Free-cutting steel and hot rolling method

Abstract

This application discloses a free-cutting steel and a hot rolling method, addressing the technical problem of low cutting efficiency of free-cutting steel in the prior art. The free-cutting steel comprises a first free-cutting portion and a second free-cutting portion disposed opposite each other along the width of the free-cutting steel. The first free-cutting portion is larger than the second free-cutting portion along the thickness of the free-cutting steel. The hot rolling method comprises: rough rolling a steel billet to obtain an intermediate billet; performing a first finish rolling on the intermediate billet to obtain a finished billet; the finished billet comprising a first rolling section and a second rolling section disposed opposite each other along the width of the rolling mill; performing a second finish rolling on the finished billet, wherein the deformation rate of the first rolling section along the height direction is less than the deformation rate of the second rolling section along the height direction during the second finish rolling, thereby obtaining the free-cutting steel; and performing a second finish rolling temperature of 850°C to 1050°C, with the second finish rolling temperature of the first rolling section being 20°C to 50°C lower than that of the second rolling section. The hot rolling method provided by this application improves the cutting efficiency of free-cutting steel.

Description

Technical Field

[0001] The present application belongs to the technical field of free-cutting steel rolling, and specifically relates to free-cutting steel and a hot rolling method. Background Art

[0002] Free-cutting steel refers to alloy steel that has a certain amount of one or more free-cutting elements, such as sulfur, phosphorus, lead, calcium, selenium, and tellurium, added to improve its machinability. MnS inclusions in free-cutting steel not only facilitate chip breakage during cutting but also lubricate the tool during machining, effectively reducing tool wear.

[0003] Free-cutting steel is generally delivered in the form of bars, wires, etc. The cross-section of bars and wires is mainly circular, with a small number of irregular shapes. Figure 2 and Figure 3 ) rods and wires are mainly non-center-symmetrical in shape. After rolling, the cutting performance of the wires is uneven, which affects the cutting efficiency. Summary of the Invention

[0004] In order to solve the current technical problem of uneven cutting performance of free-cutting steel, the present application provides a free-cutting steel and a hot rolling method.

[0005] In a first aspect of the present application, a hot rolling method is provided, which is applicable to free-cutting steel, wherein the free-cutting steel has a first free-cutting portion and a second free-cutting portion disposed opposite to each other along a width direction of the free-cutting steel, wherein along a thickness direction of the free-cutting steel, the first free-cutting portion is larger than the second free-cutting portion, and wherein the hot rolling method comprises:

[0006] Rough rolling of the steel billet to obtain an intermediate billet;

[0007] Performing a first finishing rolling on the intermediate billet to obtain a finished billet; the finished billet comprises a first rolling section and a second rolling section which are arranged opposite to each other along the width direction of the rolling mill;

[0008] The finished billet is subjected to a second finishing rolling. During the second finishing rolling process, the deformation rate of the first rolling section along the height direction is less than the deformation rate of the second rolling section along the height direction, so as to obtain the free-cutting steel; wherein: the second finishing rolling temperature is 850°C~1050°C, and the second finishing rolling temperature of the first rolling section is 20°C~50°C lower than the second finishing rolling temperature of the second rolling section.

[0009] In some embodiments, when the first finishing rolling rate does not exceed 8 m / s, the first finishing rolling temperature is 900℃~1000℃, and the second finishing rolling temperature is 850℃~950℃; when the first finishing rolling rate exceeds 8 m / s, the first finishing rolling temperature is 1000℃~1100℃, and the second finishing rolling temperature is 950℃~1050℃.

[0010] In some embodiments, when the first finishing rolling rate does not exceed 8 m / s, during the second finishing rolling process, cooling is sprayed toward the first rolling section to reduce the finishing rolling temperature of the first rolling section.

[0011] In some embodiments, during the second finish rolling process, the deformation rate of each pass of the first rolling section is the same.

[0012] In some embodiments, during the second finishing rolling process, the deformation rate of each pass of the first rolling section is 5% to 10%.

[0013] In some embodiments, during the second finish rolling process, the deformation rate of each pass in the second rolling section is the same.

[0014] In some embodiments, during the second finishing rolling process, the deformation rate of each pass in the second rolling section is 10% to 15%.

[0015] In some embodiments, during the second finish rolling process, a difference between a total deformation rate of the first rolling section and a total deformation rate of the second rolling section is 5% to 25%.

[0016] In some embodiments, the corners of the steel billet are rounded with a grinding radius of ≥15 mm, and before the rough rolling of the steel billet, the process further comprises:

[0017] First, locally heat the fillet of the steel billet so that the temperature of the fillet is 20℃~30℃ higher than other parts.

[0018] In a second aspect of the present application, there is provided a free-cutting steel obtained by the hot rolling method of the first aspect.

[0019] The hot rolling method provided according to an embodiment of the present application includes: rough rolling a steel billet to obtain an intermediate billet; performing a first finish rolling on the intermediate billet to obtain a finished billet; the finished billet has a first rolling section and a second rolling section arranged relatively to each other along the width direction of the rolling mill; performing a second finish rolling on the finished billet, and during the second finish rolling process, the deformation rate of the first rolling section along the height direction is less than the deformation rate of the second rolling section along the height direction, so as to obtain free-cutting steel; wherein: the second finish rolling temperature is 850℃~1050℃, and the second finish rolling temperature of the first rolling section is 20℃~50℃ lower than the second finish rolling temperature of the second rolling section.

