JP5614330B2 - Steel sheet for soft nitriding treatment and method for producing the same - Google Patents

Description

  The present invention relates to a steel sheet for nitrocarburizing treatment that is suitable for machine structural parts that require fatigue strength and wear resistance, such as transmission parts of automobiles, and more particularly to formability before nitronitriding treatment and after nitrocarburizing treatment. The present invention relates to a steel sheet for nitrocarburizing treatment excellent in strength stability and a method for producing the same.

  Fatigue strength and wear resistance are required for mechanical structural parts that are used in a state where stress is continuously applied for a long time, such as automobile transmission parts. Therefore, these mechanical structural parts are usually manufactured by processing a steel material into a desired part shape and then subjecting it to a surface hardening heat treatment. When the surface hardening heat treatment is performed, the steel surface is hardened and compressive residual stress is introduced into the steel surface layer portion, so that the fatigue strength and wear resistance of the parts are improved.

Typical examples of the surface hardening heat treatment include carburizing treatment and nitriding treatment. Carburization, the steel is heated to a temperature above A ₃ transformation point, a process of diffusion and osmosis (carburized) carbon in the surface layer of the steel, usually, be directly quenching the steel after carburizing in a high temperature state Thus, the surface hardening of the steel is achieved. This carburization process, for diffusing and spreading the carbon steel surface layer portion in A ₃ transformation point or more high temperature zone, the results of carbon is diffused and penetrated to a relatively deep position from the surface of the steel, greater case depth is obtained .

  However, when carburizing treatment is employed as the surface hardening heat treatment, a decrease in the accuracy of the part shape due to transformation strain and thermal strain during quenching is inevitable. Moreover, in the state of being quenched after carburizing, the toughness of the steel is significantly reduced. Therefore, when manufacturing parts by carburizing, it is essential to perform tempering (for example, press tempering) for the purpose of correcting the shape of parts and restoring toughness after quenching, which increases the number of manufacturing processes. This is extremely disadvantageous in terms of cost.

On the other hand, nitriding treatment, the steel is heated to a temperature below the A ₁ transformation point, a process of diffusion and osmosis (nitride) of nitrogen into the steel surface layer portion, without hardening as carburizing, surface hardening of steel Is intended. In other words, since the nitriding treatment has a relatively low processing temperature and does not involve a phase transformation of steel, if the nitriding treatment is performed to manufacture a part, the shape accuracy of the part can be kept good. However, in the case of gas nitriding using ammonia gas, the time required for nitriding is as long as about 25 to 150 hours, so that it is not suitable for automobile parts and the like on the premise of mass production.

  In order to advantageously solve the above-mentioned problems found in gas nitriding, in recent years, soft nitriding is becoming popular. The soft nitriding treatment is a nitriding treatment that rapidly proceeds the nitriding reaction by using a carburizing atmosphere. According to this soft nitriding treatment, the steel surface hardness obtained is lower than that of the conventional nitriding treatment (gas nitriding), but the nitriding treatment time can be greatly shortened.

  Soft nitriding is roughly classified into a method of nitriding in a salt bath and a method of nitriding in a gas. In the method of nitriding in a salt bath (salt bath soft nitriding treatment), since a cyan bath is used, measures to prevent environmental pollution are essential. On the other hand, in the method of nitriding in gas (gas soft nitriding treatment), since a mixed gas containing ammonia as a main component is used, there are few emissions that cause environmental pollution. For the above reasons, among the soft nitriding treatments, the spread rate of the gas soft nitriding treatment, particularly nitriding in gas, is increasing.

  On the other hand, machine structural parts such as automobile transmission parts are conventionally manufactured by machining intermediate products obtained by casting or forging, and processing and joining them into desired shapes. However, in recent years, steel plates (thin steel plates) have been actively used as raw materials, and the steel plates (thin steel plates) are subjected to press working or the like and formed into a desired shape for production. Thereby, a manufacturing process is shortened compared with the past, and the manufacturing cost can be significantly reduced. Against this background, there has been a growing demand for steel sheets for nitrocarburizing treatment with excellent formability, which are suitable for materials for machine structural parts such as automobile transmission parts, and various techniques have been proposed to date.

