KR19990047911A - Bismuth-sulfur free cutting steel and its manufacturing method - Google Patents

Abstract

The present invention relates to a bismuth-sulfur free cutting steel and a method for manufacturing the same, and an object thereof is to adjust the free cutting steel and austenite grain size of excellent physical properties to 60 micrometers or less to improve the high temperature ductility at 900 ° C to 60% or more. It is to provide a Bi-S-based free cutting steel manufacturing method.

The present invention for achieving the above object in weight%, carbon: 0.05-0.15%, manganese: 0.5-2.0%, sulfur: 0.15-0.40%, phosphorus: 0.01-0.10%, oxygen: 0.003-0.020%, bismuth: 0.03-0.30%, silicon: 0.01% or less, aluminum: 0.0009% or less, consisting of the remaining iron and inevitable impurities, and among the inclusions, the shear face fraction of the MnS inclusions and the MnS inclusions with bismuth adsorbed is 0.5% -2.0 %, And a bismuth-sulfur free cutting steel having a shear plane fraction of metallic bismuth inclusions of 0.030% -0.30%,

The present invention relates to a method for manufacturing Bi-S-based free-cutting steel for controlling austenite grain size of 60 µm or less in hot rolling and wire rolling using 850-1000 ° C.

In the manufacturing method of Bi-S-based free-cutting steel to control the grain size of austenite below 60㎛ during hot rolling and wire rolling using the steel containing titanium: 0.02-0.1% and nitrogen: 0.0030-0.0100% Let that point be about that.

Description

Bismuth-sulfur free cutting steel and its manufacturing method

The present invention relates to a Bi-S-based free-cutting steel used in a tool for machining a part, and a manufacturing method thereof, and more particularly, to a free-cutting steel having excellent machinability and a Bi-S-based free-cutting steel having excellent high temperature ductility during manufacturing. .

Free-cutting steel is a general-purpose material for processing small precision parts such as cameras and watches requiring excellent machinability. In recent years, lead (Pb) -sulfur (S) -based free-cutting steel has been developed in which sulfur (S) -based free-cutting steel is added with elements that improve machinability, such as lead (Pb), as production increases in automobiles, watches, and cameras. It has been put to practical use as a steel suitable for high speed and automation in cutting. However, because lead oxide is toxic to the human body, the steel developed to solve this problem is bismuth-sulfur free cutting steel.

In general, defects on the surface of hot rolled bismuth-sulfur free cutting steel wire can be classified into two types. One of them is a large scab-shaped grain, about 5-10 millimeters in length, with a uniform spacing of more than a few hundred millimeters. Another defect is fine cracks slanted obliquely in the longitudinal direction of the wire rod, which is less than 1 millimeter in length, having a distribution interval of several hundred micrometers and somewhat regular. The scab-like defects are grown through the interface between the liquid bismuth-adsorbed MnS and the matrix structure, pin / blow holes, or grain boundaries among many fine cracks. Therefore, in the related art, it has been dependent on a method of improving the ductility of the matrix structure by controlling the rolling temperature at a high level in order to suppress the formation of fine cracks, which are the origins of defects, or to suppress further growth even if fine cracks are formed. However, maintenance and management of the high rolling temperature is accompanied by problems such as constraints on the rolling equipment, deterioration in productivity.

The present invention is to solve the above problems, and its object is to provide a free-cutting steel of excellent physical properties by controlling the fraction and shape of the inclusions, while also maintaining a typical rolling temperature of about 900-1200 ℃ By controlling the reheating temperature, the austenite grain size is adjusted to 60 micrometers or less to improve the high temperature ductility, and the titanium content is 0.02-0.1% and the nitrogen content is 0.0030-0.0100% to control the austenite grain size. It is to provide a Bi-S-based free-cutting steel manufacturing method for adjusting the micrometer or less to improve the high temperature ductility of 900 ℃ to 60% or more.

Generally, free-cutting steel improves machinability by interposing non-metallic or metallic inclusions in steel, and typical non-metallic inclusions are MnS, and metallic inclusions are low melting metals such as lead and bismuth having little solid solution in steel. These inclusions act as a stress concentration source during cutting to facilitate the generation of voids and crack growth at the interface between the inclusions and the iron, reducing the force required for cutting, and also softening or melting by the cutting heat. It acts as a lubricant at the interface between the chip and the cutting tool, thereby suppressing tool wear and reducing cutting force.

