Spring steel wire rod and production method thereof

Abstract

The present application provides a spring steel wire rod and a production method thereof. In the production method, the steel billet is rolled at 940-980°C, enters finish rolling at 870-910°C, and exits finish rolling at 950-1000°C; the cooling rate of the cooling stage is controlled to be 3.5-5.0°C / s, and the temperature of exiting the cooling stage is 650-680°C; the cooling rate of the insulation stage is controlled to be 0.5-0.8°C / s, and the temperature of exiting the insulation stage is 600-630°C; the cooling rate of the air cooling stage is controlled to be 1.5-2.0°C / s, and the coiling temperature of exiting the air cooling stage and coiling is ≤450°C; the induction heating temperature and the salt bath temperature are determined according to the strength-plasticity index SE of the spring steel wire rod, and the austenitization heating temperature and the sorbite phase transformation salt bath temperature in the salt bath heat treatment process are reasonably set, so that the wire rod structure is uniform, the sorbite grain size and the interlamellar spacing fluctuate little, and the spring steel wire rod has small tensile strength fluctuation and good plasticity.

Description

Technical Field

[0001] The present application relates to the technical field of steel smelting, and in particular to a spring steel wire rod and a production method thereof. Background Art

[0002] As the automobile transportation industry develops towards high speed, heavy load and lightweight, the automobile industry has an increasing demand for high-strength, high fatigue and low-cost spring steel. At present, 1960MPa-level suspension springs are gradually replacing 1860MPa-level products to become the mainstream of the market, and some high-end models have adopted 2060MPa-level products. For high-strength suspension springs, the currently disclosed technical solutions all add expensive micro-alloy elements to the billet composition design, adopt the method of large square billet smelting, billet grinding, combined with multi-pass rolling mill controlled rolling and Stelmor line controlled cooling process to obtain the finished wire rod. The wire rod is then drawn, quenched and tempered to prepare high-strength spring steel wire above 2060MPa. However, due to the expensive alloy elements, the cost of wire rod increases, and the cross-sectional shrinkage rate of wire rod drawn into steel wire after quenching and tempering is usually not good. Summary of the invention

[0003] The purpose of the present application is to provide a spring steel wire rod and a production method thereof. The production method of the spring steel wire rod controls cooling and adds salt bath heat treatment to make the spring steel wire rod structure more uniform, thereby solving the problem of low cross-sectional shrinkage rate of steel wire drawn after quenching and tempering of the wire rod in the prior art.

[0004] In order to achieve one of the above-mentioned purposes of the invention, an embodiment of the present application provides a method for producing a spring steel wire rod, comprising heating, high-speed wire rolling, cooling and salt bath heat treatment of a steel billet, wherein:

[0005] In the high-speed wire rolling process, the steel billet is heated and then sent to the rolling mill for rolling to obtain spring steel wire rods, wherein the starting rolling temperature is 940~980℃, the finishing rolling inlet temperature is 870~910℃, and the finishing rolling outlet temperature is 950~1000℃;

[0006] In the cooling process, the spring steel wire rod is advanced and cooled through the roller conveyor, and the roller conveyor is divided into three sections: cooling section, heat preservation section and air cooling section along the advancing direction of the spring steel wire rod;

[0007] In the cooling stage, the roller speed is 0.40~0.55m / s, the cooling rate is 3.5~5.0℃ / s, and the temperature of the spring steel wire rod exiting the cooling stage is 650~680℃;

[0008] In the second insulation stage, the roller speed is 0.55~0.65m / s, the cooling rate is 0.5~0.8℃ / s, and the temperature of the spring steel wire rod exiting the second insulation stage is 600~630℃;

[0009] In the third stage of air cooling, the roller speed is 0.65~0.80m / s, the cooling rate is 1.5~2.0℃ / s, and the coiling temperature of the spring steel wire rod after exiting the third stage of air cooling and coiling is ≤450℃;

[0010] In the salt bath heat treatment process, when 2.60%≤strength-plasticity index SE<2.75%, the induction heating temperature is controlled to be 860~870℃, and the salt bath temperature is 625~635℃; when 2.75%≤strength-plasticity index SE<2.90%, the induction heating temperature is controlled to be 870~880℃, and the salt bath temperature is 615~625℃; when 2.90%≤strength-plasticity index SE≤3.05%, the induction heating temperature is controlled to be 880~890℃, and the salt bath temperature is 605~615℃; wherein, strength-plasticity index SE=[C]+0.5[Si]+0.8[Mn]+1.2[Cr], [C], [Si], [Mn] and [Cr] are the weight percentages of the elements in the spring steel wire rod respectively.

