8.8-grade large-specification cold heading steel wire rod with excellent hardenability and production method thereof

Abstract

The invention discloses a 8.8-grade large-specification cold heading steel wire rod with excellent hardenability and a production method thereof, and belongs to the technical field of steel products. The steel wire rod is prepared from the following chemical components in percentage by mass: 0.35 percent to 0.39 percent of C, less than or equal to 0.10 percent of Si, 0.60 percent to 0.85 percent of Mn, 0.20 percent to 0.40 percent of Cr, 0.15 percent to 0.25 percent of Mo, 0.05 percent to 0.10 percent of Ni, 0.020 percent to 0.040 percent of Al, 0.0040 percent to 0.0080 percent of Ca, 0.0010 percent to 0.0030 percent of Ce, less than or equal to 0.0050 percent of N and the balance of Fe and inevitable impurities. The process route comprises the steps of molten iron pretreatment, converter smelting, LF refining, continuous casting, continuous rolling, billet cleaning, heating, rolling and controlled cooling. The produced # imgabs0 cold heading steel wire rod is good in cold heading performance, the hardness of the cross section of the wire rod is larger than 45 HRC after quenching, and the cold heading steel wire rod can be used for producing 8.8-grade high-strength bolts, nuts, screws and other hexagonal flange standard parts.

Description

Technical Field

[0001] The invention belongs to the technical field of steel products, and relates to an 8.8-grade large-size cold-heading steel wire rod with excellent hardenability and a production method thereof. Background Art

[0002] Carbon cold heading steels such as SWRCH35K are mainly used to produce 8.8 grade high strength bolts, nuts, hexagonal bolts and screws and other hexagonal flange standard parts. These standard parts are widely used in machinery, electronics, automobiles, construction and other industries, requiring good cold deformation ability and requiring the hardness after quenching to meet certain performance requirements.

[0003] As people's requirements become higher, the use The following quality defects occurred during the production of 8.8 grade hexagonal flange fasteners using cold-headed steel wire rods of SWRCH35K and other specifications in accordance with national standards: (1) When the cross-section of the wire rods is different, due to different degrees of hardening, although the same quenching and tempering process is used, their mechanical properties vary greatly; and the larger the cross-section, the worse the degree of hardening and the worse the mechanical properties; (2) After heat treatment of the wire rods, it was found that the core hardness was low and the tensile strength was low. These quality problems will lead to insufficient hardenability defects in the production process of downstream users, resulting in batch scrapping, affecting the stable production and contract delivery of downstream users, etc.

[0004] CN105543675A discloses a grain-refined cold heading steel and a production process thereof. The cold heading steel has the following components by weight percentage: C 0.32-0.38%, Si 0.10-0.35%, Mn 0.60-0.90%, Cr≤0.20%, P≤0.030%, S≤0.035%, Ni≤0.20%, Al 0.010-0.040%, Ti 0.050-0.080%, Cu≤0.30%, and the remainder is Fe and inevitable impurities. It includes smelting, continuous casting and rolling processes. By adding Al and Ti elements, the grains of the material are refined and the structure is more uniform. At the same time, by selecting a reasonable time to use Al and add Ti, and then strengthening the deoxidation of molten steel, the cold forging performance of the cold heading steel is greatly improved by improving the purity of the molten steel and the grain size of the material. However, since the patented steel contains a certain amount of Ti element, it is easy to form TiN precipitation phase with the inherent N element in the steel, which deteriorates the toughness of the steel.

[0005] CN111534761A discloses a SWRCH35K hot-rolled wire rod for fire-fighting fish bolts and a production method thereof. The components of the hot-rolled wire rod are as follows by weight percentage: C 0.33-0.36, Si 0.12-0.26, Mn 0.61-0.70, P≤0.015, S≤0.010, Al 0.01-0.03, Cu≤0.1, Ni≤0.2, Cr≤0.2, As≤0.03, and the remainder is Fe and unavoidable impurities. The narrow composition control is adopted to meet the requirements of stable product performance, small dimensional tolerance, high output per unit time, and long service life. However, due to the narrow composition control process, higher requirements are put forward for steelmaking, and it is easy to have chemical composition out of range, resulting in accidents such as scrapping or downgrading of steel, which causes great trouble to the smooth operation of industrial production. Summary of the invention

[0006] In order to solve the problems of the prior art, the present invention proposes a large-size cold heading steel with excellent hardenability of 8.8 grade and a production method thereof. The large-size cold heading steel wire rod is produced by controlling the rolling process of strength and toughness matching by micro-alloying elements. The wire rod has good deformation ability and can meet the hardenability requirements of standard parts, and can be used to produce 8.8 grade high-strength bolts, nuts, screws and other hexagonal flange standard parts.

