TW201400627A - Steel sheet for soft-nitriding and method for manufacturing the same - Google Patents

Abstract

A steel sheet for soft-nitriding with excellent formability and a fatigue tolerance property is provided. The steel sheet for soft-nitriding includes the following composition and a complex structure as follows. The composition contains 0.05 mass% or more and 0.10 mass% or less of C, 0.5 mass% or less of Si, 0.7 mass% or more and 1.5 mass% or less of Mn, 0.05 mass% or less of P, 0.01 mass% or less of S, 0.01 mass% or more and 0.06 mass% or less of Al, 0.5 mass% or more and 1.5 mass% or less of Cr, 0.03 mass% or more and 0.30 mass% or less of V, and 0.005 mass% or less of N. A ratio of contents of V at solid solution and contents of V is more than 0.50, and the remainder contains Fe and inevitable impurities. The complex structure contains a ferrite and a pearlite.

Description

Steel sheet for soft nitriding treatment and method for producing same

The present invention relates to a steel sheet for soft nitriding treatment which is suitable for a mechanical structural component requiring fatigue strength or wear resistance, such as a transmission component of an automatic vehicle, and particularly excellent in formability before a soft nitriding treatment, and A steel sheet for soft nitriding treatment excellent in fatigue resistance after soft nitriding treatment and a method for producing the same.

Fatigue strength or wear resistance is required for a mechanical structural component used in a state in which a transmission component for an automatic vehicle is continuously subjected to a stress load for a long period of time. Therefore, these mechanical structural parts are usually manufactured by subjecting a steel material to a desired part shape and then performing a surface hardening heat treatment. When the surface hardening heat treatment is performed, the steel surface is hardened, and the compressive residual stress is introduced into the steel surface layer portion, so that the fatigue strength and wear resistance of the part are improved.

As a typical treatment of the surface hardening heat treatment, a carburizing treatment and a nitriding treatment are mentioned. Carburizing is a process in which steel is heated to a temperature above the A ₃ metamorphic point, and carbon is diffused and infiltrated (carburized) to the surface portion of the steel, usually by directly carburizing the steel at a high temperature. Quenching, and seeking hardening of the surface of steel. In the carburizing treatment, carbon is diffused and infiltrated into the surface layer portion of the steel in a high temperature region above the A ₃ transformation point, so that carbon diffuses from the steel surface and penetrates to a relatively deep position, and as a result, a deep surface hardened layer depth can be obtained. .

However, when carburizing treatment is employed as the above surface hardening heat treatment, A decrease in the shape accuracy of the part caused by the strain or thermal strain at the time of quenching cannot be avoided. Further, in the state where quenching is performed after carburization is maintained, the toughness of the steel remarkably decreases. Therefore, when a carburizing process is performed to manufacture a part, after quenching, it is necessary to perform tempering (for example, pressurization and tempering treatment) for the purpose of correcting the shape of the part or to restore the toughness, and the number of manufacturing steps is increased, so that the manufacturing cost is increased. Become extremely disadvantageous.

On the other hand, the nitriding treatment is a process of heating the steel to a temperature lower than the A ₁ transformation point and diffusing and permeating (nitriding) nitrogen to the surface layer portion of the steel, which is not quenched as in the case of carburizing. The surface hardening treatment of steel. That is, the nitriding treatment not only has a relatively low processing temperature, but also is not accompanied by a phase change state of steel. Therefore, when a nitriding treatment is performed to manufacture a component, the shape accuracy of the component can be favorably maintained. However, in the case of nitriding using a gas of ammonia gas, the time required for nitriding is about 25 hours to 150 hours, which is remarkably long, and thus it is not suitable for an automatic vehicle part or the like which is premised on mass production.

In recent years, soft nitriding treatment is becoming popular as a treatment that advantageously solves the above problems found in gas nitriding. The soft nitriding treatment is a nitriding treatment in which the nitriding reaction is rapidly performed by using a carburizing atmosphere, and the object to be treated is held in a treatment environment of 550 ° C to 600 ° C for several hours, and iron carbide formation is performed. The medium allows nitrogen to diffuse from the steel surface into the steel. According to this soft nitriding treatment, although the obtained steel surface hardness is lower than that of the prior nitriding treatment (gas nitriding), the nitriding treatment time can be greatly shortened.

The soft nitriding treatment is roughly classified into a method of treating in a salt bath and a method of treating in a gas. The method of treatment in a salt bath (salt bath soft nitriding treatment) requires the use of a cyanide bath, so it is necessary to prevent environmental pollution. Policy. On the other hand, in the method of treating in a gas (gas nitrocarburizing treatment), since a mixed gas containing ammonia as a main component is used, there are few emissions which cause environmental pollution. For the above reasons, the penetration rate of the gas nitrocarburizing treatment which is particularly treated in a gas during the soft nitriding treatment is gradually increasing.