[0011] As a further improvement of one embodiment of the present application, in the salt bath heat treatment process, when 10mm≤wire rod diameter D≤14mm, the wire speed is 4.5~4.8m / min; when 14mm<wire rod diameter D≤18mm, the wire speed is 3.8~4.2m / min; when 18mm<wire rod diameter D≤26mm, the wire speed is 3.2~3.5m / min.

[0012] As a further improvement of one embodiment of the present application, the cooling stage occupies 1 / 4 of the roller length, the insulation stage 2 occupies 1 / 3 of the roller length, and the air cooling stage 3 occupies 5 / 12 of the roller length.

[0013] As a further improvement of one embodiment of the present application, in the heating process, the steel billet is fed into a heating furnace for heating, wherein the temperature of the preheating section is 800~870°C, the temperature of the first heating section is 870~920°C, the temperature of the second heating section is 920~960°C, the temperature of the equalizing section is 960~1040°C, and the total heating time is 80~100min.

[0014] As a further improvement of one embodiment of the present application, the air-fuel ratio of the first heating stage and the second heating stage is 0.50~0.55, and the air-fuel ratio of the equalizing stage is 0.45~0.50.

[0015] As a further improvement of an embodiment of the present application, before the heating process, the process further includes steelmaking, continuous casting and blanking processes performed in sequence, wherein:

[0016] In the billeting process, the large square billet obtained by continuous casting is billeted into small square billets. After the small square billets are surface treated, the surface roughness Ra of the billet is ≤25μm, and the depth of the surface crack is ≤50μm.

[0017] An embodiment of the present application also provides a spring steel wire rod, whose microstructure includes ferrite and troostite, the troostite ratio is ≥92%, the troostite grain size is 8.8~10.2μm, and the troostite interlamellar spacing is 150~180nm.

[0018] As a further improvement of an embodiment of the present application, its chemical composition, in weight percentage, includes: C: 0.51~0.65%, Si: 1.40~1.70%, Mn: 0.60~0.90%, Cr: 0.60~0.85%, Cu≤0.02%, Al≤0.005%, P≤0.010%, S≤0.010%, N≤50ppm, O≤20ppm, and the rest are Fe and unavoidable impurities. The strength-plasticity index SE is 2.60~3.05%, wherein SE=[C]+0.5[Si]+0.8[Mn]+1.2[Cr], and [C], [Si], [Mn] and [Cr] are the weight percentages of the elements in the spring steel wire rod respectively.

[0019] As a further improvement of an embodiment of the present application, the tensile strength of the spring steel wire rod is 1100~1180MPa, the cross-sectional shrinkage rate is ≥52%, and the elongation after fracture is ≥16%.

[0020] As a further improvement of an embodiment of the present application, the tensile strength of the spring steel wire rod after drawing, quenching and tempering treatment is 2100~2180MPa, and the cross-sectional shrinkage rate is ≥48%.

[0021] One or more technical solutions provided by this application have at least the following technical effects or advantages:

[0022] The production method of spring steel wire rod provided by the present application is that after high-speed wire rolling, in the cooling process, in the cooling stage, austenite quickly passes through the ferrite phase region to reduce the ferrite phase transformation ratio, and the second heat preservation stage allows the austenite to fully undergo pearlite phase transformation. In the salt bath heat treatment process, according to the difference in the strength and plasticity index of the wire rod, the austenitization heating temperature and the sorbite phase transformation salt bath temperature in the salt bath heat treatment process are reasonably set, so that the sorbite grain size and the interlamellar spacing fluctuate little, and the finished wire rod has a small fluctuation in tensile strength and good plasticity. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. 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.

[0024] The present application provides a method for producing a spring steel wire rod, comprising heating, high-speed wire rolling, cooling and salt bath heat treatment of a steel billet, wherein:

[0025] In the high-speed wire rolling process, the steel billet is heated and then sent to the rolling mill for rolling to obtain spring steel wire rods, wherein the starting rolling temperature is 940~980℃, the finishing rolling inlet temperature is 870~910℃, and the finishing rolling outlet temperature is 950~1000℃;

[0026] In the cooling process, the spring steel wire rod is advanced and cooled through the roller conveyor, and the roller conveyor is divided into three sections: cooling section, heat preservation section and air cooling section along the advancing direction of the spring steel wire rod;

