JPH09170047A - Bainitic non-heat treated steel with high strength and high toughness and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bainitic non-heat treated steel, having properties equal to or higher than those of the conventional quenched-and-tempered material and also reduced in manufacturing costs, and its production. SOLUTION: A steel, having a composition in which 0.04-0.25%, by weight, C, 0.08-1.0% Si, and <=0.03% N are contained and the hardenability index H, represented by H=Cr+Mn+Ni+Mo+5(Cu+W+Zr-+V+Ti)+XB+20Nb+0.5 Si-5Al, is regulated to >=3.5 (where Mn<=3.0%, Cr<=3.0%, Ni<=4.0%, Cu<=1.0%, Mo<=2.0%, W<=0.5%, Zr<=0.5%, B<=0.01%, V<=0.3%, Nb<=0.08%, Al<=0.2%, and Ti<=0.06% are satisfied) and the balance is composed essentially of Fe, is used as a stock. At the time of production, the stock is heated to >=830 deg.C to undergo austenitization and then forged while lowering the temp. to a value in the metastable austenite region, followed by bainitic transformation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength-high toughness bainite type non-heat treated steel suitable for use as automobile parts such as flange companions, connecting rods and spindles, and a method for producing the same.

[0002]

2. Description of the Related Art
Parts such as flange companion, connecting rod, spindle, etc. are to ensure predetermined strength and toughness after hot forging JIS-S45C, SCR420, SCM420, etc.
It was manufactured by quenching and tempering. However, in this case, although the strength and toughness can be secured, there is a problem that the lead time becomes long.

On the other hand, when a non-heat treated steel, which is being actively developed at present, is used, there is a problem in that sufficient strength and toughness cannot be obtained although it is excellent in terms of lead time.

In order to solve these problems, in recent years, a technique called ausforming has been studied as one of the thermomechanical treatment techniques represented by the controlled rolling technique, and it is applied to general structural steel. In this case, ferrite transformation is extremely promoted during processing, resulting in an incomplete microstructure. For this reason, sufficient strength and toughness have not been obtained with conventional structural steels.

Further, when ausforming is applied to structural steel, deformation resistance increases during forging, so that there are few structural steels to which ausforming can be applied, such as inconvenience occurring during forging. It's a reality.

On the other hand, when the above-mentioned parts are manufactured by warm forging, it is necessary to perform softening heat treatment such as annealing in order to reduce the deformation resistance, which causes a problem of increasing the material cost.

[0007]

The non-heat treated steel of the invention of the present application was developed to solve such problems. Therefore, the non-heat treated steel of the present invention is 0.0% by weight.
4 ≦ C ≦ 0.25%, 0.08 ≦ Si ≦ 1.0%, N ≦
Hardenability index H of 0.03% represented by the following formula
However, H = Cr + Mn + Ni + Mo + 5 (Cu + W + Zr
+ V + Ti) + XB + 20Nb + 0.5Si-5Al ≧
3.0 (However, Mn ≦ 3.0%, Cr ≦ 3.0%, Ni ≦
4.0%, Cu ≦ 1.0%, Mo ≦ 2.0%, W ≦ 0.
5%, Zr ≦ 0.5%, B ≦ 0.01%, V ≦ 0.4
%, Nb ≦ 0.08%, Al ≦ 0.2%, Ti ≦ 0.0
6%, and when B is 0.0008 or more and 0.005 or less, XB = 0.5), and the balance substantially consists of Fe (claim 1).

The non-heat treated steel of claim 2 is the same as that of claim 1, further containing S, Ca, Pb, Te and Bi as free-cutting components.
One or two or more of S ≦ 0.2%, Ca ≦ 0.
05%, Pb ≦ 0.3%, Te ≦ 0.1%, Bi ≦ 0.
It is characterized by containing at 15%.

Next, claim 3 relates to a method for manufacturing the high strength-high toughness bainite type non-heat treated steel according to claim 1 or 2, wherein the material is heated to 830 ° C or higher to be austenitized. Then, at an average cooling rate of 30 ° C./min or more, 550 to 550
After forging after cooling to the range of 900 ° C, the Bs point is 300 ° C at an average cooling rate of 20 ° C / min or more.
It is characterized in that it is cooled below and then cooled to room temperature, or is subjected to bainite transformation while being kept at a constant temperature between 300 and 500 ° C. which is between the Bs point and the Bf point.

