JPH07173531A - Production of high strength, high toughness non-heat treated steel for hot forging excellent in yield ratio - Google Patents

Abstract

PURPOSE:To produce a high strength and high toughness non-heat treated steel by specifying the compsn., controlling the ferrite precipitation critical cooling velocity and executing air cooling from the hot forging finishing temp. by tempering treatment. CONSTITUTION:The compsn. of a steel is constituted of, by weight, 0.05 to 0.30% C, 0.05 to 2.0% Si, 0.3 to 5.0% Mn, 1.0 to 3.0% Cr, 0.01 to 0.5% Nb, 0.01 to 0.06% Al and 0.005 to 0.02% N, and the balance Fe with inevitable impurities. Furthermore, componental adjustment is executed in such a manner that the ferrite precipitation critical cooling velocity Vc1 obtd. by the formula I is regulated to the average cooling velocity from 800 to 500 deg.C after hot forging or below. The steel having the same compsn. is subjected to hot forging and is thereafter cooled to form its structure into a mixed one of bainite and martensite contg. no pro-eutectoid ferrite, and next, tempering is executed in the temp. range from 350 to 600 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to quenching after hot forging.
Even without heat treatment such as tempering, hot forging, cooling,
The present invention relates to a method for producing a high-strength, high-toughness non-heat treated steel for hot forging, which can be used as parts for automobiles, parts for construction machinery and the like by tempering and has an excellent yield ratio.

[0002]

2. Description of the Related Art Structural parts used in automobiles, construction machines, etc. are made of carbon steel for machine structure and alloy steel for machine structure, and are quenched and tempered after hot forging to give necessary strength and toughness. Generally, a treatment (conditioning treatment) is performed. Further, in recent years, in order to reduce the energy required for the above-mentioned heat treatment and to reduce the cost of work-in-process, for example, carbon steel for machine structure specified in JIS G 4051 or machine structure specified in JIS G4106. A non-heat treated steel in which a precipitation hardening type element such as V or Nb is added to a manganese steel for use has been developed and used as an engine part of an automobile, an undercarriage part or a part for construction machinery.

These non-heat treated steels are those which are cooled after hot forging to form a ferrite / pearlite structure, and obtain the required strength by precipitation strengthening of carbides and nitrides of V and Nb. However, these known non-heat treated steels are excellent in static strength such as tensile strength and hardness, but since the structure is mainly composed of a ferrite-pearlite structure transformed from coarse austenite, There is a problem of low ductility.

Therefore, in order to solve such a problem, a high toughness type non-heat treated steel having strength and toughness and ductility equivalent to those of conventional heat treated steel, for example, a tensile strength of 80 kgf / mm ² grade. Charpy impact value of 2mm U notch at room temperature is 6
Steels having a kgf · m / cm ² or more have been developed and some have been put to practical use (for example, JP-A-62-74055).

Further, as a high-strength component having a tensile strength of 90 kgf / mm ² or more, there is, for example, Japanese Patent Laid-Open No. 63-129949.
A high strength / high toughness type non-heat treated steel as disclosed in Japanese Patent Publication has been proposed. However, the tensile strength is 90kg
High strength and high toughness type non-heat treated steels with f / mm ² or more are water-cooled to improve strength, so there is a problem that distortion and quench cracks occur with quenching.
Therefore, there is a demand for an air-cooled type high strength, high toughness non-heat treated steel. Further, these non-heat treated steels have a low yield strength ratio as compared to those obtained by quenching and tempering alloy steel, and for example, sufficient yield strength cannot be obtained if the tensile strength is similar to that of the quenched and tempered material. It is not suitable as a component material that requires high yield strength.

[0006]

SUMMARY OF THE INVENTION The present invention has been made by paying attention to the above-mentioned problems, and its purpose is to provide sufficient toughness, which is a drawback of the conventional non-heat treated steel for hot forging. A method for manufacturing high-strength, high-toughness non-heat treated steel for hot forging that solves the problem and has high strength and high toughness with an adjusted cooling such as air cooling or air blast cooling after hot forging and an excellent yield ratio. Is to establish.