[0007] The invention discloses a 8.8-grade large-size cold heading steel wire rod with excellent hardenability, comprising the following chemical components in mass percentage (unit: wt%): C: 0.35-0.39, Si: ≤0.10, Mn: 0.60-0.85, Cr: 0.20-0.40, Mo: 0.15-0.25, Ni: 0.05-0.10, Al: 0.020-0.040, Ca: 0.0040-0.0080, Ce: 0.0010-0.0030, N: ≤0.0050, and the balance is Fe and unavoidable impurities.

[0008] The main functions of the chemical composition of the above-mentioned 8.8 grade large-size cold heading steel wire rod with excellent hardenability are:

[0009] C: C is the element with the most obvious solid solution strengthening effect in steel. As the C content increases, the strength and hardness of the steel increase, while the plasticity and toughness decrease. The C content is selected to be 0.35% to 0.39%;

[0010] Si: Si is a ferrite solid solution strengthening element, and it also causes a sharp increase in cold heading deformation resistance, significantly increases mold consumption, and is not conducive to cold processing plastic deformation. Therefore, the range of Si is ≤0.10%;

[0011] Mn: Mn plays the role of solid solution strengthening and grain refinement, and delays the transformation of pearlite and ferrite, improving the strength and work hardening properties of steel. Therefore, the range of Mn is 0.60% to 0.85%;

[0012] Cr: Cr can improve the toughness of steel and has good cold heading forming performance. At the same time, it can effectively improve the hardenability and ensure the strength and hardness of the material after quenching. Therefore, the composition range of Cr is 0.20% to 0.40%;

[0013] Mo: Mo can greatly improve the hardenability and ensure the hardness and strength of the material after quenching. In addition, this element has a strong ability to resist delayed fracture, so the content of Mo is controlled at 0.15% to 0.25%;

[0014] Ni: The lattice constant of Ni is similar to that of γ-iron, so it can form a continuous solid solution. This is beneficial to improving the hardenability of steel. Ni can lower the critical point and increase the stability of austenite, so its quenching temperature can be lowered and the hardenability is good. Ni is an element that can increase the activity of C. It can make carbon atoms gather around dislocations and increase the stacking fault energy, thereby inhibiting crack propagation and improving the toughness of steel. Generally, large-section and large-size steel products use Ni-added steel. When it is combined with Cr and Mo, the hardenability can be increased. Therefore, the Ni content is controlled at 0.05% to 0.10%;

[0015] Al: Adding a certain amount of Al (0.020% to 0.040%) to refine the grains and deoxidize, so that the steel has an extremely low oxygen content and reduces the number and size of non-metallic inclusions in the steel;

[0016] The main inclusion in Ca:Al deoxidized steel is Al2O3, and Al2O3 inclusions are hard and have sharp edges, which can easily scratch the matrix during the subsequent processing of the steel. In addition, due to its high melting point, it is easy to adhere to the inner wall of the nozzle, affecting the smooth casting and even causing serious accidents such as nozzle blockage. In order to avoid the formation of pure Al2O3 inclusions, the Ca treatment process is used to make the Al2O3 inclusions react with CaO to generate low-melting calcium aluminate inclusions. At the same time, since the MnS inclusions in the steel are transformed into CaS or CaO·Al2O3·CaS composite inclusions, the isotropy of the steel plate is improved, and the performance of the steel matrix is ​​improved. Therefore, the Ca composition is designed to be 0.0040%~0.0080%;

[0017] Ce: When heated to austenite, Ce atoms will inevitably segregate to the austenite grain boundary, inhibiting the diffusion of carbon atoms to the grain boundary and delaying the formation of cementite. It inhibits the nucleation of proeutectoid ferrite in steel and improves hardenability.

[0018] Furthermore, the size of the finished wire rod is

[0019] The production method of the above-mentioned 8.8-grade large-size cold heading steel wire rod with excellent hardenability has a process route of: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling.

[0020] Furthermore, in LF refining, the main tasks of LF refining are composition fine-tuning, temperature control and slag making operation. After the molten steel is alloyed by LF, the calcium alloy cored wire is fed into the ladle, and argon is weakly stirred after feeding the wire. The argon soft blowing flow rate is 10-30NL / min, so that the slag creeps slightly and the molten steel is not exposed. The station is left after weak stirring for 5-10 minutes.

[0021] Furthermore, the calcium-containing alloy cored wire is a double-layer pure calcium cored wire.

