JPH07102340A - Production of non-heattreated steel excellent in fatigue characteristic - Google Patents

Abstract

PURPOSE:To produce a non-heattreated steel excellent in fatigue strength, machinability, and yield strength and used for automobile and machine structural parts. CONSTITUTION:This steel has a composition containing, by weight ratio, 0.15-0.50% C, 0.005-2.00% Si, 0.40-2.00% Mn, 0.01-0.10% S, 0.0005-0.05% Al, 0.003-0.05% Ti, 0.0020-0.0200% N, 0.20-0.70% V, and one or >=2 kinds among specific amounts of Cr, Mo, Nb, Pb, and Ca. Moreover, the structure is regulated so that (ferrite + pearlite) structure comprises >=90% of the metallic structure after cooling is done after hot forging and transformation is finished, and further, aging treatment is performed at 200-700 deg.C. By this method, the production of the non-heattreated steel excellent in fatigue characteristic, machinability, and yield characteristics is made possible, and extremely large industrial effects can be produced.

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 parts for machine structures such as automobiles by hot forging. More specifically, it is excellent by subjecting a specific steel material to aging treatment after hot forging. The present invention also relates to a method for manufacturing a non-heat treated steel capable of simultaneously providing fatigue strength, machinability and yield strength.

[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 simplifying the process and reducing the manufacturing cost. These non-heat treated steels have been mainly developed to have high tensile strength (or hardness), yield strength and toughness.

Therefore, as seen in, for example, JP-A-62-205245, a non-heat treated steel using V, which is a typical element for precipitation strengthening, has been proposed. However, when such a high-strength and high-toughness non-heat treated steel is applied to mechanical parts, deterioration of machinability due to an increase in strength is a major obstacle.

The most important characteristic of mechanical parts is fatigue strength. Fatigue strength is generally said 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, the realization of a non-heat treated steel that improves fatigue strength without degrading machinability has been eagerly desired.

[0005]

To this end, improving the ratio of fatigue strength to tensile strength, that is, the durability ratio is an effective means. Therefore, as disclosed in, for example, Japanese Patent Laid-Open No. 4-176842, a method has been proposed in which a bainite-based metallographic structure 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 best, and the improvement is only at most about twice that of the conventional bainite non-heat treated steel, which had extremely poor machinability. Not done.

The present inventors first focused on a pearlite-containing structure having good machinability, and utilize the following two-step precipitation for this.

First, TiN and VN are composite-precipitated on MnS, whereby austenite crystal grains at the time of forging heating are refined, and ferrite is fine-precipitated by using the composite precipitate as a nucleation site. Next, when the pearlite is precipitated, V is further added to the ferrite matrix material in the precipitated pearlite.
Precipitate carbide or V carbonitride extremely finely.

[0009] By combining such a method utilizing two-step precipitation to obtain a metal structure having a fine structure and precipitation strengthened pearlite, a hot work having excellent fatigue strength and machinability can be obtained. Invented non-heat treated steel for forging.

However, this type of non-heat treated steel has a problem in that the yield strength is low, and it cannot be used due to the plastic deformation of the parts to which a large local stress is applied.

The present invention has a high yield ratio and machinability, which have been difficult to realize by the conventional hot forging type non-heat treated steel, that is, the method of natural cooling after hot forging, and further has a yield ratio at the same time. A method for producing a non-heat treated steel is provided.

[0012]

[Means for Solving the Problems] Generally, as a method for improving the yield strength, aging a low-temperature transformation structure containing a large amount of supersaturated solid solution elements such as a martensite (quenching) structure for refining crystal grains. There are two methods, such as fine precipitation of the solid solution element to fix dislocations.

On the other hand, in the ferrite-pearlite structure type non-heat treated steel previously invented by the present inventors, the crystal grains have already been refined by precipitating a composite precipitate, and the ferrite has a high temperature transformation structure. -It is considered that the supersaturated solid solution elements are few and the effect is small even if the pearlite structure steel is aged, and it is considered difficult to improve the yield strength.