  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%, Ti: more than 0.010% and less than 4C [%], V: 0.010 to 1.00% The steel is rolled up at 500 ° C. or higher after hot rolling, and then cold rolled at a reduction rate of 50% or higher, and recrystallization annealing is performed. A steel sheet for nitriding having the above composition and excellent formability is disclosed. In addition, according to the technology, the steel content for nitriding is excellent in formability and nitriding properties by suppressing the C content that adversely affects formability to less than 0.08% and containing Cr, Al, etc. as nitriding promoting elements. It is supposed to be.

  Patent Document 3 contains, in mass%, C: 0.03% or more and less than 0.10%, Si: 0.005 to 0.10%, Mn: 0.1 to 1.0%, Cr: 0.20 to 2.00%, and as impurities, S: 0.01% or less, P: 0.020% or less, sol.Al: 0.10% or less, N: 0.01% or less, with the balance being substantially composed of Fe, and the ferrite grain size specified by JIS G 0552 as the grain size number A steel for nitrocarburizing treatment of 5 to 12 is proposed. And according to such technology, it is said that an inexpensive steel sheet is obtained because expensive elements such as Ti and V are not added, and a steel sheet excellent in press workability is obtained by refining the crystal grain size of steel. ing.

Further, Patent Document 4 includes mass%, C: more than 0.01%, 0.09% or less, Si: 0.005 to 0.5%, 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 V: 0.01-1.0%, Ti: 0.01-1.0% and Nb: 0.01-1.0% There has been proposed a thin steel sheet for nitriding with a composition containing the above and having a grain interface area Sv per unit volume of 80 mm ^-1 or more and 1300 mm ^-1 or less. And according to such a technique, after containing nitride forming elements such as Cr, Al, V, Ti, and Nb within a range that does not impair the formability of the steel sheet, the grain boundary area per unit volume is controlled within a predetermined range. By doing so, it is said that both high surface hardness and sufficient curing depth can be obtained after nitriding.

  Moreover, in patent document 5, C: 0.03-0.10 mass%, Si: 0.5 mass% or less, Mn: 0.1-0.6 mass%, P: 0.04 mass% or less, S: 0.04 mass% or less, Al: 0.005-0.08 A steel sheet for nitrocarburizing treatment having a composition containing mass%, Cr: 0.4 to 1.2 mass%, Nb: 0.002 mass% or more and less than 0.01 mass% and N: 0.01 mass% or less has been proposed. And according to such a technique, it is said that a soft nitriding steel sheet having both workability and fatigue characteristics can be obtained by containing a small amount of Nb.

Japanese Patent Laid-Open No. 9-25513 Japanese Patent Laid-Open No. 9-25543 Japanese Patent Laid-Open No. 2003-1054889 JP 2003-277877 A JP 2009-68057 A

  However, since the techniques proposed in Patent Document 1 and Patent Document 2 contain a large amount of Al as a nitriding promoting element, there is a concern about the occurrence of internal defects and surface defects due to Al inclusions. In addition, since a large amount of Al-based slag is produced during refining, there is also a problem that the melting cost increases.

  In addition, the technique proposed in Patent Document 3 does not contain an expensive element, so that an inexpensive steel sheet for soft nitriding can be obtained. However, the tensile strength is about 420 MPa at the most, so that it is in a high stress load state. Application to the parts used is limited.

  Further, in the technique proposed in Patent Document 4, although a thin steel sheet for nitriding treatment having a tensile strength exceeding 500 MPa is obtained, the hardness distribution in the thickness direction after nitriding treatment is not taken into consideration, and actually In many cases, the durability of the parts does not reach the necessary and sufficient level when the nitriding treatment is applied.

