TW201400625A - Steel sheet for soft-nitriding and method for manufacturing the same - Google Patents

Abstract

A steel sheet for soft-nitriding with excellent formability and strength stability after soft-nitriding is provided. The steel sheet for soft-nitriding includes the following composition and the following structure. 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, 0.005 mass% or more and 0.025 mass% or less of Nb, 0.005 mass% or less of N, and the remainder contains Fe and inevitable impurities, wherein C and Nb satisfy 0.10 ≤ Nb/C ≤ 0.30 and represent contents of element respectively. The structure is a complex structure which contains a ferrite and a pearlite, and a ratio of a structure except for the ferrite and the pearlite is 1% or less, and a ratio of a polygonal ferrite contained in the ferrite is less than 50%.

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 mechanical structural parts requiring fatigue strength or wear resistance, such as a transmission part of an automatic vehicle, and particularly relates to a formability and soft nitrogen before a soft nitriding treatment. A steel sheet for soft nitriding treatment excellent in strength stability after chemical 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 of 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 steel surface layer portion in a high temperature region above the A ₃ transformation point. Therefore, carbon diffuses from the steel surface and penetrates to a relatively deep position, and as a result, a deep hardened layer depth can be obtained.

However, when carburizing treatment is used as the surface hardening heat treatment, A reduction in the shape accuracy of the part caused by the strain or thermal strain at the time of quenching is 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 utilizing a carburizing atmosphere. 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 nitriding in a salt bath and a method of nitriding in a gas. The method of nitriding in a salt bath (salt bath soft nitriding treatment) requires a cyanide-based bath, so countermeasures against environmental pollution are required. On the other hand, the method of nitriding in a gas (gas nitrocarburizing treatment) becomes an environmental pollution by using a mixed gas containing ammonia as a main component. The reason for the discharge is less. For the above reasons, the penetration rate of the gas nitrocarburizing treatment for nitriding, particularly in a gas, is gradually increasing.

On the other hand, in the prior art, parts for mechanical construction such as a transmission part of an automatic car are usually manufactured in such a manner that the intermediate product obtained by casting or forging is machined and processed and joined into a desired shape, but 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 The coiling is performed at 500 ° C or higher, or thereafter, cold rolling is performed at a reduction ratio of 50% or more, and then recrystallization annealing is performed. Further, according to this technique, the C content which adversely affects the formability is suppressed to less than 0.08%, and Cr, Al, etc. are contained. It is a nitriding promoting element, and forms a steel sheet for nitriding which is excellent in moldability and nitridability.

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 treatment which has a composition of C: 0.03 mass% to 0.10 mass%, Si: 0.5. Mass% or less, Mn: 0.1 mass% to 0.6 mass%, P: 0.04 mass% or less, S: 0.04 mass% or less, Al: 0.005 mass% to 0.08 mass%, Cr: 0.4 mass% to 1.2 mass%, Nb: 0.002 mass% or more, less than 0.01 mass%, and N: 0.01 mass% or less. Further, according to this technique, a steel sheet for soft nitriding treatment having both workability and fatigue characteristics can be obtained by containing a trace amount of Nb.

[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. 9-25543

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2003-105489

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2003-277887

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2009-68057

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, although a steel sheet for nitriding treatment having a tensile strength exceeding 500 MPa can be obtained, Considering the hardness distribution in the thickness direction after the nitriding treatment, the durability of the parts during the nitriding treatment is actually insufficient to a sufficient level.

In addition, in the technique proposed in the patent document 5, the steel sheet for soft nitriding treatment which is excellent in workability can be obtained, but the strength is up to about 400 MPa in terms of tensile strength, and therefore, similarly to the technique proposed in Patent Document 3, It is limited when applied to parts used under high stress loads.

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 portion of the steel sheet may be remarkable. Ascending, on the other hand, 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 of the inside of the steel sheet (non-nitriding portion) is greatly lowered by the soft nitriding treatment, and the soft nitriding treatment cannot be performed. The latter part gives the desired strength (fatigue strength).

For the above reasons, in the steel sheet for soft nitriding treatment, the strength of the inside of the steel sheet (non-nitriding portion) is not greatly lowered by the soft nitriding treatment, and the inside of the steel sheet before and after the soft nitriding treatment (non-nitriding portion) The change in strength is small, that is, the strength stability after the soft nitriding treatment becomes an important characteristic. However, in any of the above prior art, no research has been conducted on the strength stability 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 desired strength (tensile strength: 440 MPa or more), and formability before soft nitriding treatment and strength after soft nitriding treatment. stable A steel sheet for soft nitriding treatment excellent in qualitative properties and a method for producing the same.

