A 8.8 grade cold heading steel wire rod with guaranteed quenching property and a production method thereof

Abstract

The application discloses a kind of 8.8 grade cold heading steel wire rod of guaranteeing quenching and its production method, belong to steel product technical field.The wire rod includes following mass percentage chemical composition: 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%, the balance is Fe and unavoidable impurities.Process route is: hot metal pretreatment-converter smelting-LF refining-continuous casting-continuous rolling-billet cleaning-heating-rolling-controlling cooling.Cold heading steel wire rod produced by the application has good cold heading performance, the hardness of transverse section after quenching is greater than 50HRC, and can be used for producing 8.8 grade high-strength bolt, nut, screw and other hexagonal flange standard parts.

Description

Technical Field

[0001] This invention belongs to the field of steel product technology and relates to an 8.8 grade cold heading steel wire rod with guaranteed hardenability and its production method. Background Technology

[0002] Carbon cold heading steels such as SWRCH35K are mainly used to produce standard hexagonal flange parts such as 8.8 grade high-strength bolts, nuts, and screws. These standard parts are widely used in industries such as machinery and electronics, requiring not only good cold deformation capabilities but also hardness after quenching to meet certain performance requirements.

[0003] As people's requirements become higher, the following quality defects have occurred during the production of 8.8 grade hexagonal flange fasteners using cold heading steel wire rods such as SWRCH35K according to national standards: (1) When the wire rod cross-section is different, due to the different degrees of hardening, although the same quenching and tempering process is used, their mechanical properties differ greatly; and the larger the cross-section, the worse the degree of hardening and the worse the mechanical properties; (2) After heat treatment, the core hardness and tensile strength of the wire rod are found to be low. These quality problems will lead to insufficient hardening 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 its production process. The cold heading steel, by weight percentage, has the following composition: 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%, with the balance being Fe and unavoidable impurities. The process includes smelting, continuous casting, and rolling steps. By adding Al and Ti, the material's grain size is refined, and the microstructure becomes more uniform. Furthermore, by selecting appropriate timing for Al and Ti additions and strengthening steel deoxidation, the cold heading steel's cold upsetting performance is significantly improved through increased steel purity and grain size. However, because the patented steel contains a certain amount of Ti element, it is easy to form TiN precipitate phase with the N element inherent in the steel, which deteriorates the toughness of the steel.

[0005] CN 111534761A discloses a SWRCH35K hot-rolled wire rod for fire-fighting fish-tail bolts and its production method. The composition of the hot-rolled wire rod, by weight percentage, is: 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, with the balance being Fe and unavoidable impurities. Narrow composition control is employed to meet the requirements of stable product performance, small dimensional tolerances, high output per unit time, and long service life. However, due to the narrow composition control process, high requirements are placed on steelmaking, and it is easy for the chemical composition to exceed the range, resulting in accidents such as steel scrapping or downgrading, causing significant disruption to the smooth operation of industrial production. Summary of the Invention

[0006] To address the problems of existing technologies, this invention proposes an 8.8 grade cold heading steel wire rod with guaranteed hardenability and its production method. The cold heading steel wire rod is produced by controlling the rolling process to match strength and toughness through micro-alloying elements. The wire rod has good deformation capacity and can meet the hardenability requirements of standard parts. It can be used to produce 8.8 grade high-strength bolts, nuts, screws and other hexagonal flange standard parts.

[0007] An 8.8 grade cold heading steel wire rod with guaranteed hardenability comprises the following chemical composition by 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, with the balance being Fe and unavoidable impurities.

[0008] The main function of the chemical composition of the aforementioned 8.8 grade cold heading steel wire rod, which ensures hardenability, is as follows:

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

[0010] Si: Si is a ferrite solid solution strengthening element, but it also leads to a sharp increase in resistance to cold heading deformation, significantly increasing die wear and hindering cold working plastic deformation. Therefore, the range of Si is ≤0.10%;

[0011] Mn: Mn plays a role in solid solution strengthening and grain refinement strengthening, and delays the transformation of pearlite and ferrite, thereby 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 strength and toughness of steel, enhance its resistance to atmospheric corrosion, and provide good cold heading properties. It also effectively improves hardenability, ensuring the strength and hardness of the material after quenching. Therefore, the composition range of Cr is 0.20%–0.40%.

[0013] Mo: Mo can significantly improve hardenability, ensuring the hardness and strength of the material after quenching. Furthermore, this element possesses extremely strong resistance to delayed fracture; therefore, the Mo content is controlled between 0.15% and 0.25%.

