JP5630523B2 - Steel sheet for nitriding treatment and method for producing the same - Google Patents

Description

  The present invention relates to a steel sheet for nitriding suitable as a material for machine parts used after nitriding for improving durability, in particular, a steel sheet for nitriding excellent in formability and punchability before nitriding and its It relates to a manufacturing method.

  Mechanical parts used in automobile transmissions and the like are often used after being subjected to surface hardening treatment after forming a steel material as a desired part shape in order to improve fatigue strength and wear resistance. Typical examples of such surface hardening treatment include carburizing treatment and nitriding treatment.

The carburizing process is the most common surface hardening process. However, the carburizing treatment usually for performing hardening after diffused-permeation carbon (carburization) in the surface layer of the steel in the A ₃ transformation point or above of the steel, due to the influence of the distortion caused with the quenching from high temperature In addition, a decrease in the shape accuracy of the parts is inevitable. In addition, the toughness of the steel is significantly reduced in the state of being quenched after carburizing. Therefore, after quenching, tempering for toughness recovery and correction of part shape are essential. Therefore, when carburizing treatment is adopted, there are disadvantages that the number of processes required for manufacturing parts increases and the manufacturing cost increases.

In contrast, the nitriding treatment is usually a treatment in which the steel is heated to a temperature of about 500 to 600 ° C. lower than the A ₁ transformation point, and nitrogen is diffused and permeated (nitriding) into the surface layer portion of the steel. Thus, the surface hardening of the steel is achieved without quenching. That is, the nitriding treatment has a relatively low processing temperature and does not involve the phase transformation of the steel at the time of cooling, and therefore has an advantage that the deterioration of the part shape accuracy due to transformation strain does not occur. Further, the volume change of the steel surface layer portion due to nitriding is small, and there is also an advantage that it is easy to keep good shape accuracy of the parts.

  In the case of nitriding with ammonia gas, conventionally, the time required for nitriding is remarkably long, so that it is not suitable for automobile parts and the like on the premise of mass production. However, in recent years, a nitriding treatment called soft nitriding that allows a nitriding reaction to proceed rapidly by using a carburizing atmosphere has become widespread, and there has been a problem that the processing time that has been a problem in the conventional nitriding treatment is very long. It is being resolved.

  In this soft nitriding treatment, the object to be treated is held in a treatment atmosphere at 550 to 600 ° C. for several hours, and nitrogen is diffused and introduced from the steel material surface into the steel immediately with the formation reaction of iron carbide. According to the soft nitriding treatment, the surface hardness obtained after the treatment is lower than that of the conventional nitriding treatment, but the time required for nitriding can be greatly shortened. For these reasons, in recent years, there are many cases where soft nitriding treatment is employed as surface hardening treatment instead of carburizing treatment.

  On the other hand, mechanical parts used for automobile transmissions and the like have been conventionally manufactured by machining an intermediate product obtained by casting or forging. However, in recent years, a thin steel plate has been actively used as a material for mechanical parts, and the thin steel plate is manufactured by being pressed into a desired shape. This is because the manufacturing process can be shortened and the manufacturing cost can be reduced by substituting a sheet metal processed product of a steel plate for a part that has been manufactured by machining an intermediate product obtained by casting or forging. From such a background, the necessity of the steel sheet for nitriding processing which is excellent in a formability as a raw material steel material of an above-described machine part is increasing.

Various techniques have been proposed for nitriding steel sheets having excellent formability.
For example, in Patent Document 1 and Patent Document 2, 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% by weight ratio , Cr: more than 0.15 to 5.00%, Al: more than 0.060 to 2.00%, and further, steel having a composition containing one or two of Ti and V, is rolled up at 500 ° C. or more after hot rolling Then, a technique for producing a steel sheet for nitriding by performing cold rolling at a reduction rate of 50% or more and performing recrystallization annealing has been proposed. And according to these technologies, by suppressing the C content which adversely affects the formability to less than 0.08%, it is possible to obtain a low carbon steel sheet containing Al, Cr, Ti and / or V nitriding promoting elements at the same time. It is said that a nitriding steel plate having excellent formability and nitriding properties can be obtained.

