This paper presents the results of industrial scale researches on the influence of the chemical composition on the characteristics of high yield point plates of wieldable fine grain steels. The comparative study of the mechanical characteristics obtained by the classical process and by the controlled rolling process point out the possibility of replacing the classical heat treatment for some steel grades, for plates used metallic structures in railway bridges, high tornage travelling cranes, marine platform bases, high pressure vessels, etc

Ana DONIGA

Elisabeta VASILESCU

Ioan VASILESCU

Marian NEACSU

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

Directory of Open Access Journals

FO N D ATĂ
197 6
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
 
 
STUDY ON THE INFLUENCE OF THE CHEMICAL  
COMPOSITION IN THE THERMOMECHANICAL PROCESSING  
OF THE STEEL PRODUCTS 
 
Elisabeta VASILESCU, Ioan VASILESCU, 
Ana DONIGA, Marian NEACSU 
Faculty of Metallurgy and Materials Science 
“Dunărea de Jos” University of Galati 
email: elisabeta.vasilescu@yahoo.com, uscaeni@yahoo.com  
 
ABSTRACT 
 
This paper presents the results of industrial scale researches on the influence of 
the chemical composition on the characteristics of high yield point plates of 
wieldable fine grain steels. The comparative study of the mechanical characteristics 
obtained by the classical process and by the controlled rolling process point out the 
possibility of replacing the classical heat treatment for some steel grades, for plates 
used metallic structures in railway bridges, high tornage travelling cranes, marine 
platform bases, high pressure vessels, etc. 
 
KEY WORDS: microalloyed steel, heavy plates, thermomechanical treatment, 
controlled rolling, chemical composition, mechanical characteristics 
 
1. Introduction 
 
The growing demand of plates characteristics 
for the manufacture of components for the chemical, 
oil and naval industries requires sessions 
consideration on entering new steel grades and new 
preparation technologies able to improve the structure 
and mechanical characteristics of the plates, without 
considerably affecting the factory prices. The 
development of microalloyed steels, including their 
alloy design, processing, and application, covers the 
last four decades. During this period, microalloyed, 
high-strength, low-alloy steels became an 
indispensable class of structural steels. Their ability to 
achieve final engineering properties is as hot- rolled 
conditions eliminated the need for heat treatments, 
such as normalizing [1]. Thermomechanical rolling 
followed by accelerated cooling is the standard 
production route for steel with high strength and good 
toughness. This processing route relies on elements 
microalloying which promotes a finer grain size 
austenite conditioning. By combining grain 
refinement and precipitation hardening of elements, 
microalloying maximizes the strengthening process. 
Precipitation hardening increases strength but may 
contribute to brittleness. Grain refinement increases 
strength also improves toughness. As a result, grain 
refinement counteracts any embrittling caused by 
precipitation hardening. In practice, grain refinement 
can be achieved during hot rolling by the interaction 
between microalloying elements (niobium, vanadium, 
or titanium) and hot deformation [2]. Maximum grain 
refinement can be achieved by increasing the 
austenite grain-boundary area. This can be 
accomplished by either producing fine grains of 
austenite through repeated recrystallization between 
passes or by flattening non recrystallization austenite 
grain into pancakes. Grain refinement may be further 
enhanced by accelerating cooling after the completion 
of hot rolling [3].  
 
 
 
 
 
 
 
 
 
 
Fig.1. Increase yield strength in vanadium steels 
in terms of Nitrogen content 
 
The undercooling of austenite enhances the rate 
of ferrite nucleation and slows down the rate of 
growth. A combination of these two factors 
-  160  -
FO N D ATĂ
197 6
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
contributes to the formation of smaller grains. 
Significant strengthening is obtained by the 
precipitation of microalloying elements appearing as 
carbonitrides (or carbides) in ferrite (fig. 1) [4]. 
Since their solubility in ferrite is much less than 
in austenite, there is strong supersaturation which 
provides the driving force for precipitation. The most 
desirable are those microalloys, which contribute to 
both grain refinement and precipitation hardening. 
The combined effect of these two strengthening 
mechanisms may provide as much as 70% of the yield 
strength, accounting for the remarkable cost 
effectiveness of microalloyed steels (fig.2). Because 
these two dominant strengthening mechanisms 
operate in microalloyed steels, their carbon (or CE) 
may be very low (only 0.04 to 0.06).This low carbon 
content contributes to excellent weldability [5]. 
 
