JP3300511B2 - Manufacturing method of sub-hot forging steel excellent in toughness, durability ratio, yield ratio and machinability - 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 steel for machine structures such as automobiles by sub-hot forging, and more particularly, to a steel having a specific chemical composition at 800 to 1050.degree. After performing sub-hot forging in the temperature range described above to obtain a specific metal structure, by subjecting to aging treatment, the tensile strength exceeds 90 kgf / mm ² and the impact value at 20 ° C. is 2 kg.
The present invention relates to a method for producing a steel for sub-hot forging, characterized by having excellent durability ratio, yield ratio and machinability at f-m / cm ² or more.

[0002]

2. Description of the Related Art From the viewpoint of omitting steps and reducing manufacturing costs, application of non-heat treated steel to steel for machine structural use such as automobiles has become widespread.

[0003] These non-heat treated steels have been developed mainly with a view to having high tensile strength (or hardness), yield strength and toughness. For example, Japanese Patent Application Laid-Open No. Sho 62-20
As seen in Japanese Patent No. 5245 and the like, a non-heat treated steel using V, which is a typical element of precipitation strengthening, has been proposed. However, what really hinders the application of such a high-strength, high-toughness non-heat treated steel to mechanical steel is fatigue strength and machinability.

[0004] Fatigue strength is generally considered to depend on tensile strength, and increases as tensile strength increases. However, by increasing the tensile strength, the machinability deteriorates extremely and the tensile strength becomes 1
If it exceeds 20 kgf / mm ² , production will no longer be possible with normal production efficiency. Therefore, the realization of a non-heat treated steel that improves the fatigue strength without deteriorating the machinability has been desired.

[0005] For this purpose, it is effective means to improve the ratio of fatigue strength to tensile strength, that is, the durability ratio. Therefore, as disclosed in, for example, Japanese Patent Application Laid-Open No. 4-176842, a method has been proposed in which a bainite-based metal structure is used to reduce high-carbon island martensite and retained austenite in the structure.

However, despite such development efforts, the durability ratio is at most about 0.55, and the machinability is extremely improved to at most about twice that of conventional bainite non-heat treated steel, which is extremely poor. Not done.

The present inventors have studied the fatigue properties and machinability of several types of sub-hot forgings having a metal structure in which an appropriate amount of bainite structure is mixed with a ferrite structure. As a result, MnS—TiN and MnS were formed into a ferrite-bainite two-phase structure containing an appropriate amount of a bainite structure whose hardness was controlled by reducing C and N.
-Austenite crystal grains during forging heating are refined by VN composite precipitates; sub-hot forging is performed at a temperature of 800 to 1050 ° C to refine the austenite crystal grains by processing and recrystallization; Ti
N, MnS-VN composite precipitates are used as nucleation sites to finely precipitate ferrite from within austenite grains.
Next, V carbide or V carbonitride is precipitated extremely finely on the matrix of the precipitated ferrite and bainite to strengthen the matrix by precipitation. And a precipitation-strengthened ferrite-bainite structure is obtained, and in such a steel, the growth of fatigue cracks slows down during the transition from soft ferrite grains to hard bainite grains or vice versa. And improved durability ratio. Furthermore, it has been found that the machinability is also improved by forming a two-phase structure in which a soft ferrite phase and a hard bainite phase coexist. In addition, the toughness generally tends to decrease when the durability ratio is increased. However, by realizing a fine ferrite-bainite structure by sub-hot forging in a temperature range of 800 to 1050 ° C., the high durability ratio and the toughness are improved. And found that it is possible to achieve both.

[0008] However, in a steel having a bainite structure in a transformed state, although tensile strength and fatigue strength are improved,
There is a problem that the yield strength and the yield ratio are remarkably reduced. From such a problem, it has been difficult to apply the present invention particularly to the engine parts of an automobile which is subject to a large load on an irregular basis.

[0009]

The present invention has a tensile strength of 90 kgf / m, which has been difficult to achieve with conventional non-heat treated steel.
m ^2, greater than at 20 ° C. in the impact value 2kgf-m / cm ² or more, there is provided superior endurance ratio, a manufacturing method of the yield ratio and Anetsu forging steel with machinability simultaneously.

[0010]

The yield strength is equal to the stress at which plastic deformation starts. For example, in the case of a two-phase structure of a hard phase and a soft phase, the yield strength is determined by the yield strength of the soft phase. Therefore, the ferrite-bainite two-phase structure is mainly affected by the yield strength of the soft ferrite phase. Since the transformation of this ferrite phase is completed at a relatively high temperature, the amount of solute C and N is smaller than that of the bainite phase, which is a low-temperature transformation phase, so that an increase in yield strength due to aging cannot be expected.

