JPH02285050A - High hardness free-cutting die steel - Google Patents

Abstract

PURPOSE:To manufacture the high hardness free-cutting die steel having good mirror finishing properties, polishability, etc., by preparing a steel contg. specified ratios of C, Si, Mn, Cr, Ni, sol Al and N as essential components and contg. specified ratios of B, Ca and P as free-cutting elements. CONSTITUTION:A steel contg., by weight, 0.10 to 1.00% C, 0.05 to 1.50% Si, 0.10 to 2.00% Mn, 2.00 to 9.50% Cr, 0.25 to 5.00% Ni, 0.60 to 1.5% sol Al and 0.0015 to 0.060% N as essential components, contg. one or more kinds from the group of free cutting elements among 0.0003 to 0.01% B, 0.0005 to 0.050% Ca and 0.005 to 0.10% P and the balance Fe with inevitable impurities is refined and cast. Preferably, the steel is reheated, is rolled or forged, is thereafter heated to about 850 to 1100 deg.C, is subjected to water or oil quenching and is tempered at about 150 to 700 deg.C. In this way, the high hardness free-cutting die steel having about 50 to 70 class Hs hardness can be obtd.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention provides improvements in mirror workability, layer workability, fatigue properties, and thermal fatigue properties used in molds for glass resin-containing plastic injection molding, press molds, etc. This relates to a high-hardness free-cutting mold steel with good hardness of Hs class 50 to 70.

[Conventional technology]

As Hs grade 50-70 steel for plastic molds,
JIS 5KD61 (0,4Cl5i-5Cr 1.
25Mo-IV) and 5KDII (1,5C12Cr-I
Although there are materials equivalent to Mo -0.4V, these mold steels have high hardness and generally have low machinability.

In order to obtain a specified cutting efficiency with this type of steel material, there are two methods: 1) Adding free machinability to the steel itself using sulfide or PB (Japanese Patent Publication No. 63-66384), 2) Adding intermetallic compounds after cutting. , method of applying precipitation heat treatment of Cu alloy phase
163248), ■ Methods using high-grade cutting tools such as ceramics and cermets, etc. However, the steel material itself does not necessarily have high hardness, free machinability, mirror workability, polishing workability, fatigue properties, and thermal properties. It cannot be said that the fatigue properties are fully satisfied.

Therefore, it is desired to develop a high-hardness free-cutting mold steel having a hardness of Hs class 50 to 70 and having good specular workability, polishing workability, fatigue properties, and thermal fatigue properties.

[Problem to be solved by the invention]

For this reason, the present invention complies with the conventional JIS SKD 61 (0
,4clsi-5Cr-1,25Mo-IV) and SKD
11 (1,5C-12Cr-1Mo-0,4V)
It is an object of the present invention to provide a high-hardness free-cutting mold steel having a hardness of Hs class 50 to 70 and having good mirror finish workability, polishing workability, fatigue properties, and thermal fatigue properties.

[Means to solve the problem]

The present invention has been made to advantageously solve these problems, and the gist thereof is as follows.

(1) Weight ratio: C: 0.10-1.00% Si: 0.05-1.50% Mn: 0.10-2.00% Cr: 2.00-9.50% Ni: 0. 25-5.00% sob, Al: 0.60-1.5%N: 0
．． Free-machining element group B: O, 0003-0.01% Ca: 0.0005-0.050% P: o,
A high-hardness free-cutting mold steel containing one or more of oos to 0.10%, with the balance being Fe and inevitable impurities.

(2) Weight ratio: C: 0.10-1.00% Si: 0.05-1.50% Mn: 0.10-2.00% Cr: 2.00-9.50% Ni: 0. 25-5.00% sol, A1: 0.60-1.5% N: O, 0
Free-machining element group B: 0.0003-0.01% Ca: 0.0005-0.050% P: 0
．． 005-0.10%, and further hardness improving element group Mo: 0.05-3.00% W: 0.05-2.00% V: 0.010- 2.00% Nb: 0.005 to 0.50% Co: 0.25 to 3.00% Contains one or more of the following, and the balance is Fe and inevitable impurities for high hardness free-cutting molds steel.

