JPS61124520A - Production of regulated coarse-grained carburizing steel - Google Patents

Abstract

PURPOSE:To prevent a steel contg. prescribed percentages of Al and N from producing a hardening strain and from being made uneven owing to partial coarsening caused by making fine austenite by hot rolling the steel under prescribed conditions. CONSTITUTION:A carbon steel or a low alloy steel contg. 0.01-0.1% Al and 0.001-0.05% N is refined. The steel is heated to 850-1,150 deg.C, hot rolled at <1,000 deg.C and >=30% draft, and cooled to ordinary temp.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing steel for carburizing, and in particular, by performing a specific hot rolling, austenite is partially coarsened due to fineness. The present invention relates to a method for manufacturing a coarse-grained carburizing steel that has excellent carburizing properties and low-temperature toughness without causing non-uniformity or quenching distortion due to carbonization.

(Conventional technology) Carburizing treatment is 5 for gears and shafts! This method is widely used to improve the wear and fatigue resistance of mechanical parts, but it requires high temperatures and long periods of time, which poses many problems in terms of productivity and energy conservation.

As a countermeasure to these problems, Gachika uses a treatment temperature of 950C, which is higher than the normal processing temperature of 900 to 930C, to speed up the carburizing speed.
Steel tends to be carburized at temperatures above C. However, when carburized at high temperatures, the austenite structure of the steel material becomes coarser and a mixed grain structure is generated.
Occurrence of quenching distortion 2 Various problems such as deterioration of mechanical properties and variation in carburization hardening depth are induced.

Conventionally, in order to suppress the coarsening of the austenitic structure, it has been carried out to contain Y, kt+ Nb, Ti+, etc., which have a crystal grain refining effect, into steel to suppress the coarsening of the steel grains when austenitized. ing. However, since the coarsening suppressing function depends on the solid solution state of these additive elements and the N-temperature, suppressing the coarsening of crystal grains is easily affected by heat treatments such as cold working and softening annealing, and is not sufficiently effective. do not have. In Japanese Patent Publication No. 54-1647, the heating temperature was set at 1150 C, and Nb, which has the effect of suppressing austenite crystal grains from becoming mixed grains by finely precipitating during the dipping treatment, was added. Disperse and dissolve as much as possible in austenite, then hot-roll.

Hot processing such as hot forging is performed. This hot processing is 10
It is characterized by finishing at a high temperature of 00 tl:' or more, and cooling the austenite crystal grain size to coarse grains with a grain size number of 5 or less. The method described in Japanese Patent Publication No. 54-1647 is extremely effective in achieving refinement of austenite grains during carburization.

(Problems to be Solved by the Invention) However, in the above method, Nb and Iyt compounds (nitrides, carbides) are added at a heating temperature of 1150 C or higher, and hot working is performed at a temperature of 1000 C or higher. Molten Ntt increased, and the generation of mixed grains at the carburizing temperature could not be completely prevented.

Furthermore, addition of Nb, Ti, etc. not only increases cost but also lowers low-temperature toughness, which is not preferable. Furthermore, from the viewpoint of hardenability, it is necessary to add a larger amount of hardenability-improving elements, and there are various drawbacks such as the influence of the weight effect becomes more severe and the cost increases.

(Means for Solving the Problems) An object of the present invention is to provide a method for manufacturing coarse-grained carburizing steel that can eliminate and improve the drawbacks and problems of conventional methods. The above object can be achieved by providing a method for manufacturing coarse-grained carburizing steel as described in the claims. That is, in this invention, A-t is 0.01-0.1%, NJ-0,001-0.
05% carbon steel or low alloy steel containing 85Qc to 1
Heating to a temperature range of less than 150 C, hot pressure fi with a reduction rate of 30% or more in a temperature range of less than 1000 C! ; It relates to a method for manufacturing coarse-grained carburizing steel, which is provided with: and cooled to room temperature.

The method of the present invention will be explained in detail below.

According to the present invention, carbon steel or low alloy steel containing a suitable amount of Asahi, N is heated at 850C to less than 1150C, and then 1
After hot rolling at a temperature of less than 0.000 C and a rolling reduction of 30% or more, the copper is air-cooled to form copper with a fine structure.

