JP4252893B2 - Duplex stainless steel strip for steel belt - Google Patents

Abstract

A high-strength dual-phase stainless steel strip has a chemical composition consisting of 0.04-0.15 mass % C, 10.0-20.0 mass % Cr, 0.5-4.0 mass % Ni and the balance being Fe except inevitable impurities, and a metallurgical structure composed of 20-85 vol. % martensite grains and the balance ferrite grains with prior austenite grains controlled to 10 mu m or less in size. The stainless steel strip is conditioned to hardness of HV 300 or more. Transformation strains are uniformly distributed in a steel matrix during martensitic transformation, so that the steel strip is formed and straightened to a belt shape without L}ders band. Consequently, steel belts with fine external appearance are manufactured from the stainless steel strip.

Description

The present invention relates to a high-strength duplex stainless steel strip for steel belts that has an excellent surface shape that does not generate a Lueder's band during shape correction in a steel belt manufacturing process.

For stainless steel belts, in addition to work hardening type austenitic stainless steel in which austenitic stainless steel such as SUS301 and SUS304 is strengthened by cold rolling, low carbon martensitic stainless steel (Japanese Patent Publication No. 51-31085). Precipitation hardening type martensitic stainless steel (Japanese Patent Publication No. 59-49303) is used for convenience.

Work hardening type structures represented by SUS304 and SUS301 are metastable austenite structures, and work-induced martensite is formed by deformation. Therefore, a Luders band resulting from the formation of work-induced martensite is generated during the deformation (Journal of the Japan Institute of Metals, Vol. 55, No. 4, pages 376-382, Nisshin Steel Technical Report No. 69, pages 1-14. ) Undesirable surface irregularities occur as a steel belt material.

  The martensite system and the precipitation hardening type martensite system are transformed into a substantially martensite single phase in the cooling process from the annealing in the production process, but the shape is likely to change due to expansion accompanying the transformation. The deteriorated shape cannot be easily corrected in the belt state.

  The present invention has been devised to solve such a problem, and when a belt shape correction such as a metastable austenite system is performed, a Luders band is generated, or in the manufacturing process such as a martensite system. An object of the present invention is to provide a stainless steel strip for a steel belt having a ferrite / martensite double phase structure and an excellent surface shape without making shape correction difficult by complete transformation.

In order to achieve the object, the high-strength duplex stainless steel strip for steel belt of the present invention has C: 0.04 to 0.15% by mass, Cr: 10.0 to 20.0% by mass, Ni: 0.0. 5 to 4.0 mass%, Si: 2.0 mass% or less, Mn: 2.0 mass% or less, Al: 0.10 mass% or less, N: 0.10 mass% or less, Mo: 1.0 mass % Or less, Cu: 2.0% by mass or less, having the composition of the balance Fe and inevitable impurities, the prior austenite average particle size of 10 μm or less, and 20 to 85% by volume of martensite and the balance at room temperature after transformation Has a ferrite structure and is tempered to a hardness of HV300 or higher.

It is preferable that the prior austenite average particle diameter is 10 μm or less, and the average expansion amount when the austenite undergoes martensitic transformation in the cooling process of the annealing process is adjusted to 9% or less.
In the present specification, “steel strip” is used to include steel plates.

The inventors of the present invention investigated and examined the influence on the Lueders band generated during shape correction of the steel belt 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 have a great influence on the generation of the Lueders band. Considering the effects of strain distribution and volume expansion, it is possible to eliminate residual austenite in the stainless steel strip and to disperse and generate expansion strain throughout the steel strip when the austenite phase undergoes martensitic transformation during the cooling process in the annealing process. It was clarified that it is effective in preventing the occurrence of a dozen bands.
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 alloy component that is extremely effective for strengthening the martensite phase. Can adjust the amount of martensite generated after the high temperature heating above austenitizing temperature Ac _l point, it contributes to the intensity adjustment and high strength. The effect of adding C becomes significant when the C content is 0.04% by mass or more. However, excessive C content causes Cr carbide to precipitate at the grain boundaries in the cooling process and aging treatment after the multi- phase treatment, thereby reducing the intergranular corrosion resistance and fatigue characteristics, so the upper limit of the C content is 0.15. It was set to mass%.

