KR20150110804A - Age-hardening steel - Google Patents

Abstract

The steel sheet according to any one of the above items (1) to (3), wherein the steel sheet contains 0.05 to 0.20% of C, 0.01 to 0.50% of Si, 1.5 to 2.5% of Si, 0.005 to 0.08% of S, 0.03 to 0.50% of Cr, 0.005 to 0.05% of Al, 0.25 to 0.50% : 0 to 1.0% of Cu, 0 to 0.3% of Cu, 0 to 0.3% of Ni, 0 to 0.005% of Ca and 0 to 0.4% of Bi and the balance of Fe and impurities, , Ti <0.005% and N <0.0080%, [C + 0.3Mn + 0.25Cr + 0.6Mo 0.68], [C + 0.1Si + 0.2Mn + 0.15Cr + 0.35V + 0.2Mo & ≥0.00] has a hardness of not more than 290 HV before aging, a hardening amount of not less than 25 HV by aging, a fatigue strength of not less than 350 MPa, a notched depth of 2 mm and a notched bottom radius of 1 mm The absorbed energy at 20 ° C after aging evaluated by the Charpy impact test carried out using a standard test piece is 16 J or more, which is very suitable as a material for mechanical parts.

Description

Aging Hardening Steel {AGE-HARDENING STEEL}

The present invention relates to an age hardening steel. More specifically, the present invention relates to a method for producing a steel sheet, which is formed into a predetermined shape by hot forging and cutting and then subjected to aging hardening treatment (hereinafter simply referred to as "aging treatment"), The present invention relates to a steel which can be suitably used as a material for manufacturing mechanical parts such as automobiles, industrial machines, construction machines,

Mechanical parts such as automobiles, industrial machines, and construction machines are required to have high fatigue strength from the viewpoints of high output of the engine and weight reduction aimed at improving fuel efficiency. If the steel is provided with a high fatigue strength, it can be easily achieved by raising the hardness of the steel using alloying elements and / or heat treatment. However, in general, the above-mentioned mechanical parts are formed by hot forging, and then finished to a predetermined product shape by cutting. As a result, the steel to be the material of the mechanical parts must have high machining strength and high machinability at the same time. Generally, the higher the hardness of the material, the more excellent the fatigue strength. On the other hand, during machining, the cutting resistance and the tool life tend to fall off as the hardness of the work is higher.

Thus, in order to achieve both fatigue strength and machinability, the hardness can be suppressed to a low level in a molding step requiring good machinability, and on the other hand, an aging treatment is then carried out to increase the hardness in the final product stage, A variety of techniques are disclosed.

For example, Patent Document 1 discloses the following age hardened steel.

Namely, it is preferable to contain 0.11 to 0.60% of C, 0.03 to 3.0% of Si, 0.01 to 2.5% of Mn, 0.3 to 4.0% of Mo, 0.05 to 0.5% of V and 0.1 to 3.0% of Cr in terms of mass% , Ni: 0.005 to 0.025%, Nb: 0.5% or less, Ti: 0.5% or less, Zr: 0.5% or less, Cu: 1.0% : 0.01 to 0.20%, Ca: 0.003 to 0.010%, Pb: 0.3% or less and Bi: 0.3% or less, the balance being Fe and inevitable impurities,

4C + Mn + 0.7Cr + 0.6Mo-0.2V? 2.5,

C? Mo / 16 + V / 5.7,

V + 0.15Mo? 0.4

After the rolling, forging, or solution treatment, the temperature between 800 ° C and 300 ° C is cooled at an average cooling rate of 0.05 to 10 ° C / second, and before the aging treatment, the bainite structure An area ratio of not less than 50%, and a hardness of not more than 40 HRC, and the hardness is higher than the hardness before aging by 7HRC or more by the aging treatment.

Patent Document 2 discloses the following bainite steel.

In other words, in terms of mass%, it is preferable that C: 0.14 to 0.35%, Si: 0.05 to 0.70%, Mn: 1.10 to 2.30%, S: 0.003 to 0.120%, Cu: 0.01 to 0.40% At least one selected from the group consisting of Ti: 0.001 to 0.100% and Ca: 0.0003 to 0.0100%, wherein the steel sheet contains 0.01 to 0.50%, Mo: 0.01 to 0.30% and V: 0.05 to 0.45% , The balance being Fe and inevitable impurities,

13 [C] +8 [Si] +10 [Mn] +3 [Cu] +3 [Ni] +22 [Mo] +11 [V]

5 [C] + [Si] +2 [Mn] +3 [Cr] +2 [Mo] +4 [V]

Cr] +1.1 [Mo] +0.2 [V] &lt; / = 3.1,

2.5? [C] + [Si] +4 [Mo] + 9 [V]

[C] ≥ [Mo] / 16 + [V] / 3

Quot; bainite steel &quot; is satisfied.

Patent Documents 3 and 4 disclose age hardenable steels having a predetermined chemical composition or structure. Patent Literatures 5 and 6 disclose a method for obtaining a steel component for mechanical structure, wherein a steel material is subjected to hot forging And then aging treatment is performed at a predetermined temperature within a predetermined temperature range.

