KR100406386B1 - Method for manufacturing bismuth-sulfur based free cutting steel having superior hot rolling property - Google Patents

Abstract

PURPOSE: A method for manufacturing bismuth-sulfur based free cutting steel having superior hot rolling property is provided which substantially reduces surface cracks in an ordinary rolling manner without extra facility complementation by properly adding boron to constituents of work, thereby improving hot rolling property of the work. CONSTITUTION: The method comprises the steps of controlling dissolved oxygen in molten steel contained in tundish to 5 to 20 ppm and dissolved hydrogen to 10 ppm or less; obtaining a bloom comprising 0.03 to 0.15 wt.% of carbon, 0.03 wt.% or less of silicon, 0.90 to 1.40 wt.% of manganese, 0.060 to 0.120 wt.% of phosphorus, 0.250 to 0.350 wt.% of sulfur, 0.10 to 0.20 wt.% of bismuth, 30 to 80 ppm of boron, 0.010 wt.% or less of Al, 30 to 100 ppm of nitrogen and a balance of Fe and other inevitable impurities by continuously casting the controlled molten steel; obtaining a billet by rolling the heated bloom after heating the obtained billet to a temperature of 1,200 to 1,300 deg.C; heating the obtained billet in the temperature range of 1,150 to 1,250 deg.C; rolling the heated billet into wire rod in the temperature range of 950 to 1,100 deg.C; and coiling the wire rod in an ordinary manner.

Description

Manufacturing method of bismuth-sulfur free cutting steel excellent in hot rolling property

TECHNICAL FIELD The present invention relates to a method for producing free cutting steel used for precision mechanical parts and the like, and more particularly, to a method for producing a bismuth-sulfur (Bi-S) free cutting steel having excellent hot rolling property.

Bismuth-sulfur (Bi-S) free-cutting steels are usually used after cold drawing or annealing, after cutting, cutting and cutting according to the end use. They are mainly used for cutting parts for precision machinery parts such as automobiles and watches.

However, such a bismuth-sulfur free-cutting steel is inferior in ductility in the region of the hot rolling temperature due to the influence of the low-melting point (melting point 273 ° C) bismuth component in the process of producing billets or wire-rods by hot rolling It is very difficult to suppress the occurrence of surface cracks.

Surface cracks generated in the steel strip cause difficulty in rolling the wire, and surface cracks generated in the wire cause disconnection in the drawing process and surface defects of the final machined parts, making it difficult to use them for the required applications. You must prevent it.

Therefore, it is a very important problem to improve the high-temperature ductility in the hot-rolling temperature range to produce a steel excellent in hot-rolling property without surface cracking.

In order to prevent such defects, it is necessary to maintain the rolling reduction temperature at the high temperature during the rolling of the wire rod and to adjust the pass schedule during the rolling to secure a proper reduction ratio. However, in general rolling billet rolling, And it is almost impossible to manufacture without modification of equipment in view of energy saving and manufacturing cost.

On the other hand, as a conventional technique for improving the hot rolling property of free cutting steel to which machinability improving elements such as Pb, Te and Bi are added, there is known a method of breaking the Pb-Bi free cutting steel disclosed in Japanese Patent Application Laid- This method specifies that the rolling temperature is maintained at 1050 ° C or higher and hot rolled, and the reduction rate and the cooling rate are controlled. However, the above method does not consider the method of manufacturing the steel strip and the wire rod as continuous casting boom as the method of manufacturing the broom, and it is almost impossible to maintain the rolling temperature at 1050 ° C or more in the general rolling of the steel strip and the wire rod rolling method. Therefore, it is very difficult to manufacture a uniform piece of wire or wire without surface cracking by applying the above method in the manufacture of general steel wire and wire.

Accordingly, the present inventors have conducted research and experiments to solve the above-mentioned conventional problems, and have made the present invention based on the results. The present invention has been made in view of the above- The present invention aims at producing a bismuth-sulfur free cutting steel in which the generation of surface cracks is remarkably improved by a general rolling method without supplementing another equipment.

