Production method for improving uniformity of structure and performance of scm435 cold heading steel wire rod

Abstract

The present application relates to high-speed wire production technology field, specifically to a kind of production method for improving the uniformity of SCM435 cold heading steel rod organization performance, the method includes the steps of molten iron pretreatment-converter smelting-refining-continuous casting-billet dressing-high line heating rolling-wire drawing isothermal treatment-off-line detection warehousing in sequence.The present application is stable in temperature by heating and controlled rolling process, ensures that the initial organization state of rod before entering brine is consistent;Using the unique cooling method of completely immersed in constant temperature brine, the fundamental problem of inconsistent cooling rate in the middle and lap position of traditional Stelmor on-line scattered volume distribution rod is solved, the cooling rate is uniform and controllable, and the holding time is controlled by combining with the roller speed adjustment at the bottom of salt tank, so that the organization completely phase changes, avoid residual austenite transformed into martensite organization after off-line, so that the rod obtains uniform, certain deformation capacity bainite organization.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of high-speed wire rod production, in particular to a production method for improving the uniformity of the structure and performance of SCM435 cold heading steel wire rod. BACKGROUND

[0002] The chromium-molybdenum steel wire rod represented by SCM435 is the main raw material for producing 10.9-grade and 12.9-grade standard parts. Since the SCM435 product has high alloy content (Mn: 0.60-0.90 wt%, Cr: 0.90-1.20 wt%, Mo: 0.15-0.30 wt%), the ideal critical diameter DI for quenching is greater than or equal to 70 mm, and martensite transformation is extremely prone to occur. According to the phase transformation law curve, when the cooling speed is greater than 1 ℃ / s, austenite transforms into martensite, the martensite structure has high hardness and poor plasticity, and thus cannot be directly deformed for use. Before cold heading, the martensite structure needs to be spheroidized for 1-2 times.

[0003] At present, the conventional "two-spheroidizing and two-drawing" production process is as follows: hot-rolled wire rod-spheroidizing annealing-phosphating and saponification-coarse drawing-spheroidizing annealing-phosphating and saponification-fine drawing-cold heading forming-quenching and tempering treatment. In the spheroidizing annealing process, the wire rod generally needs to be heated to 700-780 ℃ for a long time to make the carbon elements in the martensite precipitate and the lamellar cementite in the pearlite dissolve and spheroidize, so that a spheroidized structure with uniform and easy deformation is obtained. However, the spheroidizing annealing process is a high-energy and high-time process. With the implementation of the "double carbon" policy of the country and the demand of production plants for energy saving and cost reduction, the market demand for annealing-free coarse drawing or reducing the annealing temperature before coarse drawing and shortening the annealing time is becoming increasingly strong.

[0004] In order to avoid the formation of martensite structure, long-time preservation above the martensite formation temperature Ms is required, which cannot be realized by the current high-speed wire rod air cooling line equipment. In order to meet the user's demand for reducing or eliminating the first annealing, the steel enterprises try to reduce the proportion of martensite structure by adopting hot rolling and slow cooling processes, so as to form a structure mainly composed of pearlite and ferrite, thereby improving the forming performance of the hot-rolled wire rod to meet the downstream user's demand.

[0005] The current research idea is basically similar to the rolling method of SCM435 steel disclosed in Chinese patent CN102716902A. The composition design of SCM435 alloy cold heading steel and the TMCP hot rolling process control means are optimized, the wire rod is slowly cooled at 0.4 ℃ / s through delayed cooling control, the structure of the 8-12 mm diameter SCM435 wire rod is ferrite+pearlite, the tensile strength is 660-735 MPa, the reduction of area is 48-64%, and the hardness HRB at the 1 / 2 radius of the cross section is 82-95. The plasticity of the wire rod is improved, and the first annealing process can be eliminated.

