Spring steel for profiled wire and method for producing the same

Abstract

This invention relates to a spring steel for shaped wire and its production method. The method does not consider adding expensive microalloying elements in the wire rod composition design. Instead, it finely controls the main alloying elements C, Si, Mn, and Cr by limiting the strength-ductility index (SE). The production process includes steelmaking, continuous casting of large billets, hot charging and billet preparation, hot charging and heating of small billets, high-speed wire rod controlled rolling, and Steyrmo line controlled cooling. This invention achieves uniform control of the wire rod's microstructure and properties through a rational proportion of alloying elements combined with refined controlled rolling and cooling processes, meeting the requirements for low cost and high strength in the preparation of shaped spring steel wire.

Description

Technical Field

[0001] This invention relates to a spring steel for shaped steel wire and its production method, belonging to the field of steelmaking and metal materials in the metallurgical industry. Background Technology

[0002] Spring steel is an important basic component manufacturing material, widely used in aerospace, automotive, rail transportation, and engineering machinery. In recent years, with the rapid development of the low-altitude economy and the intelligent manufacturing industry, various drones and robots are increasingly entering the market, leading to a demand for customized and irregularly shaped spring parts. Unlike traditional round helical springs, irregularly shaped spring steel wires have diverse cross-sectional shapes, such as trapezoids, rectangles, and rhombuses. Furthermore, the production of irregularly shaped spring steel wire requires large-deformation cold extrusion forming, placing high demands on the uniformity of the microstructure and properties of the base material wire rod while ensuring strength. Currently, publicly available technical solutions for this type of product all employ a small billet continuous casting production method, combined with high-speed wire rod rolling and Steyrmo line cooling processes to obtain finished wire rods. Subsequently, the wire rods undergo drawing, quenching and tempering heat treatment, and irregular-shaped rolling to obtain high-strength irregularly shaped spring steel wires.

[0003] For example, patent CN113943895A discloses a high-quality spring steel wire and its production process. The chemical composition of the spring steel wire rod is: C 0.64~0.67%, Si 0.16~0.30%, Mn 0.60~0.68%, Cr 0.30~0.33%, P ≤0.025%, S ≤0.020%. The main production process is: converter smelting, LF refining, small billet continuous casting, heating, high-speed wire rolling, controlled cooling, and slow cooling in the heat-insulating channel. The wire rod microstructure is pearlite + sorbite + ferrite, with a sorbite ratio of 1.5 and a decarburized layer ≤1.0%D; the mechanical properties include a tensile strength of 946~995MPa, a reduction of area of ​​44~47%, and an elongation after fracture of 15~19%; non-metallic inclusions are A+C ≤2.0 and B+D ≤2.0. This patent uses a steelmaking process with small billet continuous casting. The segregation at the center of the billet is only controlled to ≤1.5, which cannot guarantee that the segregation of the finished wire rod meets the requirements. The controlled cooling process is not combined with the phase transformation point for fine control, which cannot guarantee the strength of the wire rod. The shrinkage rate of the wire rod section is only 44-47%. Due to the large degree of cold deformation of the shaped steel wire, the plasticity requirement is high. The shrinkage of the base wire rod must be at least ≥50%.

[0004] Patent CN117721372A discloses a method for producing carbon spiral spring steel wire rod. The chemical composition of the spring steel wire rod is: C 0.62~0.70%, Si 0.17~0.37%, Mn 0.90~1.20%, P ≤0.02%, S 0.006~0.015%, Ni ≤0.15%, Cr ≤0.10%, Cu ≤0.20%. The main production process includes: blast furnace ironmaking, hot metal desulfurization, converter smelting, LF refining, small billet continuous casting, heating, high-speed wire rod rolling, coiling, and quenching and tempering. The small billet continuous casting size is 150×150mm. 2 The finished wire rod specifications are φ5.5 and 6.5mm. The steelmaking process of this patent uses a small billet continuous casting method, which cannot guarantee that the segregation of the finished wire rod meets the requirements; the wire rod rolling temperature is relatively high (initial rolling temperature 1096 and 1101℃), which cannot guarantee the grain refinement effect of the wire rod; the wire rod production process does not reflect the controlled cooling process, which cannot guarantee the uniformity of the microstructure and properties within the same coil; the microstructure and properties of the finished wire rod are not reflected, and it cannot be determined whether it meets the requirements of special-shaped spring steel wire.

