Free-cutting steel and hot rolling method

Abstract

The invention discloses free-cutting steel and a hot rolling method. The technical problem that in the prior art, free-cutting steel is low in cutting efficiency is solved. The free-cutting steel is provided with a first free-cutting part and a second free-cutting part which are oppositely arranged in the width direction of the free-cutting steel, and the size of the first free-cutting part is larger than that of the second free-cutting part in the thickness direction of the free-cutting steel. Performing primary finish rolling on the intermediate billet to obtain a finish-rolled billet; the finish rolling blank is provided with a first rolling part and a second rolling part which are oppositely arranged in the width direction of the rolling mill; the finish rolling blank is subjected to second finish rolling, in the second finish rolling process, the deformation rate of the first rolling part in the height direction is smaller than the deformation rate of the second rolling part in the height direction, and the free-cutting steel is obtained; the second finish rolling temperature is 850-1050 DEG C, and the second finish rolling temperature of the first rolling part is 20-50 DEG C lower than the second finish rolling temperature of the second rolling part. According to the hot rolling method, the cutting efficiency of the free-cutting steel is improved.

Description

Technical Field

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

[0002] Free-cutting steel refers to alloy steel in which a certain amount of one or more free-cutting elements such as sulfur, phosphorus, lead, calcium, selenium, tellurium, etc. are added to improve its machinability. The MnS inclusions in free-cutting steel can not only make the chips in the cutting process easy to break, but also play a role in lubricating the cutting tool during the processing, thus effectively reducing tool wear.

[0003] The delivery shapes of free-cutting steel are generally bars, wire rods, etc. The cross-sections of bar and wire rods are mainly circular, and a small part is of special shape. For bar and wire rods with special cross-sections (see Figure 2 and Figure 3 ), they are mainly non-centrosymmetric shapes. After rolling, the cutting performance of the wire rods is uneven, affecting the cutting efficiency. Summary of the Invention

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

[0005] In the first aspect of this application, a hot rolling method is provided, which is applicable to free-cutting steel. The free-cutting steel has a first free-cutting part and a second free-cutting part that are oppositely arranged along the width direction of the free-cutting steel. Along the thickness direction of the free-cutting steel, the size of the first free-cutting part is larger than that of the second free-cutting part. The hot rolling method is characterized by including: Rough rolling the steel billet to obtain an intermediate billet; Performing first finish rolling on the intermediate billet to obtain a finish-rolled billet; the finish-rolled billet has a first rolling part and a second rolling part that are oppositely arranged along the width direction of the rolling mill; Performing second finish rolling on the finish-rolled billet. During the second finish rolling process, the deformation rate of the first rolling part along the height direction is less than the deformation rate of the second rolling part along the height direction to obtain the free-cutting steel; where: the second finish rolling temperature is 850°C to 1050°C, and the second finish rolling temperature of the first rolling part is 20°C to 50°C lower than that of the second rolling part.

[0006] In some embodiments, when the first finish rolling speed does not exceed 8 m / s, the first finish rolling temperature is 900°C to 1000°C, and the second finish rolling temperature is 850°C to 950°C; when the first finish rolling speed exceeds 8 m / s, the first finish rolling temperature is 1000°C to 1100°C, and the second finish rolling temperature is 950°C to 1050°C.

[0007] In some embodiments, when the first finish rolling speed does not exceed 8 m / s, during the second finish rolling process, spray cooling is performed towards the first rolling part to reduce the finish rolling temperature of the first rolling part.

[0008] In some embodiments, during the second finish rolling process, the reduction rate of each pass of the first rolling part is the same.

[0009] In some embodiments, during the second finish rolling process, the reduction rate of each pass of the first rolling part is 5% - 10%.

[0010] In some embodiments, during the second finish rolling process, the reduction rate of each pass of the second rolling part is the same.

[0011] In some embodiments, during the second finish rolling process, the reduction rate of each pass of the second rolling part is 10% - 15%.

[0012] In some embodiments, during the second finish rolling process, the difference between the total reduction rate of the first rolling part and the total reduction rate of the second rolling part is 5% - 25%.

[0013] In some embodiments, the corners of the steel billet are rounded corners with a grinding radius ≥ 15 mm. Before rough rolling the steel billet, it further includes: First, locally heat the rounded corners of the steel billet so that the temperature at the rounded corners is 20°C - 30°C higher than other parts.

[0014] In the second aspect of the present application, an easy - cutting steel is provided, which is obtained by using the hot - rolling method of the first aspect.

