JPH0121223B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a free-cutting mold steel used with a medium hardness of H _R C of 40 to 47, particularly 43 to 46.

[Conventional technology]

When mold steel used for die plates, drawing dies, punching dies, die-casting molds, etc. contains free-cutting components, the A-based inclusions that are formed are deformed into linear shapes by plastic working. As a result, initial fracture is likely to occur due to stress concentration at the acute corners of inclusions, resulting in low toughness.

[Problems to be Solved by the Invention] The purpose of the present invention is to solve the above-mentioned problems and realize a free-cutting mold steel that has higher machinability than known products and has excellent toughness. It's about doing.
Specifically, the shape of nonmetallic inclusions generated from free-cutting components is made spherical to increase impact resistance and toughness, pre-hardened to medium hardness to facilitate complex machining, and deformation caused by heat treatment after machining. An object of the present invention is to provide a free-cutting mold steel that prevents the above problems.

[Means to solve the problem]

The free-cutting mold steel of the present invention basically has C:
0.2-0.8%, Si: 0.1-1.5%, Mn: 0.4-1.5%,
Cr: 1.0~6.0%, Mo: 0.1~1.0% and N: 0.02
~0.3% as the basic component, S as the free-cutting component:
Contains 0.05~0.3% and Te: 0.03~0.3%, with the remainder consisting of Fe and inevitable impurities, and has a hardness of H _R
C is in the range of 40-47, especially 43-46. For the above alloy composition, Ni: 0.3~
1.5% can be added. In order to adjust the hardness to a range of 40 to 47, particularly 43 to 46 in H _R C, heat treatment may be performed prior to machining to bring it into a pre-hardened state.

[Effect]

In the present invention, the reason why each alloy component is set in the above composition range is as follows. C: 0.2-0.8% It is effective in combining with carbide-forming elements such as Cr, Mo, V, Nb, Zr, and Ti to form composite carbides and improving the wear resistance necessary for tools, and It is necessary to form a solid solution in the base and impart hardness to it. If the content is less than 0.2%, this effect will not be sufficiently exhibited, and the necessary hardness will not be obtained by tempering. On the other hand, if the content exceeds 0.8%, the tempering softening resistance decreases, large inclusions appear, and the mirror finish deteriorates. Si: 0.1-1.5% This is an effective element for extending fatigue limits. Also,
Increases softening resistance in the temperature range of 200-300℃.
These effects cannot be obtained at 0.1% or less. If it exceeds 1.5%, the mold temperature will increase and machinability will decrease due to a decrease in thermal conductivity. Mn: 0.4-1.5% Reacts with S to form MnS. It goes without saying that this MnS contributes to improving machinability. However, Mn stabilizes austenite and significantly lowers the martensitic transformation point. If the content is less than 0.4%, the formation of MnS will not be completed, and excess S will react with Fe to form FeS with a low melting point, so a minimum content of 0.4% is required. If it exceeds 1.5%, the martensite transformation point will drop by about 80°C, the amount of retained austenite will increase, and dimensional changes will occur over time. In addition, the work hardening ability increases, which adversely affects machinability, so the upper limit was set at 1.5%. Cr: 1.0-6.0% This is an essential component for imparting oxidation resistance to steel, but if it is less than 1.0%, the effect is insufficient and the necessary tempering hardness cannot be obtained.
On the other hand, when it exceeds 6.0%, the carbide reaction shifts to the low temperature side, reducing temper softening resistance and impairing toughness. Cr _is also _M7O3
type of giant eutectic carbide. Since this carbide has an angular shape, when stress is applied from the outside during use, stress concentration occurs at the corners as a notch effect, and cracks are likely to occur there. For this reason,
The Cr content was determined to be in the range of 1.0 to 6.0%. Mo: 0.1-1.0% Creates fine carbides and strengthens them by dissolving them in the matrix, improving wear resistance and heat check resistance. When the Cr content is 2% or more, the tempering softening resistance improves when the Mo addition amount is 0.1% or more, but the effect is saturated when it exceeds 1%, so the Mo composition range is 0.1%.
~1.0%. N: 0.02-0.3% Like C, it reacts with elements such as Cr, Mo, V, Nb, Zr, and Ti to form nitrides, and is effective in improving wear resistance and preventing coarsening of crystal grains. There is. If the addition amount is less than 0.02%, most of the N will be in the form of carbonitrides, so this effect cannot be expected, and if it exceeds 0.3%, carbonitrides will grow enormously at the triple points of the grain boundaries.
Decrease toughness. S: 0.05~0.3% Te: 0.03~0.3% Both are important elements as components that impart free machinability.
It is necessary to add both elements in a composite state. These combine with Mn to form a Mn-sulfotellide solid solution Mn (S, Te), which becomes inclusions and is uniformly distributed in the matrix, improving machinability. Since this inclusion is harder than the MnS type, it is difficult to deform during plastic working of the base material, and only becomes oval or oval. In known similar free-cutting steels, soft MnS inclusions are the main component.
During plastic working, it stretches into a long thread-like shape with a sharp edge, so when stress is repeatedly applied and removed from the outside, a notch action occurs and premature failure occurs. On the other hand, when S and Te are added in combination, Mn (S,
Te) has a nearly spherical shape, so there are no sharp edges in the inclusions, making them less likely to become a starting point for cracks. Therefore, toughness can be improved. This inclusion shape also affects machinability.
Gives better results than thread-like ones such as. In order to obtain egg-shaped inclusions and to prevent cracking during forging, S: 0.05 to 0.3%;
Te: Composite addition within the range of 0.03 to 0.3% is required. Ni: 0.3 to 1.5% Added when desired to improve toughness and hardenability. This effect cannot be obtained if the content is less than 0.3%, and on the other hand, if the content exceeds 1.5%, the retained austenite becomes stabilized, the carbide generation reaction is delayed, and machinability decreases.

