ES2769257T3 - Aging-curable steel, and method of manufacturing components including aging-curable steel - Google Patents

Abstract

Age-hardenable steel comprising, in% by mass, C: 0.09% to 0.20%, Si: 0.01% to 0.40%, Mn: 1.5% to 2.5% , S: 0.001% to 0.045%, Cr: more than 1.00% to 2.00%, Al: 0.001% to 0.060%, V: 0.22% to 0.55%, N: more than 0.0080% to 0.0170%, optionally Mo: 0.9% or less, optionally further one or more of Cu: 0.3% or less and Ni: 0.3% or less, and optionally further one or more of Ca: 0.005% or less and Bi: 0.4% or less, and traces of Fe and impurities, with P and Ti in these impurities being P: 0.03% or less and Ti: less than 0.005%, where an area fraction of the bainitic structures is 80% or more, an effective ratio of V, which is the amount of dissolved V / total amount of V, is 0.9 or more, and a chemical composition is one in which the F1 expressed by the following formula (1 ') is 1.00 or less and the F2 expressed by the following formula (2') is 0.30 or more: F1 = C + 0.1 x Yes + 0.2 x Mn + 0.15 x Cr + 0.35 x V + 0.2 x Mo ⋯ .. (1 ') F2 = -4.5 x C + Mn + Cr - 3.5 x V -0 , 8 x Mo ⋯ .. (2 ') where, in the above formulas (1') and (2 '), the element symbols mean the content of the elements in% by mass.

Description

DESCRIPTION

Aging-curable steel, and method of manufacturing components including aging-curable steel

Technical field

The present invention relates to aging-curable steel. More specifically, it refers to steel for the production of a mechanical part for automobiles, industrial machinery, and construction machinery that is worked to obtain a predetermined shape by hot forging and machining, it is treated to achieve an aging hardening (in Hereinafter, it is simply called "aging treatment ''), and has the desired strength and toughness guaranteed by this aging treatment. Furthermore, the present invention also relates to such a one-piece production method using aging-hardenable steel .

Prior art

From the standpoints of increasing engine performance, lightening weight in order to improve fuel economy, etc., mechanical parts for automobiles, industrial machinery, construction machinery, etc. are required to have high fatigue resistance. If only steel with high fatigue resistance is provided, it can be easily achieved using alloying elements and / or heat treatment

of steel. However, in general, mechanical parts are formed by hot forging, then machined to finish with predetermined product shapes. For this reason, the steel used as material for the mechanical parts must simultaneously be endowed with a high resistance to fatigue together with sufficient machinability.

In general, the fatigue resistance improves the higher the hardness of the material. On the other hand, in machinability, the resistance to machining and the useful life tend to be lower the greater the hardness of the material. Furthermore, among the parts that make up the motor, the precisely shaped mechanical parts are required to remain unchanged in size during use. Depending on the environment of use, these precision-formed mechanical parts can be instantly subjected to higher loads compared to magnitude loads in regular use. An elastic limit is also required so that the dimensions do not change against such loads.

Therefore, various techniques have been disclosed that can provide all-in-one properties such as fatigue strength, creep resistance, and machinability by keeping hardness low in the forming stage where good machinability is required while hardness is increased by aging treatment in the final stage of the product where resistance is required.

For example, Japanese Patent Document Published No. 2006-37177A (DP 1) describes "aging hardenable steel" obtained by rolling, forging, or solutioning of steel in which the Mo and V precipitation hardening elements are present in quantities limited by specific formulas and a cooling between a temperature of 800 ° C and 300 ° C at an average cooling rate of 0.05 to 10 ° C / s, which has, before aging treatment, an area fraction of bainite structures of 50% or higher and a hardness of 40 HRC or less and having, after aging treatment, a hardness of 7 HRC or higher than the hardness before carrying out the aging treatment.

Japanese Patent Document Published No. 2011-236452A (DP 2) describes bainitic steel containing specific amounts of Mo and V as precipitation hardening elements, as an excellent steel in hot forging and machinability after hot forging and that it can increase its resistance by aging hardening after machining.

Published Japanese Patent Document No. 2000-17374A (DP 3) proposes, as an aging-hardened high-strength bainitic steel for use in hot forging, an aging-hardened high-strength bainitic steel characterized by having a limit point elastic or 0.2% elastic limit of 900 MPa or more, obtained by hot rolling or hot forging of steel containing Mo and V, then it is cooled according to the steel components, making the hardness 400 HV or less, making the structure have a bainite fraction of 70% or more, making the starting austenitic grain size 80 | jm or less, then mechanization or plastic shaping of the steel is applied as needed and a aging treatment.

Japanese Patent Document Published No. 2013-245363A (DP 4) describes a steel that exhibits high machinability and high resistance to fatigue by adjusting the content of the alloying elements to satisfy specific parameter formulas and, therefore, Thus, relatively reducing the Mo content while making the hardness before aging treatment after hot forging to be 290 HV or less and the hardness after aging treatment to be 325 HV or greater.