  In addition, the technique proposed in Patent Document 5 can provide a steel sheet for nitrocarburizing treatment with excellent workability, but its strength is at most about 400 MPa in tensile strength. Similarly, application to parts used in high stress loading conditions is limited.

  Furthermore, when soft nitriding is applied to a steel sheet, the steel sheet is usually heated to a processing temperature of about 550 to 600 ° C. and held at the processing temperature for about 1 to 5 hours. On the other hand, the strength inside the steel sheet (non-nitrided part) may decrease. Therefore, even if it has the desired strength (tensile strength) before soft nitriding, the strength inside the steel sheet (non-nitrided part) is significantly reduced by soft nitriding, which is desirable for parts after soft nitriding It is conceivable that the strength (fatigue strength) cannot be imparted.

  For the above reasons, in the nitrocarburized steel sheet, the strength inside the steel sheet (non-nitrided part) is not significantly reduced through the soft nitriding process, and the strength change inside the steel sheet (non-nitrided part) before and after the soft nitriding process is small. That is, having strength stability after soft nitriding is one important characteristic. However, in any of the above-described conventional techniques, no investigation has been made on strength stability after soft nitriding.

  The present invention advantageously solves the above-mentioned problems of the prior art, has a desired strength (tensile strength: 440 MPa or more), and has formability before soft nitriding and strength stability after soft nitriding. An object of the present invention is to provide a steel sheet for nitrocarburizing process excellent in the above and a method for producing the same.

In order to solve the above-mentioned problems, the present inventors have intensively studied various factors affecting the strength and formability of the nitrocarburizing steel sheet and the strength change inside the steel sheet (non-nitriding part) before and after the nitronitriding process. did. As a result, the following findings were obtained.
1) By making the steel sheet structure a composite structure containing ferrite and pearlite, a decrease in strength after nitrocarburizing treatment is suppressed, and a steel sheet having excellent strength stability is obtained.
2) When the ratio of polygonal ferrite in the ferrite is increased, the strength of the steel sheet is lowered, and the strength change inside the steel sheet (non-nitrided portion) before and after the soft nitriding treatment is likely to increase.
3) Concerning the steel sheet composition, the inclusion of a desired amount of Nb is effective in increasing the steel sheet strength and reducing the ratio of polygonal ferrite in the ferrite.
4) Concerning the steel sheet composition, by containing Nb and C so as to satisfy a predetermined relationship (0.10 ≦ Nb / C ≦ 0.30), the strength change inside the steel sheet (non-nitrided part) before and after soft nitriding is reduced. about.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
Mn: 0.7% to 1.5%, P: 0.05% or less,
S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% to 1.5%, Nb: 0.005% to 0.025%,
N: 0.005% or less, so that C and Nb satisfy the following formula (1), the balance is composed of Fe and inevitable impurities, and includes ferrite and pearlite. A steel sheet for soft nitriding characterized by having a composite structure having a ratio of 1% or less and a structure in which the ratio of polygonal ferrite in the ferrite is less than 50%.
Record
0.10 ≦ Nb / C ≦ 0.30 (1)
(C, Nb: content of each element (mass%))

(2) Heating the steel slab, subjecting it to hot rolling consisting of rough rolling and finish rolling, cooling after completion of finish rolling, winding, and hot rolling steel sheet,
The steel slab is in mass%,
C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
Mn: 0.7% to 1.5%, P: 0.05% or less,
S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% to 1.5%, Nb: 0.005% to 0.025%,
N: 0.005% or less, so that C and Nb satisfy the following formula (1), the balance is composed of Fe and inevitable impurities, and the heating temperature of the hot rolling is 1100 ° C. or higher and 1300 ° C. or lower The finishing temperature of the finish rolling is Ar ₃ transformation point or higher (Ar ₃ transformation point + 100 ° C.) or lower, the average cooling rate of cooling is 30 ° C./s or higher, and the winding temperature of winding is 500 ° C. or higher. A method for producing a steel sheet for nitrocarburizing treatment, characterized by being 650 ° C or lower.
Record
0.10 ≦ Nb / C ≦ 0.30 (1)
(C, Nb: content of each element (mass%))