  Moreover, in patent document 3, regarding steel plate for soft nitriding, steel plate composition is C: 0.01-0.10 mass%, Si: 0.1 mass% or less, Mn: O.1 to l.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.01 to 0.06 mass%, Cr: 0.05 to 0.50 mass%, V: 0.01 to 0.30 mass%, N: 0.01 mass% or less, with the balance consisting of Fe and inevitable impurities A technology has been proposed. According to the technique proposed in Patent Document 3, the surface is obtained by nitrocarburizing treatment by simultaneously adding Cr and V, which are low in cost and excellent in formability by reducing alloy elements, and are simultaneously nitriding promoting elements. It is said that a steel sheet for soft nitriding having excellent curing characteristics can be obtained.

Japanese Patent Laid-Open No. 9-25513 Japanese Patent Laid-Open No. 9-25543 JP 2005-171331 A

  When machine parts used in automobile transmissions are manufactured by forming thin steel sheets as raw materials, the thin steel sheet materials are blanked to a predetermined dimension prior to forming, and after forming Often pierce holes of various shapes. Therefore, the material steel plates of these parts are required to have excellent formability and excellent punchability. When the punchability of the steel sheet is deteriorated, sagging and burrs generated on the punching end face during punching process become remarkable, and the dimensional accuracy of the machine parts is impaired. In addition, micro cracks are likely to occur on the punched end face, which may adversely affect the strength characteristics of mechanical parts.

However, none of the above prior art studies the punchability of the steel sheet. In addition, the following problems remain.
The techniques proposed in Patent Documents 1 and 2 contain a large amount of Al as a nitriding promoting element. Therefore, there are concerns about the occurrence of internal defects and surface defects due to Al inclusions, and a large amount of Al slag is generated, resulting in an increase in smelting cost during refining.

  In the technique proposed in Patent Document 3, it is said that sufficient hardening characteristics can be imparted to the steel sheet for soft nitriding even if the alloy elements for nitriding promotion are reduced, but the strength of the obtained steel sheet is insufficient. Application to high-load parts is difficult.

  The present invention solves the above-mentioned problems of the prior art, and can be widely used as a material for parts of automobile transmissions, etc., and is excellent in formability before nitriding and has excellent punchability and It aims at providing the manufacturing method.

  In order to solve the above-mentioned problems, the present inventors have intensively studied various factors affecting the formability and punchability of a steel sheet in addition to the surface hardening characteristics by nitriding of the steel sheet. As a result, by adjusting the chemical composition and microstructure of the steel sheet to a predetermined range, in addition to being able to give good hardening characteristics by nitriding treatment, the steel sheet also has sufficient formability and punchability. I found out that I can do it.

The present invention has been completed by further studies based on the above-described findings, and the gist of the present invention is as follows.
[1] By mass%
C: 0.02% to 0.08%, Si: 0.1% or less,
Mn: 0.2% to 1.8%, P: 0.05% or less,
S: 0.02% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% or more and 1.5% or less, N: 0.01% or less, with the balance consisting of Fe and inevitable impurities, ferrite as the main phase, pearlite and / or bainite as the second phase, A structure in which the fraction of the entire structure is 70% or more, the average crystal grain size of the ferrite is 5 μm or more and 25 μm or less, and the average major axis in the rolling direction section of the cementite existing in the second phase is 3.0 μm or less. A steel sheet for nitriding treatment, comprising:

[2] In the above [1], in addition to the above composition, the mass is selected from V: 0.005% to 0.075%, Nb: 0.005% to 0.025%, Ti: 0.005% to 0.025%. A steel sheet for nitriding characterized by containing one or more kinds.

[3] In mass%,
C: 0.02% to 0.08%, Si: 0.1% or less,
Mn: 0.2% to 1.8%, P: 0.05% or less,
S: 0.02% or less, Al: 0.01% or more and 0.06% or less,
A steel material containing Cr: 0.5% or more and 1.5% or less, N: 0.01% or less, and the balance consisting of Fe and inevitable impurities is heated to 1050 ° C or higher and 1250 ° C or lower, and Ar ₃ transformation point or higher ( (Ar ₃ transformation point + 100 ° C) or less, and hot rolling at a finishing temperature of 750 ° C or less, cooling the temperature range from the finishing temperature to 750 ° C at a cooling rate of 40 ° C / s to 80 ° C / s, and then from 750 ° C 500 ℃ or higher
For nitriding treatment, which is cooled at a cooling rate of 15 ° C / s to 35 ° C / s in the temperature range up to the cooling stop temperature of 650 ° C or lower and wound at a winding temperature of 500 ° C to 650 ° C. A method of manufacturing a steel sheet.

