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XPF steel
A stronger, lighter more formable steel for chassis applications
Background
140
120
DP600
100
800
BS800
AHSS
60
40
Figure 1: A key measure of the formability of steel is hole-expansion capacity (HEC) – how
far the material around a stamped hole will stretch before fracturing. XPF combines the
hole-expansion performance associated with bainitic and martensitic grades, with the
elongation characteristics of dual-phase (DP) and high-strength low-alloy (HSLA) steels
1000
E690TM
HSLA
The XPF family has a single-phase ferrite microstructure with a nanoprecipitate-reinforced matrix. This novel metallurgical concept yields
several benefits over other steel grades typically found in chassis
applications. XPF steels are at least as strong as equivalent high-strength,
low-alloy steels (HSLA), and advanced multi-phase steel grades (AHSS),
but improve on both in terms of hole-expansion capacity (HEC) (Fig: 2(A))
and elongation (Fig:2(B)). In addition they are at least as easy to weld as
HSLA steels, and easier to weld than AHSS. XPF steels provide increased
product homogeneity compared to multi-phase AHSS.
XPF800
CP800
80
60
BS800
40
E690TM
20
20
600
700
800
900
1000
12
1100
14
16
18
20
22
Total elongation (%)
Tensile strength (MPa)
Figure 2:
(A) HEC versus tensile strength for
XPF from laboratory trials
(B) HEC versus total elongation for
XPF800 from laboratory trials
Laboratory trials also demonstrate that as well as providing superior
stretch-flange formability, the ductile single-phase ferrite microstructure
of XPF offers excellent fatigue. Figures 3(A) and 3(B) show fatigue curves
of XPF650 and XPF800 respectively, in comparison with HSLA and AHSS
with similar tensile strength. This comparison clearly shows the superior
fatigue performance delivered by single-phase XPF compared to
multi-phase AHSS. In fact, Figure 3(B) suggests that XPF even provides a
benefit over conventional HR-HSLA with respect to endurance limit.
1200
1200
1000
1000
600
400
10000
XPF650
800
HR-DP600
HR-FB590
100000
1000000
Cycles to failure (Nf)
(A) S-N fatigue curve of XPF650
Product concept
S500MC
XPF800
100
Stress Range (MPa)
Key
• BS: bainitic steel
• MS: martensitic steel
• FB: ferritic-bainitic
• CP: complex phase
• HSLA: high-strength
low-alloy
• DP: dual phase
CPxxx
80
Stress Range (MPa)
XPF grades
FB590
650
500
The new hot-rolled XPF family of steels promises a major breakthrough in
automotive structural materials technology. It addresses the known
challenges of current high-strength steels in terms of forming and
manufacturing by combining the mechanical strength and fatigue
resistance that designers require, with a formability that provides even
greater freedom to reduce vehicle weight without compromising
manufacturing robustness or safety standards.
120
XPFxxx
Hole-expansion capacity (%)
Hole-expansion capacity (%)
Reducing the weight of new cars in order to deliver better fuel economy
is a major focus for automakers worldwide. This focus extends to the
chassis system where engineers are striving for lower weight solutions
capable of meeting stringent safety and environmental requirements. To
meet these goals, engineers need new materials featuring high specific
strength and fatigue resistance, together with the manufacturability that
enables them to innovate.
10000000
XPF800
HR-S700MC
800
CR-CP800
600
400
10000
CR-DP800
100000
1000000
10000000
Cycles to failure (Nf)
(B) S-N fatigue curve of XPF800
Figure 3:
S-N fatigue curves (R = -1) of (A) XPF650 in comparison with hot-rolled FB590 and DP600
and (B) XPF800 in comparison with hot-rolled S700MC and cold-rolled CP800 and DP800
Besides formability and fatigue performance, weld fatigue is crucial for
chassis and suspension applications and mass-saving opportunities. In
order to assess weldability and weld fatigue at an early stage, XPF blanks
from laboratory trials were used for a gas metal arc welding study
(GMAW) with an overlap fillet type of joint configuration. This study found
that both XPF 650 (Fig 4: (A)) and 800 (Fig 4: (B)) grades had better
weldability than HSLA, and showed that material could be welded
without any defects.
