KR20140081124A - Boron-added high carbon steel for nitriding, nitriding method for the same and nitrided boron-added high carbon steel - Google Patents

Abstract

The present invention relates to boron-added high carbon steel for nitriding, a nitriding method for the same, and nitrided boron-added high carbon steel, wherein the boron-added high carbon steel includes 0.40 to 0.55 wt% of C, 1.5 wt% or less of Mn, 0.5 wt% or less of Si, 0.1 to 0.3 wt% of Cr, 0.7 to 1.2 wt% of Al, 0.0008 to 0.0060 wt% of B, 0.1 to 0.3 wt% of Ti, the remaining amount consisting of Fe and other inevitable impurities. According to the present invention, the steel made of low cost component elements is nitrided at a high temperature, thus strengthening the matrix structure of the steel and ensuring high abrasion resistance.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a boron-added high carbon steel for nitriding, a nitriding treatment method thereof, and a nitrided boron-added high carbon steel for a nitriding treatment,

The present invention relates to a high carbon steel containing boron (B) for nitriding treatment, a nitriding treatment method thereof and a high carbon steel with nitrile-treated boron (B).

Nitriding is a kind of surface hardening method which is originally carried out at a temperature of 500 to 600 ° C and is a classical surface heat treatment method used for products requiring wear resistance such as various automobile parts, machine parts and tool molds because the deformation is relatively small. In recent years, a method of simultaneously obtaining a heat treatment effect and a nitrided layer effect by quenching while cooling a steel material nitrided at a high temperature starting from stainless steel is also increasing. In order to obtain high surface hardness by such high-temperature nitriding heat treatment, alloying elements play an important role. Among these, Al, Cr and Mo are the most important elements.

Existing typical nitriding heat treatment robust SACM645 steel for, it contains a high-priced alloying element, and Al, such as Mo, Cr, they make the respective MoN or Mo ₂ N, CrN or Cr ₂ N, AlN compound nitride layer having high hardness It is advantageous in securing physical properties due to the nitriding heat treatment and has sufficient curing ability.

However, conventional nitrided heat treated steels with SACM645 and similar alloys have a disadvantage of high component cost because they contain 0.15 ~ 0.3% of very expensive Mo and 1.3 ~ 1.7% of high Cr.

In general B-added high carbon steels, these expensive elements are replaced with low-priced B to ensure hardenability. However, in the case of high-temperature nitriding heat treatment, a nitride layer is formed mainly on FeN, (Solute B) in the steel is combined with nitrogen (N) to form BN by the influence of diffused penetrated nitrogen (N) in the deep portion, so that the hard B And the curing ability expected from the B-containing high carbon steel is not ensured.

One aspect of the present invention is to provide a boron-added high carbon steel for nitriding which is capable of uniformly setting the hardenability at the surface and the deep without using a large amount of expensive elements, a method of nitriding boron- I would like to present the addition of high carbon steel.

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

In order to accomplish the above object, one aspect of the present invention provides a steel sheet comprising: 0.40 to 0.55% of C, 1.5% or less of Mn, 0.5% or less of Si, 0.1 to 0.3% of Cr, %, B: 0.0008 to 0.0060%, and Ti: 0.1 to 0.3%, and the balance Fe and other unavoidable impurities.

Another aspect of the present invention is to provide a method of manufacturing a semiconductor device, comprising: 0.40 to 0.55% of C, 1.5% or less of Mn, 0.5% or less of Si, 0.1 to 0.3% of Cr, 0.7 to 1.2% of Al, 0.0008 to 0.0060% Preparing a steel comprising 0.1 to 0.3% of Ti, the balance consisting of Fe and other unavoidable impurities; The method comprising the steel at a temperature above A ₃ chimjil nitride atmosphere heat treatment for 10 minutes or more; And cooling the steel. The present invention also provides a method for nitriding boron-added high-carbon steels.

In another aspect of the present invention, there is provided a ferritic steel comprising: 0.40 to 0.55% of C, 1.5% or less of Mn, 0.5% or less of Si, 0.1 to 0.3% of Cr, 0.7 to 1.2% of Al, 0.0008 to 0.0060% And 0.1 to 0.3% of Ti, and the balance of Fe and other inevitable impurities, nitrogen introduced into the surface of the steel by the nitriding heat treatment is combined with Al and Ti in the steel to form AlN and TiN to maintain the hardenability of the surface portion And a nitrided boron-added high-carbon steel which retains the hardening ability of the core by forming TiN by binding Ti to nitrogen at the core of the steel.