However, it has been found that when aging treatment is applied to a material in which the amounts of C and V added are adjusted, the yield strength is considerably improved, and in addition, the fatigue strength and the machinability, which are the features of the previous invention, are also improved.

The present inventors have made the following findings based on these findings.
The present invention has been completed by designing the chemical composition and metallographic structure of hot forging steel containing pearlite, and further examining the conditions for aging this material.

(1) That is, in the first aspect of the present invention, C: 0.15 to 0.50% and Si: 0.00 in terms of weight ratio.
5 to 2.00%, Mn: 0.40 to 2.00%, S:
0.01-0.10%, Al: 0.0005-0.05
%, Ti: 0.003 to 0.05%, N: 0.002
0 to .0200% V: contains 0.20 to 0.70 percent, the balance being a steel having a composition consisting of Fe and impurity elements, subjected to hot forging is heated to a temperature not lower than _C3 points A, 90% or more of the metal structure after the transformation by cooling is a ferrite + pearlite structure, which is further subjected to an aging treatment at a temperature of 200 to 700 ° C. It is a method of manufacturing quality steel.

(2) Further, in order to adjust the ferrite + pearlite structure ratio, in addition to the steel component of the above item (1), C
r: 0.02 to 1.50%, Mo: 0.02 to 1.00
% Of 1% or 2%.

(3) Further, in order to refine the crystal grains, in addition to the steel material components of the above (1) and (2), Nb:
0.001 to 0.20% is contained.

(4) Further, in order to further improve the machinability, P is added to the steel material components in the above items (1) to (3).
b: 0.05 to 0.30%, Ca: 0.0005 to 0.
One or two of 010% are contained.

[0020]

Next, in the method for producing a non-heat treated steel having excellent fatigue properties of the present invention, the chemical composition of the steel material, the metal structure after hot forging, cooling and transformation, and the conditions for aging this material are limited. The reason will be described below.

C: Controls the ferrite + pearlite structure ratio, thereby increasing the tensile strength of the final product, and
It is an important element that forms carbides or carbonitrides with V during aging treatment. If it is less than 0.15%, its effect is small,
On the other hand, if it exceeds 0.50%, the hardness becomes too high and the machinability is impaired, so the content is made 0.15 to 0.50%.

Si: an element for suppressing the deoxidation and precipitation of bainite and adjusting the ferrite + pearlite structure ratio,
If it is less than 0.005%, its effect is small, and if it exceeds 2.00%, both the durability ratio and the machinability are deteriorated.
It is set to 05 to 2.00%.

Mn: An element that causes an increase in the amount of pearlite and a decrease in the transformation temperature, and serves as a base of a composite precipitate that is a ferrite precipitation site by becoming MnS. If it is less than 0.40%, its effect is small. 2.00%
If it exceeds the limit, bainite is generated and both the durability ratio and the machinability deteriorate, so the content is made 0.40 to 2.00%.

S: MnS serves as a base of a composite precipitate that is a ferrite precipitation site and improves machinability. If it is less than 0.01%, its effect is small, and if it exceeds 0.10%. Since the durability ratio decreases, 0.
It is set to 01 to 0.10%.

Al: an element having a deoxidizing effect, 0.000
If it is less than 5%, its effect is small, and if it exceeds 0.050%, hard inclusions are formed, and both the durability ratio and the machinability deteriorate, so the content is made 0.0005 to 0.050%.

Ti: MnS is deposited as a nitride on the
It is an element that forms a composite precipitate that becomes a ferrite precipitation site. If it is less than 0.003%, its effect is small and 0.0
If it exceeds 50%, the formation of coarse hard inclusions will be promoted, and the durability ratio,
Any of the machinability will decrease, so 0.003 to 0.05
0%

N: Ti and V and elements forming nitrides or carbonitrides. If less than 0.0020%, the effect is small, and if more than 0.0200%, both durability ratio and machinability deteriorate. Although it is 0.0020 to 0.0200%, if N is excessive, the adverse effects such as formation of coarse nitride and matrix hardening are great, and 0.0070% or less is desirable.