However, in a ferrite-bainite structure material in which V is increased to some extent, a large amount of solute V can be present in ferrite. When aging treatment is performed on a ferrite-bainite structure material of a steel component in which C and N are controlled to be relatively low, fine V carbides consistent with the ferrite matrix precipitate not only in the bainite phase but also in the ferrite, and this is transformed. It was found that the yield strength was increased by hindering the movement of the introduced movable dislocations, and that the aging treatment in a specific temperature range did not cause a decrease in tensile strength and improved fatigue strength.

[0012] The present inventors have based on such knowledge,
By designing the chemical composition, forging conditions and metallographic structure of steel containing bainite, and applying aging treatment in a specific temperature range, tensile strength and toughness are high, durability ratio, yield ratio and machinability are also improved. The inventors have invented a manufacturing method capable of providing a good ideal steel for hot forging.

That is, in the first invention of the present invention, C: 0.10 to 0.35% by weight, Si: 0.15 to 2.0
0%, Mn: 0.40 to 2.00%, S: 0.03 to
0.10%, Al: 0.0005 to 0.050%, T
i: 0.003 to 0.050%, N: 0.0020 to
0.0070%, V: 0.30 to 0.70%,
The remainder is steel with a composition consisting of Fe and impurity elements.
Sub-hot forging is performed in a temperature range of 800 to 1050 ° C., and then cooled so that 80% or more of the metal structure after the transformation is completed is a ferrite-baileite structure. By performing aging treatment, a tensile strength of more than 90 kgf / mm ² , an impact value at 20 ° C. of 2 kgf-m / cm ² or more, and excellent durability ratio, yield ratio and machinability are simultaneously obtained. The second invention is a method for producing a ferrite-bainite type sub-hot forging steel. The second invention further comprises Cr: a component of the first invention steel in order to refine crystal grains, adjust a bainite structure ratio and further improve machinability. 0.02 to 1.50%, Mo: 0.02 to 1.
00%, Nb: 0.001 to 0.200%, Pb: 0.
0.05 to 0.30%, Ca: 0.0005 to 0.0100
% Or one or more of the above.

Next, in the method for producing a ferritic-bainite type sub-hot forging steel of the present invention, the chemical composition of the steel material, the forging conditions, the metal structure after the sub-hot forging and cooling to complete the transformation, and this material The reasons for limiting the conditions for aging will be described below.

C: An important element that adjusts the bainite structure ratio and thus increases the tensile strength of the final product.
If it is excessive, the strength is too high, and the impact value and machinability are remarkably deteriorated. That is, if it is less than 0.10%, the tensile strength and the fatigue strength become low, and if it exceeds 0.35%, the tensile strength becomes too high, and the impact value and machinability are remarkably reduced. And

Si: an element for adjusting the deoxidation and the bainite structure ratio. If less than 0.15%, the effect is small.
If it exceeds 2.00%, the durability ratio, the impact value, and the machinability are all reduced.

Mn: an element that becomes a base of a composite precipitate which is a precipitation site of ferrite by adjusting the bainite structure ratio and becoming MnS. When Mn is less than 0.40%, the effect is small, and when it exceeds 2.00%, the effect is small. Since a large amount of martensite is generated and the durability ratio, impact value, and machinability are all reduced, the content is set to 0.40 to 2.00%.

S: MnS is an element that serves as a base for composite precipitates, which are ferrite precipitation sites, and improves machinability. If it is less than 0.03%, its effect is small, and if it exceeds 0.10%, MnS is an element. Is excessive, and the durability ratio and the impact value are reduced.

Al: an element having an effect of deoxidizing and refining crystal grains. If less than 0.0005%, the effect is small.
If it exceeds 0.050%, a hard inclusion is formed, and the durability ratio, impact value, and machinability are all reduced.
050%.

Ti: An element that forms a nitride on MnS and forms a composite precipitate that becomes a precipitation site of ferrite. If the content is less than 0.003%, the effect is small.
If it exceeds 0%, the formation of coarse hard inclusions is promoted, and the durability ratio, impact value, and machinability are all reduced.
50%.