[0015] The hot - rolling method according to the embodiments of the present application includes: rough rolling the steel billet to obtain an intermediate billet; performing first finish rolling on the intermediate billet to obtain a finish - rolled billet; the finish - rolled billet has a first rolling part and a second rolling part oppositely arranged along the width direction of the rolling mill; performing second finish rolling on the finish - rolled billet. During the second finish rolling process, the reduction rate of the first rolling part in the height direction is less than the reduction rate of the second rolling part in the height direction to obtain an easy - cutting steel; wherein: the second finish rolling temperature is 850°C - 1050°C, and the second finish rolling temperature of the first rolling part is 20°C - 50°C lower than that of the second rolling part.

[0016] The second finish rolling temperature of the first rolling section is 20°C to 50°C lower than that of the second rolling section. The second finish rolling temperature of the first rolling section corresponding to the large head side is lower than that of the second rolling section corresponding to the small head side. In this way, the plasticity of manganese sulfide in the first rolling section is better than that in the second rolling section. During the second finish rolling process, the deformation amount of the first rolling section is small, while that of the second rolling section is large. After the second finish rolling, the deformation amount of manganese sulfide in the first rolling section is closer to that in the second rolling section. The length dimensions of manganese sulfide in the first rolling section and the second rolling section will be close, and the width dimensions will also be close, reducing the difference between the manganese sulfide sizes on the large head side and the small head side, improving the uniformity of manganese sulfide in the first easy-cutting part and the second easy-cutting part on the special-shaped cross-section of the bar and wire, making the cutting performance of the free-cutting steel more uniform and the cutting efficiency higher. Description of the Drawings

[0017] Figure 1 Shows a schematic diagram of the distribution of manganese sulfide inclusions in free-cutting steel.

[0018] Figure 2 Shows a schematic diagram of the structure of free-cutting steel with a water-drop-shaped cross-section.

[0019] Figure 3 Shows a schematic diagram of the structure of free-cutting steel with an isosceles trapezoidal cross-section.

[0020] Figure 4 Shows a process step diagram of the hot rolling method provided by this application.

[0021] Figure 5 Shows a schematic diagram of the structure of the finish-rolled billet in cooperation with the special-shaped rolling hole.

[0022] Description of the reference numerals: 10 - free-cutting steel, 11 - first easy-cutting part, 12 - second easy-cutting part; 20 - finish-rolled billet, 21 - first rolling section, 22 - second rolling section; 30 - rolling hole. Detailed Description of the Embodiment

[0023] In order to enable those skilled in the art in the technical field to which this application belongs to understand this application more clearly, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of this application.

[0024] In the first aspect embodiment of this application, a hot rolling method is provided. This method is applicable to long strip-shaped free-cutting steel materials, such as wire rods and bars, and can improve the uniformity of the cutting performance of free-cutting steel and the cutting efficiency.

[0025] 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 form of manganese sulfide in free-cutting steel can be referred to Figure 1 , Figure 1 In the figure: 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 of the manganese sulfide in the free-cutting steel is in the shape of long strips, and the extension direction of the manganese sulfide is basically consistent with the hot rolling direction. Generally speaking, technicians in this field refer to the maximum dimension of the manganese sulfide along the hot rolling direction as the length of the manganese sulfide, and the maximum radial dimension as the width. For example, the maximum dimension of the manganese sulfide along the X direction or the Y direction is called the width of the manganese sulfide.

[0026] 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 which 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, and the size of the first free-cutting portion 11 is larger than that of the second free-cutting portion 12 along the thickness direction of the free-cutting steel 10, that is, the thickness dimension of the first free-cutting portion 11 of the free-cutting steel 10 is larger than that 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, and the two sides of the end surface in the height direction are connected to the top surface and the bottom surface respectively, the thickness dimension of the first free-cutting portion 11 is larger than that 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 be smoothly transitioned. Similarly, the bottom surface can also be a curved surface, and the end surface and the bottom surface can also be smoothly transitioned. That is to say, along the width direction, the thickness dimension of the first easy-cutting portion 11 itself can be variable. For example, along the direction from the first easy-cutting portion 11 to the second easy-cutting portion 12, the thickness dimension of the first easy-cutting portion 11 first gradually increases and then gradually decreases. Of course, it can also decrease in sequence. Similar to the first easy-cutting portion 11, the thickness dimension of the second easy-cutting portion 12 itself can also be variable. For example, it decreases in sequence along the direction from the first easy-cutting portion 11 to the second easy-cutting portion 12, forming an easy-cutting steel 10 with a teardrop-shaped cross section (see Figure 2). In some other embodiments, the end face, the top face, and the bottom face may also all be flat surfaces. The end face and the top face are arranged at an angle. For example, the end face and the top face of the first machinable part 11 are arranged at an acute angle, and the end face and the top face of the second machinable part 12 are arranged at an obtuse angle. The end face and the bottom face are also arranged at an angle. For example, the end face and the bottom face of the first machinable part 11 are arranged at an acute angle, and the end face and the bottom face of the second machinable part 12 are arranged at an obtuse angle, forming a machinable steel 10 with an isosceles trapezoid cross-section (see Figure 3 ). Of course, the end face and the top face may also be perpendicular to each other, and the end face and the bottom face are perpendicular to each other. Along the width direction, the thickness dimension of the first machinable part 11 itself may be a fixed value, and the thickness dimension of the second machinable part 12 itself may be a fixed value. That is to say, the cross-section of the machinable steel 10 steel appears similar to a stepped structure.