[0027] In the cooling stage, the roller speed is 0.40~0.55m / s, the cooling rate is 3.5~5.0℃ / s, and the temperature of the spring steel wire rod exiting the cooling stage is 650~680℃;

[0028] In the second insulation stage, the roller speed is 0.55~0.65m / s, the cooling rate is 0.5~0.8℃ / s, and the temperature of the spring steel wire rod exiting the second insulation stage is 600~630℃;

[0029] In the third stage of air cooling, the roller speed is 0.65~0.80m / s, the cooling rate is 1.5~2.0℃ / s, and the coiling temperature of the spring steel wire rod after exiting the third stage of air cooling and coiling is ≤450℃;

[0030] In the salt bath heat treatment process, when 2.60%≤strength-plasticity index SE<2.75%, the induction heating temperature is controlled to be 860~870℃, and the salt bath temperature is 625~635℃; when 2.75%≤strength-plasticity index SE<2.90%, the induction heating temperature is controlled to be 870~880℃, and the salt bath temperature is 615~625℃; when 2.90%≤strength-plasticity index SE≤3.05%, the induction heating temperature is controlled to be 880~890℃, and the salt bath temperature is 605~615℃; wherein, strength-plasticity index SE=[C]+0.5[Si]+0.8[Mn]+1.2[Cr], [C], [Si], [Mn] and [Cr] are the weight percentages of the elements in the spring steel wire rod respectively.

[0031] The wire rod obtained after high-speed wire rolling is spun at 850~890℃. After the spun wire is completed, in the cooling process, the cooling rate is controlled at 3.5~5.0℃ / s in the first cooling stage, so that the austenite quickly passes through the ferrite phase area, reducing the proportion of the ferrite phase, and avoiding the abnormal martensite caused by too fast cooling rate. In the second insulation stage, the cooling rate is controlled at 0.5~0.8℃ / s to make the austenite fully undergo pearlite phase transformation, and then air cooling is performed to cool the wire rod and coil it, reducing the scratches on the inner ring during the coiling process.

[0032] In the salt bath heat treatment process, induction heating uses electromagnetic induction to generate eddy currents inside the wire rod, and relies on the energy of these eddy currents to achieve the purpose of heating. The heating rate of the wire rod during induction heating is ≥300℃ / s. Austenitization combined with salt bath treatment can obtain a higher content of troostite. In this application, the heating temperature of austenitization and the salt bath temperature of troostite phase transformation in the salt bath heat treatment process are reasonably designed according to the difference in strength and plasticity index of the wire rod.

[0033] Preferably, before the small square billet is subjected to high-speed wire rolling, it is first descaled by a high-pressure descaling machine, and the descaling water pressure is ≥20MPa.

[0034] In some embodiments, in the salt bath heat treatment process, when 10mm≤wire rod diameter D≤14mm, the wire speed is 4.5~4.8m / min; when 14mm<wire rod diameter D≤18mm, the wire speed is 3.8~4.2m / min; when 18mm<wire rod diameter D≤26mm, the wire speed is 3.2~3.5m / min. The wire speed is designed according to the wire rod specification range to ensure that the wire rod reaches the corresponding temperature during induction heating and salt bath treatment, so that the wire rod structure is homogenized, the sorbite grain size and lamellar spacing fluctuate little, and the finished wire rod has small fluctuations in tensile strength and good plasticity.

[0035] Specifically, in the cooling process, a Stelmor cooling line can be used. The Stelmor cooling line includes a roller conveyor and a heat preservation cover and a fan arranged above and below the roller conveyor. The heat preservation cover is arranged above the roller conveyor. When the heat preservation cover is closed, the wire rod can be placed between the heat preservation cover and the roller conveyor, and the fan can blow air to cool the wire rod on the roller conveyor. The roller conveyor is composed of multiple sub-rollers, each of which is provided with a fan. A heat preservation cover is arranged between two adjacent sub-rollers in the length direction of the roller conveyor, and a heat preservation cover is also arranged before the first fan and after the last fan, that is, the number of heat preservation covers is one more than the number of fans.

[0036] In some embodiments, the cooling stage occupies 1 / 4 of the roller length, the insulation stage 2 occupies 1 / 3 of the roller length, and the air cooling stage 3 occupies 5 / 12 of the roller length.

