JPH09170046A - High strength-high toughness martensitic non-heat treated steel and method for producing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a martensitic non-tempered steel having properties equal to or higher than those obtained by a conventional quenching-tempering material and being inexpensive in manufacturing cost, and a manufacturing method thereof. SOLUTION: The composition of steel is 0.02 ≦ C ≦ 0.15%, 0.0% by weight.
8 ≦ 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 + 20Nb +
0.5Si-5Al ≧ 3.5 (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 ≦ 0.3%,
Nb ≤ 0.08%, Al ≤ 0.2%, Ti ≤ 0.06%, and when B is 0.0008 or more and 0.005% or less, XB = 0.5),
The balance is a composition consisting essentially of Fe. And at the time of its production, the material is heated to 830 ° C or higher to be austenitized, then cooled to 550 to 900 ° C at a rate of 30 ° C / min or more and forged in the metastable austenite region, and then 100 ° C / min or more. It is cooled at a cooling rate of 3 to transform into martensite.

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 martensitic 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 companions, connecting rods, and spindles were manufactured by hot forging JIS-S45C, SCR420, SCM420, etc., and then performing quenching-tempering treatment to ensure predetermined strength and toughness. 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.
2 ≦ C ≦ 0.15%, 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 ≧
3.5 (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 ≦ 0.3
%, 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 producing the high-strength-high-toughness martensitic non-heat treated steel of claim 1 or 2, wherein the raw material is heated to 830 ° C or higher to be austenitized. And then at an average cooling rate of 30 ° C / min or more, 55
After cooling to a range of 0 to 900 ° C., forging is performed, and then the Mf point is 3 at an average cooling rate of 100 ° C./min or more.
It is characterized in that it is cooled to a temperature of 00 ° C. or lower to form martensite.

Further, the manufacturing method of claim 4 is the method according to claim 3, wherein after cooling to 300 ° C. or less, 600
It is characterized in that the reheating treatment is performed in the range of ℃ or less.

[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
+ 0.5Si-5Al (XB is 0.5 when B contains 0.0008 or more and 0.005 or less) is derived, and if the index H is 3.5 or more, the ausforming can be stably applied, and the final It was discovered that a martensitic non-heat treated steel with excellent strength and toughness can be obtained.

According to the present invention, a high level of strength and toughness exceeding the conventional quench-tempered material of JIS steel grade can be obtained, and therefore, when it is applied to parts such as automobiles, it becomes 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 (claim 2). ).

The steel of the present invention is basically a non-heat treated steel, and has sufficient strength and toughness without being subjected to quenching and tempering treatments. Therefore, it is more lead than the conventional quench-tempering material of JIS steel type. Time can be shortened and cost can be reduced.

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 metastable austenite region and forged there, after which it is cooled to martensite 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 being martensitic by cooling, it can be reheated to a temperature of 600 ° C. or lower, in which 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: 0.02 to 0.15% Since it is a non-heat treated steel, the upper limit was set to 0.15% so that the quenching strength would be 1500 MPa or less, which is the maximum as a structural steel. Further, the lower limit was set to 0.02% due to manufacturing restrictions.

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

N: 0.03% or less N increases the deformation resistance like C, so the upper limit is set to 0.
03%.

Mn: 3.0 or less Mn is an element that enhances the 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: 0.3% 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. 0.06
If it exceeds%, 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%, Ca: ≤0.05%, P
b: ≦ 0.3%, Te: ≦ 0.1%, Bi: ≦ 0.15
% 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 ≧ 3.5 This H is an index showing hardenability, and by setting H to 3.5 or more, the ausforming method can be stably applied, and the structure can be easily martensiticized in the subsequent cooling. You can

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 and then cooled to 550 to 900 ° C. in a metastable austenite state. Forging is performed.By processing within this temperature range, dislocations and work-induced precipitation carbides that can survive martensitic transformation can be generated, 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 martensite.

Since this method requires sufficient hardenability and forgeability, the hardenability index H is 3.5.
Materials having the above content and C content of 0.15% 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.

[0032]

Next, embodiments of the present invention will be described in detail. Table 1
For each of the steel compositions of various compositions shown in, after heating each at an austenitizing (γ-forming) temperature (830 ° C. or higher), cooling this, and forging under the conditions of a forging temperature of 750 ° C. and a surface reduction rate of 60%. Then, water cooling was carried out to convert the structure into martensite, and the hardness thereof was measured to obtain the relationship between the hardness H and the index H indicating the hardenability. The results are shown in Figure 1.

[0033]

[Table 1]

From FIG. 1, it can be seen that when the austenitizing heating temperature is 830 ° C. or more and the hardenability index H is 3.5 or more, hardness of 300 or more can be secured and the ausforming method of the present invention can be applied.

On the other hand, JIS steel grades SCR420, SC
The hardenability index H of M420 and the like is 1.8 to 2.0,
Further, 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 small 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. Hardness is greatly reduced.