According to the present invention, high-temperature nitriding heat treatment of steel composed of low-cost constituent elements including boron can strengthen the base structure of steel and ensure wear resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating a partial cross-sectional structure of a nitrided boron-added high carbon steel according to an embodiment of the present invention. FIG.

Hereinafter, the boron-added high carbon steel for nitriding treatment, the nitriding treatment method thereof, and the nitrided boron-added high carbon steel of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention.

In the present invention, there is proposed a boron-added high-carbon steel for nitriding treatment in which high-priced Mo and Cr are largely removed for the purpose of securing hardenability and physical properties in a nitriding heat treatment steel which is conventionally used.

Specifically, the boron-added high carbon steel for nitriding provided in the present invention contains 0.40 to 0.55% of C, 1.5% or less of Mn, 0.5% or less of Si, 0.1 to 0.3% of Cr, 0.7 to 1.2% , B: 0.0008 to 0.0060%, and Ti: 0.1 to 0.3%, and the balance Fe and other unavoidable impurities.

In other words, Al was mainly designed to be alloyed with nitrogen introduced in the nitriding heat treatment, so that N is bonded to Al in the steel. By increasing Ti more than general B-added high carbon steel, N penetrating deep into the nitriding heat treatment Ti was first bound to the TiN layer so that it could be hardened by the addition of B in the core and strengthened by nitriding at the surface.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

C: 0.40 to 0.55%

The steel containing a large amount of carbon is advantageous in securing the martensite structure by quenching, and the secured martensite structure contributes greatly to the strength. In the case of the present steel, since a sufficient amount of martensite is required to form a hardened structure of the deep portion supporting the nitrided layer in the cooling process, the lower limit of C is set to 0.40% in order to secure the martensite. In the case of the upper limit, in the case of addition of more than 0.55%, the effect of increasing the hardenability by the addition of B, which is an important functional group of the present invention, is gradually saturated and the effect of increasing the hardenability of the steel by B addition becomes insignificant, so it is limited to 0.40 to 0.55%.

Mn: 1.5% or less

Mn has little help for increasing the hardenability, but its effect is not so large as compared with boron (B), so that the lower limit is not required, and if it is too high, segregation such as center segregation or micro segregation becomes severe. For this reason, its content should be limited to 1.5% or less.

Si: not more than 0.5%

Si is the five major elements of steel and small amounts are naturally added during the manufacturing process and contribute to the strength increase. If the amount is too high, surface defects such as severe scale are induced in the high carbon hot-rolled steel, It will hinder sex. The steel material to be produced in the present invention is a steel material for the surface heat treatment, and when the surface is defective, the meaning of securing the wear resistance through surface heat treatment is not meaningful, so the upper limit is limited to 0.5% in order to control surface defects.

Cr: 0.1 to 0.3%

Cr has an effect of preventing the decarburization of molten steel by forming a Cr layer on the surface of the molten steel, which is effective for improving the hardenability. In the present invention, since the hardenability can be ensured through the addition of B, it is defined as described above so that an appropriate amount (0.1%) or more for use for the purpose of preventing decarburization of molten steel can be ensured.

Al: 0.7 to 1.2%

Al is one of the most important elements in the present invention. AlN layer, which is a strong nitriding layer, is formed on the surface to improve abrasion resistance. In the core part, Ti bonds with B added to secure hardenability of nitrogen (N) It plays a role of supplementary prevention together. In order to reduce the cost, the present invention greatly reduces the Mo and Cr which can harden the surface compared with the conventional SACM645 steel, so that Al must play its role in most cases. Therefore, the lower limit was defined by significantly increasing the content of Al. However, when Al is added as a large amount of element which gives considerable strain to the casting, the stability of the continuous casting such as occurrence of corner crack is impaired and the upper limit is restricted at an appropriate level.

B: 0.0008 to 0.0060%

B is a ferrite or a pearlite (or bainite) by decreasing the grain boundary energy by segregating in grain boundaries, or by the effect that the fine precipitates of Fe ₂₃ (C, B) ₆ are segregated in grain boundaries and lowering the grain boundary area It is an element that has the effect of inhibiting transformation. As in the present invention, by controlling the cooling rate after hot rolling, it is an important alloying element in the case of producing martensite or martensite and a small amount of bainite as a main phase after being rolled in the hot rolling process. In the case of bonding with nitrogen (N), BN is generated and the curing ability is lowered. Therefore, it is usually added in the same manner as Ti. When boron (B) is added in an amount of less than 8 ppm, the effect of increasing the hardenability by addition of B is insignificant. It is expected that deterioration of toughness and degradability due to grain boundary precipitation of precipitates of B are expected to be 0.0008 to 0.0060% because of the tendency that the hardening ability increase effect increases sharply as solute boron increases and gradually decreases. Respectively.