[0020] The second finishing temperature of the first rolling section is 20℃~50℃ lower than the second finishing temperature of the second rolling section, and the second finishing temperature of the first rolling section corresponding to the big head side is lower than the second finishing temperature of the second rolling section corresponding to the small head side. In this way, the plasticity of the manganese sulfide in the first rolling section is better than the plasticity of the manganese sulfide in the second rolling section. During the second finishing rolling process, the deformation of the first rolling section is small, while the deformation of the second rolling section is large. After the second finishing rolling is completed, the deformation of the manganese sulfide in the first rolling section is closer to the deformation of the manganese sulfide in the second rolling section, and the length and width of the manganese sulfide in the first rolling section and the second rolling section will be close, reducing the difference between the size of the manganese sulfide on the big head side and the size of the manganese sulfide on the small head side, improving the uniformity of the manganese sulfide in the first easy-to-cut section and the second easy-to-cut section on the special-shaped section of the rod and wire, making the cutting performance of the easy-to-cut steel more uniform and the cutting efficiency higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram showing the distribution of manganese sulfide inclusions in free-cutting steel is shown.

[0022] Figure 2 A schematic structural diagram of free-cutting steel with a teardrop-shaped cross section is shown.

[0023] Figure 3 A schematic structural diagram of free-cutting steel with an isosceles trapezoidal cross section is shown.

[0024] Figure 4 A process step diagram of the hot rolling method provided by this application is shown.

[0025] Figure 5 A schematic diagram of the structure of the finishing billet and the special-shaped rolling hole is shown.

[0026] Explanation of reference numerals: 10 - free-cutting steel, 11 - first free-cutting portion, 12 - second free-cutting portion; 20 - finishing billet, 21 - first rolling portion, 22 - second rolling portion; 30 - rolling hole. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to understand the present application more clearly, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of this application.

[0028] The first embodiment of the present application provides a hot rolling method, which is suitable for long strips of free-cutting steel, such as wires and bars, and can improve the uniformity of the cutting performance of the free-cutting steel and improve the cutting efficiency.

[0029] MnS inclusions are artificial inclusions added to free-cutting steel. They not only make the chips easier to break during cutting, but also play a role in lubricating the tool during processing, thereby effectively reducing tool wear. The morphology of manganese sulfide in free-cutting steel can be referred to Figure 1 , Figure 1 In the middle: Z direction is the hot rolling direction, X direction is the width direction of the free-cutting steel, and Y direction is the thickness direction of the free-cutting steel, which can also be called the height direction. Figure 1 It can be seen that most manganese sulfide in free-cutting steel appears in the form of long strips, and the extension direction of the manganese sulfide is basically consistent with the hot rolling direction. Generally speaking, those skilled in the art refer to the maximum dimension of manganese sulfide along the hot rolling direction as the length of manganese sulfide, and the maximum radial dimension as the width. For example, the maximum dimension of manganese sulfide along the X direction or the Y direction is referred to as the width of manganese sulfide.

[0030] See also Figure 2 as well as Figure 3 The free-cutting steel 10 has a first free-cutting portion 11 and a second free-cutting portion 12 that are arranged opposite to each other along its width direction. That is, the first free-cutting portion 11 and the second free-cutting portion 12 are located on both sides of the free-cutting steel 10 in the width direction. In the thickness direction of the free-cutting steel 10, the size of the first free-cutting portion 11 is larger than the thickness of the second free-cutting portion 12. That is, the thickness of the first free-cutting portion 11 of the free-cutting steel 10 is larger than the thickness of the second free-cutting portion 12. The first free-cutting portion 11 and the second free-cutting portion 12 may both have an end surface, a top surface, and a bottom surface. The top surface and the bottom surface are arranged opposite to each other along the height direction. Both sides of the end surface in the height direction are connected to the top surface and the bottom surface respectively. The thickness of the first free-cutting portion 11 is larger than the thickness of the second free-cutting portion 12. That is, the distance between the top surface and the bottom surface of the first free-cutting portion 11 is larger than the distance between the top surface and the bottom surface of the second free-cutting portion 12. In some embodiments, the end surface and the top surface can both be curved surfaces, and the end surface and the top surface can have a smooth transition. Similarly, the bottom surface can also be a curved surface, and the end surface and the bottom surface can also have a smooth transition. That is to say, along the width direction, the thickness of the first free-cutting portion 11 itself can be variable. For example, along the direction from the first free-cutting portion 11 to the second free-cutting portion 12, the thickness of the first free-cutting portion 11 first gradually increases and then gradually decreases. Of course, it can also decrease in sequence. Similar to the first free-cutting portion 11, the thickness of the second free-cutting portion 12 itself can also be variable. For example, it can decrease in sequence along the direction from the first free-cutting portion 11 to the second free-cutting portion 12, forming a free-cutting steel 10 with a teardrop-shaped cross section (see Figure 2In other embodiments, the end surface, top surface and bottom surface may all be planes, and the end surface and the top surface may be arranged at an angle, for example, the end surface and the top surface of the first free-cutting portion 11 may be arranged at an acute angle, and the end surface and the top surface of the second free-cutting portion 12 may be arranged at an obtuse angle, and the end surface and the bottom surface may also be arranged at an angle, for example, the end surface and the bottom surface of the first free-cutting portion 11 may be arranged at an acute angle, and the end surface and the bottom surface of the second free-cutting portion 12 may be arranged at an obtuse angle, thereby forming a free-cutting steel 10 with an isosceles trapezoidal cross-section (see Figure 3 Of course, the end surface and the top surface can also be perpendicular to each other, and the end surface and the bottom surface can also be perpendicular to each other. Along the width direction, the thickness of the first free-cutting portion 11 itself can be a constant value, and the thickness of the second free-cutting portion 12 itself can be a constant value. In other words, the cross-section of the free-cutting steel 10 appears similar to a stepped structure.

