JPH0313543A - Steel metal mold excellent in machinability - Google Patents

Abstract

PURPOSE:To produce a metal mold improved in machinability without causing deterioration in hardness by using a steel stock having specific composition and also having a dual-phase structure as a material for metal mold for molding plastics. CONSTITUTION:As a material for metal mold for molding plastics, a steel which has a composition containing, by weight, 0.05-0.20% C, 0.10-0.50% Si, 0.30-2.00% Mn, <=0.10% P, 0.9-1.5% Cu, 2.1-3.0% Ni, and 0.5-1.5% Al and having a quantitative relationship of Ni/Cu>=2.0 at this time or further containing one or >=2 kinds among <=0.25% S, <=0.004% Ca, <=0.5% Se, and <=0.5% Te is used as a stock and formed into a metal mold by means of hot forging. By allowing the above metal mold to stand to be cooled and then applying ageing treatment to this metal mold, the metal mold having a dual phase structure of ferrite and pearlite, also having about 300 Vickers hardness, and excellent in machinability can be obtained.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to mold steel, and particularly to mold steel suitable for plastic molding. (Prior Art) When long life is required as steel for molds for plastic molding, hardness of around Hv300 is required. As a material having a hardness of around Hv300, for example, Japanese Patent Application Laid-Open No. 63-38557 has been proposed. The steel according to this proposal utilizes precipitation hardening of Cu, Ni, and AM, and by adding Mo for the purpose of improving hardenability, the Hv300 level is secured. However, in the parent structure, a bainite structure is formed due to the presence of Mo added to improve hardenability. This bainite structure generally tends to deteriorate machinability. On the other hand, in recent years, emphasis has been placed on reducing machining costs for the purpose of cost reduction, and there is a demand for good machinability even in Hv300 level materials. The present invention was made in response to these demands, and aims to provide a steel for molds that has a hardness of Hv300 level and has excellent machinability. . (Means for Solving the Problems) The present inventor has conducted intensive research on the composition and structure, considering that the conventional steel described above has the necessary hardness but has a bainite structure that deteriorates machinability. As a result, by adjusting specific components, a two-phase structure of ferrite and pearlite is inevitably formed under the condition of normal solution treatment and aging treatment, and precipitation hardening of Cu, Ni, and AQ results in a Hv30
We have discovered that it is possible to ensure zero-level hardness and improve machinability, and have hereby created the present invention. That is, 1 the present invention has C: 0.05 to 0.20%, Si
:0.10~0.50%, Mn:0.30~2.00%
, P amount 0.1%, Cu: 0.9-1.5%, Ni: 2°1-3.0% and Afi: 0.5-1.5%,
Preferably, the relationship Ni/Cu≧2.0 is satisfied, and if necessary, Ss0.25%, Ca≦0.004%,
One or two of Ss≦0.5% and Te≦0.5%
The gist of this invention is a mold steel with excellent machinability, which is characterized by containing at least 100% of Fe, the remainder consisting of Fe and unavoidable impurities, and having a two-phase structure of ferrite and pearlite. The present invention will be explained in more detail below. (Function) First, the reason for limiting the chemical components in the present invention will be explained. C: C is required to be at least 0.05% in order to improve the hardenability of the steel and ensure the hardness required as a mold steel. However, if it is contained in an amount more than necessary, carbides will increase and have a negative effect on machinability, so the upper limit is set at 0.20%. Si: Si is used as a deoxidizing element, for which 0.10
% or more is required. However, since excessive content deteriorates toughness, the upper limit is set at 0.50%. Mn: Mn is an element necessary to improve hardenability and ensure the hardness required as mold steel. However, if the content is less than 0.30%, the effect is insufficient, and if the content exceeds 2,00%, machinability deteriorates. Therefore, Mn
The amount should be in the range of 0.30-2.00%. P: P is an element useful for improving machinability, but 0.
If the content exceeds 1%, hot workability will be affected. This is not preferable because it impairs toughness. Therefore, the amount of P is 0
．． 1% or less. Cu: Cu is an element necessary for precipitation hardening to ensure hardness, and for this purpose, 0.9% or more is required. However, if added in large amounts, hot workability deteriorates, so
The upper limit is set at 1.5%. Ni: Ni is required to be 2.1% or more in order to combine with AQ to form an intermetallic compound and ensure hardness. However, adding too much will make the hardness higher than necessary.
The upper limit is set at 3.0% because it has a negative effect on machinability.
shall be. AΩ: AQ combines with Ni to form an intermetallic compound. 0.5% or more is required to ensure hardness. However, if excessively added, the hardness becomes higher than necessary and oxide-based nonmetallic inclusions increase, which adversely affects machinability, so the upper limit is set at 1.5%. Although the above elements are essential components, it is preferable to control the Ni/Cu ratio to 2.0 or more, especially in order to prevent deterioration of hot workability due to Cu. Further, in the present invention, one or more of the following elements can be contained in appropriate amounts as necessary. In other words, S, Ca, Ss, and Te are all elements that improve machinability, and in order to demonstrate their performance,