On the other hand, in the past, a mechanical structural component including a transmission component of an automatic vehicle is generally manufactured by machining and joining the intermediate product obtained by casting or forging into a desired shape. In recent years, steel sheets (thin steel sheets) have been actively used as materials, and steel sheets (thin steel sheets) have been subjected to press working or the like to be molded into a desired shape to manufacture the above-described mechanical structure parts. Thereby, the manufacturing process is shortened compared with the prior art, and the manufacturing cost can be drastically reduced. In view of such a background, a steel sheet for soft nitriding treatment which is excellent in moldability of a material for a mechanical structural component such as a transmission component of an automatic vehicle has been desired, and various techniques have been proposed so far.

For example, Patent Document 1 and Patent Document 2 disclose a method for producing a steel sheet for nitriding which is excellent in moldability, and a steel sheet for nitriding which is excellent in moldability having the following composition, and a method for producing the steel sheet for nitriding In terms of weight ratio, C: 0.01% to less than 0.08%, Si: 0.005% to 1.00%, Mn: 0.010% to 3.00%, P: 0.001% to 0.150%, N: 0.0002% to 0.0100%, and Cr: 0.15% to 5.00%, Al: more than 0.060% to 2.00%, and further containing Ti: 0.010% or more, less than 4C [%], V: 0.010% to 1.00% of one or two kinds of steel after hot rolling Coiling at 500 ° C or higher, or thereafter performing cold rolling at a reduction ratio of 50% or more, and then performing Recrystallization annealing. In addition, according to this technique, the content of C which adversely affects the moldability is suppressed to less than 0.08%, and Cr, Al or the like is contained as a nitriding promoting element, thereby forming nitriding which is excellent in moldability and nitridability. Steel plate.

Further, Patent Document 3 proposes a steel for soft nitriding treatment, which has a composition containing C: 0.03% or more, less than 0.10%, Si: 0.005% to 0.10%, and Mn: 0.1% to 1.0 in terms of mass%. %, Cr: 0.20% to 2.00%, impurities: S: 0.01% or less, P: 0.020% or less, sol. Al: 0.10% or less, N: 0.01% or less, the remaining portion substantially contains Fe, and is numbered by particle size. The grain size of the ferrite iron crystals specified in JIS G 0552 is set to 5 or more and 12 or less. Further, according to this technique, since a high-priced element such as Ti or V is not added, an inexpensive steel sheet can be obtained, and a steel sheet excellent in press formability can be obtained by refining the crystal grain size of the steel.

Further, Patent Document 4 proposes a steel sheet for nitriding treatment, which has a composition containing, by mass%, C: more than 0.01%, 0.09% or less, Si: 0.005% to 0.5%, and Mn: 0.01% to 3.0%. , Al: 0.005% to 2.0%, Cr: 0.50% to 4.0%, P: 0.10% or less, S: 0.01% or less, and N: 0.010% or less, or further containing V: 0.01% to 1.0%, Ti: One or more of 0.01% to 1.0% and Nb: 0.01% to 1.0%, and the grain boundary area Sv per unit volume is set to be 80 mm ^-1 or more and 1300 mm ^-1 or less. In addition, according to this technique, not only a nitride-forming element such as Cr, Al, V, Ti, or Nb is contained in a range that does not inhibit the formability of the steel sheet, but also the grain boundary area per unit volume is controlled to a predetermined range. Both high surface hardness and sufficient hardening depth can be obtained together after the nitriding treatment.

Further, Patent Document 5 proposes a steel sheet for soft nitriding comprising C: 0.01 mass% to 0.10 mass%, Si: 0.1 mass% or less, Mn: 0.1 mass% to 1.0 mass%, and P: 0.05 mass%. Hereinafter, S: 0.01 mass% or less, Al: 0.01 mass% to 0.06 mass%, Cr: 0.05 mass% to 0.50 mass%, V: 0.01 mass% to 0.30 mass%, N: 0.01 mass% or less, and the remainder Contains Fe and unavoidable impurities. In addition, according to the technique, by containing Cr: 0.05 mass% to 0.50 mass% and V: 0.01 mass% to 0.30 mass% as the nitriding promoting element, the surface hardening property by the soft nitriding treatment is improved, and a large amount of alloy is not added. In the element, the soft nitriding steel sheet which is excellent in moldability before soft nitriding treatment and has excellent surface hardening property by soft nitriding treatment is inexpensively produced.

Further, Patent Document 6 proposes a steel sheet for soft nitriding treatment containing C: 0.04 mass% to 0.08 mass%, Si: 0.1 mass% or less, Mn: 0.05 mass% to 0.6 mass%, P: 0.03 mass % or less, S: 0.01 mass% or less, Al: 0.1 mass% or less, Cr: 0.6 mass% to 1.2 mass%, V: 0.002 mass% to less than 0.01 mass%, and N: 0.01 mass% or less, and the remainder Contains Fe and unavoidable impurities. Further, according to this technique, by containing a very small amount of V (0.002 mass% to less than 0.01 mass%), a soft nitride treatment can be used to form a nitride layer having high hardness and a small formation of a porous layer, so that processing can be obtained. A steel sheet for soft nitriding treatment which is excellent in properties and excellent in abrasion resistance.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. Hei 9-25513 [Patent Document 2] Japanese Patent Laid-Open Publication No. Hei. No. Hei. No. Hei. No. Hei. No. 2003-105489. Japanese Laid-Open Patent Publication No. 2008-280331 (Patent Document 6)

However, in the techniques proposed in Patent Document 1 and Patent Document 2, since a large amount of Al is contained as a nitriding promoting element, there is a concern that internal defects and surface defects caused by Al intervening are generated. Further, the following problem has been found: a large amount of Al-based slag is generated during refining, resulting in an increase in melting cost.