[0007]

[Means for Solving the Problems] The composition of the manufacturing method according to the present invention, which was able to solve the above problems, is C: 0.05 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.3 to 5.0%, Cr: 1.0 to 3.0%, Nb: 0.01 to 0.5%, Al: 0.01 to 0.06%, N: 0.005 to 0. 02%, or in addition to these, Ni: 0.2 to 3.0%, Mo: 0.05 to 1.0%, V: 0.03 to 0.5%, Ti: 0.003 To 0.2% of at least one element selected from the group consisting of balance Fe and inevitable impurities, and the ferrite precipitation critical cooling rate Vc ₁ determined by the following formula (I) or (II) Alternatively, steel whose composition is adjusted so that Vc ₂ is equal to or less than the average cooling rate V between 800 and 500 ° C. after hot forging. Using a material, the steel material is cooled after hot forging to a bainite that does not contain proeutectoid ferrite and a mixed structure of bainite and martensite after cooling,
Then, it has the gist of tempering in the temperature range of 350 to 600 ° C. Vc ₁ = 10 ^k1 (° C / sec) …… (I) Vc ₂ = 10 ^k2 (° C / sec) …… (II) However, k1 = 4.05- (4.5 ×% C +% Mn + 0.8 ×% Cr + 9 ×% N
b) k2 = 4.05- (4.5 x% C +% Mn + 0.8 x% Cr + 9 x% Nb + 0.5 x% Ni +
1.6 ×% Mo) (% element represents the content rate of the element in steel)

In the present invention, as other components,
S: 0.12% or less, Pb: 0.3% or less, Ca: 0.
Excellent machinability can be provided by using a steel material containing at least one element selected from the group consisting of 01% or less, Te: 0.3% or less, and Bi: 0.3% or less. . As a hot forging material used for carrying out the above method, when a steel ingot or a steel slab is produced from molten steel, 2 ° C. between 1500 and 900 ° C. after casting of molten steel is used.
Using a non-heat treated steel bar cooled at a cooling rate of at least 1 minute, hot forging is performed at a steel material heating temperature of 1300 ° C. or less during hot forging, and the forging is completed at a temperature of 850 to 1150 ° C. The features of the present invention can be more effectively achieved by using a molded product.

[0009]

As described above, in the present invention, the chemical composition of the steel material is specified, and particularly, the average cooling rate V between 800 and 500 ° C. after hot forging is used as a reference, and the above (I) and (II) are used. Ferrite precipitation critical cooling rate Vc ₁ or V obtained by the formula
A steel material whose composition is adjusted so that c ₂ is equal to or lower than the cooling rate V is used, and the microstructure after cooling is cooled by hot forging and bainite containing no proeutectoid ferrite or a mixture of bainite and martensite. Structured and then tempered by heating or using the self-heating of the forged product 350-6
By carrying out in the temperature range of 00 ° C., the non-heat treated steel for hot forging having high strength and high toughness and a high yield ratio was successfully obtained. First, the reason for defining the chemical composition of the steel material in the present invention will be clarified.

C: 0.05 to 0.30% C enhances the hardenability of the material and makes the structure after hot forging and cooling into bainite or a mixed structure of bainite and martensite, and secures the necessary strength. It is an indispensable element to achieve this, and it must be contained at least 0.05% or more. However, if it is contained excessively, not only the toughness is deteriorated but also the machinability is adversely affected.
Must be kept below 0%.

Si: 0.05 to 2.0% Si is an element useful for deoxidizing during melting and ensuring the required strength, and is particularly solid-dissolved in the matrix to improve the yield strength and fatigue strength. Improve. These effects are effectively exhibited by containing 0.05% or more, but if added excessively, the machinability is significantly deteriorated, so 2.0% is made the upper limit.

Mn: 0.3-5.0% Mn, like Si, acts effectively as a deoxidizing element during melting, and also has the effect of increasing hardenability and strength. Tensile strength of 8 under the conditions specified above
In order to obtain a high strength of 0 kgf / mm ² or more, Mn is set to 0.
Must be contained by 3% or more. However, even if added in excess of 5.0%, no further improvement in strength can be obtained,
Rather, it has an adverse effect on machinability, so 5.
The upper limit is 0%.

Cr: 1.0 to 3.0% Cr, like Mn, is an essential element for enhancing the hardenability and ensuring the required strength, and it is necessary to add 1.0% or more. . However, the effect is saturated at 3.0% or more, so addition of more than that is economically useless.

