KR101458348B1 - Untempered steel for hot casting, hot-casted untempered article and method for producing same - Google Patents

Abstract

The present invention relates to a non-tempered steel for hot forging which enables high-frequency quenching, a hot forged non-tempered product in which the structure in the component is controlled by cooling after hot forging to suppress the lowering of machinability accompanying high strength, And a manufacturing method thereof. The steel according to the present invention is characterized in that it contains 0.45 to 0.60% of C, 0.02 to 0.15% of Si, 1.50 to 3.0% of Mn, 0.0002 to 0.150% of P, 0.001 to 0.200% of S, , Al: 0.001 to 0.300%, V: 0.01 to 0.30%, Mo: 0.03 to 1.00%, and N: 0.0020 to 0.0070%, with the balance being Fe and inevitable impurities. A hot forging uncooled product having the same steel composition and capable of high frequency quenching in which the steel structure has a bainite structure in an area ratio of 95% or more and Mo carbonitride is dispersed in the steel, and a production method thereof.

Description

TECHNICAL FIELD [0001] The present invention relates to a non-tempered steel for hot forging and a hot forged uncoated material and a method for manufacturing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

TECHNICAL FIELD The present invention relates to a non-tempered steel material for hot forging processed into mechanical parts such as automobiles and industrial machines without being tempered after hot forging, hot forged uncoated products using the same, and a method for producing the same. In the hot forging state, high strength and high hardness are provided even without tempering, and high frequency quenching is possible.

BACKGROUND ART Conventionally, most of mechanical structural parts such as automobiles and industrial machines have been given high strength and toughness by hot forging from a material rod steel to a component shape, reheating, and quenching tempering treatment . In recent years, the quenching treatment process of quenching tempering has been omitted from the viewpoint of reducing the manufacturing cost. For example, as shown in Patent Document 1 and the like, in the hot forging, high strength and high toughness Has been proposed. Particularly, in a shaft component such as a crankshaft, after hot working, it is cooled, given a predetermined strength, machined into a predetermined shape by machining, and then subjected to high frequency quenching at a necessary portion, Thereby improving wear resistance and fatigue strength.

Patent Document 1 discloses that hot forged unoriented steels having a strength higher than that of a conventional crude material and a low temperature toughness were obtained in a hot forging state by using a steel material containing 1.0% Si and a large amount of S, V and N added thereto . However, there is no description about the fatigue strength and internal resistance of the non-refined steel.

So far, some reports have been made on non-tempered steels for high frequency quenching. For example, the invention described in Patent Document 2 is an invention for preventing the lowering of the surface hardness and the depth of the surface hardened layer due to the formation of residual ferrite after high frequency quenching by setting the bainite ratio to 75% or higher in the structure before high frequency quenching . However, the non-treated steel for high frequency quenching described in Patent Document 2 has no description about the fatigue strength and internal resistance, and their characteristics are not considered at all.

Further, for example, the invention described in Patent Document 3 is an invention for reducing the amount of retained austenite after high-frequency quenching. However, in the non-tempered steel for high frequency quenching described in Patent Document 3, there is no description about fatigue strength and internal resistance, and their characteristics are not considered at all. Although it is described that an appropriate amount of S, Pb, Bi, Te, Se and Ca may be added to improve machinability, it is known that the effect of improving machinability is small when the tensile strength is 1100 MPa or more.

In the application of these high strength unoriented steels to steel parts for mechanical structural purposes, what is actually an obstacle is to achieve both high fatigue strength and the opposite properties of machinability. Generally, the fatigue strength is considered to depend on the tensile strength, and the higher the tensile strength, the higher the fatigue strength. On the other hand, an increase in tensile strength lowers the machinability. Most of the steel components for mechanical structure require cutting after hot forging, which leads to a considerable increase in cutting cost. Generally, it is known that when the tensile strength is increased to increase the fatigue strength of a steel component for machine structural use, the machinability is remarkably lowered when the tensile strength exceeds 1300 MPa. Since the cost of cutting greatly increases, it is practically difficult to obtain a high strength exceeding the tensile strength of 1300 MPa. Therefore, in these mechanical structural parts, an increase in cutting cost due to a reduction in machinability is a problem of high fatigue strength, and both high fatigue strength and machinability are demanded.

