Martensitic Stainless Steel and Production Method Therefor

Abstract

A martensitic stainless steel for use in tableware, knives, scissors and the like, containing in % by weight, 0.10 to 0.50% carbon and 11 to 16% chromium, and a production method therefore. A production method for mid-carbon martensitic stainless steel in a strip-casting device comprising a pair of rolls rotating in opposite directions, edge dams respectively provided to both sides of the rolls so as to form a molten steel pool, and a meniscus shield for supplying inert nitrogen gas to the upper surface of the molten steel pool, wherein a stainless-steel thin sheet is produced by supplying a molten stainless steel of the above composition to the molten steel pool via a nozzle from a tundish, and a hot-rolled annealed strip is produced to a rolling reduction of 5 to 40% using in-line rollers, and relates to martensitic stainless steel produced by the production method

Description

TECHNICAL FIELD

An aspect of the present invention relates to a martensitic stainless steel and a production method therefor, and more particularly, to a mid-carbon martensitic stainless steel and a production method therefor, in which the mid-carbon martensitic stainless steel containing, as percentages by weight, 0.10 to 0.50% carbon and 11 to 16% chromium is produced using a strip-casting method, thereby suppressing lamination defects and having uniform hardness.

BACKGROUND ART

In general, a martensitic stainless steel is produced through the following production process. That is, a casting slab is producing by casting a molten steel, reheated and then hot-rolled. In hot-rolled state, a martensitic phase, a tempered martensitic phase, a remaining austenitic phase, and the like are mixed in the structure of the steel. Such a hot-rolled coil is decomposed into ferrite and carbide through a batch annealing process so as to be softened for the purpose of hot-rolled strip annealing, and the material softened by the hot-rolled annealing goes through a pickling process so as to remove scales formed thereon. After the pickling process is performed, the soft material is subject to cold rolling or machining and then goes through a heat treatment process, thereby producing a martensitic stainless steel.

420 series stainless steel is used as a representative of martensitic stainless steel. The 420 series stainless steel forms coarse carbide center segregation in a casting-slab production process due to the high carbon content of the steel. The carbide center segregation is a phenomenon occurring as a result of absorption and integration in a bulk molten steel pool while a micro-segregation molten steel existing between dendrites is solidified. Since the center segregation formed in the slab is not well removed in a reheating or annealing heat treatment process, the center segregation remains in a hot-rolled or cold-rolled sheet, and therefore, accompanies lamination defects in a process of shearing a strip.

In a conventional method for producing a slab with a thickness of 200 to 250 mm, the slab is produced by decreasing a casting speed by 70 to 80% as compared with an ordinary material in a casting process so as to minimize center segregation. In this case, the casting productivity is remarkably lowered. In order to employ coarse carbide formed at a central portion of the slab in the casting process, the annealing temperature and maintaining time of batch annealing should be excessively increased, and therefore, the productivity is rapidly lowered. The center segregation occurring in the continuous casting process is caused by thickened molten steel due to the accumulation of carbon while the molten steel is solidified, and therefore, a method for reducing center segregation has been reported. That is, the method for reducing the center segregation includes electromagnetic stirring, mechanical soft reduction, thermal soft reduction, and the like.

DISCLOSURE OF INVENTION

Technical Problem

Accordingly, an object of the present invention is to provide a high-carbon martensitic stainless steel in which carbide center segregation is reduced by producing a martensitic stainless steel not using a conventional continuous casting method but using a strip-casting method for directly producing a thin material by means of a pair of rolls, so that it is possible to suppress lamination defects that were the largest disadvantage of the conventional continuous casting method and to have uniform hardness.

Technical Solution

According to an aspect of the present invention, there is provided a production method for a martensitic stainless steel, wherein, in a strip-casting device comprising a pair of rolls rotating in opposite directions, edge dams respectively provided to both sides of the rolls so as to form a molten steel pool, and a meniscus shield for supplying inert nitrogen gas to the upper surface of the molten steel pool, a stainless-steel thin sheet is produced by supplying a molten stainless steel containing, as percentages by weight, 1.10 to 0.50% carbon and 11 to 16% chromium, to the molten steel pool via a nozzle from a tundish, and a hot-rolled annealed strip is produced to a rolling reduction of 5 to 40% using in-line rollers.

