KR100306138B1 - METHOD FOR MANUFACTURING Bi-S FREE CUTTING STEEL ROD WIRE WITH NO SURFACE DEFECTS - Google Patents

Abstract

PURPOSE: Provided is a rolling method of Bi-S free cutting steel rod wire with no surface microcrack due to hot brittleness. CONSTITUTION: In the rolling method of Bi-S free cutting steel rod wire comprising C 0.10wt.% or less, Si 0.05wt.% or less, Mn 1.20-1.40wt.%, P 0.0-0.09wt.%, S 0.27-0.33wt.%, Al 0.01wt.% or less, Bi 0.05-0.15wt.%, B 70-130ppm, balance Fe and other inevitable impurities in the temperature range of 1100 to 1250deg.C through rough rolling, process rolling, and finish rolling, this method is characterized in that Bi-S free cutting steel is subjected to rough rolling at a reduction ratio less than 10% through 1 to 2 pass, and through 3 to 5 pass, reduction ratio is 10-20%.

Description

Hot wire rolling method of bismuth-sulfur free cutting steel without surface defects

The present invention relates to a wire rod manufacturing method of bismuth-sulfur free cutting steel used in automobiles or precision equipment parts, and more particularly, to a hot wire rerolling method of bismuth-sulfur free cutting steel without surface defects.

Free-cutting steel is a type of steel that improves the machinability during cutting by distributing non-metallic or metallic inclusions in the base metal. Among these inclusions, non-metallic inclusions are manganese emulsion (MnS), and metallic inclusions are lead (Pb) and bismuth (Bi). Low melting point metals with little degree of solidity in the base material.

Bismuth (Bi) -sulfur (S) free cutting steel has the same machinability as sulfur (S), lead (Pb), calcium (Ca), and free cutting steel that combines these, but has a beautiful surface after cutting. Since there is no pollution problem in manufacturing, the demand is increasing rapidly.

Metallic and nonmetallic inclusions that provide free machinability act as stress concentration sources during cutting, facilitating the formation and growth of cracks at the interface between the inclusions and the substrate, reducing the force required for cutting and softening by the cutting heat. Or melted to act as a lubricant at the interface between the chip and the cutting tool to suppress the wear of the tool and to improve the machinability by reducing the cutting force.

However, these inclusions are an important factor for improving machinability during cutting, while sulfur (S) added to form a non-metallic inclusion, manganese emulsion (MnS), segregates at grain boundaries and causes hot embrittlement (FeS). It may also cause the formation of, reducing the hot ductility at high temperatures. In the case of bismuth (Bi) -sulfur (S) free cutting steel, bismuth, which is a low melting point metal with little solid solution of steel, is crystallized in a state of being adsorbed to grain boundaries or nonmetallic inclusions, which may cause hot embrittlement.

In the case of bismuth (Bi) -sulfur (S) -based free cutting steel, the temperature of hot embrittlement is in the range of 900-1050 ℃. When wire rolling is performed, cracks are generated on the surface of the billet due to the decrease in hot ductility. The characteristics are poor. However, when the wire rod is manufactured by the conventional wire rod rolling method, significant high heating is difficult due to the capability of the furnace, and thus, the steel sheet is rolled in the hot embrittlement temperature range, thereby causing surface flaws or cracks during wire rod manufacturing.

Referring to the wire rolling process for manufacturing a wire rod in a conventional steel sheet as follows.

Here, the term 'Billet' refers to a material produced by rolling a Bloom material produced by a continuous casting method in a Billet rolling machine. In addition, the "wire" refers to a product manufactured by wire rod rolling in a specific dimension (5.5 mm-34.0 mm diameter in the present invention) according to the characteristics of the use of the order (Billet).

In general, the wire rolling process for producing a wire rod from a billet (Billet) is divided into rough rolling, intermediate sand rolling and filament rolling, and uses a double continuous rolling method. At this time, the general rolling process is heated to 1000-1250 ℃ by using a milling mill, intermediate sand mill, finishing mill 27 to 29 rolling mill (Stand) of 5.5mm in diameter, each of which is composed of a double continuous by heating the steel (Billet) Manufacture wire rod of 34.0mm.

In the case of rolling the billet of bismuth (Bi) -sulfur (S) -based free cutting steel with wire rod, the rolling temperature of the material passing through the intermediate finishing mill and the finishing mill is determined by the transformation heat due to recrystallization and the rolling mill of the material. Friction heat caused by contact makes it possible to secure the temperature above 1050 ° C, and the austenite grain size (hereinafter referred to as 'AGS') becomes fine due to repeated recrystallization, so that surface defects of the material due to hot embrittlement of the material do not occur.

However, when passing through the roughing mill, the rolling temperature of the material is in the hot embrittlement temperature range (900-1050 ℃) of bismuth (Bi) -sulfur (S) -based free cutting steel, and it is immediately after extraction from the wire furnace. Since AGS is coarse, surface flaws are likely to occur. If surface flaws or cracks are generated after wire rod rolling, the product loses its value as a result of defective products and eventually, manufacturing cost increases.