International patent document WO2012 / 161323A (DP 5) describes a steel part for a structure of machine using cooling and heat treatment after hot forging to optimize V carbonitride shapes and bainitic structure shapes, which have precipitation strengthening ability and provide machinability, fatigue resistance and toughness properties all together.

Japanese Patent Document Published No. 2013-213254A (DP 6) describes a steel for cold forging and nitriding, excellent in cold forgeability and chip removal ability after cold forging, and which can provide a nitrided part Cold forged with high core hardness, high surface hardness and high effective depth of hardened layer.

French patent document FR 2990218 A1 describes an aging-hardened micro-alloyed bainitic steel.

Summary of the invention

Technical problem

By using the aging treatment to cause the fine particles of the secondary phase to precipitate in the steel, it is possible to obtain a high resistance to fatigue and a high elastic limit. In this sense, the steel hardened by the aging treatment has a reduced toughness.

Steel with reduced toughness increases its sensitivity to notches. By increasing the notch sensitivity, the fatigue resistance of steel is easily affected by small surface defects.

In addition, low-tenacity steel undergoes faster fracture progression and larger-scale fractures once fatigue cracking occurs.

Also, if the steel has too low a toughness, it becomes difficult to correct the deformation produced in the hot forging by cold work.

In the steel described in DP 1 it can be adjusted in the content of the alloying elements to satisfy specific parameter formulas in order to obtain a high aging hardening capacity, but the toughness is not considered at all.

In the steel described in DP 2, the content of the alloying elements is adjusted to satisfy specific parameter formulas in order to relatively reduce the Mo content while making the hardness before aging treatment 300 HV or less. after hot forging and that the hardness after aging treatment is 300 HV or higher. In this regard, however, steel is not designed enough to increase its hardness after aging treatment.

The steel described in DP 3 has a C content that is kept low in a range of 0.06 to 0.20%, but the V content is extremely high in a range of 0.51 to 1.00%, for which steel is noticeably strengthened by aging hardening, but is not excellent in toughness.

The steel described in DP 4 is not sufficiently designed to increase toughness and yield strength after aging treatment.

The steel described in DP 5 is not sufficiently designed to increase the yield strength after aging treatment.

The steel described in DP 6 is low in N content, so nitrides are produced insufficiently and, as a result, an excellent yield strength is not obtained.

Therefore, an object of the present invention is to provide aging hardenable steel that satisfies the following points <1> to <3>:

<1> Low hardness after hot forging related to machining strength and tool life. It should be noted that, in the following explanation, the hardness after prior hot forging will be referred to as "hardness before aging treatment".

<2> The ability to provide a mechanical part with the desired fatigue strength and yield strength, through aging treatment.

<3> High toughness after aging treatment.

Specifically, an object of the present invention is to provide aging-hardenable steel having a hardness before aging treatment of 340 HV or less, a fatigue strength, explained later, after aging treatment of 480 MPa or greater, a 0.2% yield strength, determined by the compensation method using a prescribed 0.2% plastic strain amount in a tensile test performed using a JIS 14A tensile test piece having a parallel part $ 6, 800 MPa or higher, and also has an absorption energy at 20 ° C after aging treatment, assessed by a Charpy impact test performed using a standard U-notched test piece that has a notch depth of 2mm and a 1mm notch bottom radius described in JIS Z 2242, 25J or higher.

Solution to the problem

Findings (a) to (d)

The authors of the present invention participated in evaluations and studies related to the chemical composition, structure, and effective ratio of V (amount of dissolved V / total amount of V) and the values calculated by formulas that use specific element contents to solve the above problem. Specifically, they investigated the conditions for obtaining good toughness even with steel that provides high resistance to fatigue and a high elastic limit by causing the precipitation of fine particles from the secondary phase in steel due to aging. As a result, the following findings (a) to (d) were obtained.

(a) Limitation of chemical composition (C, V, Mo, and Ti)

The elements that produce deterioration of the tenacity after the aging treatment are C, V, Mo, and Ti. Among these, Ti binds with N and / or C to form TiN and / or TiC. If TiN and / or TiC is precipitated, the fatigue resistance sometimes becomes higher, but the toughness is caused to decrease considerably. The intensity of the action of Ti in reducing toughness is extremely large compared to those of V and Mo, which are the elements that contribute to the strengthening by precipitation such as V. For this reason, Ti must be as limited as possible .

C forms cementite in steel and can become the starting point for intercrystalline fracture. Even if the treatment of steel containing an amount of V or Mo excessive with respect to the amount of C due to aging, part of the cementite remains. Both V and Mo cause carbide precipitation in the same crystalline planes of the matrix along with the aging treatment, and therefore aid the progression of intercrystalline fractures and impair toughness. Therefore, to increase toughness, it is necessary to reduce the content of C, V, and Mo.