Japanese Patent Application Laid-Open No. 2006-37177 Japanese Patent Application Laid-Open No. 2011-236452 International Publication No. 2010/090238 International Publication No. 2011/145612 International Publication No. 2012/161321 International Publication No. 2012/161323

However, if an attempt is made to obtain a high strength by depositing a fine secondary phase in the steel by aging treatment, the toughness of the steel deteriorates.

The steel with deteriorated toughness increases the susceptibility to picking. When the susceptibility to cutting is increased, the fatigue strength of the steel is more likely to be affected by fine surface scratches.

Further, in the case of a steel having a low toughness, once a fatigue crack is generated, the crack progresses faster and the fracture becomes large.

Further, in the case of attempting to cold-deform a deformation caused by hot forging, if the toughness of the steel becomes too low, it may become difficult to calibrate even by cold.

The steel disclosed in Patent Document 1 has a hardness of up to 40 HRC before the aging treatment and is very high in hardness, so it is difficult to secure the machinability, specifically, the cutting resistance is high and the tool life is short, Throw away. The steel disclosed as a specific example includes those having a hardness of not more than 40 HRC before aging, but they contain Mo of at least 1.4%, and toughness is not considered at all.

The steel disclosed in Patent Document 2 has a hardness of 300 HV or less before the aging treatment (after hot forging) while relatively decreasing the content of Mo by adjusting the content of the alloy element so as to satisfy the specific parameter formula, And the hardness is 300 HV or more. However, a method of increasing the toughness after the aging treatment is not sufficiently known.

Therefore, an object of the present invention is to provide a time-hardening steel satisfying <1> to <3> below.

<1> Hardness sufficiently low after hot forging in relation to cutting resistance and tool life. In the following description, the hardness after hot forging is referred to as &quot; hardness before aging treatment &quot;.

<2> Cured by aging treatment and capable of providing desired mechanical fatigue strength to machine parts.

<3> High toughness after aging treatment.

Specifically, the object of the present invention is to provide a method of manufacturing a steel sheet having a hardness of 290 HV or less before aging, a hardness of 25 or more at HV by aging treatment, a fatigue strength of 350 MPa or more to be described later, An aged hardening steel having an absorbed energy of 16 J or more at 20 캜 after aging evaluated by a Charpy impact test using a standard test piece with a U notch with a depth of 2 mm and a notched bottom radius of 1 mm was used to measure the Mo content in the steel to 1.0 mass % Or less.

In order to solve the above problems, the present inventors conducted the investigation using a steel whose chemical composition was variously adjusted. As a result, the following findings (a) to (c) were obtained.

(a) V has a precipitation peak of the carbide at a temperature of 750 to 700 占 폚 at the time of cooling from a high temperature. For example, in a steel containing 0.3% by mass of V and 0.1% by mass of C, it is relatively easy to suppress precipitation in hot forging, since V is not precipitated until around 850 ° C once it is solved in the matrix.

(b) The carbide of (V) tends to precipitate at the phase interface when the austenite is transformed into ferrite. Therefore, when a large amount of pro-eutectoid ferrite is produced during cooling after hot forging, the carbide of V precipitates at the phase interface and the amount of solid solution V decreases, so that the amount of solid solution V necessary for curing Can not.

(c) Therefore, in order to secure the solid solution V at the stage before the aging treatment, it is necessary to convert the bainite to the columnar phase in the structure after hot forging.

Therefore, the inventors of the present invention investigated conditions for stably increasing the area ratio of the bainite of the structure by varying the chemical composition of the steel to a steel containing 0.25 mass% or more of V. In addition, the aging hardening ability when the steel was aged was examined. As a result, the following findings (d) to (f) were obtained.

(d) The structure after hot forging has a close correlation with the contents of C, Mn, Cr and Mo. That is, if the content of the element is controlled so that the value represented by the expression (1) representing the hardenability index described later becomes a specific range, a large amount of pro-eutectoid ferrite which is detrimental to securing the solid solution V is suppressed. As a result, a structure having bainite as a main phase, that is, a bainite structure having an area ratio of 70% or more can be easily obtained, and a sufficient amount of solid solution V can be secured.

(e) When the content of C, Mn, Cr and Mo satisfies the condition that the expression (1) described in (d) is in the specific range, the hardness before the aging treatment So that the cutting resistance at the time of cutting increases and the tool life is sometimes lowered.

(f) On the other hand, if the content of C, Si, Mn, Cr, V and Mo is controlled such that the value represented by the formula (2) described below becomes a specific range, the hardness before the aging treatment can be kept low.

Therefore, the present inventors have also found that when a steel containing 0.25 mass% or more of V and containing C, Si, Mn, Cr, Mo and V satisfies the conditions described in (d) and (f) After the forging, the aging treatment was carried out. Under the condition that the absorbed energy at 20 ° C after aging evaluated by the Charpy impact test using the standard notched U-notched test piece having a notch depth of 2 mm and a notched bottom radius of 1 mm was not less than 16 J I investigated. As a result, the following findings (g) to (i) were obtained.

(g) Elements which degrade toughness after aging are C, V, Mo and Ti. Among them, Ti forms TiN and / or TiC by bonding with N and / or C. When TiN and / or TiC are precipitated, the fatigue strength may be increased, and the toughness is greatly reduced. The action force for deteriorating the toughness of Ti is very large as compared with V and Mo which are the same precipitation strengthening elements. As a result, Ti has to be extremely limited. C forms a cementite in the steel, which can be a starting point of cleavage failure. Even when a steel containing an excessive amount of V or Mo is excessively treated with respect to the amount of C, some of the cementites remain. V and Mo also precipitate carbide on the same crystal face of the matrix by the aging treatment, thereby promoting the progress of cleavage fracture and deteriorating toughness. Therefore, in order to increase the toughness, it is necessary to reduce the contents of C, V and Mo.