Sulfur added to form MnS is supersaturated if the content of manganese is insufficient or if solidification or cooling occurs rapidly even if the content of manganese is sufficient. In continuous casting, the cast steel has a metal-coagulated structure of a certain thickness, and then a columnar tablet is formed therein, and MnS is crystallized between the columnar tablets. However, in a rapidly solidified tissue called chil, sulfur is not defined as MnS and is supersaturated. Sulfur supersaturated in the seven tablets precipitates as MnS during cooling or reheating, and the remainder precipitates as FeS which segregates at grain boundaries and causes hot brittleness. In addition, the seven tablets supersaturate not only sulfur but also gas components with little solubility in the steel, so that these gas components exceeding the solid solubility in solids remain as fine bubbles (pin / blow holes).

On the other hand, bismuth, which is useful for cutting, has little solid solubility in steel and has a low melting point, so that in general carbon steel, which intentionally does not produce MnS inclusions at ordinary hot rolling temperatures, it is segregated into a very thin film shape at grain boundaries. In this case, the free-cutting steel in which MnS inclusions are intentionally produced is crystallized by adsorption to MnS inclusions and segregated in the liquid state at the interface between MnS inclusions and iron, or in austenite grain boundaries, causing hot embrittlement. The liquid bismuth present at the interface between the MnS inclusions and the iron is mainly at 900-1000 ° C, and the liquid bismuth at the austenite grain boundary mainly acts as a factor to reduce the ductility at 800-900 ° C. In particular, the degradation of ductility at 800-900 ° C is directly related to the surface defects during hot rolling. Therefore, the increase in the grain boundary area due to grain refinement can suppress the grain boundary embrittlement caused by bismuth while relatively reducing the fraction of the grain boundary area in which bismuth is segregated. In addition, only by miniaturization of the grain itself, the size of the voids generated at the triple point of the grain boundary becomes small, and recrystallization is facilitated, thereby improving ductility. Such fine grain refinement to improve the high temperature ductility is possible by suppressing the growth of the grain when reheating. Therefore, it is possible to control the reheating temperature to suppress the growth of the grains or to recrystallize the grains, even if the reheating temperature is high. It can be improved.

Starting from the theory and research results as described above, the present invention is expressed in weight% of carbon: 0.05-0.15%, manganese: 0.5-2.0%, sulfur: 0.15-0.40%, phosphorus: 0.01-0.10%, oxygen: 0.003- 0.020%, bismuth: 0.03-0.30%, silicon: 0.01% or less, aluminum: 0.0009% or less, and the shear fraction of MnS inclusions and MnS inclusions with bismuth adsorbed among the inclusions, which is composed of the remaining iron and unavoidable impurities a bismuth (Bi) -sulfur (S) free cutting steel having a section area fraction (%) of 0.5-2.0% and a shear face fraction of metallic bismuth inclusions of 0.030-0.30%;

In addition, the present invention is subjected to hot rolling and wire rolling using the steel of the composition as described above, the reheating temperature is 850-1000 ℃ Bi-S-based free-cutting steel to control the austenite grain size below 60㎛ It relates to a manufacturing method of;

In addition, the present invention uses a steel material containing titanium: 0.02-0.1% and nitrogen: 0.0030-0.0100% in the steel composition as described above Bi to control the grain size of austenite during hot rolling and wire rolling to 60㎛ or less It relates to a method for producing -S-based free cutting steel.

Next, the reason for limiting the composition of the steel used in the present invention will be explained, and the conditions for inclusion of the inclusions will be described in terms of ensuring machinability.

The carbon (C) should be added more than 0.05% to secure the surface roughness and mechanical properties of the workpiece. However, if it exceeds 0.15%, the machinability is inferior due to the increase in the hard pearlite phase.

The manganese (Mn) should be added at least 0.5% in order to secure the required amount of MnS inclusions, and to suppress hot embrittlement due to the generation of FeS at the grain boundaries during hot rolling. However, in excess of 2.0%, the hardness of the steel is increased, resulting in a decrease in machinability.

The phosphorus (P) should be 0.01% or more to improve the surface roughness of the workpiece, and should not exceed 0.1% to ensure mechanical properties and cold workability.