[Effect]

In order to improve free machinability, generally speaking, during cutting, the workpiece is embrittled by 1) inclusions that act as a stress concentration source or form a built-up cutting edge, and 2) inclusions with low deformation resistance act as lubricants for the tool. It is necessary to increase the lubrication effect that occurs, and 3) the tool protection effect in which the low melting point compound melts in two parts during cutting and the solid solution protects the cutting edge.

The present inventors have investigated inclusions and precipitates in order to obtain high hardness and good mirror workability, polishing workability, fatigue characteristics, and thermal fatigue characteristics while imparting free machinability to steel for molds. A detailed investigation was conducted regarding the As a result, we found that B and borides, Ca and calcium compounds, P and phosphides are effective, and using these inclusions and precipitates, we can improve mirror workability, polishing workability, fatigue properties, and thermal fatigue. This was a successful development of a high-hardness free-cutting cold mold steel with good properties.

The relationships between the contents of B, Ca, and P and the thrust force, which is the force in the circumferential direction during drilling, are shown in FIGS. 1, 2, and 3. All of the workpiece materials exhibit a microstructure consisting mainly of tempered martensite and a portion of paynite, and have a hardness of 57 to 61 in Hs. Figures 1 and 3
From the figure, B and P are each 0.010% by weight
Hereinafter, when the content is 0.10% or less, as the content increases, the thrust force, that is, the cutting resistance decreases, and the machinability improves.

On the other hand, when the content exceeds this, in the case of B, the machinability decreases due to improvement in grain boundary strength, and in the case of P, the machinability decreases due to solid solution hardening.

Furthermore, phosphides such as phosphorus nitride and calcium phosphide and borides such as boron carbide and boron nitride are fine amorphous precipitates with low melting points, so they partially melt during cutting and act as lubricants. At the same time, it protects the tool by covering the cutting edge. In addition, these precipitates also serve as precipitation nuclei for high-hardness intermetallic compounds and high-hardness alloy phases.

Moreover, as shown in FIG. 2, the addition of Ca reduces thrust force, that is, cutting resistance, and improves machinability as the content increases. However, excessive addition of Ca will cause
Since Ca deteriorates mirror workability, polishing workability, fatigue properties, and thermal fatigue properties, an upper limit regulation is also required for Ca. Incidentally, the above-mentioned phosphides, borides, and calcium compounds are generated during casting.

Mirror workability and polishing workability are important characteristics regarding the surface quality of steel for precision plastic molds.

Mirror finishing refers to polishing the surface with paper to #1200 after surface cutting, and then polishing with diamond paste powder with a diameter of In or less to finish the steel surface like a mirror. The processability is evaluated based on the degree of roughness on the mirror surface and the degree of distortion of the reflected image. In order to satisfy specularity, inclusions must be reduced and made finer.

In the case of polishing, the diameter of the diamond paste powder during final polishing is coarser than that during mirror finishing, ranging from several μm to more than ten μm. Polishing is the process after polishing the surface of steel.
This involves injecting plastic and transferring the polished surface of steel onto the plastic surface. Even if the polished surface of the steel material is good, if there are inclusions, the plastic surface will have convexities because the compression plastic deformability of the inclusions is higher than that of the steel material.

It is necessary that this convexity is not detected by visual inspection or observation using a magnifying glass of about 20 times, and it is necessary to reduce and miniaturize inclusions.

Fatigue cracks caused by stress differences during molding and mold release, and thermal cracks caused by the thermal cycle of molding, cooling, mold release, and molding are transferred to the plastic surface in convex shapes. For this reason, molds are also required to have good fatigue properties and thermal fatigue properties.

Regarding fatigue properties, it is effective to reduce inclusions and make them fine, and regarding thermal fatigue properties, it is effective to reduce inclusions and carbides and make them fine.

Next, the reasons for limiting the ingredients in the present invention are as follows.

C dissolves in martensite or precipitates as a carbide, and has the effect of increasing the hardness of steel.

In order to exhibit this effect, the content must be 0.10% or more, but if it exceeds 1.00%, the repair weldability of the mold will be impaired, so the content should be reduced to 0.10-1.00%. Limited.

