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Wpływ odkształcenia plastycznego na przemiany austenitu przechłodzonego stali mikrostopowej

Opiela Marek, Wojtacha Anna

Stal, Metale & Nowe Technologie

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2019

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nr 1-2

74--80

PL

W pracy przedstawiono wyniki badań wpływu odkształcenia plastycznego oraz szybkości chłodzenia na strukturę i postać krzywych przemian fazowych austenitu przechłodzonego stali mikrostopowej z Nb, Ti, V i B. Wyznaczono krzywe przemian austenitu przechłodzonego nieodkształconego plastycznie (CTPc) oraz odkształconego plastycznie (OCTPc).

EN

The aim of the paper is to investigate the influence of plastic deformation and cooling rate on the structure and shape of the supercooled austenite transformations diagrams of a Nb-Ti-V-B microalloyed steel. The CCT diagrams of underformed and plastically deformed supercooled austenite were developed. The plastic deformation of steel prior to the start of phase transformations results in a sharp acceleration of pearlitic transformation and a slight translation of bainitic transformation towards shorter times. The elaborated curves of supercooled austenite transformations of the steel create possibilities to develop an industrial technology for thermomechanically treated forgings of high mechanical properties.

2

Study of the Effect of Deformation on Transformation Diagrams of Two Low-Alloy Manganese-Chromium Steels

Kawulok R., Kawulok P., Schindler I., Opěla P., Rusz S., Ševčák V., Solowski Z.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 4

1735--1741

EN

The work deal with an assembling and comparing of transformation diagrams of two low-alloy steels, specifically 16MnCrS5 and 20MnCrS5. In this work, diagrams of the type of CCT and DCCT of both steels were assembled. Transformation diagrams were assembled on the basis of dilatometric tests realized on the plastometer Gleeble 3800, of metallographic analyses and of hardness measurements. In addition, for comparison, the transformation diagrams were assembled even with use of the QTSteel 3.2 software. Uniform austenitization temperature of 850°C was chosen in case of both steels and even both types of diagrams. In case of both steels, an influence of deformation led to expected acceleration of phase transformations controlled by diffusion and also of bainite transformation. In both cases, the kinetics of martensitic transformation was not significantly affected by deformation.

3

Wpływ odkształcenia plastycznego na postać krzywych CTPc nowo opracowanej stali mikrostopowej

Opiela M.

Inżynieria Materiałowa

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2014

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Vol. 35, nr 4

325--331

PL

W pracy przedstawiono wyniki badań wpływu odkształcenia plastycznego oraz warunków chłodzenia na mikrostrukturę i postać krzywych CTPc nowo opracowanej stali mikrostopowej. Wyznaczono krzywe przemian austenitu przechłodzonego nieodkształconego plastycznie (CTPc) oraz odkształconego plastycznie (OCTPc) stali mikrostopowej z Nb, Ti i V. Badania dylatometryczne przeprowadzono za pomocą dylatometru DIL 805A/D wyposażonego w głowicę pomiarową LVDT. Część próbek austenityzowano w temperaturze 885°C, a następnie chłodzono z tej temperatury z różnymi szybkościami od 234°C/s do 1°C/min. Natomiast w celu wyznaczenia wpływu odkształcenia plastycznego na postać krzywych CTPc próbki odkształcano w temperaturze 885°C oraz 1100°C, stosując 50% stopień gniotu, a następnie chłodzono z szybkością od 95°C/s do 1°C/min. Przeprowadzone badania wykazały, że stal ma temperaturę Ac3 = 843°C, Ac1 = 707°C oraz dość niską temperaturę Ms wynoszącą 370°C. Odkształcenie plastyczne stali w temperaturze 885°C przed rozpoczęciem przemian fazowych powoduje wyraźne przyspieszenie przemiany perlitycznej oraz słabe przesunięcie do krótszego czasu przemiany bainitycznej. Stwierdzono ponadto, że próbki odkształcone plastycznie w temperaturze 885°C przed ich kontrolowanym chłodzeniem wykazują większą twardość w porównaniu z próbkami nieodkształconymi plastycznie, chłodzonymi z tą samą szybkością. Opracowane krzywe przemian austenitu przechłodzonego badanej stali w pełni predysponują ją do wytwarzania odkuwek hartowanych bezpośrednio z temperatury końca kucia, a następnie poddawanych wysokiemu odpuszczaniu.

