BORON ALLOYED LOW CARBON STEEL SUITABLE FOR HOT FORMING / HOT
STAMPING / QUENCHING PROCESS AND PROCESS OF MANUFACTURING THE
SAME
FIELD OF INVENTION
This invention relates to boron-containing low carbon alloyed hot and cold rolled steel
suitable for hot forming / hot stamping / quenching process. More particularly the
present invention relates to such an alloy and process of manufacturing the same
which results in a unique combination of ultra high strength (as high as 1500 MPa)
and tensile ductility suited for making crash resistant structural members of
automobiles and agricultural implements after high temperature processing.
BACKGROUND ART
The challenge in the automobile industry is to build 'Low cost & safe vehicles'. Three
factors viz. fuel cost, dwindling resources and exponentially increasing traffic density
has brought about such change in thinking of automotive designers, manufacturers
and end-users. The first two concerns can be addressed by making the vehicle light
weight. The third, namely to cope with increased traffic density and thereby avoidance
of collision possibility necessitates the automobile must be equipped with provision of
passive safety. Foremost among these is the resistance to impact during collision.
Needless to state, that the car bodies will require steels with ultra high strength and
the industries have elicited strong interest in crash resistant car components.
Conventionally for fabrication of these components, advanced high Strength steel
(AHSS) steel such as Transformation Induced Plasticity (TRIP), Multiphase steels
have been used for crash resistant automotive components. It suffers from lowering of
forming properties and spring back phenomenon. To overcome these limitations and
also to enhance tensile strength level to as high as 1500 MPa, innovative processes
have been developed like hot stamping, hot forming etc, where forming is done at high
temperature followed by controlled cooling.

In the present innovation, an innovative alloy has been designed with boron suited for
high temperature forming. Strong affinity towards nitrogen in steel, whose presence in
inevitable, constrain the addition of boron as its efficacy in free form is greatly
reduced. It readily forms an intermetallic compound, boron nitride (BN)/ boron
carbonitride. Titanium is added to tie down boron for the reason that titanium has
much greater affinity towards nitrogen as compared to that towards boron. This leaves
boron in free state in austenite grains proper or segregate to austenite grain
boundaries above austenite to ferrite transformation (AC3) temperatures.
Against this backdrop, for example, Japanese Patent Application Publication No. 6-
35619 discloses a technique for producing a cold-rolled steel sheet with high
elongation by maintaining a steel containing, in percent by weight, 0.10% to 0.45% of
carbon, 0.5% to 1.8% of silicon, 0.5% to 3.0% of manganese, and 0.01% to 0.07% of
soluble aluminum in the temperature range of 350[deg.] C. to 500[deg.] C. for 1 to 30
minutes after annealing to form 5% to 10% or more of retained [gamma].
In addition, Japanese Patent Application Publication No. 62-40405 discloses a method
for producing a high strength steel sheet combining low yield stress (YP), high
elongation (El), and high bake hardenability (BH) by adjusting the cooling rate, after
annealing, of a steel containing, by weight, 0.005% to 0.15% of carbon, 0.3% to 2.0%
of manganese, and 0.023% to 0.8% of chromium to form a dual-phase structure
composed mainly of ferrite and martensite.
Furthermore, Japanese Patent No. 3969350 discloses a method for producing a high
strength steel sheet having excellent bake hardenability and excellent room-
temperature anti-aging properties by adding 0.02% to 1.5% of molybdenum to a steel
containing, in percent by mass, more than 0.01% to less than 0.03% of carbon, 0.5%
to 2.5% of manganese, and 0.0025% or less of boron and controlling the soluble
aluminum, nitrogen, boron, and manganese contents so as to satisfy sol.AI>=9.7*N
and B>=1.5*10<4>*(Mn<2>+1) to form a microstructure composed of ferrite and a
low-temperature transformed phase.