Generally, when austenitized at 900 to 1000 C, carbon steel or low alloy steel containing Aj and N becomes A2.
Grain growth is suppressed by precipitation of nitrides, resulting in fine grains. However, this temperature range is a region where n-nitride partially dissolves, so it is in a solid solution state and pretreatment.

It is significantly affected by the form of precipitates, and in reality coarse grains may occur locally, making control difficult.

According to the present invention, on the contrary, coarse austenite grains with regular grains do not cause quenching distortion or deterioration of mechanical properties, and improve hardenability and carburizability. This invention was completed by investigating the manufacturing conditions of the tissue in detail.

That is, according to the present invention, 850C is applied prior to hot rolling.
A large amount of n-nitride is precipitated by heating at a relatively low temperature of less than 1150 C to obtain fine austenite grains, followed by hot rolling. In particular, if hot rolling is carried out at a temperature lower than 1000 C, the progress of recrystallization of austenite grains will be delayed or will not occur. Furthermore, as the eriostenite grains are elongated during rolling, many lattice defects are accumulated within the grains. This effect is particularly noticeable when the rolling reduction is 30% or more.

When such austenite grains undergo r→α transformation, they become extremely fine in structure. Furthermore, solid solution M precipitates as so-called strain-induced AJLg compounds in lattice defects within austenite grains, and the amount of solid solution decreases significantly.

When such a fine structure is austenitized at the carburizing temperature, rapid grain growth occurs, resulting in well-sized austenite grains. The reason for this is that reprecipitated n-nitrides, which suppress the growth of austenite grains, are extremely reduced, and when the microstructure undergoes α→r transformation, the number of nucleation sites increases significantly.
Fine austenite grains are generated, but the driving force for growth of austenite grains increases as the grain size decreases, so coarsening occurs at lower temperatures, and coarse grains are formed in the carburizing temperature range. This is due to

As mentioned above, since the present invention utilizes the effect of inhibiting rainbow crystal grain growth and the effect of reducing the degree of coarsening of austenite grains in the microstructure, it takes n to achieve the intended purpose.
It is also necessary to control the N content and manufacturing conditions.

Next, the addition amount of n and N will be explained.

Beni is effective in suppressing the growth of austenite grains before hot rolling. However, if it is less than 0.01%, this effect is insufficient, and if it exceeds 0.10%, non-gold inclusions such as 203 will increase, so the upper limit of the added uji should be set to 0.10%.
%. It combines with red in NFi steel, precipitates as A2 nitride, and exhibits the effect of suppressing austenite grain growth. In order to make this effect noticeable, it is necessary to add 0.001% or more. The larger the amount added, the greater the effect and the better the hardenability, so it is preferable, but if it exceeds 0.05%, the effect will be saturated and the cost will increase.
The upper limit was set at 0.05%.

Next, manufacturing conditions according to the present invention will be described.

Steel is melted in a conventional manner and heated in a temperature range of 850C to less than 1150C. Below 850C, depending on the steel type, it may not completely austenitize, and when performing multi-stage rolling, the deformation resistance becomes large, increasing the load on the rolling mill and impairing productivity, so we set the lower limit of the heating temperature to 850C. On the other hand, A,1 nitride becomes almost completely dissolved when the temperature exceeds 1150 C, and the austenite grains become coarse, making it difficult to obtain fine austenite even if hot rolling is performed. It becomes difficult to obtain well-sized coarse austenite grains during carburizing due to an increase in the amount of grains and a decrease in the progress of strain-induced precipitation, making it difficult to achieve the desired purpose. The upper limit of the temperature was set to less than 1150C.

Next, hot rolling is performed at a temperature of less than 10,000 ℃ and a rolling reduction of 30% or more. If hot rolling is carried out at a temperature below 1000 C, the progress of recrystallization of austenite grains is severely delayed, and in many cases, the grains are elongated in the rolling direction and many lattice defects such as twins, deformation bands, and dislocations are introduced into the grains. be done. Among these lattice defects, twins and deformation bands act as α grain formation sites during γ→α transformation, resulting in a fine structure. Since -1 dislocations act as effective precipitation sites for precipitates, solid solution n precipitates in large amounts as Asahi nitrides, which aggregate and coarsen during the subsequent cooling process, resulting in coarsening of austenite grains during carburization. loses its inhibitory effect.