  According to the present invention, a steel sheet for nitrocarburizing treatment having a desired strength (tensile strength: 440 MPa or more) and excellent in formability before nitrocarburizing treatment and strength stability after nitrocarburizing treatment is obtained. . Therefore, even for parts used under high stress load conditions such as automobile transmission parts, it is possible to significantly reduce the manufacturing cost by using a steel plate material, and there is a remarkable industrial effect.

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the component composition of the steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.05% or more and 0.10% or less
C is an element that contributes to increasing the strength of steel through solid solution strengthening and formation of the second phase. When the C content is less than 0.05%, it is not possible to ensure the strength of the steel sheet required as a material for parts used in high stress load conditions such as automobile transmission parts. On the other hand, when the C content exceeds 0.10%, the steel sheet strength is excessively increased and formability is deteriorated. Therefore, the C content is 0.05% or more and 0.10% or less. Preferably they are 0.05% or more and 0.08% or less.

Si: 0.5% or less
Si is a solid solution strengthening element, is an element effective for increasing the strength of steel, and also acts as a deoxidizer. In order to obtain such an effect, it is preferable to contain 0.03% or more. However, if the Si content exceeds 0.5%, a hardly peelable scale is generated and the surface properties of the steel sheet are significantly deteriorated. Therefore, the Si content is 0.5% or less. Preferably, it is 0.1% or less.

Mn: 0.7% to 1.5%
Mn is a solid solution strengthening element and is an element effective for increasing the strength of steel. It also acts as an element that fixes S present as impurities in steel as precipitates and reduces the adverse effects of S on steel. When the Mn content is less than 0.7%, the desired steel plate strength cannot be ensured. On the other hand, when the Mn content exceeds 1.5%, the steel sheet strength is excessively increased and formability is deteriorated. Therefore, the Mn content is 0.7% or more and 1.5% or less. Preferably they are 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 that lowers the formability and toughness of the steel sheet. In the present invention, P is preferably reduced as much as possible. Therefore, the P content is 0.05% or less. Preferably it is 0.03% or less.

S: 0.01% or less
S, like P, is an element that lowers the formability and toughness of the steel sheet. In the present invention, S is preferably reduced as much as possible. Therefore, the S content is 0.01% or less. Preferably it is 0.005% or less.

Al: 0.01% or more and 0.06% or less
Al is an element that acts as a deoxidizer, and the Al content is set to 0.01% or more in order to reliably obtain the effect. On the other hand, when the Al content exceeds 0.06%, the deoxidation effect is saturated, and Al inclusions increase to cause internal defects and surface defects in the steel sheet. 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 is an element that has the effect of forming nitrides in steel by soft nitriding and increasing the hardness of the surface layer of the steel sheet, and is one of the important elements in the present invention. In order to make such an effect remarkable, the Cr content needs to be 0.5% or more. On the other hand, when the Cr content exceeds 1.5%, the surface hardened layer (nitrided layer) obtained by soft nitriding becomes brittle. Therefore, the Cr content is 0.5% or more and 1.5% or less. Preferably they are 0.8% or more and 1.2% or less.

Nb: 0.005% to 0.025%
Nb is precipitated in the steel as carbonitride, and is an element effective in increasing the strength of the steel sheet by particle dispersion strengthening (precipitation strengthening) and ensuring the steel sheet strength stability after the soft nitriding treatment. Is one of the important elements. When the Nb content is less than 0.005%, desired steel plate strength and steel plate strength stability cannot be ensured. On the other hand, when the Nb content exceeds 0.025%, the steel sheet strength is excessively increased and formability is deteriorated. Therefore, the Nb content is 0.005% or more and 0.025% or less. Preferably they are 0.010% or more and 0.020% or less.

