TWI642790B - Metastable Wostian iron-based stainless steel belt or steel plate and manufacturing method thereof - Google Patents

Abstract

The present invention provides a metastable Wostian iron-based stainless steel strip or steel sheet having high strength and high ductility and a method for producing the same. It is composed of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2%, Cr: 16 to 18%, and Ni: 4 to 11% by mass%. Mo: 2.5%~3.5%, and the remainder is composed of Fe and unavoidable impurities; in the 2-phase structure of α ́ phase and γ phase, the γ phase is composed of γ _T phase and γ _R phase. The total of the γ _T phase and the γ _R phase is 15 to 50% by volume, and the ratio of the γ _T phase area defined by the formula (2) (=100× (the ratio of the γ _T phase to the total area of the observation area as a whole) is 1% or more and 20 % or less; and has a 0.2% relief stress (YS) of 1400 N/mm ² to 1900 N/mm ² , and the "YS-EL value" (YS·EL) satisfies the characteristics of 21,000 to 48,000.

Description

Metastable Wostian iron-based stainless steel belt or steel plate and manufacturing method thereof

FIELD OF THE INVENTION The present invention relates to a metastable Worthfield iron-based stainless steel strip or steel sheet excellent in balance between strength and ductility and a method for producing the same.

BACKGROUND OF THE INVENTION Functional components of precision machines represented by smart phones, notebook computers, cameras, and the like, and high-durability skeleton structural parts such as automobiles and airplanes satisfy the requirements for workability and dimensional accuracy, and seek Thinning and light weight due to high strength. Further, since the load during driving of the component is increased due to the size and weight of the machine, it is required to withstand the durability of excessively severe use and excellent durability such as repeated fatigue strength.

In particular, for the skeletal structure parts for automobiles, the development of high strength and high ductility has been actively carried out. For example, γ-SUS and TWIP (Twinning Induced Plasticity) steels having a strength-ductility balance of more than 20 mass% of Mn or Ni and having a conventional TRIP (Transformation Induced Plasticity) steel grade are being developed. However, these high-strength and high-ductility steels not only increase the component cost, but also make it difficult to manufacture cold rolling of steel strips or steel sheets. In addition, most of the conventional steels do not contain Cr, so corrosion resistance is poor and rust-proof treatment is required.

Today, the low-alloy TRIP-type composite structural steel which is considered to be the most effective candidate has a TS: 980 MPa-EL: 30%, and TS: 1180 MPa-EL: about 25% (see Non-Patent Document 1). However, even with this type of steel, the performance obtained is still insufficient. A steel strip and a steel sheet having a tensile strength (YP) 1400 1400 MPa required as a structural material and having high ductility have not yet been put into practical use.

For the purpose of improving the flatness of the shape, the stainless steel strip is subjected to a melt treatment, and the cold rolling is used to cause the processing to induce the iron phase of the granules. (α phase) is generated, followed by heating at 500 ° C to 700 ° C to generate 3% _T phase (inverted state Worth iron phase) in the α' phase, and manufacturing the Vickers by the inversion state treatment A high-strength Worthfield iron-based stainless steel belt having a flatness of 400 or more.

However, the amount of γ _T phase has a high correlation with temperature, and although it varies depending on the chemical composition, if the temperature is 500 ° C or higher, the amount of γ _T phase may exceed 60%, which is difficult to obtain. Strength of 1400 N/mm ² or more. Furthermore, when the inverter state is maintained for a short period of time (for example, 1 to 5 minutes), the ductility is improved to some extent, but it is extended for a longer period of time (for example, about 5 to 15 minutes). The reduction in sex will be impatient. As described above, the inverting state treatment is a very unstable process, and it is difficult to manufacture a steel strip or a steel sheet having stable mechanical properties. In addition, the precipitation of carbides such as Cr-C or Mo-C does not proceed, so the 0.2% stress is only slightly increased. As described above, the manufacturing method of Patent Document 1 does not substantially balance the high strength and high ductility of steel.

A stainless steel sheet having a component system similar to the stainless steel belt or the steel sheet of the present invention is disclosed in Patent Document 2: Japanese Patent Laid-Open Publication No. SHO 54-120223, and is subjected to a melt treatment, 20 to 80% cold rolling, And low temperature tempering at 400 °C. However, in Patent Document 2, in order to improve corrosion resistance, 2.0% or less of Mo (1.15% of Example 9 in the specification) is added as an active component, and Mo is not added as a precipitation strengthening component in low-temperature heat treatment. In addition, it is difficult to exhibit the function of precipitation strengthening in low-temperature heat treatment by the amount of addition of Mo which is rare as described above.

Patent Document 3: JP-A-2012-201924 discloses an annealing of a stainless steel sheet at 700-1100 ° C, a cold rolling of 10% or more, and an aging treatment at 300 ° C. However, this stainless steel plate does not contain Mo, and it is not possible to exhibit the "precipitation strengthening function in low-temperature heat treatment" caused by the addition of Mo.