[4] In the above [3], in addition to the above composition, the mass is selected from V: 0.005% to 0.075%, Nb: 0.005% to 0.025%, Ti: 0.005% to 0.025%. The manufacturing method of the steel plate for nitriding processing characterized by containing 1 type, or 2 or more types.

  According to the present invention, a steel sheet having excellent formability and punchability and having good hardening characteristics by nitriding can be obtained. The steel sheet of the present invention is indeed suitable as a material for a molded part that is subjected to nitriding treatment, such as a transmission part of an automobile, and has a remarkable industrial effect. The steel sheet of the present invention is not limited to gas soft nitriding or salt bath soft nitriding, and is also suitable as a steel sheet for various nitriding treatments such as plasma nitriding, gas nitriding, carbonitriding, and nitrosulphurizing. Can be used.

First, the structure of the nitriding steel sheet of the present invention will be described.
The steel sheet of the present invention has a structure composed of ferrite as a main phase and a second phase. The second phase is pearlite and / or bainite. Furthermore, the fraction of the entire structure of the ferrite is 70% or more, the average crystal grain size of the ferrite is 5 μm or more and 25 μm or less, the average major axis in the rolling direction section of the cementite present in the second phase is 3.0 μm or less It is.

Main Phase: Ferrite The steel sheet of the present invention ensures the formability of the steel sheet by using soft ferrite as the main phase. When the main phase is other than ferrite, good formability cannot be imparted to the steel sheet. However, a steel sheet having a ferrite single-phase structure cannot secure sufficient strength as a material steel sheet that can be widely applied to transmission parts and the like of automobiles. Therefore, this invention steel plate is taken as the structure which consists of the ferrite which is a main phase, and the following 2nd phases.

Second phase: pearlite and / or bainite The remaining second phase other than ferrite is one or two selected from pearlite and bainite. The second phase in the steel sheet structure plays a role of reinforcing the strength of the steel sheet whose main phase is soft ferrite. Here, when utilizing the structure strengthening in which the second phase is martensite, the martensite is softened by the temperature rise during the nitriding treatment, and the strength fluctuation of the steel sheet increases. Therefore, in order to maintain stable steel sheet strength even after nitriding treatment maintained at about 500 to 600 ° C., the second phase in the steel sheet structure needs to be pearlite and / or bainite.

Percentage of ferrite in the entire structure: 70% or more In order to give good formability to the steel sheet, the fraction of ferrite as the main phase needs to be 70% or more. When the ferrite fraction is less than 70%, the formability of the steel sheet tends to be insufficient. Moreover, the punchability of the steel sheet is also reduced, for example, the shear surface ratio of the punched end face is reduced when the steel sheet is punched. On the other hand, if the ferrite fraction is too high, the strength of the steel sheet may not reach the required level, so the ferrite fraction is preferably 97% or less, more preferably 95% or less. preferable.

Average grain size of ferrite: 5 μm or more and 25 μm or less If the average grain size of ferrite exceeds 25 μm, the surface properties of the steel sheet will deteriorate during the forming process, or the smoothness of the punched surface will be reduced. It also leads to punching deterioration. Further, when the crystal grain size of ferrite is increased, the crystal grain boundaries are decreased, and therefore, there is a concern that the grain boundary diffusion of N during nitriding treatment is suppressed and the time required for nitriding treatment becomes longer. On the other hand, when the average crystal grain size of ferrite is less than 5 μm, the steel sheet is hardened and the formability tends to be lowered. Therefore, the average crystal grain size of ferrite is 5 μm or more and 25 μm or less. Preferably they are 5 micrometers or more and 15 micrometers or less.

Average length in the rolling direction cross section of the cementite present in the second phase: 3.0 μm or less When the average long diameter in the rolling direction cross section of the steel sheet of the cementite present in the second phase exceeds 3.0 μm, The degree of stress concentration at the interface between cementite and ferrite is increased, the occurrence of fine cracks is facilitated, and the fracture surface ratio at the punched end surface is increased. Therefore, the average major axis is 3.0 μm or less. However, when the cementite becomes extremely small, the second phase becomes harder than necessary, and the difference in hardness from ferrite as the main phase increases, so that microcracking is likely to occur at the punched end face of the steel sheet. Therefore, the average major axis is preferably 1.0 μm or more.

  Next, the reason for limiting the chemical composition of the steel sheet for nitriding treatment according to the present invention will be described. Hereinafter, “%” which is a unit of component element content means “% by mass” unless otherwise specified.