(h) In order to increase the toughness, it is necessary to make the bainite structure finer. In order to make the bainite structure finer, the transformation temperature of bainite from the austenite may be lowered. In order to lower the transformation temperature of bainite, the content of Mn and Cr, which lower the bainite transformation start temperature, may be increased.

(i) From the above, in order to impart sufficient toughness to the age hardening steel having high strength, the content of C, Mn, Cr, V, Is controlled to be not less than a specific value and the content of Ti is required to be not more than a specific value so that inclusions and precipitates harmful to toughness are not included in the steel.

The present invention has been made on the basis of the above knowledge, and the gist of the present invention resides in the age hardening steel described below.

(1) A ferritic stainless steel comprising, by mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.5 to 2.5%, S: 0.005 to 0.08%, Cr: 0.03 to 0.50% , 0 to 0.3% of Ni, 0 to 0.3% of Ca, 0 to 0.005% of Ca, 0 to 0.4% of Bi, 0.2 to 0.50% of Mo, 0 to 1.0%

The balance being Fe and impurities,

P, Ti and N in the impurities are not more than 0.03% of P, less than 0.005% of Ti and less than 0.0080% of N,

The age hardening steel having a chemical composition represented by the following formula (1): 0.68 or more, F2 represented by the formula (2): 0.85 or less, and F3 represented by the formula (3): 0.00 or more.

F1 = C + 0.3Mn + 0.25Cr + 0.6Mo ... (One)

F2 = C + 0.1Si + 0.2Mn + 0.15Cr + 0.35V + 0.2Mo ... (2)

F3 = -4.5C + Mn + Cr-3.5V-0.8Mo ... (3)

The symbol of the element in the above formulas (1) to (3) means the content by mass% of the element.

(2) The age-hardening steel according to (1) above, wherein the chemical composition comprises, by mass%, at least one element selected from the following elements <1> to <3>.

&Lt; 1 > Mo: 0.05 to 1.0%

&Lt; 2 > Cu: 0.1 to 0.3% and Ni: 0.1 to 0.3%, and

&Lt; 3 > Ca: 0.0005 to 0.005% and Bi: 0.03 to 0.4%

(3) The age-hardening steel according to (1) or (2) above, wherein the main phase is bainite and the average block size of the bainite is 15 to 60 탆.

(4) The age-hardening steel according to any one of (1) to (3), wherein the hardness is 290 HV or less.

The age hardening steel of the present invention has a hardness before aging of 290 HV or less. Furthermore, when the age hardening steel of the present invention is used, hardness of not less than 25 at HV is hardened by aging treatment after cutting, fatigue strength of not less than 350 MPa, notch depth of 2 mm and notch bottom radius of 1 mm Excellent absorption of energy at 20 deg. C after aging evaluated by Charpy impact test using standard test specimens of not less than 16 J can be ensured. As a result, the age hardening steel of the present invention can be suitably used as a material for mechanical parts such as automobiles, industrial machines, and construction machines.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing a shape of a tensile compression type fatigue test piece of one axis used in the embodiment. Fig. The numerical values in the figure indicate dimensions (unit: mm).

Hereinafter, each of the requirements of the present invention will be described in detail. In addition, "%" of the content of each element means "% by mass".

C: 0.05 to 0.20%

C is an important element in the present invention. C combines with V to form carbides and strengthen the steel. However, when the content of C is less than 0.05%, the carbide of V is hardly precipitated, so that a desired strengthening effect can not be obtained. On the other hand, if the content of C is too large, C, which is not bonded to V or Mo, increases the amount of Fe and carbide (cementite) to be formed, thereby deteriorating toughness. Therefore, the content of C was set to 0.05 to 0.20%. The content of C is preferably 0.08% or more, more preferably 0.10% or more. The content of C is preferably 0.18% or less, more preferably 0.16% or less.

Si: 0.01 to 0.50%

Si is useful as a deoxidizing element at the time of steelmaking, and has an effect of improving the strength of steel by being incorporated into the matrix. In order to sufficiently obtain such an effect, the Si content needs to be 0.01% or more. However, if the Si content is excessive, the hot workability of the steel is lowered, and the hardness before the aging treatment is increased. Therefore, the content of Si is set to 0.01 to 0.50%. The content of Si is preferably 0.06% or more. The content of Si is preferably 0.45% or less, more preferably 0.35% or less.

Mn: 1.5 to 2.5%

Mn has an effect of improving hardenability and making bainite the main phase of the structure. Further, by lowering the bainite transformation temperature, it also has an effect of increasing the toughness of the matrix by making the bainite structure finer. Further, Mn has the function of forming MnS in the steel and improving the debris processability at the time of cutting. In order to sufficiently obtain such an effect, the content of Mn should be at least 1.5%. However, since Mn is an element that is liable to segregate at the time of solidification of steel, if the content is too large, it is inevitable that the variation in hardness in parts after hot forging becomes large. Therefore, the content of Mn was set to 1.5 to 2.5%. The content of Mn is preferably 1.6% or more, and more preferably 1.7% or more. The content of Mn is preferably 2.3% or less, more preferably 2.1% or less.

