JPH02294450A - Die steel for molding plastics and its manufacture - Google Patents

Abstract

PURPOSE:To secure the prescribed hardness in the die steel and to improve its deformation resistance, weld cracking resistance, etc., at the time of working by subjecting a steel having specified contents of Mn, Mo, V, Ni, Al, etc., to hot forming and thereafter executing forced cooling and annealing under specified conditions. CONSTITUTION:A steel having the chemical components constituted of, by weight, 0.05 to 0.18% C, <=0.15% Si, 1.5 to 2.5% Mn, <=0.5% Cr, 0.2 to 0.5% Mo, 0.2 to 0.7% V, 2.5 to 3.5% Ni, 0.5 to 1.5% Al and the balance Fe with inevitable impurities is subjected to hot forming. The steel is subjected to forced cooling from 900 to 1100 deg.C and is thereafter annealed at 500 to 650 deg.C. By this treatment, the structure substantially constituted of bainite is formed and fine carbon nitride and intermetallic compounds are precipitated to regulate the hardness to the range of 35 to 45HRC. The steel has excellent machinability, wear resistance, etc., and is applicable to a metal pattern for molding plastics.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for producing mold steel for plastic molding, and more specifically, it has a hardness of HRC 35 or more and 45 or less, and has good machinability, wear resistance, and This invention relates to a method of manufacturing steel used for plastic molds that have excellent deformability during processing.

(Conventional technology) Plastic molds are used for electrical equipment, precision mechanical parts, automobiles, cosmetic containers, camera bodies, and various engineering plastic products (
Due to the increasing demand for various plastic molded products such as gears, etc.) and lenses, the production volume is rapidly increasing, and technologically speaking, there is a remarkable trend toward mass production and precision.

Under these circumstances, it may be necessary to shorten the manufacturing period of the mold, and this requirement may have to be met through processing or design changes. To this end, we need to improve machinability,
Excellent weld cracking resistance and fewer hardness discontinuities in the weld heat affected zone are now required.

It cannot be said that 851C and SCM440, which are conventional general-purpose steels for plastic molds, necessarily have sufficient properties in response to the market trend described above. In other words, the weld cracking resistance is particularly poor since it is a high carbon steel, and due to the residual stress generated during heat treatment as it is quenched or tempered at a relatively low temperature, deformation occurs when finished with precise dimensions. There is a possibility of a problem occurring.

(Problems to be Solved by the Invention) The present invention solves the above-mentioned problems by reducing the number of carbon stars in the steel and increasing the Mn content higher than that of normal steel, thereby improving machinability by changing the structure to bainite. , Mo, V,
By adding appropriate amounts of Ni and Al, we utilize the hardening caused by the precipitation of these carbides or intermetallic compounds to ensure a specified hardness, as well as plastic molding with excellent deformation resistance and weld cracking resistance during processing. The purpose is to supply steel for molds.

(Means for solving the problem) In order to achieve the above object, the present invention has a ball amount ratio of C: 0.05.
~0.08%, S1:O. 15% or less, Mn: 1.5
~2.5%, Cr: 0.5% or less, Mo: 0.2
~0.5%, V: 0.2 ~0.7%, Ni: 2.
Contains 5-3.5%, AfI: 0.5-1.5%,
The remainder has a chemical component consisting of Fe and unavoidable impurities,
It is characterized by consisting essentially of a bainite structure and having fine carbon and nitride precipitated therein. In addition, carbon/nitrides of V and Mo dissolved in solid solution can be obtained by forcibly cooling the steel having the above components from a temperature range of 900°C to 1100°C to form a bainitic structure and annealing it at a temperature of 500°C to 650°C. The gist of the invention is to precipitate an intermetallic compound of NIAl and thereby secure hardness (35 to 45 in HRC) through strain hardening.

The steel of the present invention basically has a bainite structure,
In addition to taking advantage of the good machinability of bainite, most of the residual stress due to distortion during cooling is released by annealing in the range of 500 to 850°C, so residual stress is released during the cutting process when making precision molds. This has the advantage that less distortion occurs. In addition, the content of Mn is adjusted appropriately,
By adding Mo and V, the non-recrystallized area during hot working is expanded, and the austenite grain size due to recrystallization is optimized to give graining workability and mirror finish.

Next, the reason for limiting the component range will be described.

C is a basic additive necessary to obtain the bainite structure, which is the basic structure of the steel of the present invention, and to combine with Mo and V to produce carbon and nitrides. The limit is 0.05%. Also, if it is excessively large, 900
After forced cooling from ℃ or above, the structure basically does not become bainite, but some hardened structures such as martensite are included, resulting in poor uniform machinability, grain workability, mirror finish workability, and ultimately weldability. You will lose it. This limit is 0.
18%, which was set as the upper limit.