35
XPF650
50
S355MC
40
S460MC
30
20
10
0
1000
10000
100000
XPF800
25
20
There is a small associated assembled cost increase for this performance
and manufacturing robustness improvement, but this does not factor in
any additional benefits from potential production yield improvement
that is expected when applying the XPF grades.
15
10
5
0
1000
1000000 10000000
10000
100000
1000000
10000000
Cycles to failure (Nf)
Cycles to failure (Nf)
(A) S-N weld fatigue curve of XPF650
E690TM
30
Maximum Stress / Rm weld (%)
Maximum Stress / Rm weld (%)
60
(B) S-N weld fatigue curve of XPF800
Figure 4: S-N weld fatigue curves (R = 0.1) with lap shear loading configuration based on
GMAW arc welding of (A) XPF650 compared with S355MC and S460MC and (B) XPF800
compared with E690TM. Plotted vertically is the maximum stress divided by the tensile
strength (3xHV) of the weld. Filler wire used: ER70S for XPF650 and S355MC, ER90S for
S460MC, and ER120S for XPF800 and E690TM
Material composition
Tata Steel is currently evaluating two chemistries for the XPF800 grade:
with and without the addition of Molybdenum. The addition of
Molybdenum – referred to as XPF800 (Mo+) – marginally raises the
strength of the product whilst offering a balance towards increased
hole-expansion capacity. The variant without the addition of
Molybdenum – referred to as XPF800 (Mo-) – offers a balance towards
increased total elongation.
TCO benefits for the XPF grades
Tata Steel has developed a process to review and quantify how its steel
grades perform against objectives of weight, cost and performance – the
TCO Scan (total cost of ownership). Using this method, Tata Steel
engineers concluded that XPF grades can yield two significant TCO
benefits:
1. Safer component with increased performance at the same mass:
Replacement of the multiphase grade baselines with XPF grades at
equivalent gauge can offer immediate performance benefits, including a
15% increase in collapse strength and improved manufacturing
robustness at equivalent mass through utilisation of the extra strength of
the XPF grades.
2. Mass reduction at equivalent performance:
Design optimisation through minor changes to the component geometry
helped achieve mass savings without loss in key performance metrics
such as stiffness and collapse strength:
a.XPF650 versus MP590MPa: 17% mass saving
b.XPF800(Mo-) versus MP780MPa: 4% mass saving
c.XPF800(Mo+) versus MP780MPa: 8% mass saving
All XPF grades offer mass saving potential without loss of performance,
achieved through optimising grade attributes against component shape.
The XPF650 and XPF800 (Mo+) offer the best combinations of mass
reduction against lowest TCO, with the potential to offer a cost saving
benefit of 4% for the XPF650 grade, and 3% for the XPF800 (Mo+).
Conclusions
The combination of higher strength and enhanced ductility of the new
XPF grades compared to existing multiphase grades offers two distinct
value propositions to our customers. Firstly, the XPF grades offer
improved in-service performance and manufacturing robustness at a
small cost premium – which does not factor in any additional benefits
from potential production yield improvement. Secondly, the increased
elongation and hole-expansion coefficient can enable design
optimisation to yield mass reduction at equivalent in-service
performance and cost. This new family of products will support our
customers in reducing their TCO, through offering a steel optimised for
the challenges of producing chassis components.
a.XPF650 versus MP590MPa: 15% collapse strength improvement
b.XPF800(Mo-) versus MP780MPa: 8% collapse strength improvement
c.XPF800(Mo+) versus MP780MPa: 18% collapse strength improvement
www.tatasteeleurope.com
Tata Steel
Automotive
PO Box 10000
1970 CA IJmuiden
The Netherlands
[email protected]
www.tatasteelautomotive.com
CT0515:AM:750:ENG:0614
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contained in this brochure is accurate, neither Tata Steel Europe
Limited nor its subsidiaries accept responsibility or liability for
errors or information which is found to be misleading.
Tata Steel Europe Limited is registered under number 05957565
with registered office at 30 Millbank, London, SW1P 4WY.
Copyright 2014
Tata Steel Europe Limited