S: 0.005 to 0.08%

S is required to be contained in an amount of 0.005% or more, because it combines with Mn in the steel to form MnS and improves the disposal property at the time of cutting. However, when the content of S is increased, coarse MnS is increased to deteriorate toughness and fatigue strength. Therefore, the content of S was set to 0.005 to 0.08%. The content of S is preferably 0.01% or more. The content of S is preferably 0.05% or less, more preferably 0.03% or less.

Cr: 0.03-0.50%

Cr, like Mn, has the effect of increasing the hardenability and bainite the main phase of the structure. Further, by lowering the bainite transformation temperature, it also has an effect of increasing the toughness of the matrix by making the bainite structure finer. However, if the content of Cr exceeds 0.50%, the hardenability becomes large, and the hardness before the aging treatment may exceed 290 HV depending on the size and part of the component. Therefore, the content of Cr was set to 0.03 to 0.50%. The Cr content is preferably 0.05% or more, more preferably 0.15% or more.

Al: 0.005 to 0.05%

Al is an element having a deoxidizing action, and it is necessary to set the Al content to 0.005% or more in order to obtain this effect. However, when Al is contained excessively, a coarse oxide is produced, and toughness is lowered. Therefore, the content of Al is set to 0.005 to 0.05%. The content of Al is preferably 0.04% or less.

V: 0.25 to 0.50%

V is the most important element in the steel of the present invention. V has an effect of increasing the fatigue strength by forming fine carbides in combination with C at aging treatment. Further, when Mo is contained in the steel, V is precipitated in combination with Mo by aging treatment, and the effect of increasing the age hardening ability is further enhanced. In order to obtain such an effect sufficiently, the content of V must be 0.25% or more. However, if the content of V is excessively high, the non-solidified carbonitride tends to remain in the heating at the time of hot forging, and toughness is lowered. In addition, if the content of V is excessive, the hardness before the aging treatment may increase. Therefore, the content of V was set to 0.25 to 0.50%. The content of V is preferably 0.45% or less, more preferably 0.40% or less. The content of V is preferably 0.27% or more.

Mo: 0 to 1.0%

Like Mo, Mo has a relatively low carbide precipitation temperature and is an element that is easy to use for aging hardening. Mo has an effect of enhancing hardenability, making bainite the main phase of the structure after hot forging, and increasing the area ratio thereof. Mo has a function of forming a carbide complex with V and increasing the age hardening ability. For this reason, Mo may be added as needed. However, since Mo is a very expensive element, the production cost of steel is increased and the toughness is lowered as the content is increased. Therefore, when Mo is contained, the amount is made 1.0% or less. The content of Mo is preferably 0.50% or less, more preferably 0.40% or less, still more preferably 0.30% or less.

On the other hand, in order to stably obtain the above effect of Mo, the content thereof is preferably 0.05% or more, more preferably 0.10% or more.

Both Cu and Ni have an action of increasing the fatigue strength. For this reason, when it is desired to obtain a larger fatigue strength, these elements may be contained in the range described below.

Cu: 0 to 0.3%

Cu has an effect of improving the fatigue strength. For this reason, Cu may be contained as needed. However, if the content of Cu increases, the hot workability decreases. Therefore, when Cu is contained, the content thereof is set to 0.3% or less. The content of Cu is preferably 0.25% or less.

On the other hand, in order to stably obtain the above effect of increasing the fatigue strength of Cu, the content thereof is preferably 0.1% or more.

Ni: 0 to 0.3%

Ni has an action of improving the fatigue strength. Ni also has an effect of suppressing the deterioration of hot workability caused by Cu. For this reason, Ni may be added as needed. However, if the content of Ni is increased, the above effect is saturated in addition to the cost. Therefore, when Ni is contained, the amount thereof is set to 0.3% or less. The content of Ni is preferably 0.25% or less.

On the other hand, in order to stably obtain the above effect of Ni, its content is preferably 0.1% or more.

The above-mentioned Cu and Ni may be contained in any one of them or in a combination of two kinds. , The total content of the above elements may be 0.6% in the case where the contents of Cu and Ni are respectively the upper limit values.

Both Ca and Bi have an effect of lengthening the tool life at the time of cutting. Therefore, when it is desired to further extend the tool life, these elements may be contained in the range described below.

Ca: 0 to 0.005%

Ca has the function of longevity of tool life. For this reason, Ca may be added as needed. However, when the Ca content is increased, a coarse oxide is formed and the toughness is deteriorated. Therefore, when Ca is contained, the content thereof is 0.005% or less. The content of Ca is preferably 0.0035% or less.

On the other hand, in order to stably obtain the above-mentioned effect of lengthening the tool life of Ca, the content of Ca is preferably 0.0005% or more.

Bi: 0 to 0.4%

Bi has an effect of reducing the cutting resistance and prolonging tool life. Therefore, Bi may be added as needed. However, if the Bi content is increased, the hot workability is lowered. Therefore, when Bi is contained, the amount thereof is set to 0.4% or less. The content of Bi is preferably 0.3% or less.

On the other hand, in order to stably obtain the effect of lengthening the tool life of Bi, the content of Bi is preferably 0.03% or more.