EN

The aim of the paper is to investigate the influence of plastic deformation and cooling conditions on the microstructure and shape of CCT-diagrams of newly-developed Nb-Ti-V microalloyed steel. The CCT-diagrams of undeformed and plastically-deformed supercooled austenite for Nb-Ti-V microalloyed steel were developed. A DIL 805A/D dilatometer with a LVDT-type measuring head was used to carry out the dilatometric test. A part of the specimens were austenitized at a temperature of 885°C and next cooled to ambient temperature at various rates from 234°C/s to 1°C/min. To investigate the influence of plastic deformation on the shape of CCT (Continuous Cooling Transformations) diagrams, another part of the specimens were 50% deformed at 885°C and 1100°C and cooled to ambient temperature at rate from 95°C/s to 1°C/min. The performed dilatometric research revealed that the steel is characterized by Ac3 = 843°C, Ac1 = 707°C and a relatively low Ms temperature equal to 370°C (Fig. 1). Plastic deformation of steel at the temperature of 885°C prior to the start of phase transformations results in distinct acceleration of pearlitic transformation and slight translation of bainitic transformation towards shorter times (Fig. 2). Additionally, it was found that samples plastically deformed at the temperature of 885°C prior to their controlled cooling demonstrated higher hardness values in respect to the hardness values of non-deformed samples cooled at the same rate. The developed curves of supercooled austenite transformations of the studied steel fully predispose it to the production of forgings quenched directly from the forging finish temperature and successively subjected to high temperature tempering.

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Third generation of AHSS with increased fraction of retained austenite for the automotive industry

Grajcar A., Kuziak R., Zalecki W.

Archives of Civil and Mechanical Engineering

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2012

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Vol. 12, no. 3

334--341

EN

Third generation of advanced high-strength steels for the automotive industry contains a high volume fraction of fine-grained ferrite, carbide-free bainite, martensite and retained austenite. The level of strength and ductility is highly dependent on the fraction and mechanical stability of austenitic phase. One of the methods to obtain an increased fraction of γ phase is trough its chemical stabilization by Mn. Two 0.17C–3Mn–1.5Al–0.2Si–0.2Mo steels with and without Nb microaddition were developed in the present study. The steels were subjected to the thermomechanical processing designed on the basis of the DCCT diagram (deformation – continuous cooling transformation). The paper presents the results of the multi-stage compression tests and multiphase microstructures obtained as a result of the controlled multi-stage cooling. It was found that the hot workability of a new generation of AHSS is very challenging due to high values of flow stresses required. However, the thermomechanical processing enables to obtain very fine-grained bainite-based microstructures with a fraction of retained austenite up to 20%. The highest fraction of fine grains and interlath austenite was obtained for the temperature range between 400 and 450 °C. The effect of Nb results in higher flow stresses and better distribution of all the microstructural constituents.

5

Designing of cooling conditions for Si-Al microalloyed TRIP steel on the basis of DCCT diagrams

Grajcar A., Zalecki W., Kuziak R.

