JP2000054068A - High strength prehardening steel material excellent in machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength prehardening steel material improved in machinability without damaging the advantage of being well-balanced between strength and ductility, which is the characteristic of a steel material composed essentially of martensitic structure, and particularly useble as a steel for metal mold for plastic molding. SOLUTION: This is a high strength prehardening steel material excellent in machinability, which has a structure composed essentially of martensite and in which the size of a packet constituting the structure is equal to or larger than No.8 when evaluated by austenite grain size number. To be concrete, it is preferable to regulate the chemical composition of the steel material so that it consists of, by weight, <=0.2% C, <=1.5% Si, <=2.0% Mn, 3.0-<8.0% Cr, 1.0-4.0% Ni, 0.5-2.0% Al, 0.3-3.5% Cu, and the balance essentially Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength prehardened steel material having both high strength and machinability due to a martensite structure.

[0002]

2. Description of the Related Art Conventionally, molds for molding plastic products which have excellent machinability include carbon steel, SC and the like.
M-based materials and materials containing free-cutting elements such as S and Pb are also used. As a mold material, in addition to the above-described carbon steel and SCM, there has been proposed a material in which an appropriate amount of Ni and Al is added to precipitate an intermetallic compound thereof to secure hardness (particularly). Kaihei 2-179845
issue). This structure has two phases composed of 60% or more of ferrite and the remaining pearlite, and the machinability is not sufficient.

[0003] Prehardened steel containing a relatively large amount of a specific amount of Cr, Mo and Cu as essential components has also been proposed as a material particularly suitable for a large mold material (Japanese Patent Laid-Open No. 2-263953). This material has hardness HR
The structure is increased to about C34, and the structure is adjusted to upper bainite. Although this achieves relatively good machinability without adding a free-cutting element such as S, the machinability in a wide range of hardness is not sufficient. Further, the above-mentioned materials have poor corrosion resistance, and may cause problems such as rusting during long-term storage or when left in a state where water-soluble cutting oil is adhered thereto.

[0004] On the other hand, as a mold having excellent corrosion resistance, stainless steel materials such as SUS420 and SUS630 have been used.
No. 3 discloses a similar mold steel. These are mold materials for molding highly corrosive resins such as flame-retardant resins. Although they have excellent corrosion resistance and naturally have no problems such as rusting during storage, they are inferior in machinability and increase the number of mold processing steps. , Delivery time, price, etc.

[0005]

Although the above-described pre-hardened steel material mainly composed of the upper bainite structure realizes relatively good machinability, it is necessary to reduce production costs and lead time particularly required in this field. It cannot be said that it has sufficient machinability to reduce the number of steps for cutting the mold from the viewpoint. In addition, in order to obtain an upper bainite structure that realizes stable machinability, it is necessary to control a cooling rate in a heat treatment step during manufacturing, and there is a disadvantage that a large number of heat treatment steps are required.

[0006] On the other hand, a steel material mainly composed of a martensite structure is transformed into a martensite from austenite, so that although its strength is greatly increased, its ductility and
It is used for various applications by making full use of the feature that toughness hardly decreases. However, martensite is considered to have a problem in machinability, and machining after adjusting to martensite structure is not usually performed. An object of the present invention is to solve the above problems, and without impairing the machinability without impairing the advantage of excellent strength-ductility balance, which is a characteristic of a steel material mainly composed of a martensite structure. It is an object of the present invention to provide a high-strength pre-hardened steel material which can be used as an improved steel for plastic molds.

[0007]

Means for Solving the Problems The present inventor studied the martensite structure and machinability, and adjusted the size of the martensite packet generated from the austenite structure during quenching as large as possible to thereby improve the hardness after quenching and tempering. The present inventors have found that the machinability of the steel is greatly improved and arrived at the present invention.

That is, the present invention is excellent in machinability having a structure mainly composed of martensite and having a packet size constituting the structure equal to or larger than No. 8 when evaluated by austenite grain size number. High strength pre-hardened steel.

Without deteriorating the machinability described above, for example, it is excellent in graining workability and polishing property as a steel material for plastic molding dies. In order to avoid the problem described above, C: 0.2% or less, Si: 1.5% or less, M
n: 2.0% or less, Cr: 3.0 to less than 8.0%, N
i: 1.0 to 4.0%, Al: 0.5 to 2.0%, C
u: 0.3 to 3.5%, and preferably the balance is substantially adjusted to the chemical composition of Fe.

