A production method for 8.8 grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti

Abstract

The present invention provides a method for producing 8.8-grade non-quenched and tempered cold-headed steel wire rod containing Cr and Ti. The chemical composition of the steel by weight is as follows: C = 0.25% to 0.30%, Si = 0.20% to 0.40%, Mn = 0.60% to 0.80%, P ≤ 0.015%, S ≤ 0.015%, Cr = 0.15% to 0.25%, Ti = 0.02% to 0.05%, Al = 0.020% to 0.050%, N ≤ 0.0070%, O ≤ 0.0025%, H ≤ 0.0002%, with the remainder being Fe and unavoidable impurity elements. The process flow includes converter steelmaking → LF furnace refining → VD / RH vacuum treatment → continuous casting → heating → rolling → roller air cooling → coiling → bundling → weighing → storage. Hot-rolled wire rods are processed into bolts after drawing and cold heading. They do not need to undergo tempering heat treatment. The tensile strength can reach more than 800Mpa, meeting the requirements of GBT3098.22 for 8.8 grade non-tempered steel bolts, screws and bolts.

Description

Technical Field

[0001] The invention belongs to the technical field of cold heading steel production, and relates to a production method of 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, with a Mn content generally above 1.3%. A higher Mn content easily causes positive segregation in the core of the ingot during continuous casting, resulting in the formation of martensite structure in the center of the disk. The martensite does not deform during the subsequent drawing and cold heading processes, forming microcracks, which reduces the service life of the bolt.

[0003] High Mn content increases alloy cost, and the cost advantage of using non-quenched and tempered cold heading steel instead of quenched and tempered steel is not obvious for downstream processing companies. Therefore, it is necessary to develop a non-quenched and tempered cold heading steel with good internal quality and low alloy cost. Summary of the Invention

[0004] The present invention aims to provide a method for producing 8.8 grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti.

[0005] The technical solution of the present invention:

[0006] A method for producing a grade 8.8 non-quenched and tempered cold heading steel wire rod containing Cr and Ti, wherein the chemical composition of the steel is as follows by weight: C = 0.25% to 0.30%, Si = 0.20% to 0.40%, Mn = 0.60% to 0.80%, P ≤ 0.015%, S ≤ 0.015%, Cr = 0.15% to 0.25%, Ti = 0.02% to 0.05%, Al = 0.020% to 0.050%, N ≤ 0.0070%, O ≤ 0.0025%, H ≤ 0.0002%, and the remainder is Fe and unavoidable impurity elements; the method comprises the following process steps:

[0007] 1) Converter steelmaking: End point target C ≥ 0.06%, P ≤ 0.012%, tapping temperature 1570~1600℃, re-blowing times no more than 2 times; slag blocking tapping, adding 100~200kg of pre-melted slag during the tapping process; alloying begins when 1 / 4 of the steel is tapped, and high-aluminum ferromanganese is added in the middle and late stages of tapping.

[0008] 2) LF refining: The total argon blowing time of molten steel in the 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% and 0.040% at the exit. Titanium iron is added after the deoxidation target is achieved.

[0009] 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.

[0010] 4) Continuous Casting: A tundish with an integral nozzle is used, and the tundish covering agent is a covering agent plus carbonized rice hulls. Medium-carbon steel mold slag is used, and the tundish lower nozzle is immersed 80-120mm below the mold liquid level. The tundish molten steel superheat is 25-40°C in the start-casting furnace and 20-35°C in the continuous casting furnace. The cross-sectional dimensions of the continuous casting billet are 150mm x 150mm. The casting speed is 2.5-2.7m / s, the specific water content is 0.8-1.0L / kg, the mold electric stirring current is 200-300A at 3Hz, and the end electric stirring current is 200-250A at 8Hz.

[0011] 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 discharge temperature is 1000~1050℃.

[0012] 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℃.

[0013] 7) Roller air cooling: The roller speed of the air cooling line is 0.40~0.9m / s; 8~20 insulation covers of the air cooling roller are opened, 3~10 cooling fans under the roller are turned on, and the air volume is 80%~100%. The remaining insulation covers and fans are all closed; the temperature after air cooling is 500~550℃.

