JPH0568528B2 - - Google Patents

Description

[Detailed description of the invention]

Field of Industrial Application The present invention relates to a method for manufacturing case hardened steel that does not cause coarsening of crystal grains called mixed grains or abnormally coarse grains even if carburized at a high temperature of 1000°C or higher. be. Gears used in automobiles, industrial machinery, etc. are manufactured using case-hardened steel bars and wire rods for mechanical structures, and are subjected to forging, cutting, various heat treatments, and finally carburizing heat treatment. This heat treatment for carburizing is a heat treatment that requires a long time of several hours or more, and is a heat treatment that consumes extremely energy. The present invention relates to a method for manufacturing case hardened steel in which coarsening of grains does not occur even if the temperature of the carburizing heat treatment is increased in order to shorten the time of the carburizing heat treatment. Conventional technology In order to shorten the carburizing heat treatment time of case hardening steel, it is effective to increase the temperature of carburizing heat treatment, but increasing the temperature of carburizing heat treatment causes coarsening of crystal grains, increase in hardening distortion, It was not possible to increase the temperature of the carburizing heat treatment because it would cause deterioration of the toughness of the carburized parts. In order to prevent this crystal from becoming coarse, Al, V, Nb, Ti, etc. are added, and the rolling conditions are
Many techniques have been proposed to prevent coarsening of crystal grains by combining heat treatment conditions, etc. (for example, JP-A-61-166922, JP-A-61-
No. 133366, JP-A-60-208413, JP-A-58-
No. 52424). Problems to be Solved by the Invention However, the reality is that no satisfactory technology has yet been established to stably prevent crystal grain coarsening during the complex gear manufacturing process described above. be. Means for Solving the Problems The present invention provides a method for producing case hardened steel that solves the above problems and provides a stable grain coarsening prevention effect. That is, C: 0.13-0.23%, Si: 0.15-0.35
%, Mn: 0.60−1.50%, Al: 0.02−0.100%,
N: 0.0030-0.0200%, Ti: 0.01-0.07%, and
Ti/N: 1.0/3.4, further Cr: 0.35-1.20%,
Mo: Contains 0.15-0.30% of one or two types,
In case-hardened steel for machine structures, the remainder of which is Fe and unavoidable impurities, after being melted in a conventional manner,
Continuous casting enables a temperature range of 1000℃ from the freezing point.
Coarsening of crystal grains during carburizing treatment, which is characterized by casting and cooling into slabs under conditions that result in a cooling rate of 20°C/min. or higher, and then immediately rolling them into steel bars and wire rods without blooming. This is a method for producing case hardened steel that does not cause any hardening. The key point of the present invention is that when casting molten case-hardened steel having chemical compositions limited within a specific range, the cooling rate after solidification is set high and then it is rolled into steel bars and wire rods without blooming. It has been found that by doing so, a stable effect of preventing coarsening of crystal grains can be obtained during the subsequent complicated and wide-ranging gear manufacturing process. Effect The reason why the composition of the steel is limited in the present invention is as follows. C is required to be at least 0.13% in order to obtain the required strength, but it is kept at 0.23% or less in order to ensure the internal toughness of the parts after carburizing. Si acts as a strengthening and deoxidizing element, but 0.40%
Exceeding this will result in greater decarburization during carburizing heat treatment.
0.40% or less. Mn is added in an amount of 0.60% or more to strengthen solid solution and improve hardenability, but if it exceeds 1.50%, toughness decreases. Al is necessary for deoxidizing and suppressing the growth of crystal grains, but if it is less than 0.020%, the effect will not be improved, and if it exceeds 0.100%, alumina-based nonmetallic inclusions will increase rapidly, so the content should be 0.020% to 0.100%. . N is necessary to prevent coarsening of crystal grains by combining with Al and Ti in steel to form AlN and TiN, and if it is less than 0.0030%, the effect will not be enhanced, and
Even if it is contained in a large amount exceeding 0.020%, the effect will be saturated and it is not economically desirable, so 0.0030 to 0.020
%. Ti is necessary because it combines with N in steel to suppress grain growth, and if it is less than 0.01%, it has little effect;
Adding more than 0.07% not only saturates the effect but also reduces the toughness, so the content should be 0.01% to 0.07%. Furthermore, it is important to control the content of Ti and N at a constant ratio. In other words, the Ti/N ratio is