[0027] For the sake of easy explanation, the first machinable part 11 can be understood as the big head or the large end, and the second machinable part 12 can be understood as the small head or the small end. For the long-strip machinable steel 10 with a big head and a small head, during the rolling process, the dimensional deformation rate of the big-head side along the thickness direction is less than that of the small-head side along the thickness direction. Therefore, the length dimension of manganese sulfide in the first machinable part 11 will be less than that of manganese sulfide in the second machinable part 12, that is, the length of manganese sulfide on the big-head side is short and the width is long, while the length of manganese sulfide on the small-head side is long and the width is short. So, in the long-strip steel with the thickness dimension of the first machinable part 11 being greater than that of the second machinable part 12, the dimensions (length and width) of manganese sulfide are uneven, which will make the cutting performances of the first machinable part 11 and the second machinable part 12 different. Therefore, the cutting rate can only be set according to the side with poor cutting performance during cutting, which will reduce the cutting efficiency.

[0028] The present application will be described below with reference to the accompanying drawings and specific embodiments: Please refer to Figure 4 , the hot rolling method provided by the embodiment of the present application includes: S1. Rough roll the steel billet to obtain an intermediate billet; S2. Perform the first finish rolling on the intermediate billet to obtain a finish-rolled billet 20; the finish-rolled billet 20 has a first rolling part 21 and a second rolling part 22 that are oppositely arranged along the width direction of the rolling mill; S3. Perform the second finish rolling on the finish-rolled billet 20. During the second finish rolling, the deformation rate of the first rolling part 21 along the height direction is less than that of the second rolling part 22 along the height direction to obtain a machinable steel; wherein: the temperature of the second finish rolling is 850°C to 1050°C, and the temperature of the second finish rolling of the first rolling part 21 is 20°C to 50°C lower than that of the second rolling part 22.

[0029] Rough rolling is to roll a steel billet, generally a square billet or a rectangular billet, at a temperature of 1050-1100°C to form an intermediate billet. The rough rolling steps are basically the same as the rolling process of free-cutting steel 10 bar wire with a circular cross-section, and will not be elaborated in this application. The formed intermediate billet is then subjected to first finish rolling, and this step is also basically the same as the rolling process of free-cutting steel 10 bar wire with a circular cross-section.

[0030] After the first finish rolling, it is followed by second finish rolling. The second finish rolling is carried out in a special-shaped hole rolling mill stand. The deformation rates of the first rolling part 21 and the second rolling part 22 are different, so as to form a long strip steel with a special-shaped cross-section. During the second finish rolling process, the temperature is 850°C - 1050°C, such as 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, etc. In the temperature range of 850°C - 1050°C, the plasticity of manganese sulfide inclusions in the steel matrix has a linear relationship with the temperature, that is, in the range of 850°C - 1050°C, as the temperature increases, the plasticity of manganese sulfide gradually decreases.

[0031] Please refer to Figure 5 , during the second finish rolling process, the finish rolling billet 20 deforms under the action of the special-shaped rolling hole 30. The deformation rate of the first rolling part 21 in the height direction is less than that of the second rolling part 22 in the height direction, that is to say, the deformation rate of the first rolling part 21 in the thickness direction is less than that of the second rolling part 22 in the thickness direction. Therefore, after the second finish rolling, the first rolling part 21 correspondingly forms the first free-cutting part 11, that is, the large-head side, and the second rolling part 22 correspondingly forms the second free-cutting part 12, that is, the small-head side. Here, it needs to be explained that it is not that the first rolling part 21 is completely formed into the first free-cutting part 11 after the second finish rolling, nor is it that the second rolling part 22 is completely changed into the second free-cutting part 12 after the second finish rolling. During the second finish rolling process, the deformation rate of the second rolling part 22 is large, and the matrix of the second rolling part 22 will flow towards the first rolling part 21. Here, for the convenience of explanation, it is considered that the first rolling part 21 forms the first free-cutting part 11 after the second finish rolling, and their positions are corresponding, but not exactly the same. Similarly, the second rolling part 22 forms the second free-cutting part 12 after the second finish rolling, and their positions are corresponding, nor are they exactly the same.