[0037] Preferably, the roller table is composed of 12 sub-rollers, that is, the cooling stage includes 3 sub-rollers, the insulation stage 2 includes 4 sub-rollers, and the air cooling stage 3 includes 5 sub-rollers; in the forward direction of the spring steel wire rod, the fans are provided with 1# to 12# fans, and the insulation covers are provided with 1# to 13# insulation covers. Among them, the 1# fan is fully opened, and the air volume is 260,000 m 3 / h, 2# fan opens 50~80% of the maximum air volume, 3# fan opens 10~40% of the maximum air volume, 1#~4# insulation covers are opened; 4#, 5#, 7# fans are closed, 6# fan opens 10~30% of the maximum air volume, 5# and 8# insulation covers are closed, 6# and 7# insulation covers are opened; 8#~12# fans are closed, 9#~13# insulation covers are opened. Control the opening, closing and air volume of the fans, as well as the opening and closing of the insulation covers, and work together with the roller speed to control the cooling rate and exit temperature of the spring steel wire rod in the aforementioned cooling stage 1, insulation stage 2 and air cooling stage 3.

[0038] In some embodiments, in the heating process, the steel billet is sent to a heating furnace for heating, wherein the preheating temperature is 800-870°C, the heating temperature is 870-920°C, the heating temperature is 920-960°C, the soaking temperature is 960-1040°C, and the total heating time is 80-100 minutes. The steel billet is first heated to the high-speed rolling temperature.

[0039] In some embodiments, the air-fuel ratio of the first heating stage and the second heating stage is 0.50-0.55, and the air-fuel ratio of the equalizing stage is 0.45-0.50.

[0040] In some embodiments, before the heating process, the steelmaking process also includes steelmaking, continuous casting and billeting processes performed in sequence, wherein the steelmaking process includes molten iron pretreatment, converter smelting, LF refining and RH refining to obtain molten steel that meets the composition requirements, and then continuous casting is performed to obtain a large billet, and the large billet obtained by continuous casting is billeted into a small billet. After the small billet is surface treated, the billet surface roughness Ra ≤ 25μm, and the surface crack depth ≤ 50μm. LF refining and RH refining are used for two-step refining to obtain molten steel with higher cleanliness. The surface of the small billet is ground, shot blasted, and flaw detected, so that the surface performance of the spring steel wire rod obtained by subsequent high-speed rolling is excellent, which is beneficial to the drawing of the spring steel wire rod into steel wire and the strength of the steel wire after drawing into steel wire.

[0041] The present application also provides a spring steel wire rod, whose microstructure includes ferrite and troostite, the troostite ratio is ≥92%, the troostite grain size is 8.8~10.2μm, and the troostite interlamellar spacing is 150~180nm. The troostite ratio is determined by "YB / T169-2014 High Carbon Steel Wire Rod Troostite Content Metallographic Detection Method", and the grain size is determined by the oxidation method in "GB / T 6394-2017 Metal Average Grain Size Determination Method". The spring steel wire rod has a high proportion of troostite, which makes the wire rod have high strength.

[0042] In some embodiments, the chemical composition of the spring steel wire rod includes, by weight percentage: C: 0.51~0.65%, Si: 1.40~1.70%, Mn: 0.60~0.90%, Cr: 0.60~0.85%, Cu≤0.02%, Al≤0.005%, P≤0.010%, S≤0.010%, N≤50ppm, O≤20ppm, and the rest is Fe and unavoidable impurities, and the strength-ductility index SE is 2.60~3.05%.

[0043] The effects of each chemical component have the following characteristics:

[0044] C: C is a cheap and effective strength-enhancing element, and the carburized body it forms will form dispersed fine carbide particles in the subsequent quenching and tempering treatment, further improving the strength. However, excessive addition of C will deteriorate plasticity and affect subsequent drawing and forming. In order to take into account both strength and plasticity, the C content is set to 0.51~0.65%.

[0045] Si: Si is an effective elasticity-enhancing element, which can improve the elastic modulus of the wire rod and the anti-elasticity of the finished spring. At the same time, as a strong and effective deoxidizing element, it can greatly reduce the oxygen content of molten steel and improve purity. However, excessive addition affects plasticity and increases the tendency of decarburization, so the Si content is controlled to 1.40~1.70%.

[0046] Mn: Mn is an effective solid solution strengthening and hardenability improving element, which can effectively improve strength, and MnS formed with impurity element S can enhance hot deformation ability. However, excessive Mn will lead to excessive hardenability, resulting in abnormal structure during cooling. Therefore, the Mn content is set to 0.60~0.90%.