[0031] For ease of explanation, the first free-cutting portion 11 can be understood as the large end, and the second free-cutting portion 12 can be understood as the small end. For a long, free-cutting steel strip 10 having a large end and a small end, during the rolling process, the dimensional deformation rate along the thickness direction on the large end side is smaller than that on the small end side. Therefore, the length of the manganese sulfide in the first free-cutting portion 11 is smaller than that on the second free-cutting portion 12. That is, the manganese sulfide on the large end side is shorter and longer, while the manganese sulfide on the small end side is longer and shorter. Therefore, within a long steel strip where the thickness of the first free-cutting portion 11 is greater than that of the second free-cutting portion 12, the size (length and width) of the manganese sulfide is uneven. This results in different cutting performance between the first and second free-cutting portions 11, 12. Consequently, during cutting, the cutting rate can only be set based on the side with the poorer cutting performance, which reduces cutting efficiency.

[0032] The present application is described below with reference to specific embodiments and with reference to the accompanying drawings:

[0033] See also Figure 4 The hot rolling method provided in the embodiment of the present application includes:

[0034] S1. Rough rolling the steel billet to obtain an intermediate billet;

[0035] S2, performing a first finishing rolling on the intermediate billet to obtain a finished billet 20; the finished billet 20 has a first rolling section 21 and a second rolling section 22 arranged opposite to each other along the width direction of the rolling mill;

[0036] S3. Perform a second finishing rolling on the finished billet 20. During the second finishing rolling process, the deformation rate of the first rolling section 21 along the height direction is less than the deformation rate of the second rolling section 22 along the height direction, thereby obtaining free-cutting steel; wherein: the second finishing rolling temperature is 850°C~1050°C, and the second finishing rolling temperature of the first rolling section 21 is 20°C~50°C lower than the second finishing rolling temperature of the second rolling section 22.

[0037] Rough rolling involves rolling a steel slab, typically a square or rectangular billet, at a temperature of 1050-1100°C to form an intermediate bar. The rough rolling process is essentially the same as the rolling process for round free-cutting steel bars and wire rods, and will not be described in detail in this application. The resulting intermediate bar then undergoes a first finish rolling process, which is also essentially the same as the rolling process for round free-cutting steel bars and wire rods.

[0038] After the first finishing rolling, the second finishing rolling is performed in the special-shaped hole rack. The deformation rates of the first rolling section 21 and the second rolling section 22 are different, so that long strips of steel with special-shaped cross-sections can be formed. During the second finishing rolling process, the temperature is 850°C~1050°C, for example, 855°C, 860°C, 864°C, 869°C, 870°C, 872°C, 876°C, 880°C, 882°C, 885°C, 887°C, 890°C, 893°C, 896°C, 899°C, 1000°C, 1005°C, 1009°C, 1015°C, 1018°C, 1022°C, 1025°C, 1026°C, 1028°C, 1032°C, 1035°C, 1037°C, 1041°C, 1043°C or 1047°C. Within the temperature range of 850°C~1050°C, the plasticity of the manganese sulfide inclusions in the steel matrix is ​​linearly related to the temperature. That is, within the range of 850°C~1050°C, the plasticity of the manganese sulfide gradually decreases with increasing temperature.

[0039] See also Figure 5 During the second finishing process, the finished billet 20 is deformed under the action of the special-shaped rolling hole 30. The deformation rate of the first rolling section 21 along the height direction is smaller than the deformation rate of the second rolling section 22 along the height direction. That is to say, the deformation rate of the first rolling section 21 along the thickness direction is smaller than the deformation rate of the second rolling section 22 along the thickness direction. Therefore, after the second finishing process, the first rolling section 21 forms the first easy-to-cut section 11, i.e., the large head side, and the second rolling section 22 forms the second easy-to-cut section 12, i.e., the small head side. It needs to be explained here that it is not the first rolling section 21 that completely forms the first easy-to-cut section 11 after the second finishing process, nor is it the second rolling section 22 that completely becomes the second easy-to-cut section 12 after the second finishing process. During the second finishing process, the deformation rate of the second rolling section 22 is large, and the base of the second rolling section 22 will flow toward the first rolling section 21. For the sake of convenience of explanation, it is assumed that the first rolling section 21 forms the first easy-to-cut section 11 after the second finishing process. The positions of the two are corresponding, but not completely identical. Similarly, the second rolled portion 22 forms the second easy-cutting portion 12 after the second finish rolling. The positions of the two portions correspond but are not completely identical.