For example, Patent Document 4 discloses an invention for improving the fatigue strength of the entire component by suppressing the strength improvement after hot forging and deepening the surface hardened layer by high frequency quenching in order to secure machinability.

As means for achieving both high fatigue strength and machinability, it is effective to improve the ratio of the fatigue strength to the tensile strength, that is, to improve the fatigue strength (fatigue strength / tensile strength). For example, in Patent Document 5, it has been proposed that it is effective to reduce the high-carbon island-shaped martensite and retained austenite in the structure as a metal structure of bainite-based material. However, the internal hardness is at most 0.56, and there is a limit to increase the strength without lowering the machinability. All of these fatigue strengths are low.

Patent Document 6 discloses a crankshaft capable of achieving high wear resistance and fatigue strength and high machinability, and a manufacturing method thereof. In this method, the micro-metal structure of the hot forging prior to the softening treatment is made into the structure of the bainite main body (70% or more) and the hot forged product is softened under the temperature condition of 550 to 650 占 폚 to obtain the fatigue strength And the like. The steel component is defined by using the parameter Hg which shows the specific relation of the amount of C, Si, Mn, Cr, Mo and V of the steel material in order to appropriately increase the internal hardness after softening to obtain high fatigue strength. However, the softening treatment usually needs to be carried out by exposing in an atmosphere containing nitrogen and heating in a temperature range below the austenitizing temperature, which is costly compared to the surface hardening treatment by high frequency quenching. Since the steel material for softening treatment for a certain period of time has a large amount of Si, in high frequency quenching by instantaneous induction heating only on the surface, retained austenite remains in the internal structure and high fatigue strength is obtained It does not.

Japanese Patent Application Laid-Open No. 1-198450 Japanese Patent Application Laid-Open No. 63-100157 Japanese Patent Application Laid-Open No. 11-286744 Japanese Patent Application Laid-Open No. 2005-68518 Japanese Patent Application Laid-Open No. 4-176842 Japanese Patent Application Laid-Open No. 2010-189697

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above problems and to provide a non-tempered steel for hot forging which enables high-frequency quenching and a cooling after hot forging to suppress the deterioration of workability accompanying high- And to provide a hot forged unsteered product having improved fatigue strength and a manufacturing method thereof.

The present invention relates to a method for producing a high carbon steel which is capable of obtaining a high surface hardness by performing high frequency quenching and using a steel to which a large amount of Mo is added to precipitate a large amount of Mo carbonitride in a cooling process after hot forging, The present invention has been accomplished based on the discovery that a hot forged uncooled product having a high internal strength by suppressing a defect density and suppressing a decrease in workability accompanying high strength and an improved fatigue strength is obtained. As used herein, the term " carbonitride " means carbonitride and carbide.

The gist of the present invention is as follows.

(1) A ferritic stainless steel comprising, by mass%, 0.45 to 0.60% of C, 0.02 to 0.15% of Si, 1.50 to 3.0% of Mn, 0.0002 to 0.150% of P, 0.001 to 0.200% of S, : 0.001 to 0.300%, V: 0.01 to 0.30%, Mo: 0.03 to 1.00%, and N: 0.0020 to 0.0070%, with the balance being Fe and unavoidable impurities.

(2) A high-frequency quenching method as described in (1) above, further comprising one or more of Ca: 0.0002 to 0.0100%, Te: 0.0002 to 0.1000%, and Zr: 0.0002 to 0.2000% Non - tempered steel for hot forging that can be treated.

(3) The steel sheet as described in any one of (1) to (2) above, wherein the steel structure has a bainite structure in an area ratio of 95% or more and an average size of Mo carbonitride dispersed in the steel is 4 nm or more, Lt; RTI ID = 0.0 > nm. ≪ / RTI >

(4) A steel material having the composition described in the above item (1) or (2) is heated to 1000 ° C. or more and 1250 ° C. or less for hot forging. After the hot forging, the average cooling rate ° C / sec or more and 0.80 ° C / sec or less, and a high frequency quenching treatment is applied to a site where strength is required.

