JPH07109545A - Non-heat treated steel for hot forging excellent in tensile strength, fatigue strength and machinability - Google Patents

Abstract

PURPOSE:To obtain a non-heat treated steel for hot forging excellent in tensile strength, fatigue strength and machinability by specifying the chemical components in a steel and controlling its metallic structure. CONSTITUTION:This steel contains, by weight, 0.10 to 0.35% C, 0.15 to 2.00% Si, 0.40 to 2.0% Mn, 0.03 to 0.10% S, 0.0005 to 0.050% Al, 0.003 to 0.050% Ti, 0.0020 to 0.0070% N, 0.30 to 0.70% V, and the balance Fe with inevitable impurities, and in its metallic structure after cooling at a room temp. following hot forging, the structural rate (f) of the bainitic structure is regulated to 1.4C+0.4>=f>=1.4C based on the C-contg. %. Thus, the ferrite-bainite type non- heat treated steel having high tensile strength, high fatigue strength and excellent machinability and used in hot-forged state as it is can be obtd..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a non-heat treated material that has excellent tensile strength, fatigue strength and machinability at the same time without being subjected to heat treatment such as quenching and tempering after hot forging. It is about steel.

[0002]

2. Description of the Related Art The application of non-heat treated steel is widely used for mechanical structural parts such as automobiles from the viewpoint of omitting steps and reducing manufacturing costs.

These non-heat treated steels have been mainly developed to have high tensile strength (or hardness), yield strength and toughness. Then, for example, JP-A-62-20
As seen in Japanese Patent No. 5245, non-heat treated steels using V, which is a typical element for precipitation strengthening, have been proposed. However, fatigue strength and machinability are the real obstacles to the application of such high strength and high toughness non-heat treated steel to mechanical parts.

Fatigue strength is generally considered to depend on tensile strength, and the higher the tensile strength, the higher the fatigue strength. However, the machinability is extremely deteriorated by increasing the tensile strength, and when the tensile strength exceeds 120 kgf / mm ² , it is no longer possible to produce with normal production efficiency. Therefore, it has been earnestly desired to realize a non-heat treated steel that improves fatigue strength without deteriorating machinability.

For this, it is an effective means to improve the ratio of fatigue strength to tensile strength, that is, the durability ratio. Therefore, as disclosed in, for example, Japanese Patent Laid-Open No. 4-176842, a method has been proposed in which a metal structure mainly composed of bainite is used to reduce high carbon island martensite and retained austenite in the structure.

However, in spite of such development efforts, the durability ratio is about 0.55 at most, and the machinability is improved to only about twice as high as that of the conventional bainite non-heat treated steel which is extremely poor in machinability. Not done.

The inventors of the present invention first focused on a pearlite-containing structure having good machinability, and first of all, by precipitating TiN and VN on MnS in combination, thereby finely converting the austenite crystal grains during forging heating and Ferrite is finely precipitated by using the composite precipitate as a nucleation site, and then, when pearlite is precipitated, V carbide or V carbon chamber compound is further extremely finely precipitated on the ferrite matrix material in the precipitated pearlite. A non-heat treated steel for hot forging excellent in fatigue strength and machinability was invented by combining a method utilizing the precipitation of (1) to obtain a metal structure having a fine pearlite structure and a precipitation strengthened structure. However, in this type of non-heat treated steel, the tensile strength is limited to about 100 kgf / mm ² at most, and therefore even if the durability ratio is improved, the fatigue strength is also limited.

[0008]

SUMMARY OF THE INVENTION The present invention provides a non-heat treated steel for hot forging having high fatigue strength, tensile strength and machinability, which has been difficult to realize with conventional non-heat treated steel. It is a thing.

[0009]

To obtain high fatigue strength, the easiest method is to increase tensile strength (hardness). In order to increase the tensile strength, a low temperature transformation structure such as martensite or bainite may be introduced. However, as described in the prior art, such a method causes a remarkable deterioration in machinability.

The present inventors have examined the fatigue characteristics and machinability of several types of hot forged materials having a metallic structure in which an appropriate amount of bainite structure is mixed with the ferrite structure. As a result, MnS + Ti for the purpose of refining the structure
Utilizing a composite precipitate of N + VN as a precipitation nucleus of ferrite Precipitating V carbide in a bainite structure having a ferrite-bainite two-phase structure containing an appropriate amount of bainite structure whose hardness is controlled by low C and low N From the above three points, the inventors have invented a ferrite-bainite type non-heat treated steel for hot forging which can improve tensile strength and fatigue strength and can secure machinability at an acceptable level in the current cutting process.

