MARTENSITIC STAINLESS STEEL PRODUCED BY A TWIN ROLL STRIP
CASTING PROCESS AND METHOD FOR MANUFACTURING SAME
5
Technical Field
The present invention relates to a martensitic stainless
steel produced by a twin roll strip casting process and a
method of manufacturing the same, and more particularly to a
10 martensitic stainless steel wherein center segregation,
cracking and strip breakage have been suppressed during
casting thus ensuring casting stability and also a refined
cast structure has been obtained thus enabling formation of
products having high hardness and superior edge quality, and
15 to a method of manufacturing the same.
Background Art
Typically martensitic stainless steel has superior
corrosion resistance, hardness and wear resistance, and is
20 thus employed in the production of a variety of tools or
knives.
In the case of producing martensitic stainless steel
using a continuous casting process, the severity of coarse
center segregation at the central portion of a cast product is
25 proportional to an increase in the amount of carbon, and a
PCT/KR2010/009004
RO/KR 16.12.2010
2
solid-liquid coexisting region is widely formed undesirably
weakening castability. Hence, martensitic stainless steel has
been mainly manufactured using an ingot casting process in
which an ingot is made into a slab, followed by performing reheating
and hot rolling to produce a hot rolled coil, which 5 is
then subjected to a BAF (Batch Annealing Furnace) process,
pickling and cold rolling.
However, the use of the above ingot casting process is
problematic because coarse center segregation takes place in
10 the slab due to the slow cooling rate, and the center
segregation is not well removed in the subsequent heat
treatment process and is thus left behind in the hot rolled
steel sheet, undesirably causing lamination defects in the
course of cutting the strip. Furthermore, coarse primary
15 chromium carbides are precipitated at grain boundaries, thus
generating cracks or strip breakage in the steel sheet in the
post-treatment process.
To solve the above problems, techniques are known in
which the annealing temperature is increased and the annealing
20 time is kept long in the BAF process subsequent to the hot
rolling in order to dissolve carbides, but are problematic
because a cost investment in the furnace is required
undesirably increasing the production cost and drastically
decreasing productivity.
25 Also, low-temperature casting and low-rate casting
PCT/KR2010/009004
RO/KR 16.12.2010
3
methods have been proposed, which enable the formation of a
granular equiaxed structure at the central portion of the cast
product and the rapid formation of a solidified layer of a
cast strand to thus reduce center segregation, but the nozzle
may experience blockage during casting, undesirably leading 5 to
unstable work and lowered productivity.
Therefore, there are the needs for high-quality
martensitic stainless steel and a method of manufacturing the
same, wherein center segregation is suppressed upon casting
10 and primary chromium carbides are finely precipitated at grain
boundaries, thus ensuring casting stability thanks to superior
crack resistance.
Disclosure
15 Technical Problem
Accordingly, the present invention has been made keeping
in mind the above problems encountered in the related art, and
an object of the present invention is to provide a martensitic
stainless hot rolled steel sheet having superior crack
20 resistance and a method of manufacturing the same, wherein a
twin roll strip casting process is applied and grain boundary
strengthening elements are added thus suppressing center
segregation, cracking and strip breakage thereby ensuring
casting stability, and also to provide a martensitic stainless
25 cold rolled steel sheet having high hardness and a method of
PCT/KR2010/009004
RO/KR 16.12.2010
4
manufacturing the same, wherein the uniform distribution of a
refined structure in steel may be obtained, from which knives
or tools having high hardness and edges with high quality may
be produced.
5
Technical Solution
The present invention provides a martensitic stainless
hot rolled steel sheet having superior crack resistance,
manufactured by a twin roll strip casting process and
10 comprising 0.1 ~ 1.5% of C, 12 ~ 15% of Cr, 1% or less of Ni,
0.005 ~ 0.1% of Ti, and a balance of Fe and other inevitable
impurities by wt%, wherein primary chromium carbides
precipitated at grain boundaries are fragmented and refined.
As such, the martensitic stainless hot rolled steel sheet
15 may further comprise either or both of 0.005 ~ 0.1 wt% of Mo
and 0.005 ~ 1.0 wt% of V.