[0014] Al: Adding a certain amount of Al (0.020%~0.040%) refines the grains and deoxidizes the steel, resulting in extremely low oxygen content and reducing the number and size of non-metallic inclusions in the steel.

[0015] Mg: Commonly used as a deoxidizer and desulfurizer in steelmaking. It reacts with O and S in molten steel to form magnesium oxides or sulfides, reducing the [O] and [S] levels in the molten steel to even lower levels. Inclusions formed float to the surface or are adsorbed by protective slag and removed from the molten steel, thus 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, enhancing their deformability. Therefore, the Mg content should be controlled between 0.0035% and 0.0065%.

[0016] When Ce is heated to austenitization, Ce atoms inevitably segregate towards the austenite grain boundaries, inhibiting the diffusion of carbon atoms to the grain boundaries and delaying the formation of cementite. This suppresses the nucleation of proeutectoid ferrite in steel, improving hardenability.

[0017] Furthermore, the dimensions of the finished wire rod are:

[0018] The production method for the above-mentioned 8.8 grade cold heading steel wire rod with guaranteed hardenability is as follows: hot metal 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, magnesium alloy cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring, and the soft blowing flow rate is 10-30 NL / min, which makes the steel slag slightly creep and the molten steel does not leak out. After weak stirring for 5-10 minutes, the ladle leaves the station.

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

[0021] Furthermore, during continuous casting: the tundish superheat is controlled at 20–30℃, and the casting speed is constant (the billet casting speed 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 with a stirring voltage of 100–300V, a current intensity of 100–300A, and a frequency of 3–8Hz. The electromagnetic stirring process promotes the flotation of non-metallic inclusions in the molten steel, reduces the size of inclusion particles in the steel, and ensures uniform distribution of inclusions; it reduces elemental segregation in the steel, thereby homogenizing the microstructure and mitigating banded defects; it reduces the number and depth of surface defects such as transverse cracks, longitudinal cracks, and corner cracks in the billet, providing high-quality raw materials for the production of scratch-free wire rod.

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

[0023] Furthermore, during the continuous casting process, protective casting is adopted, and a covering agent is added to the molten steel in the ladle and tundish; protective sleeves are used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer.

[0024] Furthermore, in the rolling process, the initial rolling temperature is controlled at 980–1020℃, the finishing rolling inlet temperature at 930–980℃, the sizing inlet temperature at 880–950℃, and the wire drawing temperature at 890–930℃.

[0025] Furthermore, in the controlled cooling process, the Stellmore air-cooled line cooling method is adopted: the temperature is cooled to 680-770°C at a cooling rate of 6-12°C / s before being placed into the hood. The insulation hood is completely closed, the cooling rate inside the hood is 0.5-5.0°C / s, and the temperature exiting the hood is 460-600°C. The cooling bed is air-cooled.

[0026] Furthermore, by controlling the heating, rolling, and controlled cooling processes, wire rods with ideal ferrite and pearlite structures can be obtained.

[0027] Furthermore, the coils after controlled cooling undergo finishing, inspection, and warehousing.

[0028] Furthermore, the cold heading steel wire rod is subjected to heat treatment, which is quenching treatment, with a temperature of 880-890℃ and water or oil cooling.

[0029] The beneficial effects of this invention are:

[0030] The invention produces Cold-heading steel wire rod has a ferrite area ratio of 40-50% and a pearlite area ratio of 50-60%; longitudinal banded structure ≤ grade 2; A and B type non-metallic inclusions ≤ grade 2; C and D type non-metallic inclusions ≤ grade 1. Hot-rolled wire rod exhibits the following mechanical properties: tensile strength > 600 MPa, elongation > 20%, and reduction of area > 42%. It demonstrates excellent cold-heading performance, with a 100% pass rate for 1 / 2 cold upsetting and a pass rate of over 60% for 1 / 3 cold upsetting. After quenching, the hardness of the transverse section of the wire rod is greater than 50 HRC, making it suitable for producing standard hexagonal flange parts such as 8.8 grade high-strength bolts, nuts, and screws. Attached Figure Description

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

[0032] Figure 2 The longitudinal strip-like tissue morphology of the wire rod prepared in Example 1 (×200). Detailed Implementation

[0033] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0034] Example 1

[0035] A production method for 8.8 grade cold heading steel wire rod with guaranteed hardenability, comprising the following process route: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0036] LF refining: After the molten steel is alloyed by LF, high-magnesium (containing 24wt% magnesium) cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The soft blowing flow rate is 18NL / min, which makes the steel slag slightly creep and the molten steel does not leak out. After weak stirring for 6 minutes, the ladle is removed from the station.