That is, in the first aspect of the present invention, the weight ratio of C: 0.10 to 0.35%, Si: 0.15 to 2.0.
0%, Mn: 0.40 to 2.00%, S: 0.03 to
0.10%, Al: 0.0005 to 0.050%, T
i: 0.003 to 0.050%, N: 0.0020 to
0.0070%, V: 0.30 to 0.70% is contained,
The balance has a composition consisting of Fe and an impurity element, and the microstructure ratio f of the bainite structure in the metal structure after hot forging and cooling to room temperature is 1.4C + 0.4 ≧ with respect to the carbon content C (%). It is a ferrite-bainite type non-heat treated steel to be used as it is hot forged, characterized in that f ≧ 1.4 C, and the second invention is grain refinement, bainite microstructure ratio adjustment and machinability. For further improvement, the composition of the first invention steel further contains Cr: 0.02 to 1.50%,
Mo: 0.02 to 1.00%, Nb: 0.001 to 0.
20%, Pb: 0.05-0.30%, Ca: 0.00
One or two or more of 05 to 0.010% are contained.

Next, the chemical composition of the non-heat treated ferrite-bainite type steel of the present invention and the reasons for limiting the metal structure after hot forging and cooling to room temperature will be described below.

C: It is an important element that adjusts the bainite structure ratio and thus increases the tensile strength of the final product. However, if it is too much, the strength increases too much and the machinability deteriorates significantly.
That is, if it is less than 0.10%, the tensile strength and fatigue strength are low, and if it exceeds 0.35%, the tensile strength is too high and the machinability is significantly reduced.
And

Si: an element for adjusting the deoxidation and bainite structure ratio, and if less than 0.15%, its effect is small,
If it exceeds 2.00%, both the durability ratio and the machinability deteriorate, so 0.15% to 2.00% is set.

Mn: An element which becomes the base of a composite precipitate which is a ferrite precipitation site by adjusting the bainite structure ratio and becoming MnS. If less than 0.40%, its effect is small, and if it exceeds 2.00%. Since a large amount of bainite is generated and both the durability ratio and the machinability are deteriorated, 0.
40 to 2.00%.

S: MnS is an element that serves as a base of a composite precipitate that is a ferrite precipitation site and improves machinability, and is 0.03 to 0.10%.

Al: an element having an effect of deoxidizing and refining crystal grains, and if less than 0.0005%, the effect is small,
If it exceeds 0.050%, hard inclusions are formed and both the durability ratio and the machinability deteriorate, so 0.0005 to 0.050%
And

Ti: An element that forms a nitride on the MnS to form a composite precipitate that serves as a ferrite precipitation site. If less than 0.003%, its effect is small, and 0.05
If it exceeds 0%, the formation of coarse hard inclusions is promoted and both the durability ratio and the machinability deteriorate, so the content is made 0.003 to 0.050%.

N: An element which forms a nitride or carbonitride with Ti and V. If it is less than 0.0020%, its effect is small, and if it exceeds 0.0070%, both the durability ratio and the machinability deteriorate. , 0.0020 to 0.0070%.

V: An element that forms a composite precipitate with MnS and TiN and precipitation strengthens the matrix ferrite in bainite. If it is less than 0.30%, its effect is small, and if it exceeds 0.70%, the durability ratio and the cutting ratio are reduced. Since all of the properties are deteriorated, the content is set to 0.30 to 0.70%.

The above are the reasons for limiting the chemical composition of the steel of the first invention of the present application. In the second invention of the present application, in order to refine the crystal grains, adjust the bainite structure ratio, and further improve the machinability, Cr, Mo, Nb, and P are added to the components of the first invention steel.
One or more of b and Ca are contained. The reasons for limiting these chemical components will be described below.

Similar to Cr: Mn, it is an element for adjusting the bainite structure ratio. If it is less than 0.02%, its effect is small, and if it exceeds 1.50%, a large amount of bainite is generated and the durability ratio and machinability are improved. 0.02 to 1.5 because all of them decrease
0%

Mo: An element having almost the same effect as Mn and Cr. If the content is less than 0.02%, the effect is small and 1.0
If it exceeds 0%, a large amount of bainite is generated and both the durability ratio and the machinability deteriorate, so the content is made 0.02 to 1.00%.

Nb: an element having almost the same effect as Ti and V, and if less than 0.001%, the effect is small.
If it exceeds 0.20%, both the durability ratio and the machinability deteriorate, so the content is made 0.001 to 0.20%.

Pb: an element that improves the machinability, and is 0.
If it is less than 05%, the effect is small, and if it exceeds 0.30%, the effect is saturated and the fatigue strength and the durability ratio decrease, so the content is made 0.05 to 0.30%.

Ca: Pb is an element having almost the same effect as Pb. If it is less than 0.0005%, the effect is small and 0.01
If it exceeds 0%, the effect is saturated and the fatigue strength and the durability ratio decrease, so the content is made 0.0005 to 0.010%.