Nb: 0.01 to 0.5% Nb has a function of refining austenite crystal grains as a carbide or nitride forming element, and also Nb added.
Has a function of increasing the hardenability at the time of cooling after hot forging by forming a solid solution, and these effects are effectively exhibited by adding 0.01% or more. However, their effects saturate at 0.5%, so any further addition is economically wasteful.

Al: 0.01 to 0.06% Al acts as a deoxidizing element during melting, and is also an element effective in forming nitrides and refining austenite crystal grains. In order to exert the effect effectively, 0.01% or more must be added. However, if it is contained excessively, the austenite crystal grains are rather coarsened and the toughness is deteriorated, so the upper limit is 0.06%.

N: 0.005-0.02% N has an action of combining with a nitride-forming element to refine austenite crystal grains, and in order to exert its effect effectively, 0.005% or more is added. There must be. However, even if added in excess of 0.02%, no further effect is obtained, and rather, the toughness is adversely affected by an increase in the amount of nitride-based inclusions. Therefore, in the present invention, the upper limit is 0.02%. The essential constituent elements are as described above, and the balance consists of Fe and unavoidable impurities. However, by further adding one or more of Ni, Mo, V, and Ti as appropriate as other elements as described below, It is also effective to further improve the toughness and strength, or to increase the machinability by adding an appropriate amount of one or more of S, Pb, Ca, Te and Bi.

Ni: 0.2 to 3.0%, Mo: 0.05
~ 1.0%, V: 0.03-0.5%, Ti: 0.00
3 to 0.2% of one or more kinds of Ni is an element effective for improving hardenability and toughness, and these effects are effectively exhibited by adding 0.2% or more. However, the effect is saturated at 3.0%, so any further addition is useless.
Mo, like Ni, has the effects of providing good hardenability and enhancing toughness, and these effects are effectively exhibited by adding 0.05% or more. However, the effect saturates at 1.0%, so any further addition is useless. V has the effect of forming carbides or nitrides, refining the austenite crystal grains, contributing to improving the toughness, and increasing the strength, and these effects are effectively exhibited by addition of 0.03% or more. However, their effect is 0.
Saturate at 5%. Ti suppresses the growth of austenite crystal grains at the time of heating, and when it is combined with S, Ti makes the structure fine and significantly improves the toughness. Such an effect is effectively exhibited by adding 0.003% or more, but even if it is added in excess of 0.2%, no further effect can be obtained, so the upper limit is 0.2%.

S: 0.12% or less, Pb: 0.3% or less, Ca: 0.01% or less, Te: 0.3% or less, B
i: 1% or more of 0.3% or less All of these elements are machinability improving elements, and the machinability can be improved by including each in the above range, but each exceeds the upper limit. However, no further improvement in machinability can be obtained, and rather the problem of toughness deterioration arises, so the upper limit of each is set as described above.

In the present invention, a steel material satisfying the requirements of the above chemical composition is used. After hot forging, the steel material is cooled to obtain a predetermined metallographic structure, and then reheated or tempered by utilizing the self heat of the forged product. However, in order to obtain a high level of yield ratio, strength and toughness intended in the present invention, the average cooling rate V between 800 and 500 ° C. after hot forging is performed.
With reference to the above formula (I) or (II), the ferrite precipitation critical cooling rate Vc ₁ or Vc ₂ should be equal to or less than the above average cooling rate V.
(Contents of Mn, Cr, Nb, Ni and Mo) are adjusted, and the structure after hot forging and cooling is set to bainite containing no proeutectoid ferrite or a mixed structure of bainite and martensite, and subsequent tempering is performed at 350 It is necessary to perform in the temperature range of up to 600 ° C.