N: 0.005% or less
N is a harmful element that reduces the formability of the steel sheet. N is also an element that combines with a nitriding promoting element such as Cr before the soft nitriding treatment and causes a decrease in the effective nitriding promoting element amount. Therefore, in the present invention, it is preferable to reduce the N content as much as possible, and it is 0.005% or less. Preferably it is 0.003% or less.

Furthermore, the steel sheet of the present invention contains C and Nb so as to satisfy the above range and the expression (1).
0.10 ≦ Nb / C ≦ 0.30 (1)
(C, Nb: content of each element (mass%))
The above formula (1) improves the strength of the steel plate before soft nitriding and reduces the strength change inside the steel plate (non-nitrided part) before and after soft nitriding, that is, ensures strength stability after soft nitriding. It is a requirement to be satisfied.

  As described above, in the present invention, since precipitation strengthening by Nb carbonitride is used as one of the mechanisms for increasing the strength of steel sheets, the strength change inside the steel sheet (non-nitrided part) due to soft nitriding treatment is used. In order to suppress it, it is important to suppress the fluctuation of the precipitation strengthening amount before and after the soft nitriding treatment. And in order to suppress the fluctuation | variation of the said precipitation strengthening amount, even if a steel plate passes through the thermal history of a soft nitriding treatment, the precipitation state (a particle size and volume ratio) of Nb carbonitride in a steel plate is the precipitation state before a soft nitriding treatment. It is necessary not to fluctuate greatly.

  Therefore, as a result of the study by the present inventors on the means for stabilizing the precipitation strengthening amount before and after the soft nitriding treatment, it is effective to adjust the Nb content relative to the C content in the steel so as to satisfy the above formula (1). I found out. When Nb / C is within the range of the above formula (1), it is presumed that the growth and additional precipitation of Nb carbonitride during the soft nitriding treatment are suppressed, or that they are minute 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, components other than those described above are Fe and inevitable impurities. As inevitable impurities, for example, in mass%, 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, V: 0.005% or less, Zr: 0.005% or less, Ca: 0.005% or less, Sn: 0.005% or less, O: 0.005% or less, B: 0.0005% or less, etc. are acceptable.

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 is a composite structure including ferrite and pearlite, and has a structure in which the proportion of polygonal ferrite in the ferrite is less than 50%.

  Increasing the proportion of ferrite in the steel sheet structure is effective in securing the formability of the steel sheet. However, if the steel sheet has a ferrite single-phase structure, the steel sheet strength is insufficient and can be used as a material for machine structural parts. The range is narrowed and the versatility becomes poor. On the other hand, in order to strengthen the structure by generating a second phase in the ferrite-based structure, to ensure the strength of the steel sheet, when the hard low-temperature transformation phase such as martensite and bainite is the second phase, The low temperature transformation phase is softened by the thermal history during the soft nitriding treatment, and the strength change inside the steel sheet (non-nitriding portion) before and after the soft nitriding treatment becomes very large.

  Therefore, in the present invention, in order to suppress the strength change inside the steel sheet (non-nitrided part) due to the thermal history of soft nitriding treatment, the structure of the steel sheet is composed of a composite structure in which ferrite is the main phase and the second phase is pearlite. To do. In the present invention, it is preferable that the ferrite fraction in the steel sheet structure is 80% to 95% and the pearlite fraction in the steel sheet structure is 5% to 20%. Also, the steel sheet of the present invention is ideally a composite structure composed of ferrite and pearlite, but even if other phases (structures) are inevitably generated, the distribution in the steel sheet structure is unavoidable. It is acceptable if the rates are 1% or less in total.

  Polygonal ferrite is soft and tends to grow during heating. Therefore, in a steel sheet containing a large amount of polygonal ferrite, the steel sheet strength tends to be low, and the strength inside the steel sheet (non-nitrided portion) is likely to decrease due to grain growth during soft nitriding. Therefore, in the present invention, 50% or more of the ferrite is ferrite other than polygonal ferrite, and less than 50% of the ferrite is polygonal ferrite. In the present invention, examples of ferrite other than polygonal ferrite include acicular ferrite and bainitic ferrite.