Further, in the technique proposed in Patent Document 3, since a high-priced element is not contained, an inexpensive steel sheet for soft nitriding treatment can be obtained, but the strength is about 420 MPa in terms of tensile strength, so it is applied to high stress. Parts used under load are limited.

Further, in the technique proposed in Patent Document 4, a steel sheet for nitriding treatment having a tensile strength of more than 500 MPa can be obtained, but the hardness distribution in the thickness direction after the nitriding treatment is not considered, and actually, The durability of parts during nitriding treatment is not as good as the required level.

Further, in the technique proposed in Patent Document 5, a steel sheet for soft nitriding treatment excellent in surface hardening properties by soft nitriding treatment can be obtained, but the tensile strength is also less than 390 MPa. Therefore, it is difficult to apply to parts for mechanical construction subjected to high stress, and lacks versatility.

Further, in the technique proposed in Patent Document 6, it contains Cr (0.6 mass% to 1.2 mass%) and contains a very small amount of V (0.002). Mass%~ less than 0.01 mass%) to form a high-quality nitride layer, so that a steel sheet for soft nitriding treatment excellent in abrasion resistance can be obtained, but the strength is about 400 MPa in terms of tensile strength, and therefore, the patent document The proposed technique is similarly applied to parts that are used under high stress loading conditions.

Further, when the steel sheet is subjected to a soft nitriding treatment, the steel sheet is usually heated to a treatment temperature of about 550 ° C to 600 ° C and maintained at the treatment temperature for about 1 hour to 5 hours, so that the hardness of the surface layer portion of the steel sheet may be borrowed. The strength is significantly increased by the soft nitriding treatment, and the strength of the inside of the steel sheet (non-nitriding portion) is lowered. Therefore, even if it has a desired strength (tensile strength) before the soft nitriding treatment, it is conceivable that the strength inside the thickness of the steel sheet (non-nitriding portion) is greatly lowered by the soft nitriding treatment, and it is not soft. The final product after the nitriding treatment imparts desired strength and fatigue resistance.

For the above reasons, in the steel sheet for soft nitriding treatment, the inside of the steel sheet (non-nitriding portion) has one of the important characteristics such that the desired strength is obtained even after the soft nitriding treatment. However, in any of the above prior art, no change was made in the intensity variation inside the sheet thickness seen before and after the soft nitriding treatment.

An object of the present invention is to solve the problems of the prior art described above, and to provide a soft nitriding treatment having a desired strength (tensile strength: 440 MPa or more) and excellent fatigue resistance after soft nitriding treatment. Steel plate and its manufacturing method.

In order to solve the above problems, the inventors of the present invention have treated steel for soft nitriding treatment. Efforts have been made to study various factors such as the strength of the sheet, the formability, and the change in the strength of the inside (non-nitriding portion) of the thickness of the steel sheet observed before and after the soft nitriding treatment. As a result, the following findings were obtained.

1) By setting the steel sheet structure to a composite structure containing ferrite iron and ferrite, the strength reduction after the soft nitriding treatment is suppressed, and a steel sheet excellent in strength stability can be obtained.

2) By making the steel sheet composition contain the required amount of V and setting the half of the V content to the solid solution V, not only the strength of the surface layer portion of the steel sheet is increased by the soft nitriding treatment, but also the thickness of the steel sheet (non- The strength of the nitrided portion is also increased by the soft nitriding treatment, and the fatigue resistance is improved.

3) After the soft nitriding treatment, the hardness of the inside of the steel sheet (non-nitriding portion) is increased by more than 5% of the hardness before the soft nitriding treatment, whereby the fatigue resistance is stably improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) A steel sheet for a soft nitriding treatment, comprising: a composite structure having a composition of C: 0.05% or more and 0.10% or less and Si: 0.5% or less, and Mn: 0.7% or more and 1.5% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or more, 0.06% or less, Cr: 0.5% or more and 1.5% or less, and V: 0.03% or more and 0.30% or less. N: 0.005% or less, and the ratio of the amount of solid solution V to the above V content (solid solution V amount / V content) exceeds 0.50, and the remainder contains Fe and is inevitable Impurities; the above composite structure comprises ferrite iron and ferrite.

(2) The steel sheet for nitrocarburizing treatment according to (1), which further contains, in mass%, Nb: 0.005% or more and 0.025% or less, in addition to the above composition.