N: an element which forms a nitride or a carbonitride with Ti and V. If its content is less than 0.0020%, its effect is small. On the other hand, if it exceeds 0.0070%, a large amount of VN precipitates during sub-hot forging and the solid solution V decreases, so that precipitation strengthening by VC during ferrite transformation cannot be achieved,
Since all of the durability ratio, impact value, and machinability decrease, the ratio is set to 0.1%.
0020 to 0.0070%.

V: an element that forms a nitride on MnS to form a composite precipitate that becomes a precipitation site for ferrite, and an element that strengthens the precipitation of matrix ferrite in bainite. The effect is small, and if it exceeds 0.70%, the durability ratio, the impact value, and the machinability are all reduced, so it is set to 0.30 to 0.70%.

The above is the reason for limiting the chemical composition of the steel of the first invention of the present application. In the second invention, Cr, Nb, Pb, and C are further added to the components of the first invention steel in order to refine the crystal grains, adjust the bainite structure ratio, and further improve the machinability.
One or more of a are contained. The reasons for limiting these chemical components are described below.

Almost similar to Cr: Mn, it is an element that adjusts the bainite structure ratio. If it is less than 0.02%, its effect is small, and if it exceeds 1.50%, a large amount of martensite is generated and the durability ratio, impact value, Since all of the machinability is reduced, 0.
02 to 1.50%.

Mo: An element having almost the same effect as Mn and Cr. If it is less than 0.02%, the effect is small.
If it exceeds 0%, a large amount of martensite is generated, and the durability ratio, impact value, and machinability are all reduced.
0%.

Nb: an element having substantially the same effect as Ti and V. If less than 0.001%, the effect is small.
If it exceeds 0.200%, the durability ratio, the impact value, and the machinability are all reduced, so the content is set to 0.001 to 0.200%.

Pb: an element that improves machinability,
If it is less than 05%, the effect is small, and if it exceeds 0.30%, the effect is saturated and the fatigue strength, durability ratio, and impact value are reduced.

Ca: an element having almost the same effect as Pb. If it is less than 0.0005%, the effect is small and 0.01%.
If it exceeds 00%, the effect is saturated and the fatigue strength, the durability ratio, and the impact value are reduced.
%.

The above is the reason for limiting the chemical components added in the steel of the second invention of the present application. Next, the reason for limiting the forging temperature in the present invention will be described.

First, the lower limit of the forging temperature is set to 800 ° C., because the forging load increases remarkably at a temperature lower than 800 ° C.
This is because molding becomes difficult. Further, the upper limit of the forging temperature was set to 1050 ° C., because at a forging temperature higher than 1050 ° C., the effect of refining austenite crystal grains by working recrystallization was insufficient, and the decarburization of the steel material surface became more severe.
This is because the fatigue strength of the machine component after forging is significantly reduced.

Next, the metal structure of the steel of the present invention when the transformation is completed after cooling after sub-hot forging is performed.
It is necessary that 0% or more has a two-phase structure of ferrite-bainite. Even if pearlite, martensite or retained austenite has a microstructure ratio of less than 20%, this effect is not impaired.

In order to obtain such a ferrite-bainite two-phase structure, a cooling method after sub-hot forging is not particularly specified, but natural cooling is naturally desirable from the viewpoint of equipment and manufacturing costs. The metal structure is confirmed by observing the corroded test piece with an optical microscope or the like, or measuring the microhardness of the structure with a micro Vickers hardness tester.

Finally, the reasons for limiting the conditions for aging such a material will be described. Heating temperature for aging treatment is 20
If the temperature is lower than 0 ° C., diffusion of C is difficult and the effect becomes insufficient. On the other hand, when the temperature exceeds 700 ° C., the precipitated carbides become coarse, and not only the tensile strength decreases, but also the fatigue strength decreases. Therefore, the heating temperature of the aging treatment is set to 200 to 700 ° C. The heating time does not need to be particularly limited as long as it is within this temperature range,
Desirably, it should be about 10 minutes to 2 hours. Further, the performance of the present invention can be obtained by any cooling method after the aging treatment, such as air cooling, water cooling, and oil cooling.

Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

[0035]

【Example】

(1) Influence of chemical composition of steel material
kg of steel ingot, cut out the forging material from this, once normalize by heating and cooling at 950 ° C, heat to 850 to 1100 ° C, perform sub-hot forging at a temperature of 800 to 1050 ° C, and then cool down did. Further, this material was placed in a heating furnace at 400 ° C. for 1 hour to perform aging treatment.

