Method for reducing tertiary cementite precipitation amount of low-carbon cold heading steel wire rod

Abstract

The invention belongs to the technical field of low-carbon cold forging steel, and discloses a method for reducing the tertiary cementite precipitation amount of a low-carbon cold forging steel wire rod. The wire rod comprises the following chemical components in percentage by mass: 0.01%-0.04% of C, less than or equal to 0.10% of Si, 0.20%-0.30% of Mn, less than or equal to 0.010% of P, less than or equal to 0.008% of S, 0.020%-0.050% of Al, 0.02%-0.04% of Ti and the balance of Fe and inevitable impurities. The production process comprises the steps of continuous casting, heating, rolling, spinning and cooling. By controlling cooling and adding a proper amount of Ti element, the precipitation amount and grade of the tertiary cementite in a ferrite grain boundary are reduced, the tensile strength of the obtained wire rod in a hot rolling state is smaller than or equal to 350 MPa, the grain size is larger than or equal to 7.5 grade, and the structure grade of the tertiary cementite is smaller than or equal to 2.0 grade.

Description

Technical Field

[0001] The invention belongs to the technical field of low-carbon cold heading steel and relates to a method for reducing the amount of tertiary cementite precipitation in a low-carbon cold heading steel wire rod. Background Art

[0002] In the fields of handicrafts, automobiles, electronics, new energy, etc., low-carbon cold heading steel is used to produce special-shaped products with large deformation by cold forming due to its good ductility and plasticity. However, large deformation places high demands on the overall performance of the material, requiring good plasticity, low strength, and good organizational morphology.

[0003] Patent CN103736744A discloses a hot rolling method for controlling tertiary cementite in low-carbon steel wire rod, belonging to the field of steel rolling technology. This hot rolling process method for controlling tertiary cementite in the microstructure is used to improve the wire rod's drawing limit and reduce the wire breakage rate. The specific steps are: after wire drawing, the cooling rate in the 900-750°C temperature range is controlled at 1.5-2°C / s; the air cooling line length and slow cooling distance are controlled at 85-100m, the roller conveyor is designed with four drop sections, and the roller speed adopts a 13-stage design; the cooling rate in the 570-670°C temperature range is controlled at 0.2-0.5°C / s; and the cooling rate in the 450-570°C temperature range is controlled at 0.8-1°C / s. The advantage is that the tertiary cementite in the low-carbon steel wire rod can be controlled, ensuring both the amount and size of precipitation, meeting the deep drawing requirements of the wire rod. This method is suitable for product development in high-speed wire workshops, with clear process control points and strong operability. However, it is only applicable to low-carbon steel.

[0004] Wire rod produced using existing low-carbon cold-heading steel compositions and conventional processes is prone to cracking when producing products with large deformations. Material analysis revealed that the cross-sectional microstructure showed significant tertiary cementite precipitation at grain boundaries and within the matrix. This presence reduced the material's plasticity, impacting cold forming performance and, to a certain extent, restricting the use of low-carbon cold-heading steel. The analysis suggests that by adding a certain amount of titanium to the steel, the affinity between titanium and carbon allows carbon to preferentially react with titanium to form TiC, reducing the precipitation and distribution of free cementite at ferrite grain boundaries. By utilizing the fact that titanium is a ferrite-forming element and combining controlled rolling and controlled cooling in the two-phase region to refine the material's grains and control the precipitation, distribution, and level of tertiary cementite, the wire rod exhibits excellent ductility. Summary of the Invention

[0005] To achieve the above object, the present invention provides a method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, which solves the problems existing in the prior art.

[0006] The technical solution adopted by the present invention is a method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, wherein the chemical composition of the wire rod comprises, by mass percentage, C: 0.01%-0.04%, Si≤0.10%, Mn: 0.20%-0.30%, P≤0.010%, S≤0.008%, Al: 0.020%-0.050%, Ti: 0.02%-0.04%, and the remainder is Fe and unavoidable impurities; and the production process steps include: continuous casting, heating, rolling, spinning and cooling.

[0007] Furthermore, in the key process step of rolling, the finishing mill adopts two-phase rolling, the rolling temperature is controlled between 830-870℃, and the wire drawing is carried out at 820-850℃ after rolling. After the wire drawing, one or two Stelmor roller cooling fans are turned on according to the specifications, and the fan air volume is ≤50%. The first section of the Stelmor roller is 0.25m / s, and the speed of the subsequent sections increases by 0.05m / s. The first four insulation covers are turned on and the rest are closed.

[0008] Furthermore, after spinning, the Stelmor roller cooling fans are turned on according to different specifications: one fan is turned on for specifications below Φ10.0mm, and two fans are turned on for specifications between Φ11.0mm and Φ16.0mm.