Sulfur (S) should be added at least 0.15% to form MnS inclusions for improving the surface roughness of the workpiece by growth inhibition of the BUE (Built-Up-Edge). However, in order to ensure hot workability and cold workability, it should not exceed 0.4%.

The oxygen (O) should be added at least 0.003% in order to prevent machinability from being degraded due to the stretching of the MnS-based inclusions during hot rolling. However, it should not exceed 0.020% to ensure plastic deformation of MnS inclusions during cutting.

Since bismuth (Bi) is adsorbed by bismuth alone or MnS in steel, it acts as a stress concentration source during cutting, thereby reducing chip curvature and improving chip treatability. Moreover, bismuth (Bi) has an effect of improving the surface roughness of the workpiece by increasing the area of the MnS inclusion layer on the cutting tool surface. In addition, bismuth is melted in the cutting heat, thereby reducing friction between the chip and the cutting tool and suppressing wear of the cutting tool. Therefore, less than 0.02%, the machinability is inferior. However, if it is more than 0.30%, hot workability becomes very bad.

The silicon (Si) produces SiO ₂ forming MnS inclusions and composite inclusions. Since the plastic deformation of such composite inclusions is very bad, the formation of MnS-based inclusion layers at the end of the tool can be hindered to easily generate BUE, which adversely affects the surface roughness of the workpiece. Therefore, the content of silicon should be kept as low as possible to not exceed 0.003%.

The aluminum (Al) forms Al ₂ O ₃ , Al ₂ O ₃ is easy to form MnS inclusions and composite inclusions. Since the composite inclusion has a very poor plastic deformation capability as with the SiO ₂ composite inclusion of MnS, it hinders the formation of the MnS-based inclusion layer at the end of the tool, thereby easily forming the BUE, and thus adversely affects the surface roughness of the workpiece. Al ₂ O ₃ is also very hard, which promotes wear of cutting tools. Therefore, the content of aluminum should not exceed 0.0009%.

The titanium (Ti) may be dissolved in steel or combined with nitrogen and carbon to precipitate TiN, Ti (CN), TiC, or the like. Precipitates at high temperatures in order of TiN, Ti (CN), and TiC. In other words, TiN is reclaimed at the highest temperature. The exact restocking temperature of TiN is determined by the solubility of Ti and N, but it is generally known to remain at temperatures above 1200 ° C. Therefore, in order to suppress austenite grain growth during reheating, it is advantageous to use TiN which is stable even at a higher temperature. When titanium is added at 0.02% or less, the TiN inventory temperature is low, so it is not effective for suppressing grain growth. When titanium is added at 0.1% or more, it is effective for inhibiting grain growth by TiN, but the grain boundary is relatively hardened by TiN. It becomes weaker and deteriorates hot ductility.

The nitrogen (N) is precipitated in the steel to which titanium is added, such as TiN, Ti (CN) to refine the crystal grains at high temperature, and precipitate hardening at room temperature. Therefore, in order to use TiN in order to refine the grains, an appropriate nitrogen concentration is required. If too much is added, age hardening may be caused by interaction with an electric potential, thereby causing room temperature embrittlement.

In the present invention, the fraction of inclusions is limited in order to obtain excellent physical properties.

If the shear surface fraction of the MnS inclusions adsorbed with MnS and bismuth is 0.5% or less, the machinability is poor, and the high temperature embrittlement mechanism transitions from interfacial embrittlement of grain iron and MnS to grain boundary embrittlement. If it is 2.0% or more, hot workability will fall. If the shear face fraction of the metallic bismuth inclusion is 0.030% or less, the machinability is inferior, and if it is 0.30% or more, the hot workability is inferior.

In addition, if the size of the inclusion is too small, the role of stress concentration source is reduced, there is no cutting effect and may not serve as a source of voids during hot processing. Too coarse can degrade machinability and hot workability. In addition, if the shape ratio of the inclusions (L / W, L: length of the inclusions, W: width of the inclusions) is too large or too small, the machinability is degraded. If the aspect ratio is too large, the shape ratio of the inclusions may not serve as a source of voids at high temperature deformation. Can be. Therefore, in the present invention, the MnS inclusions to which the MnS and Bi are adsorbed are preferably 5-20 µm in length and 1-10 µm in width.

Next, in manufacturing the above-mentioned free cutting steel, a free cutting steel manufacturing method excellent in high temperature ductility will be described in detail.