In order to solve this problem, when heating the billet (Billet), the wire rod temperature of the roughing mill is heated to 1300 ℃ or higher enough to ensure the hot embrittlement temperature range of 900-1050 ℃ or higher, or passes through the roughing mill. Should be reheated to above 1050 ℃. However, when heated to a high temperature of 1300 ℃ or more there is a problem that the surface of the steel piece is melted, and reheating when passing through the rough rolling machine is a limitation of the equipment.

Due to these practical difficulties, no wire rolling technology has been proposed to solve the surface scratches or cracks of wire rods due to hot embrittlement during wire rolling of bismuth (Bi) -sulfur (S) -based free cutting steel.

The present invention provides a hot wire rolling method for solving the occurrence of surface flaws or cracks in wire rods due to hot embrittlement during wire rolling of bismuth (Bi) -sulfur (S) based free cutting steel.

1 is a view showing the change of austenite grain size (AGS) by rolling mill (Stand)

It is a graph.

2 is a graph showing the change in material ductility according to the rolling temperature.

The present invention for achieving the above object is by weight (PPM), carbon (C): 0.10% or less, silicon (Si): 0.05% or less, manganese (Mn): 1.20-1.40%, phosphorus (P): 0.07 -0.09%, sulfur (S): 0.27-0.33%, aluminum (Al): 0.01% or less, bismuth (Bi): 0.05-0.15%, boron (B): 70-130PPM, and the remaining Fe and other unavoidable impurities In the method of heating the formed steel strip (Billet) to 1100-1250 ℃ hot rolled material including rough rolling, intermediate sand rolling, filament rolling, the rough rolling is carried out with a rolling reduction of less than 10% in 1-2 passes. Next, the 3-5 pass is configured to include a 10-20% rolling reduction rate.

Hereinafter, the present invention will be described in detail.

The present invention is to reduce the surface scratches or cracks of the wire rod as much as possible by reducing the pressure at the hot embrittlement temperature, it is characterized by.

Among them, carbon (C) is an effective element that guarantees the strength of the final product, and 0.10% or more is limited to 0.10% or less by increasing the hardness and machinability and degrading the ductility of the material during hot rolling.

Silicon (Si) is commonly used as a deoxidizer in molten steel manufacturing, but is limited to 0.05% or more because more than 0.05% increases hardness and degrades machinability.

Manganese (Mn) is used as a deoxidizer in molten steel and improves machinability by forming manganese sulphate (MnS), and less than 1.20% cannot form spherical good manganese sulphate (MnS) and cause brittleness. It promotes emulsification (FeS), which causes brittleness during hot rolling, and 1.40% or more is limited to 1.20-1.40% because it causes hardness increase and increase in manufacturing cost.

Phosphorus (P) is a machinability improving element, but its effect is less than 0.07%, and the hardness is increased above 0.09%, which deteriorates machinability, so it is limited to 0.07-0.09%.

Sulfur (S) is an element that improves machinability by forming inclusions with manganese (Mn), and its effect is less than 0.27% and less than 0.33%, and thus deteriorates hot workability due to brittleness, so it is limited to 0.27-0.33%.

Aluminum (Al) is limited to 0.01% or less because it forms alumina inclusions (Al ₂ O ₃ ) which are light inclusions and degrades machinability.

Bismuth (Bi) is an excellent element for machinability improvement, and its effect is insufficient because the machinability is significantly lower than 0.05%, and its specific gravity is high at 0.15% or more, which causes central segregation and deteriorates hot workability with liquid metal. Limited to -0.15%.

Boron (B) is first segregated at the grain boundary to suppress the formation of iron sulfide (FeS), which is the cause of brittleness in the grain boundary, the effect is less than 70PPM or less, and precipitates at the grain boundary above 130PPM, so it is limited to 70-130PPM.

The steel pieces formed as described above are heated at 1100-1250 ° C. in the usual manner, followed by rough rolling, in which case it is necessary to set rolling conditions in which surface defects do not occur.

The present inventors observed the microstructure at the time of rolling, and confirmed that the solution was possible by lightly depressurizing at 1-5 passes during rough rolling. That is, the AGS of the steel strip (Billet) extracted from the heating furnace becomes fine by repeated recrystallization while passing through the rolling mill, and when the furnace is extracted, when AGS rolls 5 passes at 200 μm as shown in FIG. 1, 50 μm To change. In addition, looking at the high temperature ductility according to the rolling temperature and grain size, it can be seen that when the AGS is 50㎛ ductility (RA) is more than 60 (%). Here, when ductility is 60% or more, it means a section in which hot embrittlement does not exist.

Therefore, surface defects are likely to occur in the 1-5 pass section, so it is necessary to reduce the pressure to prevent this.

Specifically, according to the present invention, when the rough rolling is performed in the 1-2 pass, the reduction rate of rolling is 10% or less, and the 3-5 pass is 20-30%.

Limiting the rolling reduction rate of 1-2 passes to 10% or less is because AGS is coarse immediately after reheating the furnace, so if rolling reduction is more than 10%, rolling is performed in a temperature range where hot ductility is weak. This is because defects are likely to occur.