(3) A method for producing a steel sheet for soft nitriding treatment, comprising: heating a steel sheet, performing hot rolling including rough rolling and final rolling, cooling after final rolling, and then performing rolling When the hot-rolled steel sheet is formed, the steel sheet has a composition containing C: 0.05% or more and 0.10% or less, Si: 0.5% or less, and Mn: 0.7% or more and 1.5% by mass%. Hereinafter, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or more, 0.06% or less, Cr: 0.5% or more and 1.5% or less, V: 0.03% or more, 0.30% or less, and N: 0.005% or less. The remaining portion contains Fe and unavoidable impurities, and the heating temperature of the hot rolling is set to be 1100° C. or higher and 1300° C. or lower, and the final rolling temperature of the final rolling is set to be equal to or higher than the Ar ₃ transformation point (Ar _{3 ).} The average cooling rate of the above cooling is 30° C./s or more, and the winding temperature of the winding is set to 500° C. or higher and 600° C. or lower.

(4) The method for producing a steel sheet for nitrocarburizing treatment according to the above aspect, which further comprises, in addition to the above composition, Nb: 0.005% or more and 0.025% or less by mass%.

According to the present invention, a steel sheet for soft nitriding treatment having a desired strength (tensile strength: 440 MPa or more) and excellent in moldability before soft nitriding treatment and fatigue resistance after nitrocarburizing treatment can be obtained. In the case of such a steel plate, it can be used for a component that is used under a high stress load state, such as a transmission component of an automatic car, and the manufacturing cost can be greatly reduced, and a special effect is obtained industrially.

Hereinafter, the present invention will be described in detail.

First, the reason for limiting the chemical composition of the steel sheet of the present invention will be described. In addition, unless otherwise indicated, the following % of the component composition is a mass %.

C: 0.05% or more and 0.10% or less

C is an element which contributes to the high strength of steel by solid solution strengthening and formation of a second phase. When the C content is less than 0.05%, the strength of the steel sheet required for the material of the parts used in the high stress load state, such as the transmission parts of the automatic vehicle, cannot be ensured. On the other hand, when the C content exceeds 0.10%, the strength of the steel sheet is excessively increased, and the formability is lowered. Therefore, the C content is set to 0.05% or more and 0.10% or less. It is preferably 0.05% or more and 0.08% or less.

Si: 0.5% or less

Si is a solid solution strengthening element and is an element effective for increasing the strength of steel, and also functions as a deacidifying agent. In order to obtain such an effect, it is preferable to contain 0.03% or more of Si. However, when the Si content exceeds 0.5%, the peeling-off scale is formed, and the surface properties of the steel sheet are remarkably deteriorated. Therefore, the Si content is made 0.5% or less. It is preferably 0.1% or less.

Mn: 0.7% or more and 1.5% or less

Mn is a solid solution strengthening element and is an element effective for increasing the strength of steel. Further, Mn also functions as an element in which S which is present as an impurity in steel is fixed as a precipitate, and the adverse effect on steel due to S is reduced. When the Mn content is less than 0.7%, the desired strength of the steel sheet cannot be ensured. On the other hand, when the Mn content exceeds 1.5%, the strength of the steel sheet is excessively increased, and the formability is lowered. Therefore, the Mn content is set to be 0.7% or more and 1.5% or less. It is preferably 1.0% or more and 1.5% or less. More preferably, it is 1.2% or more and 1.5% or less.

P: 0.05% or less

P is an element which lowers the formability and toughness of the steel sheet, and in the present invention, it is preferable to reduce P as much as possible. Therefore, the P content is made 0.05% or less. It is preferably 0.03% or less.

S: 0.01% or less

S is an element which lowers the formability and toughness of the steel sheet in the same manner as P, and in the present invention, it is preferable to reduce S as much as possible. Therefore, the S content is made 0.01% or less. It is preferably 0.005% or less.

Al: 0.01% or more and 0.06% or less

Al is an element that functions as a deacidifying agent, and in order to surely obtain the effect, the Al content is made 0.01% or more. On the other hand, when the Al content exceeds 0.06%, not only the effect as a deacidification agent is saturated, but also the Al-based intervening substance causes an internal defect and a surface defect of the steel sheet. Therefore, the Al content is set to 0.01% or more and 0.06% or less. It is preferably 0.02% or more and 0.05% or less.

Cr: 0.5% or more and 1.5% or less

Cr is an element which has an effect of forming a nitride in steel by soft nitriding treatment, thereby improving the hardness of the surface layer portion of the steel sheet, and is an important element in the present invention. In order to make such an effect remarkable, it is necessary to set the Cr content to 0.5% or more. On the other hand, when the Cr content exceeds 1.5%, the embrittlement of the surface hardened layer (nitriding layer) obtained by the soft nitriding treatment becomes remarkable. Therefore, the Cr content is set to 0.5% or more and 1.5% or less. It is preferably 0.5% or more and 1.0% or less.