As well, the primary chromium carbides may have a
thickness of 0.5 ㎛ or less.
Furthermore, center pores may be removed from the
20 martensitic stainless hot rolled steel sheet.
Also, the equiaxed structure ratio of the cross-sectional
structure of the martensitic stainless hot rolled steel sheet
may be 5 ~ 30%.
In addition, the present invention provides a martensitic
PCT/KR2010/009004
RO/KR 16.12.2010
5
stainless cold rolled steel sheet having high hardness,
manufactured by a twin roll strip casting process and
comprising 0.1 ~ 1.5% of C, 12 ~ 15% of Cr, 1% or less of Ni,
0.005 ~ 0.1% of Ti, and a balance of Fe and other inevitable
impurities by wt%, wherein spherical secondary chromi5 um
carbides are finely distributed.
As such, the martensitic stainless cold rolled steel
sheet may further comprise either or both of 0.005 ~ 0.1 wt%
of Mo and 0.005 ~ 1.0 wt% of V.
10 As well, the secondary chromium carbides may have a size
of 5 ㎛ or less, and there may be 30 or more chromium carbides
having the above size per area of 100 ㎛2.
The martensitic stainless cold rolled steel sheet may
have a hardness of 100 ~ 300 Hv.
15 In addition, the present invention provides a method of
manufacturing a martensitic stainless cold rolled steel sheet
having high hardness, comprising casting molten steel
comprising 0.1 ~ 1.5% of C, 12 ~ 15% of Cr, 1% or less of Ni,
0.005 ~ 0.1% of Ti, and a balance of Fe and other inevitable
20 impurities by wt% into a strip in a twin roll strip casting
process, rolling the strip at a rolling rate of 5 ~ 50% using
an inline rolling machine thus producing a hot rolled steel
sheet, and subjecting the hot rolled steel sheet to a BAF
PCT/KR2010/009004
RO/KR 16.12.2010
6
(Batch Annealing Furnace) process at 650 ~ 950℃ in a
reducible gas atmosphere and then to cold rolling, wherein the
cold rolling is performed multiple times and intermediate
annealing is performed between multiple times of the cold
rollin5 g.
The molten steel may further comprise either or both of
0.005 ~ 0.1 wt% of Mo and 0.005 ~ 1.0 wt% of V.
Advantageous Effects
10 According to the present invention, a twin roll strip
casting process is applied and grain boundary strengthening
elements are added thus preventing center segregation,
cracking and strip breakage upon casting to thereby ensure
casting stability, and also the uniform distribution of a
15 refined structure in steel can be obtained, from which knives
or tools having high hardness and edges with high quality can
be produced.
Description of Drawings
20 FIG. 1 illustrates the configuration wherein a twin roll
strip casting process is performed;
FIG. 2 illustrates cracks generated in martensitic
stainless steel during casting;
FIG. 3 illustrates the crack fracture surface of
PCT/KR2010/009004
RO/KR 16.12.2010
7
martensitic stainless steel during casting;
FIG. 4 illustrates primary chromium carbides precipitated
at grain boundaries of martensitic stainless steel;
FIG. 5 illustrates an equilibrium phase diagram of
martensitic stainless 5 steel;
FIG. 6 illustrates a graph of the equiaxed structure
ratio and crack generation depending on the amount of Ti in
martensitic stainless steel;
FIG. 7 illustrates center pores of a cross-sectional
10 structure of martensitic stainless steel depending on the
rolling rate of hot rolling, wherein (a) illustrates the case
before hot rolling, and (b) illustrates the case after hot
rolling at a rolling rate of 25%;
FIG. 8 illustrates grain diameter depending on the amount
15 of Ti in a martensitic stainless hot rolled steel sheet;
FIG. 9 illustrates primary chromium carbides precipitated
at grain boundaries depending on the amount of Ti in the
martensitic stainless hot rolled steel sheet; and
FIG. 10 illustrates secondary chromium carbides of a
20 martensitic stainless cold rolled steel sheet according to the
present invention.

