WO2002101108A1

WO2002101108A1 - Double phase stainless steel strip for steel belt

Info

Publication number: WO2002101108A1
Application number: PCT/JP2002/005572
Authority: WO
Inventors: Kouki Tomimura; Hiroshi Fujimoto; Kenichi Morimoto; Naoto Hiramatsu
Original assignee: Nisshin Steel Co., Ltd.
Priority date: 2001-06-11
Filing date: 2002-06-06
Publication date: 2002-12-19
Also published as: DE60205896D1; ATE303458T1; CN1227383C; KR20040014492A; US20040168750A1; EP1396552B1; EP1396552A1; JPWO2002101108A1; CN1514885A; JP4252893B2; EP1396552A4

Abstract

A high strength double phase stainless steel strip, wherein it has a chemical composition in mass %: C: 0.04 to 0.15 %, Cr: 10.0 to 20.0 mass %, Ni: 0.5 to 4.0 mass %, and balance: substantially Fe, an old austenite average grain diameter is 10 μm or less, and it has a structure after transformation at an ordinary temperature which contains martensite in an amount of 20 to 85 vol % and ferrite in the balanced amount, and a hardness of HV300 or more. The double phase stainless steel strip can be used for the production of a steel belt excellent in surface characteristics, since the transformation strain associated with martensitic transformation is dispersed uniformly and thus no Luders band is formed during the correction of the shape of a steel belt.

Description

Duplex stainless steel strip for steel belt

TECHNICAL FIELD The present invention relates to a high-strength duplex stainless steel strip for a steel belt having an excellent surface shape in which a reducer band does not occur during shape correction in a steel belt manufacturing process. Light

Rice field

Background art

Stainless steel belts include work hardened austenitic stainless steels, which are reinforced by cold rolling austenitic stainless steels such as SUS301 and SUS304, as well as low-carbon martensitic stainless steels (Japanese Patent Publication No. No. 31085) and precipitation-hardening martensitic stainless steel (Japanese Patent Publication No. 59-49303).

The work-hardened structure represented by SUS304 and SUS301 is a metastable austenitic structure, and a work-induced martensite is formed by deformation. As a result, during deformation, a reusable band is generated due to the formation of work-induced martensite (Journal of the Institute of Metals, Vol. 55, No. 4, pp. 376-382, Nisshin Steel Engineering Reports No. 69, Nos. 1--14). page). Undesired surface irregularities occur as a steel belt material.

Martensite-precipitation-hardened martensite transforms almost to a single martensite phase during the cooling process after annealing in the manufacturing process, but tends to change its shape due to expansion accompanying the transformation. A deteriorated shape cannot be easily corrected in a belt state. Disclosure of the invention

The present invention has been devised in order to solve such a problem, and the generation of a Ruder's band when correcting a belt shape such as a metastable austenite system, and the formation of a martens belt. To provide stainless steel strips for steel belts that have a ferrite / martensite dual-phase structure and excellent surface shape without making shape correction difficult due to complete transformation to martensite in the manufacturing process like a site system. The purpose is to: The high-strength duplex stainless steel strip for steel belts of the present invention has a C content of 0.04 to 0.15 mass in order to achieve the object. / ₀ , Cr: 10.0-20.0 mass. Ni: contains 0.5 to 4.0% by mass, has a prior austenite average particle size of ΙΟμιη or less, and has a volume of 20 to 85 at room temperature after transformation. / ₀ has martensite and the remainder has a ferrite structure and has been refined to a hardness of HV300 or more.

It is preferable that the average grain size of the prior austenite is less than ΙΟμπι, and the average expansion when austenite undergoes martensitic transformation in the cooling step of the annealing step is less than 9%.

In this specification, “steel strip” is used to include steel sheets. BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors investigated and examined the effects on the reducer band generated when the shape of a steel belt was corrected from various viewpoints such as composition, structure, and material. As a result, it was found that the strain distribution and volume expansion accompanying the martensitic transformation had a great influence on the generation of the Luders band. In consideration of the effects of strain distribution and volume expansion, residual austenite is eliminated from the stainless steel strip, and the expansion strain generated when the austenite phase undergoes martensitic transformation during the cooling process in the annealing step is dispersed throughout the steel strip. Was effective in preventing the occurrence of the re-use band. Hereinafter, alloy components, contents, and the like included in the duplex stainless steel strip targeted by the present invention will be described.