Si is an element that inexpensively increases the hardness of steel.

In order to exhibit this effect, the content is required to be 0.05% or more, but if it exceeds 1.50%, the toughness decreases, so the content is limited to 0.05 to 1.50%.

Mn has the effect of improving the hardness and toughness of steel at low cost, and for this purpose, it must be contained in an amount of 0.10% or more.

On the other hand, if the content exceeds 2.00%, weldability will be impaired due to the formation of huge MnS. For this reason, the content is reduced to 0.
．． It was limited to lO~2.00%.

Cr is necessary as an element to improve hardenability, and 2.00% is required to obtain this effect. On the other hand, even if the content exceeds the upper limit of 9.50%, the effect will be saturated. For this reason, the content was limited to 2.00 to 9.50%.

Sol and AI are not only necessary as deoxidizing elements during the production of thick steel plates, but also as intermetallic compound forming elements, and in order to obtain this effect, 0.60% or more is required.

On the other hand, if the content exceeds 1.5%, the cleanliness of the steel plate will decrease significantly. Therefore, the content of sol and kl is 0.60~
It was limited to 1.5%.

N precipitates as AfN during the production of thick steel plates and prevents austenite grains from becoming coarser. The amount of N required for grain refinement is 0.0015% or more. On the other hand, 0.06
If the content exceeds 0%, giant A7N precipitates and reduces toughness. Therefore, the N content is 0.0015 to 0.06
It was limited to 0%.

Ni is necessary to ensure hardness and toughness, and 0.25% is required to obtain this effect. On the other hand, 5.00
Even if it is contained in an amount exceeding the upper limit of %, the effect becomes saturated and it is not economical. Therefore, the Ni content is 0.
It was limited to 25-5.00%.

Next, B, Ca, and P, which are added as free-machining elements, will be described.

B
: 0.0003%, Ca: 0, 0005%, P: 0
．． 005% is required.

However, B: 0.01% and Ca: 0.050%, respectively.
, P: Even if the content exceeds the upper limit of 0.10%,
In order to avoid saturation of their effects or impair toughness, graining workability, polishing workability, and weldability, the respective contents were determined as described above.

Furthermore, Mo, W, v, Nb, and Co added as hardness improving element groups in the invention according to claim 2 will be described. These components are added because they have the uniform effect of improving the hardness of steel, but in order to ensure the desired effect on the above effect, the lower limit of each content should be Mo: 0. 05%, W: 0.05%, V: 0.
010%, Nb: 0.005%.

Go: 0.25% is required.

However, Mo: 3.00%, W: 2.00%, respectively.
, v:2. oo%, Nb: 0.50%, Co: 3.00
Even if the content exceeds the upper limit of %, the effect will be saturated, so each content was determined as above.

For melting, either an electric furnace or a converter may be used, and for casting, any of the ordinary ingot method, continuous casting method, and unidirectional solidification method may be used.

850-1100°C after reheating and rolling or forging
It is preferable to quench in water or oil after heating, and then temper at 150 to 700°C. In addition, 850~1100°
C is subjected to heating rephosphorization, and if necessary, the temperature is 150 to 70.
It is also possible to temper at 0°C.

〔Example〕

Table 1 shows the plate thickness and chemical composition of the inventive examples and comparative examples, and Table 2 shows the mechanical properties of this steel.

After melting the steel, it was made into slabs by a normal method, and each slab was reheated to 1050 to 1250°C and then rolled. Furthermore, it is heated to 880-1070°C and then quenched.
Tempering was performed at 00 to 650°C.

Note 1) Hardness HB (3000Kgf) is Hultgr
The value at the center of the plate thickness when using an en ball (10 mm ball, load 3000 Kgf).

To satisfy Hs-50~70, 1IB=341~5
17 is required.

2) The Charpy test piece is JIS No. 5 (5rrtm U
Notsuchi). The sampling location is in the C direction at the center of the plate thickness.

uE+20°C is the shock value of +20°C. Unit is kgfm
/cffl.

