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COMUNICAÇÃO TÉCNICA ______________________________________________________________________________________________________________________________________________________________________________________________________

Nº 174933.1

Study, in pilot plant, of the relationship process, microstructure and properties of microalloyed steels processes by conventional and thermomechanical rolling Carolina Elgert Felipe Moreno Siqueira Borges de Carvalho Ana Paola Villalva Braga Felipe Bastos José Roberto Bolota Leila Teichmann

Apresentação no SEMINÁRIO DE LAMINAÇÃO E CONFORMAÇÃO DE METAIS, 54., 2017, São Paulo. Palestra... 17 slides.

ABM WEEK 2017, Rio de Janeiro

A série “Comunicação Técnica” compreende trabalhos elaborados por técnicos do IPT, apresentados em eventos, publicados em revistas especializadas ou quando seu conteúdo apresentar relevância pública. ___________________________________________________________________________________________________

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STUDY, IN PILOT PLANT, OF THE RELATIONSHIP PROCESS, MICROSTRUCTURE AND PROPERTIES OF MICROALLOYED STEELS

PROCESSED BY CONVENTIONAL AND THERMOMECHANICAL ROLLING Carolina Elgert – Gerdau Aços Especiais Felipe Moreno – Instituto de pesquisas tecnológicas/POLI-USP

Oct 4th

• Thermomechanical Rolling Process (TMR) • Obtain ultra fine grains (UFG) • High strenght-toughness • Downsizing • High performance • High Cold formability • No heat treatment need (refined ferritic perlitic

microstructure obtained on direct cooling after rolling)

Introduction

• Definitions and strategy to obtain refine structure

Introduction

Type I Full rex

Type II No rex

RLT Tnr RST

Ar3

Type III Partial rex

Introduction

• 3 ingots were melted in vacum induced furnace – 3 chemical compositions

• Plates were machined from ingots • The rolling process were developed in pilot scale

Methodology

Chemical composition

Aço C Mn Si Cr V Nb

20MnCr5 0,17 0,22

1,10 1,40

- 0,40

1,00 1,30

20MnCr5 + Nb 0,17 0,22

1,10 1,40

- 0,40

1,00 1,30 x

20MnCr5 + Nb + V 0,17 022

1,10 1,40

- 0,40

1,00 1,30 3x x

Plate before rolling Plate after rolling

• 2 diferent rolling conditions – Conventional – Thermomechanical

Rolling and direct cooling

• 2 diferent cooling rate after rolling – Air – Forced air

reheating temperature

Microstructure – 500x

20MnCr5 20MnCr5 + Nb

Conv

entio

nal

Ther

mom

echa

nica

l

20MnCr5 + Nb + V

Etching: Nital 2%

Effect of cooling

20MnCr5 + Nb convenctional rolling and air cooled (A) and forced air(B). Magnification: 500x.

(a) (b)

Microstructure detail

20MnCr5 + Nb hot rolled and direct air cooled. (a) ferrite + degenerated perlite; (b) degenerated perlite.

Austenite grain

20MnCr5 20MnCr5 + Nb 20MnCr5 + Nb + V

Conv

entio

nal

Ther

mom

echa

nica

l

Austenite grain

20MnCr5 20MnCr5 + Nb 20MnCr5 + Nb + V

Conv

entio

nal

Ther

mom

echa

nica

l

• Elongation: The absolute value of the difference between the inertias of the major and the minor axes, divided by the sum of these inertias. The minor axis is defined as the perpendicular axis to the major axis. This measure is zero for a circle and approaches 1 for a long and narrow ellipse.

– 0 – circle. – 1 – long and narrow elipse.

• Equivalent Diameter: Gives the diameter of the circle whose area is equal to the area of the object, expressed in calibrated units.

Mensurement of austenitic structure

Elongation

Rolling condition 20MnCr5 20MnCr5 Nb 20MnCr5 Nb + V Conventional 0,51 0,68 0,67

Thermomechanical 0,69 0,73 0,76

• Increase of the elongation for the three steels in thermo-mechanical rolling condition • Larger elongation and narrower distribution for microalloyed steel

20MnCr5 20MnCr5 Nb

20MnCr5 Nb + V

• Equivalent diameters smaller for microalloyed steels in the two rolling conditions;

• Small increase in equivalent diameter for thermo-mechanical rolling as a function of elongation

Rolling condition 20MnCr5 20MnCr5 + Nb 20MnCr5 + Nb + V Conventional (µm) 36,45 28,60 26,80

Termomechanical (µm) 44,50 32,53 35,38

20MnCr5 Nb

20MnCr5 Nb + V

20MnCr5

Equivalent Diameter

Mechanical properties

• Increased yield strenght for Microalloyed Steels • Better results were obtained for the 20MnCr5+Nb alloy, conventional lamination, cooled to the calm air. • Lower toughness in the alloy 20MnCr5 + Nb + V

20MnCr5 20MnCr5 + Nb 20MnCr5 + Nb + V

• Austenitic grain elongation occurred for all the materials processed by thermomechanical rolling. A narrower morphological distribution was also observed for micro-alloyed steels than for base steel;

• The equivalent diameters for the 20MnCr5+Nb and 20MnCr5+Nb+V alloys are lower when compared to the base alloy, even in the material processed by conventional rolling.

• There was a slight increase in austenitic grain size when we compared the thermo-mechanical process with the conventional process.

• There was an increase in the yield strength of the samples with the addition of microligant elements (Nb and V) for the same mechanical processing.

• The best results were observed for the Nb microliged alloy, with thermomechanical rolling, cooled by calm air.

• Toughness decreased with the addition of vanadium • Although the tests were carried out on a pilot scale, with limitations on the amount of total

deformation used, it was possible to notice that the thermocontrolled lamination process has significant effects on the morphology of the austenitic grain delivered after rolling. There was a significant increase in the degree of grain elongation, which restricts the growth of products resulting from the decomposition of austenite during the cooling, thus refining the final microstructure.

Conclusions