JPH11269599A - High strength non-refining steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a non-refining steel having high tensile strength and an excellent endurance ratio and suitable as the stock for parts for machine structures including automotive chassis parts at a low cost. SOLUTION: A ferrite-pearlite type high strength non-refining steel is the one having a compsn. composed of, by weight, 0.20 to 0.40% C, 0.4 to 1.0% Si, 1.0 to 2.0% Mn, 0.05% P, 0.03 to 0.10% S, 0.3 to 0.8% Cr, 0.05 to 0.30% V, 0.006 to 0.10% Zr, 0.005 to 0.025% N, 0.005 to 0.07% Al, 0 to 0.10% Nb, 0 to 0.05% Ti, 0 to 0.30% Pb, 0 to 0.010% Ca, 0 to 0.30% Se, 0 to 0.10% Te, 0 to 0.30% Bi, and satisfying C+(1/10)Si+(1/5)Mn-(5/7)S+(5/22)Cr+1.65V>=0.8%, and contg. the balance Fe with impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a high-strength non-heat treated steel. More specifically, mechanical structural parts such as undercarriage parts for automobiles having high tensile strength and an excellent durability ratio (fatigue strength / tensile strength) without performing quenching and tempering after hot working. The present invention relates to a ferrite / pearlite type high-strength non-heat-treated steel suitable as a material for steel.

[0002]

2. Description of the Related Art Wheel hubs, knuckles, arms and the like as machine structural parts, especially forged parts for automobiles, are heated using carbon steel (such as S45C) or alloy steel (such as SCM440) for mechanical structures. After forming by cold forging, machining and refining treatment were performed to secure a desired shape and performance.

However, the refining process consumes a lot of energy and cost. Furthermore, quenching often occurs when medium and high C steels are quenched, and even if there is no quenching cracking, a large transformation strain occurs, resulting in a large "bending".
A straightening process for bending is required. Therefore, in recent years, non-heat-treated steels that can be used as they are in hot forging have been actively developed in order to meet the social demands for energy savings and to reduce costs and simplify the manufacturing process.

[0004] As non-heat treated steels, bainite, martensite and ferrite / pearlite non-heat treated steels are known. Of these, bainite-type and martensite-type non-heat treated steels provide high strength but low machinability. For this reason, there is difficulty in finish forming by machining,
In addition, there is a problem that "bending" becomes large due to generation of a large transformation strain, and a step of correcting the bending is required, which leads to an increase in cost. For example, Japanese Patent Application Laid-Open No. 4-1
The bainite type "non-heat treated steel for hot forging" proposed in Japanese Patent No. 76842 still has a problem in terms of the above-described machinability and bending.

[0005] Japanese Patent Application Laid-Open No. Hei 4-210449 discloses a "high-toughness non-heat-treated steel for hot forging" whose structure is mainly ferrite and bainite and partially contains pearlite. In the technique proposed in this publication, since the structure contains ferrite and pearlite, "bending" due to transformation strain is somewhat eliminated as compared with the case of bainite single phase. However, when bainite occupies a high proportion in the structure, a step of correcting "bending" due to the occurrence of transformation strain is required, and a cost increase is inevitable.

On the other hand, as a technique relating to a ferrite / pearlite type non-heat treated steel, for example, Japanese Patent Application Laid-Open No. 63-19998
No. 8, JP-A-7-70698, JP-A 7-1
No. 02340 is disclosed.

Among them, Japanese Patent Application Laid-Open No. 63-199848 discloses an attempt to strengthen ferrite by V nitride and solid solution N by limiting the Al content to less than 0.020%. Non-heat treated steel for hot forging having excellent workability and machinability ”is disclosed. However, since the non-heat treated steel proposed in this publication has a low content of Al, the effect of AlN on the grain refinement may not be obtained. It is at most 0.55.

[0008] Japanese Patent Application Laid-Open No. 7-70698 discloses "high fatigue strength free-cut non-heat treated steel" having a specific chemical composition. However, the technology proposed in this publication is 900
It is an object of the present invention to provide a non-heat treated steel excellent in machinability having a tensile strength of not less than MPa and a durability ratio of not less than 0.5, and the above non-heat treated steel satisfies the above-mentioned target. However, as is clear from the description of the example, the durability ratio is at most 0.54. This is because sufficient consideration has not been given to crystal grain refinement.