In order to solve the above problems, the inventors of the present invention have various factors such as the strength and formability of the steel sheet for soft nitriding treatment and the change in strength of the steel sheet (non-nitriding portion) which is observed before and after the soft nitriding treatment. Efforts have been made to study. 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) When the proportion of the polygonal ferrite in the ferrite is increased, the strength of the steel sheet is lowered, and the change in the strength of the inside (non-nitriding portion) of the steel sheet before and after the soft nitriding treatment tends to be large.

3) It is effective to make the steel sheet composition contain a required amount of Nb for the increase in the strength of the steel sheet and the decrease in the proportion of the polygonal ferrite iron in the above-mentioned ferrite iron.

4) The Nb and C are included in the steel sheet composition so as to satisfy a predetermined relationship (0.10 ≦ Nb/C ≦ 0.30), whereby the strength change inside the steel sheet (non-nitriding portion) before and after the soft nitriding treatment is small.

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 composition having a composition of C: 0.05% in terms of % by mass, and C and Nb satisfying the following formula (1) Above, 0.10% or less, Si: 0.5% or less, Mn: 0.7% or more, 1.5% or less, and 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, Nb: 0.005% or more, 0.025% or less, N: 0.005% or less, and the remainder contains Fe and not. Impurities to be avoided; the above-mentioned structure is a composite structure containing ferrite iron and Boride iron, the ratio of the above-mentioned fertilized iron and the other than the Borne iron is 1% or less, and the polygonal ferrite iron is occupied by the above-mentioned fertilized iron The proportion is less than 50%.

Note 0.10≦Nb/C≦0.30...(1)

(C, Nb: content of each element (% by mass))

2. 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 coiling In the case of the hot-rolled steel sheet, the steel sheet has a composition of C: 0.05% or more and 0.10% or less in such a manner that C and Nb satisfy the following formula (1). Si: 0.5% or less, Mn: 0.7% or more, 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, Nb: 0.005% or more, 0.025% or less, N: 0.005% or less, and the remainder contains Fe and unavoidable impurities, and the heating temperature of the hot rolling is set to 1100 ° C or more and 1300 ° C or less, and the final final rolling is performed. The rolling temperature is equal to or higher than the Ar ₃ transformation point and (Ar ₃ transformation point + 100 ° C) or less, and the average cooling rate of the cooling is 30° C./s or more, and the coiling temperature of the winding is set to 500° C. or higher. 650 ° C or less.

Note 0.10≦Nb/C≦0.30...(1)

(C, Nb: content of each element (% 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 strength stability after soft nitriding treatment can be obtained. Therefore, even if it is a component used for a high-stress load state, such as a transmission component of an automatic car, it is possible to use a steel plate material to greatly reduce the manufacturing cost, and to obtain a special effect 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 high in steel by solid solution strengthening and formation of a second phase. Intensified element. 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 deacidification effect is saturated, but also the Al-based intervening substance increases the internal defects and surface defects 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 one of important elements 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 0.8% or more and 1.2% or less.

Nb: 0.005% or more and 0.025% or less

Nb is an element which is precipitated into steel as a carbonitride, and is effective in enhancing the strength of the steel sheet by particle dispersion strengthening (precipitation strengthening) and is effective in securing the strength of the steel sheet after the soft nitriding treatment, and is in the present invention. One of the important elements. When the Nb content is less than 0.005%, the desired steel sheet strength and steel sheet strength stability cannot be ensured. On the other hand, if the Nb content is super When the ratio is 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.

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, it is preferred to reduce the N content as much as possible, and to set the N content to 0.005% or less. It is preferably 0.003% or less.

Further, the steel sheet of the present invention contains C and Nb in such a range as to satisfy the formula (1).

0.10≦Nb/C≦0.30......(1)

(C, Nb: content of each element (% by mass))

The above formula (1) is for improving the strength of the steel sheet before the soft nitriding treatment, and reducing the strength change of the inside (non-nitriding portion) of the steel sheet before and after the soft nitriding treatment, that is, ensuring the strength stability after the soft nitriding treatment. The necessary conditions that should be met.

As described above, in the present invention, precipitation strengthening by Nb carbonitride is used as one of the high-strength mechanisms of the steel sheet, thereby suppressing the inside of the steel sheet caused by the soft nitriding treatment (non-nitriding) In terms of the change in strength of the portion, it is important to suppress the variation in the amount of precipitation strengthening before and after the soft nitriding treatment. Further, in suppressing the fluctuation of the amount of precipitation strengthening, It is necessary to suppress in such a manner that even if the steel sheet is subjected to the thermal history of the soft nitriding treatment, the precipitation state (particle size and volume ratio) of the Nb carbonitride in the steel sheet is not greatly increased from the precipitation state before the soft nitriding treatment. change.