The Ca and Bi may be contained in any one of them or in a combination of two kinds. , The content of these elements may be 0.405% in the case where the contents of Ca and Bi are each the upper limit value, but it is preferable that the total content is 0.3% or less.

The age hardening steel of the present invention is characterized in that P, Ti and N in the impurity are composed of 0.03% or less of P, less than 0.005% of Ti and less than 0.0080% of N, Further, the above-mentioned F1 is a steel having a chemical composition of F1 of 0.68 or more, F2 of the formula (2) of 0.85 or less, and F3 of the formula (3) of 0.00 or more.

The impurities are those which are incorporated from an ore, a scrap or a manufacturing environment as a raw material when industrially producing a steel material.

P: not more than 0.03%

P is an element which is contained as an impurity and which is undesirable in the present invention. In other words, P segregates at the grain boundaries to deteriorate toughness. Therefore, the content of P was 0.03% or less. The content of P is preferably 0.025% or less.

Ti: less than 0.005%

Ti is contained as an impurity and is a particularly undesirable element in the present invention. In other words, Ti bonds with N and / or C to form TiN and / or TiC, resulting in deterioration of toughness. Particularly, when the content is 0.005% or more, the toughness deteriorates. Therefore, the content of Ti is made less than 0.005%. In order to ensure good toughness, the content of Ti is preferably 0.0035% or less.

N: less than 0.0080%

N is contained as an impurity, and in the present invention, it is an undesirable element which fixes V as a nitride. In other words, V precipitated as nitride does not contribute to the age hardening, so that the content of N should be decreased in order to suppress the precipitation of nitride. For this purpose, the content of N should be less than 0.0080%. The content of N is preferably 0.0070% or less, more preferably 0.0060% or less.

F1: More than 0.68

The age hardening steel of the present invention,

F1 = C + 0.3Mn + 0.25Cr + 0.6Mo ... (One)

F1 must be 0.68 or more.

As already described, the symbol of the element in the above formula (1) means the content by mass% of the element.

F1 is an index of hardenability. When the amount of each alloy element contained in the steel is within the above-mentioned range, if F1 satisfies the above-mentioned condition, the structure after hot forging becomes bainite as the main phase.

When F1 is less than 0.68, pro-eutectoid ferrite enters the structure after hot forging and carbide of V precipitates at the upper interface, so that the hardness before the aging treatment increases and the age hardening ability decreases.

F1 is preferably 0.70 or more, more preferably 0.72 or more. Further, F1 is preferably 1.0 or less, more preferably 0.98 or less.

F2: Not more than 0.85

The age hardening steel of the present invention,

F2 = C + 0.1Si + 0.2Mn + 0.15Cr + 0.35V + 0.2Mo ... (2)

Should be 0.85 or less.

As already described, the symbol of the element in the above formula (2) means the content by mass% of the element.

F2 is an index showing the hardness before the aging treatment. If the age-hardening steel of the present invention satisfies the above-mentioned condition of F1, the hardness before the aging treatment becomes too high, the cutting resistance at the time of cutting becomes large, and the tool life becomes short.

That is, when F2 exceeds 0.85, the hardness before the aging treatment becomes too high. In order to keep the hardness before the aging treatment to 290 HV or less, it is necessary to satisfy the condition of F2 after setting the content of each alloy element within the specified range and satisfying the condition of F1.

F2 is preferably 0.82 or less, more preferably 0.80 or less. F2 is preferably 0.55 or more, more preferably 0.60 or more.

F3: 0.00 or more

The age hardening steel of the present invention,

F3 = -4.5C + Mn + Cr-3.5V-0.8Mo ... (3)

Should be 0.00 or more.

As already described, the symbol of the element in the above formula (3) means the content by mass% of the element.

F3 is an index showing the toughness after the aging treatment. That is, only when the conditions of F1 and F2 are satisfied, the toughness after the aging treatment is lowered, and the target toughness can not be secured in some cases.

That is, when F3 is less than 0.00, the toughness after the aging treatment decreases. In order to secure the aimed toughness, it is necessary to satisfy the condition of F3 after the content of each of the above-described alloying elements is within a specified range and the condition of F1 and the condition of F2 are satisfied.

F3 is preferably 0.01 or more.

Further, if F1 is 0.68 or more and F2 is 0.85 or less, there is no need to particularly limit the upper limit of F3.

The age hardening steel of the present invention preferably has an average block size of bainite of 15 to 60 mu m. In the present invention, the &quot; block &quot; of bainite refers to a region surrounded by a boundary having an azimuth difference of 15 degrees or more when the azimuthal analysis of the structure is performed by the EBSD (Electron Back Scatter Diffraction) method. The larger the average block size of bainite is, the lower the hardness before aging, and therefore the better machinability can be obtained. On the other hand, if the average block size is too large, the toughness is lowered. The average block size is more preferably 20 m or more. The average block size is more preferably 45 μm or less, and further preferably 30 μm or less.

The method for producing the age hardening steel of the present invention is not particularly limited, and may be adjusted by a solvent and chemical composition by a general method.

Hereinafter, an example of a method for manufacturing mechanical parts such as automobiles, industrial machines, and construction machines using the age-hardening steel of the present invention produced as described above as a material is described.

First, a material to be provided for hot forging (hereinafter referred to as &quot; hot forging material &quot;) is prepared from a steel whose chemical composition is adjusted to the above-described range.

