Production method of 8.8-grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti

Abstract

The invention relates to a production method of a 8.8-grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti. The steel comprises the following chemical components in percentage by weight: 0.25%-0.30% of C, 0.20%-0.40% of Si, 0.60%-0.80% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S, 0.15%-0.25% of Cr, 0.02%-0.05% of Ti, 0.020%-0.050% of Al, less than or equal to 0.0070% of N, less than or equal to 0.0025% of O, less than or equal to 0.0002% of H and the balance of Fe and inevitable impurity elements. The technological process comprises the steps of converter steelmaking, LF furnace refining, VD / RH vacuum treatment, continuous casting, heating, rolling, roller bed air cooling, coil collecting, bundling, weighing and warehousing. The hot-rolled wire rod is machined into the bolt after being subjected to drawing and cold heading forming, quenching and tempering heat treatment is not needed, the tensile strength can reach 800 Mpa or above, and the requirements of GBT3098.22 for 8.8-grade non-quenched and tempered steel bolts, screws and bolts are met.

Description

Technical Field

[0001] The invention belongs to the technical field of cold heading steel production, and relates to a production method of a 8.8-grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti. Background Art

[0002] The existing technology for producing 8.8 grade non-tempered cold heading steel mainly uses C and Mn steel, and the Mn content is generally above 1.3%. The higher Mn content is prone to positive segregation in the core of the ingot during continuous casting, resulting in the formation of martensitic structure in the center of the disk. The martensite does not deform and forms microcracks during the subsequent drawing and cold heading process, which reduces the service life of the bolt.

[0003] The higher the Mn content, the higher the alloy cost. For downstream processing companies, the cost advantage of replacing quenched and tempered steel with non-quenched and tempered cold heading steel is not obvious. Therefore, it is necessary to develop a non-quenched and tempered cold heading steel with good internal quality and lower alloy cost. Summary of the invention

[0004] The present invention aims to provide a method for producing a 8.8-grade non-quenched and tempered cold-heading steel wire rod containing Cr and Ti. The technical solution of the present invention: A method for producing a 8.8-grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti, wherein the chemical composition weight percentage of the steel is C=0.25%-0.30%, Si=0.20%-0.40%, Mn=0.60%-0.80%, P≤0.015%, S≤0.015%, Cr=0.15%-0.25%, Ti=0.02%-0.05%, Al=0.020%-0.050%, N≤0.0070%, O≤0.0025%, H≤0.0002%, and the rest are Fe and unavoidable impurity elements; the method comprises the following process steps: 1) Converter steelmaking: end point target C ≥ 0.06%, P ≤ 0.012%, tapping temperature 1570 ~ 1600 ℃, re-blowing times not exceeding 2 times; slag blocking tapping, add 100 ~ 200 kg of pre-melted synthetic slag to make slag during the tapping process; start adding alloys when the tapping is 1 / 4, and add high aluminum ferromanganese in the middle and late stages of tapping.

[0005] 2) LF refining: The total argon blowing time of molten steel in LF furnace is ≥40min. After refining, the Ca wire is fed for 100~200m. The soft argon blowing time after feeding the Ca wire is ≥12min. The Al content is controlled between 0.020%~0.040% at the exit. After deoxidation reaches the target, titanium iron is added.

[0006] 3) VD / RH vacuum treatment: vacuum target is below 67Pa, vacuum maintenance time is ≥15 minutes, and soft blowing after breaking the air is ≥25 minutes.

[0007] 4) Continuous casting: Use an integral nozzle tundish, the tundish covering agent is a covering agent plus carbonized rice husk; the mold protection slag is medium carbon steel protection slag, and the tundish lower water port is immersed in the crystallizer liquid level at a depth of 80~120mm; the molten steel superheat of the tundish is 25~40℃ in the opening furnace and 20~35℃ in the continuous casting furnace. The cross-sectional size of the continuous casting billet is 150mm×150mm; the pulling speed is 2.5~2.7m / s, the specific water volume is 0.8~1.0L / kg, the crystallizer electric stirring current is 200~300A, the frequency is 3Hz, and the end electric stirring is 200~250A, the frequency is 8Hz.

[0008] 5) Heating: The furnace temperature of the preheating section of the heating furnace is ≤950℃, the furnace temperature of the heating section is 950~1050℃, the furnace temperature of the soaking section is 1000~1080℃, the heating time is 90~200 minutes, and the billet out of the furnace temperature is 1000~1050℃.

[0009] 6) Rolling: high-pressure water dephosphorization pressure 25~30Mpa, sizing temperature 820~850℃; after sizing, each water tank is opened evenly, and the spinning temperature is 860~880℃.

[0010] 7) Roller air cooling: The roller speed of the air-cooled line is 0.40~0.9m / s; 8~20 insulation covers of the air-cooled roller are opened, 3~10 cooling fans under the roller are turned on, the air volume is 80%~100%, and the remaining insulation covers and fans are all closed; the temperature after air cooling is 500~550℃.