1: tundish 2: nozzle
3: casting roll 4: molten steel
25 5: edge dam 6: brush roll
PCT/KR2010/009004
RO/KR 16.12.2010
8
7: strip 8: loop pit
9: meniscus shield 10: pinch roll
11: inline rolling machine (IRM)
Mode for Inven5 tion
Hereinafter, martensitic stainless steel according to the
present invention will be described in detail with reference
to the drawings.
According to the present invention, a martensitic
10 stainless hot rolled steel sheet having superior crack
resistance is manufactured using a twin roll strip casting
process. As illustrated in FIG. 1, the twin roll strip
casting process is performed by supplying molten steel 4
between a pair of rotating casting rolls 3 so that a strip
15 having a thickness of ones of mm is directly continuously
produced from the molten steel.
Specifically, the molten steel 4 having a predetermined
composition is supplied by way of a nozzle 2 between the
casting rolls 3 which are responsible for cooling while
20 rotating in opposite directions and thus solidifies thus
forming a solidified shell which is then depressed using a
roll nip, thus producing a strip 7. The strip 7 thus produced
is guided by pinch rolls 10, and rolled by means of rolling
rolls of an inline rolling machine (IRM) 11 and thus
25 manufactured into a martensitic stainless hot rolled steel
PCT/KR2010/009004
RO/KR 16.12.2010
9
sheet.
When martensitic stainless steel is conventionally
produced using a continuous casting process or an ingot
casting process, center segregation is formed undesirably
generating linear defects or planar separation, and als5 o
coarse primary chromium carbides are precipitated at grain
boundaries undesirably generating cracks or strip breakage in
the steel sheet upon post-treatment. However, when
martensitic stainless steel is produced using a twin roll
10 strip casting process, the molten steel near the roll nip is
depressed and thus squeezing flow takes place, so that molten
steel in the zone where the concentration of solute of the
central portion occurs is squeezed out, thus removing center
segregation. Moreover, the cooling rate at which the molten
15 steel is solidified is fast thus refining grains of grain
boundaries to thereby reduce the precipitation of primary
chromium carbides. Upon casting, center segregation and
cracking may be suppressed thus ensuring casting stability.
The martensitic stainless steel manufactured by the twin
20 roll strip casting process has no center segregation compared
to when the conventional casting process is used. As
illustrated in FIG. 4, primary chromium carbides are finely
precipitated at grain boundaries thus suppressing cracking and
strip breakage. As illustrated in FIGS. 2 and 3, because the
25 primary chromium carbides may cause cracks and strip breakage
PCT/KR2010/009004
RO/KR 16.12.2010
10
upon casting, grain boundary strengthening elements are added
to maximally suppress the effects of such carbides.
The martensitic stainless steel according to the present
invention comprises 0.1 ~ 1.5% of C, 12 ~ 15% of Cr, 1% or
less of Ni, 0.005 ~ 0.1% of Ti, and a balance of Fe and othe5 r
inevitable impurities by wt%. As such, 0.005 ~ 0.1 wt% of Mo
and 0.005 ~ 1.0 wt% of V may be further added alone or in
combination to the steel.
The reason why the amounts of the above components are
10 limited is given below.
C is very effective at enhancing the hardness of
stainless steel. If the amount of C is less than 0.1 wt%, the
hardness required for martensitic stainless steel cannot be
ensured. In contrast, if the amount of C exceeds 1.5 wt%,
15 comparatively coarse primary chromium carbides may be formed
thus increasing crack sensitivity and reducing corrosion
resistance. Hence, the amount of C is limited to 0.1 ~ 1.5
wt%.
Cr is added to enhance corrosion resistance. If the
20 amount of Cr is less than 12 wt%, improvements in corrosion
resistance become insignificant. In contrast, if the amount
of Cr exceeds 15 wt%, corrosion resistance may be improved but
strength is high and elongation is low thus deteriorating
processability and requiring a relatively high cost. Hence,
25 the amount of Cr is limited to 12 ~ 15 wt%.
PCT/KR2010/009004
RO/KR 16.12.2010
11
Ni is an element which produces a gamma (γ) phase. If
the amount thereof is high, the γ phase is increased and when
a coil is air cooled after hot rolling, the formation of the
martensitic phase may be promoted and thus strength and
hardness may increase whereas elongation may decrease. Hence5 ,
the amount of Ni is preferably limited to 1 wt% or less.