[0040] The second finishing temperature of the first rolling section 21 is 20°C to 50°C lower than the second finishing temperature of the second rolling section 22, for example, 22°C, 24°C, 26°C, 28°C, 30°C, 32°C, 31°C, 36°C, 37°C, 39°C, 40°C, 42°C, 45°C, 46°C, 47°C, 48°C, or 49°C. This means that in each pass of the second finishing rolling, the second finishing temperature of the first rolling section 21 is 20°C to 50°C lower than the second finishing temperature of the second rolling section 22. In other words, the second finishing temperature of the first rolling section 21 corresponding to the large end side is lower than the second finishing temperature of the second rolling section 22 corresponding to the small end side. Thus, the plasticity of the manganese sulfide in the first rolling section 21 is greater than that of the manganese sulfide in the second rolling section 22. During the second finishing rolling process, the deformation of the first rolling section 21 is small, while the deformation of the second rolling section 22 is large. After the second finishing rolling is completed, the deformation of the manganese sulfide in the first rolling section 21 is closer to the deformation of the manganese sulfide in the second rolling section 22.

[0041] Specifically, the temperature on the bullhead side is low, but the manganese sulfide has good plasticity. Thus, even if the deformation rate on the bullhead side is low during the second finishing rolling process, the manganese sulfide can be deformed and elongated as much as possible, reducing its width (the dimension of the manganese sulfide along the width of the steel material). This increases the length of the manganese sulfide and reduces its width. The temperature on the small end side is high, but the manganese sulfide has poor plasticity. Therefore, even if the deformation rate on the small end side is high during the second finishing rolling process, the deformation and elongation of the manganese sulfide can be minimized, suppressing the reduction in the width of the manganese sulfide, thereby reducing the length of the manganese sulfide and maintaining a large width. After the second finishing rolling is completed, the length and width of the manganese sulfide on the bullhead and small end sides are close, reducing the difference between the lengths of the manganese sulfide on the bullhead side and the small end side, and improving the uniformity of the length and width of the manganese sulfide in the first easy-cutting portion 11 and the second easy-cutting portion 12 of the bar or wire profile, thereby ensuring uniform cutting performance of the bar or wire.

[0042] The second finishing temperature of the first rolling section 21 is 20°C~50°C lower than the second finishing temperature of the second rolling section 22. If the second finishing temperature of the first rolling section 21 is higher than the second finishing temperature of the second rolling section 22, the plasticity of the manganese sulfide in the first rolling section 21 is worse than the plasticity of the manganese sulfide in the second rolling section 22. Then the deformation of the first rolling section 21 is small, and the elongated size of the manganese sulfide inside the first rolling section 21 is still small, which will cause the length of the manganese sulfide in the first easy-to-cut section 11 in the long steel strip to be very small; the deformation of the second rolling section 22 is large, and the plasticity of the manganese sulfide inside the second rolling section 22 is good, so the length of the manganese sulfide in the second easy-to-cut section 12 is very long, which will aggravate the unevenness of the manganese sulfide size in the first easy-to-cut section 11 and the second easy-to-cut section 12. If the second finishing temperature of the first rolling section 21 is 0-20°C lower than that of the second rolling section 22, the uniformity of the manganese sulfide size in the first and second free-cutting sections 11 and 12 will be insignificant. If the second finishing temperature of the first rolling section 21 is more than 50°C lower than that of the second rolling section 22, such as 58°C or 62°C, the plasticity of the first rolling section 21 will be much higher than that of the second rolling section 22. In the final steel product, the length of the manganese sulfide in the first free-cutting section 11 will be much longer than that in the second free-cutting section 12, deteriorating the uniformity of the manganese sulfide size across the cross section.

[0043] For steel billets, surface defects such as scale and vibration marks can be removed by grinding. The grinding depth can be 1mm to 5mm. The corners of the steel billets are ground through 3 to 7 passes to form a radius of 15mm or more. This can reduce the temperature drop at the corners during the rough rolling process. The cross-sectional dimensions of the steel billets can be (100-300) mm × (100-300) mm.

[0044] After grinding, the billet is heated in a heating furnace equipped with a preheating section, a first heating section, a second heating section, and a soaking section, arranged in sequence according to the process. The preheating section can be set at 600-700°C, the first heating section at 900-1000°C, the second heating section at 1100-1200°C, and the soaking section at 1150-1200°C. After heating in the heating furnace, the temperature deviation between the head and tail of the billet is ≤20°C, and the total residence time of the billet in the heating furnace can be 1-2 hours.

[0045] The steel billet after being heated in the heating furnace is subjected to high-pressure water dephosphorization, and the descaling water pressure is ≥18MPa.

[0046] After dephosphorization, the billet is locally heated at the fillet corners by an induction coil, raising the temperature there to 20°C to 30°C higher than the rest of the billet. The fillet corners have a large heat dissipation area, resulting in lower temperatures than other parts of the billet, making them susceptible to cracking during rolling. Prior to rough rolling, the fillet corners are locally heated to raise their temperature and compensate for the temperature drop caused by high heat exchange with air.