Further, the manufacturing method of claim 4 is characterized in that, in claim 3, the reheating treatment is performed within a range of 600 ° C. or lower after the bainite transformation.

[0011]

The invention of claim 1 relates to a steel type to which the ausforming method can be stably applied. In order to apply this ausforming, it is necessary to enhance the hardenability of steel. Therefore, the inventors of the present invention carried out research for that purpose, and H = Cr + Mn + Ni + M was used as an index showing the composition of the steel and the hardenability.
o + 5 (Cu + W + Zr + V + Ti) + XB + 20Nb
If + 0.5Si-5Al is derived and the index H thereof is 3.0 or more, the ausforming can be stably applied, and finally, a bainite type non-heat treated steel excellent in strength and toughness can be obtained. I got it.

According to the present invention, a high level of strength and toughness equal to or higher than that of the conventional quench-tempered material of JIS steel can be obtained. Therefore, when this is applied to parts such as automobiles, it is smaller than the conventional parts. Therefore, the weight can be reduced.

In the present invention, if necessary, S, C
One or more free-cutting components such as a, Pb, Te, and Bi can be added, and in this case, the machinability of the material is enhanced and the manufacturability of the product is enhanced. 2).

The steel of the present invention is basically a non-heat treated steel,
Sufficient strength and toughness can be obtained without quenching and tempering treatments. Therefore, the lead time can be shortened and the cost can be reduced as compared with the conventional quench-tempered material of JIS steel type.

Next, claim 3 relates to a method for producing the above steel, in which the material is first heated to 830 ° C. or more to austenite, and then 550 to 900 ° C. at an average cooling rate of 30 ° C./min or more. To the range of metastable austenite, and then forged there, and then cooled to bainite the structure.

As the above-mentioned material, a billet, a round bar, a coil or the like which has been rolled or forged can be used, and the processing temperature in that process need not be 900 ° C. or lower. The present invention is characterized by applying the above-mentioned ausforming method when forging into a product shape thereafter.

According to the manufacturing method of the present invention, since the material is forged in the austenite state, that is, in the soft state, it is not necessary to perform the softening heat treatment in advance to reduce the deformation resistance prior to the forging, and the processing is performed. It can be done easily. When a normalizing material or an as-rolled material without heat treatment is used, the deformation resistance at the time of forging can be reduced according to this method.

In the production method of the present invention, after the bainite is formed by cooling, it can be reheated to a temperature of 600 ° C. or lower, and in this case, the toughness of the material can be further enhanced. As a result, the balance between strength and toughness can be optimized, and it becomes possible to manufacture parts that meet the required characteristics of the applied parts.

Next, the reasons for limiting each chemical component in the present invention will be described in detail. C: 0.04 to 0.25% 1500 M, which is the maximum structural strength of structural steel because it is a non-heat treated steel.
In order to keep Pa or less and to reduce the deformation resistance,
The upper limit was 0.25%. The lower limit of 0.04% is 800MP as the lower limit of the tensile strength because it is a high strength material.
It is defined as the minimum amount for obtaining a or more.

Si: 0.08 to 1.0% Si has the effect of enhancing hardenability, but impairs workability, so the upper limit was made 1.0%.

N: 0.03% or less N, like C, suppresses the deformation resistance, so the upper limit is 0.03%.
And

Mn: 3.0 or less Mn is an element that enhances hardenability, but the upper limit was made 3.0% because it damages the furnace wall during melting.

Cr: 3.0% or less Mo: 2.0% or less W: 0.5% or less Zr: 0.5% or less V: 0.4% or less Nb: 0.08% or less These components are strong. Although it is an element that generates carbides and enhances hardenability, if too much is added, the forgeability deteriorates due to undissolved carbides, so the respective upper limits were set to the above values.

B: 0.01% or less B is added to improve hardenability. The effect is saturated at 0.01%. A hardenability effect appears at 0.001% or more.

Ti: 0.06% or less Ti is added to fix N as TiN because the hardenability effect decreases when B forms BN. 0.0
If it exceeds 6%, the cleanliness of steel is impaired.