[0005] Patent CN120796827A discloses a low-cost production method for carbon helical spring steel wire rod. The chemical composition of the spring steel wire rod is: C 0.62-0.67%, Si 0.17-0.27%, Mn 0.90-1.00%, P ≤0.030%, S ≤0.030%. The production method for the spring steel wire is as follows: converter smelting, LF refining, continuous casting of small square billets, heating, high-speed wire rolling, and controlled cooling. The initial rolling temperature of the billet is 1150-1180℃, the final rolling temperature is 880℃, and the wire drawing temperature is 860-870℃. The tensile strength of the finished wire rod is 1050-1150MPa, and the reduction of area is 36-42%. This patent also uses small square billet continuous casting in steelmaking, which cannot guarantee the level of segregation control in the core of the wire rod; the wire rod rolling process does not reflect the effect of fine and uniform control of the microstructure; the mechanical properties of the wire rod fluctuate greatly, and the section reduction rate is only 36-42%, which cannot meet the requirements of irregular spring steel wire.

[0006] Clearly, the currently disclosed production methods for spring steel wire fail to effectively control center segregation during continuous casting, and the rolling and cooling processes lack precise control over the uniformity of wire rod microstructure and properties. Furthermore, the base wire rod processing requires quenching and tempering heat treatment, which cannot meet the demand for low-cost, high-efficiency processing of irregularly shaped spring steel wire. Therefore, it is necessary to design a production method specifically for irregularly shaped spring steel wire to solve these problems. Summary of the Invention

[0007] This invention provides a spring steel for shaped steel wire and its production method. By rationally proportioning alloying elements and combining refined controlled rolling and cooling processes, the microstructure and properties of the wire rod are uniformly controlled, meeting the requirements of low cost and high strength in the preparation of shaped spring steel wire.

