Advances in SPecial Steel Lectures TMCP

(1)TMCP ? Thermo-Mechanical Controlled Processing:

(2) Ultra refinement in ferrite grain size by ultimate utilization of TMCP

WHAT IS TMCP?

In the past, the purpose of hot rolling was only to achieve the nominal dimensions like thickness, width and length.

when the quality requirement was severe, the off-line heat treatment such as normalizing or Quench and Tempering had been added.

But when the quality requirement became severer, the new process for plate rolling had to be developed.

That is TMCP.

TMCP is a microstructural control technique combining controlled rolling and cooling.

• For the TMCP process, the total control during reheating of

slab, plate rolling and cooling after plate rolling is important.

• This technology was developed early in 1980’s and it was

introduced to most of Japanese Plate mills and TMCP steels

have been widely applied to Japanese shipyards.

• According to the exact definition of TMCP,

• TMCP includes TMR (Thermo-Mechanical Rolling) and AcC

(Accelerated Cooling).

TMCPThermo Mechanical Controlled Processing

Controlled Rolling (CR) Accelerated Cooling (AcC)

TMCP

TMCP

The aim of TMCP is to get the fine and uniform microstructure with

fine grains instead of Ferrite/Pearlite banded structure of conventional

steels.

As a result, TMCP steels have higher strength and better toughness.

Fig.3 shows the relationship between tensile strength and Ceq (Carbon

Equivalent). At the same Ceq level, strength of TMCP steels is higher

than those of conventional steels. As shown in Fig.4, toughness is

improved with decrease of the grain size. Therefore, TMCP steels have

the better toughness.

AIM of TMCP

Concept of microstructure change during TMCP

Microstructure of TMCP steel

20μm 5μm

Conventional rolling TMCP

Introduction to Line Pipe Steel

• Line pipe steels are strongly required for pressurized fluid transportation over long distance.

• The innovative pipeline steel API X80, API X100 and API X120 newly developed are considering for the new generation of line pipe steels.

• The technical requirements of these steels are the finest combination of various properties; it includes

i. high strength, ii. low temperature better toughness, iii. yield ratio,iv. high weldability, v. superior H2S corrosion resistance, vi. resistance to Hydrogen Induced Cracking (HIC) and vii.better fatigue behavior etc.

• The prime difficulty in adjusting these properties arises from the fact that, they are inversely linked each other; for example,

• An increase in strength is achieved at the expenses of the low temperature toughness and yield ratio and vice versa.

• Therefore, development of high performance line pipe steels can be produced by careful control of “microstructure design”. In turn, the microstructures are controlled by designing the “fine-tuning of chemical composition” and “processing route”.

Introduction to Line Pipe Steel…Cont

• It is a valid way to achieve excellent mechanical properties of line pipe steel by improving microstructure and refining grain size by subjecting to different rolling conditions.

• The Metallurgical Phenomena occur through TMCP provides different phases such as

i. Polygonal Ferrite (PF),ii. Banitic or Acicular Ferrite (BF or AF), iii. Martensite-Austenite Constituent (MA), iv. Quasi Polygonal Ferrite (QF) or Massive Ferrite (MF), and v. Granular Banitic ferrite (BF). However, there are still disagreements and uncertainties on the

metallographical identification and classification of the phases, for example some time the AF is also considered as banite

Introduction to Line Pipe Steel…Cont

Example-1 : API X70 and X80 Grade

Resultant Microstructure

The A steel rolled in the single phase region consists of acicular ferrite (AF) and

granular bainite (GB), with the presence of a small amount of martensite–

austenite constituent (MA) .

The B steel rolled in the two phase region is mainly composed of polygonal

ferrite (PF) transformed during finish rolling, with the presence of AF and upper

bainite (UB) and a small amount of MA and cementite.

The C steel, X80 steel rolled in the single phase region, consists of AF and UB,

with a small amount of MA. Table 3 summarizes the basic microstructures of the

three steels and the volume fraction of secondary phases, such as MA and

cementite.

