FIELD OF THE INVENTION
The present invention relates to a low-carbon bainitic steel grade with high yield
strength and high yield ratio for application in seismic-proof buildings. The
invention further relates to a process of producing a low-carbon bainitic steel
grade with high yield strength and high yield ratio.
BACKGROUND OF THE INVENTION
Steels with high yield point, close to the ultimate tensile strength, capable to
withstand strong earthquake, are known to be used in construction of modern
buildings. Conventional design of seismic-resistant buildings adopts material
capable of undergoing sufficient plastic deformation thereby resisting the
structure from collapse. To the contrary, new design of these modern buildings
uses steels that would deform only in the elastic regime of the steel, and only
when a very high load will be sustained. Accordingly, any permanent damage of
the structure is avoided, and damage if any, can be restored very easily following
any occurrence of earthquake. Accordingly, steels for this application are
required to possess very high yield strength and high yield ratio.
Yield strength of steels increase dramatically with the carbon content. A large
increase in carbon content on the other hand, is known to impair weldability of
the material leading to cracks after fabrication of the structure. Carbon content

of the material is therefore required to be maintained at a minimum level in
order to assure easy fabrication. In order to achieve high-strength in the steels,
it could be difficult to maintain a small amount of carbon in the steel. The
present inventors recognized that this task can be fulfilled by generating a
bainitic microstructure instead of a polygonal ferrite microstructure that
constitutes most of the low-carbon steels.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a low-carbon bainitic steel
grade with high yield strength and high yield ratio for application in seismic-proof
buildings.
Another object of the invention is to propose a process of producing a low-
carbon bainitic steel grade with high yield strength and high yield ratio.
SUMMARY OF THE INVENTION
Accordingly, there is provided in a first aspect a low carbon bainitic steel grade
with high yield strength and high yield ratio for application in seismic-proof
buildings, characterized in that the chemical composition of the starting material

in wt% is c ≤ 0.05, Mn ≤ 3, S ≤ 0.02, P ≤ 0.02, Si ≤ 0.02, Al ≤ 0.01,
Mo ≤. 0.06, Ti ≤ 0.1, Nb ≤ 0.1, and B ≤ 0.006, and in that mechanical
properties of the produced steel comprise yield strength ≥. 670 MPa, ultimate
tensile strength ≥. 710 MPa, elongation ≥ 15%, and yield ratio ≥. 0.9.
In a second aspect, the invention provides a process for producing a low-carbon
bainitic steel grade with high yield strength and high yield ratio for application in
seismic-proof buildings, the process comprising the steps of:
producing an alloy composition containing in wt% c ≤ 0.05, Mn ≤ 3,
S ≤ 0.02, P ≤ 0.02, Si ≤ 0.02, c ≤ 0.05, Al ≤ 0.01, Mo ≤ 0.6, Ti ≤ 0.1,
Nb ≤ 0.1, and B ≤ 0.006 in an air-melting furnace, wherein the bath is
killed with aluminium and titanium and niobium being mixed in order to
retain boron in the solution;
- casting the composition to produce an ingot followed by homogenization
and forging at a temperature around 1200°C;
- heat treatment of the forged sample in a muffle furnace by heating to
about 960°C, and soaking for about 15 minutes; and
- cooling the heat-treated material in air or coil.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS AND
TABLES
Figure 1 - shows a microstructure of the new steel grade after cooling according
to the invention.
Table 1 - shows mechanical properties of the inventive steel grade after heat-
treatment.
Table 2 - shows chemical composition of the steel according to the invention.
DETAIL DESCRIPTION OF THE INVENTION
Steel making and heat treatment:
According to the invention, chemical composition of the steel is designed such
that a bainitic microstructure can be obtained from an austenite phase after
continuous-cooling from high temperatures. Secondly, the critical temperature at
which the austenite phase starts transforming into bainite phase is kept low in
order to generate a fine scale of the microstructure. The steel is therefore made
with low carbon content with an optimum addition of manganese, molybdenum
and boron.