[0008] The technical solution adopted by this invention to solve its technical problem is: A spring steel for shaped steel wire, comprising the following components by weight percentage: C 0.66~0.70%, Si 0.22~0.28%, Mn 0.92~0.98%, Cr 0.24~0.30%, Cu≤0.02%, Al≤0.01%, P≤0.015%, S≤0.012%, with the remainder being Fe and unavoidable impurities; The strength and plasticity index (SE) is limited to 2.05~2.25%, and SE = [C] + 0.8[Mn] + 1.2[Si] + 1.5[Cr]. The production method of spring steel for shaped steel wire includes the following steps: Step S1, steelmaking; after hot metal pretreatment, converter smelting, and LF refining, molten steel that meets the preset chemical composition of the spring steel for the shaped steel wire is obtained; Step S2, large billet continuous casting: After the molten steel is smelted, it is sent to the tundish for continuous casting. The tundish adopts induction heating technology. The secondary cooling section of the billet is equipped with end electromagnetic stirring and dynamic light reduction. The electromagnetic stirring frequency is set to 1.8~2.7Hz, the reduction is controlled to 18~22mm, and the continuous casting speed is set to 0.60~0.65m / min. The finished large billet has a cross-sectional size of (300~330)mm×(390~450)mm and a length of 5.5~5.8m. Step S3, hot charging and billet opening: The continuously cast billet is hot-charged into the billet opening heating furnace via a continuous automatic roller conveyor. The furnace temperature is 720~780℃. The furnace adopts segmented heating, dividing the furnace area into a preheating section, a first heating section, a second heating section, and a soaking section. The total heating time is controlled to be 210~240min. After exiting the furnace, the continuously cast billet undergoes multiple passes of continuous rolling to achieve billet opening. The temperature of the preheating section is 800~880℃, the first heating section is 880~950℃, the second heating section is 950~1030℃, the soaking section is 1030~1080℃, and the initial rolling temperature is 920~960℃. The cross-sectional dimensions of the small square billet after billet opening are 140mm×140mm. Step S4, hot conveying and heating of small square billets; after billet cutting, the small square billets are hot conveyed to the steel rolling heating furnace via roller conveyor for heating. The hot conveying temperature is 600~650℃. The furnace adopts segmented temperature control, dividing the furnace area into a preheating section, a first heating section, a second heating section, and a soaking section, and controlling the heating time to be 60~75min; among which, the temperature of the preheating section is 850~920℃, the first heating section is 920~980℃, the second heating section is 980~1040℃, and the soaking section is 1040~1100℃; Step S5, high-speed wire rod controlled rolling; the billet is produced by controlled rolling using a continuous bar and wire rolling mill. After exiting the furnace, the billet is first descaled by high-pressure water. The initial rolling temperature is set at 910~940℃, the finishing rolling inlet temperature is 840~870℃, the sizing inlet temperature is 800~830℃, and the wire drawing temperature is 810~840℃; the finished wire rod specifications are φ5~8mm. Step S6, Steyrmo line-controlled cooling; the air volume of the Steyrmo line fans is uniformly set at 260,000 m³ / h. 3 / h, two fans correspond to three insulation covers, using multi-stage cooling control, divided into rapid cooling, slow cooling, and air cooling sections. In the rapid cooling section, fan 1 is turned on at 80-100%, fan 2 at 30-80%, fan 3 at 0-80%, fan 4 is off, fan 5 at 30-60%, fan 6 at 0-40%, and all insulation covers 1-9 are open. In the slow cooling section, fans 7-10 are off, and insulation covers 12 and 14 are open. In the air cooling section, fans 11-13 are off, and all insulation covers 16-20 are open. Furthermore, in step S2, when the intermediate tundish uses induction heating, the superheat is controlled at 26~30℃ and the liquid level fluctuation is ±1.5mm. Furthermore, the C segregation index of the continuously cast billet is ≤1.07, and the central segregation, central porosity, and corner cracks are all ≤1.0 level; Furthermore, in step S3, the air-fuel ratio of the preheating section and the heating section is set to 0.62~0.65, and the air-fuel ratio of the soaking section is set to 0.57~0.60; the heating time of the soaking section is 150~180min; after the continuous casting billet is taken out of the furnace, it undergoes nine consecutive rolling to achieve billet opening treatment, and the high-pressure descaling water pressure is set to ≥18MPa; The cross-sectional dimensions of the small square billet after roughing are 140×140mm. The center segregation of the small square billet is ≤0.5 grade, the center porosity is ≤0.5 grade, and the center carbon segregation index is ≤1.04. Furthermore, in step S4, the air-fuel ratio of the preheating section and the heating section is set to 0.57~0.60, and the air-fuel ratio of the soaking section is set to 0.52~0.55; the heating time of the soaking section is ≥45min; Furthermore, in step S5, the descaling water pressure is ≥20MPa; Furthermore, in step S6, in the rapid cooling section, the roller speed is controlled at 1.20~1.25m / s, the cooling rate is 6.5~7.8℃ / s, and the temperature at the end of the rapid cooling section is 630~660℃; in the slow cooling section, the roller speed is controlled at 1.28~1.35m / s, the cooling rate is 1.2~1.8℃ / s, and the temperature at the end of the slow cooling section is 585~615℃; in the air cooling section, the roller speed is controlled at 1.08~1.15m / s, the cooling rate is 2.6~3.3℃ / s, and the winding temperature is 510~540℃. Furthermore, the obtained wire rod has a microstructure of ferrite combined with sorbite, with a sorbite ratio of ≥93%, a sorbite grain size of 9.2~11.6μm, a sorbite lamellar spacing of 165~195nm, and an anomalous martensite structure of ≤0.5 grade; Furthermore, the obtained wire rod has a tensile strength of 1120~1160MPa, a tensile strength fluctuation of ≤35MPa within the same coil, a reduction of area of ​​≥53%, a reduction of area fluctuation of ≤5% within the same coil, and an elongation after fracture of ≥18%. The special-shaped spring steel wire prepared from the wire rod through drawing and cold extrusion can meet the requirements of a tensile strength of 1850~1920MPa and an elongation after fracture of ≥8%.