[0032] The second finish rolling temperature of the first rolling section 21 is 20°C to 50°C lower than that of the second rolling section 22, such as 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. Here, it means that in each pass of the second finish rolling, the second finish rolling temperature of the first rolling section 21 is 20°C to 50°C lower than that of the second rolling section 22. That is to say, the second finish rolling temperature of the first rolling section 21 corresponding to the large end side is lower than that of the second rolling section 22 corresponding to the small end side. In this way, the plasticity of manganese sulfide in the first rolling section 21 is better than that of manganese sulfide in the second rolling section 22. During the second finish rolling process, the deformation amount of the first rolling section 21 is small, while the deformation amount of the second rolling section 22 is large. After the second finish rolling, the deformation amount of manganese sulfide in the first rolling section 21 is closer to that of manganese sulfide in the second rolling section 22.

[0033] That is to say, the temperature of the large end side is low, but the plasticity of manganese sulfide is good. In this way, even if the deformation rate of the large end side is small during the second finish rolling process, manganese sulfide can be deformed and elongated as much as possible, and the width dimension of manganese sulfide (the dimension of manganese sulfide along the width direction of the steel) becomes smaller, thereby increasing the length of manganese sulfide and decreasing the width of manganese sulfide; the temperature of the small end side is high, but the plasticity of manganese sulfide is poor. In this way, even if the deformation rate of the small end side is large during the second finish rolling process, the deformation and elongation of manganese sulfide can be reduced as much as possible, and the decrease in the width dimension of manganese sulfide can be inhibited, thereby reducing the length of manganese sulfide and maintaining a larger width dimension. After the second finish rolling, the length dimensions of manganese sulfide on the large end side and the small end side will be close, and the width dimensions will also be close, reducing the difference between the length of manganese sulfide on the large end side and the length of manganese sulfide on the small end side, and improving the uniformity of the length and width dimensions of manganese sulfide in the first easy-cutting part 11 and the second easy-cutting part 12 on the special-shaped cross-section of the bar and wire, thereby ensuring the uniformity of the cutting performance of the bar and wire.

[0034] The second finish rolling temperature of the first rolling section 21 is 20°C to 50°C lower than that of the second rolling section 22. If the second finish rolling temperature of the first rolling section 21 is higher than that of the second rolling section 22, the plasticity of manganese sulfide in the first rolling section 21 is poorer than that of manganese sulfide in the second rolling section 22. Then, the deformation amount of the first rolling section 21 is small, and the elongation size of manganese sulfide inside the first rolling section 21 is also small, which will result in a very small length of manganese sulfide in the first easy-to-cut part 11 of the long strip steel; the deformation amount of the second rolling section 22 is large, and the plasticity of manganese sulfide inside the second rolling section 22 is good. Therefore, the length of manganese sulfide in the second easy-to-cut part 12 is very long, which will exacerbate the non-uniformity of the manganese sulfide sizes between the first easy-to-cut part 11 and the second easy-to-cut part 12. If the second finish rolling temperature of the first rolling section 21 is 0°C to 20°C lower than that of the second rolling section 22, the effect of the uniformity of the manganese sulfide sizes between the first easy-to-cut part 11 and the second easy-to-cut part 12 is not obvious. If the second finish rolling 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, 62°C, etc., the plasticity of the first rolling section 21 will be much higher than that of the second rolling section 22. In the final steel, the length of manganese sulfide in the first easy-to-cut part 11 is much longer than that of manganese sulfide in the second easy-to-cut part 12, deteriorating the uniformity of the manganese sulfide sizes on the cross-section.

[0035] For the steel billet, the surface defects such as scale and vibration marks on the surface of the steel billet can be eliminated by a grinding machine first. The grinding depth of the steel billet can be 1 mm to 5 mm; the corners of the steel billet are ground through 3 to 7 passes to form a fillet with a radius ≥ 15 mm, which can reduce the temperature drop at the corners during the rough rolling process. The cross-sectional size of the steel billet can be (100 - 300) mm × (100 - 300) mm.

[0036] The ground steel billet can enter the heating furnace for heating. The heating furnace is provided with a preheating section, a first heating section, a second heating section, and a soaking section arranged in sequence according to the process. Among them, the temperature of the preheating section can be 600 - 700°C, the temperature of the first heating section can be 900 - 1000°C, the temperature of the second heating section can be 1100 - 1200°C, and the temperature of the soaking section can be 1150 - 1200°C. After the steel billet is heated by the heating furnace, the temperature deviation between the head and the tail is ≤ 20°C, and the total residence time of the steel billet in the heating furnace can be 1 h to 2 h.

[0037] The steel billet heated by the heating furnace is subjected to high-pressure water descaling, and the descaling water pressure ≥ 18 MPa.