[0006] But in actual production, the wire rod is distributed in scattered and overlapping circles in the air cooling line after wire rod rolling and laying, and the cooling speed is different between the middle sparse position and the overlapping position, so it is impossible to control the cooling speed of the middle position and the edge overlapping position consistently, thus inevitably causing differences in structure and performance. For example, increasing the roller running speed will increase the cooling speed of the wire rod, and when the cooling speed is controlled to be fast (≥1°C / s), martensite structure is easily formed in the middle sparse position; if the roller running speed is reduced, the cooling speed is slow, and the wire rod is kept at high temperature for a long time, and the overlapping position cannot complete the decomposition of all austenite when the air cooling roller is running, and after the line is cut off, the cooling speed is also increased, which is also easy to form martensite structure, thus causing the tensile strength of the same circle of the currently produced SCM435 to fluctuate in the range of 700-1050MPa, and a certain amount of quenched structure is inevitably present, and the structure and performance of the same circle are greatly different.

[0007] At the same time, considering the influence of normal composition fluctuation and segregation phenomenon of different heats on phase transition temperature and time, it is impossible to completely eliminate the martensite in the structure only by precise control of process parameters. When the user uses the first pass annealing, the drawing fracture phenomenon occurs in the local position where the martensite exists, which cannot be avoided, especially in winter, the probability of wire rod product fracture is obviously increased, which produces safety hazards to the on-site operators and equipment. Even if no fracture occurs during drawing, the quenched structure formed in the core has poor cooperative deformation ability, which causes intermittent distribution of holes in the core, which reduces the service performance of the fastener product. In addition, the same circle tensile strength and hardness fluctuate greatly, and after the first pass annealing is exempted, the uneven structure, strength and hardness of the same circle will cause the wave bending phenomenon of the finished wire.

[0008] With the deepening of research, the influence of initial structure on the structure and performance after spheroidizing annealing has attracted the attention of researchers. In 2017, the Eleventh Steel Conference Proceedings published Zheng Xiaowei's "Influence of Different Original Structure on SCM435 Spheroidizing Annealing" pointed out that the bainite structure is easier to spheroidize than the pearlite + ferrite structure; Hebei Metallurgy, 2015 (12) published Tian Xin's "Influence of Original Structure and Annealing Process on SCM435 Wire Rod Annealing Behavior" that the polygonal ferrite, pearlite, bainite and a little martensite structure obtained by low temperature rolling process is not conducive to obtaining uniformly distributed spherical pearlite. This is because the polygonal ferrite and pearlite structure is relatively stable, and it is more difficult to spheroidize than the bainite / martensite metastable structure in the spheroidizing annealing process, and the hardness after spheroidizing annealing is 3-5HRB. Therefore, for users who still use the conventional production process of spheroidizing annealing first, the different original structures of the wire rod will cause the strength and hardness to fluctuate after spheroidizing.

[0009] It is not difficult to find that although certain effects are achieved by technical transformation and process optimization of steel enterprises and steel wire drawing enterprises for reducing and exempting annealing, the fundamental problem of uniformity of rod organization and performance has not been improved under the existing production equipment and process conditions, resulting in instability and uncontrollability in the production process, and therefore how to improve the uniformity of hot-rolled rod is the bottleneck for realizing reduced and exempted annealing drawing. SUMMARY

[0010] The purpose of the present application is to provide a production method for improving the uniformity of SCM435 cold heading steel rod organization and performance to solve the problems raised in the above background.