Further, the target steel of Non-Patent Document 2 is based on the balance between tensile strength (TS) and elongation (EL) in the range of 300 ° C to 500 ° C, and the tensile strength (TS) is increased to 1750 N/mm. ² or so, but the 0.2% relief stress is only about 1250N/mm ² . Further, the target steel of Non-Patent Document 2 is an Fe-Cr-C-based steel in which the γ phase is the mother phase, and is not in the category of the metastable Worth Iron-based stainless steel to which the present invention pertains.

In the wide-purpose stainless steel containing 12% by mass or more of Cr, a metastable Worth iron-based stainless steel represented by SUS304 or SUS301 or the like is used. In particular, when strength is required, SUS301 is a steel which can reduce the Ni content and is subjected to cold working to induce metamorphism into a granulated iron (α' phase) from Worthite iron (γ phase) processing. The stainless steel is advantageous in that it focuses on various properties such as strength and workability, but when a 0.2% stress (YS) of more than 1400 N/mm ² is obtained, the elongation (EL) becomes 10% or less, and YS The -EL balance (values indexed by YS x EL) is only about 14,000. Therefore, as a material for a small-complexity part, not only does it have a sufficient balance of strength and ductility, but the reliability of the part is not sufficient.

For the purpose of increasing the strength after forming the part, SUS631 precipitation hardening type stainless steel is used as the steel grade by using a chemical composition of SUS301 as a base and adding about 1% of Al to the precipitation strengthening steel by Ni ₃ Al. . Since the steel is subjected to precipitation hardening heat treatment after the forming process, the cost of the secondary processing manufacturer increases, and there is also a problem that the molded parts are deformed or the dimensions are inconsistent due to the heat treatment. In addition, precipitation hardening causes a decrease in the ductility of the part itself, so the toughness of the part itself is lowered. Under the above circumstances, the user is required to have a material which does not require post-treatment after forming (heat treatment which is the main cause of dimensional change) and which is excellent in balance of strength and ductility.

PRIOR ART DOCUMENT Patent Document Patent Document 1: Japanese Patent Laid-Open Publication No. JP-A No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. Document 1: "Iron and Steel" Vol.100 (2014) No.1, P.82-93 Non-Patent Document 2: Nanoscale austenite reversion through partitioning, segregation and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C Steel L.Yuan et al.l Acia Malerialia 60 (2012), p.2790-2804

Disclosure of the Invention Problems to be Solved by the Invention The present inventors focused on the potential of the α' phase generated by processing induced metamorphism, and strive to increase the 0.2% stress (YS) of the metastable Worthfield iron-based stainless steel to 1400 N/ Mm ² or so.

Although the conventional metastable Wolster iron-based stainless steel can be subjected to processing-induced metamorphism or aging precipitation strengthening by cold working to balance the ductility before processing and the strength after processing, on the other hand, the cost of aging precipitation strengthening and Dimensional changes and the like are gradually becoming a problem. In electronic parts or precision parts that require particularly high precision dimensions, dimensional changes after forming can have a significant impact on the performance of the final product. Therefore, high expertise, skill and experience are required for the subsequent step of heat treatment.

Then, the present inventors have obtained the following knowledge: after 1 to 80% of cold working is used to deform the metal structure of the stainless steel to the α ́ phase, a low-temperature heat treatment at 250 to 480 ° C is performed, whereby the α ́ phase is accumulated. The strain energy in the driving force is such that the supersaturated solid solution carbon is diffused and concentrated in the γ phase which is several % by volume, and the γ phase can be made the core, and the adjacent α ́ phase inversion state is the γ _T phase. . Further, it has been found that the carbides of Cr and Mo are finely precipitated in the α ́ phase due to the heat treatment, so that the strength is further improved, and the processing induced metamorphism (TRIP) by the γ _T phase dispersion is further enhanced. The effect is to achieve a 0.2% relief stress (YS) of 1400 N/mm ² or more and an elongation (EL) of 15% or more. What is more, the ideal conditions to the scope of the invention, to take into account 1550N / mm ² or more in 0.2% yield strength (YS) and elongation of 23% or more (the EL), and implemented by (1) the request of the formula The "YS-EL balance" value is more than 35,000. "YS-EL balance" = YS × EL...(1)

The α ́ phase indicates that the processing induces the iron phase of the field. The γ _R phase represents the residual Worthfield iron phase. The γ _T phase represents the inverted state Worthfield iron phase.

An object of the present invention is to provide a steel strip or a steel sheet having high strength, high ductility, and high corrosion resistance, and a method for producing the same.