Also a Japanese Patent No. 4113036 discloses that a steel sheet having excellent
anti-aging properties at room temperature and excellent bake hardenability can be
produced using a steel containing, in percent by mass, 0.2% or less of carbon, 3.0%
or less of manganese, 0.0030% to 0.0180% of nitrogen, 0.5% to 0.9% of chromium,
and 0.020% or less of aluminum by adjusting the ratio of chromium to nitrogen to 25 or
more and the area ratio of ferrite to 80% or more.
Accordingly, it could be helpful to provide a hot and cold rolled steel with moderate
strength that solves the above problem. This steel achieves the required ultra high
strength after high temperature forming and controlled cooling process. Phase
transformation attributes of the steel is exploited to improve strength upon cooling
after deformation through hot stamping / hot forming / quenching process and tailor-
made advanced high strength steel developed.
It is a principal object of this invention to provide an alloy and article made therefrom
having a unique and desirable combination of tensile strength and ductility. The
reduction in spring back due to lath martensitic structure bestows the auto
components with greater geometrical stability along with acceptable dimensional
tolerance. Ultra high strength attained after quenching from austenite provide it with
crash resistance. The steel will have good weldability due to reduced carbon
equivalent. The scale formation can be eliminated by coating layers of aluminum and
zinc which will further result in improvement in yield in the production line.
SUMMARY OF THE INVENTION
To a large extent the foregoing objective as well as additional objectives and
advantages are realized by providing a workable low carbon low alloyed boron
containing steel, as well as a worked article made therefrom in accordance with this
invention, which consists essentially of, in weight percent, about: C-0.25 wt% max,
Mn-1.5 wt%max, P-0.030 wt% max, S-0.025 wt% max, Si-0.40 wt% max, Ti- 0.05 wt%
max, Cr-0.40 wt%max and B - 30ppm max and the balance iron.

Therefore, the principle object of the present invention is to provide boron containing
low carbon low alloyed steel with a critical cooling rate applicable during hot forming /
hot stamping / quenching process. At ambient temperature the steel blank has
predominantly ferritic-pearlitic structure with a moderate tensile strength level of 350-
550 MPa. The blank has been heated above Ac3 temperature in austenite field. The
forming has been carried out by transferring it to tooling die when the steel still has
austenitic structure. The subsequent quenching at the critical cooling rate has resulted
in martensitic structure to attain a ultra high strength. Though, this is accompanied by
decrease in ductility but it has little significance post deformation. Spring back forms
recoverable part of elastic strain in the framework of total strain. This has effect on the
geometrical stability of the formed component as this will result in compromise on
designed tolerance typically on the negative side. As all the deformation is carried out
in the austenite phase and at high temperature, wherein the reduced flow stress is
encountered, this phenomenon is minimized.
As per another object of the present invention, there is disclosed a process technology
for development of crash resistant high strength steel suitable for hot stamping / hot
forming / quenching established.
As per yet another object of the present invention, there is disclosed an alloy
chemistry, austenitising temperature and cooling rate, which are optimised for
achieving desired properties.
As per yet another object of the present invention, suitable alloy chemistry is devised
wherein it is not boron alone but its synergistic effect in presence of carbon,
manganese, chromium has resulted in significant change in hardening behaviour of
steel.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figures illustrate the scanning micrographs from samples cooled at different rates in
accordance with the present invention; wherein:

Fig 1(a) illustrates microstructures from samples cooled at slower rates of 0.5 C/s
which is completely dominated by ferrite and pearlite phase mixture;
Fig 1(b) illustrates microstructures from samples cooled at slower rates of 2 C/s which
is completely dominated by ferrite and pearlite phase mixture as well;
Fig 1(c) illustrates microstructures from samples cooled at rates of 10 C/s which is first
indication of formation of bainite; and
Fig 1(d) illustrates microstructures from samples cooled at rates of 20 C/s wherein the
bainite volume fraction has increased as can be discerned in accordance with the
present invention;
DETAILED DESCRIPTION
Boron has been found to be most effective if added in small quantity from 10ppm to
30ppm. A higher boron content will lead to formation of borocarbide precipitates and
would not support enhancement of hardenability. This is the reason that carbon
content is also kept low. However in case of steels for hot stamping a higher carbon
content is desirable as martensite as final phase is to be obtained post quenching. A
moderate amount of carbon ~0.25pct in steel has been found to be quite adequate. A
higher amount of carbon will result in the formation of plate martensite, which will not
be desirable as it will drastically reduce ductility leading to brittleness. On the other
hand, carbon in modest quantity will promote formation of lath martensite, which
besides imparting high strength, will not concede much on elongation properties,
although there will be significant decrease in ductility. This may be the reason for
avoidance of straightening or any further deformation after hot stamping. The addition
of silicon once again in quantity ~0.25pct will suppress formation of cementite and if
cooled at slower rates, the carbon will precipitate out as graphite. On the other hand
rapid cooling will just assist in the formation of martensite. Chromium is added to
increase resistance to corrosion, though it may increase strength of steel marginally.
Titanium is added to combine with nitrogen so that free boron is available for
increasing hardenability. Oxygen is kept less than 2 ppm at the time of addition of