[0009] By employing the above technical solutions, the present invention has the following beneficial effects compared to the prior art: 1. The spring steel for irregularly shaped steel wire provided by the present invention does not consider adding expensive microalloying elements such as V, Nb, Mo, and Ni in the design of the wire rod composition. By limiting the strength-plasticity index SE, the main alloying elements C, Si, Mn, and Cr are finely controlled to ensure the strength and plasticity of the finished wire rod and to enable it to process irregularly shaped spring steel wire. 2. The production method of spring steel for shaped steel wire provided by the present invention adopts a low superheat, end electromagnetic stirring and dynamic light reduction process in the continuous casting process. The billet heating adopts a diffusion homogenization method, which effectively improves the segregation of the core of the intermediate billet, improves the uniformity of the subsequent wire rod structure and properties, and reduces the fluctuation of tensile strength and reduction of area. In the billet process, the billet heating time is shortened by hot charging and hot delivery, which reduces energy waste and improves production efficiency. 3. The production method of spring steel for shaped steel wire provided by this invention, in the Stellmore controlled cooling process of the wire rod, is based on the sorbite phase transformation temperature range of the composition system and the cooling differences of wire rods of different specifications, and formulates a segmented controlled cooling process; in the rapid cooling section, the wire rod quickly passes through the ferrite phase region, and considering the back-temperature phenomenon that may occur during the sorbite phase transformation process, a two-stage cooling is adopted; in the slow cooling section, the heat preservation cover is opened at intervals to adjust the cooling rate of the slow cooling section and ensure the maximization of the sorbite phase transformation; in the air cooling section, the wire rod is cooled to a suitable coiling temperature to ensure the shape of the package and shorten the packaging time; the overall roller speed is set to be relatively fast and then slow, which on the one hand improves production efficiency, increases the spacing between wire rods, reduces the temperature difference between overlapping and non-overlapping points, and reduces the performance fluctuation of the same coil; on the other hand, before coiling, the rear wire rod is continuously squeezed forward to increase the coiling density and ensure the shape of the package. 4. The production method of spring steel for shaped steel wire provided by the present invention improves the proportion of sorbite structure in the obtained wire rod by low-temperature controlled rolling and Stellmore line controlled cooling, and effectively refines it, while avoiding the occurrence of abnormal martensite structure as much as possible. The base wire rod has excellent strength and plasticity with small fluctuations, which can effectively reduce the customer's heat treatment process, save processing costs and improve efficiency, and is fully suitable for processing and producing high-strength shaped spring steel wire. Attached Figure Description

[0010] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0011] Figure 1 This is a schematic diagram of the metallographic structure of the wire rod in Embodiment 5 of the present invention, wherein sorbite accounts for 95%, ferrite accounts for 5%, and martensite is grade 0; Figure 2 This is a schematic diagram of the sorbite lamellar spacing of the wire rod in Embodiment 5 of the present invention. Detailed Implementation

[0012] To make the objectives, technical solutions, and advantages of this invention clearer, the design scheme of the steel smelting components and the corresponding preparation process of this invention will be described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Raw materials, equipment, and operating steps not specifically described herein are all conventional technologies in the field of steel smelting.

[0013] In response to the existing production methods for spring wire steel as described in the background art, the continuous casting process in steelmaking fails to effectively control center segregation, and the rolling and cooling process does not finely regulate the uniformity of the wire rod's microstructure and properties, resulting in the base wire rod failing to meet the processing requirements of shaped spring steel wire. This application provides a spring steel for shaped wire and its production method, which rationally proportions alloying elements combined with a refined controlled rolling and cooling process to meet the requirements for preparing low-cost, high-strength shaped spring steel wire.

[0014] Firstly, in terms of chemical composition, the spring steel for shaped steel wire provided in this application, by weight percentage, includes the following components: C 0.66~0.70%, Si 0.22~0.28%, Mn 0.92~0.98%, Cr 0.24~0.30%, Cu≤0.02%, Al≤0.01%, P≤0.015%, S≤0.012%, with the remainder being Fe and unavoidable impurities.

[0015] The design principles of each chemical element are explained below: C: C is a cost-effective element for improving strength. Adding an appropriate amount can ensure the proportion of sorbite in the wire rod structure. However, excessive C will worsen plasticity and affect subsequent cold rolling forming. To balance strength and plasticity, C is set at 0.66~0.70%.

[0016] Si: Si is an effective strengthening element, and due to its strong deoxidizing effect, it can significantly reduce the oxygen content of molten steel and reduce the number of inclusions. However, excessive addition affects plasticity and increases the tendency for decarburization. Therefore, the Si content is controlled at 0.22~0.28%.