Ti which is a grain boundary strengthening element is
added to fragment primary chromium carbides of grain
boundaries or to finely precipitate them thus suppressing
10 cracking and strip breakage. If the amount of Ti is less than
0.005 wt%, the effects thereof on suppressing cracks and strip
breakage of a steel sheet are insignificant. In contrast, if
the amount of Ti exceeds 0.1 wt%, there may be clogging
wherein the stopper of a tundish is clogged due to Ti-based
15 oxides, undesirably generating casting problems. Hence, the
amount of Ti is limited to 0.005 ~ 0.1 wt%.
Mo and V may be added alone or in combination, and are
preferably added in an amount of 0.005 wt% or more to
strengthen grain boundaries and enhance corrosion resistance.
20 If the amount thereof exceeds 0.1 wt%, toughness may decrease.
Hence, the amounts of Mo and V are limited to 0.005 ~ 0.1 wt%.
As illustrated in FIGS. 8 and 9, a feature of the
martensitic stainless hot rolled steel sheet having the above
composition resulting from rolling the strip cast by the twin
25 roll strip casting process using an inline rolling machine is
PCT/KR2010/009004
RO/KR 16.12.2010
12
that primary chromium carbides precipitated at grain
boundaries are in a band shape and grains are refined and
fragmented and discontinuously distributed, so that grain
boundaries are strengthened thus suppressing cracks and strip
breakage upon casting, consequently obtaining an improv5 ed
casting completion.
The primary chromium carbides have a thickness of 0.5 ㎛
or less and are mainly distributed in the form of band-shaped
refined grains having a size of 0.05 ~ 0.30 ㎛.
10 The martensitic stainless hot rolled steel sheet is hot
rolled at a rolling rate of 5 ~ 50% using the inline rolling
machine, and thereby as illustrated in FIG. 7(b), center pores
are removed, thus suppressing brittleness due to the pores and
ensuring elongation.
15 As illustrated in the graph of FIG. 6, as the amount of
added Ti increases, the equiaxed structure ratio of the crosssectional
structure of the martensitic stainless hot rolled
steel sheet is increased. When the equiaxed structure ratio
is increased, center segregation may be decreased, and cracks
20 may be removed. As seen in the graph, when the equiaxed
structure ratio of 5% or more is ensured, cracks may be
greatly decreased upon casting, and when such a ratio is 7% or
more, cracks may be completely removed.
If the equiaxed structure ratio is less than 5%, columnar
PCT/KR2010/009004
RO/KR 16.12.2010
13
structures collide with each other thus facilitating the
generation of cracks, and upon non-uniform solidification, the
generation of cracks may be further increased. It is
difficult to technically ensure the equiaxed structure ratio
exceeding 5 ing 30%.
Meanwhile in the present invention, the martensitic
stainless hot rolled steel sheet having crack resistance is
subjected to BAF and cold rolling, thus producing a
martensitic stainless cold rolled steel sheet having high
10 hardness. Also, 0.005 ~ 0.1 wt% of Mo and 0.005 ~ 1.0 wt% of
V may be further added alone or in combination thereto.
As illustrated in FIG. 10, the martensitic stainless cold
rolled steel sheet is provided in the form of secondary
chromium carbides being in a spherical shape and finely
15 uniformly distributed, thus obtaining a high-hardness
martensitic stainless cold rolled steel sheet, from which
tools or knives having edges with high quality may be
produced.
The secondary chromium carbides have a size of 5 ㎛ or
20 less, the diameter of which is mostly 0.1 ~ 3.0 ㎛, and are
uniformly distributed. Also, a refined structure is formed in
which there are 30 or more chromium carbides having a size of
5 ㎛ or less per area of 100 ㎛2, thus manufacturing a
PCT/KR2010/009004
RO/KR 16.12.2010
14
martensitic stainless cold rolled steel sheet having a high
hardness of 100 ~ 300 Hv, from which tools or knives having
edges with high quality may be produced.