A JIS No. 4 tensile test piece, a JIS No. 1 rotary bending test piece, and a JIS No. 3 impact test piece were collected from the center of this material, and a tensile test, a rotary bending fatigue test and a Charpy impact test were performed. An optical microscope specimen was sampled from the same material, corroded with 5% nital, observed at a magnification of 200, and the bainite texture was determined. Further, a cutting test piece was sampled from the same material, a 30 mm deep blind hole was drilled using a SKH9 10 mmφ straight shank drill, and the machinability was evaluated based on the total drilling distance until the life of the drill was destroyed. The cutting speed was 50 m / min, the feed speed was 0.35 mm / rev, and the cutting oil was 7 L / min.

Table 2 shows the bainite structure ratio and performance evaluation results of each test material.

First, the heat-treated steel No. 1 was used. 42 durability ratio 0.
No. 47, the machinability of 1.00, and 1
No. 20 to No. 20 have a durability ratio of 0.56 or more, and the machinability is No. 42, which is as good as 2-3 times.

No. of Comparative Example Sample No. 21 has low fatigue strength because of low tensile strength and low durability ratio due to low C content. No. of the comparative example. In No. 22, martensite was generated because the C content was too high, and the ferrite-bainite structure ratio condition of the present invention could not be satisfied. The tensile strength was high, but the durability ratio was lower than that of the examples of the present invention, and the impact value, machinability Is also bad.

No. of Comparative Example Sample No. 23 has a low amount of Si and therefore has a low degree of deoxidation and a lower durability ratio than that of the present invention. N of Comparative Example
o. In No. 24, since the amount of Si was high, martensite was generated and the condition of the ferrite-bainite structure ratio of the present invention could not be satisfied, the durability ratio was lower than that of the examples of the present invention, and the impact value and machinability were poor.

No. of Comparative Example Sample No. 25 has a small amount of Mn, so that precipitation of composite precipitates is small, and the durability ratio and impact value are lower than those of the examples of the present invention. No. of the comparative example. In No. 26, since the amount of Mn is high, martensite is generated, and the condition of the ferrite-bainite 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 impact value and machinability are poor.

No. of Comparative Example Sample No. 27 has a low S content so that the precipitation of composite inclusions is small, the durability ratio and the impact value are lower than those of the examples of the present invention, and the machinability is poor because the effect of improving the machinability of MnS cannot be obtained. No. of the comparative example. Sample No. 28 has a high S content, resulting in excessive precipitation of MnS, and has a lower durability ratio and impact value than those of the present invention.

No. of Comparative Example Since No. 29 has a low Al content, the degree of deoxidation and the effect of crystal grain refinement are small, and the durability ratio is lower than that of the present invention. No. of the comparative example. 30 has a high Al content, so that a hard inclusion is formed, and the durability ratio is lower than that of the present invention.
The impact value and machinability are also poor.

No. of Comparative Example Sample No. 31 has a low Ti content, so that precipitation of composite precipitates is small, and the durability ratio is lower than that of the present invention. No. of the comparative example. No. 32 has a high Ti content, so that hard inclusions are formed, the durability ratio is lower than that of the example of the present invention, and the impact value and machinability are poor.

No. of Comparative Example No. 33 has a low N content and therefore has a small precipitation of composite precipitates, and has a lower durability ratio and impact value than those of the present invention. No. of the comparative example. No. 34 has a high N content, so that the matrix is hardened, the durability ratio is lower than that of the present invention, and the impact value,
The machinability is also poor.

No. of Comparative Example No. 35 has a low V content so that the precipitation of composite precipitates is small and the effect of strengthening the precipitation of matrix ferrite is small, so that the durability ratio and the impact value are lower than those of the examples of the present invention. No. of the comparative example. 36 has a high V content,
The durability ratio is lower than that of the present invention, and the impact value and machinability are also poor.

No. of Comparative Example In No. 37, since the amount of Cr is high, martensite is generated and the condition of the ferrite-bainite 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 impact value and machinability are poor.

No. of Comparative Example In No. 38, since the Mo content is high, martensite is generated and the condition of the ferrite-bainite 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 impact value and machinability are poor.

No. of Comparative Example 39 has a high Nb content,
The durability ratio is lower than that of the present invention, and the impact value and machinability are also poor.

No. of Comparative Example 40 has a high Pb content,
Although the machinability is good, the durability ratio and the impact value are poor.