[0017] Mn: Mn can effectively improve strength and hardenability of wire rod through solid solution strengthening. However, excessive addition can lead to excessive hardenability, resulting in abnormal martensitic structure during cooling. Therefore, Mn is set at 0.92~0.98%.

[0018] Cr: Cr is a strong carbide-binding element, readily combining with carbon to form alloy carbides that dissolve in the matrix and enhance its strength. Similar to Mn, excessive addition can lead to excessive hardenability and the appearance of martensitic abnormal structures during the cooling stage. Therefore, the Cr content is set at 0.24~0.30%.

[0019] Cu: Cu is an impurity element in spring steel, which tends to segregate at grain boundaries and phase boundaries, exacerbating the material's hot brittleness and affecting hot deformation performance. Therefore, Cu should be controlled to ≤0.02%.

[0020] Al: Al is a strong deoxidizing element and readily combines with O to form spinel hard inclusions, significantly reducing the fatigue performance of the finished spring. Therefore, Al should be controlled to ≤0.01%.

[0021] P: P is an easily segregating impurity element that tends to accumulate in grain boundaries and reduce grain boundary strength, thereby decreasing the plastic deformation capacity of the wire rod. Therefore, P should be controlled to be ≤0.015%.

[0022] S: S easily causes center segregation, and excessive content will produce large-sized MnS inclusions, affecting the plastic deformation capacity of the wire rod. Therefore, S should be controlled ≤0.012%.

[0023] In terms of composition design, this application offers another innovation: limiting the strength-ductility index (SE) to 2.05~2.25%, where SE = [C] + 0.8[Mn] + 1.2[Si] + 1.5[Cr]. A higher SE indicates higher strength but lower ductility; conversely, a lower SE indicates lower strength but higher ductility. Therefore, based on the requirements of finished irregularly shaped springs for the strength and plastic deformation capacity of spring steel, the SE is limited to 2.05~2.25%.

[0024] After rationally proportioning alloying elements, it is also necessary to combine refined controlled rolling and cooling processes. This application further provides a production method for spring steel for shaped steel wire, the control process of which is as follows: steelmaking → continuous casting of large billets → hot charging and billet opening → hot delivery and heating of small billets → high-speed wire rod controlled rolling → Steyrmo wire rod controlled cooling.

[0025] In the above production method, in order to solve the problem that the existing technology of using small billet continuous casting cannot guarantee that the segregation of finished wire rod meets the requirements, this application adopts large billet continuous casting. However, large billet continuous casting is prone to problems of high cost and low efficiency. Therefore, this application simultaneously designed hot charging billet opening and small billet hot delivery heating steps to improve efficiency and reduce production costs.

[0026] The production method of the spring steel for the shaped steel wire specifically includes the following steps: Step S1, steelmaking; after hot metal pretreatment, converter smelting, and LF refining, molten steel that meets the preset chemical composition of the spring steel for the shaped steel wire is obtained; Step S2, large billet continuous casting: After the molten steel is smelted, it is sent to the tundish for continuous casting. The tundish adopts induction heating technology, and the superheat is controlled at 26~30℃, with liquid level fluctuation ±1.5mm. The secondary cooling section of the billet is equipped with end electromagnetic stirring and dynamic light reduction. The electromagnetic stirring frequency is set to 1.8~2.7Hz, the reduction is controlled at 18~22mm, and the continuous casting speed is set to 0.60~0.65m / min. The finished large billet has a cross-sectional size of (300~330)mm×(390~450)mm and a length of 5.5~5.8m. The C segregation index of the continuously cast billet is ≤1.07, and the central segregation, central porosity, and corner cracks are all ≤1.0 grade.