(b) Limitation of structure

To increase toughness, it is necessary to make most of the structure of fine bainite. In addition, making the difference in orientation between the blocks that form the bainite is also essential to improve toughness. If the orientation difference between the blocks is small, even if the size of the blocks is refined, the effect of increasing the toughness is not sufficiently obtained. To widen the orientation difference between blocks, it is necessary to increase the driving force at the time of the bainitic transformation and to enhance the formation of block nuclei with large orientation differences. To obtain these effects, the content of C, Mn, Cr, and Mo must be increased.

However, C and Mo have the effect of refining the structure and increasing toughness and the action of precipitating cementite or carbides and decreasing toughness. In general, C greatly reduces toughness and Mo slightly decreases toughness.

(c) Limitation of the effective ratio of V

In order to use the precipitation strengthening by V to the maximum degree, it is necessary to limit the effective ratio of V, defined as the amount of dissolved V with respect to the total amount of V. If the effective ratio of V is low, it means that the ratio of the amount of V that contributes to precipitation strengthening is low and the strengthening capacity is low and not preferable. There is no upper limit of the effective ratio of V. The closer to 1 the better.

(d) Limitation of values calculated by formulas

Use of content of specific elements and limitation of the amount of Ti

In order to impart sufficient toughness to aging-curable steel having high strength, the contents of C, Mn, Cr, V and Mo must be controlled so that the value expressed by (2) or (2 ') that shows a toughness indicator after the aging treatment explained below, it becomes a specific value or higher. Likewise, the Ti content must have a specific value or less so that the inclusions and precipitates harmful to toughness are not present in the steel.

Findings (e) to (g)

Next, the authors of the present invention participated in additional evaluations and studies related to the values calculated by the formulas using the chemical composition and the content of specific elements. Specifically, they adjusted steel components that could ensure post-aging hardness and investigated conditions related to pre-aging hardness and hardness. after aging and the aging hardening capacity expressed by their difference. As a result, the following findings (e) to (g) were obtained.

(e) Limitation of values calculated by formulas using the content of specific elements

If the contents of C, Si, Mn, Cr, V, and Mo are controlled so that the value expressed by the formula explained below (1) or the formula (1 ') becomes a specific range, it can be avoided the hardness before the previous aging treatment increases excessively. For this reason, when machining under various conditions, machinability can be expected to allow for mass industrial production.

(f) Limitation of chemical composition (Mo, V, C)

If the contents of Mo, V, and C are reduced in order to increase toughness after aging, the driving force of carbonitride precipitation of V at the time of aging is also reduced. For this reason, the fine precipitates formed due to aging are reduced, and the hardness and elastic limit after aging decreases.

(g) Limitation of chemical composition (Mn, Cr)

If the Mn and Cr content is increased in order to increase the toughness after aging, the hardening capacity increases and the hardness before aging becomes higher. With such a structure, the structure recovers easily at the time of aging, whereby the margin of increase in hardness due to aging decreases easily. If the hardening capacity increases, the mobile displacement density remaining in the matrix after aging also increases easily, making it difficult to obtain a high yield strength.

Findings (h) to (j)

Next, the authors of the present invention participated in additional evaluations and studies related to chemical composition. Specifically, they focused on the fact that even if tenacity increases after aging by reducing C, V, and Mo contents and by increasing Mn and Cr contents, to cause sufficient amounts of hardener particles to precipitate by precipitation and In order to obtain sufficient aging hardening capacity and a high elastic limit, it is necessary to increase the precipitation hardening per unit quantity of V. In addition, the authors of the present invention participated in several studies on techniques to increase the precipitation strengthening capacity. using V and obtained the following findings (h) to (j).

(h) Limitation of chemical composition (C, N)

When using the precipitation strengthening by V to the maximum degree, it is sufficient to increase the driving force for the precipitation of carbonitrides from V. To this end, it is necessary to sufficiently guarantee the amount of C and the amount of N to be used for the V carbonitride precipitation in a range that does not hinder toughness.

(i) Limitation of chemical composition (N)

The bonding force between N and V is greater than the bonding force between C and V, so the effect of causing carbonitride precipitation of V is greater with N than with C.

(j) Limitation of chemical composition (C, N)

If the content of C and N becomes too high, V does not enter a solution even if it is heated during hot forging or ends up precipitating in the austenitic region during forging. For this reason, if the C and N content is excessively increased, on the contrary, the capacity to strengthen the precipitation is decreased.

The present invention was made based on the above findings (a) to (j) and is defined in the appended claims.

Advantageous effects of the invention

The aging curable steel of the present invention has a hardness before aging treatment of 340 HV or less. Furthermore, a mechanical part utilizing the aging-curable steel of the present invention has a fatigue strength of 490 MPa or higher due to post-machining aging treatment. In addition, the mechanical part has a toughness (absorption energy at 20 ° C after aging treatment evaluated by a Charpy impact test carried out with a standard test piece with a U-notch with a notch depth of 2 mm and a 1mm notch bottom radius) 25J or greater. Furthermore, the mechanical part has an elastic limit of 800 MPa or higher. For this reason, the aging-curable steel of the present invention can be used extremely well as a material for a mechanical part of automobiles, industrial machinery, construction machinery, etc.