C: 0.02% to 0.08%
C is an element having an effect of strengthening steel through solid solution strengthening and formation of a second phase. If the C content is less than 0.02%, sufficient steel sheet strength cannot be secured as a component material. On the other hand, when the C content exceeds 0.08%, the strength of the steel sheet is excessively increased and the formability is lowered. In addition, the fraction of the second phase is increased and it is difficult to obtain a desired form of cementite. Therefore, the C content is 0.02% or more and 0.08% or less. Preferably they are 0.04% or more and 0.06% or less.

Si: 0.1% or less
Si is an element effective for deoxidation of steel, and has an effect of strengthening steel by solid solution strengthening. In order to obtain these effects, the Si content is preferably set to 0.01% or more. However, when the Si content exceeds 0.1%, a hard-peeling scale is generated during hot rolling, and the surface properties of the steel sheet are significantly deteriorated. Therefore, the Si content is 0.1% or less. Preferably it is 0.05% or less.

Mn: 0.2% to 1.8%
Mn is an element that strengthens steel by solid solution strengthening. In addition, S present as an impurity in the steel is fixed as a precipitate, and has an effect of reducing adverse effects caused by S. If the Mn content is less than 0.2%, the above-mentioned effects cannot be obtained sufficiently, and the necessary steel sheet strength cannot be ensured. On the other hand, if the Mn content exceeds 1.8%, the strength of the steel sheet is excessively increased, and a band-like structure due to microsegregation is easily formed, resulting in a decrease in formability and punchability of the steel sheet. Therefore, the Mn content is 0.2% to 1.8%. Preferably they are 0.2% or more and 1.2% or less.

P: 0.05% or less
P is an element present as an impurity in steel, and if it is contained in a large amount, the formability and toughness of the steel sheet deteriorate. Therefore, the P content is 0.05% or less. Preferably it is 0.03% or less.

S: 0.02% or less
S is also an element present in the steel as an impurity. Therefore, the S content is 0.02% or less. Preferably it is 0.01% or less.

Al: 0.01% or more and 0.06% or less
Al is an element added for deoxidation of steel. If the Al content in the steel is less than 0.01%, a sufficient deoxidation effect cannot be obtained. On the other hand, if the Al content in the steel exceeds 0.06%, the deoxidation effect is saturated, and the possibility that internal defects and surface defects increase due to an increase in inclusions in the steel increases. Therefore, the Al content is 0.01% or more and 0.06% or less. Preferably they are 0.02% or more and 0.05% or less.

Cr: 0.5% to 1.5%
Cr has the effect of increasing the hardness of the steel sheet surface layer by forming nitrides in the steel by nitriding, and is an important alloying element in the present invention. Moreover, it also has the effect | action which refines | miniaturizes the cementite in steel. In order to fully exhibit such effects, the Cr content needs to be 0.5% or more. However, if the Cr content exceeds 1.5%, the hardened portion of the outermost layer is significantly embrittled by the nitriding treatment, while the hardening depth may be decreased. Therefore, the Cr content is 0.5% or more and 1.5% or less. Preferably they are 0.5% or more and 1.0% or less.

N: 0.01% or less
N is an element present as an impurity in steel. A large amount of N reduces the formability of the steel sheet, and may combine with a nitriding promoting element such as Cr before nitriding to reduce the hardening characteristics by nitriding. Therefore, the N content is 0.01% or less. Preferably it is 0.005% or less.

  In addition to the above component composition, the steel sheet according to the present invention may further include one or two selected from V: 0.005% to 0.075%, Nb: 0.005% to 0.025%, Ti: 0.005% to 0.025%. It may contain seeds or more.

V: 0.005% to 0.075%
V is an element that has the effect of increasing the hardness of the steel sheet surface layer by forming nitrides in the steel by nitriding. Further, since V is a carbonitride-forming element, V also has an effect of increasing the strength of steel by particle dispersion strengthening (precipitation strengthening). Therefore, in the steel sheet of the present invention, V can be contained for the purpose of controlling the hardening characteristics by nitriding treatment or adjusting the strength level of the steel sheet. In order to sufficiently exhibit the above effects, the V content is preferably 0.005% or more. On the other hand, when the V content is excessive, it causes a decrease in formability due to an excessive increase in strength of the steel sheet, embrittlement of the hardened portion due to the nitriding treatment, and is economically disadvantageous. Therefore, the V content is preferably 0.005% or more and 0.075% or less. More preferably, it is 0.025% or more and 0.075% or less.