[0011] To achieve the above purpose, the present application provides the following technical solution: a production method for improving the uniformity of SCM435 cold heading steel rod organization and performance, which comprises the steps of molten iron pretreatment, converter smelting, refining, continuous casting, billet grinding, high-speed wire heating rolling, wire laying isothermal treatment and offline detection and warehousing, wherein the method of high-speed wire heating rolling is as follows: qualified continuous casting billets are put into a heating furnace for heating, the temperature of the atmosphere in the furnace is controlled at 1120-1180℃, the actual temperature of the billets is 1040-1100℃, the residual oxygen content in the furnace is ensured to be ≤1.5% by adjusting the proportion of air and gas to avoid decarburization on the surface of the billets, and after heating, the billets are rolled into 5.5-14mm specification wire through 33 passes of rough rolling, finish rolling, pre-precision rolling, precision rolling and reducing sizing unit, and the temperature of the rolled piece is uniformly and stably controlled in the austenite region of 860-900℃ through adjusting the cooling water flow and pressure of the high-speed wire water tank during the rolling process; the method of wire laying isothermal treatment is as follows: after the rolling deformation is completed, the wire is formed into a scattered coil flat laying state through a wire laying machine, and the whole is immersed in a liquid salt solution below the bainite starting phase change temperature of 500-560℃ through a roller, a motor driven roller is arranged at the bottom of the salt solution, the roller speed can be adjusted to adjust the isothermal time of the rod in the salt solution, and at present, the isothermal time control range can be controlled in the range of 50-120s to ensure that the austenite completely undergoes bainite phase change, and the finished rod is obtained with uniform bainite organization.

[0012] Preferably, the method of molten iron pretreatment is as follows: the molten iron is subjected to desulfurization pretreatment to reduce the S content to below 0.005%.

[0013] Preferably, the method of converter smelting is as follows: the desulfurized molten iron is subjected to converter decarburization operation with scrap steel to ensure that the C and P elements meet the target requirements, and the tapping temperature is controlled at ≥1610℃.

[0014] Preferably, the method of refining is as follows: the molten steel after converter smelting is subjected to LF refining, a calcium aluminate slag system is adopted, the argon blowing intensity is strictly controlled to avoid the contact between the molten steel and air, and then vacuum treatment is performed to reduce the O and N gas content and improve the inclusion control level.

[0015] Preferably, the continuous casting production method is that the refined molten steel is protected casting, the size of the continuous casting billet is 180mm*240mm, the superheat degree and the drawing speed are controlled in the casting process, the element segregation is reduced, the subsequent organization control is provided, the billet is discharged and closely discharged and slowly cooled to prevent the billet from being cracked due to the too fast cooling speed.

[0016] Preferably, the billet grinding method is that the billet is cooled to room temperature, the shot blasting detection or the skinning treatment is performed to ensure that the surface of the billet is free of defects affecting the surface quality of the finished product.

[0017] Preferably, the finished product control target of the refined molten steel is that C is 0.33-0.38%, Si is 0.15-0.35%, Mn is 0.60-0.90%, P and S are ≤0.020%, Cr is 0.90-1.20%, Mo is 0.15-0.30%, and the rest is Fe and inevitable trace residual elements and impurities.

[0018] Beneficial effects: the present application ensures the initial organization state of the wire rod to be consistent before entering the salt solution by heating and controlling the temperature stability in the rolling process; the unique cooling method of completely immersing in the constant-temperature salt solution solves the fundamental problem of the inconsistent cooling speed of the middle part and the lap joint position of the distributed wire rod in the traditional Stelmor line, realizes the uniform and controllable cooling speed, and combines the speed adjustment control of the roller at the bottom of the salt tank to control the heat preservation time, so that the organization completely phase changes, the residual austenite is avoided to transform into the martensite organization after being discharged, and thus the wire rod obtains the uniform and certain deformation capacity of the bainite organization. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical scheme of the present application clear, complete and the advantages more clear and obvious, the following further describes the embodiments of the present application. It should be understood that the specific embodiments described here are part of the embodiments of the present application, not all the embodiments, only to explain the embodiments of the present application, and not to limit the embodiments of the present application, all other embodiments obtained by the person skilled in the art without creative labor belong to the protection scope of the present application.

[0020] Embodiment 1

[0021] The present embodiment provides a production method for improving the uniformity of the organization and performance of SCM435 cold heading steel wire rod, which comprises the steps of hot metal pretreatment, converter smelting, refining, continuous casting, billet grinding, high-speed wire heating and rolling, wire drawing isothermal treatment and offline detection and storage in sequence, and specifically:

[0022] The method of hot metal pretreatment is that the hot metal is subjected to desulfurization pretreatment to reduce the content of S to less than 0.005%.

