8.8-grade cold heading steel wire rod capable of guaranteeing hardenability and production method of 8.8-grade cold heading steel wire rod

Abstract

The invention discloses a 8.8-grade cold heading steel wire rod capable of guaranteeing 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, 0.020 percent to 0.040 percent of Al, 0.0035 percent to 0.0065 percent of Mg, 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 transverse section is larger than 50 HRC after the wire rod is quenched, 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 cold heading steel wire rod with guaranteed 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, screws and other hexagonal flange standard parts. These standard parts are widely used in machinery, electronics and other industries, which not only require good cold deformation ability, but also require the hardness after quenching to meet certain performance requirements.

[0003] As people's requirements become higher, the following quality defects occur in the process of using cold heading steel wire rods such as SWRCH35K to produce 8.8 grade hexagonal flange fasteners according to national standards: (1) When the cross-section size of the wire rod is different, due to different hardening degrees, although the same quenching and tempering process is used, the mechanical properties vary greatly; and the larger the cross-section, the worse the hardening degree and the worse the mechanical properties; (2) After heat treatment of the wire rod, it is found that the core hardness is low and the tensile strength is 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] CN 105543675A discloses a grain-refined cold heading steel and a production process thereof. The cold heading steel comprises the following components by weight: 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. The process includes smelting, continuous casting and rolling. 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] CN 111534761A discloses a SWRCH35K hot-rolled wire rod for fire-fighting fish bolts and a production method thereof. The hot-rolled wire rod has the following components 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 inevitable impurities. The narrow component 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 component 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 an 8.8-grade cold heading steel wire rod with guaranteed hardenability and a production method thereof, and produces the cold heading steel wire rod by controlling the rolling process for matching strength and toughness with microalloying 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 cold heading steel wire rod for ensuring 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, Al: 0.020-0.040, Mg: 0.0035-0.0065, 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 cold heading steel wire rod to ensure 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. Therefore, the range of C 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 performance of steel, so the range of Mn is 0.60% to 0.85%;

[0012] Cr: Cr can improve the toughness of steel, improve atmospheric corrosion resistance, and have good cold heading forming performance. At the same time, it can effectively improve 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] 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;

[0015] Mg: It is often used as a deoxidizer and desulfurizer in steelmaking. It reacts with O and S in the molten steel to generate magnesium oxide or sulfide, reducing [O] and [S] in the molten steel to a lower level. The generated inclusions float up or are adsorbed by the protective slag and removed from the molten steel, playing a role in deoxidation and desulfurization. The addition of trace amounts of Mg can improve the composition, size and morphology of inclusions in steel, and enhance the deformability of inclusions in steel. Therefore, the Mg content is controlled at 0.0035% to 0.0065%;

[0016] 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.

[0017] Furthermore, the size of the finished wire rod is

[0018] The production method of the 8.8-grade cold heading steel wire rod with guaranteed hardenability has the following process route: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling.

[0019] Furthermore, in the LF refining process: after the molten steel is alloyed by LF, the magnesium alloy cored wire is fed into the ladle. After the wire is fed, argon is weakly stirred with a soft blowing flow rate of 10 to 30 NL / min, so that the slag creeps slightly and the molten steel is not exposed. The station is left after weak stirring for 5 to 10 minutes.

[0020] Furthermore, the magnesium-containing alloy cored wire is a high-magnesium cored wire, and the magnesium content is 20-30wt%.

[0021] 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 billet 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 100-300V, the current intensity is 100-300A, 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, and then make the organization uniform, reduce the banded structure defects of the steel; reduce the number and depth of surface defects such as transverse cracks, longitudinal cracks, and corner cracks of the billet, and provide high-quality raw materials for rolling surface scratch-free wire rods;

[0022] Furthermore, the cross-sectional dimensions of the continuous casting billet are (250-350) mm×(350-450) mm.

[0023] 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.

[0024] Furthermore, in the rolling process, the starting rolling temperature is controlled at 980-1020°C, the finishing rolling inlet temperature is controlled at 930-980°C, the sizing inlet temperature is controlled at 880-950°C, and the spinning temperature is controlled at 890-930°C.

[0025] 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.

[0026] Furthermore, through the heating, rolling and controlled cooling process control, the ideal ferrite and pearlite structure wire rods are obtained.

[0027] Furthermore, the wire rod after controlled cooling is finished, inspected and stored.

[0028] 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.

[0029] Beneficial effects of the present invention:

[0030] The present invention produces Cold heading steel wire rod, ferrite area accounts for 40-50%, pearlite area accounts for 50-60%; longitudinal banded structure ≤ level 2; A, B type non-metallic inclusions ≤ level 2; C, D type non-metallic inclusions ≤ level 1; hot rolled wire rod mechanical properties, tensile strength greater than 600MPa, elongation greater than 20%, surface reduction greater than 42%, cold heading performance is good, 1 / 2 cold forging qualified rate 100%, 1 / 3 cold forging qualified rate above 60%. After quenching, the hardness of the transverse section is greater than 50HRC, which can be used to produce 8.8 grade high-strength bolts, nuts, screws and other hexagonal flange standard parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the transverse microstructure morphology of the wire rod prepared in Example 1 (×200).

[0032] Figure 2 This is the longitudinal banded structure morphology of the wire rod prepared in Example 1 (×200). DETAILED DESCRIPTION

[0033] 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.

[0034] Embodiment 1

[0035] A production method for 8.8-grade cold heading steel wire rod with guaranteed hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0036] LF refining: After the molten steel is alloyed with LF, high-magnesium (containing 24wt% magnesium) cored wire is fed into the ladle. After wire feeding, argon is weakly stirred with a soft blowing flow rate of 18NL / min, so that the slag creeps slightly and the molten steel is not exposed. The station is left after weak stirring for 6 minutes.