[0037] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 25℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.75m / min). The secondary cooling water ratio is controlled at 1.0L / kg. Protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer). Electromagnetic stirring in the crystallizer: the stirring voltage is 200V, the current intensity is 200A, and the frequency is 6Hz.

[0038] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 980℃, the finishing rolling inlet temperature is 940℃, the sizing inlet temperature is 890℃, and the wire drawing temperature is 910℃; the controlled cooling process is as follows: the wire rod is cooled to 690℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 1.5℃ / s, the exit temperature is 500℃, and the wire rod is air-cooled on the cooling bed.

[0039] Example 2

[0040] A production method for 8.8 grade cold heading steel wire rod with guaranteed hardenability, comprising the following process route: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0041] LF refining: After the molten steel is alloyed by LF, high-magnesium (containing 24wt% magnesium) cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The soft blowing flow rate is 22NL / min, which makes the steel slag slightly creep and the molten steel does not leak out. After weak stirring for 7 minutes, the ladle is removed from the station.

[0042] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 27℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.76m / min). The secondary cooling water ratio is controlled at 0.95L / kg. Protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve 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). Electromagnetic stirring in the crystallizer: the stirring voltage is 250V, the current intensity is 250A, and the frequency is 7Hz.

[0043] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 990℃, the finishing rolling inlet temperature is 950℃, the sizing inlet temperature is 900℃, and the wire drawing temperature is 900℃; the controlled cooling process is as follows: the wire rod is cooled to 720℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 2.0℃ / s, the exit temperature is 490℃, and the wire rod is air-cooled on the cooling bed.

[0044] Example 3

[0045] A production method for 8.8 grade cold heading steel wire rod with guaranteed hardenability, comprising the following process route: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0046] LF refining: After the molten steel is alloyed by LF, high-magnesium (containing 24wt% magnesium) cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The soft blowing flow rate is 25NL / min, which makes the steel slag slightly creep and the molten steel does not leak out. After weak stirring for 7 minutes, the ladle is removed from the station.

[0047] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 25℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.75m / min). The secondary cooling water ratio is controlled at 1.15L / kg. Protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer). Electromagnetic stirring in the crystallizer: The electromagnetic stirring voltage is 150V, the current intensity is 150A, and the frequency is 4Hz.

[0048] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 1000℃, the finishing rolling inlet temperature is 955℃, the sizing inlet temperature is 895℃, and the wire drawing temperature is 920℃; the controlled cooling process is as follows: the wire rod is cooled to 710℃ at a cooling rate of 9℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 2.0℃ / s, the exit temperature is 510℃, and the wire rod is air-cooled on the cooling bed.

[0049] Example 4

[0050] A production method for 8.8 grade cold heading steel wire rod with guaranteed hardenability, comprising the following process route: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0051] LF refining: After the molten steel is alloyed by LF, high-magnesium (containing 24wt% magnesium) cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The soft blowing flow rate is 28NL / min, which makes the steel slag slightly creep and the molten steel does not leak out. After weak stirring for 8 minutes, the ladle is removed from the station.

[0052] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 25℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.74m / min). The secondary cooling water ratio is controlled at 1.10L / kg. Protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon is blown for protection; a protective slag is added to the crystallizer). Electromagnetic stirring in the crystallizer: The electromagnetic stirring voltage is 250V, the current intensity is 250A, and the frequency is 7Hz.

[0053] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 1010℃, the finishing rolling inlet temperature is 940℃, the sizing inlet temperature is 900℃, and the wire drawing temperature is 925℃; the controlled cooling process is as follows: the wire rod is cooled to 715℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 3.0℃ / s, the exit temperature is 490℃, and the wire rod is air-cooled on the cooling bed.

[0054] Comparative Example

[0055] A production method for 8.8 grade cold heading steel wire rod, the process route is as follows: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0056] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm, and the constant casting speed (the casting speed of the billet is controlled at 0.75m / min); protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer and argon blowing is used for protection; protective slag is added in the crystallizer).

[0057] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 990℃, the finishing rolling inlet temperature is 940℃, and the wire exit temperature is 925℃; the controlled cooling process is as follows: the wire rod is cooled to 715℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 3.0℃ / s, the exit temperature is 490℃, and the wire rod is air-cooled on the cooling bed.