[0027] Step S3, hot charging and billet opening: The continuously cast billet is hot-charged into the billet opening heating furnace via a continuous automatic roller conveyor. The furnace temperature is 720~780℃. The furnace adopts segmented heating, dividing the furnace area into a preheating section, a first heating section, a second heating section, and a soaking section. The total heating time is controlled at 210~240min, and the soaking section heating time is 150~180min. The air-fuel ratio in the preheating section and the heating section is set at 0.62~0.65, and the air-fuel ratio in the soaking section is set at 0.57~0.60. Specifically, the temperature in the preheating section is 800~880℃, the first heating section is 880~950℃, the second heating section is 950~1030℃, and the soaking section is 1030~1080℃. The cross-sectional dimensions of the small square billet after opening are 140mm×140mm. After being discharged from the furnace, the continuously cast billet undergoes nine consecutive rolling passes to achieve billet opening. The opening rolling temperature is 920~960℃, and the high-pressure descaling water pressure is set to ≥18MPa. The cross-sectional dimensions of the small square billet after opening are 140×140mm. The center segregation of the small square billet is ≤0.5 grade, the center porosity is ≤0.5 grade, and the center carbon segregation index is ≤1.04.

[0028] Step S4: Hot conveying and heating of small square billets; after billet cutting, the small square billets are hot conveyed to the rolling mill heating furnace via roller conveyor for heating. The hot conveying temperature is 600~650℃. The furnace adopts segmented temperature control, dividing the furnace area into a preheating section, a first heating section, a second heating section, and a soaking section, controlling the heating time to be 60~75min; specifically, the preheating section temperature is 850~920℃, the first heating section is 920~980℃, the second heating section is 980~1040℃, and the soaking section is 1040~1100℃; the air-fuel ratio in the preheating and heating sections is set to 0.57~0.60, and the air-fuel ratio in the soaking section is set to 0.52~0.55; the heating time in the soaking section is ≥45min. Step S5, high-speed wire rod controlled rolling; the square billet is produced by controlled rolling using a continuous bar and wire rod mill. After exiting the furnace, the billet is first descaled by high-pressure water, with a descaling water pressure ≥20MPa. The initial rolling temperature is set at 910~940℃, the finishing rolling inlet temperature at 840~870℃, the reduction sizing inlet temperature at 800~830℃, and the wire drawing temperature at 810~840℃; the finished wire rod specifications are φ5~8mm.

[0029] One of the inventive points of this application, namely step S6, is the controlled cooling of the Steyrmo line. For the spring steel base wire rods used to produce the shaped steel wires required in this application, the next production line in the conveying process lacks a heat treatment step, resulting in products that have not undergone homogenization and exhibit large fluctuations. Therefore, to facilitate subsequent industry needs, controlled cooling is designed after the finished wire rods are obtained from high-speed wire rolling. Specifically, the air volume of the blowers on the Steyrmo line is uniformly set at 260,000 m³ / h. 3 / h, two fans correspond to three insulation covers, and multi-stage cooling is adopted, which is divided into a fast cooling section, a slow cooling section and an air cooling section. In the fast cooling section, fan No. 1 is turned on 80~100%, fan No. 2 is turned on 30~80%, fan No. 3 is turned on 0~80%, fan No. 4 is turned off, fan No. 5 is turned on 30~60%, fan No. 6 is turned on 0~40%, and all insulation covers No. 1 to No. 9 are opened; the control roller speed is 1.20~1.25m / s, the cooling rate is 6.5~7.8℃ / s, and the temperature is 630~660℃ at the end of the fast cooling section.

[0030] In the slow cooling section, fans 7 to 10 are turned off, and insulation covers 12 and 14 are turned on; the speed of the control roller is 1.28 to 1.35 m / s, the cooling rate is 1.2 to 1.8℃ / s, and the temperature at the end of the slow cooling section is 585 to 615℃.

[0031] In the air-cooling section, fans 11 to 13 are turned off, and insulation covers 16 to 20 are all opened. The control roller speed is 1.08 to 1.15 m / s, the cooling rate is 2.6 to 3.3℃ / s, and the winding temperature is 510 to 540℃.

[0032] The final wire rod has a microstructure of ferrite combined with sorbite. The sorbite content was determined to be ≥93% using the metallographic test method of "YB / T 169-2014 High Carbon Steel Wire Rod Sorbite Content". The sorbite grain size was determined to be 9.2~11.6μm and the sorbite lamellar spacing was 165~195nm using the oxidation method in "GB / T 6394-2017 Metallic Average Grain Size Determination Method". The abnormal martensite structure was ≤0.5 grade. Mechanical properties were tested using "GB / T 228.1-2021 Metallic materials - Tensile testing - Part 1: Test method at room temperature" and the results were as follows: tensile strength 1120~1160MPa, tensile strength fluctuation within the same coil ≤35MPa, reduction of area ≥53%, reduction of area fluctuation within the same coil ≤5%, elongation after fracture ≥18%; shaped spring steel wire prepared from wire rod through drawing and cold extrusion can meet the requirements of tensile strength 1850~1920MPa and elongation after fracture ≥8%.

