Process for preparing strips or sheets of high strength austenitic steel having improved fatigue strength

In the conventional process of manufacturing sheets of metastable austenitic stainless steel, a mechanical polishing step is included before or after the final temper rolling step. By insertion of this step, the fatigue resistance of the produced steel strips and sheets is remarkably improved.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to a process for preparing strips or sheets of high 
strength metastable austenitic stainless steel such as AISI Type 301 (SUS 
301 in JIS), AISI Type 201 (SUS 201 in JIS), etc. having improved fatigue 
strength, in which the strength of the materials is improved by temper 
rolling and the characteristic surface luster of the cold-rolled sheets is 
not impaired. 
BACKGROUND OF THE INVENTION 
Conventionally, plain carbon steels or low-alloy steels are used in the 
manufacturing of railroad vehicles. With these materials, in order to 
secure the strength required of materials for railroad vehicles, 
considerably thick sheets are used, which makes the weight of the vehicles 
heavy. Also vehicles made of these materials require painting for the 
purpose of protection against corrosion. Thus, much labor is required and 
much expense is incurred for periodical painting. 
Nowadays, however, temper-rolled sheets of high strength metastable 
austenitic stainless steels such as AISI Type 301 (SUS 301 in JIS), AISI 
Type 201 (SUS 201 in JIS), etc. are being used more and more for 
manufacturing railroad vehicles. Sheets of these steels have excellent 
anti-corrosion property and acquire high strength by cold rolling. By 
employment of these materials, railroad vehicles are made lighter and 
require less maintenance cost with a consequent great benefit in saving 
energy and material resources. 
Under the circumstances, expanded use of these materials is expected from 
now on. When these materials are used for railroad vehicles, however, they 
must be provided with a wider range of characteristics than when they are 
used for general purposes. That is, they must be of high strength, have 
considerably good cold workability, and also are required to have 
workhardenability because of the strength level required thereof. Further, 
they must be provided with excellent fatigue strength, because materials 
for railroad vehicles are subjected to high-frequency vibration. 
In the course of our study to develop sheets of high strength metastable 
austenitic stainless steels for railroad vehicles with improved resistance 
to fatigue without sacrificing strength, workability, workhardenability 
and corrosion resistance, we have found the process of this invention. 
Conventionally, stainless steel sheets are manufactured by annealing and 
pickling hot-rolled strips, cold-rolling the pickled strips (intermediate 
annealing optionally included), annealing and pickling or brightannealing 
the rolled strips, and finally temper-rolling them. 
We have found that it is possible to unexpectedly enhance the fatigue 
strength of the products in the abovementioned process by including a 
mechanical polishing step, which step has not been employed in the later 
stage of the cold rolling. 
This invention provides in the process for preparing cold-rolled strips or 
sheets of metastable austenitic stainless steel comprising casting a melt 
of the steel, producing a hot strip therefrom, annealing and pickling the 
obtained hot strip, cold-rolling the hot-rolled strip, wherein pickling 
and annealing are properly inserted, and finally temper-rolling the 
obtained sheets, an improved process for producing steel sheets with 
improved fatigue resistance characterized in that a mechanical polishing 
step is included before or after temper rolling. 
In the process of this invention, when bright annealing is employed as the 
final annealing, no final pickling is required. 
The polishing step may be included either before or after the temper 
rolling. However, it is preferable to carry out the polishing before the 
temper rolling for better smoothness and luster of the products. 
In the process of this invention, mechanical polishing is effected by belt 
polishing, shot peening with glass beads or steel particles, buffing, etc. 
These methods are known per se. The degree of the polishing is preferably 
several microns to some 15 microns. The usually employed polishing 
conditions give good results. Practically, the mechanical polishing can 
conveniently be conducted by way of belt polishing. 
By the process of this invention, strips or sheets of metastable austenitic 
stainless steels represented by AISI Type 301, AISI Type 201, etc., can be 
improved in their mechanical properties, especially in their fatigue 
characteristics. 
The reason why employment of a mechanical polishing step in the course of 
manufacturing cold-rolled steel sheets improves fatigue characteristics of 
the produced steel sheets is not yet well understood. Probably it is among 
the reasons that the surface defects which have been caused in the steps 
prior to the final annealing and pickling and the newly caused 
intergranular erosion during the final annealing and pickling, which will 
constitute the starting points of fatigue cracking, are removed by 
polishing, and the effect of polishing per se and some thermomechanical 
effect resulting therefrom will contribute to improvement in the fatigue 
characteristics. 