Brief description of the drawings

[Figure 1] A view showing the correlation between the hardness of a steel material before aging and a value of F1.

[Figure 2] A view showing the relationship between a Charpy impact value of a steel material after aging and an F2 value.

Description of the embodiments

Next, the requirements of the present invention will be explained in detail. It should be noted that "%" of the content of the elements means "% by mass".

Aging-curable steel

Essential components

C: from 0.09 to 0.20%

C is an important element in the present invention. C joins with V to form carbides and strengthen steel. However, if the C content is less than 0.09%, the V carbides become more difficult to precipitate, so the desired strengthening effect cannot be obtained. On the other hand, if the C content becomes too high, the amount of C that does not bind with V or Mo forming carbides with Fe (cementite) increases, so the toughness ends up degrading. Therefore, the C content is made to be from 0.09 to 0.20%. The C content is preferably made to be 0.10% or more, more preferably it is made to be 0.11% or more. Furthermore, the C content is preferably made to be 0.18% or less, more preferably it is made to be 0.16% or less.

Yes: from 0.01 to 0.40%

Si is useful as a deoxidizing element at the time of steel production and simultaneously has the effect of improving the strength of the steel by dissolving in the matrix. To obtain these effects sufficiently, the Si content is made to be 0.01% or higher. However, in steel containing Mn and Cr in large quantities, if the Si content becomes excessive, sometimes the amount of residual austenite in the structure after hot forging becomes too high and the deformation increases during aging treatment. Therefore, the Si content is made to be from 0.01 to 0.40%. The Si content is preferably made to be 0.05% or more. Furthermore, the Si content is preferably made to be 0.35% or less, more preferably 0.30% or less.

Mn: from 1.5 to 2.5%

Mn has the effect of improving hardening capacity and making the structure bainitic. Furthermore, it has the effect of reducing the bainitic transformation temperature and therefore refining the bainitic structure to improve the toughness of the matrix. In addition, Mn has the effect of forming MnS in steel to improve chip removal capacity at machining. To obtain these effects sufficiently, the Mn must have a content of at least 1.5%. However, Mn is an element that separates easily at the time of steel solidification, so if the content becomes too large, the fluctuation of hardness in one part after hot forging inevitably becomes larger. Therefore, the Mn content is made to be 1.5-2.5%. The Mn content is preferably made to be 1.6% or more, more preferably it is made to be 1.7% or more. Furthermore, the Mn content is preferably made to be 2.3% or less, more preferably it is made to be 2.1% or less.

S: 0.001 to 0.045%

The S joins with the Mn in the steel to form MnS and improves the ability to remove chips at the time of machining, so it must have a content of 0.001% or higher. However, if the S content becomes higher, the thick MnS increases and the toughness and fatigue resistance are degraded. In particular, if the S content exceeds 0.045%, the drop in toughness and fatigue resistance becomes considerable. Therefore, the content of S is made to be from 0.001 to 0.045%. The S content is preferably made to be 0.005% or more, more preferably it is made to be 0.010% or more. Furthermore, the S content is preferably made to be 0.040% or less, more preferably it is made to be 0.035% or less.

Cr: more than 1.00% to 2.00%

Cr, like Mn, has the effect of increasing hardness capacity and making the structure bainitic. Furthermore, it reduces the bainitic transformation temperature to refine the bainitic structure. It also has the effect of reducing the ease of movement of the grain boundaries to refine the austenitic grain size at the time of hot forging and, as a result, refine the bainitic structure after transformation. Cr has the effect of increasing the hardness of the matrix through its effects in refining the bainitic structure. To obtain these effects sufficiently, it must be included in more than 1.00%.

However, if the Cr content exceeds 2.0%, the hardening capacity becomes larger and the hardness before aging treatment sometimes exceeds 340 HV. Therefore, the Cr content is made to be from 1.00% to 2.00%. The Cr content is preferably made to be 1.10% or higher. Furthermore, the Cr content is preferably made to be 1.80% or less, more preferably it is made to be 1.60% or less.

Al: 0.001 to 0.060%

Al is an element that has a deoxidizing action. To obtain this effect, its content is required to be 0.001% or higher. However, if the Al content is excessive, thick oxides are formed and the toughness decreases. Therefore, the Al content is made to be 0.001 to 0.060%. The Al content is preferably made to be 0.050% or less.

