be further described below in conjunction with the embodiments.

[0038] Each embodiment is produced according to the process flow of converter-refining-continuous casting-rolling;

[0039] The chemical composition of the steel in each example is shown in Table 1, and the metallographic structure and strength results are shown in Table 2.

[0040] Embodiment 1:

[0041] The invention discloses a production method of spring square steel free of quenching and tempering. The chemical composition of the steel is as follows by weight: C: 0.23%, Si: 0.45%, Mn: 1.55%, Cr: 0.07%, Mo: 0.04%, V: 0.08%, Al: 0.031%, P: 0.012%, S: 0.001%, Cu: 0.02%, N: 0.0078%, and the rest is Fe and unavoidable impurities.

[0042] The key process steps include:

[0043] (1) Smelting: The weight of molten iron entering the converter is 81 tons, and the weight of scrap steel is 20 tons; the final molten steel C is 0.10% and P is 0.010%;

[0044] (2) Refining: The final slag basicity of LF furnace refining slag is 6.5; RH soft blowing is 30 minutes, and the outlet temperature is 1545℃.

[0045] (3) The secondary cooling water volume of continuous casting is 0.16L / KG, the tundish superheat is 24℃, the temperature of the center point of the inner arc surface of the ingot entering the straightening machine is monitored at 1110~1120℃, and the secondary cooling water distribution is fine-tuned.

[0046] (4) Rolling: The heating time of the ingot is 240 min, the temperature of the high temperature section is controlled at 1220 °C, the time is 75 min, the surface temperature of the intermediate rolling is 870 °C, the core temperature is 1020 °C, and the final rolling temperature is 780 °C;

[0047] (5) After rolling, the steel is directly cooled by water and controlled cooling, with the temperature entering the slow cooling zone at 720°C and the temperature leaving the slow cooling zone at 280°C.

[0048] Metallographic structure Figure 1 shown.

[0049] Embodiment 2:

[0050] The invention discloses a production method of spring square steel free of quenching and tempering. The chemical composition of the steel is as follows by weight: C: 0.23%, Si: 0.42%, Mn: 1.58%, Cr: 0.08%, Mo: 0.03%, V: 0.07%, Al: 0.035%, P: 0.012%, S: 0.001%, Cu: 0.03%, N: 0.0077%, and the rest is Fe and unavoidable impurities.

[0051] The key process steps include:

[0052] (1) Smelting: The weight of molten iron entering the converter is 80 tons, and the weight of scrap steel is 20 tons; the final molten steel C is 0.10% and P is 0.010%;

[0053] (2) Refining: The final slag basicity of LF furnace refining slag is 6.6; RH soft blowing for 30 minutes, the outlet temperature is 1545℃.

[0054] (3) The secondary cooling water volume of continuous casting is 0.16L / KG, the tundish superheat is 28℃, the temperature of the center point of the inner arc surface of the ingot entering the straightening machine is monitored at 1110~1120℃, and the secondary cooling water distribution is fine-tuned.

[0055] (4) Rolling: The heating time of the ingot is 270 min, the temperature of the high temperature section is controlled at 1210 °C, the time is 70 min, the surface temperature of the intermediate rolling is 860 °C, the core temperature is 1010 °C, and the final rolling temperature is 770 °C;

[0056] (5) After rolling, the steel is directly cooled by water and controlled cooling, with the temperature entering the slow cooling zone at 740°C and the temperature leaving the slow cooling zone at 320°C.

[0057] Embodiment 3:

[0058] The invention discloses a production method of spring square steel free of quenching and tempering. The chemical composition of the steel is as follows by weight: C: 0.24%, Si: 0.46%, Mn: 1.58%, Cr: 0.08%, Mo: 0.04%, V: 0.08%, Al: 0.032%, P: 0.013%, S: 0.001%, Cu: 0.02%, N: 0.0076%, and the rest is Fe and unavoidable impurities.

[0059] The key process steps include:

[0060] (1) Smelting: The weight of molten iron entering the converter is 79 tons, and the weight of scrap steel is 21 tons; the final molten steel C is 0.08% and P is 0.011%;

[0061] (2) Refining: The final slag basicity of LF furnace refining slag is 7.5; RH soft blowing is 25 minutes, and the outlet temperature is 1545℃.