Ni: 4.0% or less Cu: 1.0% or less Ni and Cu are austenite stabilizing elements and have the function of improving hardenability, but if Cu is added too much, hot workability deteriorates. Therefore, the upper limit was set to 1.0%. Also Ni
When a large amount was added, the effect of improving the hardenability converges, so the upper limit was made 4.0%.

Al: 0.2% or less Al is an element that inhibits the hardenability and is a negative factor in the formula for calculating the hardenability index, so the upper limit is 0.2%.
Limited to.

S: 0.2% or less, Ca: 0.05% or less, Pb: 0.3% or less, Te: 0.1% or less, Bi:
0.15% or less These components are components that enhance the machinability of the material, and when contained within the above ranges, the machinability of the material is enhanced and the manufacturability in the production of parts is enhanced.

H = Cr + Mn + Ni + Mo + 5 (Cu +
W + Zr + V + Ti) + XB + 20Nb + 0.5Si-
5Al ≧ 3.0 This H is an index representing hardenability, and by setting H to 3.0 or more, the ausforming method can be stably applied, and the structure can be bainite in the subsequent cooling. .

Austenitizing by heating at 830 ° C. or higher and forging at 550 to 900 ° C. In the manufacturing method of the present invention, the material is heated to 830 ° C. or higher, then cooled to 550 to 900 ° C., and forged in a metastable austenite state. By performing working within this temperature range, it is possible to generate dislocations and work-induced precipitation carbides that can survive bainite transformation, which results in a very dense structure, resulting in high strength and High toughness can be achieved at the same time. Thus, the processed structure is mainly composed of bainite.

Since this method requires sufficient hardenability and forgeability, the hardenability index H is 3.0.
Materials having the above content and C content of 0.25% or less must be used. Because forging in the low temperature austenite region significantly promotes the formation of diffusionally transformed and precipitated ferrite, the desired strength and toughness cannot be obtained, and C increases the deformation resistance during low temperature austenite. This is because it is the main factor that causes it to be suppressed as much as possible.

Next, examples of the present invention will be described in detail below. For the steel types with various compositions shown in Table 1, each is heated at each austenitizing (γ-forming) temperature (830 ° C. or higher) and then cooled to be forged under the conditions of a forging temperature of 750 ° C. and a surface reduction rate of 60%. Then, the structure was bainite by performing air cooling after that, the hardness was measured, and the relationship between the index H indicating the hardenability and the hardness was obtained. The results are shown in Figure 1.

[0033]

[Table 1]

From FIG. 1, it is possible to secure a predetermined hardness (280 HV) when the austenitizing heating temperature is 830 ° C. or higher and the hardenability index H is 3.0 or higher, and the ausforming method of the present invention can be applied. I understand.

On the other hand, JIS steel types SCR420, S
The hardenability index H of CM420 and the like is 1.8 to 2.0, and that of SNCM420 is about 3.1, which is not at a level at which stable heat treatment (ausforming) can be applied.

The reasons for these phenomena are as follows. When plastic working is applied in the low temperature austenite region, dislocations introduced by working cannot be completely eliminated, and ferrite transformation or pearlite transformation, which is diffusion transformation, is significantly accelerated. As a result, a material with a low hardenability index (low hardenability) will produce a structure called acicular ferrite, upper bainite, equiaxed ferrite, pearlite, etc. in a considerable proportion no matter how much quenching is performed after forging. Will be greatly reduced.

Next, FIG. 2 shows the relationship between the forging temperature after forging-air cooling at austenitizing heating temperature of 1000 ° C., hardness and Charpy impact value for the steel type O in Table 1. From this figure, when bainite is mainly used, the forging temperature is 5
A clear effect can be confirmed in the range of 50 to 900 ° C.

Next, for steel type O, the material was heated to 1000 ° C. for 1 minute according to the process A shown in FIG.
It cooled to -800 degreeC and measured the deformation resistance by the end surface restraint test method. For comparison, JIS-SCM420 was heated and held at 700 to 800 ° C. for 1 minute according to the process B of FIG. 3, and then the deformation resistance was measured by the end face restraint test method. Results are shown in FIG.

In this test, the as-hot-rolled structure was tested. From these results, since both steel types contain a large amount of bainite, the deformation resistance is extremely high in the process B which is a normal warm forging.