The hot forging material may be a billet obtained by crushing and rolling an ingot, a billet obtained by crushing and rolling a continuous casting material, or a bar steel obtained by hot rolling or hot forging the billet.

Then, the hot forging material is subjected to hot forging and further cutting to finish a predetermined part shape.

The hot forging may be performed by, for example, heating the hot forging material at 1100 to 1350 캜 for 0.1 to 300 minutes, forging the surface material after the finish forging to 900 캜 or higher, And the average cooling rate in a temperature range of 400 ° C to 10 ° C / min (0.2 to 1.5 ° C / sec) is cooled to room temperature. After cooling in this way, further cutting is carried out to finish with a predetermined part shape.

The average cooling rate in the temperature range of 800 to 400 ° C becomes smaller as the average block size of the bainite increases. The lower limit of the average cooling rate is preferably 20 占 폚 / min, and the upper limit is preferably 80 占 폚 / min.

Finally, aging treatment is performed to obtain mechanical parts such as automobiles, industrial machines, and construction machines having desired characteristics.

The aging treatment is carried out at a temperature of, for example, 540 to 700 ° C, preferably 560 to 680 ° C. The holding time of the aging treatment is appropriately adjusted by the size (mass) of the mechanical parts, for example, 30 to 1000 minutes.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

Strengths A to AG of the chemical compositions shown in Tables 1 and 2 were dissolved in a 50 kg vacuum melting furnace.

The steels A to W in Tables 1 and 2 are steels whose chemical composition is within the range specified in the present invention. On the other hand, the steels X to AG in Table 2 are steels whose chemical composition deviates from the conditions specified in the present invention.

&Quot; &lt; 0.001 &quot; in the column of Ti indicates that the content of Ti as an impurity is less than 0.001%.

[Table 1]

[Table 2]

The ingots of each steel were heated to 1250 ° C and hot-forged into bars having a diameter of 60 mm. Each hot-forged steel bar was once cooled in the air and cooled to room temperature. Thereafter, it was further heated at 1250 占 폚 for 30 minutes to assume forging in a component shape, and the surface temperature of the forging material at the time of finishing was 950 to 1100 占 폚 and hot forging was performed in a bar having a diameter of 35 mm. After hot forging, all were cooled in air and cooled to room temperature. The cooling rate at the time of cooling in air was measured by putting a thermocouple in the vicinity of R / 2 ("R" represents the radius of the bar) of the bar steel hot forged under the above conditions and raising the temperature to a temperature near the finish temperature of hot forging , And then measured by air cooling in air. The average cooling rate in the temperature range of 800 to 400 占 폚 after forging measured in this manner was about 40 占 폚 / min (0.7 占 폚 / sec).

For each test number, a part of the bar steel which had been finished with the above-mentioned diameter of 35 mm by hot forging and then cooled to room temperature was cut by 100 mm at both ends of the bar steel without aging treatment (that is, in the cooling state) The test pieces were cut out from the remaining central portion to examine the hardness before the aging treatment and the area ratio of bainite in the structure.

On the other hand, for each test number, the remainder of the hot-forged steel bar is subjected to an aging treatment in which the temperature is maintained at 610 to 630 DEG C for 60 to 180 minutes, and the both ends of the bar are cut off by 100 mm, , And the hardness after the aging treatment was examined. For each test number, the test piece was cut out from the bar, and the absorption energy and the fatigue strength in the Charpy impact test after the aging treatment were examined.

Hardness was measured in the following manner. First, the test piece was prepared by inserting resin so as to cross the bar steel and making the cut surface to be the inspection surface, and mirror polished. Subsequently, according to "Vickers hardness test-test method" in JIS Z 2244 (2009), ten points near the R / 2 part ("R" indicates radius) of the surface to be inspected were tested at a test force of 9.8 N And the hardness was measured. The values of the ten points were arithmetically averaged to determine Vickers hardness. When the hardness before the aging treatment was 290 HV or less, it was judged that the hardness was sufficiently low and aimed at this. Further, when the difference between the hardness after the aging treatment and the hardness at HV before the aging treatment (hereinafter referred to as &quot;? HV &quot;) was 25 or more, it was judged that the curing amount was sufficiently high.

The area ratio of the bainite of the tissue was measured in the following manner. The test piece, which was embedded in the resin used for the hardness measurement and mirror-polished, was etched away. For the test piece after etching, the structure was photographed at a magnification of 200 times using an optical microscope. The area ratio of the bainite was measured by image analysis from the photographed photograph. In the case where the area ratio of bainite is 70% or more, it is judged that the structure is sufficiently bainitized.

The toughness is judged to be sufficiently high when the absorbed energy at 20 DEG C after aging evaluated by the Charpy impact test using a standard test piece with U notched with a notch depth of 2 mm and a notched bottom radius of 1 mm is 16 J or more, .

Fatigue strength was measured by taking a tensile compression type fatigue test piece of one axis. Namely, a flat fatigue test piece having a diameter of 3.4 mm and a length of 12.7 mm in the parallel portion shown in Fig. 1 was taken parallel to the forging direction from the R / 2 portion of the bar (in the longitudinal direction of the bar) , In the atmosphere, at a stress ratio of 0.05, and at a test speed of 10 Hz. Under the above conditions, the maximum stress that the stress portion did not break in the number of repetitions of 10 ⁷ times was regarded as the fatigue strength. When the fatigue strength is 350 MPa or more, it is judged that the fatigue strength is sufficiently high.