[0038] The billet after phosphorus removal is locally heated at the rounded corners of the billet through an induction coil, so that the temperature at the rounded corners is 20°C to 30°C higher than that of other parts. The heat dissipation area at the rounded corners of the billet is large, and the temperature at the rounded corners is lower than that of other parts of the billet, which is prone to cracking during the rolling process. Before rough rolling, the rounded corners are locally heated first to raise the temperature at the rounded corners and make up for the temperature drop caused by the high heat transfer when the rounded corners are in contact with the air.

[0039] The heating temperature T at the rounded corners conforms to the following formula: Wherein, 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 the air, is the distance between the center of the equivalent circle (inscribed circle) of the cross-section of the long bar-shaped steel and the center of the circumscribed circle of the cross-section.

[0040] For rough rolling, in some embodiments, the rough rolling temperature can be 1050°C to 1100°C, and the pass reduction in the rough rolling stage can be 20% to 30%.

[0041] For the first finish rolling, in some embodiments, the first finish rolling temperature can be 900°C to 1100°C. For finish rolling, during the whole process, the temperature of the steel gradually decreases. It should be noted that the first finish rolling requires multiple passes, and the temperatures of each pass are not the same. The first finish rolling temperature here is actually the temperature of the intermediate billet at the first pass in the first finish rolling. Although the temperatures of each pass are not the same and gradually decrease as the rolling progresses, the first finish rolling temperatures of each pass all fall within the range of 1000°C to 1100°C.

[0042] For bars and wires, their rolling rates are not the same. The rolling rate of bars is slower, and the rolling rate of wires is faster. Therefore, during the first finish rolling process, the heat dissipation of bars will far exceed the heat generated by rolling deformation, and the first finish rolling temperature of bars is lower than that of wires.

[0043] Specifically, for bars, for example, when the first finish rolling rate does not exceed 8 m / s, such as 1 m / s to 5 m / s, the first finish rolling temperature can be 900°C to 1000°C, such as 920°C, 925°C, 930°C, 938°C, 940°C, 942°C, 945°C, 948°C, 950°C, 958°C, 962°C, 973°C, 978°C, 980°C, 981°C, 984°C, 987°C, 990°C, 994°C or 996°C, etc.

[0044] For wire rods, for example, when the first finishing rolling speed exceeds 8 m / s, such as 10 m / s to 30 m / s, the first finishing rolling temperature can be 1000 °C to 1100 °C, such as 1006 °C, 1010 °C, 1013 °C, 1017 °C, 1020 °C, 1022 °C, 1025 °C, 1028 °C, 1031 °C, 1034 °C, 1037 °C, 1043 °C, 1047 °C, 1054 °C, 1061 °C, 1064 °C, 1070 °C, 1074 °C, 1078 °C, 1081 °C, 1089 °C, 1090 °C, 1092 °C, 1095 °C or 1098 °C, etc.

[0045] Controlling the first finishing rolling temperature within this range can make the second finishing rolling temperature gradually decrease to within the linear region of the plastic property of manganese sulfide with temperature, facilitating the control of the size of manganese sulfide in combination with the deformation rate and improving the uniformity of the size of manganese sulfide in different deformation rate regions of the steel. If the first finishing rolling temperature is controlled too high, to a certain extent, the second finishing rolling temperature may fall into the non-linear region of the plastic property of manganese sulfide with temperature, making it difficult to control the uniformity of the size of manganese sulfide in different deformation rate regions of the steel. If the first finishing rolling temperature is controlled too low, the deformation resistance of the rolled piece increases, easily causing the rolling mill to be overloaded and damaging the production line equipment.

[0046] There may be multiple passes in the first finishing rolling process, such as 2 to 4 passes. The pass shape change in the first finishing rolling in this application is the same as that of round steel rolling, and the cross-section is circular in multiple passes of the first finishing rolling.

[0047] The second finishing rolling is carried out in a shaped hole rack. At this time, the deformation rates of the finishing billet 20 at different positions along the width direction of the rack are not the same, which means that there will be a problem of uneven length dimensions of manganese sulfide in the first rolling part 21 and the second rolling part 22 during the second finishing rolling. Controlling the second finishing rolling temperature within the temperature range of 850 °C to 1050 °C, the plasticity of manganese sulfide inclusions in the steel matrix has a linear relationship with temperature, that is, within the range of 850 °C to 1050 °C, as the temperature increases, the plasticity of manganese sulfide inclusions gradually decreases.

[0048] For bars, the finishing rolling speed is slow, and the temperature drop is large during the entire rolling process. Therefore, the second finishing rolling temperature range can be 850 °C to 950 °C, with a relatively low temperature. For wire rods, the finishing rolling speed is fast, and the temperature drop is small during the entire rolling process. Therefore, the second finishing rolling temperature range can be 950 °C to 1050 °C.