Thus alloy with optimum composition is produced using an air-melting furnace.
The bath is killed with aluminium and the alloy is made with small amounts of
titanium and niobium, in order to retain boron in the solution. The cast ingot is
then homogenized and forged at a temperature of 1200°C.
Rectangular blocks were machined from a forged sample for heat-treatment
using a muffle furnace. The blocks were heated to 960 °C, soaked for 15 minutes
and then cooled either in air or oil. The heat-treated material was then used to
carry out the mechanical tests and metallographic experiments.
Metallography and mechanical tests:
Samples were prepared from the heat-treated material using standard
metallographic techniques and etched with 2% natal. These were then visually
inspected in an optical microscope and also in a field emission gun scanning
electron microscope (FEGSEM) in order to reveal the microstructure of the
material.
The heat-treated material was used to prepare tensile test specimens of 5 mm
diameter and 25 mm gage length. The tensile test was carried out at room
temperature using a strain rate of 3.33xl0-6 S-1.

Experimental results:
Mechanical properties of the material after heat-treatment are presented in Table
1.
The new steel with optimum chemistry recorded high yield strength as well as
high yield ratio, along with substantial value of ductility.
Microstructure of the material is presented in Figure 1 is found to be essentially
similar to bainite.
Table 1: Mechanical properties of the new steel after heat treatment


Material specification in heat-treated condition:
(i) Chemistry of the steel specified is given in Table 2
(ii) Mechanical properties of the product specified are as follows:
YS ≥ 670 MPar UTS ≥ 710 MPa, %EI ≥ 15, YR ≥ 0.9
Table 2: Chemical composition of the steel specified in wt%

Process route:
(i) Steel making
(ii) Ingot casting
(iii) Forging of the cast ingot
(iv) Heat treatment of the forged material

Advantages of the invention:
(1) Optimum chemistry of the steel resulted in high yield strength and
high yield ratio of the material after heat treatment.
(2) Judicious selection of alloying elements enables generation of the
bainitic microstructure in a fine scale during continuous-cooling
from high temperature.
(3) The fine scale and the constituents of the microstructure were
Responsible for the high yield point and yield ratio of the steel.

WE CLAIM:
1. A low-carbon tainitic steel grade with high yield strength and high yield
ratio for application in seismic-proof buildings, characterized in that the
chemical compostion of the starting material in wt% is c ≤ 0.05,
Mn ≤ 3, S ≤ 0.02, P ≤ 0.02, Si ≤ 0.02, Al ≤ 0.01, Mo ≤ 0.06, Ti ≤0.1,
Nb ≤. 0.1, and B ≤ 0.06, and in that mechanical properties of the
produced steel comprise yield strength ≥ 670 MPa, ultimate tensile
strength ≥ 710 MPa, elongation ≥ 15%, and yield ratio ≥ 0.9.
2. The steel grade as claimed in claim 1, wherein after heat-treatment
reduction of area is between 60 to 70%.
3. A process for producing a low-carbon bainitic steel grade with high yield
strength and high yield ratio for application in seismic-proof buildings, the
process comprising the steps of:

- producing an alloy composition containing in wt%
c ≤ 0.05, Mn ≤ 3, S ≤ 0.02, P ≤ 0.02, Si ≤ 0.02, Al ≤ 0.01, Mo ≤ 0.06,
Ti ≤ 0.1, Nb ≤ 0.1, and B ≤ 0.06 in an air-melting furnace, wherein the
bath is killed with aluminium and titanium and niobium being mixed in
order to retain boron in the solution;
- casting the composition to produce an ingot followed by homogenization
and forging at a temperature around 1200°C;
- heat treatment of the forged sample in a muffle furnace by heating to
about 960°C, and soaking for about 15 minutes; and
- cooling the heat-treated material in air or coil.

ABSTRACT
The invention relates to a low-carbon bainitic steel grade with high yield strength and high yield ratio for application in seismic-proof buildings. The chemical composition of the starting material in wt% is c ≤ 0.05, Mn ≤ 3, S ≤ 0.02, P ≤ 0.02, Si ≤ 0.02, Al ≤ 0.01, Mo ≤ 0.06, Ti ≤ 0.01, Nb ≤ 0.01, and B ≤ 0.006, and in that mechanical properties of the produced steel comprise yield strength ≥ 670 MPa, ultimate tensile strength ≥ 710 MPa, elongation ≥ 15%, and yield ratio ≥ 0.9.