INDUSTRIAL APPLICABILITY The steel of the present invention is optimum as a material for hot forged unstructured steel parts capable of high frequency quenching with high fatigue strength while suppressing an increase in cutting cost. Further, by the manufacturing method of the present invention, high-frequency quenching hot forging uncooled products having high-density and high fatigue strength can be produced. Further, since the hot forged uncooled product of the present invention can be quenched at a high frequency when used as a part for an automobile or an industrial machine, it is possible to increase the strength of a single part of the component, thereby contributing to weight saving, fuel consumption reduction and cost reduction .

DISCLOSURE OF THE INVENTION The inventors of the present invention have extensively studied the range of the steel component, the morphology and the heat treatment conditions for the above-mentioned purpose, and found the following knowledge. In other words,

(a) It has a higher internal resistance (= fatigue strength / tensile strength) than conventional unoriented steel by dispersing fine Mo carbonitride in bainite structure of 95% or more in area ratio. The influence of the cooling rate after the hot forging is large, and the inner ratio is improved as the cooling rate is low. This is because the shorter the cooling rate is, the longer the time to stay in the temperature range where the Mo carbonitride is precipitated, the more the precipitation amount is increased. As a result, the tensile strength and the fatigue strength increase, and the smaller the cooling rate, The strength is remarkably lowered, while the fatigue strength is increased or maintained without lowering. In general, precipitation of carbonitride such as Mo used for precipitation strengthening increases not only the fatigue strength but also the tensile strength and remarkably lowers the machinability, so that the high fatigue strength and good machinability are not compatible. It has been found that by controlling the size of the Mo carbonitride to 4 nm or more and 11 nm or less by coarsening the precipitate, the fatigue strength can be increased without increasing the tensile strength that affects the machinability. However, it is necessary that the main body structure is made of bainite structure and the other elements of pro-eutectoid ferrite and retained austenite structure are less than 5% in area ratio.

(b) V, like Mo, contributes to the improvement of the inner quality by forming carbonitride. In the case of high N, V is formed at a higher temperature and stable V is formed in the cooling process after hot forging, Resulting in a reduction in the required number of parts. Low N is a necessary condition in order to sufficiently utilize the effect of improvement of the internal quality by carbonitride.

The present invention has been made based on these knowledge after repeated further examination.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the above-described steel component range will be described.

C: 0.45 to 0.60%

C is an important element for determining the strength of a steel. The alloy cost is lower than other alloying elements, so that if a large amount of C can be added, the alloy cost of the steel can be reduced. Further, the surface hardness after the high frequency quenching treatment is determined by the amount of C in the steel and the lower limit is set to 0.45% in order to obtain the required strength. However, when a large amount of C is added, retained austenite or island-shaped martensite in which C is concentrated at the boundary of lath at the time of bainite transformation is generated and the inner capacity is lowered, so the upper limit is set to 0.60%. The steel which can be subjected to the high frequency quenching treatment of the present invention refers to a steel which can have a surface hardness after high frequency quenching treatment to a required strength or higher. Therefore, in the present invention, it means a steel having a C content of 0.45% or more. In order to obtain higher strength, C content exceeding 0.5% is preferable.

Si : 0.02 to 0.15%

Si is an element that increases the amount of retained austenite in the steel due to bainite transformation in the cooling process after hot forging. In the case of performing the high frequency quenching treatment for heating only the surface layer, the retained austenite remains in the non-heated portion, and when the amount of Si exceeds 0.15%, the fatigue strength and internal resistance remarkably decrease. Therefore, the amount is limited to 0.15% or less. However, if it is suppressed to less than 0.02%, the manufacturing cost becomes large, so the lower limit is set to 0.02%.

Mn: 1.50-3.00%

Mn is an element promoting bainite transformation and is an important element for bainitizing the structure during the cooling process after hot forging. Further, it has an effect of forming a sulfide in combination with S and improving machinability. In order to exhibit these effects, the lower limit is set to 1.50%. On the other hand, when Mn is added in an amount exceeding 3.00%, the hardness of the substrate becomes large and becomes weak, so that the machinability is remarkably lowered. The upper limit is 3.00%. Particularly, the amount of Mn exceeding 2.0% is preferable because even at a slow cooling rate, the bainite structure is 95% in area ratio.