Nb: 0.005% to 0.025%
Nb is a carbonitride-forming element and has the effect of increasing the strength of steel by particle dispersion strengthening (precipitation strengthening). If the Nb content is less than 0.005%, the above effect cannot be obtained sufficiently. On the other hand, if the Nb content exceeds 0.025%, the strength of the steel sheet is excessively increased and the formability may be reduced. Therefore, the Nb content is preferably 0.005% or more and 0.025% or less. More preferably, it is 0.005% or more and 0.015% or less.

Ti: 0.005% to 0.025%
Ti is also a carbonitride-forming element and has the effect of increasing the strength of steel by particle dispersion strengthening (precipitation strengthening). If the Ti content is less than 0.005%, the above effects cannot be obtained sufficiently. On the other hand, if the Ti content exceeds 0.025%, the strength of the steel sheet is excessively increased and the formability may be deteriorated. Therefore, the Ti content is preferably 0.005% or more and 0.025% or less. More preferably, it is 0.005% or more and 0.015% or less.

  The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include Cu: 0.03% or less, Ni: 0.03% or less, Mo: 0.03% or less, Sn: 0.003% or less, Sb: 0.003% or less, O: 0.005% or less.

Next, the manufacturing method of the steel sheet for nitriding treatment of the present invention will be described.
The steel sheet of the present invention is obtained by heating and hot rolling a steel material having the above chemical composition, and then cooling and winding it.
The steel used in the present invention can be melted by any known melting method such as a converter method or an electric furnace method. The molten steel is made into a steel material (slab) by continuous casting or ingot-making / bundling rolling. If necessary, various pretreatments, secondary refining, surface treatment of steel materials, and the like can be performed.

Heating temperature of steel material: 1050 ° C. or higher and 1250 ° C. or lower If the heating temperature of the steel material is lower than 1050 ° C., it is difficult to ensure a desired finishing temperature during hot rolling. On the other hand, when the heating temperature of the steel material exceeds 1250 ° C., the energy required for heating increases and the surface properties of the steel sheet are liable to occur. Therefore, the heating temperature of the steel material before hot rolling is set to 1050 ° C. or more and 1250 ° C. or less. Preferably they are 1100 degreeC or more and 1200 degrees C or less.

In the heating of the steel material, the steel material cooled to room temperature may be reheated, or the steel material being cooled after casting may be additionally heated or kept warm.
In this invention, after heating a steel raw material to the said temperature range, rough rolling and finish rolling (hot rolling) are performed, However About rough rolling conditions should just follow a conventional method and it does not need to specifically limit.

Finishing temperature: Ar ₃ transformation point or higher (Ar ₃ transformation point + 100 ° C) or lower If the finishing temperature in the hot rolling process is lower than the Ar ₃ transformation point, an unrecrystallized ferrite structure or pancake-like structure extending in the rolling direction A coarse ferrite structure is formed, ferrite having a desired particle diameter cannot be obtained, and the formability and punchability of the steel sheet are lowered. In addition, the in-plane anisotropy of the mechanical properties of the steel sheet is increased. On the other hand, when the finishing temperature exceeds (Ar ₃ transformation point + 100 ° C.), the surface properties of the steel sheet are easily deteriorated, and the ferrite structure is easily coarsened, making it difficult to obtain ferrite having a desired particle diameter. Therefore, the finishing temperature should be not less than the Ar ₃ transformation point and not more than (Ar ₃ transformation point + 100 ° C.). It is preferably (Ar ₃ transformation point + 20 ° C.) or more and (Ar ₃ transformation point + 100 ° C.) or less. In order to secure a necessary finishing temperature, the steel plate being rolled may be additionally heated using a heating device such as a sheet bar heater or an edge heater.

Cooling rate from finishing temperature to 750 ° C: 40 ° C / s to 80 ° C / s The steel sheet after hot rolling is cooled to 40 ° C / s to 80 ° C / s in the temperature range from the finishing temperature to 750 ° C. Cool at a speed (forced cooling). Preferably, it is 45 ° C./s or more and 75 ° C./s or less. When the cooling rate in this temperature range is less than 40 ° C./s, the structure of the hot-rolled steel sheet tends to be coarsened, and ferrite or cementite having a desired shape cannot be obtained. On the other hand, when the cooling rate in this temperature range exceeds 80 ° C./s, martensite or excessively much bainite and pearlite are easily generated in the hot-rolled steel sheet, and a desired fraction of ferrite and a desired second phase are formed. Becomes difficult to obtain.