Ti: 0.1 to 0.3

Ti is added in order to prevent the nitrogen (N) from binding to the solid solution B to precipitate as BN by fixing the nitrogen present in the steel to TiN to prevent the hardening ability of the B addition steel from deteriorating. Nitrogen in the steel is usually less than 100ppm. When 0.1% or more of Ti is added, the lower limit is defined because it can be combined with most of the nitrogen present in the steel to form TiN. On the contrary, if the Ti content is too high, the toughness of the steel is markedly decreased, so the upper limit is limited to 0.3% or less.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

A steel having the above-described components is prepared and subjected to a nitriding heat treatment for 10 minutes or more in a steep atmosphere at a temperature of A ₃ or more, and quenched to room temperature to form a martensite phase.

The reason for setting the lower limit of the temperature and the time of the nitriding heat treatment is to secure the austenite, so the upper limit setting is meaningless. Further, since the purpose of cooling after the nitriding heat treatment is to produce a martensite phase, it is preferable to quench it to Ms or less and to cool to room temperature.

At this time, the submerged atmosphere can be formed by decomposing ammonia into nitrogen and hydrogen to form a nitrogen atmosphere.

2NH ₃ → 2N + 3H ₂

By such nitrification treatment, a steel having a structure as shown in Fig. 1 can be produced. That is, a nitrided layer containing a part of TiN is formed on the surface portion of the nitrided steel with AlN as the main component. Nitrogen introduced into the surface during the high-temperature nitriding heat treatment mainly bonds with Al in the steel to form an AlN compound to form a main surface hardened layer. In addition, Ti present on the surface portion bonds with nitrogen to further form a TiN layer The surface layer was strengthened.

Nitrogen concentration in the surface is very high on the surface. The next occurrence is the boundary. Since the B does not contribute to ensuring hardenability at the boundary station, the depth of the boundary zone should be as small as possible. The abundant Ti in the steel plays a role of preventing the nitrogen (N) and B penetrating into the deep part by the high-temperature nitriding heat treatment in the boundary region as well as the boundary region. That is, nitrogen (N) penetrating into the deep portion is first bound to Ti, which has strong binding force, to be expressed as TiN. As a result, B is hardly affected by the high-temperature nitriding heat treatment at the deep portion, and B is bonded to the grain boundary in a solid state to cause a solute dragging effect, thereby making it possible to improve the hardenability. Further, through the cooling step of the nitriding heat treatment, a low temperature transformation structure such as martensite can be secured in the deep portion to maintain the hardenability. It is advantageous from the viewpoint of strength that the structure of the deep part is composed of a low temperature transformation structure of 60% or more of martensite in an area fraction. When the curing treatment is performed as specified in the present invention, the tensile strength of the core portion is usually at least 1700 MPa, and the higher the strength of the core portion, the more favorable the strength of the surface is.

In addition, the hardness of the surface portion where a nitride layer is formed mainly on the basis of AlN is usually in the range of 1000 to 1200 Hv, but it may locally have a value of 1200 Hv or more in some cases. Here, the higher the hardness of the surface portion and the deep portion is, the better the upper limit is not specified. The surface hardness is preferably 1000 Hv or more, and the core hardness is preferably 500 Hv or more.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

A steel having the components shown in Table 1 below was prepared.

Comparative Example 1 is SACM645 steel, which is an expensive high alloy high carbon steel used in conventional nitriding heat treatment, Comparative Example 2 is SAPH440 having general strength, Comparative Example 3 is S45C, which is a high carbon steel having a similar amount of carbon, Comparative Example 4 is 0.5% POS10B50, which is carbon-rich B-added high carbon steel. The content of the component is in weight%.

division C Mn Si Cr Al B Ti Mo Remainder Honor 0.43 0.30 0.20 0.20 1.03 0.002 0.20 - The balance Fe and other unavoidable impurities Comparative Example 1 0.43 0.31 0.21 1.44 1.00 - - 0.21 Comparative Example 2 0.16 0.49 0.01 - - - - - Comparative Example 3 0.45 0.23 0.18 0.13 - - - - Comparative Example 4 0.51 0.42 0.23 0.17 - 0.002 0.02 -

The steels of the inventive and comparative examples prepared above were made into plate-shaped specimens having an area of 285 mm x 110 mm and a thickness of 8 mm through ordinary hot rolling. The nitriding treatment was carried out by maintaining the steep atmosphere while burning the ammonia by using the nitriding heat treatment furnace, and keeping the specimens at 820 DEG C for 25 minutes in such a steep atmosphere and cooling them.

