A method for controlling the thickness of the iron oxide scale of free-cutting steel

Abstract

The application discloses a production method for controlling the thickness of iron oxide scale of free-cutting steel, and the process steps are as follows: hot metal pretreatment, converter steelmaking, CAS treatment, LF furnace external refining, continuous casting, heating furnace heating, rolling, controlled cooling, coil collecting and finished product inspection. The composition and thickness of the surface iron oxide scale of the free-cuting steel wire rod are controlled by controlling the wire drawing temperature and the post-rolling cooling speed, the wire drawing temperature is ensured to be below 860 DEG C by adjusting the water amount of the water tank after the finishing mill, the Fe2O3 red rust is generated on the surface of the wire rod, and the proportion of Fe3O4 in the iron oxide scale on the surface of the wire rod is prevented from being increased due to the excessively high wire drawing temperature. The wire drawing temperature and the post-rolling cooling speed are controlled in the application, the red rust-like iron oxide scale is formed on the surface of the free-cutting steel wire rod, the thickness of the iron oxide scale on the surface of the free-cutting steel wire rod is controlled to be below 12 microns, the inclusions and the banded structure are uniformly distributed, and the application is suitable for producing the free-cutting steel with the specifications of 6.5-17 mm and S=0.28%-0.40%.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of metallurgical material manufacturing, and is a production method for controlling the oxide scale thickness of free-cutting steel. BACKGROUND

[0002] With the technical progress of the automobile industry, precision instrument industry and household appliance industry, mechanical processing is developing towards high speed, precision and automation. The processing cost is an important part of the manufacturing cost of parts, accounting for 40% to 60% of the manufacturing cost of parts. People hope to reduce the processing cost by improving the processing performance of steel, which promotes the increase of the yield and variety of free-cutting steel and the continuous improvement of the quality.

[0003] The prior art free-cutting steel has the characteristics of high oxygen and high sulfur. In order to avoid the phenomena of split head, cracking and winding during rolling, it is required to have long heating time, high heating temperature, high rolling temperature and the water tank cannot be fully opened during rolling, which leads to thick oxide scale on the surface of the rolled wire rod. Customers complain that it is difficult to remove the oxide scale or the surface oxide scale is not completely removed, resulting in low yield. SUMMARY

[0004] In view of the above shortcomings of the prior art, the present application aims to provide a production method for controlling the oxide scale thickness of free-cutting steel, which is suitable for producing a wire rod with a specification of 6.5 to 17 mm, has a surface oxide scale thickness of ≤12 μm, and has uniform distribution of inclusions and banded structure, so as to improve the thickness of the surface oxide scale, the distribution of inclusions and the banded structure of the wire rod.

[0005] The technical scheme of the present application is as follows:

[0006] A production method for controlling the oxide scale thickness of free-cutting steel, the process flow is molten iron pretreatment → converter steelmaking → CAS treatment → LF external refining → continuous casting → heating furnace heating → rolling → controlled cooling → coil collection → finished product inspection, the production specification is 6.5 to 17 mm, and the free-cutting steel has a sulfur element content of 0.28% to 0.40% by weight; the method comprises the following key steps:

[0007] (1) Rolling: the opening rolling temperature is 1140±50℃, the temperature before entering the finishing mill is >1000℃, the water amount of the water tank after adjusting the finishing mill is controlled to be below 860℃ to generate Fe2O3 red rust on the surface of the wire rod, so as to avoid the increase of the proportion of Fe3O4 in the oxide scale on the surface of the wire rod caused by too high wire drawing temperature;

[0008] (2)Controlled cooling: the first 5 and last 2 fans are opened at 60%~100%, the wind cooling capacity after wire drawing is increased, the wire stays at a temperature above 700℃ for a reduced time; the speed of the wind cooling roller is between 0.35~0.86m / s, so that the wire can quickly enter the heat preservation cover; the first 1~3 fans are opened at 100%, the fourth and fifth fans are opened at 70%, and the rest are closed, so that the wire can quickly cool on the wind cooling roller, thereby reducing the thickness of the iron oxide skin on the surface of the wire, obtaining the easy-to-cut steel with the thickness of the surface iron oxide skin below 12μm and uniform distribution of inclusions and banded structure.

[0009] Invention principle:

[0010] The present application reduces the thickness of the iron oxide skin on the surface of the easy-to-cut steel and improves the banded structure and sulfide inclusion morphology by rapid cooling after rolling and slow cooling in the heat preservation cover. The purpose of cooling after hot rolling is to improve the organization state of the steel and improve the performance of the steel, shorten the cooling time of the hot rolled steel, and improve the production capacity of the rolling mill. The cooling medium for controlled cooling after hot rolling can be gas, liquid and their mixture, among which liquid, especially water, is most commonly used as the cooling medium. The specific cooling method varies with product variety and the purpose of cooling after rolling.