[0011] The beneficial effects of the present invention are as follows: the present invention uses a small amount of Cr and Ti to replace part of Mn, thereby reducing the alloy cost and avoiding the occurrence of martensitic undesirable structures in the core. The wire rod produced by the present invention can meet the requirements of fasteners such as 8.8-grade bolts and screws after drawing, forming and other processes, and does not need to be subjected to quenching and tempering heat treatment, which can effectively reduce energy consumption and heat treatment costs. Compared with the prior art, the alloy content of the present invention can better control the segregation of the ingot, prevent the occurrence of abnormal structures caused by segregation, and the alloy cost will not increase. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is the metallographic structure diagram of the wire rod produced in Example 1.

[0013] Figure 2 This is the metallographic structure diagram of the wire rod produced in Example 2. DETAILED DESCRIPTION Example 1

[0014] A production method for a 8.8-grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti, wherein the chemical composition weight percentage of the steel is C=0.28%, Si=0.30%, Mn=0.70%, P=0.009%, S=0.006%, Cr=0.20%, Ti=0.03%, Al=0.03%, N=0.0065%, O=0.0010%, H=0.00015%, and the rest is Fe and unavoidable impurity elements; the process flow comprises converter steelmaking→LF furnace refining→RH vacuum treatment→continuous casting→heating→rolling→roller air cooling→coiling→bundling→weighing→warehousing, and includes the following key steps: 1) Converter steelmaking. End point target: C: 0.09%, P: 0.008%, tapping temperature: 1580℃; re-blowing times: 0. Slag blocking tapping, 146kg synthetic slag is added during tapping; alloys are added when about 1 / 4 of the steel is tapped, and high aluminum ferromanganese is added in the middle and late stages of tapping.

[0015] 2) LF refining. The total argon blowing time of molten steel in LF furnace is 45min. After refining, feed Ca wire 102m, and soft argon blowing time after feeding Ca wire is 15min. The Al content is controlled at 0.035% before leaving the station. Ferrotitanium is added after white slag is formed.

[0016] 3) VD vacuum treatment: vacuum degree 65Pa, vacuum maintenance time 17 minutes, soft blowing for 25 minutes after breaking the air.

[0017] 4) Continuous casting. The integral nozzle tundish is used. The tundish covering agent is a covering agent plus carbonized rice husk. The mold protection slag is a medium carbon steel protection slag. The tundish lower nozzle is immersed in the crystallizer liquid level to a depth of 90mm. The superheat of the molten steel in the tundish is 30℃. The cross-sectional size of the continuous casting billet is 150mm×150mm. The pulling speed is 2.6m / s, and the specific water volume is 0.8L / kg. The crystallizer electric stirring current is 200A, the frequency is 3Hz, and the end electric stirring is 250A, the frequency is 8Hz.

[0018] 5) Heating. The furnace temperature of the preheating section of the heating furnace is 780~800℃, the furnace temperature of the heating section is 1020~1040℃, the furnace temperature of the soaking section is 1060~1080℃, and the heating time is 95~100 minutes. The billet out of the furnace temperature is 1020~1040℃.

[0019] 6) Rolling. The high-pressure water dephosphorization pressure is 25Mpa, and the sizing temperature is 830~840℃. After sizing, each water tank is opened evenly, and the spinning temperature is 860~870℃.

[0020] 7) Roller air cooling. The rolling specification is 17mm, and the speed of the first section of the air-cooled roller is 0.50m / s; each section increases by 0.05m / s, and then decreases after increasing to 0.75m / s, and finally rolls at a speed of 0.55m / s; 20 insulation covers are opened, and 10 cooling fans under the rollers are opened, with an air volume of 100%, and the temperature after air cooling is 550~570℃. All other insulation covers and fans are closed.

[0021] The hot rolling properties of the wire rod produced in Example 1 are shown in Table 1. Example 2

[0022] A production method for a 8.8-grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti, wherein the chemical composition weight percentage of the steel is C=0.29%, Si=0.32%, Mn=0.70%, P=0.008%, S=0.005%, Cr=0.22%, Ti=0.028%, Al=0.032%, N=0.0060%, O=0.0015%, H=0.00015%, and the rest is Fe and unavoidable impurity elements; the process flow includes converter steelmaking→LF furnace refining→RH vacuum treatment→continuous casting→heating→rolling→roller air cooling→coiling→bundling→weighing→warehousing. The key steps are as follows: 1) Converter steelmaking. End point target: C: 0.07%, P: 0.01%, tapping temperature 1581℃; re-blowing times 0. Slag blocking tapping, adding 145kg pre-melted slag to make slag during tapping; alloys are added when about 1 / 4 of the steel is tapped, and high aluminum ferromanganese is added in the middle and late stages of tapping.