Japanese Patent Application Laid-Open No. 7-102340 discloses that a steel material having a specific chemical composition of 90% or more of ferrite + pearlite in which at least 90% of the structure is cooled after hot forging is used.
A method for producing a non-heat treated steel having excellent fatigue properties, which is subjected to aging treatment at 700 ° C., is disclosed. However, the technology proposed in this publication can omit the quenching of the refining treatment, but requires heat treatment for aging similar to tempering, so that the energy cost increases. Further, the durability ratio of the "non-heat treated steel" manufactured by the method proposed in this publication is at most 0.58 as is clear from the description in the examples.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is based on ordinary hot forging and cooling conditions.
It has a high tensile strength of 800 MPa or more and an excellent durability ratio of 0.60 or more, for example, and has no heat treatment including heat treatment including aging treatment after forging. It is an object of the present invention to provide a ferrite-pearlite type high-strength non-heat-treated steel suitable as a material for parts for machine structures including the above at low cost.

[0011]

The gist of the present invention resides in the following high-strength non-heat treated steel.

That is, "in weight%, C: 0.20 to 0.20%
0.40%, Si: 0.4 to 1.0%, Mn: 1.0 to
2.0%, P: 0.05% or less, S: 0.03 to 0.1
0%, Cr: 0.3-0.8%, V: 0.05-0.3
0%, Zr: 0.006 to 0.10%, N: 0.005
0.025%, Al: 0.005 to 0.07%, N
b: 0 to 0.10%, Ti: 0 to 0.05%, Pb: 0
~ 0.30%, Ca: 0 ~ 0.010%, Se: 0 ~
0.30%, Te: 0 to 0.10%, and Bi: 0 to 0.
Fn1 represented by the following formula, where the symbol of the element in the formula is the content in weight% of the element, is 0.8%.
%, With the balance being ferrite-pearlite high-strength non-heat treated steel consisting of the chemical composition of Fe and unavoidable impurities.

Fn1 = C + (1/10) Si + (1/5) Mn− (5/7) S + (5/22) Cr + 1.65V...

The present inventors have conducted various studies on the chemical composition and structure of the non-heat treated steel in order to achieve the above object, and have obtained the following findings.

(A) The strength of a ferritic / pearlite type non-heat treated steel having a specific chemical composition can be arranged by the above-mentioned formula. If the value of the formula is 0.8% or more, the tensile strength of 800 MPa or more is stable. Is obtained.

(B) The durability ratio of ferrite-pearlite type non-heat treated steel can be increased by refining the structure, increasing the ferrite fraction, and strengthening the ferrite.

(C) By adding Zr to precipitate fine Zr carbonitrides in the steel, it is possible to prevent austenite grains from becoming coarse during heating for hot forging, Since the number of ferrite precipitation sites increases during cooling, a fine structure can be obtained and the ferrite fraction increases.

(D) containing an appropriate amount of Mn and S
If S is generated, this MnS becomes a ferrite precipitation site in the cooling process after forging, so that the structure can be refined and the ferrite fraction can be increased.

(E) In order to increase the ferrite fraction and strengthen the ferrite, the C content should be reduced to include Si and V.

(F) The increase in the durability ratio of the ferrite-pearlite type non-heat treated steel can also be achieved by increasing the ductility (drawing) of the steel material.

(G) When Cr is contained, the lamella spacing of pearlite is reduced, and the drawing of steel material can be increased.

(H) When a tensile strength test is performed using a test piece having a circular cross section cut out of a ferrite / pearlite type non-heat treated steel material having a specific chemical composition, if a drawing of 50% or more is obtained, 0 is obtained. A durability ratio of .60 or more is obtained.

(I) The aperture of 50% or more of (h) is
Ferrite is JI in ferrite-pearlite structure
S can be obtained stably when the fine particles have an S particle number of 6 or more.

The present invention has been completed based on the above findings.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, “%” of the content of the chemical component means “% by weight”.