[0058] The wire rods produced according to the chemical composition, content, and method designed in Examples 1 to 4 and the comparative examples of this invention are shown in Table 1 for their measured chemical composition; the test results of non-metallic inclusions in the wire rods are shown in Table 2 for their non-metallic inclusions; and the mechanical properties, cold forging, and quenching hardness indices after quenching heat treatment of the hot-rolled wire rods are shown in Table 3 for their mechanical properties, cold forging, and quenching hardness indices. The heat treatment process used for the wire rods in the comparative examples and Examples 1 to 4 in Table 3 is shown in Table 4 for its heat treatment process.

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

[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, which can be used to produce standard hexagonal flange parts such as 8.8 grade high-strength bolts, nuts, and screws.

[0061] Table 1 Chemical composition (wt%)

[0062]

[0063] Table 2. Inspection Results of Non-metallic Inclusions in Steel

[0064]

[0065] Table 3 Test data of mechanical properties, cold forging, and quenching hardness of steel.

[0066]

[0067] Table 4. Process Regulations for Quenching and Heat Treatment of Wire Rod

[0068]

Claims

1. A 8.8 grade cold heading steel wire rod for ensuring hardenability, characterized in that, The chemical composition includes the following mass percentages: 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 of Fe and inevitable impurities; The production method of the 8.8-grade cold upsetting steel wire rod with guaranteed quenching property comprises the following steps: hot metal pretreatment, converter smelting, LF refining, continuous casting, continuous rolling, billet cleaning, heating, rolling, and controlled cooling. In the rolling process, the open rolling temperature is controlled to be 980-1020 DEG C, the finish rolling inlet temperature is 930-980 DEG C, the reducing inlet temperature is 880-950 DEG C, and the wire drawing temperature is 890-930 DEG C. In the controlled cooling process, the Stelmor air cooling line cooling mode is adopted: cooling to 680-770 DEG C at a cooling rate of 6-12 DEG C / s, keeping the cover closed, the cooling rate in the cover being 0.5-5 DEG C / s, and the outlet temperature being 460-600 DEG C.

2. The 8.8 grade cold upset steel wire rod for ensuring the quenching penetration according to claim 1, characterized in that, The dimensions of the wire rod are 3. The 8.8 grade cold upset steel wire rod for ensuring the quenching penetration according to claim 1, characterized in that, The wire rod has a structure of ferrite + pearlite, wherein the area ratio of ferrite is 40-50%, the area ratio of pearlite is 50-60%, the longitudinal band structure is ≤2 levels, the A and B type non-metallic inclusions are ≤2 levels, the C and D type non-metallic inclusions are ≤1 level, the tensile strength of the wire rod is greater than 600 MPa, the elongation is greater than 20%, the area reduction is greater than 42%, the 1 / 2 cold upset forging qualified rate is 100%, the 1 / 3 cold upset forging qualified rate is more than 60%, the transverse section hardness after quenching is greater than 50 HRC, and the quenching process is at a temperature of 880-890 DEG C with water cooling or oil cooling.

4. The 8.8 grade cold upset steel wire rod for ensuring the quenching penetration according to claim 1, characterized in that, In the LF refining process, after the molten steel is treated by LF alloying, the magnesium-containing alloy cored wire is fed into the ladle, argon is weakly stirred after the wire feeding, the argon flow rate is 10-30 NL / min, and the weak stirring is performed for 5-10 min before leaving the station.

5. The 8.8 grade cold-roughed steel wire rod for ensuring the quenching penetration according to claim 4, characterized in that, The magnesium-containing alloy cored wire is a high-magnesium cored wire, and the content of magnesium is 20-30 wt%.

6. The 8.8 grade cold-roughed steel wire rod for ensuring quenching penetration according to claim 1, wherein In the continuous casting process, the tundish superheat is controlled to be 20-30 DEG C, the casting billet casting speed is controlled to be 0.75±0.02 m / min, the secondary cooling water quantity is controlled to be 0.9-1.2 L / kg, the mold electromagnetic stirring is adopted, the stirring voltage is 100-300 V, the current intensity is 100-300 A, and the frequency is 3-8 Hz.

7. The 8.8 grade cold-roughed steel wire rod for ensuring the quenching penetration according to claim 6, characterized in that, In the continuous casting process, the continuous casting billet section size is (250-350) mm x (350-450) mm, the protective casting is adopted, the covering agent is added into the ladle and the tundish, the protective sleeve is adopted and argon is blown for protection from the ladle to the tundish and from the tundish to the mold, and the protective slag is added into the mold.

Images