Journal of Achievements in Materials and Manufacturing Engineering

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2011

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Vol. 45, nr 2

115--124

EN

Purpose: The aim of the research presented in the paper is to design the cooling routes after plastic deformation ensuring the multiphase structure with a high fraction of retained austenite on the basis of DCCT diagram for a new-developed Si-Al microalloyed TRIP steel. Design/methodology/approach: The CCT and DCCT diagrams were developed. Eight variants of the thermomechanical processing were designed on the basis of the DCCT diagram. The thermomechanical processing consisted of a multi-stage compression and various cooling strategies in the γ→α transformation range and isothermal holding temperature at a bainitic transformation region. Findings: The usefulness of DCCT diagram for designing the thermomechanical processing conditions for TRIP steels was demonstrated. The fraction of ferrite and retained austenite are highly dependent on a cooling path applied in the γ→α transformation region and a fraction of retained austenite especially on the isothermal holding temperature. The highest fraction of retained austenite as irregular grains located in a ferritic matrix and fine islands or interlath regions in bainitic regions were obtained at temperature of 400 and 450°C. Research limitations/implications: To determine precisely a fraction of retained austenite, the X-ray investigations has to be applied additionally to the image analysis. Practical implications: The designed cooling conditions are of great importance for the thermomechanical strategy for manufacturing the advanced high strength TRIP steels. Originality/value: The thermomechanical processing was carried out for a new group of TRIP steels with Si partially replaced by Al and containing microadditions of Nb and Ti.

6

Krzywe OCTPc jako podstawa doboru warunków chłodzenia stali o strukturze wielofazowej

Grajcar A., Zalecki W.

Inżynieria Materiałowa

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2011

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Vol. 32, nr 2

122-127

PL

Praca dotyczy projektowania kilkuetapowego chłodzenia stali typu C-Mn-Si-Al-Nb-Ti w celu uzyskania struktur wielofazowych z dużym udziałem metastabilnego austenitu szczątkowego, warunkującym wykorzystanie efektu TRIP (TRansformation Induced Plasticity) podczas kształtowania technologicznego blach na zimno. Na podstawie badań dylatometrycznych wyznaczono wykresy CTPc (czas-temperatura-przemiana) oraz OCTPc (odkształcenie-czas-temperatura-przemiana). Wykres OCTPc był podstawą opracowania czterech wariantów chłodzenia ze zróżnicowaniem warunków chłodzenia w zakresie przemiany y-a. Obróbkę cieplno-plastyczną przeprowadzono w symulatorze Gleeble 3800, stosując wytrzymanie izotermiczne stali w temperaturze 350°C przez 600 s. Dokonano szczegółowej analizy mikrostruktury stali z zastosowaniem mikroskopii świetlnej oraz wyznaczono udziały austenitu szczątkowego i ferrytu. Stwierdzono, że ciągłe chłodzenie stali w zakresie przemiany y-a z szybkością 25°C/s nie zapewnia pożądanego udziału ferrytu i austenitu szczątkowego. Uzyskanie ponad 12% austenitu szczątkowego w osnowie drobnoziarnistego ferrytu wymaga zastosowania wolnego chłodzenia w zakresie przemiany austenitu w ferryt.

EN

The work concerns the design of multi-stage cooling of C-Mn-Si-Al-Nb- -Ti-type steel in order to obtain multiphase structures with a large fraction of metastable retained austenite, which enables the utilization of the TRIP effect (TRansformation Induced Plasticity) during technological cold forming of sheets. The CCT (continuous cooling transformation) and DCCT (deformation-continuous cooling transformation) diagrams (Fig. 1 and 2) were developed on the basis of dilatometric tests. The DCCT was a basis of the elaboration of four cooling variants with a diversification of cooling conditions in a range of the y-a transformation (Fig. 3a). The thermomechanical processing was carried out using the Gleeble 3800 simulator, applying isothermal holding of steel at a temperature of 350°C for 600 s. The detailed analysis of the microstructure of steel by light microscopy was performed and the fractions of retained austenite and ferrite were determined (Tab. 1). It was found that the continuous cooling of steel in a range of the y-a transformation with a rate of 25°C/s does not guarantee of desirable fractions of ferrite and retained austenite (Fig. 3c, d). Obtaining over 12% retained austenite in a matrix of fine-grained ferrite requires applying of slow cooling in a range of the transformation of austenite into ferrite (Fig. 3a, b).