[0022] Furthermore, during the continuous casting process, the superheat of the tundish is controlled at 20-30°C, and the pulling speed is constant (the pulling speed of the ingot is controlled at 0.75±0.02m / min); the secondary cooling water volume is controlled at 0.9-1.2L / kg; the crystallizer is electromagnetically stirred: the stirring voltage is 300-500V, the current intensity is 300-500A, and the frequency is 3-8Hz; the electromagnetic stirring process can promote the floating of non-metallic inclusions in the molten steel, reduce the particle size of inclusions in the steel, and make the inclusions evenly distributed; reduce the element segregation of the steel, thereby uniformly organizing the structure and reducing the banded structure defects of the steel; reduce the number and depth of surface defects such as transverse cracks, longitudinal cracks, and corner cracks in the steel billet, and provide high-quality raw materials for rolling surface scratch-free wire rods.

[0023] Furthermore, the cross-sectional dimensions of the continuous casting billet are (250-350) mm×(350-450) mm.

[0024] Furthermore, during the continuous casting process, protective pouring is adopted, and covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the ladle to the tundish, and from the tundish to the crystallizer; protective slag is added to the crystallizer.

[0025] Furthermore, in the rolling process, the starting rolling temperature is controlled to be 1000-1040°C, the finishing rolling inlet temperature is controlled to be 950-990°C, and the wire laying temperature is controlled to be 910-930°C.

[0026] Furthermore, in the controlled cooling process, the Stelmore air cooling line cooling method is adopted: cooling to 680-770°C at a cooling rate of 6-12°C / s into the hood, all the insulation hoods are closed, the cooling rate in the hood is 0.5-5.0°C / s, the hood outlet temperature is 460-600°C, and the cooling bed is air-cooled.

[0027] Furthermore, through the heating, rolling and controlled cooling process control, the ideal ferrite and pearlite structure wire rods are obtained.

[0028] Furthermore, the wire rod after controlled cooling is finished, inspected and stored.

[0029] Furthermore, the cold heading steel wire rod is also subjected to heat treatment, wherein the heat treatment is quenching treatment, and the process includes: temperature of 880-890° C., water cooling or oil cooling.

[0030] Beneficial effects of the present invention:

[0031] The present invention produces For cold heading steel wire rod, the ferrite area accounts for 40-50%, and the pearlite area accounts for 50-60%; the longitudinal banded structure is ≤ level 2; the non-metallic inclusions of types A and B are ≤ level 2; the non-metallic inclusions of types C and D are ≤ level 2; the mechanical properties of the wire rod after hot rolling are as follows: the tensile strength is greater than 580MPa, the elongation is greater than 22%, and the surface reduction is greater than 45%; the cold heading performance is good, the qualified rate of 1 / 2 cold forging is more than 90%, and the qualified rate of 1 / 3 cold forging is more than 60%; after quenching, the hardness of the transverse section of the wire rod is greater than 45HRC, and it can be used to produce 8.8 grade high-strength bolts, nuts, screws and other hexagonal flange standard parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the transverse microstructure morphology of the wire rod prepared in Example 1 (×100).

[0033] Figure 2 This is the transverse microstructure morphology of the wire rod prepared in Example 1 (×500). DETAILED DESCRIPTION

[0034] The following non-limiting embodiments may enable a person skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0035] Embodiment 1

[0036] A production method for 8.8-grade large-size cold heading steel wire rod with excellent hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0037] LF refining: After the molten steel is alloyed with LF, the double-layer pure calcium cored wire is fed into the ladle. After wire feeding, argon gas is weakly stirred with a soft blowing flow rate of 20NL / min, so that the slag creeps slightly and the molten steel is not exposed. The station is left after weak stirring for 7 minutes.

[0038] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.75m / min); the secondary cooling water volume is controlled at 1.0L / kg; protective pouring (covering agent is added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; protective slag is added to the crystallizer); electromagnetic stirring of the crystallizer: the voltage of the electromagnetic stirring of the crystallizer is 350V, the current intensity is 350A, and the frequency is 6Hz.

[0039] Billets are rolled on a high-speed wire rod mill The rolling process of wire rod is as follows: control the starting rolling temperature to 1025℃, the finishing rolling entrance temperature to 960℃, and the spinning temperature to 915℃; the cooling control process is: cool to 690℃ at a cooling rate of 7℃ / s before entering the hood, close all the insulation hoods, cool at a cooling rate of 1.5℃ / s in the hood, and the hood outlet temperature to 500℃, and the cooling bed is air-cooled.