Table 3 shows the results of the above investigation. In addition, when the area ratio of bainite is 70% or more, the target is achieved, and when the area ratio of bainite is less than 70%, the target is indicated as &quot;? &Quot; In Table 3, &quot; absorbed energy in Charpy impact test &quot; is expressed as &quot; Charpy absorbed energy &quot;.

[Table 3]

As is evident from Table 3, in the case of "Inventive Example" of Test Nos. A1 to A23 having the chemical composition specified in the present invention, the hardness before the aging treatment was 290 HV or less, the hardness was hardened to 25 or more in HV by the aging treatment , The fatigue strength is not less than 350 MPa, the absorption energy in the Charpy impact test is not less than 16 J, and the strength and toughness after aging are both achieved. Further, since the hardness before the aging treatment is low, it is expected that the cutting resistance can be reduced and the tool life can be lengthened.

On the other hand, in the case of "Comparative Example" of Test Nos. B1 to B10 deviating from the specification of the present invention, the target performance is not obtained.

The test number B1 was such that the F1 of the steel X used was small and deviated from the provisions of the present invention, so that quenching hardness was small, pro-eutectoid ferrite exceeding 30% in area ratio was generated, and the area ratio of bainite was less than 70% . As a result, aging hardly occurs and the fatigue strength after the aging treatment is low.

Test No. B2 has a hardness of 311 HV before aging because F2 of the steel Y used is deviated from the specification of the present invention.

Test No. B3 has a small absorbed energy in the Charpy impact test after the aging treatment and is poor in toughness because F3 of the steel Z used is smaller than the specification of the present invention.

In Test No. B4, the F3 of the steel AA used satisfies the requirements of the present invention, but the amount of C deviates from the specification of the present invention so much that the toughness deterioration is remarkable. As a result, the absorption energy in the Charpy impact test after the aging treatment is small and the toughness is poor.

In Test No. B5, pro-eutectoid ferrite was precipitated because the Mn content of the steel AB used was too low, deviating from the specification of the present invention, and the bainite portion of the structure was not sufficiently refined. As a result, aging hardly occurs and the fatigue strength after the aging treatment is low. Also, the absorption energy in the Charpy impact test is small and the toughness is poor.

In Test No. B6, the amount of S of the steel AC used is too much deviated from the specification of the present invention, so that coarse MnS is increased, and deterioration of toughness is remarkable. As a result, the absorption energy in the Charpy impact test after the aging treatment is small and the toughness is poor. Also, the fatigue strength is low.

Test No. B7 has a small amount of V carbide precipitated by the aging treatment since the V content of the steel AD used is too low to deviate from the specification of the present invention. As a result, aging hardly occurs and the fatigue strength after the aging treatment is also low.

Test No. B8 shows that the amount of Ti in the steel AE used is too high, deviating from the specification of the present invention, so that the coarse TiN is increased and the toughness deterioration is remarkable. As a result, the absorption energy in the Charpy impact test after the aging treatment is small and the toughness is poor.

In Test No. B9, since the N amount of the strong AF used is too high deviating from the specification of the present invention, the nitride of V is precipitated during hot forging. As a result, aging hardly occurs and the fatigue strength after the aging treatment is also low.

In test No. B10, the nitride of V precipitates during the hot forging because the N content of the steel AG used is higher than the specification of the present invention. As a result, the fatigue strength after the aging treatment is low. However, since the amount of N is smaller than that of the strong AF, the number of nitrides of V precipitated during hot forging is small, and the age hardening progresses more than the strong AF.

Example 2

A part of bars having a diameter of 60 mm of the steel P and steel Y produced by hot forging in Example 1 and cooling to room temperature was cut out. The cut bar steel was further heated at 1250 占 폚 for 30 minutes to assume forgings in the form of a component and the surface temperature of the forged member at the time of finishing was 950 to 1100 占 폚 and hot forging was performed in a bar having a diameter of 35 mm. After the hot forging, the steel sheet was cooled to a temperature of 400 DEG C or less at various cooling rates by air cooling in the air or by using a blower and a mist.

Each test number was subjected to hot forging to have a diameter of 35 mm and then cooled to a temperature of 400 ° C. or less by using a blower and a mist and then the hardness before the aging treatment Respectively.

On the other hand, for each test number, the remainder of the hot-forged steel bar was aged at 630 캜 for 60 minutes. The hardness after the aging treatment, the absorption energy in the Charpy impact test, the fatigue strength, and the block size of the bainite structure were examined using the test pieces collected from the bars subjected to aging treatment.

The hardness before the aging treatment, the hardness after the aging treatment, the absorption energy in the Charpy impact test, and the fatigue strength were examined under the same conditions as in Example 1. These target values were the same as those in the first embodiment.

The block size of the bainite structure was measured in the following manner. The test piece embedded in the resin used for the hardness measurement was again polished using colloidal silica. For the polished test pieces, orientation analysis of the structure was carried out by the EBSD method. A region surrounded by a boundary having an azimuth difference of 15 degrees or more is defined as &quot; block &quot;, and the area of each block is obtained by image analysis.

