JPH06271930A - Production of high strength and high toughness steel excellent in fatigue property - Google Patents

Abstract

PURPOSE:To obtain high strength and high toughness steel excellent in strength and ductility and having good fatigue properties. CONSTITUTION:Steel contg., as essential elements, by weight, 0.4 to 1.2% C, 1.0 to 3.0% Si and 0.1 to 3.0% Mn and furthermore contg, as necessary 0.1 to 3.0% Cr, 0.2 to 2.0% Ni and 0.05 to 2.0% of one or >=two kinds among Mo, V, Nb and W, and the balance Fe with inevitable impurities is heated to the Ac3 point or above, is austenitized, is thereafter cooled from the same temp. to the temp. range of the Ms point to <=500 deg.C and is held in the same temp. range for required time to form a composite structure constituted of, as the main phases, bainite and retained austenite of >=10% volume ratio. The surface of the steel having the obtd. composite structure is subjected to shot peening treatment, and the retained austenitic phase on the surface layer part is subjected to strain reduced transformation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength and high-toughness steel which imparts a high compressive residual stress to the surface of the tough steel by shot peening.

[0002]

2. Description of the Related Art Shot peening is a processing method in which a shot material such as a steel ball is collided with the surface of a material at high speed to thereby plastically deform the surface layer. Since the plastically deformed layer is limited to the surface layer, compressive residual stress occurs on the surface of the material after shot peening, but the occurrence of compressive residual stress often improves fatigue properties.
For this reason, many attempts have been made to positively improve fatigue properties by using shot peening.

In the case of steel, shot peening is currently used industrially in most cases for gears and springs where fatigue characteristics are important. Charcoal quenching material is often used. Japanese Examined Patent Publication No. 1-306521 and Japanese Examined Patent Publication No. 1-306545 disclose a method of carburizing and quenching a gear to generate retained austenite in the surface layer, and subjecting this to shot peening to improve fatigue characteristics.

In the carburized and hardened material, a large amount of residual austenite is present as compared with the inside because a high concentration of carbon atoms is dissolved in the surface carburized layer. Therefore, in addition to the increase in hardness and the generation of compressive residual stress due to the plastic deformation of the surface layer due to shot peening, the compressive residual stress during the strain-induced transformation of the residual austenite in the carburized layer on the surface is superposed, which is very large. A compressive residual stress is obtained.

The reason why the compressive residual stress is generated when the strain-induced transformation of the retained austenite in the carburized layer occurs is as follows. In other words, the retained austenite after carburizing and quenching is a metastable phase at room temperature and transforms to martensite due to thermal and mechanical external factors. Since martensitic transformation involves volume expansion, when retained austenite in the structure transforms to martensite, compressive stress is applied to the surrounding phases. In the case of carburized and quenched materials, a large amount of retained austenite is present on the surface, so if it is possible to transform only the retained austenite in the surface layer by some method, a large compressive residual stress can be generated.

Therefore, the shot peening process, which is characterized in that only the surface layer is subjected to plastic deformation, is a very effective method for generating compressive residual stress in the carburized and quenched material. In this way, by performing shot peening using a carburized and quenched material, it is possible to generate a large compressive residual stress on the surface, which has a great effect on improving mechanical properties such as a significant improvement in fatigue properties. .

[0007]

However, when the carburized and hardened material is subjected to shot peening, there is a problem that the effect of shot peening becomes difficult to appear when an abnormal carburized layer is formed during the carburizing treatment. In addition, since the retained austenite in the carburized layer has low stability, the amount of retained austenite changes with time, which causes a problem that the size and shape are likely to be distorted.

In addition, since the carburized and quenched material itself is quite hard, there is a problem that its use is limited.
In other words, the material itself lacks in ductility and toughness, such as poor formability such as bending and poor impact toughness, and is suitable for carburizing and quenching heat treatment after processing such as gears, but bending after heat treatment It is difficult to apply it to parts to which machining is applied or parts to which an impact load is applied. The carburizing and quenching process requires a long process and strict line control such as carburizing gas, which inevitably leads to an increase in cost.