Below is a method of manufacturing the martensitic
stainless steel according to the present invention 5 n with
reference to the appended drawings.
The molten steel comprising 0.1 ~ 1.5% of C, 12 ~ 15% of
Cr, 1% or less of Ni, 0.005 ~ 0.1% of Ti, and a balance of Fe
and other inevitable impurities by wt% is supplied between
10 casting rolls 3 which performs cooling while rotating in
opposite directions via a nozzle 2 to solidify it, thus
forming a solidified shell, which is then depressed using a
roll nip thus producing a strip. As such, 0.005 ~ 0.1 wt% of
Mo and 0.005 ~ 1.0 wt% of V may be further added alone or in
15 combination to the molten steel.
The strip 7 thus produced is guided by pinch rolls 10 and
hot rolled by means of rolling rolls of the inline rolling
machine 11 thus forming a martensitic stainless hot rolled
steel sheet. As such, when hot rolling is not performed,
20 elongation is not ensured after BAF, and thus the steel sheet
becomes brittle to the extent that it is difficult to carry
out subsequent processes including pickling and cold rolling.
Hence, the method according to the present invention
essentially includes hot rolling.
25 As such, the rolling is preferably carried out at a
PCT/KR2010/009004
RO/KR 16.12.2010
15
rolling rate of 5 ~ 50%. If the rolling rate is less than 5%,
pores are formed at the center of the steel sheet, so that the
steel sheet becomes brittle due to the pores and the
elongation may decrease. In contrast, if the rolling rate
exceeds 50%, equipment costs may increase5 .
FIG. 7 illustrates center pores of the cross-sectional
structure of the martensitic stainless steel depending on the
rolling rate of hot rolling. As illustrated in FIG. 7(a),
pores are formed in an equiaxed zone when hot rolling is not
10 performed, and as illustrated in FIG. 7(b), all the pores are
removed from the equiaxed zone when rolling is performed at a
rolling rate of 25%.
The hot rolled steel sheet manufactured using the twin
roll strip casting process is subjected to BAF to stabilize
15 the solid solution of chromium carbides. Typically the
structure of hot rolled steel includes a martensitic phase, a
tempered martensitic phase, a ferrite phase, etc., which are
mixed together. When the BAF process is carried out, the
martensitic phase having oversaturated high-strength carbon
20 may decompose into ferrite and chromium carbides so that steel
is made soft, thereby improving processability.
The BAF process is performed in a manner of gradual
heating at an annealing temperature of 650 ~ 950℃ in a
reducible gas atmosphere and of slow cooling again in a batch
PCT/KR2010/009004
RO/KR 16.12.2010
16
type furnace.
If the annealing temperature is less than 650℃, heat
treatment effects are insignificant and ductility is not
ensured, making it possible for cracks or strip breakage to
occur in subsequent processes. In contrast if the 5 he annealing
temperature exceeds 950℃, re-dissolved chromium carbides may
excessively precipitate and the size of the precipitations may
partially increase and steel becomes too soft, making it
difficult to control chromium carbides. Hence, the annealing
10 temperature is limited to 650 ~ 950℃.
The steel sheet subjected to heat treatment in the BAF
process is pickled and cold rolled and thus transformed into
martensitic stainless steel. As such, the cold rolling is
performed multiple times and intermediate annealing is
15 performed between multiple times of the cold rolling, so that
re-decomposed spherical secondary chromium carbides are finely
uniformly distributed thus obtaining a high-hardness
martensitic stainless cold rolled steel sheet.
Examples of the present invention are described below.
20 Martensitic stainless steel comprising the components
shown in Table 1 below and a balance of Fe and other
impurities was cast into 100-ton strips having a casting width
of 1,300 mm and a casting thickness of 2 mm, and hot rolled
PCT/KR2010/009004
RO/KR 16.12.2010
17
using an inline rolling machine thus continuously producing
hot rolled steel sheets having a thickness of 1 ~ 2 mm. The
results are given in Table 2 below.