S1 is a deoxidizing element, and since the steel of the present invention contains 0.5 to 1.5% Afi to utilize the Ni-All intermetallic compound, deoxidation during refining is sufficient. Therefore, rather S iO
It is desirable that the amount of inclusions such as 2, etc. in the product S1 be as small as possible from the viewpoint of ensuring grain workability and mirror finish workability. The reason why the amount is set to 0.15% or less is that if the amount exceeds this, it is difficult to ensure the cleanliness of the steel, and there is a possibility that the result is poor grain workability and mirror finish workability.

Mn is an essential element for improving hardenability, adjusting bainite hardness according to the required hardness level, suppressing the formation of ferrite, and imparting machinability. The minimum amount to obtain these effects is 1.5%. If the amount is too large, a lower bainite or martensitic structure will be induced in the structure, making it difficult to obtain a uniform structure. This limit is 2.
Since it was 5%, this was set as the upper limit.

C' is an element that improves corrosion resistance and hardenability, but in the present invention, it is not used for the purpose of precipitation hardening, but rather is used to reduce the hardening distortion to prevent the generation of martensite and obtain a uniform bainite structure. Since this is considered to be of primary importance, we decided to keep it at a lower limit of 0.5%.

Mo precipitates fine precipitates during high-temperature tempering at 500° C. or higher to bring about precipitation hardening, and also suppresses precipitation of ferrite during cooling from 900° C. or higher to promote bainite structure. It is also an effective element for corrosion resistance in the atmosphere during use, especially for preventing pitting corrosion. If it is too large, the machinability and toughness will deteriorate, so it is set at 0.5% or less. If it is too low, the above effects cannot be obtained, so the lower limit is set at 0.2% or less.

(2) precipitates as fine carbon/nitride during annealing and hardens the steel through precipitation hardening phenomenon. The minimum amount necessary to obtain the hardness of the steel of the present invention is 0.2%, but if it is too large, the carbon/nitrides become coarse and the mirror finish is deteriorated due to surface roughness, which is a problem. This upper limit is 0. 7%.

Ni is added for the purpose of lowering the transformation point so that a bainite structure appears uniformly during cooling, and for the purpose of creating an intermetallic compound with Al and precipitation hardening it during annealing, but this effect is not sufficient at 2.5% or less. If it exceeds <3.5%, the effect will not be significant in proportion to the amount added and will not be economical. Therefore, 2.
It was set at 5 to 3.5%.

Afl is added to function as a deoxidizing element during melting and refining and to bring about precipitation hardening through its action with Ni. If the addition amount is less than 0.5%, sufficient precipitation hardening cannot be obtained, and if it exceeds 1.5%, no effect on precipitation hardening can be expected due to the balance with Ni, and there is a strong tendency for embrittlement. Therefore, the limit value was set to 0.5 to 1.5%.

Steel consisting of the limited components described above is hot-formed and heat-treated.The reason why the lower limit of the heat treatment temperature was set at 900°C is to prevent Mo, which precipitates during the subsequent annealing.
This is to sufficiently dissolve carbon/nitrides such as V and intermetallic compounds such as NiAil into austenite, and if the temperature is lower than this temperature, precipitation hardening of these precipitates during annealing cannot be expected.

Furthermore, the upper limit was set at 1100°C because heating temperatures exceeding this will cause the austenite grains to become too large, resulting in an increase in hardenability and a decrease in the transformation temperature range of the steel during forced cooling, resulting in the formation of martensitic structures. This is because the possibility of this increases, resulting in a steel that lacks the machinability, which is the objective of the steel of the present invention.

The steel of the present invention is forcibly cooled from a temperature of 900° C. to 1100° C., but the method is preferably water quenching, oil quenching, blasting, mist, etc., and is not particularly limited as long as the desired structure can be obtained.

The subsequent annealing is performed to ensure that the plastic mold steel, which is the object of the present invention, has good machinability when processed into molds and that processing distortion due to residual stress does not occur.
The temperature is set at a relatively high temperature of 500 to 650°C.

The lower limit was set at 500°C because at temperatures above this precipitation hardening due to carbon and nitrides such as Mo and V occurs most significantly, and also because the residual stress generated during the heat treatment in the previous process The reason why the upper limit was set at 650°C is that temperatures higher than this are effective for release relaxation of Because you can't get it.