Usually, Bi-S free cutting steel is hot rolled at 900 degreeC or more. There is no fear of surface defects because the high temperature ductility is over 60% at rolling temperature above 1000 ℃, but if the rolling temperature is reduced below 900 ° C, high temperature ductility is below 60%, so there may be surface defects during hot rolling. have. Therefore, in order to suppress the occurrence of surface defects during hot rolling, it is necessary to secure the high temperature ductility of the rolled material to 60% or more even at a rolling temperature of about 900 ° C.

In the present invention, by using the steel as described above in the first method to limit the reheating temperature for rolling to 850-1000 ℃ range, by adjusting the austenite grain size below 60㎛, high temperature ductility to 60% or more To secure. At this time, the holding time at the reheating temperature is preferably 1 minute or more. If the reheating temperature does not reach 850 ° C., the rolling temperature is too low to reduce ductility, and if it exceeds 1000 ° C., grains grow.

In the present invention, by using a steel material containing 0.02-0.1% of titanium and 0.0030-0.0100% of nitrogen in the steel component as described above, by adjusting the grain size of austenite to 60 µm or less, To secure more than 60%.

In order to predict the hot embrittlement, when measuring the fracture rate of high-temperature tensile specimen, the fracture material with the fracture reduction rate of 60% or more is observed. Bismuth produces voids at the interface between MnS inclusions and iron, and the voids Accompanied by this, ductile fracture occurs, in which it grows and coalesces with other adjacent grown voids. In fracture materials with a fracture reduction rate of 60% or less, voids are formed at the interface between Bi-adsorbed MnS inclusions and the ferrous iron and at the austenite grain boundaries, but these voids develop into cracks, rather than growth accompanied by plastic deformation, and other cracks adjacent to the cracks. And brittle fracture occurs. In the case of a fracture material having a fracture reduction rate of 60% or less, the grain boundary fracture rate increases as the fracture reduction rate decreases. Therefore, the fracture surface reduction rate of the tensile test specimen, the temperature at which 60% appears, can be determined as a criterion for hot embrittlement in which surface defects occur during hot rolling, and the fracture reduction rate is used as a criterion for quantitative evaluation of high temperature ductility.

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

Example

To prepare a specimen having the components shown in Table 1 below. At this time, the volume fraction of MnS inclusions adsorbed with MnS and bismuth is 1.5-2.0%, and the volume fraction of only bismuth inclusions is 0.15-0.2%. The inclusions were 5-15 micrometers long and 5-15 micrometers wide, with a length-to-width ratio of 1-2. The area fraction of bismuth inclusions was calculated from scanning electron microscope (SEM) images (200x magnification).

Steel grade Chemical component (wt%) Area fraction of inclusions (%) Length (L) and width (W) of inclusions, (micrometer, μm) C Si Mn P S Bi T [O] Ti N MnS + Bi Bi A 0.084 0.01 or less 1.20 0.08 0.32 0.10 0.0110 0.020 0.0062 1.5-2.0 0.15-0.2 L = 5-15W = 5-15L / W = 1-2 T1 0.099 " 1.25 0.08 0.27 0.099 0.0100 0.021 0.0048 T2 0.095 " 1.24 0.06 0.29 0.10 0.0095 0.025 0.0050 T3 0.10 " 1.24 0.08 0.28 0.096. 0.0097 0.034 0.0005 T4 0.091 " 1.25 0.09 0.30 0.11 0.0105 0.050 0.0056 T5 0.091 " 1.27 0.08 0.29 0.10 0.0102 0.12 0.0060

Tensile tests were performed by heating the specimens using steel grade A shown in Table 1 under reheating conditions as shown in Table 2 below. The fracture rate reduction rate for each temperature was obtained, and the results are shown in Table 2 below as high-temperature ductility. Also, the grain size of austenite was measured and the results are also shown in Table 2 below.

Steel grade Reheating Condition Austenitic grain size (micrometer, ㎛) High temperature ductility (%) Temperature (℃) Minutes 800 ℃ 900 ℃ 950 ℃ 1000 ℃ 1100 ℃ A 1250 3 121 49 38 67 72 83 1150 3 86 - 46 - - - 1050 3 79 - 47 - - - 900 3 54 - 67 - - - 900 One 52 - 75 - - - 900 10 55 - 68 - - -

As can be seen in Table 2, when the austenite hard crystal grain size is 121 micrometers, the high temperature ductility is less than 60% at a temperature of 900 ° C or less, so that a surface defect occurs when hot rolling at 900 ° C. However, when the reheating temperature is controlled to make the austenite grain size smaller than 55 micrometers, the hot ductility is increased to 60% or more, and surface defects are not generated even when hot-rolled at 900 ° C.