The present invention is intended to solve various problems associated with the above-mentioned carburized and quenched material, can be manufactured only by a simple heat treatment process, and has a stable shot peening effect. It is intended to develop a high-strength, high-toughness steel material that has the following properties: workability such as bending and impact toughness, and can generate a large compressive residual stress by shot peening.

[0010]

According to the present invention, C: 0.
4 to 1.2% by weight, Si: 1.0 to 3.0% by weight, Mn: 0.1 to 3.0% by weight as essential elements, and in some cases, further C
r: 0.1 to 3.0% by weight, Ni: 0.2 to 2.0% by weight, Mo, V, N
One or two or more of b and W: a steel containing 0.05 to 2.0% by weight, the balance being Fe and inevitable impurities, Ac ₃
After heating to a temperature above the point to austenite, it is cooled from this temperature to a temperature range above the Ms point and below 500 ° C and kept in this temperature range for the required time, and bainite and a volume ratio of 10%
The composite structure with retained austenite as the main phase was generated, and the surface of the obtained composite structure steel was shot peened to perform strain-induced transformation of the retained austenite phase in the surface layer. A method of manufacturing high strength and high toughness steel is provided. The surface layer obtained by shot peening the steel having the composite structure has a compressive residual stress of 500 N / mm ² or more.

[0011]

The occurrence of large compressive residual stress in the steel according to the present invention by shot peening is due to the strain-induced transformation of retained austenite in the mixed structure of bainite and retained austenite. The retained austenite was obtained by carburizing and concentrating very high concentration of carbon atoms in the surface layer in the case of carburized and hardened material, while the cementite peculiar to bainite transformation of Si-containing steel was obtained. It is a feature of the present invention that residual austenite is obtained by the formation of bainitic ferrite not containing it and the accompanying phenomenon of concentration of carbon atoms in untransformed austenite.

First, the behavior of the steel according to the present invention for producing retained austenite by the austempering heat treatment is as follows. When a carbon steel containing a large amount of Si is transformed into bainite, Si has an action of suppressing the formation of carbides, so that carbon atoms in bainitic ferrite are discharged into untransformed austenite. For this reason, the carbon concentration in untransformed austenite rises and the martensitic transformation temperature (Ms point) falls to below room temperature. After this point (after the isothermal transformation), martensite does not form even when the steel is cooled to room temperature, and a mixed structure of bainite and retained austenite is obtained.

The retained austenite thus produced is a metastable phase at room temperature and can give strain-induced transformation to martensite when given a thermal or mechanical external factor. The strain-induced transformation of retained austenite leads to very good ductility under certain conditions. TRIP (Transfor
mation Induced-Plasticity (transformation-induced plasticity) phenomenon, which is disclosed in Japanese Patent Publication No. 58-42246 and Japanese Patent Laid-Open No. 3-215623, which describes a method for manufacturing high ductility steel using this phenomenon.

The present inventors have paid attention to the fact that the retained austenite obtained when the carbon steel containing a large amount of Si is transformed into bainite can cause strain-induced transformation even by shot peening. Compressive residual stress occurs due to volume expansion associated with martensitic transformation. Furthermore, the retained austenite contained in this steel also works effectively to improve the strength-ductility balance, and is capable of bending and light drawing. Is.

According to the present invention, conventionally, the carburized and quenched steel is shot peened and the compressive residual stress is introduced to improve the fatigue characteristics. As a result, the heat treatment process can be greatly simplified. Further, the carburized and quenched parts were hard and poor in toughness, but in the case of the present invention, the TRIP phenomenon of retained austenite causes the material itself to have extremely high toughness, so that it is possible to secure a mild formability after processing the parts. . As described above, the present invention is considered to have optimum properties as a material to be subjected to shot peening treatment, and is extremely useful industrially.

The action of each alloying element of steel to which the method of the present invention is applied and the range of addition amount will be individually described below.