[0047] Cr: Cr is a carbide-binding element, which is easy to combine with C as a substitutional alloy. Carbides exist in cementite lamellae, which improve the matrix strength through solid solution strengthening. Similar to Mn, excessive addition will lead to excessive hardenability and abnormal structure, so the Cr content is set to 0.60~0.85%.

[0048] Cu: Cu is an impurity element for spring steel, which is easily enriched in grain boundaries and phase boundaries and causes hot brittleness, affecting the hot deformation effect. Therefore, the Cu content is controlled to be ≤0.02%.

[0049] Al: Al is a strong deoxidizing element and is easily combined with O to form spinel-like hard inclusions, which greatly deteriorates the fatigue properties of the finished spring. Therefore, the Al content is controlled to be ≤0.005%.

[0050] P: P is an impurity element that is easily segregated. It is easy to be enriched with Cu in the grain boundary and reduce the grain boundary strength and the plastic deformation ability of the quenched and tempered structure. Therefore, the P content is controlled to be ≤0.010%.

[0051] S: S can easily cause center segregation. Too high a content will produce large-sized MnS inclusions, affecting fatigue life. Therefore, the S content is controlled to be ≤0.010%.

[0052] N: N is easy to combine with alloy elements to form large-sized (C, N) composite particles. Although it can improve strength, it deteriorates plasticity. Because the addition of micro-alloy elements combined with it is not considered, the N content needs to be controlled to ≤50ppm.

[0053] O: Most of O combines with impurity elements to form inclusions, which affects fatigue performance, so the O content needs to be controlled to ≤20ppm.

[0054] SE=[C]+0.5[Si]+0.8[Mn]+1.2[Cr]: This index is the strength-plasticity index of the spring steel in this application, which comprehensively considers the influence of the added alloy elements on strength and plasticity. The higher the index, the higher the strength and the lower the plasticity; on the contrary, the lower the index, the lower the strength and the higher the plasticity. Therefore, according to the strength-plasticity requirements of spring steel, the strength-plasticity index SE is limited to 2.60~3.05%.

[0055] The spring steel wire rod provided in the present application does not contain any expensive alloying elements, and a relatively high-strength spring steel wire rod can be obtained by limiting the strength-ductility index SE and combining high-wire rolling, cooling and salt bath heat treatment.

[0056] In some embodiments, the spring steel wire rod has a tensile strength of 1100-1180 MPa, a cross-sectional shrinkage of ≥52%, and a post-fracture elongation of ≥16%. The spring tube wire rod having the above chemical composition and tissue ratio has excellent tensile strength and cross-sectional shrinkage.

[0057] In some embodiments, the spring steel wire rod has a tensile strength of 2100-2180 MPa after drawing, quenching and tempering, and a cross-sectional shrinkage of ≥48%. The spring steel wire rod has a diameter reduction of 8-10% after drawing, a quenching heating temperature of 890°C, and a tempering heating temperature of 425°C, and the quenching temperature and tempering temperature errors fluctuate by 2°C above and below the aforementioned temperatures.

[0058] The technical solution of the present application is further described below in conjunction with some specific embodiments.

[0059] Example 1

[0060] The production method of spring steel wire rod provided in Example 1 includes steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0061] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 852°C, the temperature of the first heating section is 907°C, the temperature of the second heating section is 937°C, the temperature of the soaking section is 1017°C, and the total heating time is 87 minutes; the air-fuel ratio of the first and second heating sections is 0.52, and the air-fuel ratio of the soaking section is 0.47.

[0062] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 21MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 955℃, the finishing rolling inlet temperature is 892℃, the finishing rolling outlet temperature is 975℃, and the wire drawing temperature is 877℃. The spring steel wire rod with a diameter of 18mm is obtained by rolling.

[0063] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0064] Salt bath heat treatment: The cooled spring steel wire rod is induction heated at a temperature of 862°C, and then subjected to salt bath treatment at a temperature of 627°C. During this period, the wire speed of the spring steel wire rod is 3.8 m / min.

[0065] Example 2

[0066] The production method of spring steel wire rod provided in Example 2 includes steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0067] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 846°C, the temperature of the first heating section is 893°C, the temperature of the second heating section is 934°C, the temperature of the soaking section is 1009°C, and the total heating time is 86 minutes; the air-fuel ratio of the first and second heating sections is 0.53, and the air-fuel ratio of the soaking section is 0.46.

[0068] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 22MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 957℃, the finishing rolling inlet temperature is 894℃, the finishing rolling outlet temperature is 976℃, and the wire drawing temperature is 874℃. The spring steel wire rod with a diameter of 16mm is obtained by rolling.