 
Fig.2 Strengthening mechanisms in 
hot-rolled steels 
2. Experimental conditions 
 
For experiments, test specimen of X70 steel 
grade has been used having chemical composition and 
mechanical characteristics mentioned in table no.1  
The main requirements for steels used for tubes 
and pipe-lines for gas and oil transportation are 
regulated by API 5L norm that establishes limits for 
the chemical composition as well as the minimum 
values of strength and tenacity (table no.1 presents 
only PSL2 trades). 
 
a) for each reduction of 0.01 % under the 
maximum C content specified, a 0.05% increase of 
specified Mn content is allowed: 
 
- up to 1.5% for X42 to X52 trades  
-up to 1.65% for the trades superior to X52 but 
under X70 and 
-up to 2.00% for X70 and above. 
 
b) Nb+V cannot be higher than 0.03% 
c) Nb,Vor any of their combinations is up to the 
manufacturer (tab.1). 
d) Nb+V+Ti cannot be higher than 0.15% 
(tab.1) 
e) Nb+V can’t be higher than 0.06% (tab.1) 
 
 
Table 1 Chemical composition and mechanical characteristics 
Chemical composition Rc Rm 
[%] max. [N/mm2] Grade Steel C Mn P S Ti 
Obs 
min. max. min. max. 
B  0.22 1.20 0.025 0.015 0.040 d, e 241 448 414 758 
X42  0.22 1.30 0.025 0.015 0.040 c, d 290 496 414 758 
X46  0.22 1.40 0.025 0.015 0.040 c, d 317 524 434 758 
X52  0.22 1.40 0.025 0.015 0.040 c, d 359 531 455 758 
X56  0.22 1.40 0.025 0.015 0.040 c, d 386 544 490 758 
X60  0.22 1.40 0.025 0.015 0.040 c, d 414 565 517 758 
X65  0.22 1.45 0.025 0.015 0.040 c, d 448 600 531 758 
X70  0.22 1.65 0.025 0.015 0.040 c, d 483 621 565 758 
X80  0.22 1.85 0.025 0.015 0.040 c, d 552 690 621 827 
 
 
The main trend in achieving longitudinally 
welded large diameter pipes was increasing thickness 
of the wall which led to the use of steels with low 
carbon equivalent (under 0.40) with good weldability. 
The main technological improvements of the 
plate with this destination are termomecanical rolling 
controlled process (TMCP) and accelerated cooling 
(ACC). According to ASTM A 841 / A 841M-01, the 
concept of termomecanical rolling with controlled 
process (TMCP) has developed from the older 
concept of controlled rolling (CR).[6] Subject to these 
processes are TMCP steel products with very fine 
grains in an optimal combination between the 
chemical composition and the integrated control of 
the process of heating and until the final cooling after 
finishing rolling. TMCP incorporates three processes 
(fig.3) [7]. 
-  161  -
FO N D ATĂ
197 6
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
 
Fig.3. Schematic diagram of TMCP and conventional rolling (ASTM 841 / A 841M-01) 
 
a) Termomechanical Rolling (TMR) 
Steels with fine grain are produced by rolling in 
the recrystallized and non-recrystallyzed austenitic 
domains and sometimes in the biphazic 
transformation A→F. Final reductions in general are 
very close or even below the Ar3 temperature. 
 
 
 
Fig.4. Dependence of flow limit on the cooling 
intensity and the level of steel microalloying. 
 
 
Steels guaranteeing superior strength and 
tenacity have a very fine grain structure and an 
optimal structures (F + P, F + P + B, acicular ferrite + 
B, F + M + B) obtained by accelerated cooling with 
water immediately after finishing controlled rolling 
(CR) or termomechanical rolling (TMR). 
c) Direct Quenching and Tempering (DQT) 
If for the quality of X65 or X70, we have an API 
limit flow of 448 N/mm2 and 483 N/mm2, for 
moderate content of carbon equivalent, on the 
conditions of a TM process followed by cooling in air 
to produce safe quality X80 with a limit of minimum 
flow of 552 N/mm2, accelerated cooling is used, 
despite a higher content of carbon equivalent (fig. 4) 
[8]. 
In experiments we have used the mark of X70 
steel with two microalloying variants (fig.5): NbVTi 
(classic), NbCrTi (steel HTP), the adaptation of 
rolling scheme and use of accelerated cooling, as 
follows: 
-  small power 2.5 x final thickness (steel a.) 6-8 
passes in finisher 
 - high power 3.5 x final thickness (steel b.) 9 -
11 passages in finisher 
 - final rolling temperature (ERT: Ar3 + 20-40 ° 
C and the following the heat treatment in ACC 
(accelerated cooling facility) 
- input temperature  ACC: Ar3 + 10 ÷ 20 ° C 
 - temperature drop in ACC: 80 ÷ 160 ° C  
- cooling rate: 6 ÷ 12 ° C / sec. 
 
