KR20140087279A

KR20140087279A - A cold-work tool steel with excellent hardness and impact toughness

Info

Publication number: KR20140087279A
Application number: KR1020120156907A
Authority: KR
Inventors: 강전연; 이태호; 손동민; 이동희
Original assignee: 한국기계연구원; 포스코특수강 주식회사
Priority date: 2012-12-28
Filing date: 2012-12-28
Publication date: 2014-07-09

Abstract

The present invention relates to a cold work tool steel including 1.00-1.20 wt% of carbon (C), 0.5-0.7 wt% of silicon (Si), 0.3-2.0 wt% of aluminum (Al), less than or equal to 0.5 wt% of manganese (Mn), 8.5-9.5 wt% of chrome (Cr), 0.8-1.3 wt% of [Mo]+0.5[W], and the remainder consisting of iron (Fe) and inevitable impurities. The cold work tool steel can ensure sufficient hardness and provide improved toughness.

Description

[0001] A cold-work tool steel with excellent hardness and impact toughness,

The present invention relates to the chemical composition of cold tool steel which is the material of various molds and other processing tools used for forming parts of metal materials at room temperature.

Tool steels contain a large amount of carbon and other alloying elements compared to steels for general structural purposes. They are mainly composed of reinforced phases such as carbides or intermetallic compounds, which have very high hardness, To obtain a high hardness and abrasion resistance as a whole.

In addition, tool steels are most widely used in the production of molds, cutting tools and machining tools for forming various metal and plastic parts, and thus have a wide range of physical properties, microstructure, and chemical composition depending on their use.

At this time, a large number of parts constituting the automobile, home appliance, and electronic product are manufactured by press forming or stamping at room temperature. As a mold material for this purpose, a cold-work tool steel Classified products are mainly used.

The cold tool steel is usually used in a state of hard hardened carbides precipitated in a tempered martensite structure. In order to obtain such a microstructure, quenching is performed at a high temperature, followed by a tempering heat treatment It is common.

STD11 (product name: JIS-SKD11 or AISI-D2), which is one of the common cold tool steels, is most widely used for molds and advanced cutting / cutting tools and has high hardness of 55HRC or more and excellent abrasion resistance.

However, in the case of the STD 11, chipping and cracking due to fatigue can be easily caused particularly during repetitive molding of a high-strength metal material due to a lack of overall toughness, which may cause breakage of the mold / tool and shortening the life span.

At this time, STD11 contains carbon (C) at a level of 1.5% and chromium (Cr) at a level of 12% by weight, and it is helpful for securing high hardness and excellent resistance to wear due to such a large amount of alloy element, And the toughness and fatigue resistance are lowered due to the carbides.

To this end, the cold tool steels to date have a tendency to reduce the content of carbon (C) and chromium (Cr) in alloying elements, but in this case there is a problem that the hardness after quenching is lowered due to the decrease of the amount of carbide. It is necessary to develop a cold tool steel having improved toughness while ensuring sufficient hardness.

It is an object of the present invention to provide a cold tool steel having improved toughness characteristics while ensuring sufficient hardness characteristics.

The problems to be solved by the present invention are not limited to the above-mentioned technical problems, and other technical problems which are not mentioned can be clearly understood by those skilled in the art from the following description.

In order to solve the above-mentioned problems, the present invention is characterized by comprising, by weight%, C: 1.00 to 1.20%; 0.5 to 0.7% of Si; Al: 0.3 to 2.0%; Mn: 0.5% or less; Cr: 8.5 to 9.5%; [Mo] + 0.5 [W]: 0.8 to 1.3%; And a cold tool steel containing Fe and unavoidable impurities.

Further, the present invention provides the cold tool steel wherein the Mo content is 0.8-1.1 wt% and the W content is 0.5 wt% or less.

Further, the present invention provides a method for manufacturing a semiconductor device, comprising: 0.2 to 0.5% of V; Ni: 0.3% or less; Cu: not more than 0.3%; And Nb: not more than 0.2%.

The present invention also provides a cold tool steel comprising at least one of 0.02 wt% or less of P, 0.1 wt% or less of S, 0.1 wt% or less of N, 0.02 wt% or less of Ca, or 0.02 wt% or less of B, .

According to the present invention as described above, it is possible to provide a cold tool steel having improved toughness characteristics while ensuring sufficient hardness characteristics.