V: 0.22 to 0.55%

V is the most important element in the steel of the present invention. At the time of aging treatment, V joins with C to form fine carbides, and therefore has the effect of increasing fatigue resistance. Furthermore, when steel contains Mo, V has the effect of combining with Mo and precipitating due to aging treatment and further increasing the ability of aging hardening. To obtain these effects, the V must have a content of 0.22% or higher. However, if the V content becomes excessive, even on heating at the time of hot forging, the undissolved carbonitrides easily remain, leading to a decrease in toughness. Also, if the V content becomes excessive, sometimes the hardness before the aging treatment ends up increasing. Therefore, the content of V is made to be 0.22 to 0.55%. The content of V is preferably below 0.45%, more preferably it is made to be 0.40% or less. Furthermore, the content of V is preferably made to be 0.25% or more, more preferably it is made to be 0.27% or more.

N: more than 0.0080 to 0.0170%

N has the effect of enhancing the carbonitride precipitation of V at aging and increasing the elastic limit. To obtain this effect sufficiently, the N content is made to be greater than 0.0080%. However, if the N content exceeds 0.0170%, at the time of hot forging, the V carbonitrides do not enter a solution and at the next time of aging the precipitation of a sufficient amount of V carbonitrides Fine becomes difficult, so the elastic limit decreases. Therefore, the N content is made to be above the range of 0.0080 to 0.0170%. The N content is preferably made to be 0.0090% or more, more preferably it is made to be 0.0100% or more. Furthermore, the N content is preferably made to be 0.0160% or less, more preferably it is made to be 0.0150% or less.

The aging curable steel of the present invention is composed of the above elements from C to N and residues of Fe and impurities, the P and Ti in the impurities are P: 0.03% or less and Ti: less than 0.005% , the area fraction of the bainitic structure is 80% or more, and the effective ratio of V (amount of dissolved V / total amount of V) is 0.9 or more.

Impurities

"Impurities" indicate elements that enter from the mineral starting materials and residues from the manufacturing environment, etc., when ferrous metal materials are produced at the industrial level.

P: 0.03% or less

P is an element present as an impurity and is not preferable in the present invention. That is, P separates at the grain boundaries to produce a decrease in toughness. Therefore, the P content is made to be 0.03% or less. The P content is preferably made to be 0.025% or less.

Ti: less than 0.005%

Ti is an element present as an impurity and is particularly not preferable in the present invention. That is, Ti joins with N and / or C to form TiN and / or Tic, leading to a decrease in resistance. In particular, if the content becomes 0.005% or higher, the toughness deteriorates greatly. Therefore, the Ti content is made to be 0.005%. To ensure good toughness, the Ti content is preferably made to be 0.0035% or less.

Structure

In the aging curable steel of the present invention, the area fraction of the bainitic structure is 80% or greater. As used herein, the area fraction of the bainitic structure means the area fraction in the case of observing a metallic structure in a position 1/3 deep to 1/2 deep in thickness from the surface of the steel material with an optical microscope. If the area fraction of the bainitic structure is 80% or higher, the precipitation of V is suppressed, the effective ratio of V becomes higher, and high fatigue resistance and a high yield strength at 0 can be obtained. ,two %.

Effective V ratio

The effective ratio of V (amount of dissolved V / total amount of V) is 0.9 or higher. As used herein, the "effective ratio of V" means the amount of dissolved V in the total amount present in the steel. If the effective ratio of V is 0.9 or higher, the amount of V carbonitrides that precipitate during the aging treatment becomes higher, and high fatigue resistance and a high elastic limit of 0.2 can be obtained. %.

Optional components

Next, reference will be made to optional components that may be present in the aging-curable steel of the present invention.

Mo: 0.9% or less

Mo, like V, is an element with a relatively low carbide precipitation temperature and is suitable for hardening by aging. Mo has the effect of increasing hardening capacity, making bainite the main phase of the structure after hot forging, and increasing the area fraction. Mo has the effect of forming carbides together with V to increase the aging hardening capacity in steel containing 0.22% or more of V. For this reason, Mo can be included as needed. However, since Mo is an extremely expensive element, when its content increases, the cost of producing the steel increases and the resistance also decreases. Therefore, when present, the amount of Mo is made to be 0.9% or less. The amount of Mo, when included, is preferably 0.75% or less, more preferably 0.60% or less, and even more preferably less than 0.50%. On the other hand, to stably obtain the above effect of Mo, the amount of Mo when included is desirably made to be 0.05% or greater, more desirably is made to be 0.10% or greater.

Cu: 0.3% or less

Cu has the effect of improving fatigue resistance. For this reason, Cu can be included as needed. However, if the Cu content increases, the hot workability decreases. Therefore, when Cu is present, the amount is made to be 0.3% or less. The amount of Cu, when included, is preferably made to be 0.25% or less. On the other hand, to stably obtain the effect of raising the fatigue resistance of Cu, the amount of Cu, when included, is desirably made to be 0.1% or more.