The above are the reasons for limiting the chemical components added in the steel of the second invention of the present application. Next, the reasons for limiting the metal structure of the steel of the present invention when it is cooled to room temperature after hot forging will be described.

As described above, the presence of an appropriate amount of bainite in the two-phase structure of ferrite-bainite ensures high tensile strength, high fatigue strength and machinability. The bainite microstructure ratio can be controlled by the C content and hardenability of steel and the cooling rate from the austenite region. In order to effectively utilize the bainite structure, the structure ratio f needs to be 1.4 C or more with respect to the carbon content C (%),
On the other hand, if it exceeds 1.4C + 0.4, the machinability is extremely deteriorated. Therefore, the bainite structure ratio f is set to 1.4C or more and 1.4C + 0.4 or less with respect to the carbon content C (%). If a metal structure including such bainite structure can be achieved,
Although the cooling method after hot forging is not specified, natural cooling is naturally desirable in terms of equipment and manufacturing cost. The bainite texture ratio f is determined by observing the corroded test piece with an optical microscope or the like, determining the texture hardness by a micro Vickers hardness meter, and finally measuring the area ratio.

The effects of the present invention will be described more specifically below with reference to examples.

[0030]

EXAMPLES In each of the following tables, the conditions surrounded by thick frames are examples satisfying the present invention, and the other conditions are comparative examples.

(1) Effects of chemical composition of steel material Steel having the chemical composition shown in Table 1 was melted in a high-frequency furnace to obtain 150
A forging material is cut out from a steel ingot of 9 kg,
After normalizing by heating and cooling at 50 ° C., it was heated to 1100 to 1250 ° C., hot forged at a temperature of 1050 to 1200 ° C., and then allowed to cool. A JIS No. 4 tensile test piece and a JIS No. 1 rotary bending test piece were sampled from the central portion of this material and subjected to a tensile test and a rotary bending fatigue test. Optical microscope observation specimens were taken from the same material and corroded with 5% Nital 2
The observation was performed at 00 times to obtain the bainite structure rate. Further, a cutting test piece was sampled from the same material, a blind hole having a depth of 30 mm was drilled using a 10 mmφ straight shank drill made of SKH9, and the total drilling distance until the life of the drill was destroyed was measured. The result of the measurement was that the total drilling distance of No steel, which is a conventional steel, was set to 1.00, and the machinability was evaluated by the relative ratio thereof. The cutting speed was 50 m / min, the feed speed was 0.35 mm / rev, and the cutting oil was 7 L / min.

[0032]

[Table 1]

[0033]

[Table 2]

Table 2 shows the bainite microstructure ratio and performance evaluation results of each test material.

First, heat treated steel No. 42 durability ratio of 0.
No. 47, which is an example of the present invention, has a machinability of 1.00. 1
Nos. 20 to 20 each have a durability ratio of 0.57 or more, and also have machinability of No. 20. It is about 2 to 3 times better than 42.

No. of the comparative example. Since No. 21 has a low C content and a low tensile strength and a low durability ratio, the fatigue property is poor. No. of the comparative example. In No. 22, martensite is generated because the amount of C is too high, the condition of the bainite structure ratio of the present invention cannot be satisfied, and the tensile strength is high, but the durability ratio is low and the machinability is poor as compared with the examples of the present invention.

No. of the comparative example. Since 23 has a low Si content, the degree of deoxidation is low and the durability ratio is lower than that of the examples of the present invention. Comparative example N
o. In No. 24, martensite is generated because the Si content is high, the condition of the bainite structure ratio of the present invention cannot be satisfied, the durability ratio is lower than that of the present invention example, and the machinability is also poor.

No. of the comparative example. In No. 25, since the amount of Mn is low, the precipitation of complex precipitates is small and the durability ratio is lower than that of the examples of the present invention. No. of the comparative example. In No. 26, martensite is generated because the Mn content is high, the condition of the bainite structure ratio of the present invention cannot be satisfied, the durability ratio is lower than that of the present invention example, and the machinability is also poor.

No. of the comparative example. In No. 27, since the S content is low, precipitation of composite inclusions is small, and the durability ratio is lower than that of the examples of the present invention.
Further, since the effect of improving the machinability of MnS cannot be obtained, the machinability is also poor. No. of the comparative example. 28 has a high S content, so Mn
Precipitation of S becomes excessive and the durability ratio is lower than that of the examples of the present invention.

No. of the comparative example. No. 29, which has a low Al content, is less effective in deoxidation and grain refinement, and has a lower durability ratio than the examples of the present invention. No. of the comparative example. In No. 30, hard inclusions are formed due to the high Al content, the durability ratio is lower than in the examples of the present invention, and the machinability is also poor.