C: 0.04-0.15 mass%

It is an austenite-forming element and an extremely effective alloy component for strengthening the martensite phase. Austenitizing temperature The amount of tensite can be adjusted, contributing to strength adjustment and high strength. The effect of adding C is 0.04 mass. /. It becomes remarkable at the above C content. However, excessive C content causes precipitation of Cr carbides at the grain boundaries during the cooling process and aging treatment after the multi-layering treatment and lowers the intergranular corrosion resistance and fatigue characteristics.Therefore, the upper limit of the C content is 0.15%. mass. /. Set to.

Cr: 10.0~ dynamic mass ^_0/0

It is an essential alloy component for ensuring the corrosion resistance of stainless steel, and 10.0 mass to provide the required corrosion resistance. Cr is included at / ₀ or more. But 20.0 mass. If an excessive amount of Cr exceeding / ₀ is added, the toughness and workability of the steel material decrease. As the Cr content increases, the amount of austenite-forming elements, such as C, N, Ni, Mn, and Cu, required for martensite formation and strengthening must be increased. Increasing the amount of austenite-forming elements not only increases steel strip costs, but also stabilizes austenite at room temperature, making it difficult to obtain high strength. Therefore, the upper limit of the Cr content is set to 20.0% by mass.

Ni: 0.5-4.0 mass%

It is an austenite forming element and is added to obtain a ferrite + austenite structure at high temperature (ferrite + martensite at room temperature). The amount of martensite increases according to the Ni content, and the steel material is strengthened. In addition, the addition of Ni increases the frequency of nucleation of austenite by annealing in the (austenite + ferrite) two-phase region, resulting in a fine (austenite + ferrite) mixed structure. The mechanism by which the increase of Ni influences the formation of a fine two-phase mixed structure is that the growth rate of austenite nuclei slows beyond the critical nuclei defined by classical nucleation theory, while the equilibrium diagram shows that This is probably because the number of nucleation sites increases due to the formation of new austenite nuclei in an attempt to generate austenite. The effect of adding Ni on the refinement of the two-phase mixed structure is 0.5 mass. /. It becomes remarkable at the above Ni content. However, not only is it an expensive element that raises the cost of steel, but the austenite phase formed at a high temperature due to the excessive addition of Ni does not transform into martensite during the cooling process to room temperature and becomes residual austenite, thereby reducing the strength of the steel. Lower Responsible. Therefore, the upper limit of the Ni content is set to 4.0% by mass. In the duplex stainless steel strip targeted by the present invention, in addition to C, Cr, and Ni, as necessary, austenite-forming elements such as Mn, Cu, and N and ferrite formation such as Si, Ti, Nb, and A1 are formed. Elements can be appropriately added, and each alloy component can be adjusted so that a ferrite + martensite double-phase structure can be obtained at room temperature. In addition, Mo is effective in improving corrosion resistance, Y, Ca, REM (rare earth metal) is effective in improving oxidation resistance and hot workability, and B is effective in improving various properties as long as the required strength is not reduced. It is acceptable to add alloying elements such as V. The content of the optional component is determined as follows.

Si: 2.0 mass% or less

It is a component added as a deoxidizer in the molten steel stage, but has a high solid solution strengthening ability and has a mass of 2.0. Excessive addition of Si exceeding / ₀ hardens the steel material and lowers the ductility.

Mn: 2.0 mass% or less

It is an austenite-forming element that suppresses the formation of δ ferrite at high temperatures and facilitates the formation of austenite. But 2.0 mass. If an excessive amount of Mn exceeding / ₀ is contained, retained austenite is easily generated after annealing, and when forming into a product shape, forms work-induced martensite and generates strain.

P: 0.050 mass% or less

Element harmful to hot workability, 0.050 mass. When excessive P exceeding / ₀ is contained, the adverse effect on hot workability becomes significant.