[0049] For bars and wire rods, during the second finishing rolling process, both the first rolling part 21 and the second rolling part 22 are in a state where the temperature is getting lower and lower as a whole.

[0050] For bars, since the finishing rolling speed is relatively slow and the heat dissipation area of the second rolling part 22, i.e., the small-end side, is large, it finally shows that in a certain second finishing rolling stand, the temperature of the first rolling part 21 is higher than that of the second rolling part 22. As can be seen from the foregoing content, within the second finishing rolling temperature range, there is a linear relationship between the plasticity of manganese sulfide and temperature. The higher the temperature, the poorer the plasticity of manganese sulfide. Therefore, when the temperature of the first rolling part 21 is higher than that of the second rolling part 22, the plasticity of manganese sulfide in the first rolling part 21 is lower than that in the second rolling part 22, and the subsequent deformation rate of the first rolling part 21 is less than that of the second rolling part 22, which will further deteriorate the size uniformity of manganese sulfide on the cross-section of the steel. Therefore, for bars, during the second finishing rolling process, spray cooling is applied to the first rolling part 21 to reduce the finishing rolling temperature of the first rolling part 21, so that the temperature of the first rolling part 21 is lower than that of the second rolling part 22, that is, the second finishing rolling 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 size uniformity of manganese sulfide on the cross-section of the steel.

[0051] For wire rods, although the heat dissipation area of the second rolling part 22, i.e., the small-end side, is larger and the heat dissipation is faster than that of the large-end side, due to the relatively fast finishing rolling speed and limited heat dissipation time, combined with the rolling heat generation, the second finishing rolling temperature of the second rolling part 22 is still higher than that of the first rolling part 21 despite its relatively small size. That is to say, for wire rods, it is not necessary to adopt additional cooling means to ensure that the second finishing rolling temperature of the first rolling part 21 is 20°C to 50°C lower than that of the second rolling part 22, thereby improving the size uniformity of manganese sulfide on the cross-section of the steel.

[0052] For the second finishing rolling, generally there are 2 to 4 rolling passes, and the pass profile for each pass is a non-circular special-shaped pass, such as a water droplet shape (see Figure 1 ), such as an isosceles trapezoid (see Figure 2 ). During the second finishing rolling process, the deformation rate of each pass of the first rolling part 21 is the same, and the deformation rate of each pass of the second rolling part 22 is the same, so that the rolling process is simpler and easier to implement. In some other embodiments, the deformation rate of each pass of the first rolling part 21 may also be different, and the deformation rate of each pass of the second rolling part 22 may also be different, which is not limited in this application.

[0053] Specifically, in some embodiments, the deformation rate of each pass of the first rolling section 21 during the second finish rolling process can be 5% - 10%. The deformation rate of each pass of the second rolling section 22 during the second finish rolling process can be 10% - 15%. This deformation rate can reach the target cross-section after the second finish rolling, and can also ensure the rolling sequence without the risk of rolling breakage. Within the range of 5% - 15% of this deformation amount, the plasticity of manganese sulfide inclusions has a linear relationship with the deformation rate. The higher the deformation rate, the higher the plasticity of manganese sulfide. It can be seen that in each pass of the second finish rolling, the deformation rate of the first rolling section 21 is less than that of the second rolling section 22, and the plasticity of manganese sulfide in the first rolling section 21 is lower than that of the second rolling section 22. After the second finish rolling, this will result in the length of manganese sulfide on the small end side being greater than that on the large end side. Therefore, temperature control on the large end side and the small end side is more required, that is, in each pass of the second finish rolling, the temperature on the large end side is 20°C - 50°C lower than that on the small end side. By controlling the temperature, the plasticity of manganese sulfide on the small end side is reduced, so as to reduce the length of manganese sulfide on the small end side during the deformation process, make the lengths of manganese sulfide on the large end side and the small end side closer, and improve the dimensional uniformity of manganese sulfide on the cross-section of the steel.

[0054] In some 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% - 25%, such as 6%, 7%, 9%, 10%, 12%, 14%, 15%, 17%, 19%, 20%, 22%, 23% or 24%, etc. In some 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% - 15%, the second finish rolling temperature of the first rolling section 21 is 20°C - 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% - 25%, the second finish rolling temperature of the first rolling section 21 is 30°C - 50°C lower than the second finish rolling temperature of the second rolling section 22.

[0055] The steel after the second finish rolling can be cooled and heat-insulated. The heat insulation can be carried out in a heat-insulation cover. The temperature when entering the heat-insulation cover > 800°C, the temperature when leaving the heat-insulation cover < 700°C, and after leaving the cover, it is air-cooled to room temperature for packing.