V: 0.03% or more and 0.30% or less

V is an element which has an effect of forming a nitride in steel by soft nitriding treatment, thereby improving the hardness of the surface layer portion of the steel sheet, and improving the thickness of the steel sheet by soft nitriding treatment (non-nitriding portion) The strength is the most important element in the present invention. In addition, V which is precipitated in the steel before the soft nitriding treatment also has an effect of improving the strength of the steel sheet for soft nitriding treatment by particle dispersion strengthening (precipitation strengthening). When the V content is less than 0.03%, these effects cannot be sufficiently exhibited. On the other hand, if the V content exceeds 0.30%, not only the embrittlement of the surface hardened layer (nitriding layer) obtained by the soft nitriding treatment becomes remarkable, but also the strength enhancement effect of the steel sheet is saturated, so the economical aspect is also Become disadvantageous. Therefore, the V content is set to 0.03% or more and 0.30% or less. It is preferably 0.05% or more and 0.20% or less.

N: 0.005% or less

N is a harmful element that lowers the formability of the steel sheet. Further, N is an element which combines with a nitriding promoting element such as Cr before soft nitriding treatment, and causes an effective amount of nitriding promoting element to decrease. Therefore, in the present invention Jia is trying to reduce the N content as much as possible, and the N content is set to 0.005% or less. It is preferably 0.003% or less.

Ratio of solid solution V to V content (solid solution V amount / V content): more than 0.50

The solid solution V in the steel sheet is subjected to soft nitriding treatment to increase the strength of the surface layer portion and the inner thickness portion (non-nitriding portion) of the steel sheet, and plays an important role in securing the fatigue resistance after the soft nitriding treatment. Therefore, in the present invention, the ratio of the amount of solid solution V to the content of V in the steel sheet for soft nitriding treatment, that is, the steel sheet before soft nitriding treatment is more than 0.50.

As described above, when the steel sheet is subjected to the soft nitriding treatment, the strength of the inside of the steel sheet (non-nitriding portion) may be lowered by the thermal history of the soft nitriding treatment, and it is estimated that the soft nitriding treatment cannot be performed after the soft nitriding treatment. The desired fatigue resistance is obtained. Therefore, it is important that the steel sheet for soft nitriding treatment has a characteristic that the inside of the steel sheet (non-nitriding portion) of the steel sheet subjected to the soft nitriding treatment has a desired strength.

As a method of securing the strength inside the thickness of the steel sheet after the soft nitriding treatment (non-nitriding portion), a method in which the thickness of the steel sheet due to the soft nitriding treatment is considered may be considered ( The strength of the non-nitriding portion is lowered, and the strength of the steel sheet for soft nitriding treatment is set to be high. However, if the strength of the steel sheet is excessively increased, the formability of the steel sheet is lowered, which is disadvantageous in that it is molded into a desired part shape before the soft nitriding treatment.

When a steel sheet for soft nitriding treatment is used as a material to produce a mechanical structural component that requires fatigue strength or wear resistance, the steel sheet for soft nitriding treatment is formed into a desired component shape by press working or the like, and then Soft nitriding To make the final product. Therefore, excessively increasing the strength of the steel sheet for soft nitriding treatment (the steel sheet before soft nitriding treatment) adversely affects the formability before the soft nitriding treatment, which is not preferable.

On the other hand, if the soft nitriding treatment is performed on the steel sheet for soft nitriding treatment, the strength of the inside (non-nitriding portion) of the sheet thickness can be increased before the soft nitriding treatment, and the nitrocarburizing treatment can be omitted. The former formability is lowered, and the fatigue resistance after the soft nitriding treatment is improved. Therefore, the steel sheet for soft nitriding treatment which requires the moldability before the soft nitriding treatment and the fatigue resistance after the soft nitriding treatment preferably has the following characteristics, that is, the inside of the steel sheet (non-nitriding portion) The intensity rises by soft nitriding treatment.

Therefore, the inventors of the present invention have studied the method of improving the strength of the inner thickness (non-nitriding layer) of the steel sheet by soft nitriding treatment, and as a result, it has been found that it is effective to contain the steel sheet before the soft nitriding treatment. The amount of solid solution V is required, and solid solution V is precipitated as a carbide during the soft nitriding treatment.

According to the present invention, in the present invention, after the V content in the steel sheet is set to 0.03% or more and 0.30% or less, it is necessary to set a half of the V content to be a solid solution V, that is, a solid solution V amount and a V content. The ratio (solid solution V amount / V content) is set to exceed 0.50. When the ratio of the solid solution V amount to the V content (solid solution V amount/V content) is 0.50 or less, the effect of increasing the strength inside the thickness of the steel sheet subjected to the soft nitriding treatment (non-nitriding portion) cannot be sufficiently exhibited. In addition, as a carbonitride, it is precipitated in the steel before the soft nitriding treatment, and both the securing of the strength of the steel sheet before the soft nitriding treatment and the securing of the amount of hardening by the soft nitriding treatment are considered. The upper limit of the ratio of the solid solution V amount to the V content (solid solution V amount / V content) is preferably set to 0.80.

The above is the basic composition of the present invention, but may further contain Nb in addition to the basic composition.