[0040] Embodiment 2

[0041] A production method for 8.8-grade large-size cold heading steel wire rod with excellent hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0042] LF refining: After the molten steel is alloyed with LF, the double-layer pure calcium cored wire is fed into the ladle. After wire feeding, argon gas is weakly stirred with a soft blowing flow rate of 25NL / min, so that the slag creeps slightly and the molten steel is not exposed. The station is left after weak stirring for 8 minutes.

[0043] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 27℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.76m / min); the secondary cooling water volume is controlled at 0.95L / kg; protective pouring (covering agent is added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; protective slag is added to the crystallizer); the crystallizer electromagnetic stirring voltage is 450V, the current intensity is 400A, and the frequency is 7Hz.

[0044] Billets are rolled on a high-speed wire rod mill The rolling process of wire rod is as follows: control the starting rolling temperature to 1010℃, the finishing rolling entrance temperature to 970℃, and the spinning temperature to 925℃; the cooling control process is: cool to 720℃ at a cooling rate of 7℃ / s before entering the hood, close all the insulation hoods, the cooling rate in the hood is 2.0℃ / s, the hood outlet temperature is 490℃, and the cooling bed is air-cooled.

[0045] Embodiment 3

[0046] A production method for 8.8-grade large-size cold heading steel wire rod with excellent hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0047] LF refining: After the molten steel is alloyed by LF, the double-layer pure calcium cored wire is fed into the ladle. After the wire is fed, argon gas is weakly stirred, and the soft blowing flow rate is 30NL / min, so that the slag creeps slightly and the molten steel is not exposed. After weak stirring for 6 minutes, the station is left. Among them:

[0048] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.75m / min); the secondary cooling water volume is controlled at 1.15L / kg; protective pouring (covering agent is added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; protective slag is added to the crystallizer); crystallizer electromagnetic stirring: the crystallizer electromagnetic stirring voltage is 500V, the current intensity is 480A, and the frequency is 4Hz;

[0049] Billets are rolled on a high-speed wire rod mill The rolling process of wire rod is as follows: control the starting rolling temperature to 1005℃, the finishing rolling entrance temperature to 960℃, and the spinning temperature to 925℃; the cooling control process is: cool to 710℃ at a cooling rate of 9℃ / s before entering the hood, close all the insulation hoods, the cooling rate in the hood is 2.0℃ / s, the hood outlet temperature is 510℃, and the cooling bed is air-cooled.

[0050] Embodiment 4

[0051] A production method for 8.8-grade large-size cold heading steel wire rod with excellent hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0052] LF refining: After the molten steel is alloyed by LF, the double-layer pure calcium cored wire is fed into the ladle. After the wire is fed, argon gas is weakly stirred, and the soft blowing flow rate is 15NL / min, so that the slag creeps slightly and the molten steel is not exposed. After weak stirring for 9 minutes, the station is left. Among them:

[0053] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.74m / min); the secondary cooling water volume is controlled at 1.10L / kg; protective pouring (covering agent is added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; protective slag is added to the crystallizer); crystallizer electromagnetic stirring: the crystallizer electromagnetic stirring voltage is 380V, the current intensity is 450A, and the frequency is 7Hz;

[0054] Billets are rolled on a high-speed wire rod mill The rolling process of wire rod is as follows: the starting rolling temperature is controlled at 1005℃, the finishing rolling entrance temperature is 960℃, and the spinning temperature is 925℃; the cooling control process is: cooling to 715℃ at a cooling rate of 7℃ / s before entering the hood, the insulation hood is completely closed, the cooling rate in the hood is 3.0℃ / s, the hood outlet temperature is 490℃, and the cooling bed is air-cooled.

[0055] Comparative Example

[0056] A production method for 8.8 grade cold heading steel wire rod, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0057] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.74m / min); protective sleeves are used from the large ladle to the tundish, and from the tundish to the crystallizer, and argon is blown for protection;

[0058] Billets are rolled on a high-speed wire rod mill The rolling process of wire rod is as follows: control the starting rolling temperature to 990℃, the finishing rolling entrance temperature to 940℃, and the spinning temperature to 925℃; the cooling control process is: cool to 715℃ at a cooling rate of 7℃ / s before entering the hood, close all the insulation hoods, the cooling rate in the hood is 3.0℃ / s, the hood outlet temperature is 490℃, and the cooling bed is air-cooled.

