55SiCr spring steel hot-rolled wire rod and method for improving uniformity of cleaning rod structure and mechanical property of 55SiCr spring steel hot-rolled wire rod

Abstract

The invention discloses a 55SiCr spring steel hot-rolled wire rod and a method for improving the uniformity of the cleaning rod structure and the mechanical property of the 55SiCr spring steel hot-rolled wire rod. According to the method for improving the uniformity of the cleaning rod structure and the mechanical property of the 55SiCr spring steel hot-rolled wire rod, element segregation is improved, spinning and air cooling processes are optimized, and the uniformity of the cleaning rod structure and the mechanical property of the 55SiCr spring steel hot-rolled wire rod is improved. After the temperature difference between the lap joint point and the non-lap joint point of the wire rod is reduced, the uniformity of the structure and the mechanical property of the hot-rolled wire rod can be improved, and the overall mechanical property of the wire rod can be improved; the tensile strength fluctuation of the prepared 55SiCr spring steel hot-rolled wire rod is smaller than or equal to 20 MPa, and the percentage reduction of area fluctuation is smaller than or equal to 3%.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, in particular to a 55SiCr spring steel hot-rolled wire rod and a method for improving the uniformity of the structure and mechanical properties of the 55SiCr spring steel hot-rolled wire rod. Background Art

[0002] 55SiCr is a spring steel material with excellent performance. After drawing, quenching, tempering and other treatments, the strength can reach more than 1950MPa. It is generally used to manufacture automobile suspension springs. During the driving process of the car, the suspension spring plays a role of buffering and shock absorption, and is required to have good mechanical properties, anti-elasticity and fatigue life. Most people only focus on how to improve the strength and plasticity of 55SiCr spring steel wire rod, while the uniformity of wire rod structure and mechanical properties is also very important. The uneven structure and mechanical properties of the hot-rolled wire rod of 55SiCr spring steel will be inherited to the uneven structure and mechanical properties of the manufactured spring, thereby affecting the quality of the spring. Summary of the invention

[0003] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a 55SiCr spring steel hot-rolled wire rod and a method for improving the uniformity of the organization and mechanical properties of the 55SiCr spring steel hot-rolled wire rod.

[0004] The present invention solves the technical problem by adopting the following technical solutions.

[0005] The invention provides a 55SiCr spring steel hot-rolled wire rod. The chemical components of the 55SiCr spring steel are as follows: C: 0.54%-0.56%, Si: 1.41%-1.50%, Mn: 0.61%-0.69%, Cr: 0.61%-0.69%, P: ≤0.012%, S: ≤0.011%, Ni: ≤0.010%, Cu: ≤0.05%, and the rest is Fe and inevitable impurities. The tensile strength fluctuation of the 55SiCr spring steel wire rod is ≤20MPa, and the cross-sectional shrinkage fluctuation is ≤3%.

[0006] The present invention provides a method for improving the uniformity of the microstructure and mechanical properties of the above-mentioned 55SiCr spring steel hot-rolled wire rod, comprising: adjusting the superheat of molten steel, the pulling speed and the amount of soft reduction to reduce element segregation, and reducing the temperature difference between the overlapping point and the non-overlapping point to improve the uniformity of the microstructure and mechanical properties of the wire rod.

[0007] The present invention has the following beneficial effects:

[0008] The invention provides a 55SiCr spring steel hot-rolled wire rod and a method for improving the uniformity of the organization and mechanical properties of the 55SiCr spring steel hot-rolled wire rod. The invention improves element segregation by adjusting the superheat of molten steel, the pulling speed and the amount of soft reduction, reduces the wire laying temperature and increases the roller speed to reduce the temperature difference between the overlapping point and the non-overlapping point, thereby not only improving the strength and shrinkage of the wire rod, but also significantly improving the uniformity of the organization and mechanical properties of the wire rod. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0010] Figure 1 This is the microstructure diagram of the core of the hot-rolled wire rod obtained in Example 1;

[0011] Figure 2 This is the microstructure diagram of the core of the hot-rolled wire rod obtained in Example 2;

[0012] Figure 3 This is the microstructure diagram of the core of the hot-rolled wire rod obtained in Example 3;

[0013] Figure 4 This is the microstructure diagram of the core of the hot-rolled wire rod obtained in Comparative Example 1;

[0014] Figure 5 This is the microstructure diagram of the core of the hot-rolled wire rod obtained in Comparative Example 2. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0016] The following specifically describes the 55SiCr spring steel hot-rolled wire rod and the method for improving the uniformity of the structure and mechanical properties of the 55SiCr spring steel hot-rolled wire rod provided in the embodiments of the present invention.