【Example】

Steels of the present invention (Nos. 1 to 4) and conventional steels (Nos. 5 and 6) having the compositions shown in Table 1 were prepared, the hardness was adjusted by heat treatment, and an impact test was conducted. The result is
They are also shown in Table 1. A drilling test was conducted on the steel of the present invention and the conventional steel using a SKH51 3mmφ straight shank drill. The results are shown below. The test conditions are rotation speed
1480 rpm, feed rate 0.067 mm/rev., and number of tests was 5 times.

[Table] As is clear from this result, the steel of the present invention has a drilling test result of 2 to 5 at high hardness compared to conventional steel.
Since it is twice as good, it can be easily machined when making molds for cold and warm working. Therefore, a die plate, a drawing die, a punching die, and a zinc die-casting mold were manufactured using the steel of the present invention and the conventional steel. The life of each mold is shown in Table 2. As is clear from this table, the life of molds made using the steel of the present invention is 1.5 to 3 times longer than molds made from known materials.

【table】

【table】

【Effect of the invention】

The free-cutting mold steel of the present invention has higher machinability than known mold steels, and also has improved impact resistance and toughness by adjusting the form of inclusions. This mold steel is supplied as pre-hardened steel and is H _R C40
It has a medium hardness of ~47 and has no difficulty in machining into complex shapes.

Claims (1)

[Claims] 1 C: 0.2-0.8%, Si: 0.1-1.5%, Mn: 0.4-0.4%
The basic components are 1.5%, Cr: 1.0-6.0%, Mo: 0.1-1.0% and N: 0.02-0.3%, and free-cutting components S: 0.05-0.3% and Te: 0.03-0.3%, with the balance A free-cutting mold steel consisting of Fe and unavoidable impurities, with a hardness in the range of H _R C40 to 47. 2 C: 0.2~0.8%, Si: 0.1~1.5%, Mn: 0.4~
In addition to 1.5%, Cr: 1.0-6.0%, Mo: 0.1-1.0% and N: 0.02-0.3%, Ni: 0.3-1.5% is the basic component, and S: 0.05-0.3% and Te are free-cutting components. : Free-cutting mold steel containing 0.03 to 0.3%, the balance consisting of Fe and unavoidable impurities, and having a hardness in the range of H _R C40 to 47.