[0033] To verify the feasibility of the above production method, this application provides Examples 1 to 9.

[0034] The chemical composition of the spring steel provided in Examples 1 to 9 is shown in Table 1, with the balance being Fe and unavoidable impurities.

[0035] Table 1 Chemical composition (wt%) of spring steel in Examples 1-9

[0036] The process parameters set for the large billet continuous casting and hot charging billet opening processes are shown in Table 2.

[0037] Table 2

[0038] The parameter settings for the heating process of small billets are shown in Table 3.

[0039] Table 3

[0040] The parameter settings for each step in the high-speed wire rod controlled rolling process are shown in Table 4.

[0041] Table 4

[0042] The parameter settings for the Stellmore linear cooling process are shown in Table 5.

[0043] Table 5

[0044] The final microstructure and properties of the spring steel wire rod and the properties of the shaped steel wire are shown in Table 6.

[0045] Table 6

[0046] by Figure 1 and Figure 2 Taking the metallographic structure and sorbite lamellar spacing of the wire rod obtained in Example 5 as an example, it is clear that the wire rod produced by the special-shaped steel wire spring steel and its production method provided in this application has a structure and properties that meet the requirements for low-cost and high-strength preparation.

[0047] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0048] The meaning of "and / or" as used in this application includes situations where each exists alone or both exist simultaneously.

[0049] The term "connection" as used in this application can mean a direct connection between components or an indirect connection between components through other components.

[0050] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A spring steel for shaped steel wire, characterized in that: By weight percentage, it includes the following components: C 0.66~0.70%, Si 0.22~0.28%, Mn 0.92~0.98%, Cr 0.24~0.30%, Cu≤0.02%, Al≤0.01%, P≤0.015%, S≤0.012%, with the remainder being Fe and unavoidable impurities; The strength plasticity index (SE) is limited to 2.05~2.25%, and SE = [C] + 0.8[Mn] + 1.2[Si] + 1.5[Cr].

2. The method for producing spring steel for shaped steel wire according to claim 1, characterized in that: Includes the following steps: Step S1, steelmaking; after hot metal pretreatment, converter smelting, and LF refining, molten steel that meets the preset chemical composition of the spring steel for shaped steel wire as described in claim 1 is obtained; Step S2, large billet continuous casting: After the molten steel is smelted, it is sent to the tundish for continuous casting. The tundish adopts induction heating technology. The secondary cooling section of the billet is equipped with end electromagnetic stirring and dynamic light reduction. The electromagnetic stirring frequency is set to 1.8~2.7Hz, the reduction is controlled to 18~22mm, and the continuous casting speed is set to 0.60~0.65m / min. The finished large billet has a cross-sectional size of (300~330)mm×(390~450)mm and a length of 5.5~5.8m. Step S3, hot charging and billet opening: The continuously cast billet is hot-charged into the billet opening heating furnace via a continuous automatic roller conveyor. The furnace temperature is 720~780℃. The furnace adopts segmented heating, dividing the furnace area into a preheating section, a first heating section, a second heating section, and a soaking section. The total heating time is controlled to be 210~240min. After exiting the furnace, the continuously cast billet undergoes multiple passes of continuous rolling to achieve billet opening. The temperature of the preheating section is 800~880℃, the first heating section is 880~950℃, the second heating section is 950~1030℃, the soaking section is 1030~1080℃, and the initial rolling temperature is 920~960℃. The cross-sectional dimensions of the small square billet after billet opening are 140mm×140mm. Step S4, hot conveying and heating of small square billets; after billet cutting, the small square billets are hot conveyed to the steel rolling heating furnace via roller conveyor for heating. The hot conveying temperature is 600~650℃. The furnace adopts segmented temperature control, dividing the furnace area into a preheating section, a first heating section, a second heating section, and a soaking section, and controlling the heating time to be 60~75min; among which, the temperature of the preheating section is 850~920℃, the first heating section is 920~980℃, the second heating section is 980~1040℃, and the soaking section is 1040~1100℃; Step S5, high-speed wire rod controlled rolling; the billet is produced by controlled rolling using a continuous bar and wire rolling mill. After exiting the furnace, the billet is first descaled by high-pressure water. The initial rolling temperature is set at 910~940℃, the finishing rolling inlet temperature is 840~870℃, the sizing inlet temperature is 800~830℃, and the wire drawing temperature is 810~840℃; the finished wire rod specifications are φ5~8mm. Step S6, Steyrmo line-controlled cooling; the air volume of the Steyrmo line fans is uniformly set at 260,000 m³ / h. 3 / h, two fans correspond to three insulation covers, using multi-stage cooling control, divided into rapid cooling, slow cooling and air cooling sections. In the rapid cooling section, fan 1 is turned on at 80-100%, fan 2 at 30-80%, fan 3 at 0-80%, fan 4 is off, fan 5 at 30-60%, fan 6 at 0-40%, and all insulation covers 1-9 are open. In the slow cooling section, fans 7-10 are off, and insulation covers 12 and 14 are open. In the air cooling section, fans 11-13 are off, and all insulation covers 16-20 are open.