No. of Comparative Example 41 has a high Ca content,
Although the machinability is good, the durability ratio and the impact value are poor.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

[Table 4]

(2) Effect of ferrite-bainite structure ratio change by cooling method after hot forging A steel satisfying the conditions of the chemical components of the present invention shown in No. 20 was melted in a high-frequency furnace to form a 150 kg steel ingot, and a forging material was cut out therefrom. 800 ℃
Sub-hot forging was performed at a temperature of 1050 ° C., and then cooled by the method shown in Table 3. Further, these materials were placed in a heating furnace at a temperature of 400 ° C. for one hour to perform aging treatment. A test piece was collected from the center of this material in the same manner as in Example (1), and further subjected to a tensile test, fatigue test, impact test, cutting test, and metallographic observation in the same manner as in Example (1). Was done. Table 4 shows the ferrite-bainite structure ratio and performance evaluation results of each test material.

No. Nos. 43, 44, 45 and 46 satisfy the conditions of the present invention with a ferrite-bainite structure ratio of 0.8 or more in proportion, and have a tensile strength of 90 kgf / m.
m ² , the impact value at 20 ° C. is 2 kgf-m / cm ² or more, and the durability ratio is 0.56 or more, and the machinability is no. 48, which is approximately 2.5 times as good.

No. Reference numeral 47 denotes a structure mainly composed of martensite by increasing the cooling rate. Although the tensile strength is increased, the durability ratio and impact value are extremely low, the machinability is poor, and the tool life is short.

[0059]

[Table 5]

[0060]

[Table 6]

(3) Influence of change in aging temperature Steel of the same chemical composition as in Example (2) was melted in a high-frequency furnace to form a 150 kg steel ingot, from which a forging material was cut out, and then heated and released at 950 ° C. once. After normalizing cold, 850-11
Sub-hot forging was performed at a temperature of 800 to 1050 ° C. by heating to 00 ° C., and then allowed to cool. Further, this material was placed in a heating furnace at a temperature shown in Table 5 for 1 hour to perform aging treatment. These materials were subjected to a tensile test, a fatigue test, an impact test, a cutting test, and a metal structure observation in the same manner as in Example (1). Table 6 shows the performance evaluation results of each test material.

No. 50, 51 and 52 satisfy the aging temperature range of the present invention of 200 to 700 ° C,
In each case, the durability ratio of 0.58 or more is ensured, and the machinability is also the current tempered steel No. 2. This is approximately 2.5 times as good as 54.

No. 49 is a case where the aging temperature is lower than the range of the present invention, and the yield ratio and the durability ratio are inferior. No.
53 is a case where the aging temperature exceeds the range of the present invention,
Poor durability ratio.

[0064]

[Table 7]

[0065]

[Table 8]

[0066]

As described above, the steel of the present invention has a ferritic-bainite two-phase structure to obtain high tensile strength, secure machinability, and is formed of MnS, Ti nitride and V nitride. Simultaneously with the refinement of the metal structure and the strengthening of the ferrite matrix in bainite with V carbides (or carbonitrides) using composite precipitates to ensure excellent impact value and high durability, The present invention provides a very ideal method for producing a steel for sub-hot forging, which can obtain even higher yield strength by applying aging treatment to N, which is extremely effective in industry.

Continuation of the front page (56) References JP-A-5-156354 (JP, A) JP-A-64-56821 (JP, A) JP-A-64-55328 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. A weight ratio of C: 0.10 to 0.35% Si: 0.15 to 2.00% Mn: 0.40 to 2.00% S: 0.03 to 0.10% Al : 0.0005% to 0.050% Ti: 0.003% to 0.050% N: 0.0020% to 0.0070% V: 0.30% to 0.70%, the balance being Fe and impurity elements 8 for steel composition
Forging is performed in a temperature range of 00 to 1050 ° C., and then cooled to ensure that at least 80% of the metal structure after transformation is a ferrite-bainite structure.
By performing the aging treatment at a temperature of 0 to 700 ° C, the tensile strength exceeds 90 kgf / mm ² and the impact value at 20 ° C is 2 kgf.
A method for producing a ferrite-bainite type subhot forging steel excellent in toughness, durability ratio, yield ratio, and machinability, characterized by being at least −m / cm ² . 2. The composition further comprises Cr: 0.02 to 1.50% Mo: 0.02 to 1.00% Nb: 0.001 to 0.200% Pb: 0.05 to 0.30% Ca: 2. A ferrite-bainite having excellent toughness, durability ratio, yield ratio and machinability according to claim 1, wherein a steel material containing one or more of 0.0005 to 0.0100% is used. A method for producing steel for sub-hot forging.