Means for Solving the Problem The medium-stable Wolster iron-based stainless steel strip or steel sheet according to the present invention is characterized by containing C: 0.05 to 0.15%, Si: 0.05 to 1%, and Mn: 2% or less in mass%. Cr: 16~18%, Ni: 4~11%, Mo: 2.5%~3.5%, Cu: 0.4%~1.0%, and the remainder is composed of Fe and unavoidable impurities; with α ́ phase and γ The 2-phase structure of the phase, the γ phase is composed of the γ _T phase and the γ _R phase, and the total of the γ _T phase and the γ _R phase is 15 to 50% by volume, and the γ _T phase area ratio defined by the formula (2) is 1% or more and 20% or less; and having a 0.2% relief stress (YS) of 1400N/mm ² to 1900N/mm ² , and the "YS-EL balance" value obtained by the formula (1) satisfies at least 21,000 to 48000. characteristic.

The method for producing the medium-stable Wolster iron-based stainless steel strip or steel sheet according to the present invention comprises the following steps: cold-working the stainless steel strip or steel sheet having the above composition to induce the granulated iron phase from the Vostian iron phase (γ phase) forming process ( a step of α ́ phase), and a low-temperature heat treatment in a range of 250 ° C to 480 ° C for a stainless steel strip or a steel sheet having a process-induced granulated iron phase (α ́ phase) to induce Ma Tiansan from the aforementioned processing The step of the iron phase (α ́ phase) formed in the iron phase forming step is a step of growing the iron phase (γ _T phase) of the Vostian. "YS-EL balance" = YS × EL (1) γ _T phase area ratio (%) = 100 × (γ _T phase accounts for the total area ratio of the observation area as a whole) ... (2)

However, the α ́ phase indicates that the processing induces the volcanic iron phase, the γ phase represents the γ _T phase and the γ _R phase, and the γ _T phase represents the inverter state of the Vs field with an area of 5 μm ² or more and 20 μm ² or less per particle. The iron phase and the γ _R phase represent the Worthfield iron phase other than the γ _T phase, respectively, and YS represents 0.2% of the stress, and EL represents the elongation.

The structure containing the phases thereof has both the 0.2% stress (YS) of more than 1400 N/mm ² and the elongation (EL) of more than 15%. The present inventors presume that the former is obtained by the α ́ phase which is hardened by precipitation of carbides of Cr and/or Mo, and the latter is obtained by the TRIP effect of the γ _T phase dispersed in the α ́ phase.

Hereinafter, the metastable Worthfield iron-based stainless steel strip or steel sheet of the present invention will be described.

(Regarding composition) The stainless steel strip or steel sheet according to the present invention is a metastable Worth iron-based stainless steel containing the following chemical composition: C: 0.05% to 0.15%, Si: 0.05% to 1%, and Mn: 2% or less by mass% Cr: 16~18%, Ni: 4~11%, Mo: 2.5%~3.5%, Cu: 0.4%~1.0%.

In order to impart the required strength to the α ́ phase after the processing of the cold rolling delay and the metamorphosis, 0.05% or more of C is added. However, if more than 0.15% of C is added, the Worthfield iron phase will be stabilized, and the processing-induced metamorphism of the cold rolling delay will become difficult to perform, and the secondary workability such as perforation will be deteriorated, so the upper limit of C is required. It is 0.15% or less.

Si is an important element in the production of steel as a deoxidizing material, so it is necessary to add 0.05% or more. However, if more than 1% of Si is added, the rolling property or toughness is lowered, so the upper limit is made 1%.

Mn is an element which stabilizes the iron phase of Vostian together with Ni. If a large amount is added, a structure having a processing-induced α ́ phase of 50% or more cannot be obtained in a general cold rolling. Therefore, in the present invention, the upper limit of Mn is regulated to 2%. Although the lower limit of Mn is not particularly regulated, it should be set to 0.1% as a countermeasure against hot cracking of hot rolling.

In order to impart corrosion resistance as stainless steel, 16% or more of Cr is added. However, if more than 18% is added, the Worthfield iron phase will be stabilized, so that a sufficient amount of processing-induced metamorphic α ́ phase will not occur in the general cold rolling step. Therefore, in the present invention, the upper limit of Cr is set to 18%.

Ni is a Worstian iron stabilizing element, and a predetermined amount must be added to maintain the cold rolled pre-extension tissue in a metastable Worth iron state. In the present invention, as a lower limit for forming a metastable Worst iron phase after the melt treatment, 4% or more of Ni is added. However, if Ni is added in excess of 11%, the Worthite iron phase becomes stable, and it becomes impossible to obtain a process-induced metamorphic α ́ phase which is 50% or more by volume after the general cold rolling delay. organization. Therefore, the upper limit of Ni is limited to 11%.

Mo is an important element in the present invention. It is known that Mo is an effective element for improving the pitting resistance of stainless steel, and is also an important element for enhanced precipitation of low-temperature heat treatment in the present invention. In the present invention, the lower limit of the precipitation strengthening of the α ́ phase obtained by the Mo carbide is regulated to be 2.5% or more. Further, if the amount of Mo added is increased, not only the precipitation strengthening property is saturated, but also the alloy cost. The surface is also unfavorable, so the upper limit of the regulation Mo is 3.5%.