3. The method for producing spring steel for shaped steel wire according to claim 2, characterized in that: In step S2, when the intermediate tundish uses induction heating, the superheat is controlled at 26~30℃ and the liquid level fluctuation is ±1.5mm.

4. The method for producing spring steel for shaped steel wire according to claim 3, characterized in that: The C segregation index of the continuously cast billet is ≤1.07, and the central segregation, central porosity, and corner cracks are all ≤1.0 level.

5. The method for producing spring steel for shaped steel wire according to claim 2, characterized in that: In step S3, the air-fuel ratio of the preheating section and the heating section is set to 0.62~0.65, and the air-fuel ratio of the soaking section is set to 0.57~0.60; the heating time of the soaking section is 150~180min; after the continuous casting billet is delivered from the furnace, it undergoes nine consecutive rolling passes to achieve billet opening treatment, and the high-pressure descaling water pressure is set to ≥18MPa. The cross-sectional dimensions of the small square billet after roughing are 140×140mm. The center segregation of the small square billet is ≤0.5 grade, the center porosity is ≤0.5 grade, and the center carbon segregation index is ≤1.

04.

6. The method for producing spring steel for shaped steel wire according to claim 2, characterized in that: In step S4, the air-fuel ratio of the preheating section and the heating section is set to 0.57~0.60, and the air-fuel ratio of the soaking section is set to 0.52~0.55; the heating time of the soaking section is ≥45min.

7. The method for producing spring steel for shaped steel wire according to claim 2, characterized in that: In step S5, the descaling water pressure is ≥20MPa.

8. The method for producing spring steel for shaped steel wire according to claim 2, characterized in that: In step S6, in the rapid cooling section, the roller speed is controlled at 1.20~1.25m / s, the cooling rate is 6.5~7.8℃ / s, and the temperature at the end of the rapid cooling section is 630~660℃; in the slow cooling section, the roller speed is controlled at 1.28~1.35m / s, the cooling rate is 1.2~1.8℃ / s, and the temperature at the end of the slow cooling section is 585~615℃; in the air cooling section, the roller speed is controlled at 1.08~1.15m / s, the cooling rate is 2.6~3.3℃ / s, and the winding temperature is 510~540℃.

9. The method for producing spring steel for shaped steel wire according to claim 2, characterized in that: The obtained wire rod has a microstructure of ferrite combined with sorbite, with a sorbite ratio of ≥93%, a sorbite grain size of 9.2~11.6μm, a sorbite lamellar spacing of 165~195nm, and an anomalous martensite structure of ≤0.5 grade.

10. The method for producing spring steel for shaped steel wire according to claim 2, characterized in that: The obtained wire rod has a tensile strength of 1120~1160MPa, a tensile strength fluctuation of ≤35MPa within the same coil, a reduction of area of ​​≥53%, a reduction of area fluctuation of ≤5% within the same coil, and an elongation after fracture of ≥18%. The special-shaped spring steel wire prepared from the wire rod through drawing and cold extrusion can meet the requirements of a tensile strength of 1850~1920MPa and an elongation after fracture of ≥8%.

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