[0056] Based on the same technical concept as the first aspect, the second aspect embodiment of the present application provides an easy-cutting steel 10 obtained by hot rolling using any implementation method of the first aspect.

[0057] The easy-cutting steel 10 provided by the embodiment of the present application is a long-strip steel, and the cross-sectional area of the easy-cutting steel 10 can be 100mm 2 ~5000mm 2The free-cutting steel 10 has a first free-cutting portion 11 and a second free-cutting portion 12 that are oppositely arranged along its own width direction. Along its own thickness direction, the size of the first free-cutting portion 11 is larger than that of the second free-cutting portion 12.

[0058] The hot rolling method of the free-cutting steel provided by the present application will be further described below in conjunction with specific embodiments.

[0059] Examples 1 to 5 and Comparative Examples 1 to 3 Examples 1 to 5 provide a hot rolling method for free-cutting steel. The chemical composition (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%, and the balance is Fe and unavoidable impurities. Before rolling, the surface of the steel billet is treated to eliminate surface defects such as surface scale and vibration marks, and the grinding depth is 1.5 mm; the corners are ground through 5 passes to form a fillet with an R of 20 mm. The ground billet is heated, where the preheating section temperature is 650 °C, the first heating section temperature is 950 °C, the second heating section temperature is 1150 °C, the soaking section temperature is 1180 °C, the temperature deviation between the head and tail of the steel billet is 18 °C, and the total furnace residence time is 1.5 h. The heated billet is descaled, and the descaling water pressure is 20 MPa. After descaling, the corner temperature of the billet is raised to 1220 °C through an induction coil. The reheated steel billet is successively subjected to rough rolling, first finish rolling, and second finish rolling in a special-shaped pass to form a steel bar with a teardrop-shaped cross-section, and the parameters are shown in Table 1 and Table 2. The rolled steel is cooled, sent to a cooling bed and then into a heat preservation cover after rolling, the temperature entering the heat preservation cover is 820 °C, the temperature leaving the heat preservation cover is 680 °C, and it is air-cooled to room temperature after leaving the cover and then packed.

[0060] Among them, Examples 1, 2, 5, Comparative Example 1, Comparative Example 2, and Comparative Example 3 are all hot rolling of bars; Examples 3 and 4 are all hot rolling of wire rods; In Examples 1, 2, 5, Comparative Example 1, and Comparative Example 2, during the second finish rolling process, the first rolling portion, that is, the large-head side, is spray-cooled; In Examples 3, 4, and Comparative Example 3, during the second finish rolling process, no spray-cooling treatment is performed on the large-head side during the second finish rolling process.

[0061] Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Samples were taken from the free-cutting steels provided in Examples 1 to 5 and Comparative Examples 1 to 3. For each sample, one sample was taken at the edge and core positions of the first free-cutting part and the second free-cutting part respectively. The Feature function of a scanning electron microscope was used to statistically analyze the morphology data of manganese sulfide inclusions in the rolled products (along the rolling direction). The statistical area for each sample was 1 cm², and the number density of MnS inclusions was approximately 3000 per mm². Since the number density of MnS inclusions in the free-cutting steel is very high, it can be considered that the statistically analyzed 1 cm² area represents the control level of the MnS morphology in the entire sample. The equivalent circle diameter of manganese sulfide in the first free-cutting part and the second free-cutting part of each sample was statistically analyzed, as well as the percentage of the number of MnS corresponding to different length (width, aspect ratio) intervals in the first free-cutting part and the second free-cutting part. The absolute value of the difference between the proportion of manganese sulfide in a certain length or width range in the first free-cutting part and the proportion of manganese sulfide in the corresponding range in the second free-cutting part was calculated. The data are shown in Tables 3 to 12.

[0062] In steel, manganese sulfide will appear in a long strip shape. In Table 3, the equivalent circle diameter of manganese sulfide inclusions refers to the diameter of a circle obtained by converting the total surface area of the manganese sulfide inclusions into a circle with the same area. Generally speaking, the closer the equivalent circle diameters of different manganese sulfides are, to a certain extent, it can reflect that the sizes of different MnS inclusions are closer.

[0063] From the data in Table 3, it can be seen that in the steels provided in Examples 1 to 5, the difference in the equivalent circle diameter of manganese sulfide between the first free-cutting part and the second free-cutting part is 0.117 - 0.135 μm. The small difference indicates that, to a certain extent, the uniformity of the length and width dimensions of manganese sulfide in the first free-cutting part and the second free-cutting part is better.