C: 0.20 to 0.40% C is an element effective for securing a high tensile strength of 800 MPa or more in a ferrite-pearlite structure. To obtain the effect, a content of 0.20% or more is required. However, if the content exceeds 0.40%, the ferrite fraction decreases, and the durability ratio falls below 0.60. Therefore, the content of C is set to 0.20 to 0.4.
0%.

Si: 0.4-1.0% Si has the effect of promoting deoxidation and also forming a solid solution in ferrite to strengthen ferrite and increase the durability ratio.
In order to sufficiently exhibit the above effects, it is necessary to make the Si content 0.4% or more. However, when Si is excessively added, the above-described effect is not only saturated, but also decarburization of the steel material surface is advanced by heating for forging, and the surface strength is reduced. In particular, if the Si content exceeds 1.0%, the decarburization of the steel material surface becomes remarkable. Therefore, the content of Si is set to 0.4 to 1.0%.

Mn: 1.0 to 2.0 Mn has a deoxidizing effect and an effect of increasing strength. Further, MnS combined with S serves as a precipitation site of ferrite in a cooling process after hot forging, and contributes to refinement of the structure and increase in the ferrite fraction. In order to secure such an effect, the content of Mn needs to be 1.0% or more.
However, even if the content exceeds 2.0%, the effect is saturated and the cost increases. Further, the hardenability becomes too high, and bainite and island-like martensite are formed, so that the machinability decreases. For this reason, the Mn content is set to 1.0 to 2.0%.

P: 0.05% or less P is contained as an impurity in steel, and need not be added as an essential component. When added, it has the effect of improving machinability and durability ratio. To ensure this effect,
It is preferable that the content of P is 0.010% or more.
However, when the content exceeds 0.05%, the toughness is significantly reduced. Therefore, the content of P is set to 0.05% or less.

S: 0.03 to 0.10% S enhances the machinability of steel and combines M with Mn.
nS acts as a precipitation site of ferrite in the cooling process after hot forging, and contributes to refinement of the structure and increase in the ferrite fraction. However, if the content is less than 0.03%, the effect of addition is poor. On the other hand, if the content exceeds 0.10%, the above effect is saturated and the toughness is reduced. Therefore, the S content is set to 0.03 to 0.10%.

Cr: 0.3-0.8% Cr has the effect of increasing the strength as a solid solution strengthening element, reducing the pearlite lamella spacing, increasing ductility (drawing), and increasing the durability ratio. However, if the content is less than 0.3%, the effect of addition is poor. On the other hand, if the content exceeds 0.8%, the effect is saturated and the cost increases.
Further, the hardenability becomes too high, and bainite and island-like martensite are formed, so that the machinability decreases. For this reason, the Cr content is set to 0.3 to 0.8%.

V: 0.05 to 0.30% V is a precipitation strengthening element and has the effect of strengthening ferrite and increasing the durability ratio. However, the content is 0.0
If it is less than 5%, it is difficult to obtain the above effect. On the other hand, if the content exceeds 0.30%, the effect is saturated, and only the cost is increased, which impairs economic efficiency. Therefore, the content of V is set to 0.05 to 0.30%.

Zr: 0.006% to 0.10% Zr combines with C and N and precipitates as Zr carbonitride to prevent coarsening of austenite crystal grains and to prevent the above Zr carbonitride from being forged. During cooling, it becomes a ferrite precipitation site and contributes to the refinement of the structure and an increase in the ferrite fraction. However, its content is 0.006%
If it is less than 30, the desired effect cannot be obtained. On the other hand, when Zr is 0.1
Even if the content exceeds 0%, the effect is saturated and the cost is increased. Therefore, when the Zr content is 0.006
０．１0.10%.

N: 0.005 to 0.025% N is combined with V and Zr together with C to form V carbonitride or Zr.
It has the effect of precipitating as carbonitride and increasing the strength. or,
The above-mentioned AlN combined with Zr carbonitride and Al prevents the austenite grains from being coarsened, so that it is also effective in refining the structure. However, if the content is less than 0.005%, it is difficult to obtain the above effects. On the other hand, if the content exceeds 0.025%, the effect is not only saturated, but also causes deterioration of hot workability. Therefore, the content of N is set to 0.
005 to 0.025%.