In addition, one or two or more of elements such as Ti or Al may be added for the purpose of precipitation strengthening. Although the amount of each element added differs depending on the balance with other elements, it is generally suitable for 0.1% to 3.5%. Further, in order to improve the corrosion resistance of the α ́ phase after the processing-induced metamorphism, it is preferable to add Cu: 0.4 to 1.0% by mass%. When Cu is less than 0.4%, the effect of remarkably improving the corrosion resistance cannot be confirmed. Conversely, if Cu exceeds 1.0%, problems such as hot-breaking hot-breaking and other manufacturing steps are likely to occur.

The steel strip or the steel sheet of the present invention may contain unavoidable impurities, that is, P, N, S, O, etc., and the purpose of the present invention is not hindered as long as the impurity amount is contained in a general manufacturing step. Therefore, it can be tolerated.

(About metal structure) In the two-phase structure of the α ́ phase and the γ phase, the γ phase is composed of a γ _T phase and a γ _R phase, and the γ _T phase is in the two-phase structure of the α ́ phase and the γ phase. The total of the γ _R phase is 15 to 50% by volume (the α ́ phase is 50 to 85% by volume), and the γ _T phase area ratio defined in the formula (2) is (=100×(γ _T phase occupies the entire observation area). The total area ratio)) is 1% or more and 20% or less.

Here, when the total of the γ _T phase and the γ _R phase is less than 15% by volume (the α ́ phase exceeds 85% by volume), the γ phase is insufficient, the TRIP effect is lost, and the elongation is lowered.

Conversely, if the total of the γ _T phase and the γ _R phase exceeds 50% by volume (α ́ phase is less than 50% by volume), the γ phase becomes excessive, and the TRIP effect disappears and the strength decreases.

If the γ _T phase area ratio is less than 1%, the γ phase will be insufficient, the TRIP effect will disappear and the elongation will decrease.

If the γ _T phase area ratio exceeds 50%, the γ phase becomes excessive, and the TRIP effect disappears and the strength decreases.

(Regarding Characteristics) The metastable Worth Iron-based stainless steel strip or steel sheet having the above composition and metal structure may have the following characteristics: 0.2% of the undulating stress (YS) is 1400 N/mm ² to 1900 N/mm ² , and is 1550 N/mm. ² ~ 1900N / mm ² is ideal; "YS-EL balance" (= YS × EL) value of at least 21000 ~ 48000, and to meet 35000 ~ 48000 is ideal.

(About the manufacturing method) The stainless steel strip or the steel sheet of the above composition is subjected to cold working, and the iron phase (α ́ phase) is induced by the Worthite iron phase (γ phase), and the stainless steel strip is applied in the range of 250 ° C to 480 ° C. steel or low temperature heat treatment, from the foregoing strain-induced martensite phase is formed martensite phase formed in step (α'-phase) growth of an austenite phase (γ _T phases), can be prepared having whereby The aforementioned metal structure and properties are used to stabilize the Worthfield iron-based stainless steel strip or steel sheet.

The present inventors presume that the above-described characteristics of the metastable Worthfield iron-based stainless steel strip or steel sheet of the present invention are due to the following mechanism. That is, a low-temperature heat treatment is performed in the state of such a metal structure, whereby the strain energy accumulated in the α ́ phase induced by the γ phase processing during cold working is used as the driving force, and the supersaturated solid solution C in the α ́ phase is The fine γ _R phase which is the core of the inverter state is diffused and concentrated, and the growth of the γ phase proceeds. Further, by maintaining the predetermined temperature, the precipitation hardening phenomenon of the α ́ phase proceeds. By controlling the above phenomenon with various parameters, it is possible to achieve both high strength due to the α ́ phase and high ductility due to the processing induced metamorphosis of the γ phase. In other words, it is possible to satisfy the characteristic that the value of "YS-EL balance" of the formula (1) is 21,000 or more.

Incidentally, when the α ́ ratio after cold working is less than 50%, the strain energy accumulated in the α ́ phase is low, so that the diffusion and concentration of C from the α ́ phase to the γ phase do not occur. Therefore, the cold working rate is low and the difference density in the α ́ phase is low, so the balance between the strength and the elongation, that is, the value of "YS-EL balance" does not exceed the value of the conventional material. "YS-EL balance" = YS × EL...(1)

(Regarding the volume ratio) The evaluation of the granulated iron phase (α ́ phase) and the Worthite iron phase (γ phase) of the present invention was carried out using an electron backscatter diffraction method (EBSD). EBSD is an area of 0.05 mm × 0.05 mm or more in a plane perpendicular to the rolling direction of the steel material (so-called RD surface) when the number of crystal grains contained in the observation area is at least 1000 or more. The orientation difference of 5° or more is defined as a grain boundary, and the area ratio calculated based on the result of the Phase measurement in this case is converted into a volume ratio. The same is true regarding the volume %.