Cooling rate from 750 ° C to cooling stop temperature: 15 ° C / s to 35 ° C / s Cooling stop temperature: 500 ° C to 650 ° C
The temperature range from 750 ° C to the cooling stop temperature is cooled (forced cooling) at a cooling rate of 15 ° C / s to 35 ° C / s. Preferably, it is 15 ° C./s or more and 25 ° C./s or less. When the cooling rate in this temperature range is less than 15 ° C./s, the structure of the hot-rolled steel sheet is easily coarsened, and it becomes difficult to obtain ferrite and cementite having a desired shape. On the other hand, when the cooling rate in this temperature range exceeds 35 ° C./s, the progress of ferrite transformation is suppressed, and a ferrite having a desired fraction cannot be obtained.

  When the cooling stop temperature is less than 500 ° C., martensite and excessively much bainite are generated, so that the steel plate becomes hard and the formability of the steel plate is lowered, or the strength of the steel plate after nitriding is unstable. Become. On the other hand, when the cooling stop temperature exceeds 650 ° C., the pearlite becomes coarse, and a cementite having a desired shape cannot be obtained. Therefore, the cooling stop temperature is set to 500 ° C. or more and 650 ° C. or less. Preferably they are 500 degreeC or more and 600 degrees C or less.

  In addition, the steel plate cooled until it reaches the cooling stop temperature may be immediately wound, or may be allowed to cool for a short time until it is wound by a winder (coiler). Here, the term “cooling” refers to air cooling in the atmosphere without forced cooling by water injection. However, in order to drain the cooling water remaining on the steel plate, spraying high-pressure water or compressed air onto the steel plate being allowed to cool for a very short time is acceptable because the temperature drop of the steel plate is very small.

Winding temperature: 500 ° C or more and 650 ° C or less When the winding temperature is less than 500 ° C, martensite and excessively much bainite are generated, the steel plate becomes hard, and the formability of the steel plate decreases. The steel sheet strength after nitriding becomes unstable. On the other hand, when the coiling temperature exceeds 650 ° C., the pearlite is coarsened, and a cementite having a desired shape cannot be obtained. Therefore, the winding temperature is set to 500 ° C. or more and 650 ° C. or less. Preferably they are 500 degreeC or more and 600 degrees C or less.

  The steel sheet after winding is used after removing the oxide scale by pickling or shot peening. Moreover, you may give the temper rolling for shape correction and adjustment of surface roughness. By performing such oxide scale removal and temper rolling, the effects of the present invention are not impaired.

  A steel material obtained by melting steels A to L containing the constituent elements shown in Table 1 and the balance being Fe and inevitable impurities is hot-rolled under the conditions shown in Table 2 to obtain a sheet thickness of 2.3 mm. The hot-rolled steel sheet. Next, the obtained hot-rolled steel sheet was pickled and descaled, and then subjected to temper rolling with an elongation of 0.5%. Samples were taken from each hot-rolled steel sheet after temper rolling and subjected to microstructure observation, tensile test, and punching test. Furthermore, the hot-rolled steel sheet after temper rolling was subjected to nitriding treatment, and the hardness test was performed on the hot-rolled steel sheet after nitriding treatment.

(1) Microstructure observation The microstructure of the steel sheet was obtained by taking a sample of a sheet thickness cross section parallel to the rolling direction at a 1/4 width position from the steel sheet before nitriding, mirror-polishing and corroding with nital. It confirmed with the optical microscope or the scanning electron microscope using the image which image | photographed the board thickness 1/4 position by the suitable magnification of 500-5000 times.
The ferrite fraction in the microstructure was obtained by using the above-mentioned image and obtaining the area ratio of the ferrite by image analysis, which was defined as the ferrite fraction.
The average crystal grain size of ferrite was calculated from the grain size number by obtaining the crystal grain size in accordance with the method specified in Japanese Industrial Standard JIS G 0551-2005 using the above image.
The average major axis of cementite present in the second phase (perlite and / or bainite) was calculated by calculating the major axis of each cementite within the observation range using the image and calculating the arithmetic mean. These results are also shown in Table 2.

