JP3762543B2 - Manufacturing method of non-tempered steel for hot forging and non-tempered hot forged product and hot forged non-tempered product - Google Patents

Description

【００３１】
【表２】

【００３２】
【表３】

【００３３】
【発明の効果】
本発明の鋼は高降伏比、および高靱性を備えた高周波焼入れ可能な熱間鍛造非調質鋼部品用の素材として最適である。また、本発明の製造方法により、高降伏比、および高靱性を有する高周波焼入れ可能な熱間鍛造非調質品を製造することができる。さらに、本発明の熱間鍛造非調質品は、自動車用あるいは産業機械用の部品として使用するとき、高周波焼入れ可能であるため、部品の一層の高強度化が可能で、車両の軽量化、燃費低減、および低コスト化に貢献できる。
【図面の簡単な説明】
【図１】０．３５％Ｃ−２．００％Ｍｎ−０．１５％Ｃｒ−０．２０％Ｖ−０．０１５％Ｔｉ−０．０１１０％Ｎの組成の鋼において、Ｓｉ量を変え、１４７０Ｋに加熱、放冷した場合の残留オーステナイト量を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-heat treated steel material for hot forging that is processed into machine parts such as automobiles and industrial machines without being subjected to a tempering treatment after hot forging, and a hot work using the same. The present invention relates to a method for producing a non-forged tempered product and a non-tempered product for hot forging, and particularly has high strength and high toughness while being hot forged and can be induction-quenched.
[0002]
[Prior art]
Many automotive and industrial machine parts are used after processing steel bars hot and then refining the structure by quenching and tempering (tempering) to increase strength and toughness. For this reason, machine parts that are used without the tempering treatment, so-called hot forged non-tempered parts, are rapidly spreading. Recently, in order to protect the global environment, there has been a demand for lower fuel consumption of automobiles. However, one of the effective measures to achieve lower fuel consumption of automobiles is to reduce the vehicle weight. Improvement in the size is aimed at reducing the size and weight of parts.
[0003]
Various high-strength non-tempered steels have been studied to meet these demands, but especially for automotive parts whose shapes are becoming more complex in recent years, quenching strain is often used after air forging. No bainite-type non-tempered steel is suitable.
[0004]
[Problems to be solved by the invention]
However, it is difficult to say that sufficient research has been conducted on non-heat treated steel with a bainite structure. In general, bainite-type non-tempered steel as it is hot forged is austenite that cannot be transformed to room temperature because it does not have enough time to pass through the bainite transformation temperature range during air cooling after hot forging, The austenite structure remains in the bainite structure. Since this soft retained austenite exists, low yield strength and toughness are the disadvantages of bainite-type non-heat treated steel. As a countermeasure, it is possible to transform the austenite structure into ferrite and carbide by tempering, but there is no effective means for improving the yield strength and toughness in the state of hot forging. In addition, in the case of ferritic pearlite type non-heat treated steel, it is known that as the carbon content increases, the strength increases and the toughness decreases, so far it has been considered that the same tendency is also observed for the bainite type. It was. Therefore, there are few bainite type non-tempered steels containing high carbon exceeding 0.30%. On the other hand, the shafts are used by hardening the surface layer by induction hardening, but 0.30% or more of carbon is necessary to obtain sufficient hardness by induction hardening. Therefore, if a bainite-type non-tempered steel containing more than 0.30% carbon and not requiring tempering can be obtained, the range of application to various parts will be greatly expanded.
[0005]
JP-A-8-319536 has already disclosed a bainite-type non-tempered steel containing more than 0.30% carbon, which is mainly due to the refinement of retained austenite structure and lath structure. A steel having a strength of 1000 MPa or more and a high yield strength and impact value. However, if the performance equivalent to that of the tempered state can be obtained by reducing the retained austenite itself, higher performance can be achieved.
[0006]
Therefore, the present invention is a material for bainite type hot forged parts that has high strength, particularly high yield strength and high toughness, and can be induction hardened after hot forging, that is, non-tempered for hot forging. An object of the present invention is to provide a steel, a method for producing a hot forged non-tempered product using the steel, and a hot forged non-tempered product.
[0007]
[Means for Solving the Problems]
The non-heat treated steel for hot forging of the present invention is
% By mass
C: more than 0.30 to 0.60%,
Si: 0.02 to 0.17%
Mn: 1.65 to 3.00%,
Cr: 0.02-2.00%,
N: 0.0080 to 0.0200%
Including,
Al: 0.005 to 0.050%,
Ti: 0.005 to 0.050%
One or two of them,
V: 0.01 to 0.30%,
Nb: 0.01-0.30%
It is a non-tempered steel for hot forging that can be induction-hardened, including one or two of them, the balance being Fe and inevitable impurities.
[0008]
In addition, the method for producing a hot forged non-heat treated product of the present invention is capable of induction hardening characterized by processing and cooling the non-heat treated hot forged steel of the present invention at a temperature of 1270 K or higher. This is a method for producing a hot forged non-heat treated product.
[0009]
Furthermore, the non-heat treated product for hot forging of the present invention is
% By mass
C: more than 0.30 to 0.60%,
Si: 0.02 to 0.17%
Mn: 1.65 to 3.00%,
Cr: 0.02-2.00%,
N: 0.0080 to 0.0200%
Including,
Al: 0.005 to 0.050%,
Ti: 0.005 to 0.050%
One or two of them,
V: 0.01 to 0.30%,
Nb: 0.01-0.30%
It is a hot forged non-tempered product that includes one or two of them, the balance is made of Fe and inevitable impurities, and 80% or more of the volume of the steel structure can be induction hardened with a bainite structure.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the main cause of the decrease in yield strength and impact value is the presence of retained austenite that sometimes exceeds 10%. Austenite is softer than bainite and causes a decrease in yield point. Therefore, in the present invention, bainite transformation was promoted during cooling after hot forging, and as a result, retained austenite was reduced.
[0011]
The first reason that austenite remains is that the temperature and sufficient time required for complete completion of the bainite transformation during continuous cooling after hot forging cannot be taken. In particular, in actual hot forging, in order to ensure productivity, a method of blast cooling on a conveyor after forging is often employed, and the time during which steel is maintained in the bainite transformation temperature range is limited. ing.
[0012]