V: An important element that forms a composite precipitate with MnS and TiN, precipitates and strengthens ferrite in pearlite, and further forms a carbide or carbonitride by aging treatment. The effect is small, and if it exceeds 0.70%, all of the durability ratio, the machinability and the yield ratio decrease, so it is set to 0.20 to 0.70%.

The above are the reasons for limiting the chemical composition of the steel of the first invention of the present application.

Next, in the invention of the above item (2), in order to adjust the ferrite + pearlite structure ratio, the steel material component of the item (1) further contains one or two kinds of Cr and Mo. The reasons for limiting the chemical components of are described below.

Cr: Mn is an element that causes an increase in the amount of pearlite and a decrease in the transformation temperature in the same manner as Mn. If it is less than 0.02%, its effect is small, and if it exceeds 1.50%, bainite is generated and the durability ratio and cutting ratio are increased. Because all of the sex decreases
0.02 to 1.50%.

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

In the invention of the above item (3), Nb is further contained in the steel material component of the item (1) or (2) in order to refine the crystal grains. The reason for limiting Nb is as follows.

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

In the invention of the above item (4), in order to further improve the machinability, the steel material components of the items (1) to (3) further contain one or two kinds of Pb and Ca. The reasons for limiting these chemical components will be described below.

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 durability ratio decreases.
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 0.0
If it exceeds 10%, the effect is saturated and the durability ratio decreases, so the content is made 0.0005 to 0.010%.

In hot forging a steel material having these chemical components, the heating temperature is set to a temperature of A _C3 point or higher,
Hot working shall be performed in the austenite single phase region.
This is because the deformation resistance of the steel material becomes high in areas other than the austenite single phase region, the life of the tool used for forging is extremely reduced, and the deformability is low and problems such as forging cracking occur.

However, the higher the temperature, the lower the deformation resistance during processing. Therefore, it should preferably be performed at a temperature of 1100 ° C. or higher. In consideration of coarsening of austenite crystal grains, 130
It should be below 0 ° C.

Next, in the steel of the present invention, the metal structure is the one when the transformation is completed by cooling after hot forging. In order to achieve improvement in machinability and fatigue strength, the metal structure is 9
It is necessary that 0% or more has a ferrite + pearlite structure. Even if there is a low temperature transformation structure such as bainite having a structure ratio of less than 10% or residual austenite, this effect is not impaired.

If such a ferrite-pearlite two-phase structure can be obtained, the cooling method after hot forging is not specified, but natural cooling is naturally desirable from the viewpoint of equipment and manufacturing cost. The metal structure is confirmed by observing the corroded test piece with an optical microscope or the like, and measuring the microhardness of the structure with a micro Vickers hardness measuring machine.

Finally, the reasons for limiting the conditions for aging such a material will be described.

When the heating temperature of the aging treatment is less than 200 ° C.,
The diffusion of C is difficult and the effect is insufficient. On the other hand, when the temperature exceeds 700 ° C., the precipitated carbides are coarsened and not only the tensile strength is lowered but also the fatigue strength is lowered. Therefore, the heating temperature of the aging treatment is set to 200 to 700 ° C.

The heating time is not particularly limited as long as it is within this temperature range, but it should preferably be about 10 minutes to 2 hours. Further, the performance of the present invention can be obtained by a cooling method after aging treatment such as air cooling, water cooling, oil cooling, or the like.

[0045]

EXAMPLES The effects of the present invention will be more specifically described below with reference to examples. In each of the tables listed below, No. with parentheses in the section column is added. Are examples satisfying the conditions of the present invention, and other examples are comparative examples.

A. Regarding the influence of steel chemical composition, Table 1
Steel with the chemical composition shown in Table 4 was melted in a high-frequency furnace to 150
A steel ingot of kg is cut out, a forging material is cut out from this, and after normalizing by heating and cooling at 950 ° C. once, 1100 to 1250 ° C.
Then, it was hot-forged at a temperature of 1050 to 1200 ° C. and then left to cool.