Next, FIG. 2 shows the relationship between the forging temperature after forging-water cooling and the hardness and the Charpy impact value for steel type G in Table 1 at the austenitizing heating temperature of 1000 ° C. From this figure, when martensite is the main constituent, a clear effect can be confirmed in the forging temperature range of 550 to 900 ° C.

Next, for steel type G, process A shown in FIG.
After heating the material to 1000 ° C. for 1 minute according to
After cooling to 800 ° C., the deformation resistance was measured by the end face 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 thermo-mechanical treatment, is followed, the bainite structure and the like formed after hot rolling disappear during the austenitizing and heating, and a soft austenite single phase with a small amount of C and a high deformability is obtained during forging. Therefore, the workability is improved. Therefore, it can be confirmed that when the steel of the present invention is forged according to the manufacturing process of the present invention, sufficient workability can be secured even if the material supplied has a as-rolled structure with poor workability.

Next, FIG. 5 shows the present non-heat treated steel and JIS-quenching of structural steel-tempered material and the example steel of the present invention (martensite type non-heat treated steel) manufactured according to the manufacturing process of the present invention.
It is a figure showing the strength-toughness balance. From this figure,
It can be clearly seen that the inventive steels have properties that surpass not only ordinary martensitic non-heat treated steels, but also JIS-structural steel quenching-tempering materials.

Next, the relationship between the forging temperature and the hardness and the impact value was determined for the steel type X 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, the improvement in toughness can be clearly confirmed by forging at 550 to 900 ° C. and subsequent quenching. The manufacturing conditions are heating temperature: 1000 ° C., cooling method up to forging temperature: air cooling (cooling rate of 30 ° C./min or more), workability: 45%, cooling method after forging: water cooling (100 ° C./min or more Cooling rate)
And

[0043]

[Table 2]

<Manufacturing Example of Specific Product> Next, the flange companion 10 shown in FIG. 7 which is one of the important parts for automobiles was manufactured according to the process shown in FIG. The manufacturing conditions at that time were three as shown in Table 3, and the characteristics of two steel types G and B having different hardenability indexes H were compared and examined.

[0045]

[Table 3]

Here, in order to investigate the material characteristics in the case of following each process, a Charpy impact test and a hardness test were performed using impact test pieces cut out from the flange portion A and the drawn portion B shown in FIG. 9 and 10 show the hardness of the flange portion A and the drawn portion B in each steel type.

As is clear from these results, when the hardenability index H is low, when the process 3 which is a thermomechanical process or the process 2 which is a low temperature hot forging process is performed, a predetermined hardness is obtained at all. Absent. On the other hand, in the case of steel type G which is the example steel of the present invention, sufficient hardness is obtained regardless of which process is used.

FIG. 11 shows the measurement results of the toughness of the steel type G. From this figure, the highest value is shown when the process 3 which is the thermomechanical treatment process is followed, and the effect of the present invention is sufficiently exhibited. I know that

Although the embodiment of the present invention has been described in detail above, 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.

FIG. 7 is a view showing a flange companion which is an example as an applied part of the present invention.

8 is an explanatory view of a manufacturing process of the flange companion of FIG. 7. FIG.

9 is a diagram illustrating a comparative example material B according to various processes.
It is a figure showing hardness obtained when a flange companion is manufactured according to the process shown in.

FIG. 10 is a diagram showing hardness obtained when a flange companion is manufactured according to various processes by using a steel type G which is an example material of the present invention and according to various processes.

FIG. 11 is a diagram showing impact values obtained when a flange companion is manufactured according to various processes according to various processes by using steel type G which is an example material of the present invention.

[Explanation of symbols]

 10 Flange companion

Claims (4)

[Claims] 1. In weight%, 0.02 ≦ C ≦ 0.15%,
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 ≧ 3.5 (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%, A
1 ≦ 0.2%, Ti ≦ 0.06%, and B is 0.0
When the content is 008 or more and 0.005% or less, XB =
High strength-high toughness martensitic non-heat treated steel with 0.5) and the balance 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 martensitic non-heat treated steel characterized by containing e ≦ 0.1% and Bi ≦ 0.15%. 3. The method for producing a high strength-high toughness martensitic non-heat treated steel according to claim 1 or 2, wherein the raw material is 830.
After heating to ℃ or more to austenite, 30 ℃ /
Forging after cooling to a range of 550 to 900 ° C at an average cooling rate of not less than min, and then cooling to an Mf point of 300 ° C or less at an average cooling rate of not less than 100 ° C / min to form martensite. A method for producing a high strength-high toughness martensite type non-heat treated steel characterized by: 4. The high-strength / high-toughness martensitic non-heat treated steel according to claim 3, which is characterized by performing reheating treatment in a range of 600 ° C. or lower after the forging is performed after the martensite conversion. Production method.