More specifically, the cold tool steel of the present invention has a low overall carbide content and a small carbide size compared to STD11, which is a general purpose material. Especially, the number of coarse carbides formed in the solidification process is small, Also, due to the more effective strengthening effect of the fine carbides and the coarsening delay during the tempering heat treatment, it is possible to effectively secure a hardness equal to or higher than that of STD11 with a small amount of carbide.

FIG. 1 is a graph showing a change in volume at the time of austenite-ferrite transformation according to the amount of Al added to a cold tool steel according to the present invention. FIG.
FIG. 2A is a graph showing changes in hardness value after quenching and tempering (Q / T) according to the amount of Al added to cold tool steel according to an experiment of the present invention. FIG. (CCN) according to the amount of addition.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described hereinafter. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

The terminology used herein is for the purpose of illustrating embodiments and is not intended to be limiting of the present invention. In the present specification, the singular form includes plural forms unless otherwise specified in the specification. The terms " comprises "and / or" comprising "used in the specification do not exclude the presence or addition of one or more other elements in addition to the stated element.

Unless defined otherwise, all terms (including technical and scientific terms) used herein may be used in a sense commonly understood by one of ordinary skill in the art to which this invention belongs. Also, commonly used predefined terms are not ideally or excessively interpreted unless explicitly defined otherwise.

Hereinafter, preferred embodiments of the present invention will be described in detail.

The invention is characterized by the alloy composition of cold tool steels.

More specifically, the cold tool steel according to the present invention has a weight percent (Wt%), a steel composition comprising C: 1.00 to 1.20%, Si: 0.5 to 0.7%, Al: 0.3 to 2.0%, Mn: Mo: 0.8 to 1.1%, W: not more than 0.5%, V: 0.2 to 0.5%, Ni: not more than 0.3%, Cu: not more than 0.3% , Nb: not more than 0.2%, and Fe and unavoidable impurities.

In addition to the above-mentioned composition, at least one of 0.02 wt% or less of P, 0.1 wt% or less of S, 0.1 wt% or less of N, 0.02 wt% or less of Ca, or 0.02 wt% or less of B .

The cold tool steel according to the present invention is quenched in the austenite region at a constant temperature (high temperature) and then quenched in the atmosphere or various refrigerants, and then tempered and maintained at a constant temperature, for example, Through the heat treatment step, a tempered martensite can be obtained in which a carbide is dispersed in a martensite base.

The main elements of the above-mentioned alloying elements are as follows.

[C: 1.00 to 1.20%]

It forms carbides in the microstructure, is dissolved in the matrix during the high temperature heat treatment, and forms martensite during the cooling process. It imparts hardness and hardenability to the material, but can cause embrittlement when added excessively.

[Si: 0.5 to 0.7%]

The effect of strengthening employment of base is large, and it contributes to improvement of machinability.

[Al: 0.3 to 2.0%]

Hardening by hardening of the solid solution and effect of grain refinement and refinement of the carbide, and retarding the coarsening of the carbide during the tempering heat treatment to suppress the decrease of the hardness. Further, by expanding the austenite lattice, the effect of reducing the volume expansion during the transformation from austenite to ferrite or martensite is reduced, thereby reducing the heat treatment deformation.

FIG. 1 is a graph showing a change in volume at the time of austenite-ferrite transformation according to the amount of Al added to a cold tool steel according to the present invention. FIG. Referring to FIG. 1, it can be seen that there is no volume change at 1.5W% of Al.

[Cr: 8.5 to 9.5%]

As the main forming element of carbide, it binds with C to give hardness to the material and contributes to improvement of corrosion resistance. The formed carbides are mainly M ₇ C ₃ and M ₂₃ C ₆ . The addition of a large amount of Cr increases the coarsening of the carbide, which may lead to deterioration of the impact toughness. Further, it is a tendency to avoid the addition of a large amount due to the P added to Cr alloy in the electric furnace.

[Mo] + 0.5 [W]: 0.8 to 1.3% (Mo: 0.8 to 1.1%, W: 0.5% or less)

It is solved in the carbide to improve the physical properties of the carbide and contributes most to the improvement of the tempering resistance which suppresses the decrease in the hardness during the tempering process. The atomic weight of W is about twice that of Mo, so the effect of addition is about 0.5 times at the same mass fraction. The addition of Mo and W makes it possible to reduce C and Cr, but there is a problem that the cost of the material increases greatly. Therefore, in the present invention, [Mo] wt% + 0.5 [W] wt% ] +0.5 [W] = 0.8 to 1.3%. In this case, Mo may be 0.8-1.1% and W may be 0.5% or less.