C is an austenite stabilizing element and is an element essential for causing bainite transformation. The content of retained austenite (hereinafter,
(It may be abbreviated as γ _R ), and if the C content is 0.4% or less, high strength cannot be obtained with γ _R contained. Further, if the amount of C is 1.2% or more, the amount of γ _R produced is too large, which not only significantly reduces YS, but also deteriorates the toughness of the material. Therefore, in order to obtain a high strength while containing an appropriate amount of γ _R and to use it for the subsequent prestress processing, the amount of C must be in the range of 0.4 to 1.2%.

Si is an element that suppresses the formation of carbides,
It acts extremely effectively in obtaining stable γ _R having a high C concentration. If the Si content is less than 1.0%, the above effect is diminished. On the contrary, if the Si content exceeds 3.0%, not only bainite transformation is significantly suppressed, but also the manufacturability such as hot rolling-cold rolling deteriorates. . Therefore, the Si content is limited to the range of 1.0 to 3.0%.

Mn is an austenite stabilizing element, and has an action of suppressing the formation of pearlite by improving the hardenability. However, if the amount of Mn is less than 0.1%, the hardenability is insufficient and sufficient γ _R cannot be obtained due to the formation of pearlite. Moreover, if the amount of Mn exceeds 3.0%, the bainite transformation rate becomes slow, and it is also impossible to obtain sufficient γ _R. For this reason, the amount of Mn is 0.1-3.0
%.

Cr is used when softening annealing of hot rolled sheet is required,
It has the effect of suppressing graphitization that occurs during annealing. It also acts to delay bainite transformation and is an element that facilitates control of bainite transformation. In order to prevent the above graphitization, the Cr content must be at least 0.1%, but even if it is added in excess of 3.0%, no further effect can be expected and it is difficult to spheroidize the cementite during softening annealing. However, since the bainite itself tends to deteriorate the toughness, its content is in the range of 0.1 to 3.0%.

Mo, V, Nb and W are elements that greatly change the morphology of bainite transformation, and when added in appropriate amounts, the bainite structure is refined and TS and toughness are enhanced. Furthermore, since V and Nb have the function of refining the austenite grain size when heated to the austenite region, they can promote the bainite transformation. M
When the content of o, V, Nb and W is less than 0.05%, the effects of improving TS and refining the austenite grain size are small, and 2.
Even if added in excess of 0%, no further effect can be expected, which rather hinders the formation of a sound bainite structure.
Limited to 05-2.0%.

Ni, like Mn, is an austenite-forming element and has the effect of suppressing the formation of pearlite and the like by improving the hardenability, but at the same time, it has a large effect of stabilizing the retained austenite, so that the amount of C A sufficient amount of retained austenite can be obtained even in the case of a low component steel or a small amount of bainite transformation. Therefore, by adding Ni, it is possible to obtain a large amount of retained austenite at a softer strength level. However, since Ni has a large effect of promoting graphitization, addition of more than 2.0% must be avoided. In addition, the effect of stabilizing retained austenite is diminished when the added amount is less than 0.2%. Therefore, the amount of Ni added is 0.2 to 2.0.
%.

In order to make a steel having the above-mentioned composition into a steel having a composite structure, after austenitizing in the temperature range of Ac ₃ point or more, it is retained in the temperature range of Ms point or more and 500 ° C. or less to perform bainite transformation. It is necessary to have a mixed structure of γ _R and bainite with a volume ratio of 10% or more. The austenitizing temperature must be Ac ₃ or higher in order to sufficiently generate the transformation product and secure the strength. Bainite does not form when the holding temperature exceeds 500 ℃. In addition, the holding temperature is Ms
Below the point, martensite is formed prior to the formation of bainite, so that the mixed structure containing γ _R , which is a feature of the present invention, cannot be obtained. Therefore, the holding temperature is above the Ms point and is 500 ° C.
It is limited to the following temperature range.