[Table 1]
Components Steel Sheet C Cr Ni Ti Mo V (Swit% ) Mn P S N
Inventive Steel 1 0.357 12.21 - 0.005 - 0.034 0.29 0.65 0.020 0.0018 0.030
Inventive Steel 2 0.448 13.22 - 0.011 0.023 - 0.28 0.67 0.021 0.0020 0.029
Inventive Steel 3 0.553 13.19 - 0.016 0.022 0.011 0.31 0.65 0.022 0.0018 0.029
Inventive Steel 4 0.652 14.18 0.21 0.022 - - 0.30 0.66 0.022 0.0019 0.030
Inventive Steel 5 0.754 13.20 0.34 0.034 - - 0.29 0.65 0.021 0.0020 0.030
Inventive Steel 6 0.850 13.21 0.67 0.049 0.049 - 0.30 0.64 0.020 0.0015 0.031
Inventive Steel 7 0.951 14.19 0.35 0.078 0.016 0.026 0.30 0.65 0.021 0.0018 0.029
Inventive Steel 8 1.253 13.20 0.28 0.097 - 0.042 0.31 0.64 0.020 0.0019 0.028
Comparative
Steel 1 0.652 13.22 - - - - 0.30 0.65 0.020 0.0018 0.030
Comparative
Steel 2 0.653 13.20 - 0.002 - - 0.28 0.66 0.022 0.0017 0.031
Comparative
Steel 3 0.651 13.21 - 0.004 - - 0.31 0.65 0.021 0.0020 0.030
Comparative
Steel 4 0.657 13.21 - 0.127 - 0.012 0.29 0.65 0.020 0.0018 0.029
Comparative
Steel 5 0.650 13.18 0.21 0.151 0.023 - 0.30 0.66 0.021 0.0020 0.028
5
[Table 2]
Hot Rolling
Steel
Sheet
Average
Thickness
(㎛) of
Primary
Chromium
Carbide
Equiaxed
Structure
Ratio
(%)
Crack
Generation
Clogging
of
Stopper
of
Tundish
Rolling
Rate
(%)
Pore
Generation
Inventive
Steel 1 0.28 5.2 Δ X 6 X
Inventive
Steel 2 0.21 7.5 X X 10 X
Inventive
Steel 3 0.15 8.1 X X 15 X
Inventive
Steel 4 0.14 10.3 X X 20 X
Inventive
Steel 5 0.12 12.6 X X 25 X
Inventive
Steel 6 0.10 16.8 X X 30 X
PCT/KR2010/009004
RO/KR 16.12.2010
18
Inventive
Steel 7 0.07 21.7 X X 40 X
Inventive
Steel 8 0.05 25.1 X Δ 48 X
Comparative
Steel 1 0.74 2.8 O X 0 O
Comparative
Steel 2 0.61 3.2 O X 2 O
Comparative
Steel 3 0.55 3.8 O X 55 X
Comparative
Steel 4 0.04 27.4 X O 60 X
Comparative
Steel 5 0.03 29.2 X O 65 X
* Generation of cracks: O (poor), Δ (good), X (no generation)
* Clogging of stopper of tundish: O (clogging), Δ (good), X
(excellent)
As is apparent from Tables 1 and 2, Inventive Steels 1 t5 o
8 wherein the amounts of components of steel including Ti and
so on as grain boundary strengthening elements fall in the
ranges of the present invention had primary chromium carbides
having a thickness of 0.5 ㎛ or less which were finely
10 precipitated at grain boundaries, and could ensure an equiaxed
structure ratio of 5 ~ 30%, so that cracks were not generated
or the extent of crack generation was good. Furthermore, the
stopper of the tundish did not clog thus exhibiting superior
castability.
15 However, Comparative Steels 1 to 3 wherein Ti is not
added or is added in a very small amount had cracks propagated
along the grain boundaries, and Comparative Steels 4 and 5
wherein an excess of Ti is added did not generate cracks but
PCT/KR2010/009004
RO/KR 16.12.2010
19
caused clogging due to Ti-based oxides making it difficult to
apply casting.
As is apparent from Table 2, when the rolling rate of 5 ~
50% was applied upon hot rolling as in Inventive Steels 1 to
8, pores of the central portion of steel were removed thu5 s
suppressing brittleness and ensuring elongation.