Further, this material was placed in a heating furnace at a temperature of 400 ° C. for 1 hour and subjected to an aging treatment, and 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 was conducted.

Further, a cutting test piece was sampled from this material,
Blind holes with a depth of 30 mm were drilled using a 10 mmφ straight shank drill made by SKH9, and the machinability was evaluated by the total drilling distance until the life of the drill was destroyed. The cutting speed is 50m / min and the feed speed is 0.3.
5 mm / rev. The cutting oil was set to 7 L / min. An optical microscope observation test piece was taken from the material before the aging treatment, corroded with 5% nital and observed at 200 times. Tables 5 to 8 show the ferrite + pearlite microstructure ratios and performance evaluation results of each test material.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

[0053]

[Table 5]

[0054]

[Table 6]

[0055]

[Table 7]

[0056]

[Table 8]

First, the current tempered steel No. which is given as a comparative example. No. 41, which is an example of the present invention, has a durability ratio of 0.47 and a machinability of 1.00. The durability ratios of (1) to (19) are all 0.
56 or more, and the machinability is also No. It is 2.5 to 4 times better than 41.

No. of the comparative example. No. 20 has a low C content and thus a low tensile strength and a low durability ratio, and therefore has poor fatigue properties. No. of the comparative example. In No. 21, bainite was generated because the C content was too high, the condition of ferrite + pearlite structure ratio of the present invention was not satisfied, and the tensile strength was high, but the durability ratio was low and the machinability was poor compared to the examples of the present invention. is there.

No. of the comparative example. No. 22 has a low Si content and therefore has a low degree of deoxidation, and has a lower durability ratio than the examples of the present invention. No. of the comparative example. In No. 23, since the amount of Si is high, bainite is generated, the condition of the ferrite + pearlite structure ratio of the present invention cannot be satisfied, 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. 24, 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. 25, since the amount of Mn is high, bainite is generated, the condition of ferrite + pearlite structure ratio of the present invention cannot be satisfied, 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. 26, since the amount of S is low, the precipitation of complex precipitates 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. 27 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. Since 28 has a low Al content, the degree of deoxidation and the effect of refining crystal grains are small, and the durability ratio is lower than in the examples of the present invention. No. of the comparative example. In No. 29, since the amount of Al 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. No. 30 has a small amount of Ti because of the low amount of Ti, and has a lower durability ratio than the inventive examples. No. of the comparative example. No. 31 has a high Ti content, so that hard inclusions are formed, 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. 32, 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. 33, since the amount of N is high, the matrix is hardened, 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. 34, since the V content is low, the precipitation of composite precipitates is small, and the effect of precipitation strengthening the matrix ferrite is small, so that the durability ratio is lower than that of the examples of the present invention. No. of the comparative example. The sample No. 35, which has a high V content, has a lower durability ratio than the examples of the present invention and has poor machinability.

No. of the comparative example. In No. 36, since the amount of Cr is high, bainite is generated, the condition of the ferrite + pearlite structure ratio of the present invention cannot be satisfied, 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 Mo is high, bainite is generated, the condition of the ferrite + pearlite structure ratio of the present invention cannot be satisfied, the durability ratio is lower than that of the examples of the present invention, and the machinability is also poor.

Comparative Example No. 38 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.

Comparative Example No. 39 has a high Pb content,
The machinability is good but the durability ratio is poor.

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

B. Regarding the influence of the change in metal structure due to the cooling method after hot forging, No. The steel having the chemical composition shown in (19) is melted in a high-frequency furnace to form a 150 kg steel ingot, and a forging material is cut out from the steel ingot and temporarily heated and cooled at 950 ° C. to normalize, and then heated to 1100 to 1250 ° C. 1050-
Hot forging was performed at a temperature of 1200 ° C., and then the same as in Table 9
It cooled by the method shown in.