[0014] The present invention has the beneficial effects of replacing part of the manganese with a small amount of Cr and Ti, reducing alloy costs and avoiding the occurrence of martensite undesirable structure 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 and forming processes, eliminating the need for quenching and tempering heat treatment, effectively reducing energy consumption and heat treatment costs. Compared with the prior art, the alloy content of the present invention better controls the segregation of the ingot, preventing the occurrence of abnormal structures caused by segregation, and also preventing an increase in alloy costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the metallographic structure diagram of the wire rod produced in Example 1.

[0016] Figure 2 This is the metallographic structure diagram of the wire rod produced in Example 2. DETAILED DESCRIPTION Example 1

[0017] A method for producing 8.8-grade non-quenched and tempered cold-heading steel wire rod containing Cr and Ti, wherein the chemical composition of the steel is as follows by weight: 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 remainder is Fe and unavoidable impurity elements. The process 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:

[0018] 1) Converter steelmaking. Endpoint targets: C: 0.09%, P: 0.008%, tapping temperature: 1580°C; reblowing: 0. Slag-blocking tapping, with 146 kg of synthetic slag added during tapping. Alloying began when approximately 1 / 4 of the steel was tapped, with high-aluminum ferromanganese added in the middle and late stages of tapping.

[0019] 2) LF refining. The total argon blowing time in the LF furnace is 45 minutes. After refining, the molten steel is fed with 102m of Ca wire. After feeding the Ca wire, the soft argon blowing time is 15 minutes. The Al content is controlled at 0.035% at the exit. Ferrotitanium is added after the white slag is formed.

[0020] 3) VD vacuum treatment: Vacuum degree 65Pa, vacuum maintenance time 17 minutes, soft blowing for 25 minutes after breaking the air.

[0021] 4) Continuous casting. A tundish with an integral nozzle was used. The tundish covering agent consisted of a covering agent and carbonized rice hulls. The mold powder used was a medium-carbon steel mold powder. The tundish outlet was immersed 90 mm below the mold liquid level. The tundish molten steel superheated to 30°C. The cross-sectional dimensions of the continuous casting strand were 150 mm × 150 mm. The casting speed was 2.6 m / s, and the specific water content was 0.8 L / kg. The mold electric stirring current was 200 A at a frequency of 3 Hz, and the end electric stirring current was 250 A at a frequency of 8 Hz.

[0022] 5) Heating: The furnace temperature in the preheating section of the heating furnace is 780-800°C, the heating section is 1020-1040°C, and the soaking section is 1060-1080°C. The heating time is 95-100 minutes. The billet discharge temperature is 1020-1040°C.

[0023] 6) Rolling. High-pressure water dephosphorization pressure is 25 MPa, and the temperature before sizing is 830-840°C. After sizing, each water tank is opened evenly, and the spinning temperature is 860-870°C.

[0024] 7) Roller air cooling. The rolling gauge is 17mm. The roller speed of the air cooling line is 0.50m / s in the first section. The speed increases by 0.05m / s in each section until it reaches 0.75m / s, where it then decreases, ultimately converging at 0.55m / s. Twenty insulation covers are open, and ten cooling fans under the rollers are running at 100% air volume. The temperature after air cooling is 550-570°C. All other insulation covers and fans are closed.

[0025] The hot rolling properties of the wire rod produced in Example 1 are shown in Table 1. Example 2

[0026] A method for producing 8.8-grade non-quenched and tempered cold-headed steel wire rod containing Cr and Ti. The chemical composition of the steel is as follows: 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 remainder is Fe and unavoidable impurities. The process includes converter steelmaking → LF furnace refining → RH vacuum treatment → continuous casting → heating → rolling → roller air cooling → coiling → bundling → weighing → storage. The key steps are as follows:

[0027] 1) Converter steelmaking. Endpoint targets: C: 0.07%, P: 0.01%, tapping temperature 1581°C; 0 reblowing times. Slag-blocking tapping, with 145 kg of pre-melted slag added during tapping. Alloying began when approximately 1 / 4 of the steel was tapped, with high-aluminum ferromanganese added in the middle and later stages of tapping.