Here, the average cooling rate between 800 and 500 ° C.
It was decided from the results of many experiments that the degree V was set as the standard.
Light precipitation critical cooling rate Vc₁ , Vc₂ Organize relationships
From the results, it can be seen that the best correspondence can be obtained in the above temperature range.
It is based on the confirmation result of. In addition, ferrite precipitation
Field cooling rate Vc₁ Or Vc₂ Is the above (I), (II)
What is determined by the chemical composition of the steel as shown in the formula
However, these Vc ₁ Or Vc₂ Is the above average cooling speed
If the temperature exceeds V, the structure after hot forging and cooling is the primary eutectoid blower.
And perlite and are contemplated by the present invention.
It is impossible to obtain a metallic structure after cooling like
Sufficient toughness and yield ratio even if the bending conditions are set appropriately
Non-heat treated steel cannot be obtained.

Further, tempering performed after hot forging / cooling is performed by reheating or utilizing self-heating of the forged product, and the treatment temperature of the tempering depends on the yield ratio of the non-heat treated steel obtained or Significantly affects strength and toughness. That is, in the low temperature range where the temperature during tempering is less than 350 ° C., the yield strength cannot be sufficiently increased because the amount of precipitates that prevent migration of transition is small, while tempering is performed at a high temperature exceeding 600 ° C. Therefore, the above-mentioned low carbon steel defined in the present invention cannot obtain sufficient strength and toughness, and the purpose of high strength and high toughness cannot be achieved.

However, as described above, 8 after hot forging
The structure after cooling is performed after hot forging using a steel material whose composition is adjusted so that the ferrite precipitation critical cooling rate Vc ₁ or Vc ₂ becomes V or less based on the average cooling rate V between 00 and 500 ° C. Is a bainite that does not contain proeutectoid ferrite or a mixed structure of bainite and martensite, and when this is tempered in the temperature range of 350 to 600 ° C., it is an excellent non-heat treated steel in terms of yield ratio, strength, and toughness. Can be obtained. The tempering treatment time is not particularly limited, but is generally about 1 to 3 hours.

As the above-mentioned hot forged material used in the present invention, when a steel ingot or a steel slab is produced from molten steel, it is cooled at a cooling rate of 2 ° C./minute between 1500 and 900 ° C. after casting the molten steel. Using a cooled non-heat treated steel rod, hot forging was performed by setting the steel material heating temperature during hot forging to 1300 ° C or lower.
It is desirable to use a product that has been forged at a temperature of 0 to 1150 ° C. and formed into a predetermined shape. However, when the cooling rate in the above temperature range becomes lower than 2 ° C./minute, the sulfides and oxides are aggregated and coarsened, and a fine crystal structure cannot be obtained, so that the toughness of the forged material tends to be rapidly deteriorated. Because there is.

When the temperature during forging heating exceeds 1300 ° C., the arrow-shaped crystal grains become coarse and the toughness is adversely affected. Furthermore, the forging finish temperature is 1150 ° C
When the temperature is higher than, the crystal grains are coarsened and the toughness is adversely affected, and when the temperature is lower than 850 ° C., the deformation resistance during forging becomes large and the life of the mold is significantly shortened, which is practical. Disappear.

In the present invention, the formed material which is hot forged under the above-mentioned conditions is used, and the material is cooled under the above-mentioned conditions and then re-heated or tempered by utilizing the self-heating under the predetermined conditions. Provides hot forging steel that has strength and toughness equal to or higher than that of conventional tempered steel without heat treatment such as quenching and tempering, and with excellent yield ratio I was able to do it.

[0026]

EXAMPLES Next, examples of the present invention will be shown, but the present invention is not limited by the following examples, and may be carried out with appropriate modifications within a range compatible with the gist of the preceding and following description. Of course, it is possible, and all of them are included in the technical scope of the present invention.

Example 1 Steel having the chemical composition shown in Table 1 was melted in a VIF smelting furnace having a capacity of 150 kg, and then 1500 was used when a steel ingot was manufactured from the molten steel.
Each ingot obtained by changing the cooling rate from ℃ to 900 ℃ by steam spray and cooling water shower is heated to 1250 ℃, the forging end temperature is 1150 ℃ and the diameter is 50 mm.
It was forged into a round bar. Each of the obtained round bars was cut into a length of 75 mm, then heated to 1150 to 1350 ° C., hot forged to a diameter of 25 mm at a forging end temperature of 1050 to 1250 ° C., and air-cooled. Then, after further tempering treatment at 500 ° C. for 2 hours, tensile test pieces and JIS No. 3 impact test pieces were cut out by cutting and subjected to physical property tests at room temperature. The results are shown in Table 2.