Next, the manufacturing method of this invention steel plate is demonstrated.
In the present invention, a steel slab having the above composition is heated, subjected to hot rolling consisting of rough rolling and finish rolling, cooled after completion of finish rolling, and wound into a hot rolled steel sheet. At this time, the heating temperature is set to 1100 ° C to 1300 ° C, the finishing temperature is set to Ar ₃ transformation point or higher (Ar ₃ transformation point + 100 ° C) or lower, the average cooling rate of cooling is set to 30 ° C / s or higher, and the coiling temperature is set. It is preferable that the temperature is 500 ° C. or higher and 650 ° C. or lower.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. In addition, after melting, it is preferable to form a steel slab (slab) by a continuous casting method due to problems such as segregation, but as a steel slab by a known method such as an ingot-bundling rolling method or a thin slab continuous casting method. Also good. Furthermore, if necessary, various pretreatments, secondary refining, surface treatment of steel pieces, and the like may be performed.

Steel slab heating temperature: 1100 ° C or higher and 1300 ° C or lower The steel slab obtained as described above is subjected to rough rolling and finish rolling. In the present invention, Nb is sufficiently resolidified in the steel slab before rough rolling. It needs to be dissolved. When the heating temperature of the billet is less than 1100 ° C., the Nb carbonitride cannot be sufficiently decomposed to re-dissolve Nb, and the desired effect obtained by containing Nb is exhibited. I can't. On the other hand, if the heating temperature of the steel slab exceeds 1300 ° C., the energy required for heating the steel slab increases, which is disadvantageous in terms of cost. Therefore, the heating temperature of the steel slab before rough rolling is 1100 ° C. or higher and 1300 ° C. or lower. Preferably they are 1150 degreeC or more and 1250 degrees C or less.

  When heating the steel slab before rough rolling, the steel slab after casting may be heated after cooling to room temperature, or the steel slab during cooling after casting may be additionally heated or kept warm. Further, when the steel slab after casting maintains a sufficient temperature and Nb is sufficiently dissolved in the steel, direct rolling may be performed without heating the steel slab. The rough rolling conditions are not particularly limited.

Finishing temperature: Ar ₃ transformation point or more (Ar ₃ transformation point + 100 ° C) or less When the finishing temperature is less than Ar ₃ transformation point, a ferrite structure that extends in the rolling direction and an unrecrystallized ferrite structure are formed. The moldability of the is reduced. In addition, the in-plane anisotropy of the mechanical properties of the steel sheet becomes strong, and uniform forming becomes difficult. On the other hand, when the finishing temperature exceeds (Ar ₃ transformation point + 100 ° C.), the surface properties of the steel sheet tend to deteriorate. Therefore, the finishing temperature should be not less than the Ar ₃ transformation point and not more than (Ar ₃ transformation point + 100 ° C.). Here, the finishing temperature refers to the steel plate temperature at the final pass exit side of finish rolling.

In order to ensure the finishing temperature, a steel plate being rolled may be additionally heated using a heating device such as a sheet bar heater or an edge heater. Note that the Ar ₃ transformation point of steel can be obtained by measuring the heat shrinkage in the cooling process from the austenite temperature range and creating a heat shrinkage curve, or by calculating from the alloy element content. You can also.

Average cooling rate: 30 ° C / s or more Optimization of the average cooling rate is important for obtaining the desired structure of the steel sheet. In the present invention, cooling is started immediately (within 1 s) after finishing rolling. The average cooling rate from the finishing temperature to the coiling temperature is 30 ° C / s or more. When this average cooling rate is less than 30 ° C./s, a large amount of polygonal ferrite that easily occurs in a high temperature region is generated, and a steel sheet having a desired structure cannot be obtained. Further, the crystal grains may be excessively coarsened, and the strength and ductility of the steel sheet may be reduced. Furthermore, in the present invention, the strength of the steel sheet is increased by precipitating Nb carbonitride in the steel sheet, but when the average cooling rate is less than 30 ° C./s, the Nb carbonitride becomes coarse, The desired steel plate strength may not be obtained. Therefore, the average cooling rate is set to 30 ° C./s or more.