The present inventor predicts that precipitation promotion of carbides precipitated during bainite transformation shortens bainite transformation time, and finely disperses carbide forming elements (Mo, V, Nb, etc.) to become carbide precipitation sites. A possible element (Ti) and an element (Si) that suppresses carbide formation were selected, and the influence of these on the amount of retained austenite was investigated. As a result, after heating at 1200 ° C., Si has a very large influence on the amount of retained austenite when held at a constant temperature in the bainite transformation region for a certain period of time. Furthermore, as shown in FIG. It was found that by limiting to less than%, the retained austenite was sufficiently reduced under normal cooling conditions after hot forging.
[0013]
In steels of various structures, it is already known that a relatively large amount of Si suppresses precipitation and growth of carbides. However, in bainite steel containing more than 0.30% carbon, the amount of retained austenite after continuous cooling is reduced. There is no report about a significant decrease in retained austenite when Si is suppressed to less than 0.20 wt% in an example in which the influence of Si content is examined, particularly in a medium carbon steel.
[0014]
Furthermore, in the present invention, refinement of the bainite structure by the dispersion of Ti and Al nitride is combined. The fine dispersion of TiN and AlN is very effective for making bainite packets finer by making the austenite structure finer during reheating, and is effective for obtaining high yield strength and impact value.
[0015]
Next, the reasons for limiting the invention will be described.
[0016]
C: C is a toughening element of steel. If it is 0.30% or less, a large amount of alloy is required to improve the tensile strength, the cost is increased, and the deformation resistance during hot forging is increased, so that the life of the forging die is shortened. Furthermore, if it is 0.30% or less, the surface hardness when induction-quenched is less than that normally required, and is not practical for induction hardening. If it exceeds 0.60%, the yield strength is lowered, and the ferrite and pearlite structure are easily transformed during forging cooling.
[0017]
Si: Since Si is an element that suppresses carbide precipitation and increases retained austenite, it is limited to 0.17% or less . However, if Si is suppressed to less than 0.02%, the manufacturing cost of steel becomes great, so the lower limit is made 0.02%.
[0018]
Mn: Mn is necessary for improving the hardenability and transforming the as-forged and cooled structure into bainite. If it is less than 1.65, hot forging and the structure as it is allowed to cool are hard to transform into bainite. If it exceeds 3.00%, martensite transforms and becomes harder than necessary, reducing toughness and machinability. Let
[0019]
Cr: Works in the same way as CrMn, but in combination with Mn, V, etc. is effective for reducing the transformation start temperature of bainite and making the structure finer. When a large amount of Cr is added, martensite is transformed, so the content is made 2.00% or less. Moreover, suppressing Cr to less than 0.02% in actual operation does not have an effect that can be expected for cost.
[0020]
Al, Ti: Al and Ti precipitate and disperse in the steel as nitrides, thereby preventing the austenite structure from coarsening during forging reheating and increasing yield strength and toughness. The addition amounts necessary for preventing coarsening are Ti: 0.005% or more and Al: 0.005% or more. However, if added in a large amount, the precipitate becomes coarse and the steel becomes brittle, so the upper limit is Ti: 0. 0.050%, Al: 0.050%.
[0021]
N: N is an element that refines bainite packets and increases yield strength by forming various nitrides to prevent coarsening of the austenite structure during hot forging, and it must be at least 0.0080% or more. It is. However, the effect is saturated even if added over 0.0200%.
[0022]
V, Nb: V and Nb lower the bainite transformation temperature at the time of forging and increase the toughness by using the bainite structure as it is as it is forged as a fine structure, and partly precipitate at the time of cooling after forging to strengthen the steel. . In order to exert these effects, it is necessary to add 0.01% or more of either V or Nb. However, the upper limit of each is set to 0.30% in order to reduce costs.
[0023]
P is included as an impurity and is not added intentionally. From the viewpoint of improving the manufacturability and toughness of steel, 0.05% or less is desirable.
[0024]
The steel of the present invention can be aged after hot forging to exhibit precipitation strengthening of V and Nb, and in order to expect precipitation strengthening, it is efficient to temper at a temperature of about 800K or more after forging. is there.
[0025]
In the method for producing a hot forged non-tempered product of the present invention, a temperature of 1270 K or more is used in order to make steel austenitic single phase and lower the deformation resistance during hot forging to make the forging die life a practical length. Process with. Further, if the size of the automobile parts, said without hot forging microalloyed steels of the present invention take special cooling after processing, it can be a tissue composed mainly of bainite to air or .
[0026]
In the hot forged non-heat treated product of the present invention, high strength and high toughness can be obtained by making 80% or more of the volume of the structure a bainite structure. If the bainite structure is less than 80% due to cooling conditions, the mechanical properties may be deteriorated due to other mixed structures. For example, when the mixed structure is ferrite or pearlite, the tensile strength decreases, and when the mixed structure of martensite and austenite is mixed, the tensile strength increases and the toughness decreases.
[0027]
An appropriate amount of S, Pb, Bi, Te, Se, and Ca may be added to improve machinability. In order to improve machinability, S: 0.02 to 0.10%, Pb: 0.005 to 0.50%, Bi: 0.010 to 0.50%, Te: 0.001 to 0. Addition of 20%, Se: 0.010 to 0.50% is desirable, especially when considering the machinability in cutting using a carbide tool, the addition of Ca: 0.0004 to 0.0050% is effective Is.
[0028]
Note that S forms MnS to improve machinability, and also prevents coarsening of prior austenite grains and improves toughness.
[0029]
【Example】
Steels having various compositions shown in Tables 1 and 2 were melted in a 150 kg vacuum melting furnace and formed into a thickness of 40 mm. After holding these steels at a temperature of 1475K for 600S, they were immediately subjected to 50% forging and cooling, and a tensile test and an impact test were conducted. Moreover, the bainite fraction of the steel was measured from an optical micrograph. As a result, as shown in Table 3, the steel of the present invention had a yield ratio of 0.75 or more and a good impact value of 54 J / cm ² or more.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
[Table 3]

