JPH0959745A - Steel free from coarsening of crystalline grain - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel, in which coarsening of crystalline grains is prevented, by specifying the amount of carbides of Nb, etc., in a steel having a specific composition in which respective contents of C, N, Al, Nb, etc., are specified. SOLUTION: A steel, which contains, by mass, 0.05-1.2% C, <=3.00% Si, <=2.50% Mn, <=2.50% Cr, <=2.50% Ni, <=2.00% Mo, <=0.010% N, <=0.060% Al, and one or <=2 kinds among 0.005-0.20% Nb, 0.005-0.050% Ti, 0.010-0.20% Ta, 0.010-0.2O% Hf, 0.010-0.20% Zr, and 0.010-1.00% V and in which carbides (or nitrides, carbonitrides, etc.) of any of Nb, Ti, Ta, Hf, Zr, and V are precipitated by >=5pieces/10μm<2> in a matrix, is prepared. By this method, the occurrence of the coarsening of austenitic crystalline grains, resulting from heat treatment, can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel for preventing coarsening of austenite crystal grains caused by heat treatment such as carburizing and quenching, induction hardening, and quenching.

[0002]

2. Description of the Related Art Most of special steels such as carbon steel, case hardening steel, toughness steel, bearing steel, spring steel, tool steel, etc. are subjected to quenching treatment, carburizing quenching treatment, or induction hardening depending on the use application. Various heat treatments such as treatment are performed. But,
When these heat treatments are performed, the material is heated and held at a high temperature, so that the austenite crystal grains grow and there is a problem that the crystal grains are coarsened. It is known that when coarse grains are generated, the strength / toughness and fatigue properties of steel are deteriorated, which is a problem in practical use.

In recent years, there have been increasing opportunities to manufacture parts by cold forging aiming at cost reduction. However, when a part manufactured by cold working is carburized, crystal grains may partially grow abnormally, which causes a problem of promoting distortion of the part. At the same time, it is known that the strength and toughness decrease when these abnormally grown crystal grains are generated, and it is necessary to prevent the crystal grains from coarsening.

Various studies have been carried out in order to prevent the coarsening of these crystal grains. Today, steels which mainly use fine particles of fine precipitates dispersed in a steel material and utilize the pinning effect of these precipitates have been developed. Proposed. In the case of general-purpose steel such as carbon steel or low alloy steel, Al is added and deoxidized in order to reduce oxygen in the steel, and this Al combines with nitrogen in the steel to form AlN. Since this Al precipitate also provides the pinning effect, coarsening of crystal grains can be suppressed in the temperature range of 900 ° C. or lower.

However, these steels cannot necessarily suppress the coarsening of crystal grains, and even the steels currently proposed are not in a situation where sufficient coarsening characteristics can be obtained. Nowadays, it is considered to perform heat treatment at a higher temperature in order to shorten the heat treatment time, and it is desired to develop a material having excellent crystal grain coarsening characteristics even in a higher temperature region.

[0006]

The present invention has been made in view of the above circumstances, and the object of the present invention is to carry out heat treatment such as carburizing, induction hardening, or quenching. To provide a steel that prevents the occurrence of austenite grain coarsening that occurs,
To this end, it is an object to provide a steel in which the elements added to the steel are optimized and the form and amount of the precipitates dispersed in the matrix are optimized to prevent the coarsening of crystal grains.

[0007]

Means for Solving the Problems The present inventor has studied combinations of various alloying elements, and in order to prevent the growth of austenite crystal grains at the time of high temperature heating, precipitates dispersed in the base material of steel. It has been found that the grain size can be prevented from becoming coarse by optimizing the morphology and amount. The present inventor has studied in detail the precipitate morphology (composition) that affects the coarsening property, and as a result, Nb carbide, Nb nitride, or Nb carbonitride is precipitated in the matrix and at least 5 /
It has been found that by distributing 10 μm ² or more, it is possible to prevent the crystal grains from becoming coarse even during high temperature heating such as quenching treatment or carburizing treatment. Also, Ti, T
It was confirmed that similar effects could be obtained with a, Hf, Zr, and V.

It has also been found that a precipitate having a composite composition of the above precipitate and Al is also effective for improving the coarsening property. In addition, in elements such as Ta, Hf, Zr, Ti, and V, their carbides, nitrides, carbonitrides, and Al
It was found that the precipitate consisting of the composite composition of was effective in preventing coarsening.

That is, in the grain coarsening preventing steel of the present invention, the content of alloying elements is% by mass, C: 0.05% to 1.2% Si: 3.00% or less Mn: 2.50% Below Cr: 2.50% or less Ni: 2.50% or less Mo: 2.00% or less N: 0.010% or less Al: 0.060% or less, and further Nb: 0.005% to 0 .20% Ti: 0.005% to 0.050% Ta: 0.010% to 0.20% Hf: 0.010% to 0.20% Zr: 0.010% to 0.20% V: 0 0.010% to 1.00%, containing 1 or 2 or more kinds, and N in the matrix
b, Ti, Ta, Hf, Zr, or V is characterized by precipitating 5/10 μm ² or more of any of carbides, and Nb, Ti, Ta, Hf, Zr, V
5 or 10 μm ² or more of the nitrides or carbonitrides of the above are deposited in the base material.
It is characterized in that carbides / nitrides of b, Ti, Ta, Hf, Zr, and V, precipitates having a composite composition of carbonitride and Al are precipitated in the base material in an amount of 5/10 μm ² or more.