  The upper limit of the average cooling rate is not particularly defined, but is preferably set to 100 ° C./s or less in order to avoid the shape failure of the steel sheet due to strong water cooling. In addition, after the steel plate is cooled until it reaches the winding temperature, forced cooling by water injection or the like is not particularly necessary, and it may be allowed to cool in the atmosphere until winding.

Winding temperature: 500 ° C. or higher and 650 ° C. or lower Optimization of the winding temperature is important for making the steel sheet a desired structure. When the coiling temperature is less than 500 ° C., a low-temperature transformation phase is generated and the steel sheet is hardened, the formability is lowered, and the steel sheet strength stability after the soft nitriding treatment is also lowered. On the other hand, when the coiling temperature exceeds 650 ° C., the amount of polygonal ferrite increases and a desired steel sheet structure cannot be obtained. Therefore, the winding temperature is set to 500 ° C. or more and 650 ° C. or less. Preferably they are 550 degreeC or more and 650 degrees C or less.

The hot-rolled steel sheet obtained as described above is used as a steel sheet for soft nitriding treatment after removing the oxide scale by pickling, shot peening or the like. Moreover, even if the temper rolling for the purpose of shape correction or surface roughness adjustment is performed, the effect of the present invention is not impaired.
The steel sheet for soft nitriding of the present invention can be applied to both gas soft nitriding and salt bath soft nitriding.

  Steel having the composition shown in Table 1 was melted and ingot-formed / bundled and rolled into steel pieces. These steel slabs were heated, then subjected to rough rolling and finish rolling, cooled, wound up, and formed into hot-rolled steel sheets having a sheet thickness of 2.9 mm. In addition, the heating temperature of steel slab, the finishing temperature, the average cooling rate from the finishing temperature to the winding temperature, and the winding temperature in the above are as shown in Table 2.

  The hot-rolled steel sheet obtained above was pickled and descaled, and subjected to temper rolling with an elongation of 0.5%. And the test piece was extract | collected from the steel plate after temper rolling, and it used for the following evaluation.

(I) Microstructural observation After temper rolling, a sample of the sheet thickness cross section parallel to the rolling direction at the 1/4 width position of the steel sheet was sampled, mirror polished and corroded with nital, and then the 1/4 thickness position was measured. Images were taken at an appropriate magnification of 500 to 3000 times with an optical microscope or a scanning electron microscope. Using the obtained structure photograph, image analysis and ferrite area ratio to the entire structure (area ratio of the entire ferrite including polygonal ferrite), polygonal ferrite area ratio, pearlite area ratio, and other structure types and their The area ratio was determined and used as the respective fractions. Further, the ratio of polygonal ferrite in the ferrite ((polygonal ferrite fraction / ferrite fraction) × 100 (%)) was determined from the ferrite fraction and polygonal ferrite fraction obtained as described above.
The results obtained are shown in Table 3.

(Ii) Tensile test JIS Z 2241 (JIS Z 2241 (1998) No. 5 test piece taken so that the tensile test direction is the rolling direction at the 1/4 width position of the steel plate after temper rolling. 1998), tensile strength (TS) and elongation at break (El) were measured, and a balance between strength and elongation (TS × El) was obtained. In this example, a steel sheet having a tensile strength (TS): 440 MPa or more and a strength / elongation balance (TS × El): 17 GPa ·% or more was evaluated as having high strength and good formability.