(Characteristics) A stainless steel belt or steel sheet having the composition and metal structure of the present invention is characterized in that the 0.2% stress (YS) is 1400 N/mm ² or more and the elongation (EL) is 15% or more. By satisfying the above, the value of "YS-EL Balance" will be at least 21,000. Further, the ideal conditions in the range of the present invention are such that a 0.2% stress (YS) of 1550 N/mm ² or more and an elongation (EL) of 23% or more are achieved, and a characteristic of more than 35,000 in terms of a YS-EL balance value can be achieved. These are characteristics which are excellent in strength and ductility which are not obtained in a conventional stainless steel belt or steel sheet.

(Production Method) An example of a method for producing the metal structure and characteristics of the present invention and a conventional method for producing a stainless steel strip which has been conventionally carried out will be described below.

First, a conventional stainless steel belt or steel sheet method which has been conventionally carried out will be briefly described, and an example of a method for producing a stainless steel belt or a steel sheet according to the present invention will be described.

The conventional method for precipitating and strengthening the metastable Wolster iron-based stainless steel strip (such as SUS631 (17-7PH)) is to follow the usual method (such as 85% rolling reduction) to roll the conventionally produced method. The stainless steel strip of the final whole process, after which, is subjected to a solid solution heat treatment. This solid-melting heat treatment is carried out by subjecting the steel strip to a melt treatment at, for example, 1100 ° C, followed by water cooling. Next, the metamorphic treatment of the granulated iron was carried out. Specifically, the steel strip is rolled at a rolling reduction ratio of, for example, 60%. Thereafter, precipitation hardening treatment is performed at, for example, 475 ° C to precipitate and strengthen with an intermetallic compound. By the above treatment, a stainless steel strip having a 0.2% relief stress (YS) of about 1400 N/mm ² can be obtained, and the elongation (EL) is a low value of about 1 to 10%. This is because the processes are not intended for the inverter state. Further, when the reaction state is performed at a temperature equal to or higher than the precipitation hardening temperature, for example, 500 ° C or higher, the elongation (EL) is expected to increase, but the 0.2% stress (YS) is decreased. This is because the above treatment not only promotes the inversion state but also promotes the solidification of the precipitated intermetallic compound to the parent phase. Therefore, the above treatment cannot be used to obtain a 0.2% stress (YS) of 1400 N/mm ² or more.

A suitable example of the method for producing the stainless steel strip or steel sheet of the present invention will be described below.

First step: In this first step, a stainless steel belt (such as SUS631 (17-7PH)) having the composition of the present invention obtained by a conventional means is cold-rolled. This cold rolling is intended to increase the ratio of the α ́ phase by processing induced metamorphosis. Therefore, although the processing rate varies depending on the composition of the steel strip and the thickness of the steel strip, the processing ratio is preferably in the range of 20% to 90%, and the processing ratio of 30% or more is preferable.

Second step: Next, the rolled stainless steel strip is subjected to a solution heat treatment. The purpose of this heat treatment is to make the α ́ phase inversion state induced by the cold working into the γ _T phase, so that the supersaturation of C present in the α ́ phase is uniformly dispersed in the γ phase, and homogenization is followed by Ma Tian San. Metal structure in iron metamorphosis. The heat treatment temperature of the solid solution varies depending on the composition of the stainless steel strip, etc., but is, for example, in the range of 900 ° C to 1150 ° C, and preferably 1000 ° C or more. Then, it is heated and then quenched (for example, water cooled).

Step 3: Next, carry out the metamorphosis treatment of the granulated iron. The rolling reduction ratio (processing rate) in this treatment varies depending on the required characteristics or the composition of the steel strip, the thickness of the steel strip, etc., and is 0% to 60% for the steel or steel strip before processing. The range is preferably in the range of 5% to 40%.

If the rolling reduction ratio exceeds 60%, the core of the inverter state, that is, the γ phase, will be insufficient, and the organization of the scope of the present invention cannot be obtained by the subsequent inverter state treatment.

Step 4: In the third step, the metamorphic treatment of the granulated iron is carried out in accordance with the required characteristics, and the steel strip or the steel sheet subjected to the above treatment is in the range of 250 ° C to 480 ° C, preferably 300 ° C to 450 ° C. The range is subjected to low temperature heat treatment. If the heat treatment temperature is lower than 250 ° C, the diffusion and concentration of the supersaturated solid solution carbon in the α ́ phase do not sufficiently occur, and the γ phase cannot grow, so that the increase in the strength ductility balance cannot be expected. Further, if the temperature exceeds 480 ° C, the diffusion of the super-saturated solid-melting carbon in the α ́ phase is promoted, and the stable γ phase is excessively grown, and the TRIP effect is hard to occur. There is a case where the ductility is lowered and the strength is also lowered. In this regard, the steel strip or the steel sheet subjected to the first to fourth steps described above may have a balance between the strength (YS) and the elongation (EL) due to a change in the ratio of the α ́ phase to the γ phase, and the characteristics of the present invention can be obtained. .