[0047] The heating temperature T at the fillet conforms to the following formula:

[0048]

[0049] in, h is the convective heat transfer coefficient, V / A is the ratio of the volume to the surface area of ​​the billet, ρ is the density of the billet, c p is the specific heat capacity of the billet, v is the cooling rate of the billet in air, It is the distance from the center of the equivalent circle (inscribed circle) of the cross section of the long steel bar to the center of the circumscribed circle of the cross section.

[0050] For rough rolling, in some embodiments, the rough rolling temperature may be 1050-1100° C., and the pass deformation in the rough rolling stage may be 20%-30%.

[0051] For the first finishing rolling, in some embodiments, the first finishing rolling temperature can be 900℃~1100℃. For finishing rolling, the temperature of the steel is gradually reduced throughout the entire process. It should be noted that the first finishing rolling requires multiple passes, and the temperature of each pass is different. The first finishing rolling temperature here is actually the temperature of the intermediate billet at the first pass in the first finishing rolling. Although the temperature of each pass is different and will gradually decrease as the rolling progresses, the first finishing rolling temperature of each pass falls within the range of 1000℃~1100℃.

[0052] The rolling rates of bars and wires are different. The rolling rate of bars is slower, while that of wires is faster. Therefore, during the first finishing rolling process, the heat dissipated by the bars will far exceed the heat generated by rolling deformation, and the first finishing rolling temperature of bars is lower than that of wires.

[0053] Specifically, for bars, for example, when the first finishing rolling rate does not exceed 8 m / s, such as 1 m / s~5 m / s, the first finishing rolling temperature can be 900℃~1000℃, for example, 920℃, 925℃, 930℃, 938℃, 940℃, 942℃, 945℃, 948℃, 950℃, 958℃, 962℃, 973℃, 978℃, 980℃, 981℃, 984℃, 987℃, 990℃, 994℃ or 996℃, etc.

[0054] For wire, for example, when the first finishing rolling rate exceeds 8 m / s, such as 10 m / s~30 m / s, the first finishing rolling temperature can be 1000℃~1100℃, for example, 1006℃, 1010℃, 1013℃, 1017℃, 1020℃, 1022℃, 1025℃, 1028℃, 1031℃, 1034℃, 1037℃, 1043℃, 1047℃, 1054℃, 1061℃, 1064℃, 1070℃, 1074℃, 1078℃, 1081℃, 1089℃, 1090℃, 1092℃, 1095℃ or 1098℃, etc.

[0055] Controlling the first finishing temperature within this range allows the second finishing temperature to gradually decrease to within the linear region of the manganese sulfide's plasticity and temperature, facilitating control of the manganese sulfide size in conjunction with the deformation rate, and improving the uniformity of the manganese sulfide size across different deformation rate regions within the steel. If the first finishing temperature is controlled too high, the second finishing temperature may, to a certain extent, fall within the nonlinear region of the manganese sulfide's plasticity and temperature, making it difficult to control the uniformity of the manganese sulfide size across different deformation rate regions within the steel. If the first finishing temperature is controlled too low, the deformation resistance of the rolled piece increases, which can easily cause rolling mill overload and damage production line equipment.

[0056] There may be multiple passes in the first finishing rolling process, such as 2 to 4 passes. The hole shape change of the first finishing rolling in this application is the same as the hole shape change of round steel rolling, and the cross section is circular in multiple passes of the first finishing rolling.

[0057] During the second finishing rolling process, the finished slab 20 undergoes varying deformation rates at different locations along the width of the mill. This results in uneven lengths of manganese sulfide inclusions in the first and second rolling sections 21 and 22. The second finishing rolling process is performed within the 850°C to 1050°C temperature range. The plasticity of manganese sulfide inclusions within the steel matrix exhibits a linear relationship with temperature; within this range, the plasticity of the manganese sulfide inclusions decreases as the temperature increases.

[0058] For bars, the finishing rate is slow, and the temperature drop is large during the entire rolling process. Therefore, the second finishing temperature range can be 850℃~950℃, which is a relatively low temperature. For wire rods, the finishing rate is fast, and the temperature drop is small during the entire rolling process. Therefore, the second finishing temperature range can be 950℃~1050℃.

[0059] For bars and wires, during the second finishing rolling process, the first rolling section 21 and the second rolling section 22 are in a state of gradually decreasing temperature as a whole.

[0060] For bar steel, due to the relatively slow finishing speed, the second rolling section 22 (i.e., the small end side) has a large heat dissipation area. This ultimately results in the temperature of the first rolling section 21 being higher than that of the second rolling section 22 in a particular second finishing stand. As previously mentioned, within the second finishing temperature range, the plasticity of manganese sulfide exhibits a linear relationship with temperature: the higher the temperature, the worse the plasticity of the manganese sulfide. Therefore, when the temperature of the first rolling section 21 is higher than that of the second rolling section 22, the plasticity of the manganese sulfide in the first rolling section 21 is lower than that in the second rolling section 22. Consequently, the subsequent deformation rate of the first rolling section 21 is lower than that of the second rolling section 22, further deteriorating the dimensional uniformity of the manganese sulfide across the cross-section of the steel. Therefore, for the bar, during the second finishing rolling process, spray cooling is applied to the first rolling section 21 to reduce the finishing temperature of the first rolling section 21, so that the temperature of the first rolling section 21 is lower than that of the second rolling section 22, that is, the second finishing temperature on the large end side is 20°C to 50°C lower than that on the small end side, so as to improve the uniformity of the manganese sulfide size on the cross section of the steel.