P: 0.0002 to 0.150%

P is an inevitable impurity in steel and usually contains 0.0002% or more, so the lower limit is 0.0002% or more. When P is added in a large amount, P is segregated in the grain boundary of old austenite and promotes cracking after high frequency quenching, so the upper limit is 0.150%. Preferably 0.100% or less, and more preferably 0.050% or less.

S: 0.001-0.200%

S has an effect of forming sulphide with Mn to improve the machinability. In order to exhibit the effect, the lower limit is set to 0.001%. S has an effect of forming a sulfide with Mn to improve machinability and also has an effect of suppressing the growth of austenite grains and maintaining toughness. In order to exhibit these effects, the lower limit is set to 0.001%. However, depending on the amount of Mn, the addition of a large amount increases anisotropy in mechanical properties, so the upper limit is 0.200%.

Cr : 0.02 to 1.00%

Cr, like Mn, is an element effective for promoting bainite transformation. In order to exhibit the effect, Cr is set to 0.02%. However, when a large amount of Cr is added, the Fe-based carbide is stabilized and the surface hardness in the case of high-frequency quenching is lowered. Therefore, the upper limit is set to 1.00%.

Al : 0.001 to 0.300%

Al is precipitated and dispersed in the steel as a nitride, thereby preventing the coarsening of the austenite structure at the time of forging reheating and also preventing the coarsening of the bainite structure thereafter. In addition, Al bonds with oxygen during machining and adheres to the tool surface, which is effective in preventing tool wear. In order to exhibit these effects, the lower limit is set to 0.001%. It is preferably 0.050% or more, and more preferably 0.100%. On the other hand, when it exceeds 0.300%, a large amount of hard inclusions are formed, and both the inner and machinability are lowered. Therefore, the upper limit is 0.300%.

V: 0.01 to 0.30%

V is an element effective for promoting bainite transformation and is an element effective for forming carbonitride and precipitating and strengthening bainite structure to increase strength and durability. In order to exhibit this effect, a content of 0.01% or more is required. On the other hand, if it exceeds 0.30%, the effect becomes saturated, so the upper limit is 0.30%.

Mo : 0.03 to 1.00%

Mo is not only an element effective for promoting bainite transformation but also has the highest solubility in austenite as compared with alloying elements such as V, Ti and Nb which can obtain precipitation strengthening by alloy carbide, A large deposition amount of the cargo is obtained. In general, precipitation of carbonitride such as Mo, which is used for precipitation strengthening, is not preferable because not only the fatigue strength but also the tensile strength are increased and the machinability is remarkably lowered. However, when the size of the Mo carbonitride is controlled to 4 nm or more and 11 nm or less, it can be seen that the tensile strength that affects the machinability is not increased but the fatigue strength can be increased, that is, the fatigue strength and the durability are improved . In order to exhibit this effect, a content of 0.03% or more is required. On the other hand, if it exceeds 1.00%, the effect becomes saturated, so the upper limit is set to 1.00%.

N: 0.0020 to 0.0070%

N is generally used to form a nitride with V to prevent coarsening of the austenite structure during hot forging, but V nitride is the nucleus of pro-eutectoid ferrite, rather promoting the transformation of pro-eutectoid ferrite, Thereby deteriorating the rigidity. To suppress the formation of V nitride, the upper limit of the amount of N is set to 0.0070%. Further, since 0.0020% or more is normally contained as an unavoidable impurity in the steel, the lower limit is set to 0.0020%.

Ca: 0.0002 to 0.0100%, Te: 0.0002 to 0.1000%, Zr: 0.0002 to 0.2000%

Ca, Te and Zr all form oxides and serve as nuclei for crystallization of Mn sulfides and have the effect of uniformly finely dispersing Mn sulfides. Further, there is an effect of reducing the anisotropy of mechanical properties by suppressing the distortion of Mn sulfide after rolling and hot forging by solving any element or Mn sulfide and lowering its deformability. In order to exhibit these effects, the lower limits of Ca, Te, and Zr are set to 0.0002%, respectively. On the other hand, when Ca is 0.0100%, Te is 0.1000%, and Zr is more than 0.2000%, hard inclusions such as oxides and sulfides are produced in a large amount, and the durability and machinability are lowered. Therefore, the upper limit of Ca is set to 0.0100%, the upper limit of Te is set to 0.1000%, and the upper limit of Zr is set to 0.2000%.