Therefore, the present inventors have studied the method of stabilizing the precipitation strengthening amount before and after the soft nitriding treatment, and as a result, it has been found that it is effective to adjust the Nb content in the steel relative to the C content in a manner satisfying the above formula (1). . It is presumed that when Nb/C is in the range of the above formula (1), the growth and additional precipitation of the Nb carbonitride in the nitrocarburizing treatment are suppressed, or are small and balanced. Therefore, in the present invention, C and Nb are adjusted so as to satisfy 0.10 ≦ Nb / C ≦ 0.30.

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 V: 0.005% or less, in terms of % by mass, Zr: 0.005% or less, 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 structure which is a composite structure containing ferrite iron and a ferritic iron, and the proportion of the polygonal ferrite iron in the ferrite iron is less than 50%.

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, by making the ferrite core When the second phase is formed in the structure to promote the strengthening of the structure, and when the strength of the steel sheet is secured, in the case where the hard low-temperature metamorphic phase such as the granulated iron or the toughened iron is the second phase, the low-temperature metamorphic phase is caused by the soft nitriding. The heat history at the time of the treatment is softened, and the strength change inside the steel sheet (non-nitriding portion) before and after the soft nitriding treatment is extremely large.

Therefore, in the present invention, in order to suppress the change in the strength of the inside of the steel sheet (non-nitriding portion) caused by the thermal history of the soft nitriding treatment, the structure of the steel sheet is set as a composite structure which will be ferrite iron As the main phase, the second phase is set to benefic. Furthermore, in the present invention, it is preferable that the ferrite fraction in the steel sheet structure is 80% or more and 95% or less, and the iron fraction in the steel sheet structure is preferably 5% or more and 20% or less. . Further, the steel sheet of the present invention is preferably a composite structure comprising ferrite iron and ferritic iron. However, even if other phases (tissue) are inevitably formed, the other phase (tissue) is in the steel sheet structure. If the total score is 1% or less, it is acceptable.

Further, the ferrite iron of the polygonal shape is soft and the crystal grains are easily grown when heated. Therefore, in a steel sheet containing a large amount of polygonal ferrite iron, the strength of the steel sheet tends to be low, and the strength of the inside of the steel sheet (non-nitriding portion) due to grain growth during the soft nitriding treatment is likely to occur. Therefore, in the present invention, 50% or more of the above-mentioned fertilized iron is used as the fertilized iron other than the polygonal ferrite iron, and less than 50% of the fertilized iron is made into the polygonal ferrite iron. Further, in the present invention, examples of the ferrite iron other than the polygonal ferrite iron include needle-shaped ferrite iron or toughened ferrite iron.

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 650 ° 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 it may be produced by a known method such as agglomerate-block rolling method or thin billet continuous casting method. Chenggang 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 Nb in the steel sheet before rough rolling. When the heating temperature of the steel sheet is less than 1,100 ° C, the Nb carbonitride cannot be sufficiently decomposed to re-dissolve Nb, and the above-described desired effects obtained by containing Nb cannot be exhibited. 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 rough rolling is heated, the steel sheet after casting can be used. After cooling to room temperature, heating is performed, and the steel sheet in the middle of cooling after casting may be additionally heated or kept warm. Further, when the cast steel sheet is maintained at a sufficient temperature and Nb is sufficiently solid-dissolved in the steel, the steel sheet may be directly rolled without heating. 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 is less than the Ar ₃ metamorphic 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 is enhanced, 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 (A _r3 transformation point + 100 ° C). Here, the term "final rolling temperature" as used herein means the temperature of the steel sheet on the exit side of the final pass 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 order to make the steel sheet into a desired structure. 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 the coiling. The average cooling rate up to the temperature is set to 30 ° C / s or more. When the average cooling rate is not full At 30 ° C / s, the polygonal ferrite iron which is easily generated in the high temperature region is generated in a large amount, and the steel sheet having the desired structure cannot be obtained. Further, there is a case where the crystal grains are excessively coarsened, and the strength or ductility of the steel sheet is lowered. Further, in the present invention, the Nb carbonitride is deposited in the steel sheet to increase the strength of the steel sheet. However, when the average cooling rate is less than 30 ° C/s, the Nb carbonitride becomes coarse. The situation of the desired steel plate strength cannot be obtained. Therefore, the above average cooling rate is set to 30 ° 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, 650 ° C or less