In fig 6, 7, 8, Rm, Rc and KV variation is 
presented according to Cech., sheet thickness and 
cooling water flow. 
 
 
-  162  -
FO N D ATĂ
197 6
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.5. Rc and Rm distribution in groups of thickness, cooling intensity and type of steel 
 
 
 
 
Fig.6. Distribution of yield strength for groups of thicknesses,  
Cech and ACC flow rates [weighted average] 
 
 
 
Te
ns
ile
 st
re
ng
th
 [M
Pa
] 
Y
ie
ld
 st
re
ng
th
 [M
Pa
] 
Flow ACC 
Thickness 
Y
ie
ld
 st
re
ng
th
 [M
Pa
] 
     Cechiv 
 Thickness 
 Flow ACC
-  163  -
FO N D ATĂ
197 6
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
 
 
Fig.7. Distribution Rm limit for groups of thicknesses, carbon equivalent  
and ACC water flow [weighted average] 
 
Obs. It highlights the cumulative influences of 
carbon equivalent, the cooling intensity and thickness of 
the final level of resistance (Rc and Rm).  
The condition still requirs a minimum intensity of 
cooling water flow. 
Obs. Generally the level of tenacity in terms of 
fracture energy KV is excellent, with values above the 
minimum level (about 3 times and even  more). 
 
Fig.8. KV Energy distribution for groups of thicknesses, carbon equivalent 
 and ACC water flow [weighted average] 
 
4. Conclusions 
 
A statistical overview for the 2 families of steels 
for trade X70 is as follows:  
1. Steel matrix NbCrTi called HTP (High 
Temperature steel) presents a lower cost than those 
NbVTi and it is feasible to produce heavy plates X70 
mark to with max. 25.4 mm thickness when using a 
minimum of 2400 m3 of water in the ACC. 
2. To get a good tenacity and mechanical 
strength depends on the following factors:  
- carbon equivalent  
Te
ns
ile
 st
re
ng
th
 
         Cechiv 
    Thickness Flow ACC 
-  164  -
FO N D ATĂ
197 6
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 1 – 2009, ISSN 1453 – 083X 
 
 
- Temperature at the end of rolling,  
- fall in temperature in ACC, 
 - water flow rate and coolig rate in the ACC 
board. 
3. To increase the thickness of the sheet is 
necessary the correlation with the content of carbon 
equivalent. 
References 
 
[1]. Cuida, O - Influenţa Elementelor Reziduale si de microaliere 
asupra plasticităţii oţelunlor, Ed. IMAGE, Bucureşti - 2000 
[2]. Goth, W - Alloying Effects Of Molybdenum in Steels, Irons and 
Alloys , Ntern. Molybd. Encycl., Vol III- 1980  
[3]. Pickering, F. - Microalloying '75, Int. Symposium On HSLA 
Steels Union Carbide, Corp. New York, Nr.9 - 1975 
[4]. Sanno, T., Tsamura, T. - Effect of Nb Contents and Cold 
Working on Grain Coarsenir-of High Strength Steel, The 
Sumimoto Search, Nr.42 – 1990 
[5]. *** ISIJ Int. Vol.33 - 1993 
[6]. Niobium - Science And Technology, Proceedings of the Int. 
Symposium Niobium, Florida, USA – 2001 
[7]. *** Prezent si perspectivă în fabricarea tablelor groase şi 
conducte de diametru mare, sudate longitudinal, Sinteză Docum. 
ICEM 1994 
[8]. *** Specificatie pentru conducte – Specificatie API 5L, editia 
43, 2004. 
 
-  165  -


Study on the Influence of the Chemical Composition in the Thermomechanical Processing of the Steel Products

https://www.gup.ugal.ro/ugaljournals/index.php/mms/article/download/3088/2770

Study on the Influence of the Chemical Composition in the Thermomechanical Processing of the Steel Products

Abstract

Similar works

Full text

Available Versions

Directory of Open Access Journals