[0062] (3) The secondary cooling water volume of continuous casting is 0.16L / KG, the tundish superheat is 30℃, the temperature of the center point of the inner arc surface of the ingot entering the straightening machine is monitored at 1110~1120℃, and the secondary cooling water distribution is fine-tuned.

[0063] (4) Rolling: The heating time of the ingot is 250 min, the temperature of the high temperature section is controlled at 1210 °C, the time is 65 min, the surface temperature of the intermediate rolling is 880 °C, the core temperature is 1020 °C, and the final rolling temperature is 800 °C;

[0064] (5) After rolling, the steel is directly cooled by water and controlled cooling, with the temperature entering the slow cooling zone at 740°C and the temperature leaving the slow cooling zone at 340°C.

[0065] Embodiment 4:

[0066] The invention discloses a production method of spring square steel free of quenching and tempering. The chemical composition of the steel is as follows by weight: C: 0.24%, Si: 0.44%, Mn: 1.54%, Cr: 0.07%, Mo: 0.05%, V: 0.07%, Al: 0.035%, P: 0.010%, S: 0.001%, Cu: 0.04%, N: 0.0071%, and the rest is Fe and unavoidable impurities.

[0067] The key process steps include:

[0068] (1) Smelting: The weight of molten iron entering the converter is 81 tons, and the weight of scrap steel is 19 tons; the final molten steel C is 0.08% and P is 0.011%;

[0069] (2) Refining: The final slag basicity of LF furnace refining slag is 7.0; RH soft blowing is 26 minutes, and the outlet temperature is 1545℃.

[0070] (3) The secondary cooling water volume of continuous casting is 0.16L / KG, the tundish superheat is 27℃, the temperature of the center point of the inner arc surface of the ingot entering the straightening machine is monitored at 1110~1120℃, and the secondary cooling water distribution is fine-tuned.

[0071] (4) Rolling: The heating time of the ingot is 240 min, the temperature of the high temperature section is controlled at 1220 °C, the time is 70 min, the surface temperature of the intermediate rolling is 850 °C, the core temperature is 1000 °C, and the final rolling temperature is 750 °C;

[0072] (5) After rolling, the steel is directly cooled by water and controlled cooling, with the temperature entering the slow cooling zone at 730°C and the temperature leaving the slow cooling zone at 360°C.

[0073] Table 1 Chemical composition of steel in each example (%)

[0074]

[0075] Table 2 Metallographic structure and strength results of steel samples in various embodiments and comparative examples

[0076]

[0077]

[0078] Comparative Example 1:

[0079] A method for producing square spring steel without quenching and tempering, wherein the chemical composition of the steel is as follows by weight: C: 0.17%, Si: 0.40%, Mn: 1.45%, Cr: 0.02%, Mo: 0.01%, V: 0.01%, Al T : 0.031%, P: 0.012%, S: 0.001%, Cu: 0.02%, N: 0.0038%, and the rest are Fe and inevitable impurities.

[0080] The process steps are consistent with those in Example 1. The detection performance is shown in Table 2.

[0081] Comparative Example 2:

[0082] A method for producing square spring steel without quenching and tempering, wherein the chemical composition of the steel is as follows by weight: C: 0.23%, Si: 0.40%, Mn: 1.38%, Cr: 0.03%, Mo: 0.01%, V: 0.01%, AlT : 0.035%, P: 0.012%, S: 0.001%, Cu: 0.03%, N: 0.0037%, and the rest are Fe and inevitable impurities.

[0083] The process steps are consistent with those in Example 2. The detection performance is shown in Table 2.

[0084] Comparative Example 3:

[0085] The chemical composition of the steel is consistent with that of Example 3.

[0086] The difference between the process steps and Example 3 is that the staggered temperature rolling technology is not used under this composition system. The square steel obtained by conventional continuous rolling is controlled at a final rolling temperature of 880°C, an upper cooling bed temperature of 820°C, a cooling bed temperature of 500°C, and a cooling rate of 1°C / S.

[0087] As shown in Table 2, the surface of the detected organization has a brittle structure containing bainite, the core has coarse grains, and the impact is low.

[0088] Comparative Example 4:

[0089] The chemical composition of the steel is consistent with that of Example 4.

[0090] The difference between the process steps and Example 4 is that the staggered temperature rolling technology is not used under this composition system. The square steel obtained by conventional continuous rolling is controlled at a final rolling temperature of 860°C, an upper cooling bed temperature of 830°C, a cooling bed temperature of 500°C, and a cooling rate of 1°C / S.