(Iii) Cross Section Hardness Test A specimen was collected from the steel sheet after the temper rolling, and the Vickers hardness (HVc) at the position of the thickness 1/2 was measured by a method based on JIS Z 2244 (2009).
<Measurement method>
Test force: 0.98N
Measurement location: 5 locations

(Iv) Soft nitriding treatment A small piece was collected from the steel sheet after the temper rolling and subjected to gas soft nitriding treatment under the following conditions.
Soft nitriding atmosphere: Equivalent ratio mixed gas of ammonia gas and endothermic metamorphic gas Processing temperature: 570 ° C
Treatment time: 3 hours In addition, after holding the said treatment time (3 hours) at the said treatment temperature (570 degreeC), the small piece was oil-cooled (oil temperature: 70 degreeC). And the small piece after oil cooling was used for the following evaluation.

For the small pieces after oil cooling, the Vickers hardness (HV0.1) at a depth of 0.1 mm from the plate surface was measured according to JIS G 0563 (1993). Moreover, the practical nitrided layer depth based on the rule of JIS G 0562 (1993) was measured. In this example, Vickers hardness (HV0.1): 500 or more and practical nitride layer depth: 0.25 mm or more were evaluated as having good surface hardening characteristics.
Further, the Vickers hardness (HVc ′) at the plate thickness 1/2 position (non-nitrided portion) was measured by the same method as in (iii) above. And from the Vickers hardness (HVc) at the plate thickness 1/2 position before soft nitriding treatment obtained in (iii) above, and the Vickers hardness (HVc ′) at the plate thickness 1/2 position after soft nitriding treatment, Change rate of Vickers hardness at the center of the plate thickness before and after soft nitriding treatment: (HVc′−HVc) / HVc × 100 (%) was obtained. In this example, the case where the absolute value of the rate of change was 5.0% or less was evaluated as good (◯) with good strength stability after nitrocarburizing treatment, and the others were evaluated as x.

  As is clear from Table 4, in the examples of the present invention, good results were obtained in all of strength, moldability, surface hardening characteristics by soft nitriding treatment, and strength stability. On the other hand, in a comparative example in which the steel composition and structure do not satisfy the requirements of the present invention, sufficient results are not obtained in any of the above characteristics.

Claims (2)

% By mass
C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
Mn: 0.7% to 1.5%, P: 0.05% or less,
S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% to 1.5%, Nb: 0.005% to 0.025%,
N: 0.005% or less, so that C and Nb satisfy the following formula (1), the balance is composed of Fe and unavoidable impurities, and includes ferrite and pearlite, and a structure other than the ferrite and pearlite. A steel sheet for soft nitriding characterized by having a composite structure having a ratio of 1% or less and a structure in which the ratio of polygonal ferrite in the ferrite is less than 50%.
Record
0.10 ≦ Nb / C ≦ 0.30 (1)
(C, Nb: content of each element (mass%)) When heating the steel slab, subjecting it to hot rolling consisting of rough rolling and finish rolling, after finishing rolling, cooling, winding, and hot rolling steel sheet,
The steel slab is in mass%,
C: 0.05% or more and 0.10% or less, Si: 0.5% or less,
Mn: 0.7% to 1.5%, P: 0.05% or less,
S: 0.01% or less, Al: 0.01% or more and 0.06% or less,
Cr: 0.5% to 1.5%, Nb: 0.005% to 0.025%,
N: 0.005% or less, so that C and Nb satisfy the following formula (1), the balance is composed of Fe and inevitable impurities, and the heating temperature of the hot rolling is 1100 ° C. or higher and 1300 ° C. or lower The finishing temperature of the finish rolling is Ar ₃ transformation point or higher (Ar ₃ transformation point + 100 ° C.) or lower, the average cooling rate of cooling is 30 ° C./s or higher, and the winding temperature of winding is 500 ° C. or higher. A method for producing a steel sheet for nitrocarburizing treatment, characterized by being 650 ° C or lower.
Record
0.10 ≦ Nb / C ≦ 0.30 (1)
(C, Nb: content of each element (mass%))