Further, the PH stainless steel is intended for the precipitation of an intermetallic compound, and an intermetallic compound is precipitated if an inverse heat treatment is to be carried out in the vicinity of a precipitation hardening temperature (for example, 500 ° C) which is generally used. Although the strength (YS) will rise, the ductility (EL) is significantly reduced. Therefore, in the case of the PH stainless steel or the like which will precipitate the intermetallic compound, even in the range of the treatment conditions of the present invention, compared with the metastable Worthfield iron-based stainless steel other than the above-mentioned PH stainless steel, it is required to be at a low temperature (such as 250). Heat treatment is carried out at °C~300 °C). It has been found that the increase in the γ _T phase and the precipitation of carbides caused by such low-temperature heat treatment can achieve both high strength and high ductility.

Further, it has been found that when a precipitation hardening heat treatment is performed at a temperature (for example, 500 ° C) which is generally performed after the forming process, the diffusion of the solute atoms is promoted to accelerate the precipitation of the intermetallic compound, and further development is expected. The strength increases.

In view of the above, the inventors of the present invention have focused on the PH stainless steel represented by the above-mentioned SUS631 as the metastable Worth iron-based stainless steel strip or steel sheet which is excellent in the balance between the strength and the ductility.

By satisfying the conditions from the first step to the fourth step, a metastable Worthfield iron-based stainless steel strip or steel sheet having a characteristic that the YS-EL balance value is at least more than 21,000 can be produced.

According to the manufacturing method of the present invention, it is possible to manufacture stainless steel having two characteristics which cannot be achieved by a conventional method without largely deviating from the range of the secondary processing steps which are generally carried out, and without greatly increasing the manufacturing cost or the environmental burden. Belt or steel plate. Further, the production process shown in the first step or the second step may be repeated in accordance with the state of the raw material, and then the mashed iron metamorphosis treatment shown in the third step may be performed.

However, the method of producing the stainless steel strip or the steel sheet of the above embodiment is merely an example, and the present invention is not limited to this one.

Advantageous Effects of Invention According to the present invention, it is possible to achieve both the characteristics of the metastable Worth Iron-based stainless steel, that is, the strength, and the ductility of the high-formability steel sheet at a high level.

The stainless steel strip or steel sheet of the present invention makes it possible to use a part that is not achievable by a conventional high-strength material and which requires extremely high strength in construction, or a part that is more complicated in design.

The basement is stable. The Wostian iron-based stainless steel has a large content of Cr and Ni. Compared with the high-strength and high-ductility materials, the corrosion resistance is more advantageous. Therefore, the surface treatment for the purpose of preventing rust is not required after processing. The situation is also possible. Thus, it is not only strength and ductility, but it is also expected to be used in applications requiring corrosion resistance.

Up to now, in the well-known metastable Worthfield iron-based stainless steel strip, the 0.2% drop stress (YS) increases and the elongation (EL) decreases as the cold rolling rate increases. Therefore, not only the workability is poor, but also the precipitation change hardening material cannot avoid the dimensional change caused by the heat treatment after the processing.

In this regard, the use of the metastable Worthfield iron-based stainless steel strip of the present invention can not only achieve a high 0.2% relief stress (YS) of more than 1400 N/mm ² , but also an elongation (EL) of more than 15%.

MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described based on the embodiments. However, the invention is not limited to the embodiments thereof.

EXAMPLES Hereinafter, examples and comparative examples will be described together.

The steel grade 1 of the example was prepared, and the Mo content was 2 to 4 of the comparative steel grade. The chemical composition is shown in Table 1. Next, for the steel of the example of Table 1, the steel having the metal structure of the present invention (the sample identifying 1 to 5) and the steel having the metal structure outside the range of the present invention (the sample of the identification 6, 7) were produced. ). The metal structure of its steel is shown in Table 2. Further, the manufacturing conditions of the steels thereof are shown in Table 3. The hardness (HV), tensile strength (Ts), 0.2% stress (YS), and elongation (EL) of the obtained steel (samples 1 to 7) were measured and shown in Table 4, respectively. . Further, in Tables 1 to 4, the numerical value "*" attached to the left side indicates a value outside the range of the present invention. [Table 1] [Table 2] [table 3] [Table 4]

From the above results, it is understood that the samples of the identifications 1 to 5 of Table 4 satisfy the 0.2% stress (YS) exceeding 1400 N/mm ² , and the γ phase exhibits an elongation (EL) of more than 15%. On the other hand, in the comparative examples, the samples of the identifications of 6, 7 could not satisfy both the 0.2% stress (YS) and the elongation (EL). The metallographic images of the samples of the above identified 1 to 7 are shown in Figs.

Next, the steel grade 1 having the composition of the present invention of Table 1 was prepared, and steel grades 2 to 4 having a composition outside the range of the present invention were prepared, and a stainless steel belt was produced according to various production conditions shown in Table 6. The metal structure is shown in Table 5, and its characteristics are shown in Table 7. In Tables 5 and 7, the symbol "*" described at the forefront of the numerical value means that the numerical value is a value outside the range of the present invention.