In the present invention, an element for improving corrosion resistance, an element for improving toughness, or an element for improving machinability can be added within a range that does not impair the basic function of the above-mentioned structure and chemical composition. For example, as the corrosion resistance improving element, Mo: 1.0% or less, and as the toughness improving element,
V: 0.5% or less by weight ratio, and as a machinability improving element,
S: 0.20% or less can be contained. Also,
In the present invention, in order to further improve the hardness of the matrix, it is preferable that the weight ratio satisfies 0.2% <Si ≦ 1.5%. Of course, a toughness improving element such as Co ≦ 1.0% may be added.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, one of the most important features of the present invention is that the martensite packet size generated from the austenite structure during quenching is evaluated by austenite grain size number and is equal to or equal to No. 8. It is a bigger adjustment. There are various forms of martensite in steel, but martensite appearing in most practical heat treatment steels has a lath shape. Lath martensite has a very fine structure (having a width of about 0.2 μm), but individual lath crystals do not act as one crystal grain like a ferrite structure.

This is because a large number of lath martensites having almost the same crystal orientation (same variant) tend to be formed adjacent to each other, and the boundary where these laths are united becomes a small-angle grain boundary. In an optical microscope, one austenite grain is divided by several packets, and each packet is further divided by several substantially parallel strip-shaped blocks.

A packet is a region composed of a group of many laths arranged in parallel (that is, the same crystallographic plane), and a block is a region composed of a group of laths which are parallel and have the same crystal orientation. Thus, the packet or block is the basic organizational unit that governs the martensite toughness. In the case of carbon steel or low alloy steel, the toughness can be considered to be mainly controlled by the packet, because the development of the block is insufficient. Specifically, it will have the organization shown in FIG.

As described above, in the lath martensite structure, the packet is a structural unit corresponding to the crystal grain in the ferrite structure, and these become finer as the matrix austenite becomes finer. In other words, in the past developments represented by super-strength steels, intensive efforts were made to refine the lath martensite structure in order to achieve even higher strength without impairing ductility and toughness. Deterioration was promoted.

In the present invention, the packet size of martensite, which is a basic structural unit of mechanical properties, is evaluated using an austenite crystal grain size number and adjusted to be equal to or larger than No. 8, so that the martensite structure is mainly contained. It has realized that the machinability after quenching and tempering is greatly improved without impairing the advantage of excellent balance between strength and ductility, which is a feature of the steel material to be quenched. That is, they have found a martensite structure that can be used as a prehardened steel with excellent renovation properties. In practice, it is possible to combine excellent machinability with a hardness of 35 to 45 HRC.

The basis of the preferred component range in the present invention is that a low-C martensite structure obtained by solid-dissolving Cr or Mo and the like in order to impart excellent corrosion resistance without impairing machinability is used. The main base is
This is to increase the hardness by precipitating intermetallic compounds and carbides during tempering. The reason for limiting the preferable component range of the steel material for a mold of the present invention will be described.

C: 0.20% or less C is an element effective for preventing the formation of ferrite and improving hardness and strength. If the content exceeds 0.20%, carbides are formed and the wear of the tool at the time of cutting is increased, or the corrosion resistance is deteriorated because the amount of Cr in the matrix is reduced.
The following is assumed. If the content is less than 0.02%, sufficient strength may not be ensured in some cases. Therefore, the content is preferably set to 0.02% or more. Si: 1.5% or less, preferably 0.2 <Si ≦ 1.5
% Si is usually used as a deoxidizing agent, but on the other hand, it improves machinability while reducing toughness. Therefore, it is preferably 1.5% or less in consideration of the balance between the two. More desirably, 0.2 <Si ≦ 1.5% in order to improve the hardness of the matrix without impairing the balance between the above-mentioned actions.

Mn: 2.0% or less Mn is used as a deoxidizing agent in the same manner as Si, and also has the effect of increasing the hardenability and inhibiting the formation of ferrite. Since the machinability is reduced, the content is set to 2.0% or less. Cr: 3.0 to less than 8.0% Cr is an element effective for imparting corrosion resistance, and it is necessary to contain 3.0% or more to show a clear effect.
However, when the content is 8.0% or more, the corrosion resistance is further improved, but the formation of ferrite is increased and the required hardness cannot be secured, or the machinability is deteriorated due to excessive toughness, so that the content is 3.0 to 8. 0.0%.