If it is less than 1.0, the effect of preventing crystal grain coarsening will not be sufficient;
On the other hand, if it exceeds 3.4, the amount of TiC precipitated at low temperatures will increase, impairing toughness, so it is set to 1.0 to 3.4. One or two types of Cr and Mo are added for solid solution strengthening and hardenability improvement. Mo is added especially when hardenability is required. Cr is 0.35% or more,
Mo exhibits its effect at 0.15% or more, but 1.20%
Addition of Cr in excess of 0.30% and Mo in excess of 0.30% reduces toughness. When manufacturing steel with a limited composition as described above by the continuous casting method, the solidification conditions are extremely heavy to keep the ti precipitates in a fine state, which plays an important role in preventing coarsening of crystal grains. Therefore, it is important to perform solidification cooling in a temperature range of 1000°C from the freezing point under conditions that result in a cooling rate of 20°C/min or more. Furthermore, the continuously cast slabs obtained in this way need to be rolled into products, such as steel bars and wire rods, without being subjected to blooming. It must be avoided because it does not have the effect of preventing grain coarsening. If it is necessary to improve machinability, well-known machinability improving elements such as S, Pb, Bi,
Even if Ca or the like is added, the effectiveness of the present invention is not impaired in any way. EXAMPLES The present invention will be explained in more detail by giving examples below. Example 1 Steel having the chemical composition shown in Table 1 was melted in a converter, and an 8t steel ingot (average cooling rate of 4 to 1000℃ after solidification) was produced.
℃/min), continuous cast piece with a cross section of 350 x 560 mm (9℃/min)
min), and 160×160mm cross-sectional continuous slab (same 45℃/
min) and cooled. 8t steel ingot and continuous slab of 350×560mm are 160×160mm
After hot blooming rolling, the steel slab was rolled into a steel bar with a diameter of 32 mm. One of the 160 x 160 mm continuous slabs was hot rolled into a 120 x 120 mm slab and cooled, and the other was immediately rolled into a 32 mm diameter steel bar without blooming. These steel bars with a diameter of 32 mm were formed into bevel gear blanks through annealing and cold casting processes, normalized and cut, and then vacuum carburized at 1050°C. This gear was cut in the axial direction, and the presence or absence of coarsening of crystal grains was examined by microscopic observation. As the results are shown in Table 2, no coarsening of crystal grains was observed in the gear produced by the method of the present invention. Example 2 Steel having the chemical composition shown in Table 3 was melted in a converter, and continuous slabs with a cross section of 200 x 200 mm (average cooling rate of 31°C/min until 1000°C after solidification) and a cross section of 160 x 160 mm were produced. It was cast into a continuous cast piece (47℃/min) and cooled.
One of the 200 x 200 mm continuous slabs was hot-blush-rolled into a 160 x 160 mm steel slab, and the other was immediately rolled into a 75-mm diameter steel bar without blooming. On the other hand, the continuous slab of 160 x 160 mm was immediately rolled into a steel bar with a diameter of 75 mm without being subjected to blooming. These steel bars with a diameter of 75 mm were formed into spur gear stock materials by hot forging, followed by normalizing and gear cutting processes, followed by carburizing heat treatment and quenching at 1050°C. After cutting this gear, coarsening of crystal grains was investigated using a microscope. As shown in Table 4, no coarsening of crystal grains was observed in the gears using the case hardened steel of the present invention.

【table】

【table】

【table】

[Table] Effect of the invention As stated above, Al, N, Ti,
By adjusting Ti/N and controlling the cooling conditions after solidification and the subsequent manufacturing conditions, coarsening of crystal grains does not occur even when carburizing heat treatment is performed in an extremely high temperature range.

Claims (1)

[Claims] 1 C: 0.13-0.23%, Si: 0.40% or less, Mn:
0.60−1.50%, Al: 0.02−0.100%, N: 0.0030−
0.0200%, Ti: 0.01-0.07% and Ti/N: 1.0-
3.4, further Cr: 0.35−1.20%, Mo: 0.15−0.30%
Case-hardening steel for machine structures containing one or two of the above, with the remainder consisting of Fe and unavoidable impurities, is melted in a conventional manner and then continuously cast.
It is characterized by casting and cooling into slabs at a temperature range of 1000℃ from the solidification point at a cooling rate of 20℃/min. or more, and then immediately rolling them into steel bars and wire rods without performing blooming rolling. A method for manufacturing case hardened steel that does not cause coarsening of crystal grains during carburizing heat treatment.