[0023] The method of converter smelting is that: desulfurized molten iron and scrap steel are subjected to converter carbon reduction operation to ensure that C and P elements meet the target requirements, and the tapping temperature is controlled to be greater than or equal to 1610 DEG C.

[0024] The method of refining is that: the molten steel after converter smelting is subjected to LF refining, a calcium aluminate slag system is adopted, the argon blowing intensity is strictly controlled to avoid the contact between the molten steel and air, and then vacuum treatment is performed to reduce the content of O and N gas and improve the control level of inclusions. The control target of the molten steel after refining is that: C: 0.35%, Si: 0.24%, Mn: 0.70%, P: 0.011%, S: 0.005%, Cr: 0.96%, Mo: 0.16%, and the rest is Fe and inevitable trace residual elements and impurities.

[0025] The production method of continuous casting is that: the molten steel after refining is subjected to protective casting, the size of the continuous casting billet is 180mm*240mm, the superheat degree and the drawing speed are stabilized during the casting process to reduce element segregation and provide a basis for subsequent organization control, the billet is discharged and slowly cooled after being closely discharged to prevent cracks caused by too fast cooling speed of the billet.

[0026] The method of billet grinding is that: after the billet is cooled to room temperature, shot blasting detection or skinning treatment is performed to ensure that the surface of the billet is free of defects affecting the surface quality of the finished product.

[0027] The method of high-speed wire heating rolling is that: the qualified continuous casting billet is put into a heating furnace for heating, the temperature of the atmosphere in the furnace is controlled to be 1156 DEG C, the actual temperature of the billet is 1040 DEG C, the residual oxygen content in the furnace is controlled to be less than or equal to 1.5% by adjusting the proportion of air and fuel gas to avoid decarburization of the surface of the casting billet, after heating, the billet is rolled into 5.5-14mm specification wire through 33 passes of rough rolling, finish rolling, pre-precision rolling, precision rolling and reducing sizing unit, wherein the open rolling temperature is 1075 DEG C, the precision rolling inlet temperature is 879 DEG C, and the reducing sizing inlet temperature is 882 DEG C, and the rolling process is adjusted by adjusting the flow and pressure of the cooling water of the high-speed wire water tank to ensure that the temperature of the rolled piece is uniformly and stably in the austenite region of 860-900 DEG C during the whole rolling process.

[0028] The method of isothermal treatment of wire drawing is that: after the rolling deformation is completed, the wire is formed into a scattered coil flat state by a wire drawing machine, the whole is immersed in a liquid salt solution below the start temperature of the bainite phase change 532 DEG C through a roller, a motor driven roller is arranged at the bottom of the salt solution, the roller speed can be adjusted to adjust the isothermal time of the coil in the salt solution, and at present, the isothermal time can be 105s, the austenite is ensured to completely undergo bainite phase change, the wire drawing temperature is 890 DEG C, and the finished coil is obtained with uniform bainite structure.

[0029] The wire rod is naturally aged for 72 hours at room temperature, and then the structure and performance are detected. The proportion of bainite ferrite content is ≥95%. Seven samples are evenly divided from the same circle for mechanical property detection, and the results are shown in Table 1. The tensile strength is 956-998 MPa, the average is 977 MPa, and the fluctuation is 44 MPa. The reduction of area is 45-53%, the average is 49.6%, and the fluctuation is 8%.

[0030] Table 1 shows the mechanical property detection results of the SCM435 wire rod obtained in Example 1 from the same circle

[0031] Sample No. 1 2 3 4 5 6 7 Tensile strength / MPa 972 956 968 972 998 985 991 Elongation / % 50 53 49 45 49 49 52

[0032] Example 2

[0033] The embodiment provides a production method for improving the uniformity of the structure and performance of SCM435 cold heading steel wire rod, which comprises the steps of molten iron pretreatment, converter smelting, refining, continuous casting, billet grinding, high-speed wire heating rolling, wire drawing isothermal treatment and offline detection and warehousing, and specifically:

[0034] The method of molten iron pretreatment is that the molten iron is subjected to desulfurization pretreatment to reduce the content of S to less than 0.005%.