This invention is applicable to the hot strips made by conventional casting 
as well as continuous casting. Today continuous casting is widely employed 
and it should be understood that the process of this invention is more 
commonly applied to the continuous casting hot strips, although the 
embodiments described hereinafter are described with respect to 
conventional casting hot strips because of the laboratory scale practice. 
Now the invention is explained specifically by way of working examples with 
reference to the attached drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION 
An AISI Type 301 steel heat and an AISI Type 201 steel heat of the standard 
composition were melted, cast, bloomed and hot-rolled into 3.8 mm thick 
hot strips by the conventional process. The compositions of the steel 
heats are given in the following Table 1. 
TABLE 1 
______________________________________ 
Steel 
C Si Mn P S Ni Cr N 
______________________________________ 
301 0.060 0.54 1.02 0.025 
0.005 7.23 16.94 -- 
201 0.065 0.52 6.24 0.028 
0.006 4.52 16.54 0.14 
______________________________________ 
The two kinds of hot-rolled strips obtained were each divided into two 
groups. One half of each was worked in accordance with this invention, and 
the other half was worked in accordance with the conventional process, 
both into H.T. sheets and S.T. sheets, respectively. 
The working steps were the same for both the AISI Type 301 steel and the 
AISI Type 201 steel. The H.T. (hard-tempered) sheets were prepared by 
annealing and pickling the hot-rolled strip (3.8 mm thick), cold-rolling 
it down to 2.78 mm, annealing and pickling again and finally 
temper-rolling down to 2.00 mm thickness in accordance with the 
conventional process. In the process of this invention, however, a belt 
polishing step was included before the temper rolling. 
The S.T. (soft-tempered) sheets were prepared by annealing and pickling the 
hot-rolled strip (3.8 mm thick), reducing the thickness down to 2.78 mm by 
the primary cold rolling, further reducing the thickness down to 2.28 mm 
by the secondary cold rolling after the inserted annealing and pickling, 
repeating the annealing and pickling and finally temper-rolling the sheet 
down to 2 mm thickness in accordance with the conventional process. In the 
process of this invention, however, a belt polishing step was included 
before the temper rolling. 
The mechanical properties and fatigue characteristics of the thus prepared 
cold-rolled sheets were measured. The mechanical properties are summarized 
in Table 2. The fatigue characteristics were determined and evaluated by 
the reversed plane bending fatigue test using a Schenck type fatigue 
testing machine. The results are summarized in the attached drawing. 
TABLE 2 
__________________________________________________________________________ 
Mechanical Properties of Samples Sheets 
Bending 
Yield Strength (radius of 
0.2% off-set 
Tensile Strength 
Elongation 
curvature when 
Fatique Limit 
Steel Temper 
(Kg/mm.sup.2) 
(Kg/mm.sup.2) 
(%) bent at 180.degree.) 
(Kg/mm.sup.2) 
__________________________________________________________________________ 
Conventional Process 
AISI HT 74.5 99.6 28.5 1/2 T ND 
43.6 
301 ST 53.9 87.6 37.2 0 T ND 40.1 
AISI HT 79.6 111.5 25.3 1 T ND 43.0 
201 ST 48.2 95.6 38.5 0 T ND 40.1 
Invention Process 
AISI HT 76.2 101.2 27.9 1/2 T ND 
52.6 
301 ST 54.3 89.1 35.6 0 T ND 48.4 
AISI HT 79.0 110.9 24.9 1 T ND 50.0 
201 ST 49.0 95.8 37.9 0 T ND 47.2 
__________________________________________________________________________ 
T: thickness 
ND: no defect 
As is apparent from the table and the drawing, there are no differences 
found between the cold-rolled sheets of the conventional process and those 
of the process of this invention in 0.2% off-set yield strength, tensile 
strength, elongation and bending property. But the products of this 
invention generally exhibit an enhancement of 7-9 kg/mm.sup.2 in fatigue 
resistance, and also an enhancement of 7-9 kg/mm.sup.2 in fatigue limit. 
Industrial Applicability 
This invention produces stainless steel sheets with improved fatigue 
resistance suitable for manufacturing railroad vehicles by addition of a 
simple working step to the conventional process. Therefore, only a very 
small additional cost is required for producing materials with improved 
fatigue resistance.