Nb: 0.005% or more and 0.025% or less

Nb is an element which is precipitated as a carbonitride and is effective in improving the strength of the steel sheet by particle dispersion strengthening (precipitation strengthening), and may be contained as needed. When the Nb content is less than 0.005%, such an effect cannot be sufficiently exhibited. On the other hand, when the Nb content exceeds 0.025%, the strength of the steel sheet is excessively increased, and the formability is lowered. Therefore, the Nb content is made 0.005% or more and 0.025% or less. It is preferably 0.010% or more and 0.020% or less.

In the steel sheet of the present invention, the components other than the above are Fe and unavoidable impurities. As an unavoidable impurity, for example, Cu: 0.05% or less, Ni: 0.05% or less, Mo: 0.05% or less, Co: 0.05% or less, Ti: 0.005% or less, and Zr: 0.005% or less, in terms of % by mass, Ca: 0.005% or less, Sn: 0.005% or less, O: 0.005% or less, and B: 0.0005% or less.

Next, the reason for limiting the structure of the steel sheet of the present invention will be described.

The steel sheet of the present invention has a composite structure comprising ferrite iron and ferrite.

In order to ensure the formability of the steel sheet, it is effective to increase the proportion of the ferrite iron in the steel sheet structure. However, if the steel sheet is a single-phase structure of the ferrite-grained iron, the strength of the steel sheet is insufficient, and it is used as a material for mechanical structural parts. The scope of application is small and lacks versatility. On the other hand, when a second phase is formed in the structure of the ferrogranulated iron body to secure the strength of the steel sheet, when the hard low-temperature metamorphic phase such as 麻田散铁 or toughened iron is used as the second phase, the above low temperature The metamorphic phase softens due to the thermal history of the soft nitriding treatment, and the thickness of the steel sheet is internal (non- The strength of the nitrided portion is greatly reduced.

Therefore, in the present invention, in order to suppress the decrease in the strength of the inner thickness (non-nitriding portion) of the steel sheet due to the thermal history of the soft nitriding treatment, the structure of the steel sheet is set as the following composite structure, and the composite structure will be Fertilizer iron is used as the main phase, and the second phase is set as the Borne iron. Furthermore, in the present invention, it is preferable that the iron content of the fertilizer in the steel sheet structure is 80% or more and 95% or less, and the iron content of the wave is set to be 5% or more and 20% or less. In addition, the steel sheet of the present invention is preferably a composite structure containing ferrite iron and ferrite. However, even when other phases (structures) are inevitably formed, if the total fraction is 1% or less, It can also be tolerated.

Next, a method of producing the steel sheet of the present invention will be described.

In the present invention, a steel sheet having the above composition is heated, and hot rolling including rough rolling and final rolling is performed, and after cooling at the end of the final rolling, it is cooled and then wound up to obtain a hot rolled steel sheet. In this case, it is preferable to set the heating temperature to 1100 ° C or more and 1300 ° C or less, and to set the final rolling temperature to be equal to or higher than the Ar ₃ transformation point or (Ar ₃ transformation point + 100 ° C), and to set the average cooling rate of cooling. The temperature is 30 ° C / s or more, and the coiling temperature is set to 500 ° C or more and 600 ° C or less.

In the present invention, the method of melting the steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, after the melting, it is preferable to form a steel sheet (slab) by a continuous casting method due to problems such as segregation, but a known casting method such as an agglomerate-block rolling method or a thin billet continuous casting method may be used. Made of steel sheet. Further, various pretreatments or secondary refining, surface repair of steel sheets, and the like may be performed as needed.

Heating temperature of steel sheet: 1100 ° C or more, 1300 ° C or less

The steel sheet obtained as described above is subjected to rough rolling and final rolling, but in the present invention, it is necessary to sufficiently re-dissolve V in the steel sheet before rough rolling. When the heating temperature of the steel sheet is less than 1100 ° C, it is difficult to sufficiently decompose the V carbonitride and re-dissolve V again, so that the above-described desired effect obtained by containing V cannot be exhibited. In addition, it is also difficult to ensure the necessary final rolling temperature. On the other hand, if the heating temperature of the steel sheet exceeds 1300 ° C, the energy required for heating the steel sheet increases, which is disadvantageous in terms of cost. Therefore, the heating temperature of the steel sheet before rough rolling is set to 1100 ° C or more and 1300 ° C or less. It is preferably 1150 ° C or more and 1250 ° C or less.

When the steel sheet before the rough rolling is heated, the steel sheet after the casting is cooled to a normal temperature and then heated, and the steel sheet in the middle of cooling after casting may be additionally heated or insulated. Further, when the cast steel sheet is maintained at a sufficient temperature and V is sufficiently solid-dissolved in the steel, it may be directly rolled without heating the steel sheet. Further, it is not necessary to particularly limit the rough rolling conditions.