On the other hand, when the process A, which is a thermomechanical treatment, is followed, the bainite structure and the like formed after hot rolling all disappear during austenitizing and heating, and during forging a soft austenite single phase with a small amount of C and high deformability is formed. Therefore, the workability is improved.

Therefore, when the steel of the present invention is forged according to the manufacturing process of the present invention, it can be confirmed that sufficient workability can be ensured even if the material supplied has a structure as rolled that has poor workability.

Next, FIG. 5 is a diagram showing the strength-toughness balance of the present non-heat treated steel and JIS-quenched / tempered structural steel and the example material of the present invention (bainite type non-heat treated steel).

From this figure, the steel of the present invention outperforms the ordinary bainite type non-heat treated steel, and has the strength-toughness balance equivalent to JIS-Quenched structural steel-Tempered material. You can clearly see that.

Next, the relationship between the forging temperature and the hardness and the impact value was determined for the steel type Y having the chemical composition shown in Table 2 containing the free-cutting component. Results are shown in FIG. As can be seen from the figure, although the toughness is slightly reduced as compared with the case where there is no free-cutting component, improvement in toughness can be clearly confirmed by forging at 550 to 900 ° C. and subsequent quenching. The manufacturing conditions were heating temperature: 1000 ° C., cooling method up to forging temperature: air cooling (30 ° C./min or more), workability: 45%, cooling method after forging: air cooling (20 ° C./min or more).

[0045]

[Table 2]

The embodiment of the present invention has been described in detail above, but this is merely an example, and the present invention can be implemented in variously modified modes without departing from the spirit of the invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a hardenability index and hardness obtained in an example of the present invention.

FIG. 2 is a diagram showing the relationship between forging temperature, hardness, and impact value obtained in the example of the present invention.

FIG. 3 is a diagram showing a patterning of the manufacturing process of the present invention in comparison with a conventional manufacturing process.

FIG. 4 is a diagram showing deformation resistance during forging when the example material of the present invention is processed according to the pattern shown in FIG. 3.

FIG. 5 is a diagram showing the balance between the tensile strength and the impact value of the material of the present invention in comparison with the conventional material.

FIG. 6 is a diagram showing the relationship between forging temperature, hardness, and impact value obtained in the example material of the present invention containing a free-cutting component.

Claims (4)

[Claims] 1. In weight%, 0.04 ≦ C ≦ 0.25%,
0.08 ≦ Si ≦ 1.0%, N ≦ 0.03%, and the hardenability index H represented by the following formula is H = Cr + Mn + Ni
+ Mo + 5 (Cu + W + Zr + V + Ti) + XB + 20
Nb + 0.5Si-5Al ≧ 3.0 (Mn ≦ 3.0
%, Cr ≦ 3.0%, Ni ≦ 4.0%, Cu ≦ 1.0
%, Mo ≦ 2.0%, W ≦ 0.5%, Zr ≦ 0.5%,
B ≦ 0.01%, V ≦ 0.4%, Nb ≦ 0.08%, A
1 ≦ 0.2%, Ti ≦ 0.06%, and B is 0.0
When it is included in the range of 008 to 0.005, XB = 0.
5), and the balance is a high strength-high toughness bainite type non-heat treated steel consisting essentially of Fe. 2. The method according to claim 1, further comprising one or more of S, Ca, Pb, Te and Bi as free-cutting components, S ≦ 0.2%, Ca ≦ 0.05%, Pb ≦ 0.3. %, T
A high strength-high toughness bainite type non-heat treated steel characterized by containing e ≦ 0.1% and Bi ≦ 0.15%. 3. The method for producing a high strength-high toughness bainite type non-heat treated steel according to claim 1 or 2, wherein the material is heated to 830 ° C. or higher to be austenitized and then heated to 30 ° C./min or higher. After forging after cooling to the range of 550 to 900 ° C. at the average cooling rate, it is then cooled to the Bs point of 500 ° C. or less at the average cooling rate of 20 ° C./min or more, and then cooled to room temperature or Bs. 300 between the point and the Bf point
A method for producing a high-strength, high-toughness bainite type non-heat treated steel, which is characterized by carrying out a bainite transformation while maintaining a constant temperature between 500 ° C and 500 ° C. 4. The method for producing a high strength-high toughness bainite type non-heat treated steel according to claim 3, wherein reheat treatment is performed in a range of 600 ° C. or lower after the bainite transformation.