[0035] The method of converter smelting is that the desulfurized molten iron is subjected to converter carbon reduction operation with scrap steel to ensure that the contents of C and P meet the target requirements, and the tapping temperature is controlled to be greater than or equal to 1610 DEG C.

[0036] The method of refining is that the molten steel after converter smelting is subjected to LF refining, a calcium aluminate slag system is adopted, the argon blowing intensity is strictly controlled to avoid the contact between the molten steel and air, and then vacuum treatment is performed to reduce the contents of O and N and improve the inclusion control level. The target control of the molten steel after refining is that the contents of C, Si, Mn, P, S, Cr and Mo are 0.34%, 0.22%, 0.70%, 0.010%, 0.005%, 0.97% and 0.16% respectively, and the rest is Fe and inevitable trace residual elements and impurities.

[0037] The production method of continuous casting is that the molten steel after refining is subjected to protective casting, the size of the continuous casting billet is 180 mm*240 mm, the superheat and the drawing speed are stabilized and controlled during the casting process to reduce element segregation and provide a basis for subsequent structure control, and the billet is closely discharged and slowly cooled after offline to prevent cracks caused by too fast cooling speed of the billet.

[0038] The method of billet grinding is that the billet is cooled to room temperature, and then shot blasting detection or skinning treatment is performed to ensure that the surface of the billet is free of defects affecting the surface quality of the finished product.

[0039] The method of high-speed wire heating rolling is as follows: qualified continuous casting billets are put into a heating furnace for heating, the temperature of the atmosphere in the furnace is controlled at 1182°C, the actual temperature of the billets is 1040°C, the residual oxygen content in the furnace is controlled at ≤1.5% by adjusting the proportion of air and gas to avoid decarburization on the surface of the billets, after heating, the billets are rolled into 5.5-14mm wire through 33 passes of rough rolling, finish rolling, pre-precision rolling, precision rolling and reducing and sizing unit, wherein, the open rolling temperature is 1094°C, the precision rolling inlet temperature is 887°C, and the reducing and sizing inlet temperature is 886°C, and the rolling process is adjusted by adjusting the flow and pressure of the cooling water of the high-speed wire water tank to ensure that the temperature of the rolled piece is uniformly and stably in the austenite region of 860-900°C during the whole rolling process.

[0040] The method of isothermal treatment of wire drawing is as follows: after the rolling deformation is completed, the wire is formed into a scattered coil and laid flat state by a wire drawing machine, the whole is immersed in a liquid salt solution below the start temperature of the bainite phase change of 542°C through a roller, a motor-driven roller is arranged at the bottom of the salt solution, the roller speed can be adjusted to adjust the isothermal time of the coil in the salt solution, and the isothermal time of 55s can be realized at present, the austenite is ensured to completely undergo bainite phase change, the wire drawing temperature is 895°C, and the finished coil is obtained with uniform bainite structure.

[0041] After the coil is stored at room temperature for 72 hours for natural aging, the bainite ferrite content ratio is ≥95%, the same circle is evenly divided into 7 samples for mechanical property detection, and the results are shown in Table 2, wherein the tensile strength is 976-1038MPa, the average value is 1006MPa, and the fluctuation is 62MPa; the reduction of area is 42-50%, the average value is 46.4%, and the fluctuation is 8%.

[0042] Table 2 shows the mechanical property detection results of the SCM435 coil obtained in Example 2

[0043] Sample No. 1 2 3 4 5 6 7 Tensile strength / MPa 998 1021 1038 1010 976 992 1006 Elongation / % 48 46 42 45 50 47 47

[0044] Although the above describes the specific embodiments of the present application in order to enable those skilled in the art to understand the present application, the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, all applications created by utilizing the concept of the present application are within the protection scope of the present application as long as various changes are within the spirit and scope of the present application defined and determined by the appended claims.