[0059] According to the chemical composition, content and method of the embodiments 1 to 4 and the comparative example of the present invention, the measured chemical composition is shown in Table 1; the test results of the non-metallic inclusions in the wire rod are shown in Table 2; the mechanical properties of the hot-rolled wire rod, and the hardness index after cold upsetting and quenching heat treatment are shown in Table 3. Among them, the heat treatment process system used for the wire rods of the comparative example and embodiments 1 to 4 in Table 3 is shown in Table 4.

[0060] The transverse microstructure of the hot-rolled wire rod obtained in Example 1 is shown in Figure 1 , Figure 2 , its metallographic structure is ferrite + pearlite structure.

[0061] The large-size cold heading steel wire rod produced by the above method has good cold heading performance; the hardness of the transverse section of the wire rod after quenching is greater than 45HRC, and can be used to produce 8.8 grade high-strength bolts, nuts, screws and other hexagonal flange standard parts.

[0062] Table 1 Chemical composition (wt%)

[0063]

[0064] Table 2 Inspection results of non-metallic inclusions in steel

[0065]

[0066] Table 3 Mechanical properties, cold forging and quenching hardness test data of steel

[0067]

[0068] Table 4 Wire rod quenching heat treatment process system

[0069]

Claims

1. A large-size cold-heading steel wire rod with excellent hardenability, grade 8.8, characterized in that: The invention comprises the following chemical compositions in mass percentage: C: 0.35% to 0.39%, Si: ≤0.10%, Mn: 0.60% to 0.85%, Cr: 0.20% to 0.40%, Mo: 0.15% to 0.25%, Ni: 0.05% to 0.10%, Al: 0.020% to 0.040%, Ca: 0.0040% to 0.0080%, Ce: 0.0010 to 0.0030%, N: ≤0.0050%, and the balance is Fe and unavoidable impurities.

2. The 8.8-grade large-size cold-heading steel wire rod with excellent hardenability according to claim 1, characterized in that: The size of the wire rod is 3. The 8.8-grade large-size cold-heading steel wire rod with excellent hardenability according to claim 1, characterized in that: The structure of the wire rod is ferrite + pearlite, wherein the area of ​​ferrite accounts for 40-50%, and the area of ​​pearlite accounts for 50-60%; the longitudinal banded structure is ≤ level 2; the non-metallic inclusions of types A and B are ≤ level 2; the non-metallic inclusions of types C and D are ≤ level 2; the tensile strength of the wire rod is greater than 580MPa, the elongation is greater than 22%, the surface reduction rate is greater than 45%, the qualified rate of 1 / 2 cold forging is more than 90%, and the qualified rate of 1 / 3 cold forging is more than 60%; after the wire rod is quenched, the hardness of the transverse section is greater than 45HRC; wherein, the quenching process: temperature 880-890°C, water cooling or oil cooling.

4. The method for producing the 8.8-grade large-size cold-heading steel wire rod with excellent hardenability according to any one of claims 1 to 3, characterized in that: include: Hot metal pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling; In the rolling process, the starting rolling temperature is controlled to be 1000-1040°C, the finishing rolling inlet temperature is controlled to be 950-990°C, and the wire laying temperature is controlled to be 910-930°C; In the controlled cooling process, the Stelmore air cooling line cooling method is adopted: cooling to 680-770℃ at a cooling rate of 6-12℃ / s before entering the hood, the insulation hood is completely closed, the cooling rate in the hood is 0.5-5.0℃ / s, the hood outlet temperature is 460-600℃, and the cooling bed is air-cooled.

5. The production method according to claim 4, characterized in that: In the LF refining process, after the molten steel is alloyed by LF, the calcium alloy cored wire is fed into the ladle. After feeding, argon is weakly stirred with an argon flow rate of 10 to 30 NL / min. The station is left after weak stirring for 5 to 10 minutes.

6. The preparation method according to claim 5, characterized in that: The calcium-containing alloy cored wire is a double-layer pure calcium cored wire.

7. The preparation method according to claim 4, characterized in that: During the continuous casting process, the superheat of the tundish is controlled at 20-30°C, the billet pulling speed is controlled at 0.75±0.02m / min; the secondary cooling water volume is controlled at 0.9-1.2L / kg; electromagnetic stirring of the crystallizer is adopted: the stirring voltage is 300-500V, the current intensity is 300-500A, and the frequency is 3-8Hz.

8. The preparation method according to claim 7, characterized in that: During the continuous casting process, the cross-sectional size of the continuous casting billet is (250-350) mm × (350-450) mm; protective pouring is adopted, and covering agent is added to the molten steel in the ladle and the tundish; protective casing is used from the ladle to the tundish, and from the tundish to the crystallizer, and argon is blown for protection; protective slag is added to the crystallizer.

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