[0017] In the first aspect, an embodiment of the present invention provides a 55SiCr spring steel hot-rolled wire rod, the chemical composition mass percentage of the 55SiCr spring steel is: C: 0.54%~0.56%, Si: 1.41%~1.50%, Mn: 0.61%~0.69%, Cr: 0.61%~0.69%, P: ≤0.012%, S: ≤0.011%, Ni: ≤0.010%, Cu: ≤0.05%, and the rest is Fe and unavoidable impurities, and the tensile strength fluctuation of the 55SiCr spring steel wire rod is ≤20MPa, and the cross-sectional shrinkage fluctuation is ≤3%.

[0018] In some optional embodiments, the microstructure of the 55SiCr spring steel hot-rolled wire rod is: troostite+pearlite+a small amount of ferrite, wherein the proportion of troostite is greater than or equal to 85%.

[0019] In some optional embodiments, the size of the 55SiCr spring steel wire rod is φ5.5 mm to φ19 mm.

[0020] In the second aspect, an embodiment of the present invention provides a method for improving the uniformity of the microstructure and mechanical properties of the above-mentioned 55SiCr spring steel hot-rolled wire rod, comprising: adjusting the superheat of molten steel, the pulling speed and the amount of light reduction to reduce element segregation, and reducing the temperature difference between the overlap point and the non-overlap point to improve the uniformity of the microstructure and mechanical properties of the wire rod.

[0021] During the production process of 55SiCr spring steel wire rod, due to the problems of element segregation and uneven cooling, the microstructure and mechanical properties of the wire rod are quite different, and wire breakage often occurs during the subsequent drawing and spring winding processes.

[0022] The solidification process of the ingot is a process of growth from the outside to the inside. Due to the different solubility of elements in the liquid and solid phases, element segregation will occur in the center of the ingot. During the rolling production process, the hot-rolled wire rod will be laid flat on the air-cooled roller for cooling and transported to the coiling barrel for coiling. The contact between the wire rod circles will form overlapping points. At the overlapping points, multiple circles of wire rods are in close contact, the heat dissipation is slow and the temperature is high. At the non-overlapping points, the loose spread heats up quickly and the temperature is low. In this way, there will be a large temperature difference between the overlapping points and the non-overlapping points. The main reasons affecting the uniformity of the microstructure and mechanical properties of the hot-rolled wire rod of 55SiCr spring steel are element segregation and the large difference in cooling rate between the overlapping points and the non-overlapping points.

[0023] The embodiment of the present invention provides a method for improving the uniformity of the microstructure and mechanical properties of the above-mentioned 55SiCr spring steel hot-rolled wire rod, which includes: improving element segregation by adjusting the superheat of molten steel, the pulling speed and the amount of light reduction, reducing the wire laying temperature and increasing the roller speed to reduce the temperature difference between the overlapping point and the non-overlapping point, which can not only improve the strength and surface shrinkage of the wire rod, but also significantly improve the uniformity of the microstructure and mechanical properties of the wire rod.

[0024] In some optional embodiments, the steps include: continuously casting molten steel with qualified ingredients to obtain a large square billet with a size of 320mm×425mm, then rolling the billet into a rolled billet with a cross-sectional size of 165mm×165mm, and then rolling the rolled billet into a wire rod with a size of φ5.5mm~φ19mm, spinning the straight wire rod into a wire rod, and winding the wire rod after spinning into a continuous coil on a Stelmor air-cooled line roller, and controlling the cooling of the coil on the roller to obtain a 55SiCr spring steel hot-rolled wire rod.

