JP3514018B2 - Method for producing high-strength and high-toughness martensitic non-heat treated steel - Google Patents

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 a high-strength, high-toughness martensitic non-heat treated steel suitable for use as automobile parts such as flange companions, connecting rods and spindles.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
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]

Means for Solving the Problems The present application inventions are those that have been developed to solve such problems. Thus, the manufacturing method of the invention of the present application is 0.02 ≦ C ≦ 0.
15%, 0.08 ≦ Si ≦ 1.0%, N ≦ 0.03% ,
Mn ≦ 3.0%, Cr ≦ 3.0%, Al ≦ 0.2%,
0.0008 ≦ B ≦ 0.01%, Ti ≦ 0.06% der
Or further Ni ≦ 4.0%, Cu ≦ 1.0%, M
o ≦ 2.0%, W ≦ 0.5%, Zr ≦ 0.5%, V ≦
One or more of 0.3% and Nb ≦ 0.08% are contained, the hardenability index H ≧ 3.5 represented by the following formula, and the balance is
A steel material consisting essentially of Fe is heated to 830 ° C. or higher to be austenitized, cooled to a range of 550 to 900 ° C. at an average cooling rate of 30 ° C./min or higher, and then forged, and thereafter. M at an average cooling rate of 100 ° C / min or more
It is characterized in that it is cooled to 300 ° C. or lower, which is point f, to form martensite (claim 1). H = Cr + Mn + N
i + Mo + 5 (Cu + W + Zr + V + Ti) + XB + 2
0Nb + 0.5Si-5Al (XB = 0.5 when B is contained in 0.0008 or more and 0.005% or less)

Further, the manufacturing method of claim 2 is characterized in that in claim 1 , after the forging, after converting to martensite, reheating treatment is performed in a range of 600 ° C. or lower.

Next, the manufacturing method of claim 3 is the same as that of claims 1 and 2.
In any one of the above, the steel material further contains one or more of S, Ca, Pb, Te, and Bi as free-cutting components, S ≦ 0.2%, Ca ≦ 0.05%, Pb ≦ 0. Three
%, Te ≦ 0.1%, Bi ≦ 0.15%.

[0010]

[Effect of the action and INVENTION The invention of claim 1 is to apply the ausforming techniques. In order to apply this ausforming, it is necessary to enhance the hardenability of steel. Therefore, the inventors of the present invention have conducted research for that purpose by setting the composition of steel to the above composition and using H = Cr + Mn + Ni + Mo + 5 (Cu + W + Z) as an index showing hardenability.
r + V + Ti) + XB + 20Nb + 0.5Si-5Al
(XB is 0.5 when B is 0.0008 or more and 0.005 or less is included), and if the index H is 3.5 or more, the ausforming can be stably applied, and finally strength and toughness can be obtained. It was learned that excellent martensitic non-heat treated steel 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. Therefore, when this is applied to parts such as automobiles, it becomes smaller than the conventional parts. Therefore, the weight can be reduced.

The steel obtained according to 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, compared with the conventional JIS-quenched-tempered steels. Lead time can be shortened and cost can be reduced.

[0013] In the production how of the present invention, the first material 830
Heated above ℃ to austenite, then 30 ℃ /
With an average cooling rate of more than a minute, to a range of 550 to 900 ° C,
That is, the structure is cooled to a metastable austenite region, forged there, and then cooled to martensite the structure.

As the above-mentioned material, a billet, a round bar, a coil or the like obtained by rolling or forging 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 ( Claim 2) . 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.

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

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.
It was set to 03%.

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: 0.3% 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 Since Ti reduces the hardenability effect when B forms BN, it is added to fix N as TiN. 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 550 to 90 ° C.
It is cooled to 0 ° C and is forged in a metastable austenite state. By performing working in this temperature range, dislocations and work-induced precipitation carbides that can survive martensitic transformation can be generated, which makes it extremely possible. A dense structure is obtained, and as a result, 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. This is because forging in the low temperature austenite region significantly promotes the formation of ferrite that transforms and precipitates diffusely, so that 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.

[0031]

EXAMPLES Examples of the present invention will be described in detail below. Table 1
After heating each austenitizing (γ-forming) temperature (830 ° C. or higher) for each of the steel types having various compositions shown in (4) and cooling the steel, 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.

[0032]

[Table 1]

From FIG. 1, it can be seen that when the austenitizing heating temperature is 830 ° C. or higher and the hardenability index H is 3.5 or higher, a hardness of 300 or higher 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 thermomechanical treatment, is followed, the bainite structure and the like formed after hot rolling disappear during 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. If you forging according to the production process of the present invention, therefore, it can be confirmed that the material to be supplied can ensure a tissue that is an even sufficient workability with remains poor rolling workability.

Next, FIG. 5 shows the strength of the present non-heat treated steel and JIS-hardening of structural steel-tempering material and the steel of the present invention (martensite type non-heat treated steel) manufactured according to the manufacturing process of the present invention. -A diagram showing the toughness balance. From this figure, it can be clearly seen that the steel according to the present invention has characteristics that surpass not only ordinary martensitic non-heat treated steel but also JIS-quenched structural steel-tempered material. it can.

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

[0042]

[Table 2]

<Production Example of Specific Product> Next, the flange companion 10 shown in FIG. 7 which is one of the important parts for automobiles was produced 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.

[0044]

[Table 3]

Here, in order to investigate the material characteristics when each process was followed, 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, the prescribed hardness is obtained at all when the process 3 which is the heat treatment process and the process 2 which is the low temperature hot forging are performed. Absent. On the other hand, in the case of steel type G which is the steel of the present invention example , 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 obtained 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 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

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-6-299285 (JP, A) JP-A-4-371547 (JP, A) JP-A-7-97657 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C21D 8/00 C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. In weight%, 0.02 ≦ C ≦ 0.15%,
0.08 ≦ Si ≦ 1.0%, N ≦ 0.03% , Mn ≦
3.0%, Cr ≦ 3.0%, Al ≦ 0.2%, 0.00
08 ≦ B ≦ 0.01%, Ti ≦ 0.06% ,
In addition, Ni ≦ 4.0%, Cu ≦ 1.0%, Mo ≦ 2.
0%, W ≦ 0.5%, Zr ≦ 0.5%, V ≦ 0.3%,
One or more of Nb ≦ 0.08% is contained, the hardenability index H ≧ 3.5 represented by the following formula, and the balance is substantially F:
The steel material consisting of e is heated to 830 ° C or higher to be austenitized, and then 5 at an average cooling rate of 30 ° C / min or higher.
High strength characterized by cooling to a range of 50 to 900 ° C., forging, and then cooling to an Mf point of 300 ° C. or less at an average cooling rate of 100 ° C./min or more to form martensite − Manufacturing method of high toughness martensitic non-heat treated steel. H = Cr + Mn + Ni + Mo + 5 (Cu + W + Zr + V
+ Ti) + XB + 20Nb + 0.5Si-5Al (XB = 0.5 when B is contained in 0.0008 or more and 0.005% or less) 2. The high strength-high toughness martensite type non-heat treated steel according to claim 1 , wherein after the forging process is converted to martensite, reheat treatment is performed in a range of 600 ° C. or less. Production method. 3. The steel according to claim 1 ,
The material is S, Ca, Pb, Te, B as a free-cutting component.
One or more of i is S ≦ 0.2%, Ca ≦ 0.
05%, Pb ≦ 0.3%, Te ≦ 0.1%, Bi ≦ 0.
A high-strength toughness martensitic non-heat treated steel characterized by containing 15%.