titanium and boron during steel refining. The chemical composition is specified below.
Composition (in the Description, % Refers to Percent by Mass) and the rest percent by
mass is Iron.

In the disclosed process technology, continuous cast slabs of 200 mm thickness were
processed into hot rolled coils of 2.5 mm thickness which were subsequently cold
rolled to 0.8 mm to 2 mm thicknesses. The base steel material (before subjecting to
hot deformation and controlled cooling) has a ferritic-pearlitic structure microstructure
with a tensile strength of approximately 450MPa
Manufacturing Conditions
Steel having the above composition is made in basic oxygen furnace (BOF), thereafter
the secondary refining is done in ladle furnace (LF). Steel is cast through continuous
casting route and concast slabs are subjected to hot-rolling and cold-rolling in a usual
manner. Cold rolled coils are annealed through batch annealing route.
Hot Rolling
Hot rolling is carried out in a usual manner, for example, at a slab heating temperature
of 1,200[deg.] C. to 1,300[deg.] C, a finish rolling temperature is around 860 - 880
[deg.] C, the cooling rate after hot rolling is preferably 20[deg.] C./sec on run out table
and average cooling rate i.e from last rolling strand to coiling, is preferably 7-10[deg.]
C./sec, and the coiling temperature is preferably 660 - 680[deg.] C. or lower.
Cold Rolling
In cold rolling, the rolling reduction is 50% to 70%.

Annealing
The cold-rolled steel coils are annealed through batch annealing route. The annealing
temperature is 600 [deg.] C/sec is attained in 9-10 hrs , intermediate holding for 13-14
hrs, heating to 680 - 690 [deg,] C, final holding for 14-15 hrs and thereafter cooling.
Boron alloying in steels induces good hardenability at even at low cooling rates. To
find out the critical cooling rate for transformation to martensite, samples were
subjected to dilation test using Gleeble 3500. The dimensional changes in these
specimens during cooling were recorded using an ultra-sensitive dilatometer provided
with the simulator and the output obtained as dilatation plots. Based on the dilation
plots obtained with varying cooling rates, CCT diagram was generated. Critical cooling
rate for martensitic transformation was evolved as 30 C/sec on basis of dilation plot
and microstructural evaluation. Figure 1 shows the scanning micrographs from
samples cooled at different rates as indicated in the figure. The microstructures of
Figure 1 (a) and (b) from samples cooled at slower rates of 0.5 C/s and 2 C/s is
completely dominated by ferrite and pearlite phase mixture. The first indication of
formation of bainite can be perceived in Figure 1 (c). The bainite volume fraction has
increased as can be discerned from Figure 1 (d) obtained from sample cooled at the
rate of 20 C/s.
Samples were tested in thermal simulator by soaking them at 950[deg.] C for 300s.
Samples were then deformed to true strains of 0.22 and 0.69 corresponding to
reduction of 20pct. and 50pct. Three cooling rates viz. 5, 20 and 50C/s were employed
to study effect on mechanical properties and microstructure. It was found that cooling
rates though has a threshold for the formation of martensite phase unbound
accelerated cooling may not yield the desire property combinations. Study was also
carried out to understand the influence of boron on the Hardenability of unalloyed and
low alloyed steel. Increase in strength with increase in cooling rate was found 1.5
times for unalloyed boron added steel whereas it is 2.5 times for low alloyed steel.
Samples were successfully subjected to water quench using critical component B
pillar dummy die at Diede Bilbao, Spain. There is emerging trend to fabricate tube for
side impact / twist beam also subjected to heat treatment after forming. This grade is
very much suitable for high strength agricultural equipments.