The martensitic stainless hot rolled steel sheet thus
manufactured was pickled and then subjected to BAF for a long
period of time at 650 ~ 950℃, followed by conducting cold
10 rolling multiple times and intermediate annealing between
multiple times of the cold rolling. Ultimately, as
illustrated in the distribution of secondary chromium carbides
of the martensitic stainless cold rolled steel sheet of FIG.
10, chromium carbides were precipitated in a spherical shape
15 and were thus finely uniformly distributed, and also chromium
carbides having a diameter of 5 ㎛ or more were not observed
in such a refined structure. The martensitic stainless steel
having the above refined structure has a very high hardness of
100 ~ 300 Hv, from which tools or knives having edges with
20 high quality can be produced.
PCT/KR2010/009004
RO/KR 16.12.2010
20

We claim:-
1. A martensitic stainless hot rolled steel sheet having
superior crack resistance, manufactured by a twin roll strip
casting process and comprising 0.1 ~ 1.5% of C, 12 ~ 15% 5 of
Cr, 1% or less of Ni, 0.005 ~ 0.1% of Ti, and a balance of Fe
and other inevitable impurities by wt%, wherein primary
chromium carbides precipitated at grain boundaries are
fragmented and refined.
10
2. The martensitic stainless hot rolled steel sheet of
claim 1, further comprising either or both of 0.005 ~ 0.1 wt%
of Mo and 0.005 ~ 1.0 wt% of V.
15 3. The martensitic stainless hot rolled steel sheet of
claim 1, wherein the primary chromium carbides have a
thickness of 0.5 ㎛ or less.
4. The martensitic stainless hot rolled steel sheet of
20 claim 1, wherein center pores are removed from the martensitic
stainless hot rolled steel sheet.
5. The martensitic stainless hot rolled steel sheet of
claim 1, wherein an equiaxed structure ratio of a crossPCT/
KR2010/009004
RO/KR 16.12.2010
21
sectional structure of the martensitic stainless hot rolled
steel sheet is 5 ~ 30%.
6. A martensitic stainless cold rolled steel sheet having
high hardness, manufactured by a twin roll strip castin5 g
process and comprising 0.1 ~ 1.5% of C, 12 ~ 15% of Cr, 1% or
less of Ni, 0.005 ~ 0.1% of Ti, and a balance of Fe and other
inevitable impurities by wt%, wherein spherical secondary
chromium carbides are finely distributed.
10
7. The martensitic stainless cold rolled steel sheet of
claim 6, further comprising either or both of 0.005 ~ 0.1 wt%
of Mo and 0.005 ~ 1.0 wt% of V.
15 8. The martensitic stainless cold rolled steel sheet of
claim 6, wherein the secondary chromium carbides have a size
of 5 ㎛ or less, and there are 30 or more chromium carbides
having the size of 5 ㎛ or less per area of 100 ㎛2.
20 9. The martensitic stainless cold rolled steel sheet of
claim 6, wherein the hardness of the martensitic stainless
cold rolled steel sheet is 100 ~ 300 Hv.
10. A method of manufacturing a martensitic stainless
PCT/KR2010/009004
RO/KR 16.12.2010
22
cold rolled steel sheet having high hardness, comprising:
casting molten steel comprising 0.1 ~ 1.5% of C, 12 ~ 15%
of Cr, 1% or less of Ni, 0.005 ~ 0.1% of Ti, and a balance of
Fe and other inevitable impurities by wt% into a strip in a
twin roll strip casting 5 ng process;
rolling the strip at a rolling rate of 5 ~ 50% using an
inline rolling machine thus producing a hot rolled steel
sheet; and
subjecting the hot rolled steel sheet to a BAF (Batch
10 Annealing Furnace) process at 650 ~ 950℃ in a reducible gas
atmosphere, and then to cold rolling,
wherein the cold rolling is performed multiple times and
intermediate annealing is performed between multiple times of
the cold rolling.
15 11. The method of claim 10, wherein the molten steel
further comprises either or both of 0.005 ~ 0.1 wt% of Mo and
0.005 ~ 1.0 wt% of V.
Dated this 18th day of June 2012