[0064] As can be seen from the data in Table 10, Table 11, and Table 12, the difference in the proportion of manganese sulfide within each length range in the first free-cutting part and the corresponding proportion within each length range in the second free-cutting part is 0.02% - 0.82%, indicating high length uniformity. The difference in the proportion of manganese sulfide within each width range in the first free-cutting part and the corresponding proportion within each width range in the second free-cutting part is 0.12% - 1.15%, indicating high width uniformity. The difference in the proportion of manganese sulfide within each length-width ratio range in the first free-cutting part and the corresponding proportion within each length-width ratio range in the second free-cutting part is 0 - 0.49%. From this, it can be known that the manganese sulfide length and width within the large end and small end of the free-cutting steel provided in Embodiments 1 to 5 of the present application have good uniformity, the cutting performance of the free-cutting steel is uniform, the cutting rate is fast, and the efficiency is high.

[0065] Comparative Example 1 provides a hot rolling method for free-cutting steel, in which the second finishing temperature of the large end is 5°C lower than that of the small end, far lower than 20°C. In the steel obtained after hot rolling by the hot rolling method provided in Comparative Example 1, the uniformity of manganese sulfide within each length range and each width range in both the first free-cutting part and the second free-cutting part is inferior to that in Embodiments 1 to 5.

[0066] Comparative Example 2 provides a hot rolling method for free-cutting steel, in which the second finishing temperature of the large end is 60°C lower than that of the small end, far higher than 50°C. In the steel obtained after hot rolling by the hot rolling method provided in Comparative Example 1, the uniformity of manganese sulfide within each length range and each width range in both the first free-cutting part and the second free-cutting part is inferior to that in Embodiments 1 to 5.

[0067] Comparative Example 3 provides a hot rolling method for free-cutting steel, in which the second finishing temperature of the large end is 5 - 10°C higher than that of the small end, contrary to the rule of the second finishing temperature of the large end being lower and the second finishing temperature of the small end being higher in the present application. In the steel obtained after hot rolling by the provided hot rolling method, the uniformity of manganese sulfide within each length range and each width range in both the first free-cutting part and the second free-cutting part is inferior to that in Embodiments 1 to 5.

[0068] The hot rolling method provided by the present application controls the temperature of the first rolling part (the large end) to be 20°C - 50°C lower than the temperature of the second rolling part (the small end) during the second finishing process, so that the plasticity of manganese sulfide in the first rolling part is higher than that in the second rolling part, making the manganese sulfide sizes in the first free-cutting part and the second free-cutting part more uniform after the second finishing, thereby making the cutting performance of the free-cutting steel more uniform and improving the cutting efficiency.

[0069] In this application, unless otherwise clearly defined or limited, the first feature being "on" or "under" the second feature may include direct contact between the first and second features, or may include the first and second features not being in direct contact but in contact through additional features therebetween. Moreover, the first feature being "above", "over" and "on top of" the second feature includes the first feature being directly above and diagonally above the second feature, or merely indicating that the horizontal height of the first feature is higher than that of the second feature. The first feature being "under", "beneath" and "underneath" the second feature includes the first feature being directly below and diagonally below the second feature, or merely indicating that the horizontal height of the first feature is less than that of the second feature.

[0070] In the description of this application, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise" is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing this application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to this application.

[0071] In this application, unless otherwise clearly defined or limited, the terms "connected", "fixed", etc. shall be understood in a broad sense. For example, "fixed" may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and may be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0072] In addition, in this application, the descriptions such as "first", "second", etc. are only for descriptive purposes and should not be understood as indicating or implying their relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second" may explicitly or implicitly include one or more features. In the description of this application, the meaning of "a plurality" is two or more, unless otherwise specifically and clearly defined.

[0073] Although the embodiments of this application have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and purposes of this application, and the scope of this application is defined by the claims and their equivalents.

Claims

1. A hot rolling method applicable to free-cutting steel, wherein the free-cutting steel has a first free-cutting portion and a second free-cutting portion oppositely arranged along the width direction of the free-cutting steel, and along the thickness direction of the free-cutting steel, the size of the first free-cutting portion is larger than that of the second free-cutting portion, and it is characterized in that, The hot rolling method described above includes: Rough rolling the steel billet to obtain an intermediate billet; Performing first finish rolling on the intermediate billet to obtain a finish-rolled billet; the finish-rolled billet has a first rolling part and a second rolling part oppositely arranged along the width direction of the rolling mill; Performing second finish rolling on the finish-rolled billet. During the second finish rolling process, the deformation rate of the first rolling part along the height direction is less than that of the second rolling part along the height direction to obtain the free-cutting steel; wherein: the second finish rolling temperature is 850°C to 1050°C, and the second finish rolling temperature of the first rolling part is 20°C to 50°C lower than that of the second rolling part.

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

3. The hot rolling method according to claim 2, wherein When the first finish rolling speed does not exceed 8 m / s, during the second finish rolling process, spray cooling is performed towards the first rolling part to reduce the second finish rolling temperature of the first rolling part.

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

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

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

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

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

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

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

Images