Al: 0.005 to 0.07% Al has a deoxidizing effect. Further, Al is A together with N
It has the effect of forming 1N to prevent coarsening of austenite grains and to refine the structure to increase the durability ratio. But,
If the content is less than 0.005%, the effect of addition is poor,
When the content exceeds 0.07%, oxide-based inclusions increase to cause a decrease in tool life during cutting, and the subcutaneous subsurface inclusions lower fatigue characteristics. Therefore, the content of Al is set to 0.005 to 0.07%.

Nb: 0 to 0.10% Nb may not be added. If added, it has the effect of forming nitrides and carbonitrides to refine the crystal grains and increase the strength and durability ratio. In order to ensure this effect, Nb should be set to 0.1.
Desirably, the content is at least 003%. But,
If the content exceeds 0.10%, the hot workability of steel will be greatly reduced. Therefore, the content of Nb is reduced to 0.
０．１0.10%.

Ti: 0 to 0.05% Ti need not be added. The addition prevents the austenite crystal grains from becoming coarse, and contributes to the refinement of the structure and the increase in the ferrite fraction. To ensure this effect, the content of Ti is desirably 0.003% or more. However, even if the content exceeds 0.05%, the above effect is saturated and the cost increases. Therefore, the Ti content was set to 0 to 0.05%.

Pb: 0 to 0.30% Pb may not be added. If added, it has the effect of enhancing machinability. In order to surely obtain this effect, the content of Pb is preferably set to 0.01% or more. However, if the content exceeds 0.30%, the hot workability and the fatigue strength are significantly reduced. Therefore, the Pb content was set to 0 to 0.30%.

Ca: 0 to 0.010% Ca may not be added. If added, it has the effect of enhancing machinability. In order to surely obtain this effect, the content of Ca is preferably set to 0.0005% or more. However, when the content exceeds 0.010%, the hot workability and the fatigue strength are significantly reduced. Therefore, the Ca content was set to 0 to 0.010%.

Se: 0 to 0.30% Se need not be added. If added, it has the effect of enhancing machinability. In order to surely obtain this effect, the content of Se is preferably set to 0.01% or more. However, if the content exceeds 0.30%, the hot workability and the fatigue strength are significantly reduced. Therefore, the content of Se is set to 0 to 0.30%.

Te: 0 to 0.10% Te need not be added. If added, it has the effect of enhancing machinability. To ensure this effect, the content of Te is preferably set to 0.001% or more. However, if the content exceeds 0.10%, the hot workability and the fatigue strength are significantly reduced. Therefore, the content of Te is set to 0 to 0.10%.

Bi: 0 to 0.30% Bi may not be added. If added, it has the effect of enhancing machinability. To ensure this effect, the content of Bi is preferably set to 0.001% or more. However, if the content exceeds 0.30%, the hot workability and the fatigue strength are significantly reduced. Therefore, the content of Bi is set to 0 to 0.30%.

Fn1: 0.8% or more The strength of a ferrite / pearlite type non-heat treated steel having a specific chemical composition can be summarized by fn1 represented by the above equation.
If this value is 0.8% or more, the tensile strength is 800 MPa.
The above high strength is stably obtained. Therefore, fn1
Was specified to be 0.8% or more. The machinability decreases as the tensile strength increases, and in the case of a ferrite-pearlite structure, the machinability decreases significantly when the tensile strength exceeds 1100 MPa. If the value of fn1 is 1.2% or less, it becomes easy to suppress the tensile strength to 1100 MPa or less. Therefore, the value of fn1 is preferably set to 1.2% or less.

After the steel having the above chemical composition is melted by a normal method, it is subjected to, for example, hot forging by a normal method (or hot rolling or forging by a normal method). After that, it is further formed into a predetermined shape (by subjecting it to hot forging by a usual method), and is further machined as necessary to be finished into a predetermined shape part such as a wheel hub, a knuckle, an arm or the like. The cooling after the hot forging by a usual method for forming into a predetermined shape may be performed at a cooling rate such as a ferrite-pearlite structure, for example, air cooling or cooling. The “ordinary hot forging method” described above refers to a method of heating to 900 to 1300 ° C. and then forging.