Ni: 1.0 to 4.0% Ni lowers the transformation point, uniformly forms a martensite structure as a main structure during cooling, and forms and precipitates an intermetallic compound with Al. When less than 1.0%, this effect is not observed, and is 4.0%.
%, The effect is not remarkable in proportion to the amount of addition, and austenite is formed and becomes unnecessarily viscous to deteriorate machinability. Therefore, the content is set to 1.0% to 4.0%. Al: 0.5 to 2.0% Al has an action of bonding with Ni to form and precipitate an intermetallic compound NiAl, thereby increasing the hardness. To achieve this effect, 0.5% or more is required. However, even if it exceeds 2.0%, N
The effect of precipitation hardening cannot be expected from the point of balance with i, forming hard oxide-based inclusions, causing tool wear, and impairing mirror polishing, graining, etc. 2.0%.

Cu: 0.3 to 3.5% Cu is considered to form a solid solution (ε phase) in which a small amount of Fe is dissolved, and contributes to precipitation hardening like Ni. 0.3% or more is necessary for the effect. However, Cu
On the other hand, 3.5% is considered to reduce the toughness or to infiltrate the crystal grain boundaries of the base material at a high temperature, thereby deteriorating hot workability.
It was as follows. Mo: 1.0% or less Mo is extremely effective in improving corrosion resistance due to solid solution, and may be added as necessary. However, since carbides are formed to increase tool wear, the upper limit is made 1.0%.

V: 0.5% or less V is effective for refining the crystal grains and has an effect of improving the toughness of the material, and has the effect of further improving the properties of the steel of the present invention.
It is added as needed, but if it is contained in a large amount, carbides are formed and tool wear is increased. Therefore, the upper limit is set to 0.5%. S: 0.20% or less S combines with Mn to form MnS inclusions and improve machinability. However, MnS is easily added as a starting point of pitting corrosion and deteriorates corrosion resistance. But,
Even if it exceeds 0.20%, improvement in machinability corresponding to the decrease in corrosion resistance cannot be expected, so the upper limit was made 0.20%.

[0022]

EXAMPLES (Example 1) A test steel having the components shown in Table 1 was melted in a 30 kg high-frequency melting furnace to obtain 40 mm x 40 m.
After forging a square bar of m, the bar was subjected to a heat treatment and subjected to an experiment. In addition,
Sample F corresponds to SUS420, and sample G corresponds to SUS630. Heat treatment is shown below for Type 1, Type 2, Ty
The pe3 is quenched, and then tempered for 1 hour at an appropriate temperature of 520 ° C to 580 ° C in 20 ° C increments, and then air-cooled to obtain a hardness of 40HRC ± 5 for all the heat-treated materials. It is. Type 1: Heated at 1000 ° C for 1 hour, then about 20 ° C / mi
Air cooling at a cooling rate of n Type 2: heating at 1100 ° C. for 1 hour, then slowly cooling at a cooling rate of about 5 ° C./min Type 3: heating at 1000 ° C. for 1 hour, then ausforming in the cooling process, then about 100 ° C. Air cooling at min cooling rate

[0023]

[Table 1]

Table 2 shows the results of measuring the packet size, machinability, hardness, and corrosion resistance of martensite in the microstructure of the sample obtained by the heat treatment. FIG. 2 is a microstructure photograph showing the difference in the packet size of typical steel type A in which the martensite packet size was adjusted by heat treatment. As shown in FIG. 2, it can be confirmed that the packet size is increased by the heat treatment. In addition, the packet size of martensite in the actual measurement evaluation was determined by first comparing the optical microscopic structure with the standard particle size diagram at 100 times specified by ASTM,
These measurements were performed on six photographs in each sample to determine the average packet size.

The evaluation of the machinability was carried out by performing an end mill cutting test and determining the maximum wear width (Vbm) of the tool flank at a cutting length of 6 m.
ax) was measured. Cutting conditions are 2-flute φ10 high speed end mill, cutting speed 23m / min, feed speed 0.06mm.
/ Wet, using a wet method. As a corrosion resistance test, a salt spray test (5% NaCl, 35 ° C., 1 hour), a tap water immersion test (room temperature, 1 minute immersion, and then left in the air) were performed. Good: No rusting, ○ (Good: Less than 10% rusting area ratio), × (Poor:
Rust area ratio 30% or more), Δ (middle: rust area ratio 10 to 10)
(Less than 30%).