The following results can be seen from the experimental results shown in Tables 5 to 7. That is, with respect to the steel of the example, as long as the low-temperature heat treatment temperature does not exceed 500 ° C, the desired characteristics cannot be obtained regardless of the length of the heat treatment. However, in the case where the low-temperature heat treatment temperature is 500 ° C, if the heat treatment time becomes long, it becomes impossible to obtain desired characteristics. Further, as long as the low-temperature heat treatment is not performed, the desired characteristics cannot be obtained.

On the other hand, in the steel of the comparative example, even if the low-temperature heat treatment was performed under an appropriate temperature condition, the desired characteristics could not be obtained. [table 5] [Table 6] [Table 7]

Fig. 8 is a graph showing changes in YS × EL values depending on the time of the low-temperature heat treatment temperature when the sample of the steel of Example 1 was used and the steps shown in Table 6 were carried out.

As is clear from Fig. 8, in the case where the low-temperature heat treatment temperature exceeds 480 ° C, especially if the low-temperature heat treatment time becomes long, the intended YS × EL value cannot be obtained. On the contrary, it is understood that in the case where the low-temperature heat treatment temperature is lower than 250 ° C, especially if the low-temperature heat treatment time is short, the intended YS × EL value cannot be obtained. Then, it is understood that as long as it is in the range of 300 ° C to 450 ° C, it is not substantially related to the length of the low-temperature heat treatment time, and the desired YS × EL value can be stably obtained.

Fig. 9 is a graph showing changes in YS × EL values depending on the temperature of the low-temperature heat treatment time when the sample of the steel of Example 1 was used and the steps shown in Table 6 were carried out.

As is clear from Fig. 9, in the case of 300 ° C, the value of YS × EL is maintained at a low level at a value of 22,000 or more, and in the case of 400 ° C, the value of YS × EL is maintained at a high level at a value of 29000 or more. On the other hand, in the case of 500 ° C, the YS × EL value decreases from 37,000 to about 20,000 as the low-temperature heat treatment time becomes longer. As described above, when the temperature is 50 ° C or higher, the temperature is deteriorated due to the low-temperature heat treatment time, and thus the quality is unstable.

INDUSTRIAL APPLICABILITY The present invention is based on the following chemical composition of the Fort Worth iron-based stainless steel as a base: C content is 0.05 to 0.15%, Si content is 0.05 to 1%, and Cr content is Ni. The content is 16-20%, 4-11%, Mo content is 2.5%~3.5%, and Cu content is 0.4%~1.0%. Then, for the metastable Worthfield iron-based stainless steel, more than 50% of the processing obtained by cold working induces the granulated iron phase (α ́ phase) as the mother phase, and is preferably a stainless steel strip or steel sheet having the following metal structure. : a two-phase structure of a process induced α ́ phase and a γ phase (γ _R phase + γ _T phase) obtained by performing a low-temperature heat treatment at 250 ° C to 480 ° C, and the γ _T phase area ratio defined by the above formula (2) It is 1% or more and 20% or less, and the remaining phase is composed of α ́ and γ _R .

Such a method of inverting a metal structure using a low-temperature heat treatment of 480 ° C or lower to make a Ni or Mn of 11% or less is a novel technique which has not been hitherto, and according to the above-mentioned structure obtained by the above-mentioned method, The 0.2% relief stress (YS) exceeding 1400 N/mm ² is satisfied due to the α ́ phase, and the γ phase has an elongation (EL) of more than 15%.

The basement is stable. The Wostian iron-based stainless steel has a large content of Cr and Ni. Compared with the high-strength and high-expansion steel plate of the conventional iron base, the corrosion resistance is also superior, so it is not only strength or workability. It can be expected to be used in applications requiring corrosion resistance. Further, depending on the use of the required hardness, a stainless steel strip or a steel sheet having a HV of 450 or more in addition to the above characteristics can be obtained.

Fig. 1 is a view showing a metal structure of a sample of the identification 1 described in Table 2 below, in place of the micrograph of the drawings.

Fig. 2 is a view showing a metal structure of a sample of the identification 2 described in Table 2 below, in place of the microscope photograph of the drawings.

Fig. 3 is a view showing a metal structure of a sample of the identification 3 described in Table 2 below, in place of the microscope photograph of the drawings.

Fig. 4 is a view showing a metal structure image of the sample of the identification 4 described in Table 2 below, instead of the micrograph of the drawing.

Fig. 5 is a view showing a metal structure of a sample of the identification 5 described in Table 2 below, in place of the microscope photograph of the drawings.

Fig. 6 is a view showing a metal structure of a sample of the identification 6 described in Table 2 below, in place of the microscope photograph of the drawing.

Fig. 7 is a view showing a metal structure of a sample of the identification 7 described in Table 2 below, in place of the microscope photograph of the drawings.

Fig. 8 is a graph showing changes in the YS × EL value at different times of the low-temperature heat treatment temperature using the steel sample 1 of the present invention described in Table 1 below. Further, in the figure, the broken line indicates that the low-temperature heat treatment time is 15 minutes, the solid line is 60 minutes, and the one-point chain line is 360 minutes.