• EBSD = Electron Backscattered Diffraction

Thermo-mechanical control processing (TMCP) refers to a multi-

stage deformation schedule, both above and below the non-

recrystallization temperature (Tnr), followed by accelerated

cooling.

Repeated recrystallization above the Tnr produces fine austenite

grains which are subsequently rolled below the Tnr to obtain

pancake shaped austenite grains which can then transform into

very fine ferrite or bainite following the fast cooling.

Example-2

The steel used in this study was laboratory made by hot rolling of a 5 in. thick ingot. The alloy contained 0.056%C, 1.97%Mn,

and 0.41%Mo, microalloyed with Nb + Ti + V (less than 0.13%). Four thermomechanical cycles were designed as shown in Fig. 1.

Above mentioned TMCP Cycle:

Four thermo-mechanical cycles are shown in Fig. 1.

The slab was reheated at 1180 ◦C for 2.5 h and then rolled in two stages; rough rolling and

finish rolling followed by accelerated cooling and then slow cooling to simulate the coiling

process.

An overall grain refinement was expected by rolling above Tnr temperature (Tnr temperature

924 ◦C). Rolling was finished in the austenite region (above Ar3, 698 ◦C). The rough rolling ∼was started at about 1125 ◦C and finished 1010 ◦C. Finish rolling was performed at various ∼temperatures (between about 875 and 700 ◦C) in several passes. The rolled steels were then

cooled to about 450 ◦C at 30 ◦C/s (Ar1 = 368 ◦C). The rolled materials were finally furnace

cooled and had a thickness of 14 mm.

Application of TMCP steels

Historical progress of TMCPC.Ouchi:ISIJ Int., 41,(2001),542.

Concluding Remark

Though metallurgical phenomena such as recovery,

recrystallization, precipitation, and transformation are

individually simple, as described in the textbooks, the infinite

combinations of these phenomena and processing

parameters are believed to further improve the various

properties of advance steel plates.

End of TMCP

What would be happenedby ultra refinement in grain size?

20μm 5μm

Conventional rolling TMCP

1μm

UFG

Advantages and technical problems involved in ultrafine grain steels

Advantage Technical problemIncrease of yield strengthImprovement of toughnessIncrease of fatigue strengthImprovement of corrosion resistance propertyImprovement of grain boundary failure resistance property

Increase of yield ratioDecrease of uniform elongationProperties of weld and HAZHigh temperature propertiesCreation of ultrafine grain in heavy section products

C.Ouchi:CAMP-ISIJ vol ,(1998), .

International projects involved in ultrafine grain steels

・ Japan 1) Ferrous Super Metal Project 2) STX 21・ China ・ Korea HIPERS 21

Univ. of Manchester (U.K.)Univ. of Deakin and BHP (Australia)

Ferrous Super Metal Project

(1) Objective: Ultra refinement of grain size under 1μm, in carbon/low alloy steel(2) Key technology: Large-strain deformation higher than 50% per pass(3) Fund: ＄ 15M for 1997 - 2001 from Japanese government (4) Participants: Nippon Steel, NKK, Kawasaki Steel, Sumitomo Metal Industries, Kobe Steel

Three types of large-strain deformation

Ar1Ar3

Conventional TMCP

Ar1Ar3

TypeⅠ

Ac3Ac1

TypeⅡ

Ar1Ar3

Reverse transfomationafter heavy deformationin α region

Transformation/recrystrallizationafter heavy deformationin (α +γ ) or (α +θ ) region

Transformationafter heavy deformationin extremely under cooledγ region

TypeⅢUltimate utilization of TMCP

Achievement in the Ferrous Super Metal Project

Some interesting metallurgical phenomena caused by heavy deformation has been found out 1) Strain assisted low temperature diffusional transformation 2) Spontaneous reverse transformation due to adiabatic deformation heating

M.Niikura et al: Jour. of Mat. Proc. Tech,117 (2001), 341.

End of Lecture