The appropriateness of the coiling temperature is important in making the steel sheet into the desired structure. When the coiling temperature is less than 500 ° C, a low temperature metamorphic phase is formed and the steel sheet is hardened, whereby the formability is lowered, and the strength stability of the steel sheet after the soft nitriding treatment is also lowered. On the other hand, if the coiling temperature exceeds 650 ° C, the amount of polygonal ferrite grains increases, and the desired steel sheet structure cannot be obtained. Therefore, the coiling temperature is set to 500 ° C or more and 650 ° C or less. It is preferably 550 ° C or more and 650 ° 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 rust scale by pickling, shot peening or the like. In addition, even if the shape is corrected or the surface roughness is adjusted For the purpose of temper rolling, 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 of the composition shown in Table 1 was melted, and then subjected to agglomeration and block rolling to obtain a steel sheet. After heating the steel sheets, rough rolling and final rolling were carried out, followed by cooling and coiling to obtain a hot rolled steel sheet having a thickness of 2.9 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) Organizational 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 whole ferrite iron containing the polygonal ferrite iron), the area ratio of the polygonal ferrite iron, and the wave are obtained. The area ratio of the incoming iron, as well as the types of other organizations and the area ratios of these, are used as their respective rates. In addition, according to the ferrite iron fraction obtained by the above method and the polygonal ferrite iron fraction, the proportion of the polygonal ferrite iron in the ferrite iron is obtained ((polygon ferrite iron fraction/fertilizer iron) Fraction) × 100 (%)).

The results obtained are shown in Table 3.

(ii) Tensile test

For the 1/4 position of the steel sheet after the temper rolling, the test piece No. 5 specified in JIS Z 2201 (1998) in which the tensile test direction is the rolling direction is taken, and is carried out in accordance with JIS Z 2241 (1998). The specified tensile test, and the tensile strength (TS) and elongation at break (E1) were measured, and then the strength-elongation balance (TS × E1) was determined. Further, in the present example, a steel sheet having a tensile strength (TS) of 440 MPa or more and a strength-elongation balance (TS × E1): 17 GPa·% or more was evaluated as a steel sheet having high strength and good formability. .

(iii) 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

Measuring position: 5 parts

(iv) soft nitriding treatment

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 mixed gas of ammonia gas and endothermic conversion gas

Processing temperature: 570 ° C

Processing time: 3 hours

Furthermore, the above processing time is maintained at the above processing temperature (570 ° C) After (3 hours), the tablets were 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 nitride 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 nitride layer depth of 0.25 mm or more was evaluated as a steel sheet having excellent surface hardening characteristics.

Further, the Vickers hardness (HVc') at the plate thickness 1/2 position (non-nitriding portion) was measured by the same method as (iii) above. Then, according to the Vickers hardness (HVc) at the plate thickness 1/2 position before the soft nitriding treatment determined by the above (iii), and the Vickers position at the 1/2 position after the soft nitriding treatment The hardness (HVc') was used to determine the rate of change in Vickers hardness at the center portion of the thickness before and after the soft nitriding treatment: (HVc' - HVc) / HVc × 100 (%). In the present example, the steel sheet having an absolute value of the change rate of 5.0% or less was evaluated as a steel sheet (○) having good strength stability after the soft nitriding treatment, and the others were evaluated as ×.

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 strength stability. 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 (2)

A steel sheet for a soft nitriding treatment, comprising: a composition having a composition of C: 0.05% or more and 0.10 in a mass %, wherein C and Nb satisfy the following formula (1); % or less, Si: 0.5% or less, Mn: 0.7% or more, 1.5% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or more, 0.06% or less, and Cr: 0.5% or more and 1.5% or less , Nb: 0.005% or more, 0.025% or less, N: 0.005% or less, and the remainder contains Fe and unavoidable impurities; the above-mentioned structure is composed of ferrite iron and Boride iron, and the above fertilized iron and Boron iron The proportion of the tissue is less than 1% of the composite structure, and the proportion of the polygonal ferrite iron in the above fertilized iron is less than 50%; note that 0.10≦Nb/C≦0.30...(1)(C, Nb : content of each element (% 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 of C: 0.05% or more and 0.10% or less, in which C and Nb satisfy the following formula (1) in terms of % by mass, and Si: 0.5% or less, Mn: 0.7% or more, 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, 1.5% or less, Nb: 0.005% Above, 0.025% or less, N: 0.005% or less, and the remaining part contains Fe and unavoidable impurities, and the heating temperature of the hot rolling is set to 1100 ° C or more and 1300 ° C or less, and the final rolling of the final rolling is performed. The temperature is set to be equal to or higher than the Ar ₃ transformation point, and the Ar ₃ transformation point is +100 ° C or lower, and the average cooling rate of the cooling is set to 30° C./s or more, and the coiling temperature of the winding is set to 500° C. or higher and 650° C. or lower. , 0.10 ≦ Nb / C ≦ 0.30 (1) (C, Nb: content of each element (% by mass)).