[Nb: 0.2% or less, V: 0.2 to 0.5%]

It forms a fine, high-strength MC carbide to increase the strength of the base.

[Cu: 0.3% or less]

Forms extremely fine Cu precipitates to increase the strength of the matrix and improve the tear resistance.

The cold tool steel of the present invention as described above has a low overall carbide content and a small carbide size compared with STD11, which is a general purpose material. Especially, the number of coarse carbides formed in the solidification process is small, .

Further, due to the more effective strengthening effect of the fine carbide and the coarsening delay during the tempering heat treatment, even a small amount of carbide can effectively secure a hardness equal to or higher than that of STD11.

That is, as will be described later, the cold tool steel according to the present invention has a hardness comparable to or higher than that of conventional STD11, for example, when tempering at 520 ° C. is performed in 1030 ° C. quenching, which is a general heat treatment condition of general- Impact toughness can be improved by more than 60%.

The combination of hardness for abrasion resistance and toughness for chipping and crack resistance of the cold tool steel according to the present invention can be varied through various changes of quenching and tempering conditions. As a result of development of a new alloy composition, Excellent balance of physical properties under general heat treatment conditions.

Hereinafter, the properties of the cold tool steel according to the present invention will be compared through the preferred embodiments of the present invention. However, the present invention is not limited to the following embodiments.

[Experimental Example]

First, in this Experimental Example, a method of manufacturing cold tool steel is described in order to compare physical properties according to the alloy composition of the cold tool steel. Stress relieving heat treatment step → hot forging step → spheroidizing heat treatment step → quenching step → tempering heat treatment The cold tool steel was produced.

More specifically, the test materials to be described later were obtained by casting ingots having a weight of about 30 Kg using a vacuum induction melting furnace, and then maintained at 870 ° C for 4 hours to prevent cracks in the material after solidification Cooled to 680 ° C at a rate of 30 ° C per hour, and slowly cooled.

Forging work was performed to homogenize the material and to remove defects in the solidification structure. Specifically, the material was kept in a preheating furnace heated to 200 to 300 ° C for 1 hour, and then heated in a preheating furnace heated to 400 to 500 ° C Hour, and then maintained in a heat treatment furnace heated to 1130 ° C for 2 hours, and each side was forged with each rod having a length of 40 to 50 mm and then air-cooled. After that, to prevent cracks due to the delay of the heat treatment, after cooling for 5 hours at 680 ° C, cooling to 500 ° C at a rate of 20 ° C per hour, gradual cooling was performed.

Next, the spheroidizing heat treatment was performed at 870 占 폚 for 4 hours, then cooled to 680 占 폚 at a rate of 30 占 폚 / hour, and slowly cooled.

Next, quenching was carried out by maintaining the temperature at 1030 ° C. for 30 minutes, followed by air cooling. The tempering treatment was carried out at a relatively high temperature of 520 ° C. at a temperature of 520 ° C. in consideration of high temperature surface treatments (nitriding, After keeping the time, the mixture was air-cooled to room temperature, maintained at 520 DEG C for 2 hours, and then air-cooled.

The compositions of the test materials obtained through chemical composition analysis are shown in Table 1 below.

division Example 1 Example 2 Example 3 Comparative Example 3 Comparative Example 1 Comparative Example 2 C 1.07 1.17 1.11 1.10 1.55 1.16 Si 0.58 0.60 0.61 0.59 0.26 0.56 Mn 0.41 0.40 0.42 0.42 0.30 0.43 Ni 0.20 0.20 0.20 0.19 0.20 0.19 Cr 9.08 9.07 9.22 8.97 11.4 9.06 Mo 1.02 1.12 1.01 1.03 0.81 1.05 W 0.42 0.45 0.47 0.50 - 0.38 V 0.29 0.30 0.31 0.32 0.20 0.31 Nb 0.16 0.15 0.16 0.16 - 0.14 Al 0.40 1.01 2.00 2.70 - 0.04 Cu 0.20 0.18 0.21 0.24 0.15 0.19 P 0.011 0.007 0.004 0.003 0.02 0.006 S 0.0012 0.004 0.003 0.005 0.003 0.003

The comparative example 1 corresponds to the chemical composition of the general-purpose STD 11, and is an experimental example for comparing the physical properties of the cold tool steel and general-purpose STD 11 according to the present invention.

In addition, in order to confirm the effect of the change in Al content, the chemical compositions of Comparative Examples 2 and 3 and Examples 1 to 3 were made similar except for the Al content. In the case of Comparative Examples 2 and 3 and Examples 1 to 3, the content of C and Cr was limited in order to ensure impact toughness.