Tensile tests were performed by heating the specimens using the steel grades shown in Table 1 under reheating conditions as shown in Table 3 below. When the tensile test of the material having the austenite grain size and each of the austenitic grain size was measured, the fracture rate reduction rate was determined, and the results are shown in Table 3 below.

Steel grade Reheating Condition Austenitic grain size (micrometer, ㎛) High temperature ductility (%) Temperature (℃) Minutes 800 ℃ 900 ℃ 950 ℃ 1000 ℃ 1100 ℃ A 1250 3 121 49 38 67 72 83 59 44 81 87 88 90 T1 T2 60 43 77 84 86 87 T3 85 42 43 50 56 75 55 45 75 80 82 85 T4 54 41 40 45 55 60 T5

As can be seen in Table 3 above, even when the reheating temperature is high to 1250 ℃, in the steel grades adjusted with titanium and nitrogen (steel grades T1, T2, T4), TiN inhibits the growth of austenite grain size when reheating, thus austenite crystals The particle size is 60 micrometers or less, and the 900 ° C high temperature ductility of the steel grade is 60% or more, so that there is no fear of surface defects during hot rolling. However, even if titanium is added, steel grade with low nitrogen concentration (steel grade T3) does not precipitate TiN, which inhibits the growth of austenite grain size. Therefore, the grain size of austenite is 85 micrometers and 900 ℃ high temperature ductility is less than 60%. Hot rolling at ionic degrees causes surface defects. In addition, even if the nitrogen concentration is adjusted, even if the titanium is excessively added (steel grade T5), even if the precipitate of TiN refines the austenite grain size to 54 micrometers, the precipitate of excessive TiN will refine the austenite grain size to 54 micrometers. Even if the TiN due to excessive precipitation of crystal grains on the grain boundary is too large, the high temperature ductility of 900 ℃ is less than 60% may cause surface defects during hot rolling.

As described above, according to the present invention, in manufacturing the Bi-S-based free cutting steel, by controlling the reheating temperature or by adding an appropriate amount of Ti and N, the free cutting steel having excellent high temperature ductility during manufacturing and excellent physical properties can be obtained with high productivity. The effect is provided.

Claims (4)

By weight%, carbon (C): 0.05-0.15%, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.40%, phosphorus (P): 0.01-0.10%, oxygen (O): 0.003 -0.020%, bismuth (Bi): 0.03-0.30%, silicon (Si): 0.01% or less, aluminum (Al): 0.0009% or less, consisting of the remaining iron (Fe) and unavoidable impurities, among the inclusions contained therein, A bismuth-sulfur free cutting steel, characterized in that the shear face fraction of MnS inclusions and bismuth-adsorbed MnS inclusions is 0.5% -2.0%, and the shear face fraction of metallic bismuth inclusions is 0.030% -0.30%. The method of claim 1, The MnS inclusions adsorbed with the MnS and Bi have a length of 5-20 µm and a width of 1-10 µm. By weight%, carbon (C): 0.05-0.15%, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.40%, phosphorus (P): 0.01-0.10%, oxygen (O): 0.003 -0.020%, bismuth (Bi): 0.03-0.30%, silicon (Si): 0.01% or less, aluminum (Al): 0.0009% or less, hot rolling using a steel material consisting of the remaining iron (Fe) and unavoidable impurities, A method for producing a Bi-S free-cutting steel, characterized in that the reheating temperature is controlled to 850-1000 ° C. to control the grain size of austenite to 60 μm or less. By weight%, carbon (C): 0.05-0.15%, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.40%, phosphorus (P): 0.01-0.10%, oxygen (O): 0.003 -0.020%, Bismuth (Bi): 0.03-0.30%, Silicon (Si): 0.01% or less, Aluminum (Al): 0.0009% or less, Titanium: 0.02-0.1% and Nitrogen: A method for producing a Bi-S free-cutting steel, characterized in that the grain size of austenite is controlled to 60 µm or less during hot rolling and wire rolling using a steel material containing 0.0030-0.0100%.