[0025] In some optional embodiments, the continuous casting of molten steel with qualified composition to obtain a large square billet with a size of 320 mm×425 mm includes: the continuous casting process adopts full-process protective casting, the superheat T of the molten steel in the tundish is controlled to be 10°C~20°C, the billet drawing speed v is 0.6m / min~1.3m / min, and the superheat T and the billet drawing speed v satisfy the formula T×v=13.

[0026] In some optional embodiments, the light pressure reduction of the tensioning and straightening roller is controlled to be 14 mm to 20 mm.

[0027] In some optional embodiments, the spinning temperature is controlled to be 740° C. to 800° C. A low spinning temperature will increase the risk of abnormal tissue, and a high spinning temperature will increase the temperature difference between the overlapping point and the non-overlapping point of the wire rod, which is not conducive to uniform temperature and tissue.

[0028] In some optional embodiments, the roller speed of the Stelmor air cooling line is controlled to be 0.6m / s to 1.6m / s. A low roller speed will increase the stacking degree between the wire rods and increase the cooling speed difference between the overlapping points and the non-overlapping points. A too fast roller speed will cause the wire rods to be spread too loosely, which will increase the risk of abnormal tissue and be detrimental to stable and smooth production.

[0029] In some optional embodiments, the wire rod after spinning is rolled into a continuous coil on a roller, and the coil is cooled on the roller, including: before the temperature of the wire rod drops to 660°C ~ 690°C, the fan frequency is controlled to 5Hz ~ 50Hz to quickly cool the wire rod, after the wire rod enters the temperature recovery stage, the fan frequency is adjusted to 0Hz ~ 20Hz to make the wire rod cooling rate <3°C / s, when the temperature of the wire rod drops to 640°C ~ 685°C, the heat preservation cover and the fan are turned off for slow cooling.

[0030] The present invention will be further described below in conjunction with the embodiments.

[0031] Example 1

[0032] The molten iron is desulfurized by KR molten iron → smelted in converter → refined in LF furnace → vacuum treated in RH to produce molten steel with qualified composition. The continuous casting process adopts full protection casting, and the cross-sectional size of the continuous casting billet is a large square billet of 320mm×425mm. The superheat of the molten steel in the tundish is stable at 15℃, the billet drawing speed is 0.87m / min, and the draw roll reduction is 18mm.

[0033] Large square billet red delivery → heating furnace → large bar billet rolling → billet surface peeling and grinding → billet inspection → qualified billet with a cross-sectional size of 165mm×165mm. The billet is heated in the heating furnace and rolled into a φ7mm wire rod. The water cooling intensity is adjusted to make the wire laying temperature 760℃, and the roller speed of the Stelmore air cooling line is controlled to be 1.5m / s. The wire laying temperature and roller speed are kept stable during the production process, and the first 4 of the 25 fans on the Stelmore air cooling line are turned on, including: 1# fan frequency is 15Hz, 2# fan frequency is 5Hz, 3# fan frequency is 5Hz, after the wire rod enters the temperature recovery stage, 4# fan frequency is 5Hz, and when the wire rod temperature drops to 680℃, the insulation cover is closed for slow cooling.

[0034] After trimming the uncooled sections and the parts with surface defects at the head and tail of the wire rod, take one circle of wire rod from each of the head and tail circles, and evenly cut each circle into 6 samples to test the microstructure and mechanical properties, analyze and compare the uniformity of the wire rod structure and mechanical properties. The microstructure diagram of the core of the hot-rolled wire rod is as follows: Figure 1 As shown, the organization is relatively uniform.

[0035] Example 2

[0036] The molten iron is desulfurized by KR molten iron → smelted in converter → refined in LF furnace → vacuum treated in RH to produce molten steel with qualified composition. The continuous casting process adopts full protection casting, and the cross-sectional size of the continuous casting billet is a large square billet of 320mm×425mm. The superheat of the molten steel in the tundish is stable at 18℃, the billet drawing speed is 0.72m / min, and the draw roll reduction is 17mm.