[0009] Furthermore, the wire rod produced by the method has a hot-rolled tensile strength of ≤350 MPa, a grain size ≥7.5, and a tertiary cementite structure level ≤2.0.

[0010] The beneficial effects of the present invention are as follows: based on user requirements for low-carbon cold-heading steel hot-rolled wire rods and in combination with existing equipment capabilities, the present invention adds a certain amount of Ti element to the steel, utilizes the affinity of Ti and C, causes the C element to preferentially form TiC with the Ti element, and reduces the precipitation and distribution of free cementite at ferrite grain boundaries; and conducts research on steelmaking, rolling and cooling processes to produce wire rods with a hot-rolled tensile strength of ≤350 MPa, a grain size of ≥7.5, and a tertiary cementite structure level of ≤2.0. The wire rod has broad application prospects and huge economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0012] Figure 1 1 is a metallographic structure diagram of the wire rod according to Example 1 of the present invention;

[0013] Figure 21 is a metallographic structure diagram of the wire rod according to Example 2 of the present invention;

[0014] Figure 3 1 is a metallographic structure diagram of the wire rod of Comparative Example 1 of the present invention;

[0015] Figure 4 2 is a metallographic structure diagram of the wire rod of Comparative Example 2 of the present invention;

[0016] Figure 5 3 is a metallographic structure diagram of the wire rod of Comparative Example 3 of the present invention;

[0017] Figure 6 It is the metallographic organization diagram of the wire rod of comparative example 4 of the present invention. DETAILED DESCRIPTION

[0018] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0019] The main contents of the present invention, such as component control, steelmaking, rolling, and cooling methods, are further described below in conjunction with the embodiments.

[0020] Generally, a method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod includes the following steps in order: composition design, steelmaking, heating, rolling, controlled cooling, finishing, determination, packaging, weighing, and warehousing.

[0021] Example 1:

[0022] A production method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, with a rolling specification of Φ8.0mm.

[0023] As shown in Table 1, the chemical composition of the wire rod includes, by mass percentage: C: 0.02%, Si: 0.05%, Mn: 0.23%, P: 0.008%, S: 0.006%, Al: 0.031%, Ti: 0.028%, and the rest is Fe and unavoidable impurities.

[0024] The key process steps include: the temperature of the finishing mill is controlled at 840-860℃ for rolling, and the wire drawing temperature is 825-840℃; after rolling, it is cooled through the Stelmor controlled cooling line, the first Stelmor roller cooling fan is opened at 40%, and the second is opened at 20%, the first section of the Stelmor roller is 0.25m / s, and the speed of the subsequent sections increases by 0.05m / s, the first four insulation covers are opened and the rest are closed.

[0025] like Figure 1As shown in Table 2, the wire rod obtained by the method has a grain size of 8.0 and a tertiary cementite structure of 1.0. The tensile strength of the wire rod in the hot-rolled state is shown in Table 2.

[0026] Example 2:

[0027] A production method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, with a rolling specification of Φ12.0mm.

[0028] As shown in Table 1, the chemical composition of the wire rod includes, by mass percentage, C: 0.03%, Si: 0.07%, Mn: 0.22%, P: 0.009%, S: 0.004%, Al: 0.037%, Ti: 0.023%, and the rest is Fe and unavoidable impurities.

[0029] The key process steps include: the temperature of the finishing mill is controlled at 840-855℃ for rolling, and the wire drawing temperature is 830-840℃; after rolling, it is cooled through the Stelmor controlled cooling line, the first Stelmor roller cooling fan is opened at 50%, the second at 30%, the first section of the Stelmor roller is 0.25m / s, and the speed of the subsequent sections increases by 0.05m / s, the first four insulation covers are opened and the rest are closed.

[0030] like Figure 2 As shown in Table 2, the wire rod obtained by the method has a grain size of 8.0 and a tertiary cementite structure of 1.5. The tensile strength of the wire rod in the hot-rolled state is shown in Table 2.

[0031] Table 1 Chemical composition content of each embodiment

[0032] Serial number C％ Si% Mn% P％ S％ Ti% Al％ Example 1 0.02 0.05 0.23 0.008 0.006 0.028 0.031 Example 2 0.03 0.07 0.22 0.009 0.004 0.023 0.037 Comparative Example 1 0.03 0.07 0.24 0.009 0.004 0.0052 0.028

[0033] Table 2 Hot-rolled tensile strength of wire rods in various embodiments

[0034]

[0035] Comparative Example 1:

[0036] A production method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, with a rolling specification of Φ8.0mm.

[0037] As shown in Table 1, the chemical composition of the wire rod includes, by mass percentage, C: 0.03%, Si: 0.07%, Mn: 0.24%, P: 0.009%, S: 0.004%, Al: 0.028%, Ti: 0.0052%, and the rest is Fe and unavoidable impurities.