[0091] As shown in Table 2, the surface of the detected organization has a brittle structure containing bainite, the core has coarse grains, and the impact is low.

[0092] In summary, the square steel obtained by conventional continuous rolling has a brittle structure containing bainite on the surface, coarse grains in the core, a strength between 570 and 600 MPa, and a low impact strength of 25 to 35 J. The spring square steel produced by the technical solution of the present invention has a pearlite + ferrite structure, a hot rolled tensile strength of more than 600 MPa, a yield strength of more than 400 MPa, a surface reduction of more than 50%, and a low temperature impact (-20°C) V-type of more than 60 J.

[0093] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for producing spring square steel without quenching and tempering, characterized in that: The chemical composition of the steel is as follows by weight: C: 0.20% to 0.30%, Si: 0.30% to 0.80%, Mn: 1.5% to 2.0%, Cr: 0.05% to 0.10%, Mo: 0.01% to 0.05%, V: 0.04% to 0.10%, Al: 0.020% to 0.050%, P≤0.020%, S≤0.020%, Cu≤0.20%, N: 0.0060% to 0.0100%, and the rest is Fe and unavoidable impurities. The key process steps include: (1) Smelting: The converter controls the weight of molten iron entering the furnace, controls the target composition of the final molten steel to be C ≥ 0.08%, P ≤ 0.012%, retains the steel for tapping, prohibits the addition of oxidized slag, and adds a composite deoxidizer for slag formation and deoxidation during the tapping process; (2) Refining: LF basicity is controlled at 6.0-8.

0. VN alloy is added in the early stage of refining to adjust the V in the steel to the target value. Before the end of LF furnace refining, aluminum wire is fed to adjust the aluminum in the molten steel to 0.050%-0.070%. Before leaving the station, calcium wire is fed to treat the molten steel with calcium, and then a covering agent is added to protect the molten steel. The RH is vacuumed to below 67Pa and the billet is opened for circulation nitrogen addition. After the vacuum is maintained for 15 minutes, the molten steel is broken for hydrogen determination to control the hydrogen content to ≤1.5ppm. The RH is opened for circulation nitrogen addition to control the N content in the steel before leaving the station within the target range. The soft blowing time before leaving the station is 25-35 minutes. (3) Continuous casting: weak cooling is adopted, the specific water volume is 0.16L / KG, the target value of the superheat of the tundish is 20-30℃, the temperature entering the straightening machine is 1100-1150℃, and the billet is piled and cooled for 24h; (4) Rolling: The heating temperature in the soaking section is controlled at 1180-1220°C, the surface temperature in the intermediate rolling section is controlled at 850-880°C, the core temperature is controlled at 1000-1050°C, and the final rolling temperature is controlled at 750-800°C; (5) Slow cooling after rolling: spray cooling after rolling, control the upper cooling bed temperature at 720±20℃, close-packed slow cooling on the cooling bed, and control the temperature of the slow cooling zone below 500℃.

2. The method for producing spring square steel without quenching and tempering as claimed in claim 1, characterized in that: In the (1) smelting stage, the weight of molten iron entering the furnace is 80t±1 ton, and the weight of scrap steel is 20±1 ton.

3. The method for producing spring square steel without quenching and tempering as claimed in claim 1, characterized in that: In the (2) refining stage, the first continuous casting furnace is fed with 100 meters of calcium wire, and the continuous casting furnace is fed with 80 meters of calcium wire.

4. The method for producing spring square steel without quenching and tempering as claimed in claim 1, characterized in that: In the (4) rolling stage, the reduction ratios of the first and second passes are greater than 60%, and the cumulative reduction ratio is greater than 80%.

5. The method for producing spring square steel without quenching and tempering as claimed in claim 1, characterized in that: The post-rolling slow cooling stage (5) in which the slow cooling rate is from 700-740°C to 500°C is 0.05°C / S.

6. A method for producing spring square steel without quenching and tempering as claimed in any one of claims 1 to 5, characterized in that: The spring square steel produced has pearlite + ferrite as the matrix, hot rolled tensile strength is above 600MPa, area reduction is above 50%, low temperature impact (-20℃) V-type is above 60J, and austenite grain size is ≥8 levels.

Images