[0037] Large square billet red delivery → heating furnace → large bar billet rolling → billet surface peeling and grinding → billet inspection → qualified billet with a cross-sectional size of 165mm×165mm. The billet is heated in the heating furnace and rolled into a wire rod of φ15mm. The water cooling intensity is adjusted to make the wire laying temperature 780℃, and the roller speed of the Stelmore air cooling line is controlled to be 1.5m / s. The wire laying temperature and roller speed are kept stable during the production process, and the first 5 of the 25 fans on the Stelmore air cooling line are turned on, including: 1# fan frequency is 50Hz, 2# fan frequency is 50Hz, 3# fan frequency is 40Hz, after the wire rod enters the temperature recovery stage, 4# fan frequency is 15Hz, 5# fan frequency is 10Hz, and when the wire rod temperature drops to 665℃, the insulation cover is closed for slow cooling.

[0038] After trimming the uncooled sections and the parts with surface defects at the head and tail of the wire rod, take one circle of wire rod from each of the head and tail circles, and evenly cut each circle into 6 samples to test the microstructure and mechanical properties, analyze and compare the uniformity of the wire rod structure and mechanical properties. The microstructure diagram of the core of the hot-rolled wire rod is as follows: Figure 2 As shown, the organization is relatively uniform.

[0039] Example 3

[0040] The molten iron is desulfurized by KR molten iron → smelted in converter → refined in LF furnace → vacuum treated in RH to produce molten steel with qualified composition. The continuous casting process adopts full protection casting, and the cross-sectional size of the continuous casting billet is a large square billet of 320mm×425mm. The superheat of the molten steel in the tundish is stable at 20℃, the billet drawing speed is 0.65m / min, and the draw roll reduction is 15mm.

[0041] Large square billet red delivery → heating furnace → large bar billet rolling → billet surface peeling and grinding → billet inspection → qualified billet with a cross-sectional size of 165mm×165mm. The billet is heated in the heating furnace and rolled into a wire rod of φ17mm. The water cooling intensity is adjusted to make the wire laying temperature 790℃, and the roller speed of the Stelmore air cooling line is controlled to be 1.5m / s. The wire laying temperature and roller speed are kept stable during the production process, and the first 6 of the 25 fans on the Stelmore air cooling line are turned on, including: 1# fan frequency is 50Hz, 2# fan frequency is 50Hz, 3# fan frequency is 40Hz, 4# fan frequency is 25Hz, after the wire rod enters the temperature recovery stage, 5# fan frequency is 15Hz, 6# fan frequency is 10Hz, and when the wire rod temperature drops to 658℃, the insulation cover is closed for slow cooling.

[0042] After trimming the uncooled sections and the parts with surface defects at the head and tail of the wire rod, take one circle of wire rod from each of the head and tail circles, and evenly cut each circle into 6 samples to test the microstructure and mechanical properties, analyze and compare the uniformity of the wire rod structure and mechanical properties. The microstructure diagram of the core of the hot-rolled wire rod is as follows: Figure 3 As shown, the organization is relatively uniform.

[0043] Comparative Example 1

[0044] The molten iron is desulfurized by KR molten iron → smelted in converter → refined in LF furnace → vacuum treated in RH to produce molten steel with qualified composition. The continuous casting process adopts full protection casting, and the cross-sectional size of the continuous casting billet is a large square billet of 320mm×425mm. The superheat of the molten steel in the tundish is stable at 25℃, the billet drawing speed is 0.62m / min, and the draw roll reduction is 12mm.

[0045] Large square billet red delivery → heating furnace → large bar billet rolling → billet surface peeling and grinding → billet inspection → qualified billet with a cross-sectional size of 165mm×165mm. The billet is heated in the heating furnace and rolled into a φ7mm wire rod. The water cooling intensity is adjusted to make the wire laying temperature 850℃, and the roller speed of the Stelmore air cooling line is controlled to 0.5m / s. During the production process, the wire laying temperature and roller speed are kept stable, and the first two of the 25 fans on the Stelmore air cooling line are turned on, including: 1# fan frequency is 20Hz, 2# fan frequency is 10Hz, and when the wire rod temperature drops to 680℃, the insulation cover is closed for slow cooling.