Cr: 10.0-20.0 mass%
It is an essential alloy component for securing the corrosion resistance as stainless steel, and Cr is contained at 10.0 mass% or more in order to provide the necessary corrosion resistance. However, when an excessive amount of Cr exceeding 20.0 mass% is added, the toughness and workability of the steel material are lowered. As the Cr content increases, the amount of austenite-forming elements such as C, N, Ni, Mn, and Cu necessary for the formation of martensite and high strength is inevitably increased. Increasing the amount of austenite forming elements not only increases the cost of the steel strip, 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 (ferrite + martensite at room temperature) at a high temperature. The amount of martensite increases according to the Ni content, and the strength of the steel material is increased. In addition, the addition of Ni increases the austenite nucleation frequency in (austenite + ferrite) two-phase annealing, and as a result, a fine (austenite + ferrite) two-phase mixed structure is obtained. The mechanism of the increase of Ni on the formation of a fine two-phase mixed structure is that the growth rate of austenite nuclei is slower than the critical nuclei defined by classical nucleation theory, while the stable amount is This is considered to be due to an increase in the number of nucleation sites in order to form austenite nuclei in an attempt to generate austenite. The effect of adding Ni on the refinement of the two-phase mixed structure becomes significant when the Ni content is 0.5 mass% or more. However, it is an expensive element that raises the cost of steel, and of course, the austenite phase generated at a high temperature due to excessive addition of Ni does not transform into martensite in the cooling process to room temperature and becomes retained austenite, thus reducing the strength of the steel. Responsible. Therefore, the upper limit of the Ni content is set to 4.0% by mass.

The dual-phase stainless steel strip to which the present invention is directed, C, Cr, in addition to Ni, M n, Cu, austenite-forming element or Si in N, ferrite forming elements Al and added pressure, ferrite + martensite at room temperature each alloy component as duplex structure of the site is obtained Ru is adjusted. In addition, Mo that is effective for improving corrosion resistance is added within a range that does not reduce the required strength . Furthermore , Y, Ca, REM (rare earth metal) effective for improving oxidation resistance and hot workability, B, V and other alloy elements effective for improving various properties , and ferrite forming elements such as Ti and Nb are appropriately used. It may be added. The contents of these essential components and optional components are determined as follows.

Si: 2.0% by mass or less Si is a component added as a deoxidizer in the molten steel stage, but has a high solid solution strengthening ability. Excessive addition of Si exceeding 2.0% by mass hardens the steel material and reduces ductility. Let

Mn: 2.0% by mass or less An austenite-forming element that suppresses the formation of δ ferrite in a high temperature range and facilitates the formation of austenite. However, when an excessive amount of Mn exceeding 2.0% by mass is contained, retained austenite is likely to be generated after annealing, and processing-induced martensite is generated when processing into a product shape, which also causes distortion.

P: 0.050% by mass or less An element harmful to hot workability. When excessive P exceeding 0.050% by mass is included, an adverse effect on hot workability becomes remarkable.

S: 0.020% by mass or less This is a component that easily segregates at the crystal grain boundaries, embrittles the grain boundaries, and decreases hot workability. In order to suppress the adverse effects caused by S, it is preferable to limit the upper limit of the S content to 0.020% by mass.

Al: 0.10% by mass or less Al is a component added as a deoxidizer at the molten steel stage, but excessive addition of Al exceeding 0.10% by mass increases nonmetallic inclusions and causes toughness reduction and surface defects. It becomes.

N: 0.10% by mass or less An austenite forming element that suppresses the formation of δ ferrite in a high temperature range and promotes the formation of an austenite phase. However, if an excessive amount of N exceeding 0.10% by mass is contained, retained austenite tends to be generated after annealing. Residual austenite transforms into processing-induced martensite at the stage of processing into the product shape, and causes distortion. It is also a component which raises the intensity | strength of a cold-rolled annealing material, and ductility falls according to the increase in N content.

Mo: 1.0% by mass or less Mo is an alloy component effective for improving corrosion resistance. However, excessive addition of Mo exceeding 1.0% by mass delays solid solution strengthening and dynamic recrystallization at high temperatures, resulting in hot workability. Reduce.

Cu: 2.0% by mass or less Cu is an unavoidable impurity mixed from scrap as a melting raw material. However, when the Cu content is excessive, there is an adverse effect on hot workability and corrosion resistance. The adverse effect caused by Cu is suppressed by regulating the Cu content to 2.0 mass% or less.

Ti: 0.01 to 0.50 mass%
Nb: 0.01 to 0.50 mass%
V: 0.01-0.30 mass%
Zr: 0.01-0.30 mass%
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 capturing oxygen in steel as an oxide. Will reduce productivity. Therefore, the content of each alloy component is as follows: Ti: 0.01 to 0.50 mass%, Nb: 0.01 to 0.50 mass%, V: 0.01 to 0.30 mass%, Zr: 0.00. It is preferable to select in the range of 01 to 0.30 mass%.