The reasons for limiting each alloying element will be described below. C: 0.05% to 1.2% C is Nb, Ti, Ta, Hf, Zr contained in steel,
It is an essential element for precipitating carbides by combining with V or the like, and at least 0.0 in order to obtain a predetermined number of precipitations.
It is necessary to contain 5%. If C is contained in excess of this amount, carbides are precipitated, but C added as steel is generally 1.2%, and the upper limit of C amount is 1.2% in the present invention as well.

Si: 3.00% or less Mn: 2.50% or less Cr: 2.50% or less Ni: 2.50% or less Mo: 2.00% or less Each element of Si, Mn, Cr, Ni and Mo Is an element added for the purpose of improving mechanical properties of steel such as improvement of strength properties such as tensile strength and fatigue and toughness, or improvement of hardenability properties. Therefore, the element amount may be selected within a range suitable for each application. Si, Mn, Cr, N
The upper limit of the amount of each of i and Mo added is that the hot workability deteriorates,
As a range that does not deteriorate manufacturability such as machinability and forgeability, 3.00%, 2.50%, 2.50%,
2.50% and 2.00% are specified.

N: 0.010% or less By containing N, nitrides or carbonitrides of Nb, Ta, and Ti can be deposited, so that the effect of suppressing coarsening of crystal grains is obtained. The purpose of the present invention is to obtain a substantially carbide-based precipitate, and it is desirable that the content of N is small. In particular, when an element having a strong tendency to form a nitride such as Ti or Nb is added as in the present invention when the amount of N in the steel is increased, a large nitride is formed during solidification, and the originally intended carbide is formed. Since it becomes difficult to obtain a precipitate in the system, the upper limit of the N content is set to 0.010.
Regulated to%.

Al: 0.060% or less Al is an element that plays an important role in the present invention, and is added to obtain a precipitate necessary for suppressing the coarsening of crystal grains. In the present invention, the main purpose is not to obtain a precipitate of AlN as in the prior art, but N
The purpose is to obtain a precipitate in which Al and a carbide or carbonitride composed of b, Ti, or the like is combined. In particular, since the precipitate formed in combination exhibits stable properties even in a high temperature range and is found to be effective in preventing the coarsening of crystal grains, the range of 0.005% to 0.03% is preferable. It is desirable to add. In addition, even if added over 0.06%, the effect is saturated, and in order to promote the occurrence of cracks during casting, the upper limit of the added amount was restricted to 0.06%.

Nb: 0.005% to 0.20% Ti: 0.005% to 0.050% Ta: 0.010% to 0.20% Hf: 0.010% to 0.20% Zr: 0 0.010% to 0.20% V: 0.010% to 1.00% Nb, Ti, Ta, Hf, Zr, and V are elements that play an important role in the present invention, and are predetermined precipitates. In order to obtain the morphology and amount of Ti and improve the coarsening property, at least Nb: 0.005% and Ti: 0.005%, respectively.
%, Ta: 0.010%, Hf: 0.010%, Zr:
It is necessary to add 0.010% and V: 0.01% individually or in combination. However, in order to saturate the effect even if added excessively, reduce the hot workability and cold workability, and unnecessarily increase the manufacturing cost of the raw material, the upper limits of the respective addition amounts were specified.

Details of Morphology of Complex Precipitates and Quantity of Precipitates Steels containing four elements of Nb, Al, C and N are roughly classified into AlN, NbC (or NbN, Nb).
It was confirmed in the present invention that a composite carbonitride of (C, N)) and Al.Nb is precipitated in the matrix. As a result of studying the morphology of the precipitates remaining after heating the steel having these precipitates to a high temperature, most of the precipitates remaining when heated and maintained at a high temperature of 900 ° C or higher are Nb-based precipitates. I found that it was a thing.

When AlN among the precipitates is heated and maintained in a temperature range of 900 ° C. or higher, the AlN itself becomes large or becomes a solid solution in the matrix, which contributes to prevention of coarsening of crystal grains. It was confirmed that they did not. Therefore, when only AlN is deposited, a sufficient effect of suppressing coarsening cannot be obtained.

On the other hand, Nb carbide or NbN, Nb
It was confirmed that when Nb-based precipitates such as (C, N) were present in the matrix, these precipitates remained even after high temperature heating, and that coarsening of crystal grains did not occur. At the same time, it was confirmed that precipitates having a composite composition of Nb and Al also remained after heating at high temperature, and it was found that these precipitates effectively act to suppress the coarsening of crystal grains.

That is, it was confirmed that the form of the precipitate for improving the coarsening property was any of Nb carbide, Nb carbonitride, Nb nitride, and a precipitate of Al / Nb composite composition.