[0046] After trimming the uncooled sections and the parts with surface defects at the head and tail of the wire rod, take one circle of wire rod from each of the head and tail circles, and evenly cut each circle into 6 samples to test the microstructure and mechanical properties, analyze and compare the uniformity of the wire rod structure and mechanical properties. The microstructure diagram of the core of the hot-rolled wire rod is as follows: Figure 4 As shown in Figure 1, bainite and martensite abnormal structures appear inside the wire rod.

[0047] Comparative Example 2

[0048] The molten iron is desulfurized by KR molten iron → smelted in converter → refined in LF furnace → vacuum treated in RH to produce molten steel with qualified composition. The continuous casting process adopts full protection casting, and the cross-sectional size of the continuous casting billet is a large square billet of 320mm×425mm. The superheat of the molten steel in the tundish is stable at 25℃, the billet drawing speed is 0.62m / min, and the draw roll reduction is 12mm.

[0049] Large square billet red delivery → heating furnace → large bar billet rolling → billet surface peeling and grinding treatment → billet inspection → qualified billet with a cross-sectional size of 165mm×165mm. The billet is heated in the heating furnace and rolled into a wire rod of φ17mm. The water cooling intensity is adjusted to make the wire laying temperature 850℃, and the roller speed of the Stelmore air cooling line is controlled to be 0.7m / s. The wire laying temperature and roller speed are kept stable during the production process, and the first 5 of the 25 fans on the Stelmore air cooling line are turned on, including: 1# fan frequency is 50Hz, 2# fan frequency is 40Hz, 3# fan frequency is 30Hz, 4# fan frequency is 20Hz, and 5# fan frequency is 10Hz. When the wire rod temperature drops to 670℃, the insulation cover is closed for slow cooling.

[0050] After trimming the uncooled sections and the parts with surface defects at the head and tail of the wire rod, take one circle of wire rod from each of the head and tail circles, and evenly cut each circle into 6 samples to test the microstructure and mechanical properties, analyze and compare the uniformity of the wire rod structure and mechanical properties. The microstructure diagram of the core of the hot-rolled wire rod is as follows: Figure 5 As shown, the elements in the core are obviously segregated and the structure is obviously uneven.

[0051] The test results of the microstructures and mechanical properties of Examples 1 to 3 of the present invention and Comparative Examples 1 to 2 are shown in Table 1.

[0052] Table 1 Test results of organization and mechanical properties of Examples 1 to 3 of the present invention and Comparative Examples 1 to 2

[0053]

[0054]

[0055]

[0056]

[0057] Combining Table 1 and Figure 1-5 It can be seen that the 55SiCr spring steel hot-rolled wire rod prepared by the scheme provided in Examples 1-3 has uniform structure and excellent mechanical properties, with a tensile strength fluctuation of ≤20MPa and a cross-sectional shrinkage fluctuation of ≤3%. However, the 55SiCr spring steel hot-rolled wire rod produced by the method provided in the comparative example has poor uniformity in rod structure and mechanical properties, with a tensile strength fluctuation of ≤96MPa and a cross-sectional shrinkage fluctuation of ≤17%, and is prone to abnormal bainite and martensite structures.

[0058] From the above, it can be seen that the embodiment of the present invention provides a 55SiCr spring steel hot-rolled wire rod and a method for improving the uniformity of the microstructure and mechanical properties of the 55SiCr spring steel hot-rolled wire rod. By adjusting the superheat degree and light reduction amount of the continuous casting tundish, the segregation of the ingot is improved, and then the element segregation of the hot-rolled wire rod is improved, and the uniformity of the microstructure and mechanical properties of the hot-rolled wire rod is promoted; the spinning temperature is appropriately lowered, the starting temperature before the cooling rate of the overlap point and the non-overlap point begins to be uneven, and the time from spinning to the phase change occurs is reduced, thereby reducing the temperature difference in the phase change process caused by the cooling rate difference between the overlap point and the non-overlap point, and then promoting the uniformity of the microstructure and mechanical properties of the hot-rolled wire rod; the air-cooled roller speed is appropriately increased to make the wire rod spread more loosely and evenly, reducing The overlapping areas are stacked, thereby reducing the cooling rate difference between the overlapping points and the non-overlapping points, thereby promoting the improvement of the uniformity of the microstructure and mechanical properties of the hot-rolled wire rod; from the results of the embodiments, it can be found that the present invention improves element segregation, optimizes the spinning and air cooling processes, and reduces the temperature difference between the overlapping points and the non-overlapping points of the wire rod, which can not only improve the uniformity of the microstructure and mechanical properties of the hot-rolled wire rod, but also improve the overall mechanical properties of the wire rod; after the scheme provided by the present invention is implemented, the fluctuation range of the tensile strength can be controlled within 20MPa, and the fluctuation range of the cross-sectional shrinkage rate can be controlled within 3%.