B: 0.0010 to 0.0100 mass %
The transformation phase of the hot-rolled sheet is uniformly dispersed, and the transformation phase is refined in the multiphase annealing stage. The addition effect of B becomes significant at 0.0010% by mass or more, but excessive addition exceeding 0.0100% by 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% by mass or less Y, Ca, and REM are effective alloy components for improving hot workability. If excessively added, surface defects are likely to occur. When adding Y, Ca, and REM, the upper limit is preferably restricted to Y: 0.02 mass%, Ca: 0.05 mass%, and REM: 0.1 mass%, respectively.

  The component-adjusted stainless steel controls the structure, prior austenite grains, expansion coefficient during martensite transformation, and the like in order to suppress the influence of strain and volume expansion during the martensite transformation on the occurrence of the Lueders band.

Structure: 20 to 85% by volume of martensite and the remaining ferrite The amount of martensite at room temperature of 20 to 85% by volume corresponds to the amount of austenite at a high temperature of 20 to 85% by volume. The austenite phase undergoes martensitic transformation in the cooling process to room temperature, but transformation strain due to the transformation dislocation in the produced martensite and volume expansion associated with the martensitic transformation is introduced into the stainless steel after cooling.
In the martensitic transformation, if the prior austenite grains are refined to increase the grain boundary surface area of the prior austenite grains / ferrite grains in the high temperature range, the strain due to the martensitic transformation is uniformly dispersed, and the soft ferrite grains in the surrounding area are dispersed. Transformation distortion is absorbed. As a result, deformation due to transformation strain appearing on the outer surface of the steel strip is reduced. When a steel belt-shaped stainless steel strip with uniform transformation / absorption of transformation strain is corrected by applying 1-2% tensile strain, the uniform transformation of fine transformation strain is absorbed by the strain during correction, and the Luders band The stainless steel strip is uniformly deformed without any occurrence.

  In order to suppress the generation of Luders bands by actively absorbing fine transformation strains that are uniformly dispersed into processing strains during shape correction, the amount of martensite that is effective for accumulating transformation strains should be adjusted to 20% by volume or more. is important. If the amount of martensite is less than 20% by volume, the tensile strain of 1 to 2% applied in the shape correction stage exceeds the accumulated amount of transformation strain, and a Luders band appears on the surface of the steel strip. When the amount of martensite is small, it means that soft ferrite is excessive, and the strength of the stainless steel strip tends to be insufficient. Conversely, an excessive amount of martensite is completely transformed into martensite in the cooling stage of the annealing process, and shape deterioration due to transformation distortion is likely to appear, and processing during shape correction becomes difficult. Therefore, the upper limit of the martensite amount is restricted to 85% by volume.

Average grain size of prior austenite: 10 μm or less When the prior austenite grains are refined, the martensite and ferrite grains produced in the cooling stage of the annealing process become smaller and the martensite transformation region is dispersed. The accompanying distortion is uniformly dispersed. As a result, non-uniform deformation during steel belt correction is suppressed, and a Luders band is not generated. Uniform dispersion of transformation strain, and thus suppression of the Lueders band, can be effectively achieved by setting the average particle size of the prior austenite to 10 μm or less.

Average expansion coefficient associated with martensitic transformation: 9% or less When the austenite phase undergoes martensitic transformation, the crystal structure becomes f. c. c. To b. c. c. Or b. c. t. To change. As the crystal structure changes, the atomic filling rate of the crystal structure changes and the stainless steel strip is transformed and expanded. The expansion coefficient due to transformation is not simply proportional to the amount of martensite produced by transformation, but depends on the distribution of martensite and ferrite. The smaller the average grain size of the prior austenite and the larger the interfacial area of martensite / ferrite after transformation, in other words, the more finely distributed the transformation martensite, the more the transformation strain is absorbed in the surrounding soft ferrite. Transformation distortion is accumulated.

  Absorption / accumulation of transformation strain reduces the apparent bulk expansion. By utilizing the effect that the transformation martensite is refined to suppress transformation distortion, non-uniform deformation during the steel belt correction is prevented, and the Luders band does not occur. For this purpose, prior austenite is refined to an average grain size of 10 μm or less, the grain size of the martensite / ferrite two-phase structure after transformation is reduced to increase the martensite / ferrite grain interface area, and the average expansion amount Needs to be 9% or less.

Hardness: HV300 or higher Although the hardness of the multiphase stainless steel is adjusted by adjusting the C, Ni content and martensite content, it has high fatigue strength due to high responsiveness, high speed, and small pulley in the usage environment. In the required use as a steel belt, a material hardness of HV300 or higher is required.