(2) Tensile test (evaluation of formability)
The formability of the steel sheet was evaluated by the ductility by a tensile test. Tensile tests were conducted using JIS Z 2241-2011 No. 5 test specimens taken from steel sheets before nitriding so that the test direction was the rolling direction at the 1/4 width position of the steel sheet. The measurement was performed in accordance with the provisions of 2241-2011, and the tensile strength (TS) and elongation at break (EL) were measured to calculate the strength-elongation balance (TS × EL). Here, a steel sheet having a strength elongation balance value of 16 GPa ·% or more was determined to have good formability.

(3) Punching test (evaluation of punchability)
A 50mm-diameter disk-shaped test piece is punched from the steel sheet before nitriding (clearance: 5% of the steel plate thickness), the shear surface ratio at the punched end face of the test piece is measured, and the fracture surface area is measured. The presence or absence of microcracks was confirmed. When the shear surface ratio was 60% or more and no crack was observed in the fracture surface area, it was determined that the steel sheet had good punchability.

(4) Hardness test (Evaluation of surface hardening characteristics by nitriding treatment)
The hot rolled steel sheet after temper rolling was subjected to gas soft nitriding treatment, and the cross-sectional hardness (nitrided layer cross-sectional hardness) of the steel sheet after gas soft nitriding treatment was measured. As the nitriding gas, a gas obtained by mixing ammonia (NH ₃ ) and an endothermic metamorphic gas in an equal ratio was used. The gas soft nitriding temperature was 570 ° C., the holding time at the gas soft nitriding temperature was 150 minutes, and oil cooling was performed after the holding. For the cross-sectional hardness of the steel sheet, a sample with a thickness cross-section parallel to the rolling direction of the steel sheet after gas soft nitriding was taken, and a depth of 0.2 mm from the surface of the steel sheet in accordance with the provisions of JIS Z 2244-2009. Vickers hardness (HV0.1) was measured. When the value of the Vickers hardness measured here was 250 or more, it was determined that the surface hardening property of the steel sheet by nitriding treatment was good.
These results are shown in Table 3.

  Each steel plate (invention example) suitable for the present invention is a steel plate having good formability, excellent punchability of the steel plate, and excellent surface hardening characteristics by nitriding treatment. On the other hand, in each steel sheet (comparative example) in which the chemical composition and microstructure of the steel are outside the scope of the present invention, any of the characteristics of formability, punchability, surface hardening characteristics by nitriding treatment, or all characteristics The level is inadequate.

Claims (4)

% By mass
C: 0.02% to 0.08%, Si: 0.1% or less,
Mn: 0.2% to 1.8%, P: 0.05% or less,
S: 0.02% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% or more and 1.5% or less, N: 0.01% or less, with the balance consisting of Fe and inevitable impurities, ferrite as the main phase, pearlite and / or bainite as the second phase, A structure in which the fraction of the entire structure is 70% or more, the average crystal grain size of the ferrite is 5 μm or more and 25 μm or less, and the average major axis in the rolling direction section of the cementite existing in the second phase is 3.0 μm or less. A steel sheet for nitriding treatment, comprising:   In addition to the above composition, the composition further contains one or more selected from the group consisting of V: 0.005% to 0.075%, Nb: 0.005% to 0.025%, Ti: 0.005% to 0.025%. The steel sheet for nitriding according to claim 1, wherein % By mass
C: 0.02% to 0.08%, Si: 0.1% or less,
Mn: 0.2% to 1.8%, P: 0.05% or less,
S: 0.02% or less, Al: 0.01% or more and 0.06% or less,
A steel material containing Cr: 0.5% or more and 1.5% or less, N: 0.01% or less, and the balance consisting of Fe and inevitable impurities is heated to 1050 ° C or higher and 1250 ° C or lower, and Ar ₃ transformation point or higher ( (Ar ₃ transformation point + 100 ° C) or less, and hot rolling at a finishing temperature of 750 ° C or less, cooling the temperature range from the finishing temperature to 750 ° C at a cooling rate of 40 ° C / s to 80 ° C / s, and then from 750 ° C Cooling at a cooling rate of 15 ° C / s or more and 35 ° C / s or less, and winding at a winding temperature of 500 ° C or more and 650 ° C or less. A method of manufacturing a steel sheet for nitriding treatment.   In addition to the above composition, the composition further contains one or more selected from the group consisting of V: 0.005% to 0.075%, Nb: 0.005% to 0.025%, Ti: 0.005% to 0.025%. The method for producing a steel sheet for nitriding treatment according to claim 3.