Ti, Ta, Hf, Zr and V have exactly the same functions as Nb and can be added in place of Nb. Similar to Nb, these elements are C and N in steel.
It was confirmed that it has the effect of suppressing the coarsening of the crystal grains by combining with and precipitating carbides, carbonitrides, and nitrides. It was also confirmed that the same effect can be obtained even if two or more elements are added at the same time. These elements can be selected depending on the required coarsening temperature and material cost.

Further, as a result of investigating the relationship between the amount of precipitates and the coarsening temperature, it was confirmed that the coarsening temperature was improved as the amount of precipitation was increased. N to increase the coarsening temperature
b It is desirable to deposit a large amount of precipitates such as carbides, but the number of precipitates should be at least 5/10 μm in order to prevent coarsening of crystal grains even when heated and held in a temperature range of 900 ° C. or higher. ² is required. In addition to Nb carbide, the total sum of Nb nitride, Nb carbonitride, and Nb-Al composite composition precipitates is 5/10
It was confirmed that even with μm ² , almost the same coarsening characteristics were obtained.

[0021]

Example A material having the chemical composition (mass%) shown in Table 1 was melted using a vacuum induction furnace. Each material was melted, cast into a 30 kg ingot, and hot-forged into a round bar having a diameter of 32 mm. This round bar is 760 ℃
After holding for 6 hours at 650 at a cooling rate of 30 ℃ / h
A spheroidizing annealing treatment was performed to gradually cool to ℃. further,
The spheroidized annealed material is machined to have a diameter of 25 mm,
A cold-working test piece having a height of 37.5 mm was produced.

[0022]

[Table 1]

With respect to the material having the chemical composition shown in Table 1 after the spheroidizing annealing of steel, an extraction replica was prepared and the morphology of precipitates and the number of precipitates per unit area were determined by an electron microscope. Then, in order to investigate the coarsening property of each material, the cold working test piece was subjected to upset processing with a working ratio of 70% by a cold forging press, and then 900 ° C to 11 ° C.
It was heated to a temperature range of 00 ° C. and held for 30 minutes (water cooling after holding), and the crystal grain size of the heating / holding material was measured. The coarsening temperature is defined as the coarsening temperature, which is the maximum heating temperature at which the crystal grains are measured at each heating temperature and coarse grains of No. 4 or less in JIS austenite grain size number are not observed.

[0024]

[Table 2]

Table 2 shows the precipitates except AlN observed in the raw material and the coarsening temperature. In the case of the invention steels, all of them are 5 pieces / 10 μm ² or more of Nb precipitates (or T).
While i, Ta, Hf, Zr, and V-based precipitates are obtained, AlN is the only precipitate observed in the comparative steels. Further, the larger the number of precipitates, the higher the coarsening temperature tends to be.
A coarsening temperature of 0 ° C. or higher is obtained, whereas the comparative steels all have a coarsening temperature of <900 ° C.

In this way, Nb, Ti, Ta, Hf, Z
It was made possible to significantly improve the coarsening property by obtaining a predetermined amount of r, V-based carbides, carbonitrides, nitrides, and precipitates compounded with Al.

[0027]

EFFECTS OF THE INVENTION According to the present invention, it is possible to obtain a crystal grain coarsening preventive steel which does not coarsen even at high temperature, by optimizing the selection of specific elements and the morphology and amount of the precipitates that precipitate on the substrate. It is possible to provide. According to the present invention, coarsening of crystal grains is prevented, and it is possible to reduce the occurrence of strain after heat treatment and improve the strength and toughness of parts. Further, since the crystal grains are not coarsened even if the heat treatment is performed at a higher temperature, the heat treatment can be performed at a high temperature, and the treatment time can be shortened.

Claims (3)

[Claims] 1. The alloying element content is% by mass, C: 0.05% to 1.2% Si: 3.00% or less Mn: 2.50% or less Cr: 2.50% or less Ni: 2 .50% or less Mo: 2.00% or less N: 0.010% or less Al: 0.060% or less, and further Nb: 0.005% to 0.20% Ti: 0.005% to 0 0.050% Ta: 0.010% to 0.20% Hf: 0.010% to 0.20% Zr: 0.010% to 0.20% V: 0.010% to 1.00% Contains 1 or 2 or more types and N in the substrate
A grain coarsening prevention steel characterized by precipitating 5/10 μm ² or more of any one of b, Ti, Ta, Hf, Zr and V carbides. 2. The steel according to claim 1, wherein Nb,
Steel for preventing crystal grain coarsening, characterized in that nitrides or carbonitrides of Ti, Ta, Hf, Zr, V are precipitated in the base material in an amount of 5/10 μm ² or more. 3. The steel according to claim 1, wherein N is contained in the base material.
Crystal grains characterized in that carbides / nitrides of b, Ti, Ta, Hf, Zr, and V, and precipitates having a composite composition of carbonitride and Al are precipitated in the base material in an amount of 5/10 μm ² or more. Anti-coarsening steel.