> Salient Features of Innovation
• Boron containing low carbon low alloyed steel suitable for hot stamping / hot
forming / quenching process developed.
• It is not boron alone but its synergistic effect in presence of carbon, manganese,
chromium has resulted in significant change in hardening behaviour of steel.
• Alloy chemistry, austenitising temperature and cooling rate are optimised for
achieving desired properties
Although the foregoing description of the present invention has been shown and
described with reference to particular embodiments and applications thereof, it has
been presented for purposes of illustration and description and is not intended to be
exhaustive or to limit the invention to the particular embodiments and applications
disclosed. It will be apparent to those having ordinary skill in the art that a number of
changes, modifications, variations, or alterations to the invention as described herein
may be made, none of which depart from the spirit or scope of the present invention.
The particular embodiments and applications were chosen and described to provide
the best illustration of the principles of the invention and its practical application to
thereby enable one of ordinary skill in the art to utilize the invention in various
embodiments and with various modifications as are suited to the particular use
contemplated. All such changes, modifications, variations, and alterations should
therefore be seen as being within the scope of the present invention as determined by
the appended claims when interpreted in accordance with the breadth to which they
are fairly, legally, and equitably entitled.

CLAIMS:
1. Boron alloyed low carbon steel composition suitable for hot forming / hot stamping /
quenching process, comprises of C-0.25 wt% max, Mn-1.5 wt% max, P-0.030 wt%
max, S-0.025 wt% max, Si-0.40 wt% max, Ti- 0.05 wt% max, Cr-0.40 wt% max and
B - 30ppm max and the balance iron.
2. A process for the manufacture of steel bearing the boron alloyed low carbon steel
composition as claimed in claim 1, the process comprising:
(i) providing the boron containing low carbon low alloyed steel composition in
basic oxygen furnace (BOF);
(ii) providing secondary refining is done in ladle furnace (LF) and continuous
casting to form slab;
(iii) concast slabs as prepared in (ii) are subjected to hot-rolling and cold-rolling;
(iv) thermo mechanical processing / deformation by hot and controlled rolling of
the cast slabs wherein performing hot rolling of the cast slab at a at a slab heating
temperature of 1,200[deg.] C. to 1,300[deg.] C and a finish rolling temperature is
around 860 - 880 [deg.] C;
(v) performing cold rolling at the cooling rate after hot rolling at 20[deg.] C./sec
on run out table and average cooling rate i.e from last rolling strand to coiling, is
preferably 7-10[deg.] C./sec, and the coiling temperature is preferably 660 -
680[deg.] C. or lower; and
(vi) performing annealing through batch annealing route with temperature of 600
[deg.] C/sec, attained in 9-10 hrs with intermediate holding for 13-14 hrs alongwith
heating to 680 - 690 [deg,] C and final holding for 14-15 hrs and thereafter cooling
is carried out.
3. A process for the manufacture of steel bearing the boron containing low carbon low
alloyed steel composition as claimed in claim 2, wherein, hot rolling is carried out by
heating the slab preferably at temperature of 1250[deg.] C and the finish rolling
temperature is around 860 - 880 [deg.] C.

4. A process for the manufacture of steel bearing the boron containing low carbon low
alloyed steel composition as claimed in claim 2, wherein, under cold rolling, the
rolling reduction is 50% to 70%.
5. A process for the manufacture of steel bearing the boron containing low carbon low
alloyed steel composition as claimed in claim 2, wherein continuous cast slabs of
200 mm thickness are processed into hot rolled coils of 2.5 mm thickness which
were subsequently cold rolled to 0.8 mm to 2 mm thicknesses.
6. A process for the manufacture of steel bearing the boron containing low carbon low
alloyed steel composition as claimed in claim 2, wherein the steel composition
comprises of C-0.25 wt% max, Mn-1.5 wt% max, P-0.030 wt% max, S-0.025 wt%
max, Si-0.40 wt% max, Ti- 0.05 wt% max, Cr-0.40 wt% max and B - 30ppm max
and the balance iron

ABSTRACT

The present invention relates to Boron alloyed low carbon steel composition suitable
for hot forming / hot stamping / quenching process, comprises of C-0.25 wt% max,
Mn-1.5 wt% max, P-0.030 wt% max, S-0.025 wt% max, Si-0.40 wt% max, Ti- 0.05
wt% max, Cr-0.40 wt% max and B - 30ppm max and the balance iron.