[0061] For wire rods, although the second rolling section 22 (i.e., the small end side) has a larger heat dissipation area and faster heat dissipation than the large end side, the relatively high finishing speed limits heat dissipation time. Combined with the heat generated by rolling, the second finishing temperature of the second rolling section 22 remains higher than that of the first rolling section 21, despite its smaller size. This means that for wire rods, the second finishing temperature of the first rolling section 21 can be maintained at 20°C to 50°C lower than that of the second rolling section 22 without the need for additional cooling measures, thereby improving the uniformity of manganese sulfide size across the cross-section of the steel.

[0062] For the second finishing rolling, there are generally 2 to 4 rolling passes, and the pass shape of each pass is a non-circular special-shaped pass shape, such as a teardrop shape (see Figure 1 ), such as the isosceles trapezoid (see Figure 2 During the second finishing rolling process, the deformation rate of each pass in the first rolling section 21 is the same, and the deformation rate of each pass in the second rolling section 22 is the same. This simplifies the rolling process and facilitates implementation. In other embodiments, the deformation rate of each pass in the first rolling section 21 may be different, and the deformation rate of each pass in the second rolling section 22 may be different, which is not limited in this application.

[0063] Specifically, in some embodiments, the deformation rate of each pass of the first rolling section 21 during the second finishing rolling process can be 5% to 10%. The deformation rate of each pass of the second rolling section 22 during the second finishing rolling process can be 10% to 15%. This deformation rate can achieve the target cross-section after the second finishing rolling, while also ensuring the rolling sequence and eliminating the risk of breakage. Within the deformation range of 5% to 15%, the plasticity of the manganese sulfide inclusions is linearly related to the deformation rate, with the higher the deformation rate, the higher the plasticity of the manganese sulfide. It can be seen from this that in each pass of the second finishing rolling, the deformation rate of the first rolling section 21 is less than that of the second rolling section 22, and the plasticity of the manganese sulfide in the first rolling section 21 is lower than that of the second rolling section 22. After the second finishing rolling is completed, this will cause the length of the manganese sulfide on the small head side to be greater than the length of the manganese sulfide on the big head side. Therefore, this requires more temperature control on the big head side and the small head side, that is, in each pass of the second finishing rolling, the temperature on the big head side is 20℃~50℃ lower than the temperature on the small head side. The plasticity of the manganese sulfide on the small head side is reduced by temperature control, so as to reduce the length of the manganese sulfide on the small head side during the deformation process, so that the lengths of the manganese sulfide on the big head side and the small head side are closer, thereby improving the uniformity of the manganese sulfide size on the cross section of the steel.

[0064] In certain embodiments, during the second finish rolling process, the difference between the total deformation rate in the thickness direction of the first rolling section 21 and the total deformation rate in the thickness direction of the second rolling section 22 is 5% to 25%, such as 6%, 7%, 9%, 10%, 12%, 14%, 15%, 17%, 19%, 20%, 22%, 23%, or 24%. In certain embodiments, when the difference between the total deformation rate of the first rolling section 21 and the total deformation rate of the second rolling section 22 is 5% to 15%, the second finish rolling temperature of the first rolling section 21 is 20°C to 30°C lower than the second finish rolling temperature of the second rolling section 22. When the difference between the total deformation rate of the first rolling section 21 and the total deformation rate of the second rolling section 22 is 16% to 25%, the second finish rolling temperature of the first rolling section 21 is 30°C to 50°C lower than the second finish rolling temperature of the second rolling section 22.

[0065] The steel after the second finishing rolling can be cooled and kept warm. The heat preservation can be carried out in the heat preservation cover. The temperature entering the heat preservation cover is greater than 800℃, and the temperature leaving the heat preservation cover is less than 700℃. After leaving the cover, it is air-cooled to room temperature for packaging.

[0066] Based on the same technical concept as the first aspect, the second embodiment of the present application provides a free-cutting steel 10 obtained by hot rolling using the hot rolling method of any embodiment of the first aspect.

[0067] The free-cutting steel 10 provided in the embodiment of the present application is a long strip of steel. The cross-sectional area of ​​the free-cutting steel 10 can be 100 mm 2 ~5000mm 2The free-cutting steel 10 has a first free-cutting portion 11 and a second free-cutting portion 12 disposed opposite to each other along its width direction. Along its thickness direction, the first free-cutting portion 11 is larger than the second free-cutting portion 12 .

[0068] The hot rolling method for free-cutting steel provided in the present application is further described below with reference to specific embodiments.