When the component steel of the present invention is transformed into bainite under the heat treatment conditions to produce a composite structure having a retained austenite with a volume ratio of 10% or more and bainite as a main phase,
This composite structure has good strength and ductility, and by applying shot peening, a compressive residual stress of 500 N / mm ² or more can be generated in the surface layer of the steel material. When the volume ratio of retained austenite is less than 10%, the effect of volume expansion introduced into the structure is small and the compressive residual stress generated is small even if the strain-induced transformation of retained austenite occurs. Therefore, the volume ratio of retained austenite is 10
% Or more is required.

[0025]

[Examples] Table 1 shows the chemical component values (% by weight) of the test materials. Steel Nos. A, B and C are comparative steels in which the content of any one of the components is out of the range specified in the present invention.
D, E, F, G and H are steels within the composition range defined in the present invention. In both cases, normal hot rolling, softening annealing,
A steel plate having a thickness of 2 mm was obtained through cold rolling.

Specimens cut into a predetermined size from these steel sheets were subjected to austempering, quenching and tempering, or carburizing and quenching. In the austempering heat treatment, after heating to a temperature of Ac ₃ points or more to austenite, it is placed in a bath kept at a certain temperature in the range of Ms point or more and 500 ° C. or less, and kept at that temperature for a predetermined time, This is a process of cooling to room temperature. The austempering heat treatment conditions applied to each steel are the heat treatment N shown in Table 2.
o. Any of 1 to 6. In addition, the conditions of quenching and tempering treatment performed for comparison are shown in heat treatment Nos. 7 and 8 of Table 3, and the conditions of carburizing and quenching are shown in heat treatment No. 9 of Table 4.

Steels A to H were subjected to any of heat treatment Nos. 1 to 9 and had mechanical properties as they were (JIS 13
The tensile test value according to B) was measured, and the retained austenite amount (γ _R amount) was measured by X-ray diffraction. The results are shown in Table 5.

Table 5 also shows the measurement results of the maximum compressive residual stress value and the plane bending fatigue limit of the surface layer portion of the steel material after shot peening after shot peening each heat treated material. For shot peening, steel shot with a grain size of 0.6 mmφ and hardness of HRC 60 was used, and peening was performed at an arc height of 0.4 mmA. The compressive residual stress value was measured for the steel material after peening using X-ray diffraction, and the compressive residual stress value at a depth of 50 μm from the surface of the steel material is shown in Table 5.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

From the results shown in Table 5, the following can be seen.

The comparative steels A and B are steels which are used after being subjected to normal quenching and tempering heat treatment, and no Si is added. In the case of Comparative Examples A.7 and B.7 using this, since retained austenite is not generated because the heat treatment condition is quenching and tempering, high ductile toughness is not obtained, and compressive residual stress due to shot peening is not obtained. The amount of occurrence of is small and high fatigue strength cannot be obtained.

In the case of Comparative Example B.2, although the heat treatment conditions were within the range specified by the present invention, Comparative Steel B having a lower Si content than that specified by the present invention was used, and residual austenite was formed. do not do. Therefore, it is not possible to obtain high ductility toughness.

Comparative Steel C is a steel which is normally carburized and quenched, but as seen in Comparative Example C.9, the retained austenite generated in the surface layer causes compressive residual stress due to shot peening, resulting in high fatigue. Strength can be obtained. However, the retained austenite in this surface layer is not effective in improving the ductility and toughness of the base metal itself, and is inferior in tensile ductility and impact toughness.

On the other hand, the steels of the present invention D, E, F, G, H
Inventive material D, which has been subjected to austempering heat treatment against
3, E.3, F.2, G.2-4 and H.5 are extremely excellent in strength ductility and impact toughness due to the formation of a mixed structure of bainite and retained austenite.
Due to the generation of compressive residual stress due to strain-induced transformation of the retained austenite in the surface layer due to shot peening, its fatigue properties are extremely excellent.

However, even in the case of the steels D, E, F, G and H of the present invention, excellent properties cannot be obtained when the heat treatment is performed under the conditions outside the range specified in the present invention. For example, in the case of Comparative Example D.8, the retained austenite cannot be obtained because the quenching and tempering treatment is performed instead of the austempering. Therefore, although the strength is high, the effects of ductile toughness and shot peening cannot be obtained.