The final material for the mold is machine-cut gold. The work is done as a mold, but this requires a particularly elaborate finish, so
Good machinability is required.

In the present invention, the base structure is basically bainite as a means for improving machinability, and the generation of martensite is suppressed as much as possible.

In this case, in the bainite structure, carbon such as Mo and V
In precipitation hardening using nitrides or Ni-/l intermetallic compounds, there is an upper limit to the degree of hardness, and machinability cannot be expected unless the hardness reaches that level. The upper limit of this hardness is HRC45, and obtaining a hardness exceeding this requires crystallization of a high carbon martensitic structure, which results in significant deterioration of machinability.

On the other hand, molds need wear resistance in order to increase their service life, and the demands of the mold market tend to increase hardness accordingly. In particular, the minimum hardness of the break-hardened mold steel of the present invention was set to HRo35 due to its wear resistance.

In producing the steel of the present invention, the steel is melted and refined using conventional methods, such as converter furnaces, electric furnaces, and vacuum melting furnaces. It is desirable to employ a consumable electrode type remelting means such as VAR or ESR.

Hot forming is carried out by conventional economical means such as rolling or forging. It can also be used as so-called break-hardened steel by subjecting it to a predetermined heat treatment after forming. The heat treatment may be carried out by cooling once after hot molding and then reheating to a temperature of 900°C or higher, or by performing forced cooling from a temperature of 900°C or higher immediately after hot molding.

Next, the present invention will be explained based on examples.

(Example) Table 1 shows the chemical components, heat treatment conditions, and hardness (HRc) after heat treatment of the present invention and comparative examples. Steels 1 to 7 in the table are steels subject to the present invention, and steels with the same composition as Nα2, Na5, k6, and Nα7 were treated under different heat treatment conditions within the range of the present invention.

On the other hand, in the comparative example, the chemical components of k8 and k9 were within the scope of the present invention, but the heat treatment conditions were outside the scope of the present invention. Moreover, the chemical components of NalO to I2 are outside the limited range of the present invention. Invention example NO. 1~No. No. 7 fully satisfies the hardness targeted by the present invention, but Comparative Examples Nα8 to No. 12 both indicate hardness that is outside the scope of the present invention.

Comparative example Magnet 13 is an example of conventional steel JIS SCM4 with a hardness of HR, 42, and the machinability of this steel is 10.
The machinability of the steel of the present invention is expressed as an index with 0 being taken as an index.

Furthermore, Photo 1 is a micrograph of Nα1 steel at 200 times magnification as an example of the microstructure of the steel of the present invention, and it shows that it is basically an upper bainitic steel.

Furthermore, an example in which the maximum hardness under the weld bead was measured as one data indicating weldability is shown together with conventional steel Ni13 as a comparative example, and it is clearly shown that the steel of the present invention is extremely superior.

(Effect of the invention) As explained above, the present invention lowers the carbon content and
By adjusting each component such as n, Mo, V, Ni, Ap, etc., it has excellent machinability and moderate strength, and is suitable as a steel material for plastic molds. In addition, it has good weldability and little deformation during processing, making it extremely useful as a precision mold.

[Brief explanation of the drawing]

FIG. 1 is a 200 times magnified microscope photograph showing Group 1a (upper bainite structure) of the present invention. ￥ 11,] Sub-agent Patent Attorney Hiroaki Tamura Procedural Amendment (Voluntary) July 13, 1999

Claims (1)

[Claims] (1) In weight%, C: 0.05-0.08%, Si: 0
．． 15% or less, Mn: 1.5-2.5%, Cr: 0.5
% or less, Mo: 0.2-0.5%, V: 0.2-0.7
%, Ni: 2.5-3.5%, Al: 0.5-1.5%
, has a chemical composition consisting of the remainder Fe and unavoidable impurities, is substantially composed of a bainite structure, and has a hardness of H_R_C35~, which is characterized by having fine carbon/nitrides and intermetallic compounds precipitated therein. Plastic molding mold (2) weight% in the range of 45, C: 0.05 to 0.0
8%, Si: 0.15% or less, Mn: 1.5-2.5%
, Cr: 0.5% or less, Mo: 0.2 to 0.5%, V:
0.2-0.7%, Ni: 2.5-3.5%, Al: 0
．． After hot forming, a steel having a chemical composition of 5 to 1.5% and the balance consisting of Fe and unavoidable impurities was heated to a temperature of 900 to 1.
Forced cooling is performed from a temperature of 100℃, then 500℃
A method for producing mold steel for plastic molding having a hardness in the range of H_R_C35 to 45, characterized by annealing at a temperature range of 650°C.