The rolling reduction ratio necessary to fully exhibit these effects is 30% in the case of a small plate. Since this value is the lower limit at which twins and deformation bands are substantially introduced into the austenite grains, the rolling reduction during hot rolling was set at 30% or more.

After hot rolling under the above-mentioned conditions, a fine structure is obtained by cooling to room temperature. Although there are no particular limitations on the carburizing conditions, a treatment temperature of 9,000 or higher is desirable in order to fully achieve the intended purpose of the present invention.

Next, the steel applied to the method of the present invention will be described. The steel is a carbon steel or a low alloy steel, and its composition is not particularly limited, but it is a carburizing steel generally called a case hardening steel, and usually has a C: 0.05 to 0. 3%, st: 4. o
% or less, Mn: Q, carbon steel containing 3 to 1.8%, or if necessary Ni: 4.5% or less.

Cr: 2% or less, Moat% or less, Nb: 0.1%
below.

Low alloy steel containing one or more of V: 0.2% or less and B: 0,005% or less is applied to the method of the present invention.

Next, the method of the present invention will be described in detail with reference to examples.

Table 1 of Examples is a table showing the chemical composition of the five types of steel of Graves 1 to 5 used in the test. Table 2 shows that the steel of 1 to 5 mm in Table 1 was melted by the usual method, and after being bloomed into a 150 square billet, 30111 m+! When finishing a 15' steel bar, hot rolling was performed under the rolling conditions shown in Table 2. Then 251111. Machine cut to g and gas carburized (930CX)
4hr -85 (Ic x 30 min - 70 tl" oil cooling, carbon potential = 0.95%) was carried out to measure the effective hardening depth (JIS GO559) and austenite grain size. Also, for the purpose of investigating mechanical properties. The Charpy test (JIS No. 4 test piece) is carried out using the following method.

These results are shown in Table 2. Incidentally, the mixed grain generation rate is the ratio of the total area of coarse grains to the total field area, which is obtained by observing 100 fields of view using an optical microscope after the test material generates austenite grains.

As is clear from Table 2, the heating temperature of test materials &l, A3, and A13 is outside the range of the present invention, the generation rate of mixed grains is high, and the low-temperature toughness decreases, whereas steel No. 1 ．． Steel No. 5 was subjected to the method of the present invention to obtain flat plate 2.411. M1
In No. 2, there is no generation of mixed grains and the low temperature toughness is high.

In sample materials A4 and &lO, which used Perfume 2 and Perfume 4 in which either n or N is outside the scope of the present invention, mixed grains were generated in both samples, and the effective hardness depth and low-temperature toughness were decreases slightly. On the other hand, this problem does not occur in sample material A8, which uses perfume 3 in which the amounts of Aj and H added are within the range.

Sample materials A5, JK 6, and &7, whose rolling reduction ratios are outside the range of the present invention at less than 1000 C, have a high incidence of mixed grains, whereas test materials A8 and Black 9, which are within the range, have poor low-temperature toughness. Excellent quality and no mixed grains.

(Effect of the invention) As explained above, Beni is 0.01 to 0.1%, N is o,
Charcoal purple steel or low alloy steel f containing ooi ~ 0.05%
: Heated to a temperature range of 850 C to less than 1150 C.
By applying hot rolling with a reduction rate of 30% or more in the temperature range below 000C and cooling to room temperature, unevenness and quenching distortion caused by partial coarsening due to austenite refinement can be prevented, and the process can be completed immediately. It has become possible to manufacture coarse-grained carburizing steel that has high carburizability and low-temperature toughness, and its effects are extremely large.

Claims (1)

[Claims] 1. Al: 0.01-0.1%, N: 0.001-0.
05% carbon steel or low alloy steel at 850℃ to 1
A method for producing a coarse-grained carburizing steel, which comprises heating to a temperature range of less than 150°C, hot rolling at a reduction rate of 30% or more in a temperature range of less than 1000°C, and cooling to room temperature.