It is necessary to add S at 0.25% or less, Ca at 0.004% or less, and Se and Te at 0.5% or less each. However, when S exceeds 0.25%, segregation becomes significant and homogeneity is lost. Furthermore, since the amount added cannot be determined to improve machinability, it is set to 0.25% or less. Also, Ca. If Sa and Te each exceed the upper limit, the machinability improvement effect cannot be determined depending on the amount added, so Ca
≦0.004%, Se≦0.5%, and Te≦0.45%. The steel of the present invention having the above-mentioned chemical composition inevitably becomes a two-phase structure of ferrite and pearlite when subjected to normal solution treatment and aging treatment, and is hardened by precipitation of Cu, Ni, and Afl.
While maintaining v300 level hardness, machinability can be significantly improved. In addition, the composition system of the steel of the present invention is low in C and does not contain hardenability-improving elements such as Cr and Mo, so hardness does not improve much due to hardening, and therefore, weld cracking etc. occur during welding. There are also advantages that are unlikely to occur. Next, examples of the present invention will be shown. (Example) Steel having the chemical composition shown in Table 1 was melted by a conventional method,
After hot forging to a thickness of 3001111, it was allowed to cool. Furthermore, after heating and holding at 890°C, it was air cooled, and then tempered (aging treatment) at 525°C for 5 hours. The hardness of the obtained samples was examined, and the machinability was evaluated by conducting a machinability test under the conditions shown in Table 2 to examine the cutting length until the tool life. The results are also listed in Table 1. In Table 1, Il! n1~&5 and 415~Na21
is the steel of the present invention, and Nα6 to Nα14 are comparative steels, which are considered as follows. For each of the steels of the present invention, Hv3
00 level hardness can be ensured, and moreover, it has excellent machinability in that the cutting length up to the tool life is more than 1.3 times longer than that of comparative steels. Note that all of the steels of the present invention had a two-phase structure of ferrite and pearlite. On the other hand, as explained below, none of the comparative steels satisfy the required hardness level and machinability at the same time. In addition,
Among the comparative steels, Na7, &9, &11, and Nα13 are Hy
Because it was not possible to secure the hardness of 300 level, and because Ha 10 cracked during hot working,
No machinability test was conducted. First, comparative steel! 16 is M for improving hardenability. This is a conventional steel with addition of Mo, and due to the addition of Mo, it has a bainite structure and has poor machinability. Comparative steel &7 is an example of low C and H
Hardness at the V300 level cannot be ensured. Comparative mNct8 has a large amount of C and has too high hardness, resulting in poor machinability. Comparative steel surface 9 has a small amount of Cu, so precipitation hardening of Cu is small, and the HV300 level cannot be ensured. Comparative steel 1o has too much Cu, Ni/Cu≧2.0
Since this steel does not satisfy the above requirements, cracks occur during hot forging, making it unsuitable for use as mold steel. Comparative steel Nα11 has a small amount of Ni, so precipitation hardening of N1-AQ is not sufficient, and hardness at the Hv300 level cannot be ensured. Comparative steel &12 has a large amount of Ni, has large precipitation hardening of N1-Afl, becomes too hard, and has poor machinability. Comparative steel Nn13 is an example with a small amount of AQ, and N1-A
Since the precipitation hardening of Q is not sufficient, the Hv300 level cannot be secured. Comparative steel Na14 has a large amount of AQ, so precipitation hardening of Ni-Al2 is large, and the steel becomes too hard, resulting in poor machinability. In addition, oxide-based nonmetallic inclusions in Afi also have an adverse effect on machinability.

[Left below]

Table 2 Cutting Conditions (Effects of the Invention) As detailed above, according to the present invention, machinability is significantly improved while ensuring the same hardness (Hv300 level) as conventional steel for molds. Since it can be improved, the effect is remarkable.

Claims (3)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.05 to 0.2
0%, Si: 0.10~0.50%, Mn: 0.30~
2.00%, P≦0.1%, Cu: 0.9-1.5%,
A coating characterized by containing 2.1 to 3.0% Ni and 0.5 to 1.5% Al, with the remainder consisting of Fe and unavoidable impurities, and having a two-phase structure of ferrite and pearlite. Steel for molds with excellent machinability. (2) The steel further has S≦0.25%, Ca≦0.00
4%, Se≦0.5%, and Te≦0.5%. The mold steel with excellent machinability according to claim 1, which contains one or more of the following. (3) The steel for molds having excellent machinability according to claim 1 or 2, wherein the steel has a composition satisfying the relationship of Ni/Cu≧2.0.