JPS6154864B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention includes Al extrusion dies, Al die casting molds,
Used in tools for hot processing dies, such as hot forging dies, and has excellent crack resistance and progress when nitrided, and combined with excellent softening resistance and high-temperature strength, it has a long life. This relates to a tool steel for hot working that provides the following properties. Currently, 5% Cr-based SKD61 and 62 are commonly used as tool steels for hot working, and for applications that require even longer tool life, medium to low Cr-based 3Cr-3Mo steels are used. Steel (such as AISIH10) is used. However, in 5Cr-based SKD61 and 62, the core part that supports the nitrided layer does not have sufficient high-temperature strength or softening resistance, and as a result, in applications where the temperature of the mold surface is high during use, it is difficult to extend the life by nitriding treatment. has reached its limit. On the other hand, 3Cr, which has excellent high-temperature strength at the core,
When applying 3Mo steel, a relatively deep nitrided layer is formed, and for applications where there is little stress concentration due to the shape and where excessive thermal shock does not occur, SKD61 and 62 are used.
However, in the case of Al extrusion dies and other dies with sharp corners, or in applications subject to severe forging stress or thermal shock, the nitride layer may develop prematurely. There is a tendency for sharp cracks to form in the core, which quickly progress to the core and lead to large cracks. In addition, in the case of even lower Cr, sufficient nitrided surface hardness could not be obtained, and there was a limit to the life-improving effect of nitriding. The present invention has excellent nitriding properties such as high nitrided surface layer hardness and high resistance to the occurrence of initial cracks in the nitrided layer and its progress. The core's excellent high-temperature strength, combined with the core's excellent high-temperature strength, allows it to withstand molds with shapes that easily cause stress concentration, high heating temperatures, and severe working stress and cooling conditions. The present invention relates to a new tool steel for hot working that is applicable to various types of hot working molds used, does not cause cracking or settling, and provides a long service life. In other words, the present invention uses Cr as the basis for achieving both excellent nitriding properties and high-temperature strength properties.
Based on the finding that it is necessary to control the percentage to more than 4.00% and less than 4.80%,
4.8Cr-Mo(W)--Medium V is the basic component, and Si, Mn, etc. are added depending on the purpose and application.Excellent nitriding properties due to the above setting of Cr content,
It has excellent softening resistance and high temperature strength due to the appropriate combination of Cr amount, carbide precipitation strengthening element Mo (W), and V amount. From this point, the V amount is medium V0.60 ~ 1.0
% content is an essential condition, and by combining the hardenability improvement effect of Mn addition and the oxidation resistance and surface roughness improvement effect of Si addition, it enables air cooling quenching and further improves usability. It is characterized by giving. Table 1 shows the chemical compositions of the steel of the present invention and the conventional steel.

【table】 Table 2 shows the tempering softening resistance of the steels according to the invention.

[Table] It can be seen that the steel of the present invention has better softening resistance than the conventional steel. This is due to appropriate setting of the amount of Cr and appropriate adjustment of the amount of medium C to medium V and Mo (W). Softening resistance is an important property that is important not only to prevent heat-induced softening during use, but also to prevent core hardness from decreasing due to the heat effects of repeated nitriding treatments. Table 3 shows the high temperature strength of the steel of the present invention. It can be seen that the steel of the present invention is clearly superior in high temperature strength to the conventional steel. This is the setting of Cr content (over 4.00% and below 4.80%)
This is due to moderate adjustment of medium C-medium V and the amount of Mo (W).

[Table] Table 4 shows the hardness behavior of the steel of the present invention upon nitriding. 3Cr-3Mo-0.5V steel (AISIH10) for reference
The test results are also shown. It is recognized that the steel of the present invention has a surface hardness similar to that of conventional steel, and the nitriding depth showing Hv600 is slightly larger than that of conventional steel. On the other hand, 3Cr-3Mo-0.5V steel exhibits a nitriding depth of Hv600 that is nearly twice that of conventional steel.

【table】

[Table] Table 5 shows the bending breaking load of the nitrided bending test piece of the steel of the present invention. The sample was a flat plate, and after being treated with tuftride at 570℃ for 5 hours, it was loaded using a single point loading method in the center.
The breaking load is determined and expressed as a ratio, with that of conventional steel SKD62 set as 100. 3Cr―3Mo― for reference
Test results for 0.5V steel are also listed. It can be seen that the steel of the present invention has a bending breaking load in a range equal to or higher than that of conventional steel or similar to conventional steel.