Fig. 9 is a graph showing changes in the YS × EL value depending on the temperature of the low-temperature heat treatment time using the steel sample 1 of the present invention described in Table 1 below. Further, in the figure, the broken line indicates that the low-temperature heat treatment temperature is 300 ° C, the solid line is 400 ° C, and the one-point chain line is 500 ° C.

Claims (8)

A metastable Worthfield iron-based stainless steel strip or steel sheet characterized by C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni, by mass% : 4~11%, Mo: 2.5%~3.5%, Cu: 0.4%~1.0%, and the remainder is composed of Fe and unavoidable impurities; 2-phase structure with α ́ phase and γ phase, γ phase γ _T is constituted with phase γ _R, γ _T relative to the phase γ _R is 15 to 50% by volume, and γ _T relative to the area defined by the following formula 2 ratio of 1% or more and 20% or less; And the 0.2% undulation stress (YS) is 1400N/mm ² ~ 1900N/mm ² , and the "YS-EL balance" value obtained by the formula (1) satisfies at least the characteristics of 21000 to 48000, "YS-EL balance" = YS × EL ... (1) γ _T phase area ratio (%) = 100 × (γ _T phase occupies the total area ratio of the observation area as a whole) ... (2) However, α ́ phase indicates processing induced yoma iron phase, γ The phase represents a phase in which the γ _T phase and the γ _R are added together, and the γ _T phase represents an inverted state Worthfield iron phase having an area of 5 μm ² or more and 20 μm ² or less per particle, and the γ _R phase represents a γ _T phase, respectively. Vostian iron phase, and YS represents 0.2% of the stress, and EL represents the elongation. The stainless steel strip or steel sheet according to claim 1, which has the aforementioned 0.2% stress (YS) of 1550 N/mm ² to 1900 N/mm ² , and the "YS-EL balance" value obtained by the above formula (1) satisfies at least 35000~48000 features. A stainless steel strip or a steel sheet according to claim 1, which further contains, in mass%, one or two selected from the group consisting of Al: 0.1% to 3.5% and Ti: 0.1% to 3.5%, in place of a part of Fe. The stainless steel belt or steel sheet according to any one of claims 1 to 3, which has an HV of 450 or more. A method for manufacturing a metastable Worthfield iron-based stainless steel strip or steel sheet, comprising the steps of: preparing a stainless steel strip or a steel sheet in a mass %, containing C: 0.05 to 0.15%, Si : 0.05~1%, Mn: 2% or less, Cr: 16~18%, Ni: 4~11%, Mo: 2.5%~3.5%, Cu: 0.4%~1.0%, and the rest is made of Fe and cannot a step of preventing the impurities from being formed by cold working on the stainless steel strip or the steel sheet to form a volume of the volcanic iron phase (α ́ phase) by forming a volume of 50% by volume or more from the Worthite iron phase (γ phase); Forming a stainless steel strip or steel sheet with a processing-induced granulated iron phase (α ́ phase), and performing a low-temperature heat treatment in the range of 250 ° C to 480 ° C to form a granulated iron phase formed from the ferritic iron phase formation step induced by the aforementioned processing (α ́ phase) is a step of growing a Wolster iron phase (γ _T phase), and the manufacturing method has the following metal structure and mechanical properties: a 2-phase structure having an α ́ phase and a γ phase, and the γ phase is γ _R and γ _T relative phase configuration, γ _T relative to the phase γ _R is 15 to 50 vol%, γ _T relative to the area and the definition of the formula (2) in a ratio of 1% And 20% or less of the metallic structure; and having a 0.2% yield strength (YS) of ^{1400N / mm 2 ~ 1900N / mm} 2, and in formula (1) obtained by the "YS-EL balance" value of at least 21,000 ~ 48,000 satisfied Mechanical characteristics, "YS-EL balance" = YS × EL (1) γ _T phase area ratio (%) = 100 × (γ _T phase accounts for the total area ratio of the observation area as a whole) ... (2) However, α The phase indicates that the processing induces the loose iron phase of the field, the γ phase represents the phase of the addition of the γ _T phase and the γ _R phase, and the γ _T phase represents the iron phase of the inverted state of the Vostian with an area of 5 μm ² or more and 20 μm ² or less per particle. _{The R} phase represents the Worthfield iron phase other than the γ _T phase, and YS represents 0.2% of the stress, and EL represents the elongation. The manufacturing method of claim 5, which has a 0.2% relief stress (YS) of 1550 N/mm ² to 1900 N/mm ² , and the "YS-EL balance" value obtained by the above formula (1) satisfies at least 35,000 to 48,000. Characteristics. The manufacturing method of claim 5, wherein the stainless steel strip or the steel sheet further contains, in mass%, one or two selected from the group consisting of Al: 0.1% to 3.5% and Ti: 0.1% to 3.5%, in place of a part of Fe. . The manufacturing method according to any one of claims 5 to 7, wherein the HV of the stainless steel strip or the steel sheet is 450 or more.