[0026]

[Table 2]

Each sample had a hardness of 40 ± 5 by heat treatment.
It satisfies HRC. Samples A to E have good machinability when the packet size constituting the structure has a size larger than 8 when evaluated by austenite grain size number, but smaller than that It can be seen that the machinability is deteriorated in the size. Also, in the case of the conventional steels F and G, when the packet size was 8, a tendency of improvement in machinability was confirmed. Further, in the conventional steels F and G, when the packet size was finer than No. 8, the machinability was very poor, and the conventional steel G was chipped. Furthermore, the corrosion resistance of various steel types with different packet sizes varies slightly depending on each steel type and test conditions.However, any steel type can be found during long-term storage or when left with water-soluble cutting oil attached. It can be seen that it has corrosion resistance to such an extent that problems such as rust do not occur.

Example 2 30 kg of a test steel having the components shown in Table 3 was melted in a high-frequency melting furnace, and was heated to 40 mm × 40 mm.
After forging a square bar of mm, the bar was subjected to a heat treatment and subjected to an experiment. Heat treatment is performed to obtain hardness of 40HRC ± 5, and quenching is performed for 100 hours.
After heating at 0 ° C. for 1 hour, air-cooling is performed, followed by tempering at an appropriate temperature of 520 ° C. to 580 ° C. in steps of 20 ° C. for 1 hour, followed by air cooling.

[0029]

[Table 3]

The obtained sample was measured and evaluated for packet size, hardness, corrosion resistance and machinability in the same manner as in Example 1. Table 4 shows the results. In the preferred composition range of the present invention, C: 0.2% or less, Si: 1.5% or less, Mn: 2.
0% or less, Cr: 3.0 to less than 8.0%, Ni: 1.0
-4.0%, Al: 0.5-2.0%, Cu: 0.3-
Sample 1 containing 3.5% and substantially satisfying the balance of Fe
In No. 9, the hardness satisfying the hardness of 40 ± 5 HRC was obtained by the heat treatment, and the machinability and the corrosion resistance were excellent.

On the other hand, samples 11, 13, 14, and 15
However, the hardness could not be increased because of insufficient precipitation hardening elements such as Ni, Cu, and Al. Sample 1
The samples Nos. 0, 12, and 16 have a packet size of No. 9 which is easily reduced to particles, and are inferior in machinability as compared with the samples having compositions within the preferable composition range of the present invention. In addition, a sample 14 with low Cr,
Sample 17 having a large amount of S is easily rusted, and Sample 7 to which S is added is used.
Samples 13 and 15 in which 8, 9, C are high and Cr is low are also easily rusted.

[0032]

[Table 4]

[0033]

According to the present invention, since the workability of a steel material having a martensite structure as a main component after heat treatment is remarkably improved, the number of steps for cutting a die from the viewpoint of reduction of production cost and lead time is reduced. It becomes a high-strength pre-hardened steel material that is indispensable to steel. In particular, by satisfying the preferred composition range of the present invention, it is possible to have a hardness of 35 to 45 HRC, excellent corrosion resistance, and dramatically improve machinability without impairing the advantage of excellent strength-ductility balance. It is very useful as a steel material for plastic molds that can be formed.

[Brief description of the drawings]

FIG. 1 is a schematic view of a metal microstructure of a steel material according to the present invention.

FIG. 2 is a metal microstructure photograph and a schematic diagram showing a typical microstructure and a packet of the steel material of the present invention.

Claims (7)

[Claims] 1. A structure mainly composed of martensite, wherein a packet size constituting the martensite structure is equal to or larger than No. 8 when evaluated by an austenite grain size number. High-strength pre-hardened steel with excellent machinability. 2. C: 0.2% or less by weight ratio, Si: 1.
5% or less, Mn: 2.0% or less, Cr: 3.0 to 8.0
%, Ni: 1.0-4.0%, Al: 0.5-2.
The high-strength pre-hardened steel material excellent in machinability according to claim 1, wherein the steel material contains 0% and Cu: 0.3 to 3.5%, and the balance substantially consists of Fe. 3. The high-strength pre-hardened steel material having excellent machinability according to claim 2, comprising 0.02 to 0.2% of C by weight. 4. The high-strength prehardened steel material excellent in machinability according to claim 2, wherein Mo: 1.0% or less is contained in a weight ratio. 5. The high-strength pre-hardened steel material excellent in machinability according to claim 2, which contains V: 0.5% or less by weight. 6. The high-strength prehardened steel material excellent in machinability according to any one of claims 2 to 5, wherein the steel material contains S: 0.20% or less by weight. 7. The high-strength prehardened steel material excellent in machinability according to claim 2, wherein the weight ratio satisfies 0.2% <Si ≦ 1.5%.