S: 0.020 mass% or less

It is a component that tends to segregate at crystal grain boundaries, embrittles the grain boundaries, and reduces hot workability and the like. The upper limit of the S content is 0.020 mass to suppress the adverse effects caused by S. It is preferable to regulate to / ₀ .

A1: 0.10 mass% or less

0.10 mass, added as a deoxidizer in the molten steel stage. Excessive addition of A1 in excess of / ₀ increases nonmetallic inclusions, causing a decrease in toughness and surface defects. N: 0.10 mass% or less

It is an austenite-forming element and suppresses the formation of δ-ferrite at high temperatures and promotes the formation of an austenite phase. But 0.10 mass. If an excessive amount of Ν exceeding / ₀ is contained, retained austenite is likely to be formed after annealing. Retained austenite transforms into work-induced martensite at the stage of processing into the product shape, which also causes distortion. It is also a component that increases the strength of the cold-rolled annealed material. 延 The ductility decreases as the content increases.

Mo: 1.0 mass% or less

Although an effective alloy component for improving corrosion resistance, excessive addition of Mo exceeding 1.0% by mass delays solid solution strengthening and dynamic recrystallization at high temperatures, and reduces hot workability.

Cu: 2.0 mass% or less

It is an unavoidable impurity that is mixed in from the raw material scrap, etc. However, if the Cu content is excessive, hot workability and corrosion resistance will be adversely affected. The adverse effect caused by Cu is that the Cu content is 2.0 mass. It is suppressed by regulating to / ₀ or less.

Ti: 0.01-0.50 mass% Nb: 0.01-0.50 mass. / ₀

V: 0.01~0.30 mass% Zr: 0.01~0.30 mass ^_0/0

Ti, Nb, and V precipitate solid solution C as carbides to improve workability, and Zr is an alloy component that improves workability and toughness by trapping oxygen in steel as oxide, but is excessively added. Reduces productivity. Therefore, the content of each alloy component is Ti: 0.01 to 0.50 mass. Nb: 0.01-0.50 mass. V: 0.01 to 0.30 mass. Zr: 0.01 to 0.30 mass. It is preferable to select within the range of / ₀ .

B: 0.0010-0.0100 晳 ° /. Less than

It has the effect of uniformly dispersing the transformed phase of the hot-rolled sheet and making the transformed phase finer in the dual-phase annealing step. The effect of adding B becomes significant at 0.0010 mass% or more, but excessive addition exceeding 0.0100 mass% adversely affects hot workability and weldability.

Y: 0.02% by mass or less Ca: 0.05% by mass or less

REM (rare earth metal): 0.1 mass. /. j¾ below Y, Ca, and REM are effective alloy components for improving hot workability, but if they are added excessively, surface flaws are likely to occur. When adding Y, Ca, REM, preferably Y: 0.02 mass. /. , Ca: 0.05 mass. /. , REM: 0.1 mass. /. Each has an upper limit. In the stainless steel whose composition is adjusted, the structure, the prior austenite grains, the expansion rate during the martensitic transformation, and the like are regulated in order to suppress the influence of distortion and volume expansion during martensitic transformation on the generation of the Reuse band.

Tissue: 20-85 volumes. / ₀ martensite and balance ferrite

The amount of martensite at room temperature is 20 to 85% by volume, and the amount of austenite at high temperature is 20 to 85% by volume. /. Hit. The austenite phase undergoes martensitic transformation in the course of cooling to room temperature, but transformation dislocations in the formed martensite and transformation strains caused by volume expansion accompanying martensitic transformation are introduced into the cooled stainless steel.

During the martensitic transformation, when the former austenite grains are refined to increase the grain boundary surface area of the ferrite grains in the high temperature range, the strain caused by the martensite transformation is uniformly dispersed and transformed into the surrounding soft ferrite grains. The distortion is absorbed. As a result, deformation due to transformation strain that appears on the outer surface of the steel strip is reduced. When the strain is corrected by applying 1 to 2% tensile strain to a stainless steel strip in the form of a steel belt in which the transformation strain is uniformly dispersed and absorbed, the finely transformed transformation strain that is uniformly dispersed is absorbed by the distortion during straightening, and the Ruders band is used. The stainless steel strip is uniformly deformed without the occurrence of cracks.