[0023] 2) LF refining. The total argon blowing time of molten steel in LF furnace is 45 minutes. After refining, feed Ca wire for 100m, and soft argon blowing time after feeding Ca wire is 15 minutes. The Al content is controlled at 0.030% before leaving the station. Ferrotitanium is added after white slag is formed.

[0024] 3) RH vacuum treatment: vacuum degree 65Pa, vacuum time 20 minutes, soft blowing for 30 minutes after breaking the air.

[0025] 4) Continuous casting. The integral nozzle tundish is used. The tundish covering agent is a covering agent plus carbonized rice husk. The mold protection slag is a medium carbon steel protection slag. The tundish lower nozzle is immersed in the crystallizer liquid level to a depth of 90mm. The superheat of the molten steel in the tundish is 35℃. The cross-sectional size of the continuous casting billet is 150mm×150mm. The pulling speed is 2.7m / s, and the specific water volume is 1.0L / kg. The crystallizer electric stirring current is 300A, the frequency is 3Hz, and the end electric stirring is 250A, the frequency is 8Hz.

[0026] 5) Heating. The furnace temperature of the preheating section of the heating furnace is 800~820℃, the furnace temperature of the heating section is 1020~1030℃, the furnace temperature of the soaking section is 1050~1070℃, and the heating time is 101~105 minutes. The billet out of the furnace temperature is 1030~1050℃.

[0027] 6) Rolling. The high-pressure water dephosphorization pressure is 25Mpa, and the sizing temperature is 820~830℃. After sizing, each water tank is opened evenly, and the spinning temperature is 860~870℃.

[0028] 7) Roller air cooling. The rolling specification is 11mm. The air-cooled roller speed is 0.40m / s; 14 air-cooled roller insulation covers are opened, and 4 cooling fans under the rollers are opened, with an air volume of 100% and an inlet temperature of 530~550℃. All other insulation covers and fans are closed.

[0029] The hot rolling properties of the wire rod produced in Example 2 are shown in Table 1.

[0030] Table 1 Test results of hot rolling performance of wire rod produced in the embodiment and performance of fine wire after drawing .

Claims

1. A method for producing a 8.8 grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti, characterized in that: The chemical composition of steel by weight percentage is C=0.25%~0.30%, Si=0.20%~0.40%, Mn=0.60%~0.80%, P≤0.015%, S≤0.015%, Cr=0.15%~0.25%, Ti=0.02%~0.05%, Al=0.020%~0.050%, N≤0.0070%, O≤0.0025%, H≤0.0002%, and the rest are Fe and unavoidable impurity elements; The process steps include: 1) Converter steelmaking: End point target C ≥ 0.06%, P ≤ 0.012%, tapping temperature 1600 ~ 1630 ℃, re-blowing times no more than 2 times; slag blocking tapping, add 100 ~ 300 kg of pre-melted synthetic slag to make slag during tapping; add alloy when tapping is 1 / 4, and add high aluminum ferromanganese in the middle and late stages of tapping; 2) LF refining: The total argon blowing time of molten steel in LF furnace is ≥40min. After refining, the Ca wire is fed for 100~200m. The soft argon blowing time after feeding the Ca wire is ≥12min. The Al content is controlled between 0.020%~0.040% at the exit. Titanium iron is added after deoxidation reaches the target. 3) VD / RH vacuum treatment: vacuum target is below 67Pa, vacuum maintenance time is ≥15 minutes, soft blowing after breaking the air is ≥25 minutes; 4) Continuous casting: Use an integral tundish with a water nozzle, and the tundish covering agent is a covering agent plus carbonized rice husk; the mold protection slag is medium carbon steel protection slag, and the tundish lower water port is immersed in the crystallizer liquid level at a depth of 80~120mm; the superheat of the molten steel in the tundish is 25~40℃ for the opening furnace and 20~35℃ for the continuous casting furnace. The cross-sectional size of the continuous casting billet is 150mm×150mm; the pulling speed is 2.5~2.7m / s, the specific water volume is 0.8~1.0L / kg, the crystallizer electric stirring current is 200~300A, the frequency is 3Hz, and the end electric stirring is 200~250A, the frequency is 8Hz; 5) Heating: The furnace temperature of the preheating section of the heating furnace is ≤950℃, the furnace temperature of the heating section is 950~1050℃, the furnace temperature of the soaking section is 1000~1080℃, the heating time is 90~200 minutes, and the billet out of the furnace temperature is 1000~1050℃; 6) Rolling: high-pressure water dephosphorization pressure 25~30Mpa, sizing temperature 820~850℃; after sizing, each water tank is opened evenly, and the spinning temperature is 860~880℃; 7) Roller air cooling: The roller speed of the air-cooled line is 0.40~0.9m / s; 8~20 insulation covers of the air-cooled roller are opened, 3~10 cooling fans under the roller are turned on, the air volume is 80%~100%, and the remaining insulation covers and fans are all closed; the temperature after air cooling is 500~550℃.

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