[0069] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0070] Salt bath heat treatment: The cooled spring steel wire rod is subjected to induction heating at a temperature of 885°C, and then subjected to salt bath treatment at a temperature of 611°C, during which the wire speed of the spring steel wire rod is 4.0 m / min.

[0071] Example 3

[0072] The production method of spring steel wire rod provided in Example 3 includes steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0073] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 833°C, the temperature of the first heating section is 904°C, the temperature of the second heating section is 946°C, the temperature of the soaking section is 1015°C, and the total heating time is 88 minutes; the air-fuel ratio of the first and second heating sections is 0.52, and the air-fuel ratio of the soaking section is 0.47.

[0074] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 21MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 950℃, the finishing rolling inlet temperature is 893℃, the finishing rolling outlet temperature is 977℃, and the wire drawing temperature is 873℃. The spring steel wire rod with a diameter of 15mm is obtained by rolling.

[0075] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0076] Salt bath heat treatment: The cooled spring steel wire rod is induction heated at a temperature of 874°C, and then subjected to salt bath treatment at a temperature of 618°C. During this period, the wire speed of the spring steel wire rod is 4.2m / min.

[0077] Example 4

[0078] The production method of spring steel wire rod provided in Example 4 includes steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0079] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 847°C, the temperature of the first heating section is 906°C, the temperature of the second heating section is 931°C, the temperature of the soaking section is 1022°C, and the total heating time is 87 minutes; the air-fuel ratio of the first and second heating sections is 0.51, and the air-fuel ratio of the soaking section is 0.47.

[0080] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 21MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 962℃, the finishing rolling inlet temperature is 889℃, the finishing rolling outlet temperature is 987℃, and the wire laying temperature is 879℃. The spring steel wire rod with a diameter of 12mm is obtained by rolling.

[0081] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0082] Salt bath heat treatment: The cooled spring steel wire rod is induction heated at a temperature of 883°C, and then subjected to salt bath treatment at a temperature of 610°C. During this period, the wire speed of the spring steel wire rod is 4.7m / min.

[0083] Example 5

[0084] The production method of spring steel wire rod provided in Example 5 comprises steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0085] Heating: The billet is sent into a heating furnace for heating, wherein the temperature of the preheating section is 849°C, the temperature of the first heating section is 897°C, the temperature of the second heating section is 944°C, the temperature of the soaking section is 997°C, and the total heating time is 90 minutes; the air-fuel ratio of the first and second heating sections is 0.52, and the air-fuel ratio of the soaking section is 0.46.

[0086] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 22MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 967℃, the finishing rolling inlet temperature is 887℃, the finishing rolling outlet temperature is 974℃, and the wire drawing temperature is 871℃. The spring steel wire rod with a diameter of 22mm is obtained by rolling.

[0087] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0088] Salt bath heat treatment: The cooled spring steel wire rod is subjected to induction heating at a temperature of 866°C, and then subjected to salt bath treatment at a temperature of 630°C, during which the wire speed of the spring steel wire rod is 3.4m / min.

[0089] Example 6

[0090] The production method of spring steel wire rod provided in Example 6 comprises steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0091] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 856°C, the temperature of the first heating section is 893°C, the temperature of the second heating section is 951°C, the temperature of the soaking section is 996°C, and the total heating time is 88 minutes; the air-fuel ratio of the first and second heating sections is 0.53, and the air-fuel ratio of the soaking section is 0.48.

[0092] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 26MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 963℃, the finishing rolling inlet temperature is 893℃, the finishing rolling outlet temperature is 988℃, and the wire drawing temperature is 881℃. The spring steel wire rod with a diameter of 26mm is obtained by rolling.

[0093] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0094] Salt bath heat treatment: The cooled spring steel wire rod is induction heated at a temperature of 878°C, and then subjected to salt bath treatment at a temperature of 622°C. During this period, the wire speed of the spring steel wire rod is 3.2m / min.

[0095] Example 7

[0096] The production method of spring steel wire rod provided in Example 7 comprises steelmaking to obtain molten steel having the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0097] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 839°C, the temperature of the first heating section is 906°C, the temperature of the second heating section is 928°C, the temperature of the soaking section is 1011°C, and the total heating time is 91 minutes; the air-fuel ratio of the first and second heating sections is 0.51, and the air-fuel ratio of the soaking section is 0.47.