[0037] 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 stirring voltage is 200V, the current intensity is 200A, and the frequency is 6Hz.

[0038] 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 980℃, the finishing rolling entrance temperature is 940℃, the sizing entrance temperature is 890℃, and the spinning temperature is 910℃; the cooling process is: cooling to 690℃ at a cooling rate of 7℃ / s before entering the hood, the insulation hood is completely closed, the cooling rate in the hood is 1.5℃ / s, the hood outlet temperature is 500℃, and the cooling bed is air-cooled.

[0039] Embodiment 2

[0040] A production method for 8.8-grade cold heading steel wire rod with guaranteed hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0041] LF refining: After the molten steel is alloyed with LF, high-magnesium (containing 24wt% magnesium) cored wire is fed into the ladle. After wire feeding, argon is weakly stirred with a soft blowing flow rate of 22NL / min, so that the slag creeps slightly and the molten steel is not exposed. The station is left after weak stirring for 7 minutes.

[0042] 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); electromagnetic stirring of the crystallizer: the stirring voltage is 250V, the current intensity is 250A, and the frequency is 7Hz.

[0043] 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 950℃, the sizing entrance temperature to 900℃, and the spinning temperature to 900℃; 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.

[0044] Embodiment 3

[0045] A production method for 8.8-grade cold heading steel wire rod with guaranteed hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0046] LF refining: After the molten steel is alloyed with LF, high-magnesium (containing 24wt% magnesium) 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 7 minutes.

[0047] 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); electromagnetic stirring of the crystallizer: the electromagnetic stirring voltage is 150V, the current intensity is 150A, and the frequency is 4Hz.

[0048] 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 1000℃, the finishing rolling entrance temperature is 955℃, the sizing entrance temperature is 895℃, and the spinning temperature is 920℃; the cooling process is: cooling to 710℃ at a cooling rate of 9℃ / s before entering the hood, the insulation hood is completely closed, the cooling rate in the hood is 2.0℃ / s, the hood outlet temperature is 510℃, and the cooling bed is air-cooled.

[0049] Embodiment 4

[0050] A production method for 8.8-grade cold heading steel wire rod with guaranteed hardenability, the process route is: molten iron pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling. Among them:

[0051] LF refining: After the molten steel is alloyed with LF, high-magnesium (containing 24wt% magnesium) cored wire is fed into the ladle. After wire feeding, argon gas is weakly stirred with a soft blowing flow rate of 28NL / min, so that the slag creeps slightly and the molten steel is not exposed. The station is left after weak stirring for 8 minutes.

[0052] 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; quality protective slag is added to the crystallizer); electromagnetic stirring of the crystallizer: the electromagnetic stirring voltage is 250V, the current intensity is 250A, and the frequency is 7Hz.

[0053] 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 940℃, the sizing entrance temperature to 900℃, 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.

[0054] Comparative Example

[0055] 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:

[0056] Continuous casting: The cross-sectional size of the continuous casting billet is 320mm×410mm, with constant pulling speed (the pulling speed of the billet is controlled at 0.75m / min); 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 ladle to the tundish, and from the tundish to the crystallizer; protective slag is added to the crystallizer).

[0057] 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.

[0058] The measured chemical composition of the wire rod produced according to the chemical composition, content and method of Examples 1 to 4 and the comparative example of the present invention is shown in Table 1; the test results of non-metallic inclusions in the wire rod are shown in Table 2; the mechanical properties of the hot-rolled wire rod, and the quenching 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 Examples 1 to 4 in Table 3 is shown in Table 4.

[0059] The transverse microstructure of the hot rolled wire rod obtained in Example 1 is shown in Figure 1 , the longitudinal banded structure morphology is shown Figure 2 , its metallographic structure is ferrite + pearlite structure.

[0060] The wire rod produced by the above method has good cold heading performance; after quenching, the hardness of the transverse section of the wire rod is greater than 50HRC, and can be used to produce 8.8 grade high-strength bolts, nuts, screws and other hexagonal flange standard parts.

[0061] Table 1 Chemical composition (wt%)

[0062]

[0063] Table 2 Inspection results of non-metallic inclusions in steel

[0064]

[0065] Table 3 Mechanical properties, cold forging and quenching hardness test data of steel

[0066]

[0067] Table 4 Wire rod quenching heat treatment process system

[0068]

Claims

1. A 8.8 grade cold heading steel wire rod with guaranteed hardenability, 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%, Al: 0.020% to 0.040%, Mg: 0.0035% to 0.0065%, Ce: 0.0010 to 0.0030%, N: ≤0.0050%, and the balance is Fe and unavoidable impurities.

2. The 8.8 grade cold heading steel wire rod with guaranteed hardenability according to claim 1, characterized in that: The size of the wire rod is 3. The 8.8 grade cold heading steel wire rod with guaranteed 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 1; the tensile strength of the wire rod is greater than 600MPa, the elongation is greater than 20%, the area reduction rate is greater than 42%, the qualified rate of 1 / 2 cold forging is 100%, 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 50HRC; wherein, the quenching process: temperature 880-890°C, water cooling or oil cooling.

4. The method for producing the 8.8 grade cold heading steel wire rod with guaranteed 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 980-1020°C, the finishing rolling inlet temperature is controlled to be 930-980°C, the sizing inlet temperature is controlled to be 880-950°C, and the wire laying temperature is controlled to be 890-930°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 preparation method according to claim 4, characterized in that: In the LF refining process, after the molten steel is alloyed by LF, the magnesium 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 magnesium-containing alloy cored wire is a high-magnesium cored wire, and the magnesium content is 20-30wt%.

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 100-300V, the current intensity is 100-300A, 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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