[0059] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A 55SiCr spring steel hot-rolled wire rod, characterized in that: The chemical composition mass percentage of the 55SiCr spring steel is: C: 0.54% to 0.56%, Si: 1.41% to 1.50%, Mn: 0.61% to 0.69%, Cr: 0.61% to 0.69%, P: ≤0.012%, S: ≤0.011%, Ni: ≤0.010%, Cu: ≤0.05%, and the rest is Fe and unavoidable impurities, and the tensile strength fluctuation of the 55SiCr spring steel wire rod is ≤20MPa, and the cross-sectional shrinkage fluctuation is ≤3%.

2. The 55SiCr spring steel hot-rolled wire rod according to claim 1, characterized in that: The microstructure of the 55SiCr spring steel hot-rolled wire rod is: troostite+pearlite+a small amount of ferrite, wherein the proportion of troostite is greater than or equal to 85%.

3. The 55SiCr spring steel hot-rolled wire rod according to claim 1, characterized in that: The size of the 55SiCr spring steel wire rod is φ5.5mm~φ19mm.

4. A method for improving the uniformity of the microstructure and mechanical properties of the 55SiCr spring steel hot-rolled wire rod according to any one of claims 1 to 3, characterized in that: include: Adjust the superheat of molten steel, pulling speed and light reduction to reduce element segregation, and reduce the temperature difference between overlapping points and non-overlapping points to improve the uniformity of wire rod structure and mechanical properties.

5. The method according to claim 4, characterized in that The method comprises the following steps: continuously casting molten steel with qualified ingredients to obtain a large square billet with a size of 320 mm×425 mm, rolling the large square billet into a rolled billet with a cross-sectional size of 165 mm×165 mm, rolling the rolled billet into a wire rod with a size of φ5.5 mm to φ19 mm, spinning the straight wire rod into a wire rod, rolling the wire rod after spinning into a continuous coil on a Stelmor air-cooled line roller, and controlling the cooling of the coil on the roller to obtain a 55SiCr spring steel hot-rolled wire rod.

6. The method according to claim 5, characterized in that The method comprises the following steps: the continuous casting process adopts full-process protection casting, the superheat T of the molten steel in the tundish is controlled to be 10°C to 20°C, the billet drawing speed v is controlled to be 0.6m / min to 1.3m / min, and the superheat T and the billet drawing speed v satisfy the formula T×v=13.

7. The method according to claim 6, characterized in that Control the light pressure of the straightening roller to 14mm~20mm.

8. The method according to claim 5, characterized in that The spinning temperature is controlled at 740℃~800℃.

9. The method according to claim 5, characterized in that The Stelmor air cooling line roller speed is controlled to be 0.6m / s~1.6m / s.

10. The method according to claim 5, characterized in that The wire rod after spinning is rolled into a continuous coil on a roller table, and the continuous coil is cooled on the roller table, including: before the temperature of the wire rod drops to 660℃~690℃, the fan frequency is controlled to 5Hz~50Hz to quickly cool the wire rod; after the wire rod enters the temperature recovery stage, the fan frequency is adjusted to 0Hz~20Hz to make the cooling rate of the wire rod less than 3℃ / s; when the temperature of the wire rod drops to 640℃~685℃, the heat preservation cover and the fan are closed for slow cooling.

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