In order to actively absorb the finely transformed strains uniformly dispersed into the processing strains at the time of shape correction and to suppress the occurrence of reuse bands, adjust the amount of martensite effective for accumulating transformation strains to 20% by volume or more. This is very important. Martensite volume is 20 volumes. /. If it is less than 1%, the tensile strain of 1% to 2% applied in the shape correction stage exceeds the accumulated amount of transformation strain, and a Ruder's band appears on the steel strip surface. Low martensite content means excess soft ferrite, It tends to run out. Conversely, an excessive amount of martensite completely transforms into martensite in the cooling stage of the annealing process, which tends to cause shape deterioration due to transformation strain, and also makes processing at the time of shape correction difficult. Therefore, the upper limit of martensite is restricted to 85% by volume.

Average particle size of former austenite: 10um or less

When the prior austenite grains are refined, the grain size of martensite and ferrite generated in the cooling stage of the annealing process is reduced, and the martensitic transformation region is dispersed, so that the strain accompanying martensitic transformation is uniformly dispersed. As a result, non-uniform deformation during straightening of the steel belt is suppressed, and no Ruders band is generated. The uniform dispersion of the transformation strain and, consequently, the suppression of the Ruders band can be made effective by reducing the average particle size of the former austenite to ΙΟμιη or less.

Average expansion rate due to martensitic transformation: 9% or less

When the austenite phase transforms into martensite, the crystal structure changes from f.c. to b.c.c. or b.c.t. As the crystal structure changes, the atomic packing of the crystal structure changes and the stainless steel strip undergoes transformation expansion. The expansion rate due to the transformation is not simply proportional to the amount of martensite generated by the transformation, but depends on the distribution of martensite and fluoride. As the average grain size of the former austenite is smaller and the grain boundary area of the transformed martensite ferrite is larger, in other words, as the transformed martensite is more finely distributed, the transformation strain is absorbed by the surrounding soft ferrite and the inside of the ferrite Transformation strain is accumulated in

Due to the absorption and accumulation of transformation strain, the apparent bulk expansion of the bulk is reduced. Transformation By utilizing the effect of martensite refinement to suppress transformation distortion, non-uniform deformation at the time of steel belt straightening is prevented, and no reuse band is generated. For this purpose, the former austenite is refined to an average grain size of ΙΟμηι or less, the grain size of the transformed martensitic ferrite is reduced, the grain boundary area of martensite ferrite is increased, and the average expansion is increased. Must be 9% or less.

Hardness: HV300 above By adjusting the C and Ni contents and the martensite content, the hardness of the duplex stainless steel is refined, but high responsiveness in the operating environment, high speed, and high fatigue strength due to the use of small pulleys are required. For use as a steel belt, a material hardness of HV300 or more is required. Next, the present invention will be specifically described with reference to examples.

Stainless steel having the composition shown in Table 1 was vacuum-melted, forged, forged, and hot-rolled to a thickness of 3.0 mm. In the table, steel type numbers No .:! To 5 are stainless steels having the composition specified in the present invention, and steel type numbers Nos. 6 to 8 are stainless steels having compositions outside the range specified in the present invention.

For steel grade No .:! ~ 7, diffusion annealing at 780 ° C for 8 hours, cold rolling to a thickness of 1.0mm after pickling, and further soaking at 1050 ° C for 1 minute, then air-cooled dual phase annealing It was pickled again. For steel type No. 8, a 2.0 mm thick hot-rolled steel sheet equivalent to SUS301 was annealed at 1050 ° C for 60 seconds and then cold rolled to a 1.0 mm thickness.

Type of stainless steel used in Examples D

The underline indicates that the value is out of the specified range of the present invention.

For each stainless steel strip, the microstructure quantification, Vickers hardness of the surface (load 1 kg), and former austenite grain size were investigated. Ferrite and martensite are hydrofluoric acid

2: Etched with nitric acid 1: glycerin 1 etchant and quantified by the point count method. The amount of austenite was measured by a magnetic method. The old austenite particle size was observed with an electron microscope and measured by the section method. The average coefficient of expansion due to martensite transformation is determined by measuring the amount of one-way transformation expansion during the cooling process in laboratory-assisted dual-phase annealing and converting the measured value to the cube of 3 to convert it to a volume expansion coefficient. Was. Table 2 shows the survey results.