Further, this material was placed in a heating furnace having a temperature of 400 ° C. for 1 hour and subjected to an aging treatment, and these materials were subjected to a tensile test, a fatigue test, a cutting test and a metallographic structure in the same manner as in Example a. Observed. Table 10 shows the performance evaluation results of each test material.

[0073]

[Table 9]

[0074]

[Table 10]

No. 42, 43 and 44 have a ferrite + pearlite structure ratio of 0.9 or more, which satisfies the conditions of the present invention. All of them have a durability ratio of 0.56 or more, and have a machinability as a comparative example. The current tempered steel No. It is almost four times as good as 47.

No. 45 and 46 have a low temperature transformation structure mainly composed of bainite or martensite by increasing the cooling rate.Although the tensile strength is high, the durability ratio is extremely low and the machinability is poor and the tool life is long. Extremely small.

C. Regarding the effect of changes in aging temperature,
No. of Table 2 The steel having the chemical composition shown in (19) is melted in a high-frequency furnace to form a 150 kg ingot, and the forging material is cut out from it and then normalized by heating and cooling at 950 ° C. for 1100.
It was heated to ˜1250 ° C., hot forged at a temperature of 1050 to 1200 ° C., and then left to cool.

Further, this material was placed in a heating furnace having a temperature shown in Table 11 for 15 minutes to 1 hour to perform an aging treatment. Tensile test, fatigue test, cutting test and metallographic observation were performed on these materials by the same method as in Example a. Table 12
The performance evaluation results of each test material are shown in.

[0079]

[Table 11]

[0080]

[Table 12]

No. Nos. 49, 50 and 51 satisfy the aging temperature range of the present invention, which is 200 to 700 ° C., all have a durability ratio of 0.56 or more, and have machinability of No. It is almost 4 times as good as 53.

No. No. 48 is when the aging temperature is below the range of the present invention, and the durability ratio is poor. In addition, No. No. 52 is the case where the aging temperature exceeds the range of the present invention, and the durability ratio is inferior.

[0083]

As described above, the non-heat treated steel produced by the production method of the present invention has a machinability by having a ferrite + pearlite two-phase structure and is formed from MnS, Ti nitride and V nitride. By improving the fineness of the metal structure and strengthening the ferrite matrix in pearlite with V carbide (or carbonitride) at the same time by using the composite precipitate that is produced, the durability ratio, that is, the fatigue property is improved without impairing the machinability. Then, by performing aging treatment after adjusting the C and V contents, it is possible to increase the yield ratio by V carbide precipitation, and at the same time improve the fatigue properties, machinability and yield properties that have long been desired. It is possible to manufacture a satisfactory non-heat treated steel, which is extremely effective in industry.

Claims (4)

[Claims] 1. A weight ratio of C: 0.15 to 0.50%, Si: 0.005 to 2.
00%, Mn: 0.40 to 2.00%, S: 0.01
~ 0.10%, Al: 0.0005-0.05%, T
i: 0.003 to 0.05%, N: 0.0020 to
0.0200% V: contains 0.20 to 0.70 percent, the balance being a steel having a composition consisting of Fe and impurity elements, it is heated to a temperature not lower than _C3 points A subjected to hot forging, cooled 90% or more of the metal structure after the transformation is completed is a ferrite + pearlite structure, and this is further subjected to an aging treatment at a temperature of 200 to 700 ° C., which is a non-heat treated steel having excellent fatigue properties. Manufacturing method. 2. The steel composition further comprises Cr: 0.02 to 0.02.
The method for producing a non-heat treated steel having excellent fatigue properties according to claim 1, wherein a steel containing one or two of 1.50% and Mo: 0.02 to 1.00% is used. . 3. The steel composition further comprises Nb: 0.001.
The method for producing a non-heat treated steel having excellent fatigue properties according to claim 1 or 2, characterized in that a steel containing 0.1 to 0.20% is used. 4. The steel composition further comprises Pb: 0.05-
A non-heat treated steel having excellent fatigue properties according to claim 1, wherein a steel containing one or two of 0.30% and Ca: 0.0005 to 0.010% is used. Production method.