Ni: 0.3% or less

Ni has the effect of improving fatigue resistance. Furthermore, Ni also has the effect of suppressing the drop in hot workability due to Cu. For this reason, Ni can be included as needed. However, when the Ni content increases, the cost increases and, in addition, the above effects are also saturated. Therefore, when Ni is included, it is made in an amount that is 0.3% or less. The amount of Ni, when included, is preferably made to be 0.25% or less. On the other hand, to stably obtain the above effects of Ni, the amount of Ni when present is desirably made to be 0.1% or more.

The above Cu and Ni can be included as a single type of one of the same or as two types combined. The total content of the elements, when included, may be 0.6% of the case where the Cu and Ni contents are at their respective upper limit values.

Ca: 0.005% or less

Ca has the effect of lengthening the useful life. For this reason, Ca can be included according to need. However, if the Ca content becomes higher, thick oxides are formed and toughness is reduced. Therefore, the amount of Ca, when included, is made to be 0.005% or less. The Ca content, when included, is preferably made to be 0.0035% or less. On the other hand, to stably obtain the effect of Ca on increasing useful life, the amount of Ca when included is desirably made to be 0.0005% or more.

Bi: 0.4% or less

Bi has the effect of reducing machining resistance and increasing service life. For this reason, the Bi can be included according to the need. However, if the Bi content increases, it causes a drop in hot workability. Therefore, the amount of Bi, when included, is made to be 0.4% or less. The amount of Bi, when included, is preferably made to be 0.3% or less. On the other hand, to obtain the effect of Bi in prolonging the useful life, the amount of Bi, when included, is desirably made to be 0.03% or more.

The above Ca and Bi can be included as a single type of one of the same or as two types combined. The total content of the elements, when included, can be made to be 0.405% of the case in which the contents of Ca and Bi are at their respective upper limit values, but preferably it is made to be 0.3% or lower.

Values calculated by formulas using specific element contents: F1 (F1 ') and F2 (F2') The aging-curable steel of the present invention satisfies the conditions of the aforementioned chemical composition (essential components and optional components), structure and ratio Effective of V. In addition, the F1 (F1 ') and F2 (F2') values calculated by formulas using specific element contents must be 1.00 or less and 0.30 or more, respectively.

First, the calculated F1 (F1 ') value will be explained using a formula that uses the content of specific elements.

That is, when the optional elements from Mo to Bi are not present, F1 is expressed by the formula F1 = C 0.1 x Si 0.2 x Mn 0.15 x Cr 0.35 x V ..... ( one)

is 1.00 or less, and when one or more optional Mo to Bi elements are present, F1 'is expressed by the formula

F1 '= C 0.1 x Si 0.2 xM n 0.15 x Cr 0.35 x V 0.2 x M or ..... (1')

is 1.00 or less.

It should be noted that, in formula (1) and formula (1 ') above, the symbols for the elements mean the content of those elements in% by mass.

F1 and F1 'are indicators that show hardness before aging treatment. If the aging-curable steel of the present invention satisfies the conditions related to formulas F1 and F1 'above, the hardness before aging treatment is not too high, the machining resistance at machining is not high, and the longer lifespan.

F1 and F1 'are preferably 0.97 or less, more preferably 0.95 or less. Furthermore, F1 and F1 'are preferably 0.60 or greater, more preferably 0.65 or greater.

Figure 1 is a graph showing the relationship between hardness before aging (ordinate; HV) and the F1 values of various types of steel (abscissa). As can be seen from the graph in Figure 1, a strong primary positive correlation is found between the two. If F1 <1.00 or less, the hardness before aging is considered to be <340 HV.

Next, we will explain the value F2 (F2 ') calculated using a formula that uses specific element contents.

That is, when the optional elements from Mo to Bi are not present, F2 is expressed by the formula F2 = -4.5 x C Mn Cr - 3.5 x V ..... (2)

is 0.30 or greater and when one or more optional Mo to Bi elements are present, F2 'is expressed by the formula

F2 '= -4.5 x C Mn Cr - 3.5 x V -0.8 x Mo ..... (2')

is 0.30 or less.

It should be noted that, in formula (2) and formula (2 ') above, the symbols for the elements mean the contents of those elements in% by mass.

F2 and F2 'are indicators that show toughness after aging treatment. That is, by satisfying the condition of F1 or F1 ', sometimes the toughness after aging decreases and the target toughness cannot be guaranteed, so it is necessary to separately prescribe F2 and F2'.

Figure 2 is a view showing a relationship between a Charpy impact value of a steel material after aging and an F2 value. As shown in this figure, a positive correlative relationship is observed between the Charpy impact value (J) after aging treatment and the F2 value (abscissa). When F2 or F2 'is less than 0.30, the toughness after the aging treatment is not sufficiently obtained. In order to obtain an elastic limit of 800 MPa or higher, while guaranteeing the desired toughness, it is necessary to ensure that the contents of the above alloying elements that are within the prescribed ranges satisfy the conditions of F1 and F1 'and satisfy the conditions of F2 or F2 '.