Next, the present invention will be described specifically by way of examples.
Stainless steel having the composition shown in Table 1 was vacuum melted, cast and forged, and then hot rolled to a thickness of 3.0 mm. In the table, steel type no. Nos. 1 to 5 are stainless steels having the composition defined in the present invention, steel type nos. 6 to 8 are stainless steels having a composition outside the range defined in the present invention.
Steel type no. 1-7, diffusion annealing at 780 ° C. for 8 hours, cold rolling to a plate thickness of 1.0 mm after pickling, further 1050 ° C. × 1 minute soaking / air cooling dual phase annealing, pickling again did. Steel type no. In No. 8, a hot-rolled steel sheet having a thickness of 2.0 mm corresponding to SUS301 was annealed at 1050 ° C. for 60 seconds, and then cold-rolled to a thickness of 1.0 mm.

  Each stainless steel strip was examined for structure quantification, surface Vickers hardness (load 1 kg), and prior austenite grain size. Ferrite and martensite were etched with an etching solution of hydrofluoric acid 2: nitric acid 1: glycerin 1 and quantified by a point count method. The amount of austenite was measured by a magnetic method. The prior austenite particle size was observed with an electron microscope and measured by a section method. The average expansion coefficient resulting from martensitic transformation is obtained by measuring the amount of unidirectional transformation expansion during the cooling process in the multi-phase annealing performed in the laboratory, converting the measured value to the third power, and converting it to the volume expansion coefficient. It was. The survey results are shown in Table 2.

  A test piece having a width of 50 mm and a length of 200 mm, whose length direction coincides with the rolling direction, was cut out from a stainless steel strip having a thickness of 1 mm and subjected to a test for simulating actual steel belt correction. In the simulation test, tensile strain was applied to a maximum of 5% at a strain rate of 1 mm / min using a tensile tester, and the presence or absence of a Luders band was observed. In addition, in order to be close to the usage environment of the stainless steel belt subjected to the bending stress at the pulley portion, a bending stress having a radius of 50 mm was applied to the test specimen 10 times before the tensile strain was applied. The test results are also shown in Table 2.

From the results of the investigation in Table 2, it can be seen that a Lueders band does not occur even when the shape of a steel belt made from a stainless steel strip according to the present invention is corrected.
On the other hand, comparative steel No. In No. 6, the old austenite nuclei were not sufficiently formed because the Ni content was insufficient, and the generation of Lueders bands due to the prior austenite grains having an average particle size exceeding 10 μm and the average expansion amount exceeding 9% by volume Is called. Comparative steel No. In No. 6, the Ni content was too small, so that the strength was insufficient, and cracks occurred in the repeated bending test prior to the tensile strain addition test.

Comparative steel No. In No. 7, since the C content is insufficient and the amount of martensite is small, the transformation distortion is small, and the amount of distortion necessary for uniform deformation during belt correction is insufficient. Therefore, non-uniform deformation, in other words, a Luders band is likely to occur. Comparative steel No. As in the case of No. 6, although the Ni content is low, the C content is also low, so no cracks were generated at the repeated bending test stage.
Comparative steel No. with excessive Ni content No. 8 had a large amount of retained austenite, and as a result of work-induced martensitic transformation during tensile deformation, a Luders band was generated.

  As explained above, the transformation that occurs when the austenite phase undergoes martensitic transformation during the cooling process in the annealing process by refining the prior austenite grains and increasing the grain interfacial area of the ferrite / martensite double phase structure. Strain is uniformly dispersed and absorbed and accumulated in the soft ferrite phase. The accumulated transformation strain is absorbed by processing strain applied during shape correction in the steel belt manufacturing stage, and does not cause the generation of Luders band. In this way, a high-strength stainless steel strip for steel belts is provided that has superior surface shape and has no Luders band as well as shape stability compared to conventional work-hardening and precipitation-hardening stainless steel belt materials. Is done.

Claims (3)

C: 0.04-0.15 mass%, Cr: 10.0-20.0 mass%, Ni: 0.5-4.0 mass%, Si: 2.0 mass% or less, Mn: 2.0 mass% or less, Al: 0.10 wt% or less, N: 0.10 wt% or less, Mo: 1.0 wt% or less, Cu: includes 2.0 mass% or less, the remaining portion F e and unavoidable impurities A duplex stainless steel strip for steel belts characterized in that the former austenite average grain size is 10 μm or less, the normal temperature structure after transformation is 20 to 85% by volume of martensite, the remaining ferrite, and the hardness is HV300 or more. .   Stainless steel is further Ti: 0.01 to 0.50 mass%, Nb: 0.01 to 0.50 mass%, V: 0.01 to 0.30 mass%, B: 0.0010 to 0.0100 mass% %, Y: 0.02% by mass or less, Ca: 0.05% by mass or less, REM (rare earth metal): 0.1% by mass or less. Phase stainless steel strip.   The duplex stainless steel strip for a steel belt according to claim 1 or 2, wherein an average expansion amount when the austenite undergoes martensitic transformation in the cooling process of the annealing process is 9% or less.