The hardness value (unit: HRC) and impact toughness value (unit: J / cm 2) of the test materials after quenching and tempering (Q / T) were measured. The impact toughness was evaluated by the Charpy impact test using 10R C-type notch, taking into consideration the characteristics of the brittle material. A detailed description of this can be found in Metals Handbook, 10th ed., Materials Park OH 2000, vol. 8, pp. 785-86.

The measured values are shown in Figs. 2A and 2B.

FIG. 2A is a graph showing changes in hardness value after quenching and tempering (Q / T) according to the amount of Al added to cold tool steel according to an experiment of the present invention. FIG. (CCN) according to the amount of addition.

Referring to FIGS. 2A and 2B, it can be seen that the hardness and the impact toughness increase simultaneously with the increase of the Al content up to the Al content of 1% level (Comparative Example 2, Example 1 and Example 2).

In this case, it can be confirmed that Examples 1 and 2, in which Al is added at 0.4 and 1.01%, respectively, are superior to Comparative Example 2 in which Al is added at 0.04 wt% as an impurity level and Comparative Example 1 is indicated as STD11.

Compared to Comparative Example 2 in which the hardness was improved by 6.8% and the impact toughness was improved by 127% compared with STD11 of Comparative Example 1 in Comparative Example 1 and the Al content was extremely low in almost the same composition, the hardness was 5.3% 160% improvement.

On the other hand, when the Al content exceeded 2% (Example 3) and reached 3% (Comparative Example 3), it was confirmed that the impact toughness was remarkably improved, but the hardness was drastically decreased. And hardenability is not secured in quenching due to the stabilizing effect.

When the Al content exceeds 2% in the composition range of the cold tool steel according to the present invention, the base structure of the base structure after quenching is not martensite but the composite structure of martensite, bainite, bainite and ferrite, Alternatively, it can be confirmed that the hardness is greatly lowered due to the soft ferrite structure. Therefore, in the present invention, it is preferable that the Al content is 2% or less.

However, the optimum Al content may vary depending on the change in the content of other elements affecting the hardenability, and the Al content, which includes the Al content of 1.5% which minimizes the volume change during the austenite phase transformation, is 0.3 to 2.0% It can be expected that an optimum performance as a cold tool steel is exhibited.

That is, even in the case of Example 3 where the Al content is 2.0 wt%, it is possible to secure sufficiently high hardness (impact strength of 55 HRC or more) and impact toughness by upward adjustment of Mn, Ni, Cu, C and N as the austenite stabilizing elements . Also, by controlling the heat treatment temperature, the hardness value of Example 3 can be largely adjusted up to the present level of 47HRC. Accordingly, it is expected that the present invention will exhibit the best performance as a cold tool steel with an Al content of 1.5%, which minimizes the volume change during the austenite phase transformation, and an Al content of 0.3 to 2.0%.

According to the present invention, as compared with STD11, which is a general purpose material, the overall content of carbide is low and the size of carbide is small. Especially, the number of coarse carbides generated in the solidification process is small, Further, due to the more effective strengthening effect of the fine carbide and the coarsening delay during the tempering heat treatment, even a small amount of carbide can effectively secure a hardness equal to or higher than that of STD11.

Therefore, the cold tool steel according to the present invention can significantly improve the impact toughness while having a similar or higher hardness to the conventional STD11, when subjected to the tempering treatment at 520 ° C for 1030 ° C quenching, which is the general heat treatment condition of the general-purpose STD 11.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, . It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

Claims

By weight,
C: 1.00 to 1.20%;
0.5 to 0.7% of Si;
Al: 0.3 to 2.0%;
Mn: 0.5% or less;
Cr: 8.5 to 9.5%;
[Mo] + 0.5 [W]: 0.8 to 1.3%; And
Cold tool steel containing Fe and unavoidable impurities.

The method according to claim 1,
Wherein the Mo is 0.8 to 1.1 wt%, and the W is 0.5 wt% or less.

The method according to claim 1,
V: 0.2 to 0.5%;
Ni: 0.3% or less;
Cu: not more than 0.3%; And
Nb: not more than 0.2%.

The method according to claim 1,
0.02% or less P, 0.1% or less S, 0.1% or less N, 0.02% or less Ca, or 0.02% or less B by weight.

The method of claim 3,
0.02% or less P, 0.1% or less S, 0.1% or less N, 0.02% or less Ca, or 0.02% or less B by weight.