The interface of the blocks is a complex shape with irregularities. Thereby, when the observation surface of the tissue is formed so as to cut off the vicinity of the concave-convex end portion of the block, there is a case where it is observed that there is another block nested in one block. In this case, the measurement accuracy of the area of the block is lowered. In order to eliminate such an influence, when a block is completely contained in another block on the sectional view, it is regarded as a single block, and the smaller contained block is ignored and only the larger block The area was determined.

The diameter of a circle having the same area is defined as the size of the block for each block in which the area is measured in this way. The average block size was calculated from the size of each block in the area of 30000 탆 ² analyzed by the EBSD method.

When calculating the average block size, the weight of each block was evaluated by the area of the block. That is, for each of the n blocks 1 to n in the analysis area, the respective sizes are D1, D2, ... , Dn (占 퐉), the respective areas are S1, S2, ... , And Sn (mu m ² ), the average block size was (D1 x S1 + D2 x S2 + ... + Dn x Sn) / 30000. The average block size was aimed at 15 to 60 탆.

Table 4 shows the results of the above investigations. Test number C1 is the test number A16 of Table 3. The cooling rate shown in Table 4 is an average cooling rate in a temperature range of 800 to 400 占 폚 under cooling after hot forging a bar having a diameter of 35 mm. The average cooling rate was measured in the same manner as in Example 1.

[Table 4]

As is apparent from Table 4, in the case of the &quot; present invention &quot; of Test Nos. C1 to C6 having the chemical compositions defined in the present invention, the average block size of the bainite is within the target range of 15 to 60 mu m, Was 290 HV or less. As a result, a good machinability can be expected. The hardness is hardened to 25 or more by HV by the aging treatment, the fatigue strength is 350 MPa or more, and the absorption energy in the Charpy impact test is 16 J or more to attain the target, and the strength and toughness after the aging treatment are compatible . In Test Nos. C1 to C6, the area ratio of bainite before the aging treatment was 70% or more, and the target was achieved.

In the present invention, Test Nos. C1 to C4 show the average cooling rate (10 to 90 占 폚 / min, that is, 0.2 to 1.5 占 폚 / sec ). Test numbers C5 and C6 showed an average cooling rate faster than an example of this average cooling rate. Comparing test numbers C1 to C6, the slower the average cooling rate, the greater the average block size of the bainite. Further, it is known that the larger the average block size of bainite is, the lower the hardness before the aging treatment, and the better the machinability can be expected.

On the other hand, in the case of the &quot; comparative example &quot; of the test number D1 deviating from the specification of the present invention, the target performance is not obtained. That is, in Test No. D1, F2 of the steel Y used was deviated from the specification of the present invention. As a result, the average block size of bainite is as small as 9.9 mu m and the hardness before aging becomes 320 HV, which is hard. Therefore, it is considered that the machinability is inferior. Also, the absorption energy in the Charpy impact test is as small as 12J, and the toughness is poor.

Industrial availability

The age hardening steel of the present invention has a hardness of 290 HV or lower before the aging treatment and can be expected to reduce the cutting resistance and increase the service life of the tool. Furthermore, when the age hardening steel of the present invention is used, hardness of not less than 25 at HV is hardened by aging treatment after cutting, fatigue strength of not less than 350 MPa, notch depth of 2 mm and notch bottom radius of 1 mm Excellent absorption of energy at 20 deg. C after aging evaluated by Charpy impact test using standard test specimens of not less than 16 J can be ensured. As a result, the age hardening steel of the present invention can be suitably used as a material for mechanical parts such as automobiles, industrial machines, and construction machines.

Claims (4)

Si: 0.01 to 0.35%, Mn: 1.5 to 2.05%, S: 0.005 to 0.08%, Cr: 0.03 to 0.50%, Al: 0.005 to 0.05%, V: 0 to 0.3% of Ni, 0 to 0.005% of Ca, 0 to 0.4% of Bi, 0.2 to 0.50% of Mo, 0 to 1.0% of Mo, 0 to 0.3%
The balance being Fe and impurities,
P, Ti and N in the impurities are not more than 0.03% of P, less than 0.005% of Ti and less than 0.0080% of N,
The age hardening steel having a chemical composition represented by the following formula (1): 0.68 or more, F2 represented by the formula (2): 0.85 or less, and F3 represented by the formula (3): 0.00 or more.
F1 = C + 0.3Mn + 0.25Cr + 0.6Mo ... (One)
F2 = C + 0.1Si + 0.2Mn + 0.15Cr + 0.35V + 0.2Mo ... (2)
F3 = -4.5C + Mn + Cr-3.5V-0.8Mo ... (3)
The symbol of the element in the above formulas (1) to (3) means the content by mass% of the element. The method according to claim 1,
The age hardening steel according to any one of <1> to <3>, wherein the chemical composition contains, as mass%, at least one element selected from the following elements <1> to <3>.
&Lt; 1 > Mo: 0.05 to 1.0%
&Lt; 2 &gt; Cu: 0.1 to 0.3% and Ni: 0.1 to 0.3%, and
&Lt; 3 &gt; Ca: 0.0005 to 0.005% and Bi: 0.03 to 0.4% The method according to claim 1 or 2,
Wherein the main phase is bainite and the average block size of the bainite is 15 to 60 占 퐉. The method according to any one of claims 1 to 3,
An age hardening steel having a hardness of 290 HV or less.

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