F2 and F2 'are preferably 0.45 or higher, more preferably they are 0.60 or higher.

It should be noted that if F2 becomes high, the hardness before aging often becomes high as well. However, as long as F1 is controlled at 1.00 or less, even if F2 is high, the hardness before aging will not be too high and the machinability will not degrade. Consequently, there is no need to set an upper limit for F2. Similarly, if F1 'is 1.00 or less, there is no need to particularly set an upper limit for F2'.

Aging-curable steel production method

The method of producing the age-curable steel of the present invention is not particularly limited. A general method can be used to melt the steel and adjust the chemical composition.

Parts production method using aging-hardenable steel

Next, an example of the production method of a mechanical part for automobiles, industrial machinery, construction machinery, etc., will be shown, using as material the aging-curable steel of the present invention, which is produced in the following way.

First, a material used for hot forging (hereinafter referred to as "material for use in hot forging") is prepared from a steel with a chemical composition adjusted to the above-mentioned range. As the material for use in hot forging, there may be used a billet obtained by roughing an ingot, a billet obtained by roughing a continuous casting material, or steel rods obtained by hot rolling or hot forging of these billets, etc.

The material for use in hot forging is then hot forged and machined to finish the worked material with a predetermined part shape. It should be noted that hot forging is done, for example, by heating the material for use in hot forging to a temperature of 1,100 to 1,350 ° C for a time period of 0.1 to 300 minutes, then the temperature is allowed to from the surface after final forging falls to 900 ° C or more, and then cools to room temperature at an average cooling rate of 10 to 90 ° C / min in the temperature region of 800 to 400 ° C.

Furthermore, the cooled worked material obtained therefrom is subsequently machined to be finished in a predetermined part shape.

Finally, the worked material is subjected to aging treatment to obtain a mechanical part for an automobile, industrial machinery, construction machinery, etc. provided with the desired characteristics. The aging treatment is carried out, for example, in the temperature region of 540 to 700 ° C, preferably of 560 to 680 ° C. The retention time of the aging treatment is adjusted according to the size (mass) of the mechanical part for immersion, although it can be from 30 to 1,000 minutes.

Example 1

Steels 2 to 10, 13, 14 and 16 to 27 of the chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace. Steels 2 to 10, 13, 14, 16, and 17 in Table 1 are steels with chemical compositions within the ranges prescribed in the present invention. On the other hand, steels 18 to 27 in Table 1 are steels with chemical compositions outside of the conditions prescribed by the present invention.

The ingots of the various steels were heated to 1,250 ° C, then hot forged to form 60 mm diameter steel bars. The hot forged steel rods were cooled to room temperature in the atmosphere. After that, they were heated at 1,250 ° C for 30 minutes, then visualizing the forge in the form of a piece, they were hot forged forming steel bars with a diameter of 35 mm, while the surface temperatures of the forged bars at the time of finishing they were maintained from 950 to 1,100 ° C. After hot forging, all rods were cooled to room temperature in the atmosphere. The cooling rate at the time of allowing the rods to cool in the atmosphere was then measured by burying a thermocouple near R / 2 of the hot forged steel rods under the above conditions ("R" indicates the radius of the rods steel), again raising the temperature to near the finishing temperature of the hot forge, then allowing the rods to cool in the atmosphere. The average cooling rate in the temperature region of 800 to 400 ° C after forging measured in this way was approximately 40 ° C / min.

For each steel, a portion of the hot-forged steel rods up to a diameter of 35 mm was then cooled to room temperature, in the state not subjected to an aging treatment (i.e. in the state in which it was cooled) , the two final parts of the steel rods were cut to 100 mm in length, then the test pieces of the remaining central parts were cut and the hardness was determined before the aging treatment.

On the other hand, for each steel, the rest of the hot-forged steel rods were subjected to aging keeping them at a temperature of 600 to 630 ° C for a period of time of 60 to 180 minutes, the two final parts of The steel rods were 100mm in length, then the test pieces were cut from the remaining central parts and the hardness was determined after the aging treatment. In addition, for each steel, the test pieces were cut from the steel rods and the absorption energy was determined in a Charpy impact, fatigue strength, and yield strength test after the aging treatment.

Hardness was measured in the following manner. First, a steel rod was cut, buried in resin so that the cutting surface was the measured surface, then polished with a mirror finish to prepare a test piece. Next, based on the "Vickers Hardness Test - Test Method" according to JIS Z 2244 (2009), 10 points were measured near the R / 2 Part of the measured surface ("R" indicating the radius) to determine the hardness with a test force of 9.8 N. The values of the 10 previous points were arithmetically averaged to obtain the Vickers hardness. When the hardness before aging treatment was 340 HV or less, mass production was considered to be industrially possible even with machined parts in various conditions. This was considered the objective. The test piece after hardness measurement was corroded with Nital and the structure was observed, so the structure of the test piece of each steel was also mainly bainitic with some mixed MA structures.