[0069] Examples 1 to 5 and Comparative Examples 1 to 3

[0070] Examples 1 to 5 provide a hot rolling method for free-cutting steel, wherein the chemical composition (by mass percentage) of the free-cutting steel is C: 0.05%, Si: 0.05%, Mn: 1.10%, S: 0.31%, P: 0.06%, Pb: 0.29%, with the remainder being Fe and unavoidable impurities. Before rolling, the steel slab is surface treated to remove surface defects such as surface oxide scale and vibration marks, and the grinding depth is 1.5 mm. The corners are ground through five passes to achieve a 20 mm radius. The ground slab is heated, with a preheating stage temperature of 650°C, a first heating stage temperature of 950°C, a second heating stage temperature of 1150°C, a soaking stage temperature of 1180°C, a head-tail temperature deviation of 18°C, and a total furnace dwell time of 1.5 hours. The heated slab is descaled with a descaling water pressure of 20 MPa. After descaling, the slab corner temperature is raised to 1220°C using an induction coil. The steel billets after reheating were subjected to rough rolling, first finishing rolling, and second finishing rolling with a special pass to form a teardrop-shaped cross-section steel. The parameters are shown in Tables 1 and 2. The rolled steel was cooled and placed on a cooling bed in an insulation cover. The temperature entering the insulation cover was 820°C and the temperature exiting the insulation cover was 680°C. After exiting the cover, it was air-cooled to room temperature and then packaged.

[0071] Among them, Example 1, Example 2, Example 5, Comparative Example 1, Comparative Example 2 and Comparative Example 3 are all hot-rolled bars; Example 3 and Example 4 are all hot-rolled wires;

[0072] In Example 1, Example 2, Example 5, Comparative Example 1 and Comparative Example 2, during the second finishing rolling process, the first rolling portion, i.e., the large end side, is spray-cooled;

[0073] In Example 3, Example 4 and Comparative Example 3, during the second finishing rolling process, the big end side was not subjected to spray cooling treatment.

[0074] Table 1

[0075]

[0076] Table 2

[0077]

[0078] Table 3

[0079]

[0080] Table 4

[0081]

[0082] Table 5

[0083]

[0084] Table 6

[0085]

[0086] Table 7

[0087]

[0088] Table 8

[0089]

[0090] Table 9

[0091]

[0092] Table 10

[0093]

[0094] Table 11

[0095]

[0096] Table 12

[0097]

[0098] Samples of the free-cutting steels provided in Examples 1 to 5 and Comparative Examples 1 to 3 were collected. For each sample, one sample was taken from the edge center of the first and second free-cutting parts. The morphological data of manganese sulfide inclusions in the rolled product (along the rolling direction) were analyzed using the scanning electron microscope's Feature function. A statistical area of ​​1 cm² was collected for each sample, and the number density of MnS inclusions was approximately 3,000 / mm². Because the number density of MnS inclusions in free-cutting steel is very high, the statistical area of ​​1 cm² can be considered representative of the control level of MnS morphology for the entire sample. The equivalent circular diameters of the manganese sulfide inclusions in the first and second free-cutting parts of each sample, as well as the percentages of MnS inclusions in different length (width, aspect ratio) intervals of the first and second free-cutting parts, were analyzed. The absolute value of the difference between the manganese sulfide percentage within a certain length or width range of the first free-cutting part and the manganese sulfide percentage within the corresponding range of the second free-cutting part was calculated. The data are shown in Tables 3 to 12.

[0099] In steel, manganese sulfide appears in the form of long strips. In Table 3, the equivalent circle diameter of manganese sulfide inclusions refers to the diameter of a circle with the same area as the total surface area of ​​the manganese sulfide inclusions. Generally speaking, the closer the equivalent circle diameters of different manganese sulfides are, the closer the sizes of different MnS inclusions are to a certain extent.

[0100] It can be seen from the data in Table 3 that in the steels provided by Examples 1 to 5, the difference in the equivalent circle diameter of manganese sulfide between the first free-cutting portion and the second free-cutting portion is 0.117-0.135 μm. The small difference reflects, to a certain extent, that the length and width uniformity of the manganese sulfide in the first free-cutting portion and the second free-cutting portion are better.

[0101] From the data in Tables 10, 11, and 12, it can be seen that the difference between the proportion of manganese sulfide within each length range in the first free-cutting portion and the proportion within each corresponding length range in the second free-cutting portion is 0.02% to 0.82%, indicating high length uniformity. The difference between the proportion of manganese sulfide within each width range in the first free-cutting portion and the proportion within each corresponding width range in the second free-cutting portion is 0.12% to 1.15%, indicating high width uniformity. The difference between the proportion of manganese sulfide within each aspect ratio range in the first free-cutting portion and the proportion within the corresponding aspect ratio range in the second free-cutting portion is 0 to 0.49%. Therefore, it can be seen that the manganese sulfide length and width uniformity in the large and small heads of the free-cutting steels provided in Examples 1 to 5 of the present application are good, and the free-cutting steels have uniform cutting performance, fast cutting rate, and high efficiency.

[0102] Comparative Example 1 provides a hot rolling method for free-cutting steel, wherein the second finishing rolling temperature for the large end is 5°C lower than that for the small end, significantly lower than 20°C. In the steel obtained by the hot rolling method provided in Comparative Example 1, the uniformity of manganese sulfide in all length and width ranges in both the first and second free-cutting portions was inferior to that of Examples 1 to 5.

