Cold work embrittlement of IF steel

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EURO PEAN COMMISSION SCIENCE RESEARCH DEVELOPMENT technical steel research v'::;:-;.--::..-\::-V-:'i,-;.;:::ïï-Mechanical working (rolling) Cold work embrittlement of IF steel h Report EUR 18642 EN STEEL RESEARCH EUROPEAN COMMISSION Edith CRESSON, Member of the Commission responsible for research, innovation, education, training and youth M DG XII/C.2 — RTD actions: Industrial and materials technologies — Materials and steel W f Contact: Mr H. J.-L. Martin Address: European Commission, rue de la Loi 200 (MO 75 1/10), B-1049 Brussels — Tel. (32-2) 29-53453; fax (32-2) 29-65987 European Commission technical steel research Mechanical working (rolling) Cold work embrittlement of IF steel M. P. Sidey British Steel pic Welsh Technology Centre Port Talbot West Glamorgan SA13 2NG United Kingdom J. Neutjens CRM Rue Ernest Solvay, 11 B-4000 Liège Contract No 7210-EC/806/208 1 October 1993 to 30 September 1996 Final report Directorate-General Science, Research and Development 1998 EUR 18642 EN LEGAL NOTICE Neither the European Commission nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information. A great deal of additional information on the European Union is available on the Internet. It can be accessed through the Europa server (http://europa.eu.int). Cataloguing data can be found at the end of this publication.

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EURO PEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
technical steel research
v'::;:-;.--::..-\::-V-:'i,-;.;:::ïï-
Mechanical working (rolling)
Cold work
embrittlement
of IF steel
h
Report
EUR 18642 EN STEEL RESEARCH EUROPEAN COMMISSION
Edith CRESSON, Member of the Commission
responsible for research, innovation, education, training and youth
M
DG XII/C.2 — RTD actions: Industrial and materials technologies —
Materials and steel
W f
Contact: Mr H. J.-L. Martin
Address: European Commission, rue de la Loi 200 (MO 75 1/10),
B-1049 Brussels — Tel. (32-2) 29-53453; fax (32-2) 29-65987 European Commission
technical steel research
Mechanical working (rolling)
Cold work embrittlement of IF steel
M. P. Sidey
British Steel pic
Welsh Technology Centre
Port Talbot
West Glamorgan SA13 2NG
United Kingdom
J. Neutjens
CRM
Rue Ernest Solvay, 11
B-4000 Liège
Contract No 7210-EC/806/208
1 October 1993 to 30 September 1996
Final report
Directorate-General
Science, Research and Development
1998 EUR 18642 EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information.
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int).
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Publications of the European Communities, 1998
ISBN 92-828-5150-8
© European Communities, 1998
Reproduction is authorised provided the source is acknowledged.
Printed in Luxembourg
PRINTED ON WHITE CHLORINE-FREE PAPER SUMMARY
COLD WORK EMBRITTLEMENT OF IF STEEL
British Steel pic
ECSC Agreement No. 7210.EC/806
Final Summary Report
The aim of the present work was to study in detail the effects of chemistry and processing of
IF steels on the susceptibility to cold work embrittlement, in both coated and non-coated IF
steels, over a range of strength levels. The work was intended to identify the conditions that
should be employed to control the phenomenon at an acceptably low level.
The work has been carried out jointly by Centre de Recherches Métallurgiques (CRM) in
Belgium and the Welsh Technology Centre of British Steel pic. The former concentrated its
efforts on Ti/Nb IF steels, using laboratory facilities to produce and process the experimental
steels. British Steel produced the Ti IF steels using commercial facilities to manufacture and
process the experimental steels in the form of 1 tonne ingots.
The effects of additions of manganese, phosphorus, silicon and boron, as well as variations
in sulphur content, have been covered in these studies.
The work has confirmed that there is competition between Ρ and C or Β to locate at grain
boundaries. When Ρ migrates to the grain boundaries, it lowers their strength and hence
causes embrittlement, giving a poor performance in cold work embrittlement tests. The
presence of Ρ on the grain boundaries has been confirmed by Auger Spectroscopy of
susceptible steels that have been fractured "in situ" in the microscope. The potential for Ρ
to segregate to grain boundaries increases with increasing Ρ content as would be expected.