When the non-heat-treated steel of the present invention is used, the ferrite is finely divided into JIS grain size of 6 or more by hot forging according to the above-mentioned ordinary method and subsequent cooling at a cooling rate to obtain a ferrite-pearlite structure. Since a grain structure is obtained, a high tensile strength and a good durability ratio can be secured as shown in Examples described later.

[0046]

EXAMPLE 150 kg of steel having the chemical composition shown in Tables 1 and 2 was used.
It was melted by a usual method using a vacuum melting furnace. Steels 1 to 15 in Table 1 are steels of Examples of the present invention whose chemical compositions are within the range specified in the present invention, and Steels 16 to 31 in Table 2
Is a steel of a comparative example whose one of the chemical compositions is out of the range of the content specified in the present invention.

[0047]

[Table 1]

[0048]

[Table 2]

These steels were made into billets by a usual method, and then heated to 1200 to 1250 ° C.
It was hot forged into a round bar having a diameter of 50 mm at a finishing temperature of ° C.
Then, the round bar hot forged to a diameter of 50 mm was cut to a length of 100 mm, and further cut to 1200 mm by a high frequency heating device.
° C, and the diameter is 30 using a hot forging press.
It was formed into a round bar of mm. The hot forged round bar having a diameter of 30 mm was air-cooled to room temperature (room temperature).

An Ono-type rotary bending fatigue test piece having a parallel part diameter of 8 mm was cut out from the center of the thus obtained round bar and subjected to a fatigue test at room temperature (room temperature) and in the atmosphere at 3000 rpm to obtain a fatigue strength. Limit (σw) was determined.
Further, a JIS No. 4 tensile test piece was cut out from the center of the round bar, and a tensile test was performed at room temperature to measure the yield strength (YS), tensile strength (TS), elongation, and drawing. In addition, diameter 30m
A test piece having a thickness of 20 m and a thickness of 20 mm was cut out, and the structure at the center was observed with an optical microscope.

Table 3 shows the test results.

[0052]

[Table 3]

Steels 1 to 15 of the present invention all have a ferrite-pearlite structure and the ferrite is fine grains having a JIS particle size number of 6 or more. The above durability ratio is obtained.

On the other hand, the steel of the comparative example in which one of the components is out of the range of the content specified in the present invention has at least a tensile strength not reaching 800 MPa or a durability ratio reaching 0.60. Not.

A 30 mm diameter round bar of the steel of the present invention was used as a test piece, a JIS high speed tool steel SKH51 φ6 mm drill was used, and a water-soluble lubricant was used to feed 0.15 mm / rev. The drilling test was performed under the conditions of a rotation speed of 980 rpm. As a result, a tensile strength of 110
Since it was 0 MPa or less, it was confirmed that there was no problem in machinability.

[0056]

The use of the ferritic / pearlite type high-strength non-heat treated steel of the present invention stably provides a high tensile strength of 800 MPa or more and a durability ratio of 0.60 or more. It can be used as a material for parts for automobiles, especially as a material for wheel hubs, knuckles and arms as forged parts for automobiles.

Claims (1)

[Claims] C. 0.20 to 0.40% by weight, S
i: 0.4 to 1.0%, Mn: 1.0 to 2.0%, P:
0.05% or less, S: 0.03 to 0.10%, Cr:
0.3-0.8%, V: 0.05-0.30%, Zr:
0.006 to 0.10%, N: 0.005 to 0.025
%, Al: 0.005 to 0.07%, Nb: 0 to 0.1
0%, Ti: 0 to 0.05%, Pb: 0 to 0.30%,
Ca: 0 to 0.010%, Se: 0 to 0.30%, T
e: 0 to 0.10% and Bi: 0 to 0.30%,
Furthermore, a ferrite / pearlite type high-strength non-heat treated steel comprising fn1 represented by the following formula of 0.8% or more and the balance being a chemical composition of Fe and unavoidable impurities. fn1 = C + (1/10) Si + (1/5) Mn- (5/7) S + (5/22) Cr + 1.65V where the symbol of the element in the formula is the weight% of the element. Indicates the content.