[0103] Comparative Example 2 provides a hot rolling method for free-cutting steel, wherein the second finishing rolling temperature of the large end is 60°C lower than that of the small end, significantly higher than 50°C. The hot rolling method provided in Comparative Example 1 yields a steel material in which the uniformity of manganese sulfide across all length and width ranges in both the first and second free-cutting portions is inferior to that of Examples 1 to 5.

[0104] Comparative Example 3 provides a hot rolling method for free-cutting steel, wherein the second finishing temperature for the large end portion is 5-10°C higher than that for the small end portion, contrary to the pattern observed in the present application, where the second finishing temperature for the large end portion is lower and the second finishing temperature for the small end portion is higher. In the steel obtained by hot rolling using the provided hot rolling method, the uniformity of manganese sulfide across all length and width ranges in both the first and second free-cutting portions was inferior to that observed in Examples 1 to 5.

[0105] The hot rolling method provided in the present application controls the temperature of the large end of the first rolling section to be 20°C to 50°C lower than the temperature of the small end of the second rolling section during the second finishing rolling process, thereby making the manganese sulfide plasticity of the first rolling section higher than that of the second rolling section. This allows the manganese sulfide sizes of the first free-cutting section and the second free-cutting section to be more uniform after the second finishing rolling, thereby making the cutting performance of the free-cutting steel more uniform and improving the cutting efficiency.

[0106] In this application, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Moreover, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0107] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise" and "counterclockwise" indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0108] In this application, unless otherwise specified or limited, the terms "connect," "fix," etc. should be understood broadly. For example, "fix" can mean fixed connection, detachable connection, or integration; it can mean mechanical connection or electrical connection; it can mean direct connection or indirect connection through an intermediate medium; it can mean internal communication between two elements or interaction between two elements. Those skilled in the art will understand the specific meanings of the above terms in this application based on specific circumstances.

[0109] In addition, the terms "first," "second," and so on, used in this application are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, features specified as "first" or "second" may explicitly or implicitly include one or more features. In the description of this application, "plurality" means two or more, unless otherwise specifically defined.

[0110] Although the embodiments of the present application have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to the embodiments without departing from the principles and intent of the present application, and that the scope of the present application is defined by the claims and their equivalents.

Claims

1. A hot rolling method for free-cutting steel containing manganese sulfide inclusions, wherein the free-cutting steel comprises a first free-cutting portion and a second free-cutting portion disposed opposite each other along the width direction of the free-cutting steel, wherein the first free-cutting portion is larger than the second free-cutting portion along the thickness direction of the free-cutting steel, wherein: The hot rolling method comprises: Rough rolling of the steel billet to obtain an intermediate billet; Performing a first finishing rolling on the intermediate billet to obtain a finished billet; the finished billet comprises a first rolling section and a second rolling section which are arranged opposite to each other along the width direction of the rolling mill; The finished billet is subjected to a second finishing rolling process. During the second finishing rolling process, the deformation rate of the first rolling section along the height direction is less than the deformation rate of the second rolling section along the height direction, so as to obtain the free-cutting steel; wherein: the second finishing rolling temperature is 850°C to 1050°C, and within the second finishing rolling temperature range, as the temperature increases, the plasticity of the manganese sulfide inclusions gradually decreases; the second finishing rolling temperature of the first rolling section is 20°C to 50°C lower than the second finishing rolling temperature of the second rolling section.

2. The hot rolling method according to claim 1, characterized in that When the first finishing rolling rate does not exceed 8 m / s, the first finishing rolling temperature is 900° C. to 1000° C., and the second finishing rolling temperature is 850° C. to 950° C.; When the first finishing rolling rate exceeds 8 m / s, the first finishing rolling temperature is 1000° C. to 1100° C., and the second finishing rolling temperature is 950° C. to 1050° C.

3. The hot rolling method according to claim 2, characterized in that When the first finishing rolling rate does not exceed 8 m / s, during the second finishing rolling process, cooling is sprayed toward the first rolling section to reduce the second finishing rolling temperature of the first rolling section.

4. The hot rolling method according to any one of claims 1 to 3, characterized in that During the second finishing rolling process, the deformation rate of each pass of the first rolling section is the same.

5. The hot rolling method according to claim 4, characterized in that During the second finishing rolling process, the deformation rate of each pass in the first rolling section is 5% to 10%.

6. The hot rolling method according to claim 4, characterized in that During the second finishing rolling process, the deformation rate of each pass in the second rolling section is the same.

7. The hot rolling method according to claim 6, characterized in that During the second finishing rolling process, the deformation rate of each pass in the second rolling section is 10% to 15%.

8. The hot rolling method according to any one of claims 1 to 3, characterized in that During the second finishing rolling process, a difference between a total deformation rate of the first rolling section and a total deformation rate of the second rolling section is 5% to 25%.

9. The hot rolling method according to any one of claims 1 to 3, characterized in that: The corners of the steel billet are rounded with a grinding radius of ≥15 mm. Before the rough rolling of the steel billet, the method further comprises: First, locally heat the fillet of the steel billet so that the temperature of the fillet is 20℃~30℃ higher than other parts.

10. A free-cutting steel, characterized in that: The method is obtained by the hot rolling method according to any one of claims 1 to 9.

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