And, if the reduction ratio of 3-5 passes is less than 10%, even if rolling is performed in the temperature range where the hot ductility is weak due to AGS, it does not affect the surface scratches or cracks of the material, but it is limited to 27 or 29 rolling mills during wire rolling. In order to manufacture wire rod product size that meets customer's usage within rolling reduction rate, rolling load increases with increasing rolling reduction rate in finishing mill after rough rolling mill, and it is difficult to control exact dimension as wire rod is rolled at high speed. Because. In addition, the rolling reduction ratio is limited to 20% or less because there is still a possibility of hot embrittlement although the AGS is fine during the 1-2 pass rolling.

Hereinafter, the embodiment of the present invention will be described in detail.

EXAMPLE

In weight% (PPM), carbon (C): 0.071%, silicon (Si): 0.027%, manganese (Mn): 1.34%, phosphorus (P): 0.079%, sulfur (S): 0.30%, aluminum (Al ): 0.003 or less, bismuth (Bi): 0.14%, boron (B): 80PPM, nitrogen (N): 33PPM was used to prepare a wire rod using a steel sheet (Billet). At this time, the manufacturing conditions of the present invention material (1-3) after heating the temperature in the range of 1100-1250 ℃, the rolling temperature of the material in the double inverted roughing mill is rough rolling in the hot embrittlement temperature range (900-1050 ℃) Roll reduction rate of 1-2 pass (# 1- # 2Stand) is less than 10% for rolling reduction rate, and 3-5 pass (# 3- # 5Stand) for 10-20% rolling reduction rate Was carried out.

In addition, the comparative material (4-6) is 20% by 1-2 pass (# 1- # 2Stand) reduction rate of the rolling mill (Stand) of the rough rolling in the hot embrittlement temperature range (900-1050 ℃) of the material With the reduction of rolling ratio in the following range, 3-5 passes (# 3- # 5Stand) were wire-rolled at a reduction ratio of 10-30%. And both the present invention and the comparative material was subjected to wire rolling with a rolling mill reduction rate of 15% or more after rough rolling. The invention material and the comparative material thus produced were examined for surface flaws or cracks in the wire rod product, and the results are shown in Table 1 below.

At this time, the measurement of the heating temperature was measured by the surface of the surface temperature of the billet (Billet) in the heating furnace by a radio thermometer, and the rolling temperature of the material by each rolling mill to the rolling mill rolling in the hot embrittlement temperature range (900-1050 ℃) The measurement of rolling reduction ratio was calculated by measuring the width and height of the raw materials using calipers at the exit side of each rolling mill.

In addition, the measurement method of the occurrence of the flaw was measured by visual observation of the surface of the product subjected to the wire rolling, and the surface flaw or crack found by the visual observation was evaluated whether or not the flaw regardless.

division Heating temperature (℃) Reduction rate of rolling reduction per pass of rough rolling mill (%) Fault Occurrence (X: No O: O) 1 pass 2 pass 3 pass 4 pass 5 pass Invention One 1231 10 10 16 19 16 X 2 1215 9 10 19 17 15 X 3 1192 8 9 13 18 20 X Comparative material 4 1236 12 17 15 16 18 O 5 1225 8 10 12 25 18 O 6 1197 10 15 16 22 24 O

As shown in Table 1, in the case of the comparative material (4-6), the reduction rate for the rolling mill (Stand) of the rough rolling is a reduction ratio of 20% or less in the 1-2 pass, and 10-30% in the 3-5 pass. As a result of rolling the wire at a reduction rate of rolling, surface scratches or cracks occurred in the wire rod.

On the other hand, the present invention material (1-3) is a rolling reduction rate of the rolling mill (Stand) of 1-2 rolling in the rolling reduction rate of 10% or less, 3-5 passes in a 13-20% rolling reduction rate of the wire rod rolling As a result, surface flaws or cracks in the wire rod product could not be observed.

As described above, according to the present invention, the wire rod product does not have any surface defects or cracks after the wire is rolled, and does not heat the billet to 1300 ° C., which exceeds the capacity limit of the furnace, and does not reheat when passing the rough mill. Not only was it possible to manufacture bismuth (Bi) -sulfur (S) -based free-cutting steel wires without surface flaws or cracks, but it was also possible to manufacture wire rods with excellent high-temperature ductility that would significantly reduce manufacturing costs due to no defects. There is.

Claims (1)

By weight% (PPM), carbon (C): 0.10% or less, silicon (Si): 0.05% or less, manganese (Mn): 1.20-1.40%, phosphorus (P): 0.07-0.09%, sulfur (S): 0.27-0.33%, aluminum (Al): 0.01% or less, bismuth (Bi): 0.05-0.15%, boron (B): 70-130PPM, and the brittle made of remaining Fe and other unavoidable impurities 1100-1250 In the method of hot wire rolling including rough rolling, intermediate sand rolling, filament rolling, and the like, the rough rolling is performed in a rolling reduction ratio of 10% or less in a 1-2 pass, followed by a 10- 3-5 pass. A method for hot wire rolling of bismuth (Bi) -sulfur (S) -based free-cutting steel which is free from surface defects, characterized by a 20% rolling reduction ratio.

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