Next, the reason for limiting the steel structure of the hot forged untreated product will be described.

(95% or more bainite structure in area ratio)

The reason why the structure is defined as a bainite structure having an area ratio of 95% or more is that when the main body structure is a bainite structure, ferrite, retained austenite or island-shaped martensite, , The inner appearance is remarkably lowered. The smaller the amount of remaining portion structure, the higher the inner portion is, and preferably the area ratio is 97% or more.

(The average size of the Mo carbonitride dispersed in the steel is 4 nm or more and 11 nm or less)

The reason why the average size of the Mo carbonitride in the bainite structure is set to 4 nm or more is because when the average size is less than 4 nm, the high fatigue strength is high but the tensile strength is high, This is because it is not possible to achieve a balance between work and work. More preferably, the average size thereof is 8 nm or more. Further, the reason why the upper limit value of the average size of the Mo carbonitride is defined as 11 nm is because when the average size exceeds 11 nm, not only the tensile strength but also the fatigue strength are remarkably lowered, so that the high fatigue strength can not be attained. The shape of the Mo carbonitride is needle-shaped, and the size of the Mo carbonitride used in the present specification is the length in the longitudinal direction.

Next, reasons for limiting the above-described method of manufacturing a hot forged untreated product will be described.

(Heating the steel material to 1000 ° C or more and 1250 ° C or less)

The reason why the steel material having the above-mentioned composition is heated to 1000 ° C. or more and 1250 ° C. or less is to sufficiently precipitate the carbonitrides of Mo and V in the cooling process. Mo and V are heated by heating before hot forging So as to sufficiently dissolve them in the steel. When the heating temperature is lower than 1000 占 폚, Mo and V can not be sufficiently dissolved in the steel, the precipitation strengthening amount in the subsequent cooling process is small, and the fatigue strength and internal resistance become small. On the other hand, raising the heating temperature more than necessary promotes the growth of the austenite grains, and the formed structure becomes coarse during the subsequent cooling process, resulting in deterioration of the inner temperature. Therefore, the upper limit of the heating temperature is set to 1250 캜.

(After the hot forging, the average cooling rate to 200 DEG C or less is cooled to 0.05 DEG C / sec or more and 0.80 DEG C / sec or less)

The reason why the average cooling rate is not less than 0.05 DEG C / sec and not more than 0.80 DEG C / sec after the hot forging is 200 DEG C or less is to prolong the stay time in the temperature range where Mo carbonitride is precipitated, So as to control the size of the carbonitride. When the average cooling rate is 0.80 ° C / second or more, the precipitation amount of Mo carbonitride is not sufficiently obtained, and the effect of improving the strength and internal resistance is small. Particularly, in order to coarsen the Mo carbonitride and have a high solid content, the average cooling rate is preferably 0.50 ° C / sec or less. More preferably 0.30 ° C / second or less. On the other hand, when the average cooling rate is less than 0.05 占 폚 / sec, pro-eutectoid ferrite having an area ratio of 5% or more is generated at the boundary of the bainite lath, thereby significantly lowering the fatigue strength and durability.

Further, according to the present invention, a hot forged uncooled product capable of high frequency quenching with high fatigue strength is obtained, but in order to secure sufficient machinability, the tensile strength is preferably 1300 MPa or less.

The present invention will be described in detail below with reference to Examples. These examples are for the purpose of describing the technical significance and effects of the present invention, and do not limit the scope of the present invention.

Example

The steel of chemical composition shown in Table 1 was dissolved in a 150 kg vacuum melting furnace. This was rolled into a bar steel having a diameter of 100 mm, and then a forging test piece was cut out and subjected to forging and heat treatment under the conditions shown in Table 2. After the hot forging, the cooling method up to 200 캜 was air-cooled or furnace-cooled, and the cooling rate was controlled by changing the diameter of the test piece. The average cooling rate was obtained by dividing the value obtained by subtracting 200 캜 from the temperature of the test piece after hot forging divided by the time required for cooling to 200 캜 after hot forging. In addition, for comparison, a conventional steel S55C was dissolved and heat-treated so as to have the same tensile strength as the present invention. The test piece was heated to 1100 占 폚, then cooled to room temperature, and further heat-treated at 450 占 폚 for 1 hour. The underlined portions in Tables 1 and 2 are outside the scope of the present invention.