[0028]

[Table 1]

[0029]

[Table 2]

As is clear from Tables 1 and 2, those forged under the conditions satisfying the preferred requirements of the present invention (No.
1-3), high values were obtained in all of the tensile strength, impact value and yield ratio. On the other hand, any of the cooling conditions of the steel ingot, the heating temperature during forging, and the forging end temperature deviates from the preferred requirements of the present invention (No. 4, 5).
Indicates that the impact value and the yield ratio are clearly low.

Example 2 After melting the steel shown in Table 3 using a VIF furnace having a capacity of 150 kg, a temperature of 1500 ° C. to 9 ° C. at the time of manufacturing a steel ingot from the molten steel
The steel ingot produced at a cooling rate up to 00 ° C of 2.25 ° C / min was heated to 1250 ° C, and the forging end temperature was set to 115
The rod was forged into a round bar having a diameter of 50 mm at 0 ° C. Next, this round bar was cut into a length of 75 mm, heated to 1250 ° C., hot-forged to a diameter of 25 mm at a finishing temperature of 1150 ° C., and then air-cooled. At this time, the average cooling rate between 800 and 500 ° C was constant at 1.2 ° C / sec. Then 50
A tempering treatment was performed at 0 ° C. for 2 hours, a tensile test piece and an impact test piece specified in JIS No. 3 were cut out by cutting, and a physical property test was performed at room temperature. Ferrite precipitation critical cooling rate of used steel and metal structure after forging and cooling,
Table 4 shows the results of the physical property test.

[0032]

[Table 3]

[0033]

[Table 4]

In Tables 3 and 4, each of the test steels A to J is an example satisfying the prescribed requirements in terms of chemical composition, cooling conditions after hot forging, and metal structure after cooling. Tensile strength and impact value ,
In each of the yield ratios, a value equal to or higher than that of the conventional hardened / tempered material (tempered material: sample steel X, Y) was obtained. Further, the sample steel Z is a conventional non-heat treated steel, has an excessive amount of C and an insufficient amount of Cr, and has a metallographic structure composed of ferrite and pearlite. Low strength, impact value and yield ratio. The test steels K to R are comparative examples lacking any of the requirements specified in the present invention, and all have problems in performance as shown below.

Steels K to P tested: Ferrite precipitation cooling rate V
c ₁ and Vc ₂ are average cooling rates after hot forging (800 to 5
(Between 00 ° C.) V and below, and the structure does not contain any ferrite, but the sample steel K lacks the amount of C, the sample steel N lacks the amount of Cr, and the sample steel O Since the amount of Nb is insufficient, the tensile strength is particularly low and the yield ratio is not sufficient. The steel sample L has an excessive amount of C and thus has a low impact value. The test steels M and P have relatively good results in terms of tensile strength, impact value, and yield ratio, but the test steel M has a Mn content exceeding the specified range, and the test steel P Despite the fact that the Nb amount exceeds the specified range and is added in a large amount, the effect of improving the physical properties commensurate with that is not obtained, which is disadvantageous in terms of steel cost. Moreover, it has been confirmed that these elements, if added excessively, significantly reduce machinability and deteriorate productivity. The test steels Q and R have extremely large ferrite precipitation critical cooling rates Vc ₁ and Vc ₂ with respect to the average cooling rate V, and the structures after hot forging and cooling are not appropriate, so that the impact values are extremely low.

Example 3 Among the steels shown in Table 3, the sample steel I was melted in a VIF melting furnace having a capacity of 150 kg, and then the cooling rate from 1500 ° C. to 900 ° C. was 2.25 ° C./min. As a diameter of 50 mm
It was forged into a round bar. This forged material was cut into 75 mm, heated to 1250 ° C., hot forged to a diameter of 25 mm at the same temperature, air-cooled, and further tempered at 100 to 650 ° C. for 2 hours. The physical properties of the obtained tempered material were measured by the same method as described above, and the results shown in FIG. 1 were obtained. As is clear from FIG. 1, even when a steel material satisfying specified requirements in chemical composition etc. is used, the impact value and the proof stress ratio are insufficient when the tempering temperature is lower than 350 ° C, while 600 ° C.
It can be seen that the impact value and the tensile strength are rapidly deteriorated when the value exceeds.