Final rolling temperature: above Ar ₃ metamorphic point, (Ar ₃ metamorphic point + 100 ° C) or less

When the final rolling temperature in the final rolling is less than the Ar ₃ transformation point, the ferrite iron structure extending in the rolling direction and the non-recrystallized ferrite iron structure are formed, and the formability of the steel sheet is lowered. Further, the in-plane anisotropy of the mechanical properties of the steel sheet becomes strong, and it is difficult to perform a homogeneous forming process. On the other hand, if the final rolling temperature exceeds (Ar ₃ transformation point + 100 ° C), the surface properties of the steel sheet tend to deteriorate. Therefore, the final rolling temperature is set to be equal to or higher than the Ar ₃ transformation point and (Ar ₃ transformation point + 100 ° C). Further, the term "final rolling temperature" means the temperature of the steel sheet on the exit side of the final passage of the final rolling.

In order to secure the above final rolling temperature, a steel plate during rolling may be additionally heated by a heating device such as a thin plate heater or an edge heater. Further, the Ar ₃ metamorphic point of the steel can be determined by measuring the heat shrinkage during the cooling process from the Wörthfield iron temperature zone and preparing a heat shrinkage curve, or can be obtained by estimating the content of the alloy element. .

Average cooling rate: 30 ° C / s or more

It is important to optimize the average cooling rate in terms of ensuring the solid solution V in the steel sheet. In the present invention, immediately after the final rolling is finished, cooling is started immediately (within 1 s), and the final rolling temperature is taken up to coiling. The average cooling rate up to the temperature is set to 30 ° C / s or more. When the average cooling rate is less than 30 ° C / s, the carbonitride in the presence of V precipitates during the cooling process, and the desired amount of solid solution V cannot remain in the steel sheet. Further, there is a case where the crystal grains are excessively coarsened, and the strength or ductility of the steel sheet is lowered. Therefore, the above average cooling rate is set to 30 ° C / s or more. It is preferably 40 ° C / s or more.

The upper limit of the average cooling rate is not particularly limited. However, in order to avoid the shape defect of the steel sheet caused by strong water cooling, it is preferably 100 ° C / s or less. In addition, after the steel sheet is cooled to the coiling temperature, it is not particularly necessary to use forced cooling such as water injection, and it is only necessary to place it in the atmosphere and cool it to coil.

Coiling temperature: 500 ° C or more, 600 ° C or less

The appropriateness of the coiling temperature is important in securing the solid solution V in the steel sheet and making the steel sheet into a desired structure. When the coiling temperature is less than 500 ° C, a low temperature metamorphic phase is formed and the steel sheet is hardened, the formability is lowered, and The method avoids a decrease in the strength inside the thickness of the steel sheet (non-nitriding portion) caused by the thermal history of the soft nitriding treatment. On the other hand, when the coiling temperature exceeds 600 ° C, the V carbonitride is precipitated in a large amount after coiling, and the required amount of solid solution V may not remain in the steel sheet. Therefore, the coiling temperature is set to 500 ° C or more and 600 ° C or less. It is preferably 520 ° C or more and 580 ° C or less.

The hot-rolled steel sheet obtained by the above-described method is used as a steel sheet for soft nitriding treatment after removing scale by pickling, bead blasting or the like. Further, even if the temper rolling is performed for the purpose of shape correction or surface roughness adjustment, the effects of the present invention are not impaired.

Further, the steel sheet for soft nitriding treatment of the present invention can be applied to any of gas nitrocarburizing treatment and salt bath nitrocarburizing treatment.

[Example]

The steel containing the chemical components shown in Table 1 was melted, and then subjected to agglomeration and block rolling to obtain a steel sheet. After the steel sheets were heated, rough rolling and final rolling were carried out, and immediately after the final rolling, cooling was performed, and then coiling was carried out to obtain a hot-rolled steel sheet having a thickness of 3.2 mm. The heating temperature of the steel sheet, the final rolling temperature, the average cooling rate from the final rolling temperature to the coiling temperature, and the coiling temperature are shown in Table 2.

The hot-rolled steel sheet obtained by the above method was pickled to remove rust, and then subjected to temper rolling at an elongation of 0.5%. Then, a test piece was taken from the steel sheet after the temper rolling, and was subjected to the following evaluation.

(i) amount of solid solution V

The amount of solid solution V is obtained by subtracting the V amount from the V content, which is a test piece taken from the plate width of the steel sheet after the temper rolling, and then the test piece is taken in the electrolyte solution. The amount of V in the precipitate in the steel obtained by constant current electrolysis of the test piece.

(ii) Group observation

A sample having a plate thickness profile parallel to the rolling direction at a plate width of 1/4 of the steel sheet after temper rolling is mirror-polished and etched with a nital etch solution, and then subjected to optical microscopy or scanning. The electron microscope is used to take a plate thickness of 1/4 position at an appropriate magnification of 500 times to 3000 times. Using the obtained photograph of the tissue, and by image analysis, the area ratio of the fertilized iron to the whole of the tissue, the area ratio of the ferrite, and the types of other tissues and the area ratios thereof are determined as the respective fractions. . The results obtained are shown in Table 3.