[0011] One of the important purposes of controlled cooling is to further improve the strength of the material without reducing the toughness of the material. Controlled rolling is particularly effective in improving the strength and toughness of low-carbon steel, low-alloy steel and micro-alloy steel. The steel after high-temperature finish rolling is in a fully recrystallized state of austenite, and if it is slowly cooled (air cooled) after rolling, the deformed austenite grains will grow during the cooling process, and the coarse ferrite structure will be obtained after phase transition. Due to slow cooling, the pearlite transformed from austenite is thick and the interlamellar spacing is thick. The mechanical properties of this organization are relatively low. For steel after low-temperature finish rolling, the austenite is in the unrecrystallized temperature region at finish rolling, and the Ar3 temperature is increased due to deformation, and the austenite quickly phase changes after finish rolling to form ferrite. This ferrite formed at high temperature grows very quickly. If slow cooling is used after rolling, the ferrite has enough time to grow, and at room temperature, relatively coarse ferrite will be formed, thereby reducing the effect of controlled rolling on grain refinement. Rapid cooling after rolling is essentially that the deformed austenite structure refined after controlled rolling is rapidly cooled, the phase change structure changes accordingly, and the size, quantity and precipitation site of the precipitates in the steel change, thereby improving the strength and toughness of the steel.

[0012] The key point of the present application is to control the composition and thickness of the surface oxide scale of the free-cutting steel wire rod by controlling the wire rod temperature and the cooling speed after rolling. The water amount of the water tank after the finishing mill is adjusted to ensure that the wire rod temperature is below 860℃, and the Fe2O3 red rust is generated on the surface of the wire rod, so as to avoid the increase of the proportion of Fe3O4 in the surface oxide scale of the wire rod caused by the excessively high wire rod temperature. The red rust-like oxide scale is formed on the surface of the free-cutting steel wire rod, so that the thickness of the surface oxide scale of the free-cutting steel wire rod is controlled to be below 12 μm, and the inclusions and banded structure are uniformly distributed, which is suitable for producing the free-cutting steel with the specification of 6.5-17 mm and S=0.28%-0.40%. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 Fig. 1 is a drawing of the surface oxide scale of the wire rod of Example 1, and the thickness of the surface oxide scale of the wire rod is 9 μm.

[0014] Figure 2 Fig. 2 is a drawing of the surface oxide scale of the wire rod of Example 2, and the thickness of the surface oxide scale of the wire rod is 10 μm.

[0015] Figure 3 Fig. 3 is a drawing of the banded structure of the wire rod of Example 1, and the banded structure of the wire rod is 0.5 level.

[0016] Figure 4 Fig. 4 is a drawing of the banded structure of the wire rod of Example 2, and the banded structure of the wire rod is 0.5 level. DETAILED DESCRIPTION

[0017] The content of the present application will be further illustrated in combination with the examples.

[0018] Example 1: Production of 9.0 mm Free-cutting Steel

[0019] Key process steps:

[0020] (1) Rolling: the opening rolling temperature is 1130℃, the temperature before entering the finishing mill is >1050℃, and the wire rod temperature is 855℃;

[0021] (2) Controlled cooling: the controlled cooling process parameters are shown in Table 1.

[0022] The actually measured thickness of the surface oxide scale of the wire rod is 7-9 μm, and the banded structure is 0.5 level; the oxide scale thickness of the original process is 18-21 μm, and the banded structure is 1.5 level.

[0023] Example 2: Production of 14.0 mm Free-cutting Steel

[0024] Key process steps:

[0025] (1) Rolling: the opening rolling temperature is 1140℃, the temperature before entering the finishing mill is >1040℃, and the wire rod temperature is 865℃;

[0026] (2) Controlled cooling: The controlled cooling process parameters are shown in Table 2.

[0027] The measured wire surface oxide scale thickness is 10-11 μm, and the banded structure is 0.5 grade; the original process oxide scale thickness is 19-22 μm, and the banded structure is 1.5 grade.

[0028] Table 1 Controlled cooling process parameters of Example 1

[0029] .

[0030] Table 2 Controlled cooling process parameters of Example 2

[0031] .

Claims

1. A production method for controlling the scale thickness of free-cutting steel, the process flow being hot metal pretreatment → converter steelmaking → CAS treatment → LF secondary refining → continuous casting → heating furnace heating → rolling → controlled cooling → coil collection → finished product inspection, characterized in that: The production specification is 6.5-17mm, and the easy cutting steel has the weight percentage content of sulfur element S=0.28%-0.40%; The method comprises the following key steps: (1) rolling: the opening rolling temperature is 1140±50 DEG C, the temperature before entering the finishing rolling is greater than 1000 DEG C, the water amount of the water tank after adjusting the finishing rolling machine is controlled to be below 860 DEG C, and the Fe2O3 red rust is generated on the surface of the wire rod; (2) controlled cooling: the first 5 and last 2 fans are opened by 60%-100%, the air cooling capacity after the wire rod is discharged is increased, the wire rod stays at the temperature above 700 DEG C for a short time, the roller way speed of each section of the air cooling roller way is between 0.35-0.86 m / s, the wire rod is quickly entered into the heat preservation cover, the first 1-3 heat preservation covers are opened by 100%, the fourth 4-5 heat preservation covers are opened by 70%, and the rest are all closed; the thickness of the surface iron oxide skin of the easy cutting steel is below 12 mu m, the inclusions and the banded structure are uniformly distributed.

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