3) Symbols for tool wear resistance, mirror workability, polishing workability, fatigue properties, and thermal fatigue properties indicate: ◎Very good, ○Good, and X: Poor.

4) Tool wear resistance was determined by the flank wear width of the two-sided end mill.

The cutting specifications are: tool diameter φ10.2mm, tool material carbide, water-soluble cutting fluid, rotation speed 69Orpm, feed rate 69ffI.
Il/1lin1 cutting speed 26m/min.

The evaluation index is that the cutting time until the maximum wear width becomes 0.2 cylinder is ◎ when it exceeds 180 m1n, and O when it is 120 to 180 Ilin.
Less than 130 min is indicated by x.

5) Mirror workability was examined by polishing the polished surface to 174 μm with diamond powder, visually or by observing with a 20x magnifying glass, and then examining the surface roughness.

The evaluation criteria for ◎ and O1× are shown in the table below.

6) Polishing workability was examined by visual observation or observation using a 20x magnification of the polished surface and plastic surface after polishing with diamond powder to a depth of 3 μm. Evaluation was made based on the presence or absence of grain unevenness.

◎: There is no grain unevenness when viewed with a 20x magnifying glass, ○: There is no grain unevenness when visually inspected, but there is some grain unevenness when viewed with a 20x magnifying glass.
Visually, the presence of grain unevenness is indicated by an x.

7) Fatigue test is ITC based on ASTM E8134ζ
A T test piece (with a mechanical notch) was used.

To determine fatigue life, the number of repetitions until the fatigue crack length from the notch tip becomes 0.2 mm at a load of 5 TON is N = 3.
The minimum value of 3X10' times is ◎, lXl0'~3X1
0' times are O11 x l O Less than 4 times are indicated by x.

8) Thermal fatigue test was performed using ASTM E8131TCT specimen (
(with mechanical notch) was used.

The temperature was maintained at 30°C and 400°C for 30 seconds at 60-second intervals, and the occurrence and development of thermal cracks from the tip of the mechanical notch was observed. 3
More than X10' times ◎, lX10' to 3X10' times ○, l
Less than Xl0' times is indicated by x.

As is clear from Table 2, the tool according to the present invention has good tool wear resistance, and shows good levels of mirror workability, polishing workability, fatigue properties, and thermal fatigue properties. . On the other hand, none of the comparative examples satisfy all of these characteristics.

〔Effect of the invention〕

As described in detail above, by using B, Ca, P and their compounds, the hardness is 50 to 70 Hs with excellent mirror workability, polishing workability, fatigue properties, and thermal fatigue properties.
A high-grade, high-hardness, free-cutting mold steel can be obtained.

[Brief explanation of drawings]

Figure 1 is a diagram showing the influence of B content on thrust force in a drill test, Figure 2 is a diagram showing the influence of Ca content on thrust force in a drill test, and Figure 3 is a diagram showing the influence of Ca content on thrust force in a drill test. It is a figure showing the influence of P content on thrust force of a test. Figure 1 Figure 2

Claims (2)

[Claims] (1) Weight ratio: C: 0.10-1.00% Si: 0.05-1.50% Mn: 0.10-2.00% Cr: 2.00-9.50% Ni: 0. 25-5.00% sol. The basic components are Al: 0.60-1.5% N: 0.0015-0.060%, free-machining element group B: 0.0003-0.01% Ca: 0.0005-0.050% A high-hardness free-cutting mold steel containing one or more of P: 0.005 to 0.10%, with the balance being Fe and inevitable impurities. (2) Weight ratio: C: 0.10-1.00% Si: 0.05-1.50% Mn: 0.10-2.00% Cr: 2.00-9.50% Ni: 0. 25-5.00% sol. The basic components are Al: 0.60-1.5% N: 0.0015-0.060%, free-machining element group B: 0.0003-0.01% Ca: 0.0005-0.050% Contains one or more of P: 0.005 to 0.10%, further hardness improving element group Mo: 0.05 to 3.00% W: 0.05 to 2.00% V: Contains one or more of the following: 0.010-2.00% Nb: 0.005-0.50% Co: 0.25-3.00%, with the balance consisting of Fe and inevitable impurities. Steel for cutting dies.