Claims

1. A production method for improving the uniformity of the structure and properties of SCM435 cold heading steel wire rod, said method comprising, in sequence, the steps of hot metal pretreatment - converter smelting - refining - continuous casting - billet dressing - high-speed wire heating rolling - wire laying isothermal treatment - offline detection and warehousing, characterized in that, The method of high linear heating rolling is that qualified continuous casting billets are put into a heating furnace for heating, the temperature of the atmosphere in the furnace is controlled to 1120-1180 DEG C, the actual temperature of the billets is 1040-1100 DEG C, the residual oxygen content in the furnace is ensured to be less than or equal to 1.5% by adjusting the proportion of air and gas, the surface decarburization of the billets is avoided, after heating, the billets are rolled into 5.5-14 mm wire rods through 33 passes of rough rolling, finish rolling, pre-precision rolling, precision rolling and reducing sizing unit, the temperature of the rolled pieces is uniformly and stably controlled in the austenite region of 860-900 DEG C through adjusting the flow and pressure of the cooling water of the high-speed wire rod water tank during the rolling process; the method of isothermal treatment of wire rod laying is that after rolling deformation, the wire rod is laid in a scattered coil flat state through a laying head, the whole is immersed in a liquid salt solution of 500-560 DEG C through a roller, a motor-driven roller is arranged at the bottom of the salt solution, the roller speed can be adjusted, so as to adjust the isothermal time of the coil in the salt solution, the isothermal time is controlled in the range of 50-120 s, the complete bainite phase change of the austenite is ensured, and the finished coil obtains uniform bainite structure.

2. The production method for improving the uniformity of the structure performance of SCM435 cold heading steel wire rod according to claim 1, characterized in that, The method of hot metal pretreatment is that the hot metal is subjected to desulfurization pretreatment to reduce the S content to less than or equal to 0.005%.

3. The production method for improving the uniformity of the structure and properties of SCM435 cold heading steel wire rod according to claim 2, characterized in that, The method of converter smelting is that the desulfurized hot metal is subjected to converter carbon reduction operation with scrap steel to ensure that the C and P elements meet the target requirements, and the tapping temperature is controlled to be greater than or equal to 1610 DEG C.

4. The production method for improving the uniformity of the structure and properties of SCM435 cold heading steel wire rod according to claim 3, characterized in that, The method of refining is that the molten steel after converter smelting is subjected to LF refining, a calcium aluminate slag system is adopted, the argon blowing intensity is strictly controlled to avoid the contact between the molten steel and air, and then vacuum treatment is carried out to reduce the O and N gas content and improve the inclusion control level.

5. The production method for improving the uniformity of the structure and properties of SCM435 cold heading steel wire rod according to claim 4, characterized in that, The production method of continuous casting is that the molten steel after refining is subjected to protective casting, the size of the continuous casting billet is 180 mm*240 mm, the superheat and the drawing speed are stably controlled during the casting process to reduce the element segregation and provide a basis for subsequent structure control, the billet is closely discharged and slowly cooled after being taken offline to prevent cracks caused by too fast cooling speed of the billet.

6. The production method for improving the uniformity of the structure performance of SCM435 cold heading steel wire rod according to claim 1, characterized in that, The method of billet grinding is that after the billet is cooled to room temperature, shot blasting detection or skinning treatment is carried out to ensure that the surface of the billet has no defects affecting the surface quality of the finished product.

7. The production method for improving the uniformity of the structure and properties of SCM435 cold heading steel wire rod according to claim 4, characterized in that: The finished product control target of the molten steel after refining is that C is 0.33-0.38%, Si is 0.15-0.35%, Mn is 0.60-0.90%, P and S are less than or equal to 0.020%, Cr is 0.90-1.20%, Mo is 0.15-0.30%, and the rest is Fe and inevitable trace residual elements and impurities.