The tensile strength (TS), the yield strength (YS) and the elongation (EL) of the specimens subjected to the nitriding treatment were measured by the above method, and the hardness of the surface portion and the deep portion was measured. [Table 2].

division Surface hardness (Hv) Deep part hardness (Hv) Tensile properties TS (Mpa) YS (Mpa) EL (%) Inventory 1 1080 584 1820 1705 3.4 Comparative Example 1 1175 565 1778 1642 5.2 Comparative Example 2 485 153 455 389 28.3 Comparative Example 3 520 295 1023 915 6.8 Comparative Example 4 508 365 1158 1036 5.1

The results of Table 2 show that the inventive examples show similar physical properties (TS, YS, EL) to those of conventional expensive nitriding heat treatment steels (Comparative Example 1). Since Mo is not present and Cr is relatively small, Mo ₂ N compound or the like can not be formed on the surface, and CrN is not sufficiently formed. Therefore, the surface hardness is insufficient as compared with the existing expensive nitriding heat treatment steel (Comparative Example 1) The surface hardness was secured by AlN formation, and the core properties were rather superior to the conventional steel.

In the case of Comparative Example 1, although the physical properties are sufficient, there is a problem that the cost is high and the economical efficiency is low.

In the case of Comparative Example 2 in which general steel was simply nitrided at a high temperature, only a nitrogen compound layer showing relatively low hardness such as FeN or Fe ₂ N is formed on the surface because there is no alloy element suitable for nitriding, Are not produced properly and show softness properties compared with other specimens of high carbon steel series.

In the case of Comparative Example 3, the high-temperature nitriding treatment of general high carbon steel can secure the physical properties of the steel sheet with a thin thickness, but since the heat transfer to the steel sheet is slow and the hardening ability of the steel is small, The structure is not completely formed and exhibits a relatively low strength as compared with the embodiment. On the surface, only a nitrogen compound layer such as FeN or Fe ₂ N is formed similarly to the case of nitriding heat treatment of general steel.

In the case of Comparative Example 4, the boron (B) bonds with the nitrogen (N) penetrated to the deep portion when the B-containing high carbon steel is subjected to the high-temperature nitriding heat treatment and the curing ability is lowered. Thus, formation of the martensite phase The physical properties of the core portion are smaller than those of the embodiment of the present invention, and the hardness of the surface is also lowered by the same principle as that of Comparative Examples 2 and 3.

While the present invention has been described in connection with certain exemplary embodiments and drawings, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood that various modifications and changes may be made without departing from the scope of the appended claims.

Claims (5)

The steel sheet contains 0.40 to 0.55% of C, 1.5% or less of Mn, 0.5% or less of Si, 0.1 to 0.3% of Cr, 0.7 to 1.2% of Al, 0.0008 to 0.0060% of B and 0.1 to 0.3% Boron-added high-carbon steels for nitriding comprising Fe and other unavoidable impurities. The steel sheet contains 0.40 to 0.55% of C, 1.5% or less of Mn, 0.5% or less of Si, 0.1 to 0.3% of Cr, 0.7 to 1.2% of Al, 0.0008 to 0.0060% of B and 0.1 to 0.3% Preparing a steel comprising the remainder Fe and other unavoidable impurities;
The method comprising the steel at a temperature above A ₃ chimjil nitride atmosphere heat treatment for 10 minutes or more; And
And a step of cooling the steel. The steel sheet contains 0.40 to 0.55% of C, 1.5% or less of Mn, 0.5% or less of Si, 0.1 to 0.3% of Cr, 0.7 to 1.2% of Al, 0.0008 to 0.0060% of B and 0.1 to 0.3% And nitrogen introduced by the nitriding heat treatment is combined with Al and Ti in the steel to form AlN and TiN to maintain the hardenability of the surface portion and Ti is added to the core portion of the steel, A nitrided boron-added high carbon steel which retains the hardenability of the core by combining with nitrogen to form TiN. The method of claim 3,
Wherein the hardness of the surface portion is 1000 Hv or more and the hardness of the core portion is 500 Hv or more. The method of claim 3,
Wherein the texture of the deep portion is a low temperature transformation structure of 60% or more of martensite in an area fraction.

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