Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-12-03
2004-07-20
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S327000, C148S608000
Reexamination Certificate
active
06764555
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength meta-stable austenitic stainless steel strip composed of a dual-phase structure of austenite and martensite exhibiting excellent flatness with Vickers hardness of 400 or more. The invention also relates to a manufacturing method thereof.
Martensitic, work-hardened or precipitation-hardened stainless steel has been typically used as a high-strength material with a Vickers hardness of 400 or more.
Martensitic stainless steel such as SUS 410 or SUS420J2 is hardened by quenching from a high-temperature austenitic phase to induce martensite transformation. Since the steel material is adjusted to a Vickers hardness of 400 or more by heat-treatment such as quenching-tempering, its manufacturing process necessitates such the heat-treatment. The steel strip unfavorably reduces its toughness after quenching and changes its shape due to the martensite transformation. These disadvantages put considerable restrictions on manufacturing conditions.
Work-hardened austenitic stainless steel such as SUS 301 or SUS 304 is often used instead, in the case where deviation of shape causes troubles on usage. The work-hardened austenitic stainless steel has an austenitic phase in a solution-treated state and generates a deformation-induced martensite phase effective for improvement of strength during cold-rolling thereafter.
Although the surface of a steel strip is flattened by cold-rolling, the dependency of hardness on a rolling temperature is great, and the surface flatness varies irregularly along a lengthwise direction or rolling direction of the steel strip. As a consequence, it is difficult to uniformly flatten the steel strip under stable conditions by cold-rolling from commercial point of view.
A degree of transformation from austenite to deformation-induced martensite depends on a rolling temperature, even if a stainless steel strip such as SUS 301 or SUS 304 is cold-rolled at the same reduction ratio. When the steel strip is cold-rolled at a high temperature, generation of the deformation-induced martensite is suppressed, resulting in poor hardness of the cold-rolled steel strip. Conversely, a lower rolling temperature accelerates transformation to deformation-induced martensite and raises hardness of the cold-rolled steel strip. Increasing hardness causes an increase of deformation resistance, and so makes it difficult to flatten the steel strip in a uniform manner.
SUMMARY OF THE INVENTION
The present invention provides a high-strength austenitic stainless steel strip exhibiting excellent flatness with Vickers hardness of 400 or more. Improved flatness is attained by a volumetric change during the phase reversion from deformation-induced martensite to austenite so as to suppress shape deterioration caused by martensitic transformation, rather than flattening the steel strip while in a martensitic phase.
The high-strength austenitic stainless steel strip proposed by the present invention has a composition consisting of C up to 0.20 mass %, Si up to 4.0 mass %, Mn up to 5.0 mass %, 4.0-12.0 mass % Ni, 12.0-20.0 mass % Cr, Mo up to 5.0 mass %, N up to 0.15 mass %, optionally at least one or more of Cu up to 3.0 mass %, Ti up to 0.5 mass %, Nb up to 0.50 mass %, Al up to 0.2 mass %, B up to 0.015 mass %, REM (rare earth metals) up to 0.2 mass %, Y up to 0.2 mass %, Ca up to 0.1 mass % and Mg up to 0.10 mass %, and the balance being Fe plus inevitable impurities with the provision that a value Md(N) defined by the formula (1) is in a range of 0-125.
Md(N)=580-520C-2Si-16Mn-16Cr-23Ni-26Cu-300N-10Mo (1)
The steel strip has a dual-phase structure of austenite and martensite, which involves a reversed austenitic phase at a ratio more than 3 vol. %.
The newly proposed austenitic stainless steel strip is manufactured as follows: A stainless steel strip having the properly controlled composition is solution-treated, cold-rolled to generate a deformation-induced martensite phase, and then re-heated at 500-700° C. to induce a phase reversion, whereby an austenitic phase is generated at a ratio of 3 vol. % or more in a matrix composed of the deformation-induced martensite. When the steel strip is treated in this manner to achieve the austenitic phase reversion of 3 vol. % or more and then placed under a load of 785 Pa or more, the flatness of the strip is improved.
DETAILED DESCRIPTION OF INVENTION
The inventors have researched and examined, from various aspects, effects of conditions for manufacturing a meta-stable austenitic stainless steel strip, which generates deformation-induced martensite during cold-rolling, on hardness and flatness of the steel strip. As a result of the research, the inventors have found that heat-treatment to promote reversion from deformation-induced martensite to austenite causes a volumetric change of the steel strip which is effective for improving flatness. High strength and excellent flatness are gained by properly controlling the composition of the steel as well as controlling the conditions for reversion. In the specification of the present invention, the wording “a steel strip” of course involves a steel sheet, and the same reversion to austenite is realized during heat-treatment of a steel sheet.
The composition of the austenitic stainless steel together with the conditions of reversion will become apparent from the following explanation.
C up to 0.20 mass %
C is an austenite former, which hardens a martensite phase and also lowers a reversion temperature. As the reversion temperature decreases, reversion to austenite is more easily controlled at a proper ratio suitable for improvement of flatness and hardness. However, precipitation of chromium carbides at grain boundaries is accelerated in a cooling step after solution-treatment or during aging as the C content increases. Precipitation of chromium carbides causes degradation of intergranular corrosion cracking resistance and fatigue strength. In this sense, an upper limit of C content is determined at 0.20 mass %, so as to inhibit precipitation of chromium carbides by conditions of heat-treatment and a cooling speed.
Si up to 4.0 mass %
Si is a ferrite former, which dissolves in a martensite matrix, hardens the martensitic phase and improves strength of a cold-rolled steel strip. Si is also effective for age-hardening, since it promotes strain aging during aging-treatment. However, excessive additions of Si cause high-temperature cracking and also various troubles in the manufacturing process, so that an upper limit of the Si content is determined at 4.0 mass %.
Mn up to 5.0 mass %
Mn is effective for suppressing generation of &dgr;-ferrite in a high-temperature zone. An initiating temperature for reversion falls as the Mn content increases, so that a ratio of reversed austenite can be controlled with ease. However, excessive addition of Mn above 5.0 mass % unfavorably accelerates generation of deformation-induced martensite during cold-rolling, and makes it impossible to use the reversion for improvement of flatness.
Ni: 4.0-12.0 mass %
Ni inhibits generation of &dgr;-ferrite in a high-temperature zone, the same as Mn, and lowers an initiating temperature for reversion, the same as C. Ni also effectively improves precipitation-hardenability of a steel strip. These effects become apparent at a Ni content not less than 4.0 mass %. However, excessive additions of Ni above 12.0 mass % unfavorably accelerate generation of deformation-induced martensite during cold-rolling and thus makes it difficult to induce the reversion necessary for flattening.
Cr: 12.0-20.0 mass %
Cr is an alloying element used for improvement of corrosion resistance. Corrosion resistance is intentionally improved at a Cr content of 12.0 mass % or more. However, excessive additions of Cr cause too much generation of &dgr;-ferrite in a high-temperature zone and requires the addition of austenite formers such as C, N, Ni, Mn and Cu. An increase of the austenite formers stabilizes the austenitic phase at room temperature and makes it difficult to generate
Fujimoto Hiroshi
Hiramatsu Naoto
Morimoto Ken-ichi
Tomimura Kouki
Nisshin Steel Co. Ltd.
Webb Ziesenheim Logsdon Orkin & Hanson. P.C.
Yee Deborah
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