[0028] 2) LF refining. The total argon blowing time in the LF furnace is 45 minutes. After refining, the molten steel is fed with a 100-meter Ca wire. After feeding the Ca wire, the soft argon blowing time is 15 minutes. The Al content is controlled at 0.030% at the exit. Ferrotitanium is added after the white slag is formed.

[0029] 3) RH vacuum treatment: vacuum degree 65Pa, vacuum time 20 minutes, soft blowing for 30 minutes after breaking the air.

[0030] 4) Continuous casting. A tundish with an integral nozzle was used. The tundish covering agent consisted of a covering agent and carbonized rice hulls. The mold powder used was a medium-carbon steel mold powder. The tundish outlet was immersed 90 mm below the mold liquid level. The tundish molten steel superheated to 35°C. The cross-sectional dimensions of the continuous casting billet were 150 mm × 150 mm. The casting speed was 2.7 m / s, and the specific water content was 1.0 L / kg. The mold electric stirring current was 300 A at a frequency of 3 Hz, and the end electric stirring current was 250 A at a frequency of 8 Hz.

[0031] 5) Heating: The furnace temperature in the preheating section of the heating furnace is 800-820°C, the heating section is 1020-1030°C, and the soaking section is 1050-1070°C. The heating time is 101-105 minutes. The billet discharge temperature is 1030-1050°C.

[0032] 6) Rolling. High-pressure water dephosphorization pressure is 25 MPa, and the temperature before sizing is 820-830°C. After sizing, each water tank is opened evenly, and the spinning temperature is 860-870°C.

[0033] 7) Roller air cooling. The rolling gauge is 11mm. The roller speed on the air-cooling line is 0.40m / s. 14 insulation covers are open, and 4 cooling fans below the rollers are running at 100% air volume. The temperature inlet to the insulation covers is 530-550°C. All other insulation covers and fans are closed.

[0034] The hot rolling properties of the wire rod produced in Example 2 are shown in Table 1.

[0035] Table 1 Test results of hot rolling performance of wire rod produced in Example and performance of finished wire after drawing

[0036] .

Claims

1. A method for producing 8.8 grade non-quenched and tempered cold heading steel wire rod containing Cr and Ti, characterized by: The chemical composition of steel is as follows by weight: 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 frequency no more than 2 times; tapping with slag blocking, adding 100-300kg of pre-melted synthetic slag during tapping; alloy is added when 1 / 4 of the steel is tapped, and high-aluminum ferromanganese is added in the middle and late stages of tapping; 2) LF refining: The total argon blowing time of molten steel in the LF furnace is ≥40 minutes. After refining, the Ca wire is fed for 100-200m. The soft argon blowing time after feeding the Ca wire is ≥12 minutes. The Al content of the steel leaving the station is controlled between 0.020% and 0.040%. After the deoxidation target is achieved, ferrotitanium is added. 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; 4) Continuous Casting: A tundish with an integral nozzle is used, 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 nozzle is immersed in the mold liquid level to a depth of 80-120mm. The superheat of the tundish molten steel is 25-40℃ in the opening furnace and 20-35℃ in the continuous casting furnace. The cross-sectional dimensions of the continuous casting billet are 150mm×150mm. The casting speed is 2.5-2.7m / s, the specific water content is 0.8-1.0L / kg, the mold electric stirring current is 200-300A, the frequency is 3Hz, and the end electric stirring current 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 discharge 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, spinning temperature 860~880℃; 7) Roller air cooling: The roller speed of the air cooling line is 0.40~0.9m / s; 8~20 insulation covers of the air cooling roller are opened, 3~10 cooling fans under the roller are turned on, and the air volume is 80%~100%. The remaining insulation covers and fans are all closed; the temperature after air cooling is 500~550℃.

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