1 is a graph showing the rolling properties of the inventive material and the comparative material by temperature

2 is a graph showing the degree of occurrence of surface defects for the inventive material and the comparative material

According to the present invention, there is provided a method for producing a bismuth-sulfur free cutting steel, comprising the steps of: controlling dissolved oxygen in molten steel in a tundish to 5-20 ppm and dissolved hydrogen in a tundish to 10 ppm or less; The steel according to any one of claims 1 to 3, wherein the carbon steel is 0.03-0.15%, silicon is 0.03% or less, manganese is 0.90-1.40%, phosphorus is 0.060-0.120%, sulfur is 0.250-0.350%, bismuth is 0.10-0.20% A billet consisting of 80 ppm of aluminum, 0.010% of aluminum or less, 30-100 ppm of nitrogen, Fe of the remainder, and other unavoidable impurities is heated to 1200 to 1300 캜 and then the billet is rolled at a temperature of 1150 to 1250 캜 Sulfur-free cutting steel excellent in hot-rolling property, which is obtained by heating at a temperature of 950 to 1100 ° C, and then rolling it by a conventional method.

Hereinafter, the present invention will be described in detail.

First, in order to obtain a free cutting steel having excellent hot rolling property in accordance with the present invention, it is necessary to ensure that there is no defect in the inside of the broom or steel ellipse. To this end, the amount of dissolved oxygen and the amount of dissolved hydrogen in the tundish in the continuous casting There is a need.

In the present invention, the dissolved oxygen in the molten steel in the tundish is limited to 5-20 ppm and the dissolved hydrogen is limited to 10 ppm or less so as to uniformly distribute the inclusions generated by oxygen and hydrogen gas which cause internal defects in the material. When the dissolved oxygen in the molten steel is less than 5 ppm, the shape of the inclusions is uneven. If the dissolved oxygen exceeds 20 ppm, surface defects such as pin holes and blue holes are generated on the surface of the cast steel. If the dissolved hydrogen in the molten steel exceeds 10 ppm, internal cracks are caused.

The reason for limiting the numerical value of the composition of the bloom obtained by the continuous casting under the above conditions will be described as follows.

The reason for limiting the carbon (C) content to 0.03-0.15% is that it is difficult to secure sufficient strength of the material at a content of 0.03% or less, and material ductility at an increase of 0.15% or more because of the increase in strength.

It is preferable to limit the manganese (Mn) content to 0.90-1.40% and the sulfur (S) content to 0.250-0.350%. If Mn is less than 0.90% and S is added in an amount less than 0.250%, formation of MnS inclusions is insufficient and cutting workability is impaired. If Mn is added in excess of 1.40%, the strength is excessively increased to deteriorate cutting workability. If S is added in excess of 0.350%, FeS is formed and cracks are generated during hot working.

The reason why the phosphorus (P) content is limited to 0.060-0.120% and the nitrogen (N) content is limited to 30-100 ppm is that the nitrogen or phosphorus is contained in the ferrite structure at 30 ppm or more and 0.060% or more respectively, Chip chip is easy to crush, but when it is added excessively, material ductility is lowered, so the upper limit is limited.

The Bi content is preferably limited to 0.10-0.20%. When the Bi content is 0.10% or more, the steel is present around the Bi lips or MnS inclusions in the steel, and frictional heat is generated at the tip of the tool during the cutting process, 273 ℃) Lubrication reduces the cutting resistance, reduces tool wear and improves the cutting processability, and if it exceeds 0.20%, it lowers the high temperature ductility and prevents hot brittleness.

It is preferable to limit the boron (B) content to 30-80 ppm. When the boron content is 80 ppm or less, preferential segregation occurs around grain boundaries and nonmetallic inclusions to suppress void formation to improve high temperature ductility. This is because the effective boron amount is small and the ductility improving effect is insufficient.

The silicon (Si) and aluminum (Al) components are combined with oxygen in the steel to form inclusions of silicon oxide (SiO ₂ ) and alumina (Al ₂ O ₃ ) to increase tool wear during cutting, It is preferable to limit the upper limit to 0.03% and 0.010%, respectively.

On the other hand, a billet is produced by heating and then rolling a bloom formed as described above, and a wire rod is manufactured by heating and then rolling the produced billet. At this time, In the occurrence of defects (cracks), the low-melting-point bismuth components existing around the crystal grain boundaries and non-metallic inclusions are present in a liquid phase during hot rolling, causing microcracks on the contact surface with the steel due to plastic deformation, causing surface cracking.