[0037]

EFFECTS OF THE INVENTION The present invention is configured as described above, and has strength and toughness equal to or higher than that of conventional tempered steel even if it is not tempered without tempering treatment such as quenching and tempering. It has become possible to provide a non-heat treated steel for hot forging which has an excellent yield ratio.

[Brief description of drawings]

FIG. 1 is a graph of experimental results showing the effect of temperature during tempering treatment performed after hot rolling and cooling on physical properties.

Claims (6)

[Claims] 1. C: 0.05 to 0.30% (meaning weight%, the same applies hereinafter), Si: 0.05 to 2.0%, Mn: 0.3 to 5.0%, Cr: 1.0-3.0%, Nb: 0.01-0.5%, Al: 0.01-0.06%, N: 0.005-0.02%, and the balance Fe and unavoidable. A steel material which is composed of impurities and whose composition is adjusted so that the ferrite precipitation critical cooling rate Vc ₁ obtained by the following formula (I) is not more than the average cooling rate V between 800 and 500 ° C. after hot forging is used. , The steel material is hot forged and then cooled to obtain a structure after cooling as bainite containing no proeutectoid ferrite or a mixed structure of bainite and martensite, and then 350 to 600 ° C.
A method for producing high-strength, high-toughness non-heat treated steel for hot forging with excellent yield ratio, which is characterized by tempering in the temperature range of. Vc ₁ = 10 ^k1 (° C / sec) (I) However, k1 = 4.05- (4.5 ×% C +% Mn +0.8 ×% Cr +9 ×% N
b) (% element represents the content rate of the element in steel) 2. A steel material containing S: 0.
It contains at least one element selected from the group consisting of 12% or less, Pb: 0.3% or less, Ca: 0.01% or less, Te: 0.3% or less, and Bi: 0.3% or less. The manufacturing method according to claim 1. 3. When producing a steel ingot or a steel slab from molten steel, a non-heat treated steel rod is used which is cooled at a cooling rate of 2 ° C./min or more between 1500 to 900 ° C. after casting of molten steel, Hot-forging is performed at a steel material heating temperature of 1300 ° C. or less during hot forging, and forging is finished at a temperature of 850 to 1150 ° C. and formed into a predetermined shape.
The manufacturing method described in. 4. C: 0.05 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.3 to 5.0%, Cr: 1.0 to 3.0%, Nb : 0.01 to 0.5%, Al: 0.01 to 0.06%, N: 0.005 to 0.02%, Ni: 0.2 to 3.0%, Mo: 0 0.05 to 1.0%, V: 0.03 to 0.5%, Ti: 0.003 to 0.2%, containing at least one element selected from the group consisting of balance Fe and inevitable impurities. And the composition is adjusted so that the ferrite precipitation critical cooling rate Vc ₂ obtained by the following formula (II) is equal to or less than the average cooling rate V between 800 and 500 ° C. after hot forging,
Yield characterized by cooling the steel material after hot forging to obtain a structure after cooling as bainite containing no proeutectoid ferrite or a mixed structure of bainite and martensite, and then tempering in a temperature range of 350 to 600 ° C. High strength and high toughness non-heat treated steel for hot forging with excellent ratio. Vc ₂ = 10 ^k2 (° C / sec) (II) However, k2 = 4.05- (4.5 ×% C +% Mn + 0.8 ×% Cr + 9 ×% Nb + 0.5
(×% Ni + 1.6 ×% Mo) (% element represents the content rate of the element in steel) 5. The steel material further comprises S: 0.1 as another component.
It contains at least one element selected from the group consisting of 2% or less, Pb: 0.3% or less, Ca: 0.01% or less, Te: 0.3% or less, and Bi: 0.3% or less. The manufacturing method according to claim 3. 6. When producing a steel ingot or a steel slab from molten steel, a non-heat treated steel rod cooled at a cooling rate of 2 ° C./min or more between 1500 to 900 ° C. after casting of molten steel is used, Hot forging is performed at a steel material heating temperature of 1300 ° C. or lower during hot forging, the forging is finished at a temperature of 850 to 1150 ° C., and a product formed into a predetermined shape is used.
The manufacturing method described in.