The tensile strength and the fatigue strength of the tensile test specimen No. 14 of JIS Z 2201 and the specimen No. 1 rotary bending fatigue test specimen of JIS Z 2274 were obtained from the central portion of these forgings. Here, the fatigue strength is defined as the long-lasting stress amplitude without breaking at 10 ⁷ rotations in the rotational bending fatigue test. Further, the ratio of the obtained fatigue strength and tensile strength was determined as the internal resistance (fatigue strength / tensile strength).

A test piece for tissue observation was collected from the 1/4 thickness portion in the L direction of the forging material. The area ratio of bainite was determined by grinding the specimen until the specimen became mirror-finished, and then performing repera etching to confirm the texture of the remaining elements other than bainite, such as pro-eutectoid ferrite, retained austenite and island-shaped martensite, An optical microscope photograph was photographed every 10 fields and then calculated by image analysis.

The average size of the Mo carbonitride was measured by a transmission electron microscope after 15,000 times of transmission electron microscope photographs were taken every 10 days, and the observed Mo carbonitride length Direction was determined by image analysis, and the average value thereof was determined.

The inventive steels Nos. 1-18 all have a bainite structure with an area ratio of 95% or more, an average size of Mo carbonitride of 4.6 nm or more and 10.8 nm or less, and an inside content of 0.58 or more. The tensile strength is 1300 MPa or less in order to secure the machinability. However, as apparent from comparison with the tensile strength at the same level, the high fatigue strength is realized in comparison with the carbon steel of the carbon steel of the prior art No. 28.

On the contrary, in Comparative Examples Nos. 23, 24 and 27, the content of any one of C, Si, or N was large, and No. 21 was within the specified steel composition range, The amount of residual portion such as ferrite or retained austenite is large in the lath boundary, and the average size of Mo carbonitride is out of the specification, so that strength and durability are low. In Nos. 19 and 22, the steel composition or the heat treatment condition is outside the stipulated condition, sufficient precipitation strengthening is not obtained, and the inside resistance is low. No. 20 has a higher heating temperature than necessary, so that the bainite structure is coarsened, and the inside property is rather low. In No. 25, Mn is added more than necessary, the tensile strength is high, and cutting is very difficult. On the other hand, in No. 26, Al is added more than necessary, and the fatigue strength and internal resistance are lowered.

As is evident from this, the toughness and fatigue characteristics of the comparative example and the conventional example, which satisfy all the conditions specified in the present invention, are excellent.

Claims (4)

In terms of% by mass,
C: more than 0.50% and not more than 0.60%
Si: 0.02 to 0.15%
Mn: 1.50 to 3.00%
P: 0.0002 to 0.150%,
S: 0.001-0.200%,
Cr: 0.02 to 1.00%
Al: 0.001 to 0.300%
V: 0.01 to 0.30%
Mo: 0.03 to 1.00%
N: 0.0020 to 0.0070%,
And a balance of Fe and unavoidable impurities, wherein the high-frequency quenching treatment is possible. 3. The composition according to claim 1, wherein, in mass%
Ca: 0.0002 to 0.0100%
Te: 0.0002 to 0.1000%,
Zr: 0.0002 to 0.2000%
, And at least one of them is further contained in the non-tempered steel for hot forging capable of high frequency quenching treatment. Characterized in that the average grain size of the Mo carbonitride dispersed in the steel is in the range of 4 nm or more and 11 nm or less and the steel structure has the steel structure according to any one of claims 1 to 3, Hot forging uncooled product capable of high frequency quenching. A steel material having the composition described in claim 1 or 2 is heated to 1000 ° C. or more and 1250 ° C. or less for hot forging and the average cooling rate up to 200 ° C. after the hot forging is 0.05 ° C./second or more , And cooling to 0.80 ° C / second or less, and a high-frequency quenching treatment is applied to a site where strength is required.