In addition, the oxidized crystal grains during the heating of the broom and the billet may cause microcracks during the rolling process and may be connected to the low melting point Bi particles present in the steel to generate cracks.

The FeS layer oxidized at the surface layer has a melting point of 1180 ° C and forms a process with Fe. The process temperature is very low at 1080 ° C, which is equivalent to cracking at the temperature of the hot rolling, , The microcracks around the inclusions and the fine defects generated by the grain boundary oxidation may cause surface cracks in the steel strip and the wire rod due to a sudden drop in temperature during the rolling process and a lack of ductility during high-tension reduction rolling.

Therefore, in the present invention, the rolling temperature and the ash time are limited to the appropriate range in order to minimize the drop in temperature and the calculation of the billet during the rolling of the billets and wire rods.

Specifically, the heating temperature for the broom formed according to the present invention is 1200 to 1300 ° C, and the heating temperature of the billet is preferably 1150 to 1250 ° C because, due to the sufficient degree of cracking of the blank, Or high-temperature ductility by high-temperature rolling at 950-1100 deg. C in rolling the wire rod to prevent the occurrence of surface defects, thereby preventing elongation of nonmetallic inclusions in the final wire rod, thereby improving machinability. If the heating temperature is out of the above range, if the heating temperature is lower than the above range, a rapid deterioration of the ductility due to the subsequent lowering of the rolling temperature leads to a decrease in surface defects. On the other hand, And the surface oxidation and grain boundaries of the material may be dissolved to cause surface defects on the wire.

The wire rolling of the present invention is preferably carried out at a high temperature of about 950-1100 DEG C because if the wire rolling temperature is lower than 950 DEG C, surface cracking during rolling is facilitated due to a drastic decrease in ductility, Surface defects due to surface oxidation and crystal grain boundary melting are caused, and the refractory of the heating furnace is also damaged.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

A molten steel having a composition as shown in Table 1 below was continuously cast to obtain a steel part room (250x330 mm). The above steels were reheated and rolled by rolling under the conditions shown in Table 2 below to produce a piece of steel (160x160 mm). Each piece was heated and held again, followed by hot rolling and winding to produce a wire rod (5.5 to 34.0 mmØ).

The standard hot rolled tensile specimen was processed in the wire thus manufactured, and the high temperature ductility (section reduction ratio) was measured for each hot rolling temperature zone temperature, and this is shown in FIG.

Surface defects were measured for the rolled steel strip and the wire rod, and the results are shown in Table 3 and FIG. 2, respectively.

[Table 1]

[Table 2]

[Table 3]

As shown in FIG. 1, in the case of the inventive material using the inventive steel having a boron content of 31 ppm and 46 ppm, the high temperature ductility is remarkably improved in the range of 900-1050 ° C., which is the rolling temperature region, as compared with the comparative material.

In addition, as shown in FIG. 2 and Table 3, the incidence and size of surface defects in the inventive material (1-4) are remarkably reduced in comparison with the comparative material (1-4) Able to know.

INDUSTRIAL APPLICABILITY As described above, the present invention can produce bismuth-sulfur free cutting steel excellent in hot rolling property in which the generation of surface defects is remarkably improved by improving the high temperature ductility by adjusting the titration boron content and controlling the production conditions.

Claims (1)

A method of producing a bismuth-sulfur free cutting steel, The molten steel in the tundish is controlled to have a dissolved oxygen concentration of 5-20 ppm and a dissolved hydrogen concentration of 10 ppm or less and a controlled molten steel is continuously cast to form a composition containing 0.03-0.15% carbon, 0.03% silicon or less, manganese: 0.30-1.40% phosphorus: 0.260-0.120% sulfur: 0.250-0.350% bismuth: 0.10-0.20% boron: 30-80ppm aluminum: 0.010% nitrogen: 30-100 ppm nitrogen, the balance Fe and other unavoidable impurities Is heated at a temperature of 1200 to 1300 DEG C and then rolled into a billet, and the resulting billet is heated at a temperature of 1150 to 1250 DEG C, then subjected to wire rolling at a temperature of 950 to 1100 DEG C, By weight based on the total weight of the bismuth-sulfur free cutting steel.