Further, in the case of Comparative Example D.1, since the isothermal holding temperature in the austemper was too low and was below the Ms point, a large amount of martensite was formed and a mixed structure of bainite and retained austenite was not formed. In addition, strength-ductility and toughness are low. In the case of Comparative Example D.6, pearlite was formed during holding because the isothermal holding temperature was too high, and retained austenite was not formed, so the strength-ductility toughness is low.

[0041]

As is apparent from the above examples, according to the present invention, a steel material excellent in strength-ductility and toughness and having a high compressive residual stress due to shot peening and high fatigue strength. A tough steel material can be obtained.

Claims (5)

[Claims] 1. A steel comprising C: 0.4 to 1.2% by weight, Si: 1.0 to 3.0% by weight, Mn: 0.1 to 3.0% by weight, the balance being Fe and inevitable impurities, heated to a temperature of Ac ₃ or higher. After austenitizing, it is cooled from this temperature to a temperature range above the Ms point and below 500 ° C and held in this temperature range for the required time to form a composite structure consisting of bainite and retained austenite with a volume ratio of 10% or more as the main phase. , A method for producing high-strength, high-toughness steel with excellent fatigue properties, which consists of subjecting the surface of the obtained composite steel to shot peening and subjecting the retained austenite phase in the surface layer to strain-induced transformation. Wherein C: 0.4 to 1.2 wt%, Si: 1.0 to 3.0 wt%, Mn: 0.1 to 3.0 wt%, Cr: 0.1 to 3.0 wt%, the steel balance consisting of Fe and unavoidable impurities Ac ₃ After being heated to a temperature above the point to austenite, it is cooled from this temperature to a temperature range above the Ms point and below 500 ° C and held in this temperature range for the required time to maintain bainite and residual austenite with a volume ratio of 10% Of high strength and high toughness steel with excellent fatigue properties by forming a composite structure with a phase and subjecting the surface of the resulting composite structure to shot peening to cause strain-induced transformation of the retained austenite phase in the surface layer . 3. C: 0.4 to 1.2% by weight, Si: 1.0 to 3.0% by weight, Mn: 0.1 to 3.0% by weight, Cr: 0.1 to 3.0% by weight, M
one or more of o, V, Nb, W: 0.05 to 2.0% by weight,
After heating the steel with the balance Fe and unavoidable impurities to a temperature of Ac ₃ or higher to austenite, it is cooled from this temperature to a temperature range of Ms or higher and 500 ° C or lower and kept at this temperature for the required time. The bainite and the retained austenite with a volume fraction of 10% or more as the main phase are formed, and the steel surface of the obtained composite structure is shot-peened to transform the retained austenite phase in the surface layer into strain-induced transformation. Of high strength and high toughness steel with excellent fatigue characteristics. 4. C: 0.4 to 1.2% by weight, Si: 1.0 to 3.0% by weight, Mn: 0.1 to 3.0% by weight, Cr: 0.1 to 3.0% by weight, N
i: 0.2 to 2.0% by weight, one or more of Mo, V, Nb, W: 0.05 to 2.0% by weight, the balance being Fe and unavoidable impurities, and heating the steel to a temperature of Ac ₃ or higher. After austenitizing, it is cooled from this temperature to a temperature range above the Ms point and below 500 ° C and held in this temperature range for the required time to form a composite structure consisting of bainite and retained austenite with a volume ratio of 10% or more as the main phase. , A method for producing high-strength, high-toughness steel with excellent fatigue properties, which consists of subjecting the surface of the obtained composite steel to shot peening and subjecting the retained austenite phase in the surface layer to strain-induced transformation. 5. The surface layer portion subjected to shot peening is 50
The method for producing a high-strength and high-toughness steel according to claim 1, 2, 3 or 4, which has a compressive residual stress of 0 N / mm ² or more.