JPS6153404B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a hot work tool steel for metal working which has excellent heat crack resistance. For example, when rolling a seamless steel pipe, JIS G is used as a rolling tool steel inserted inside the pipe.
4404D6 type (SKD 6) is used, and the heat treatment methods include slow cooling annealing at 820-870℃, 1000-870℃ slow cooling annealing,
Air-cooled quenching at 1050℃ and air-cooled tempering at 550-650℃ are recommended, but this heat treatment alone does not eliminate grain boundary cracks that occur during the cooling process after rolling, which can become the starting point for cracks during quenching. Alternatively, carbides precipitated at grain boundaries become starting points and new cracks occur during quenching. These cracks appear as thermal cracks when the tool steel is used in a location where it is subjected to thermal cycles, and are the cause of short tool life. The present invention aims to prevent thermal cracks in tools by specifying the heat treatment of hot work tool steel, and to stop the growth of thermal cracks and prevent damage to tools. The present invention is directed to tool steel equivalent to JIS G 4404D class 6, whose chemical composition is C0.20~
0.45wt%, Si0.30~1.20wt%, Mn0.20~0.80wt
%, Cr4.0~0.6wt%, Mo0.30~1.50wt%, V0.30
~1.50 wt%, but may contain other components as long as they do not significantly alter the transformation temperature and structure. The heat treatment of the present invention consists of seven steps from cooling during hot rolling to final tempering. That is, (a) hot rolling is completed at a temperature of 800°C or higher (b) slow cooling is performed from 800°C to 650°C over a period of 180 minutes or more (c) from a temperature of 650°C or lower to 900°C or higher Raising the temperature to a temperature of 1100℃ or less, cooling to a temperature of 200℃ or less and quenching (d) Holding in a temperature range of 750℃ or more and 850℃ or less for 120 minutes or more to refine the carbide and cool it (e) Surface layer (f) followed by cooling and quenching to 200°C or less; (g) heating and tempering to a temperature of 575°C or more and 680°C or less. The reason why rolling is completed at 800°C or higher in (a) is to prevent cracking during rolling and during the cooling process.
The reason for slow cooling in (b) is to prevent cracking during gas cutting during and after cooling. The condition is to keep it below 30%. Condition (c) indicates a practical austenitizing temperature since the Ac ₃ transformation point of this steel is about 880°C, and the upper limit is limited by coarsening of crystal grains. The quenching in this step promotes the refinement or spheroidization of carbides in the next step, but the quenching in this step causes intergranular cracks and quenching cracks due to the gradual cooling in step (b). can be prevented. The Ms point of this steel type is approximately 300°C, and the Mf point is 150°C, so it must be cooled to below 200°C for quenching. The step (d) refines or spheroidizes carbides, but at the same time eliminates intergranular cracks. Since Ac ₃ is approximately 820°C, this temperature range indicates a range suitable for spheroidizing and sintering. Condition (e) is a step in which the surface layer is raised to the austenitizing temperature, for example, the surface layer is rapidly heated using a high frequency heating furnace, medium frequency heating furnace, Matsufuru furnace, etc., and the surface layer is heated in the quenching step of (f). For example depth 10
It is designed to be hardened to mm. In order to prevent grain boundary cracking and quenching cracking in this process, (a) to (d)
Heat treatment is effective. Step (g) is to adjust the hardness of the surface layer to suit the application of hot work tool steel.
The lower temperature limit is the temperature at which the tempering effect is industrially sufficient, and the upper temperature limit corresponds to the hardness of Hv320, which is generally required for use as tool steel, and depending on the use, it is around 700℃ (equivalent to Hv280). can rise to. In addition, in the method of the present invention, steam is injected into the furnace in the final tempering process, which thickens the oxide scale layer, and this scale corrodes the base material in a wave-like manner, making it difficult to peel off and acting as a heat insulating layer during rolling. do. This scale can be generated not only inside the furnace but also by humidifying the cooling bed outside the furnace. Example C: 0.38%, Si: 0.94%, Mn 0.40%, P:
0.008%, S: 0.0006%, Cr: 5.2%, Mo: 0.86
%, V: 0.38%, Al: 0.035% steel treated by the conventional method shown in Table 1 and an example treated by the present invention will be compared. The cross section of the material after rolling is 210
mmφ, and in Example 1, the cooling rate was slightly slower than air cooling, and the structure was bainitic, with intergranular cracks being observed here and there. In other examples, some bainite is present, but no intergranular cracking is observed. After quenching (e), intergranular cracks occur in both Examples 1 and 2 using the conventional method, but no intergranular cracks are observed in the method of the present invention. The thermal crack test is (d)
After the process, the test piece was cut to 100mmφ, (e), (f),
After heat treatment (g), high frequency heating is applied to
℃ was repeated 5000 times, and then judged based on appearance and cross section. In the conventional method, thermal cracks are large, whereas in the method of the present invention, they are minute, with a depth of 0.05 mm or less, which is not enough to cause a practical problem. The heating temperature was determined using a 102mmφ mandrel manufactured in the same manner as the thermal crack test piece.
1200℃, outer diameter 140mm, wall thickness 16mm steel pipe with wall thickness 4.0mm
1.0 below the mandrel surface when rolled in 6 passes up to
The temperature was measured at a position of mm. The maximum temperature at this time is shown in the table, and when steam was blown into the furnace during tempering (g), the temperature was 410°C, which is lower than the others. Furthermore, when the thermal crack test is continued to be strengthened, in the conventional method, the thermal cracks that occur in the hardened layer at the surface progress to the unhardened layer in the center, but as in the method of the present invention, the carbide is spheroidized. As a result, the growth of thermal cracks is stopped at the boundary with the unhardened portion.

[Table] As described above, by applying heat treatment including cooling after completion of rolling of hot work tool steel according to the present invention, thermal cracking of the tool can be prevented and tool life can be improved.
Furthermore, by stopping the growth of thermal cracks, damage to tools can be eliminated and serious accidents can be effectively prevented.

Claims (1)

[Claims] 1 C0.20-0.45wt%, Si0.30-1.20wt%,
Mn0.20~0.80wt%, Cr4.0~6.0wt%, Mo0.30~
The process of heat treatment of steel containing 1.50wt% and V0.30~1.50wt% (a) completing hot rolling at a temperature of 800℃ or higher; (b) heating from 800℃ to 650℃ for 180 minutes or longer (c) Raising the temperature from a temperature of 650°C or lower to a temperature of 900°C or higher and 1100°C or lower, cooling and quenching to a temperature of 200°C or lower (d) Temperature of 750°C or higher and 850°C or lower (e) Heating the surface layer to 900°C or more and 1100°C or less (f) Subsequently cooling to 200°C or less and quenching (g) 575°C or more , a method for producing hot work tool steel with excellent heat cracking resistance, characterized by comprising heating to a temperature of 680°C or less and tempering. 2. The method for producing hot work tool steel with excellent heat cracking resistance according to claim 1, wherein step (g) is carried out in an atmosphere to which H ₂ O is added.