No. of the comparative example. In No. 31, since the amount of Ti is low, the precipitation of complex precipitates is small and the durability ratio is lower than that of the examples of the present invention. No. of the comparative example. In No. 32, since the amount of Ti is high, hard inclusions are formed, the durability ratio is lower than that of the examples of the present invention, and the machinability is also poor.

No. of the comparative example. In No. 33, since the amount of N was low, the precipitation of complex precipitates was small, and the durability ratio was lower than that of the examples of the present invention.
No. of the comparative example. In No. 34, the matrix is hardened due to the high N content, the durability ratio is lower than that of the examples of the present invention, and the machinability is also poor.

Comparative Example No. No. 35, which has a low V content, has less precipitation of composite precipitates and a small effect of precipitation strengthening the matrix ferrite, and therefore has a lower durability ratio than the examples of the present invention. No. of the comparative example. Since 36 has a high V content, the durability ratio is lower than that of the examples of the present invention and the machinability is also poor.

No. of the comparative example. In No. 37, since the amount of Cr is high, martensite is generated and the bainite structure ratio condition of the present invention cannot be satisfied, and the durability ratio is lower than that of the present invention example and the machinability is also poor.

No. of the comparative example. In No. 38, since the amount of Mo is high, martensite occurs and the bainite structure ratio condition of the present invention cannot be satisfied, and the durability ratio is lower than that of the present invention example, and the machinability is also poor.

No. of the comparative example. 39 has a high Nb content,
The durability ratio is lower than that of the examples of the present invention and the machinability is also poor.

No. of the comparative example. 40 has a high Pb content,
The machinability is good but the durability ratio is poor.

No. of the comparative example. 41 has a high Ca content,
The machinability is good but the durability ratio is poor.

[0049]

[Table 3]

[0050]

[Table 4]

(2) Effect of change in bainite microstructure ratio by cooling method after hot forging Steel having the chemical composition shown in Table 1 was melted in a high-frequency furnace to obtain 150
A forging material is cut out from a steel ingot of 9 kg,
After normalizing by heating and cooling at 50 ° C., heating was performed at 1100 to 1250 ° C., hot forging was performed at a temperature of 1050 to 1200 ° C., and then, cooling was performed by the method shown in Table 3 as well. The tensile strength, the fatigue strength, the machinability and the bainite structure rate were determined from the center of this material by the same method as in Example 1. Table 4 shows the bainite microstructure ratio and performance evaluation result of each test material.

No. 45, 46, 47 and 48 are examples of the present invention in which the texture ratio f, which is the condition of the bainite structure ratio of the present invention, satisfies 1.4C + 0.4 or more and 1.4C or less with respect to the carbon content C (%). And the durability ratio is 0.
No. 55, which is a current tempered steel that secures 55 or more and has machinability.
It is about 2.5 times as high as 42.

No. Reference numerals 43 and 44 are cooling rates that are small, and most of the structures are ferrite or ferrite + spherical cementite, and the bainite structure ratio is small. Therefore, the tensile strength itself is low, but ferrite +
The effect due to bainite two-phase organization disappears, the durability ratio is as low as 0.54 or less, and the machinability is poor as compared with the examples of the present invention.

On the other hand, No. 49 has a structure mainly composed of martensite by increasing the cooling rate, and although the tensile strength is increased, the durability ratio is extremely low, and the machinability is poor, and the tool life is short.

[0055]

[Table 5]

[0056]

[Table 6]

[0057]

As described above, the steel of the present invention has a ferrite-bainite two-phase structure to obtain high tensile strength and secures machinability, and is formed from MnS, Ti nitride and V nitride. Improving the durability ratio, that is, the fatigue property without damaging the machinability by simultaneously refining the metal structure using the composite precipitate and strengthening the ferrite matrix in bainite with V carbide (or carbonitride). This makes it possible to present a non-heat treated steel for hot forging, which satisfies the long-awaited high tensile strength, high fatigue strength, and machinability, and is extremely effective in industry.

Claims (2)

[Claims] 1. A weight ratio of C: 0.10 to 0.35% Si: 0.15 to 2.00% Mn: 0.40 to 2.00% S: 0.03 to 0.10% Al : 0.0005 to 0.050% Ti: 0.003 to 0.050% N: 0.0020 to 0.0070% V: 0.30 to 0.70% and the balance Fe and impurity elements The composition ratio f of the bainite structure in the metal structure having the composition and subjected to hot forging and cooling to room temperature is 1.4C + 0.4 ≧ f ≧ 1.4C with respect to the carbon content C (%). Characteristic Ferrite-bainite type non-heat treated steel used as hot forged. 2. A further component is Cr: 0.02 to 1.50% Mo: 0.02 to 1.00% Nb: 0.001 to 0.20% Pb: 0.05 to 0.30% Ca: The ferrite-bainite non-heat treated steel used as hot forged according to claim 1, characterized in that it contains one or more of 0.0005 to 0.010%.