[Table] This is based on the appropriate setting of the Cr value of the steel of the present invention, and is mainly due to the suppressing effect of excessive internal diffusion of nitrogen during nitriding, and the combined effect with core toughness. . The nitriding properties of the steel of the present invention are particularly important for preventing a decrease in nitriding toughness during repeated nitriding. Table 6 shows the machinability of the steel of the present invention. The tool life in turning is expressed as a ratio, with that of conventional steel set as 100. It can be seen that the steel of the present invention exhibits almost the same machinability as the conventional steel without the addition of Ni. An increase in Mn decreases machinability, and the amount added is limited by this aspect.

[Table] Table 7 shows the seizure critical load in the high temperature seizure resistance test of the steel of the present invention. The specimen was a cylindrical specimen that had been quenched and tempered (H _R C45) → nitrided → air oxidized, and then the end face was pressed against a steel material (counterpart material) heated to 700°C while rotating at high speed. The maximum load (critical load) at which seizure does not occur in the case of steel is calculated, and the value of conventional steel is set as 100, and it is expressed as a ratio.

[Table] It can be seen that the steel of the present invention exhibits a seizure critical load clearly superior to that of the conventional steel. This has a high nitrided surface hardness of the conventional steel class, a slightly deeper nitrided layer than conventional steel,
This is due to the excellent high-temperature strength of the base material that supports the nitrided layer, the lubrication of the oxide film formed on the surface, and the effect of carbides dispersed in the protective base material. Next, the reason for limiting the composition of the steel of the present invention will be described. C maintains the hardenability and quenching and tempering hardness of the steel of the present invention, forms carbides with Cr, W, Mo, V, etc. to refine grains, improve wear resistance, and improves wear resistance during tempering. It is added to form a special carbide that is finer than the matrix and to provide resistance to tempering softening and high-temperature strength. If it is too high, it will reduce the toughness and also the high temperature strength, so it should be 0.45% or less, and if it is too low, the effect of the above addition will not be obtained.
Must be 0.30% or more. A more preferable range of C is 0.35
~0.45%. Si has the effect of improving the oxidation resistance of the steel of the present invention, and is added depending on the purpose and use. If it is too large, it lowers the thermal conductivity and also causes a decrease in ductility, so the content should be 1.20% or less. A more preferable range of Si is 1.0% or less. Mn is added in consideration of the purpose, application, mold dimensions, etc., in order to supplement the hardenability of the steel of the present invention. In general
The standard value is 0.60% or less, and the amount added should be adjusted taking into account the above factors. If it is too large, the machinability will be reduced and the _A1 transformation point will also be lowered, so the content should be 1.20% or less. A more preferable range of Mn is 1.00% or less. The content range of Cr has been determined from the viewpoints of the steel of the present invention, which has excellent nitriding toughness and excellent high-temperature strength properties, as well as imparting hardenability that enables air-cooling hardening, and improving wear resistance due to carbide dispersion. Setting is extremely important. If it is too large, the high-temperature strength properties will deteriorate and the nitriding depth will decrease, making it impossible to maintain the characteristics of the steel of the present invention, so the content should be 4.80% or less. If it is too low, it will lead to a decrease in toughness during nitriding, making it difficult to maintain the most important feature of the steel of the present invention, so it is set to exceed 4.00%. Mo and W are important additive elements that combine with C to form carbides to improve wear resistance, and also form and disperse fine carbides during tempering to impart softening resistance and high-temperature strength. Mo has an advantage over W in terms of toughness, while W has an advantage over Mo in terms of wear resistance and high temperature strength properties.
Depending on the intended use, additives may be added singly or in combination.
If it is too large, coarse carbides are formed and the toughness is lowered, so the content should be 3.50% or less (1/2W+Mo), and if it is too low, the effect of the above addition cannot be obtained, so the content should be 1.50% or more. A more preferable range is (1/2W+Mo)
It is 1.80 to 2.80%. Like W and Mo, V forms fine carbides that are difficult to agglomerate during tempering, and is extremely important for imparting tempering softening resistance and high-temperature strength, especially in high-temperature ranges. Further, V is an essential element for refining crystal grains and improving toughness, and without V, the effects of improving the softening resistance and high-temperature strength of Mo and W cannot be fully exhibited. If it is too large, it will form coarse carbides and cause a decrease in toughness.
The content should be 1.00% or less, and if it is too low, the effect of the above addition cannot be obtained, so the content should be 0.60% or more. As described above, the steel of the present invention has excellent crack resistance and high-temperature strength properties during nitriding, does not cause a decrease in core hardness during repeated nitriding, and is suitable for hot Al extrusion. This is a new hot working tool steel that can be applied to dies and other molds whose shape tends to cause stress concentration, and provides a long service life without cracking or sagging.

Claims (1)

[Claims] 1 C0.30~0.45%, Si1.20% or less, Mn1.20% or less, Cr4.00% or more and 4.80% or less, (1/2W+Mo)
Hot working tool steel consisting of 1.50~3.50%, V0.60~1.00% balance Fe and normal impurities.