[0098] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 21MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 958℃, the finishing rolling inlet temperature is 894℃, the finishing rolling outlet temperature is 973℃, and the wire drawing temperature is 874℃. The spring steel wire rod with a diameter of 14mm is obtained by rolling.

[0099] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0100] Salt bath heat treatment: The cooled spring steel wire rod is induction heated at a temperature of 870°C, and then subjected to salt bath treatment at a temperature of 635°C. During this period, the wire speed of the spring steel wire rod is 4.5m / min.

[0101] Example 8

[0102] The production method of spring steel wire rod provided in Example 8 comprises steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0103] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 861°C, the temperature of the first heating section is 901°C, the temperature of the second heating section is 939°C, the temperature of the soaking section is 1006°C, and the total heating time is 85 minutes; the air-fuel ratio of the first and second heating sections is 0.52, and the air-fuel ratio of the soaking section is 0.46.

[0104] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 22MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 961℃, the finishing rolling inlet temperature is 886℃, the finishing rolling outlet temperature is 978℃, and the wire laying temperature is 876℃. The spring steel wire rod with a diameter of 10mm is obtained by rolling.

[0105] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0106] Salt bath heat treatment: The cooled spring steel wire rod is induction heated at a temperature of 890°C, and then subjected to salt bath treatment at a temperature of 615°C. During this period, the wire speed of the spring steel wire rod is 4.8m / min.

[0107] Example 9

[0108] The production method of spring steel wire rod provided in Example 9 includes steelmaking to obtain molten steel with the chemical composition shown in Table 1, continuous casting of the molten steel and blanking to obtain small square billets, and the small square billets undergo the following steps:

[0109] Heating: The billet is sent into the heating furnace for heating, wherein the temperature of the preheating section is 845°C, the temperature of the first heating section is 896°C, the temperature of the second heating section is 944°C, the temperature of the soaking section is 1011°C, and the total heating time is 87 minutes; the air-fuel ratio of the first and second heating sections is 0.53, and the air-fuel ratio of the soaking section is 0.47.

[0110] High-speed wire rolling: After the small square billet is heated, it is descaled by a high-pressure descaling machine with a descaling water pressure of 20MPa. After descaling, it is sent to the rolling mill for rolling. The starting rolling temperature is 959℃, the finishing rolling inlet temperature is 887℃, the finishing rolling outlet temperature is 986℃, and the wire drawing temperature is 879℃. The spring steel wire rod with a diameter of 20mm is obtained by rolling.

[0111] Cooling: The spring steel wire rod is fed into the Stelmor cooling line for cooling. The relevant parameters of each cooling stage are shown in Table 2.

[0112] Salt bath heat treatment: The cooled spring steel wire rod is induction heated at a temperature of 868°C, and then subjected to salt bath treatment at a temperature of 631°C. During this period, the wire speed of the spring steel wire rod is 3.5m / min.

[0113] Table 1 Chemical composition of spring steel wire rods of Examples 1 to 9 (wt%)

[0114]

[0115] Table 2 Stelmor cooling process of spring steel wire rods in Examples 1 to 9

[0116]

[0117] Table 3 Microstructure and properties of spring steel wire rods and properties of quenched and tempered steel wires in Examples 1 to 9

[0118]

[0119] Table 3 lists the microstructure and properties of spring steel wire rods and the properties of quenched and tempered steel wires in Examples 1 to 9, wherein the drawing reduction from spring steel wire rod to spring steel is 8 to 10% of the diameter of the spring steel wire rod, and then the spring steel is obtained by quenching and heating at 890°C and tempering at 425°C, wherein the error between the quenching temperature and the tempering temperature is 2°C. The mechanical properties of spring steel are tested using "GB / T 228.1-2021 Tensile test of metallic materials Part 1: Room temperature test method".

[0120] It should be understood that although this specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each implementation mode may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0121] The series of detailed descriptions listed above are only specific descriptions of feasible implementation methods of the present application. They are not intended to limit the scope of protection of the present application. Any equivalent implementation methods or changes that do not deviate from the technical spirit of the present application should be included in the scope of protection of the present application.