[0033]
【The invention's effect】
The steel of the present invention is optimal as a raw material for hot forged non-tempered steel parts capable of induction hardening with high yield ratio and high toughness. In addition, by the production method of the present invention, it is possible to produce a hot forged non-tempered product having a high yield ratio and high toughness and capable of induction hardening. Furthermore, the hot forged non-heat treated product of the present invention can be induction hardened when used as a part for automobiles or industrial machines, so that the parts can be further strengthened, and the weight of the vehicle can be reduced. Contributes to reducing fuel consumption and cost.
[Brief description of the drawings]
FIG. 1 shows a steel having a composition of 0.35% C-2.00% Mn-0.15% Cr-0.20% V-0.015% Ti-0.0110% N, and the amount of Si is changed. It is a figure which shows the amount of retained austenite at the time of heating and cooling to 1470K.

Claims (3)

% By mass
C: more than 0.30 to 0.60%,
Si: 0.02 to 0.17%
Mn: 1.65 to 3.00%,
Cr: 0.02-2.00%,
N: 0.0080 to 0.0200%
Including,
Al: 0.005 to 0.050%,
Ti: 0.005 to 0.050%
One or two of them,
V: 0.01 to 0.30%,
Nb: 0.01-0.30%
Non-tempered steel for hot forging containing one or two of them, the balance being Fe and unavoidable impurities and capable of induction hardening.   A method for producing a hot forged non-heat treated product capable of induction hardening, comprising processing and cooling the non-heat treated steel for hot forging according to claim 1 at a temperature of 1270K or higher. % By mass
C: more than 0.30 to 0.60%,
Si: 0.02 to 0.17%
Mn: 1.65 to 3.00%,
Cr: 0.02-2.00%,
N: 0.0080 to 0.0200%
Including,
Al: 0.005 to 0.050%,
Ti: 0.005 to 0.050%
One or two of them,
V: 0.01 to 0.30%,
Nb: 0.01-0.30%
Among these, a hot forged non-tempered product including one or two of them, the balance being Fe and inevitable impurities, and capable of induction hardening in which 80% or more of the volume of the steel structure is a bainite structure.

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