Hardness after aging treatment was evaluated by a Charpy impact test performed with a standard U-notched test piece with a notch depth of 2 mm and a lower notch radius of 1 mm. When the absorption energy at a test temperature of 20 ° C was 25J or more, it was considered high enough. This was considered the objective.

Fatigue resistance was determined by manufacturing an Ono rotary flex fatigue test specimen with a parallel part diameter of 8 mm and a length of 106 mm. That is, the above test piece was taken so that the center of the fatigue test piece becomes the R / 2 part of a steel rod. An Ono rotary flexion fatigue test was performed eight times in air at room temperature with the condition that the stress ratio was -1. The maximum value of the stress amplitude was taken as the fatigue resistance before the break occurred until the number of repetitions reached 1.0 x 107. When the fatigue resistance was 490 MPa or more, it was considered that fatigue resistance was high enough and this became the goal.

A tensile test was performed using a 14A tensile test specimen according to the JIS standard that had a parallel part $ 6, the yield limit was found at 0.2% by the compensation method using a prescribed plastic deformation of 0, 2%, and the elastic limit became the same. When the elastic limit was 800 MPa or more, it was considered high enough and became the target. Table 2 shows the results of the evaluations.

Table 2

As can be seen from Table 2, in the case of the "invention examples" of tests Nos. A2 to A10, A13, A14, A16 and A17 that have the chemical composition, structure and effective ratio of V of (amount of V dissolved / total amount of V) prescribed in the present invention and the values calculated by the formulas that use the contents of specific elements, the hardness before the aging treatment becomes 340 HV or less, while due to the aging treatment , the fatigue resistance becomes 510 MPa or more, the elastic limit becomes 815 MPa or more, and the absorption energy in the Charpy impact test becomes 36J or more. For this reason, all the target values are reached, so that both strength and toughness can be obtained after the aging treatment, and the hardness before the aging treatment is also low, so a drop in strength can be expected. machining and a longer service life.

In contrast to this, in the case of the "comparative examples" of tests Nos. B1 to B10, which are outside the requirements of the present invention, at least one of the specific actions cannot be obtained.

Industrial applicability

The aging-curable steel of the present invention can guarantee adequate hardness before aging treatment (340 HV or less) and promises a drop in machining resistance and a longer service life. Furthermore, if the aging-curable steel of the present invention is used, due to the aging treatment performed after machining, adequate fatigue strength (490 MPa or more), yield strength (800 MPa or more) can be ensured ) and impact value (25J or more). For this reason, the aging curable steel of the present invention can be used very suitably as a material for mechanical parts in automobiles, industrial machinery, construction machinery, etc.

Claims (5)

1. Steel hardenable by aging that comprises, in mass % , C: from 0.09 % to 0.20%, Si: from 0.01 % to 0.40%, Mn: from 1.5% to 2, 5%, S: 0.001% to 0.045%, Cr: more than 1.00% to 2.00%, Al: 0.001% to 0.060%, V: 0.22% to 0.55%, N: more than 0.0080% to 0.0170%, optionally Mo: 0.9% or less, optionally one or more of Cu: 0.3% or less and Ni: 0.3% or less, and also optionally one or more of Ca: 0.005% or less and Bi: 0.4% or less, and residues of Fe and impurities, with P and Ti in these impurities P: 0.03% or less and Ti: less than 0.005%, where an area fraction of the bainitic structures is 80% or greater, an effective ratio of V which is the amount of dissolved V / total amount of V, is 0.9 or more, and A chemical composition is one in which the F1 expressed by the following formula (1 ') is 1.00 or less and the F2 expressed by the following formula (2') is 0.30 or more: F1 = C 0.1 x Si 0.2 x Mn 0.15 x Cr 0.35 x V 0.2 x M or ..... (1 ') F2 = -4.5 x C Mn Cr - 3 , 5 x V -0.8 x Mo ..... (2 ') where, in the previous formulas (1') and (2 '), the symbols of the elements mean the content of the elements in% in mass. 2. The aging-curable steel according to claim 1, comprising Mo: 0.9% or less. 3. The aging-curable steel according to claim 1 or 2, comprising one or more of Cu: 0.3% or less and Ni: 0.3% or less. 4. Aging curable steel according to any one of claims 1 to 3, comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less. 5. A method of producing a part using an aging-curable steel, comprising: a forging step for heating the aging-curable steel according to any one of claims 1 to 4 at a temperature of 1,100 to 1,350 ° C during a period of time from 0.1 to 300 minutes, then it is forged so that the surface temperature after finishing the forging is 900 ° C or more, then it is cooled to room temperature while making the average speed cooling in a temperature region of 800 to 400 ° C is a speed of 10 to 90 ° C / min, a machining stage that machines the steel after forging, and an aging treatment step that maintains the steel after machining in the temperature region of 540 to 700 ° C for a period of time of 30 to 1,000 minutes.