[0038] The key process steps include: the temperature of the finishing mill is controlled at 840-860℃ for rolling, and the wire drawing temperature is 820-840℃; after rolling, it is cooled through the Stelmor controlled cooling line, the first Stelmor roller cooling fan is opened at 40%, and the second is opened at 20%, the first section of the Stelmor roller is 0.25m / s, and the speed of the subsequent sections increases by 0.05m / s, the first four insulation covers are opened and the rest are closed.

[0039] like Figure 3 As shown in Table 2, the wire rod obtained by the method has a grain size of 8.0 and a tertiary cementite structure of 2.5. The tensile strength of the wire rod in the hot-rolled state is shown in Table 2.

[0040] Comparative Example 2:

[0041] A production method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, with a rolling specification of Φ12.0mm.

[0042] The chemical composition of the wire rod is consistent with that of Example 2.

[0043] The key process steps include: the temperature of the finishing mill is controlled at 870-890℃ for rolling, and the wire drawing temperature is 850-870℃; after rolling, it is cooled through the Stelmor controlled cooling line, the first Stelmor roller cooling fan is opened at 50%, the second is opened at 30%, the first section of the Stelmor roller is 0.25m / s, and the speed of the subsequent sections increases by 0.05m / s, the first four insulation covers are opened and the rest are closed.

[0044] like Figure 4 As shown in Table 2, the wire rod obtained by the method has a grain size of 7.0-8.0 and a tertiary cementite structure of 2.5. The tensile strength of the wire rod in the hot-rolled state is shown in Table 2.

[0045] Comparative Example 3:

[0046] A production method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, with a rolling specification of Φ8.0mm.

[0047] The chemical composition of the wire rod is consistent with that of Example 1.

[0048] The key process steps include: controlling the rolling temperature at 840-860℃ in the finishing mill and the spinning temperature at 830-850℃; cooling through the Stelmor controlled cooling line after rolling, the first Stelmor roller cooling fan is opened at 40% and the second at 20%, the first section of the Stelmor roller is 0.35m / s, and the speed of the subsequent sections increases by 0.05m / s, the first four insulation covers are opened and the rest are closed.

[0049] like Figure 5 As shown in Table 2, the wire rod obtained by the method has a grain size of 8.0-8.5 and a tertiary cementite structure of 1.0. The tensile strength of the wire rod in the hot-rolled state is shown in Table 2.

[0050] Comparative Example 4:

[0051] A production method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, with a rolling specification of Φ12.0mm.

[0052] The chemical composition of the wire rod is consistent with that of Example 2.

[0053] The key process steps include: controlling the rolling temperature at 840-860℃ in the finishing mill and the spinning temperature at 830-850℃; cooling through the Stelmor controlled cooling line after rolling, opening the first Stelmor roller cooling fan at 50%, the first section of the Stelmor roller at 0.15m / s, and the subsequent sections increasing in speed by 0.02m / s, opening the first three insulation covers and closing the rest.

[0054] like Figure 6 As shown in Table 2, the wire rod obtained by the method has a grain size of 7.0-8.0 and a tertiary cementite structure of 2.0-2.5. The tensile strength of the wire rod in the hot-rolled state is shown in Table 2.

[0055] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are included in the scope of protection of the present invention.

Claims

1. A method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod, characterized in that: The chemical composition of the wire rod includes, by mass percentage, C: 0.01%-0.04%, Si≤0.10%, Mn: 0.20%-0.30%, P≤0.010%, S≤0.008%, Al: 0.020%-0.050%, Ti: 0.02%-0.04%, and the rest is Fe and unavoidable impurities; the production process steps include: continuous casting, heating, rolling, spinning and cooling.

2. A method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod according to claim 1, characterized in that: In the key process step of rolling, the finishing mill adopts two-phase zone rolling, the rolling temperature is controlled between 830-870℃, and the wire drawing is carried out at 820-850℃ after rolling. After the wire drawing, one or two Stelmor roller cooling fans are turned on according to the specifications, and the fan air volume is ≤50%. The first section of the Stelmor roller is 0.25m / s, and the speed of the subsequent sections increases by 0.05m / s. The first four insulation covers are turned on and the rest are closed.

3. A method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod according to claim 2, characterized in that: After spinning, the Stelmor roller cooling fans are turned on according to different specifications: one fan is turned on for specifications below Φ10.0mm, and two fans are turned on for specifications between Φ11.0mm and Φ16.0mm.

4. A method for reducing the amount of tertiary cementite precipitation in low-carbon cold heading steel wire rod according to claim 3, characterized in that: The wire rod produced by the method has a hot-rolled tensile strength of ≤350 MPa, a grain size of ≥7.5, and a tertiary cementite structure level of ≤2.0.

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