From a 1 mm thick stainless steel strip, a test piece with a width of 50 mm and a length of 200 mm whose length direction matches the rolling direction was cut out and subjected to a test to simulate actual steel belt straightening. In the simulation test, tensile strain was applied up to 5% at a strain rate of ImmZ using a tensile tester, and the presence or absence of a Lüders band was observed. Before the tensile strain was applied, a bending stress with a radius of 50 mm was applied to the test specimen 10 times in order to approximate the operating environment of the stainless steel belt which receives bending stress at the pulley. The test results are shown in Table 2.

Table 2: Microstructure and test results of each stainless steel strip

An underline indicates that the value is out of the range defined in the present invention.

* 1 indicates that cracking occurred during bending.

From the investigation results in Table 2, it can be seen that even if the steel belt made from the stainless steel strip according to the present invention was straightened, no reuse band was generated.

On the other hand, in comparative steel No. 6, the core of old austenite was not sufficiently formed due to insufficient Ni content, and the average austenite grains with an average particle size exceeding ΙΟμπι and the average expansion amount exceeding 9% by volume It seems that a re-band has been generated. In addition, the comparative steel No. 6 had insufficient strength due to too low Ni content, and cracks occurred in the repeated bending test stage prior to the tensile strain addition test. Comparative steel No. 7 has a small amount of martensite due to a lack of C content, so there is little transformation distortion, and the amount of distortion necessary for uniform deformation during belt straightening is insufficient. As a result, non-uniform deformation, in other words, a Luder's band, tends to occur. Similar to comparative steel No. 6, cracks did not occur during the repeated bending test because the Ni content was low but the C content was low.

Comparative steel No. 8, which has an excessive Ni content, has a large amount of retained austenite, and a work-induced martensitic transformation occurs during tensile deformation, resulting in the formation of a ruder band. Industrial applicability

As described above, by reducing the size of the prior austenite grains to increase the grain boundary area of the double phase structure of ferrite Z martensite, the martensitic transformation of the austenite phase during the cooling process in the annealing process Transformation strain is uniformly dispersed and absorbed and accumulated in the soft ferrite phase. The accumulated transformation strain is absorbed by the processing strain applied during shape correction in the steel belt manufacturing stage, and does not cause the generation of a reuse band. In this way, compared to conventional work hardening and precipitation hardening stainless steel belt materials, high strength stainless steel strips for steel belts that have excellent surface stability and excellent surface shape, as well as shape stability. Is provided.

Claims

The scope of the claims

1. C: 0.04 to 0.15% by mass, Cr: 10.0 to 20.0% by mass. Ni: 0.5-4.0% by mass, with the balance being substantially Fe, the average austenite grain size is less than ΙΟμπι, and the normal temperature structure after transformation is 20-85 martensite. / _0, balance ferrite, hardness not less than HV300, multi-phase stainless steel strip for steel belt.

2. Stainless steel: Si: 2.0 mass% or less, Mn: 2.0 mass%. /. Below, P: 0.050 mass. /. Below, S: 0.020 mass. /. Below, A1: 0.10 mass. /. Below, N: 0.10 mass. /. Mo: 1.0 mass% or less, Cu: 2.0 mass%. / ₀ or less, Ti: 0.01 to 0.50 mass. Nb: 0.01 to 0.50 mass ⁰ V: 0.01 to 0.30 mass. Zr: 0.01-0.30 mass. Hmm

B: 0.0010 to 0.0100% by mass or less, Y: 0.02% by mass or less, Ca: 0.05% by mass or less, REM (rare earth metal): 0.1% by mass or less of one or more steel belts. For duplex stainless steel strip.

3. The duplex stainless steel strip for a steel belt according to claim 1, wherein the average expansion amount when austenite undergoes martensitic transformation in the cooling step of the annealing step is 9% or less.