Where C is available, for example, in incompletely stabilised IF steels or in ULC steels, then
it segregates to grain boundaries in preference to P, thereby strengthening them and
reducing susceptibility to CWE. When present, Β produces a similar effect to C.
Cont'd
Cover Pages: 1 Signed by: M.P. Sidey
Text Pages:83Approved by: Mr. J.G. Simpson
Figure Pages: 48 Manager, Product Technology Group
AppendicesPages:However, the beneficial effects of C and Β are only operative for short annealing treatments,
such as continuous annealing. Where extended heat treatments such as batch annealing
are involved, then there is time for Ρ to segregate to grain boundaries, thereby displacing or
overriding the beneficial effects of C and/or B.
Although it has been demonstrated that it is possible to produce solute carbon in stabilised
IF steels by lowering the coiling temperature, the potential benefit of this free carbon is lost
by reprecipitation during the subsequent annealing treatment, including continuous
annealing.
The use of a high annealing temperature has also been demonstrated as a means of
generating solute C, through dissolution of carbide precipitates, but only if followed by rapid
cooling to prevent reprecipitation. Such a process might be feasible on a continuous
annealing line but is unlikely to be practicable on a galvanising/galvannealing line.
Another way of achieving solute carbon in the fully processed steel is to use incompletely
stabilised IF steels. Theen is sufficient to protect against embrittlement for
conventional (i.e. low) phosphorus contents, but is not so effective at high phosphorus
levels, after continuous annealing treatments. However, solute carbon contents have an
adverse effect on rm. Unstabilised IF steels also have bake hardening potential.
The addition of Β has been shown to be effective in providing protection against
embrittlement in stabilised IF steels, particularly at conventional, low phosphorus levels,
after continuous annealing. Boron also appears to be effective against high phosphorus
contents after continuous annealing, except where a higher strength is achieved by
combined additions of manganese and silicon.
Charpy V-notch impact tests on the hot rolled steels have shown that phosphorus increases
the transition temperature whereas the presence of free carbon lowers the transition
temperature. This result is analogous to the cold work embrittlement phenomenon in the
cold rolled and annealed condition.
Evidence for the precipitation of phosphides i.e. Fe (Ti, Nb) Ρ has been found in both the hot
rolled and cold rolled and continuously annealed, high phosphorus Ti/Nb IF steels. Such
precipitates were found at grain boundaries but it is unclear whether they play any direct role
in the fracture process.
Both studies indicate that high manganese contents caused a deterioration in the cold work
embrittlement performance, although to a lesser degree than phosphorus. The embrittling
effect of manganese is believed to be due to the higher matrix strength of these steels.
The strengthening effects of Ρ Μη & Si have been derived in both studies. Interestingly,
there are some differences: for example CRM report that 1 % Mn increases tensile strength
by approx. 55MPa whereas BS found the average increase was approx. 30 MPa. CRM
report a virtual doubling of the strengthening effect of 0.1% P, on tensile strength between
the hot and cold rolled conditions (67 and 125 MPa respectively). BS on the other hand
found the strengthening effect was reasonably constant for a variety of processing
conditions (86 MPa on average). However both studies agreed that the strengthening effect
of 0.1% Si on tensile strength was 15 MPa.
In the absence of a defined standard for conducting the test for assessing susceptibility to
secondary cold work embrittlement, it is not surprising that the transition temperatures determined by CRM and BS differed for the same steels. The comparison of the
procedures revealed that an important factor in the differences was whether or not the cups
are trimmed to remove ears after forming. There were also important differences in the cup
forming operation, particularly with respect to the die profile radius and the punch/die
clearance. These factors can influence the compressive hoop stresses generated within the
cup wall, although the surface strains do not show great differences with respect to the
surface strains found for the same nominal drawing ratio. Thus no attempt should be made
to compare transition temperatures determined by different investigators in the absence of
an agreed standard procedure. CONTENTS
1. INTRODUCTION 17
2. WORK AT CRM8
2.1 Description of activities
2.2 Results and discussion 23
2.3 Conclusions 4
3. WORK AT BRITISH STEEL6
3.1 Description of activities
3.2 Results and discussion9
3.3 Conclusions 5
REFERENCES 60
TABLES3
FIGURES 101