(iii) Tensile test

For the 1/4 position of the steel sheet after the temper rolling, the test piece No. 5 (the gauge length L: 50 mm) specified in JIS Z 2201 (1998) in which the tensile test direction is the rolling direction is adopted. A tensile test according to the regulation specified in JIS Z 2241 (1998) was carried out, and tensile strength (TS) and elongation (El) were measured, and then the strength and elongation balance (TS × El) were determined. Furthermore, in the present example, tensile strength (TS): 440 MPa or more, strength, elongation balance (TS × El): 17 GPa. Steel sheets of % or more were evaluated as steel sheets having high strength and good formability.

(iv) section hardness test

A test piece was taken from the steel sheet after the above-mentioned temper rolling, and the Vickers hardness (HVc) at a position of 1/2 of the thickness was measured by the method according to JIS Z 2244 (2009).

<Measurement method>

Test force: 0.98 N measurement site: 5 sites

(v) Soft nitriding treatment test

A small piece was taken from the steel sheet after the above-mentioned temper rolling, and the gas nitrocarburizing treatment under the conditions shown below was carried out.

Soft nitriding environment: equal ratio of ammonia gas and endothermic conversion gas. Mixed gas treatment temperature: 580 °C Processing time: 2.5 hours

Furthermore, the above processing time is maintained at the above processing temperature (580 ° C) After (2.5 hours), the pellet was oil-cooled (oil temperature: 70 ° C). Then, the oil-cooled pellets were subjected to the following evaluation.

For the oil-cooled pellet, the Vickers hardness (HV0.1) at a depth of 0.1 mm from the surface of the sheet was measured in accordance with JIS G 0563 (1993). Further, the practical gasification layer depth according to the regulations of JIS G 0562 (1993) was measured. In the present example, a steel sheet having a Vickers hardness (HV0.1) of 500 or more and a practical nitrided layer depth of 0.40 mm or more was evaluated as a steel sheet having excellent surface hardening characteristics.

Further, by the same method as the above (iv), the Vickers hardness (HVc') at the plate thickness 1/2 position (non-nitriding portion) is measured to represent the hardness of the inside of the steel sheet (non-nitriding portion). . Then, according to the Vickers hardness (HVc) at the plate thickness 1/2 position before the soft nitriding treatment determined by the above (iv), and the Vickers position at the 1/2 position after the soft nitriding treatment The hardness (HVc') was used to determine the rate of increase in Vickers hardness at the center portion of the thickness obtained by the soft nitriding treatment: (HVc' - HVc) / HVc × 100 (%). In the present example, a steel sheet having a Vickers hardness increase rate of more than 5.0% was evaluated as a steel sheet (○) having good fatigue resistance after soft nitriding treatment, and the others were evaluated as ×. The results obtained are shown in Table 4.

As is clear from Table 4, the present invention gave good results in all aspects of strength, formability, surface hardening characteristics by soft nitriding treatment, and fatigue resistance. On the other hand, the comparative examples in which the steel composition or the structure did not satisfy the requirements of the present invention did not give sufficient results in any of the above characteristics.

Claims (4)

A steel sheet for soft nitriding treatment, comprising the following composition and composite structure, wherein the composition contains C: 0.05% or more and 0.10% or less, Si: 0.5% or less, and Mn: 0.7% or more and 1.5 in mass%. % or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or more, 0.06% or less, Cr: 0.5% or more, 1.5% or less, V: 0.03% or more, 0.30% or less, and N: 0.005% or less And the ratio of the amount of solid solution V to the above-mentioned V content (solid solution V amount / V content) exceeds 0.50, and the remainder contains Fe and unavoidable impurities; the above composite structure contains ferrite iron and ferrite. The steel sheet for nitrocarburizing treatment according to the first aspect of the invention, further comprising, in addition to the above composition, Nb: 0.005% or more and 0.025% or less by mass%. A method for producing a steel sheet for soft nitriding treatment, comprising: heating a steel sheet, performing hot rolling including rough rolling and final rolling, cooling after completion of final rolling, and winding and winding In the hot-rolled steel sheet, the steel sheet has a composition containing C: 0.05% or more and 0.10% or less, Si: 0.5% or less, and Mn: 0.7% or more and 1.5% or less, in terms of % by mass. : 0.05% or less, S: 0.01% or less, Al: 0.01% or more, 0.06% or less, Cr: 0.5% or more, 1.5% or less, V: 0.03% or more, 0.30% or less, N: 0.005% or less, and the remaining portion parts of Fe and inevitable impurities, the heating temperature in the hot rolling is made at least 1100 ℃, less 1300 ℃, the above-described final rolling final rolling temperature is set to ₃ transformation point Ar, Ar ₃ transformation point + 100 Below °C, the average cooling rate of the above cooling is 30 ° C / s or more, and the winding temperature of the winding is set to 500 ° C or more and 600 ° C or less. The method for producing a steel sheet for nitrocarburizing treatment according to the third aspect of the invention, which further comprises, in addition to the above composition, Nb: 0.005% or more and 0.025% or less by mass%.