Claims

1. A method for producing a spring steel wire rod, characterized in that: The process includes heating, high-speed wire rolling, cooling and salt bath heat treatment of the steel billet, wherein: In the high-speed wire rolling process, the steel billet is heated and then sent to the rolling mill for rolling to obtain spring steel wire rods, wherein the starting rolling temperature is 940~980℃, the finishing rolling inlet temperature is 870~910℃, and the finishing rolling outlet temperature is 950~1000℃; In the cooling process, the spring steel wire rod is advanced and cooled through the roller conveyor, and the roller conveyor is divided into three sections: cooling section, heat preservation section and air cooling section along the advancing direction of the spring steel wire rod; In the cooling stage, the roller speed is 0.40~0.55m / s, the cooling rate is 3.5~5.0℃ / s, and the temperature of the spring steel wire rod exiting the cooling stage is 650~680℃; In the second insulation stage, the roller speed is 0.55~0.65m / s, the cooling rate is 0.5~0.8℃ / s, and the temperature of the spring steel wire rod exiting the second insulation stage is 600~630℃; In the third stage of air cooling, the roller speed is 0.65~0.80m / s, the cooling rate is 1.5~2.0℃ / s, and the coiling temperature of the spring steel wire rod after exiting the third stage of air cooling and coiling is ≤450℃; In the salt bath heat treatment process, when 2.60%≤strength-plasticity index SE<2.75%, the induction heating temperature is controlled to be 860~870℃, and the salt bath temperature is 625~635℃; when 2.75%≤strength-plasticity index SE<2.90%, the induction heating temperature is controlled to be 870~880℃, and the salt bath temperature is 615~625℃; when 2.90%≤strength-plasticity index SE≤3.05%, the induction heating temperature is controlled to be 880~890℃, and the salt bath temperature is 605~615℃; wherein, strength-plasticity index SE=[C]+0.5[Si]+0.8[Mn]+1.2[Cr], [C], [Si], [Mn] and [Cr] are the weight percentages of the elements in the spring steel wire rod respectively; The chemical composition of the spring steel wire rod obtained after the steel billet is heated, rolled at high speed, cooled and subjected to salt bath heat treatment includes, by weight percentage, C: 0.51-0.65%, Si: 1.40-1.70%, Mn: 0.60-0.90%, Cr: 0.60-0.85%, Cu≤0.02%, Al≤0.005%, P≤0.010%, S≤0.010%, N≤50ppm, O≤20ppm, and the rest are Fe and unavoidable impurities. The strength and plasticity index SE is 2.60-3.05%.

2. The method for producing spring steel wire rod according to claim 1, characterized in that: In the salt bath heat treatment process, when the wire rod diameter D is 10mm≤14mm, the wire speed is 4.5~4.8m / min; when the wire rod diameter D is 14mm<18mm, the wire speed is 3.8~4.2m / min; when the wire rod diameter D is 18mm<26mm, the wire speed is 3.2~3.5m / min.

3. The method for producing spring steel wire rod according to claim 1, characterized in that: The first cooling section occupies 1 / 4 of the roller length, the second insulation section occupies 1 / 3 of the roller length, and the third air cooling section occupies 5 / 12 of the roller length.

4. The method for producing spring steel wire rod according to claim 1, characterized in that: During the heating process, the steel billet is sent to a heating furnace for heating, wherein the temperature of the preheating section is 800~870°C, the temperature of the first heating section is 870~920°C, the temperature of the second heating section is 920~960°C, the temperature of the equalizing section is 960~1040°C, and the total heating time is 80~100min.

5. The method for producing spring steel wire rod according to claim 4, characterized in that: The air-fuel ratio of the first and second heating stages is 0.50~0.55, and the air-fuel ratio of the equalizing stage is 0.45~0.

50.

6. The method for producing spring steel wire rod according to claim 1, characterized in that: Before the heating process, the process also includes steelmaking, continuous casting and billeting, which are carried out in sequence. In the billeting process, the large square billet obtained by continuous casting is billeted into small square billets. After the small square billets are surface treated, the surface roughness Ra of the billet is ≤25μm, and the depth of the surface crack is ≤50μm.

7. A spring steel wire rod produced by the method for producing a spring steel wire rod according to claim 1, characterized in that: Its microstructure includes ferrite and troostite, the troostite ratio is ≥92%, the troostite grain size is 8.8~10.2μm, and the troostite lamella spacing is 150~180nm.

8. The spring steel wire rod according to claim 7, characterized in that: The spring steel wire rod has a tensile strength of 1100-1180 MPa, a cross-sectional shrinkage of ≥52%, and an elongation after fracture of ≥16%.

9. The spring steel wire rod according to claim 7, characterized in that: The tensile strength of spring steel wire rod after drawing, quenching and tempering is 2100~2180MPa, and the cross-sectional shrinkage rate is ≥48%.

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