The martensitic stainless steel may contain, as percentages by weight, 0.1 to 1.0% silicon (Si), 0.1 to 1.0% manganese (Mn), over 0 to 0.1% nickel (Ni), over 0 to 0.04 sulfur (S), and over 0 to 0.05 phosphorus (P), and Fe and other unavoidable impurities as remnants.

A hot-rolled annealed sheet may be produced by performing batch annealing on the hot-rolled annealed strip at a temperature of 700 to 950° C. under a reducing gas atmosphere.

The hot-rolled annealed strip may have a hardness difference of 90 Hv or less between a carbide segregation portion and a non-segregation portion at a sectional portion in the thickness direction thereof when measuring their Vickers hardness values with a load of 100 g.

According to another aspect of the present invention, there is provided a martensitic stainless steel produced by means of a production method, wherein, in a strip-casting device comprising a pair of rolls rotating in opposite directions, edge dams respectively provided to both sides of the rolls so as to form a molten steel pool, and a meniscus shield for supplying inert nitrogen gas to the upper surface of the molten steel pool, a stainless-steel thin sheet is produced by supplying a molten stainless steel containing, as percentages by weight, 1.10 to 0.50% carbon and 11 to 16% chromium, to the molten steel pool via a nozzle from a tundish, and a hot-rolled annealed strip is produced to a rolling reduction of 5 to 40% using in-line rollers.

The martensitic stainless steel may contain, as percentages by weight, 0.1 to 1.0% silicon (Si), 0.1 to 1.0% manganese (Mn), over 0 to 0.1% nickel (Ni), over 0 to 0.04 sulfur (S), and over 0 to 0.05 phosphorus (P), and Fe and other unavoidable impurities as remnants.

A hot-rolled annealed sheet may be produced by performing batch annealing on the hot-rolled annealed strip at a temperature of 700 to 950° C. under a reducing gas atmosphere.

The hot-rolled annealed strip may have a hardness difference of 90 Hv or less between a carbide segregation portion and a non-segregation portion at a sectional portion in the thickness direction thereof when measuring their Vickers hardness values with a load of 100 g.

The stainless steel produced by means of the production method may be a thin sheet of which thickness is 1 to 5 mm.

Advantageous Effects

As described above, according to the present invention, a martensitic stainless steel is produced, so that it is possible to reduce center segregation and to suppress lamination defects. Further, the martensitic stainless steel produced as described above has uniform hardness in the entire structure thereof.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a general strip-casting process.

FIG. 2 is a scanning electron microscope (SEM) photograph showing a sectional microstructure of a slab with a thickness of 200 mm, cast using a continuous casting method, in which carbide segregation is etched black at a central portion in the thickness direction of the slab.

FIG. 3 is a low-magnification SEM photograph showing a sectional microstructure of a hot-rolled sheet cast through strip casting and continuously made at a high temperature using in-line rollers just after the casting, which shows a structure of equiaxed crystals formed at a central portion in the thickness direction of the sheet and a structure of columnar crystals formed at a surface layer portion of the sheet.

FIG. 4 is a low-magnification SEM photograph showing a microstructure of a hot-rolled annealed strip cast using the continuous casting method and batch annealed, in which carbide center segregation is formed in the shape of a black band.

FIG. 5 is an SEM photograph showing a microstructure obtained by magnifying the black band shown in FIG. 4, in which the band of carbide segregation is formed to a thickness of about 20 μm.

FIG. 6 is a low-magnification SEM photograph showing a microstructure of a hot-rolled annealed strip cast through strip casting and batch annealed, in which the formation of the band-shaped center segregation shown in FIG. 4 is suppressed.

FIG. 7 is an SEM photograph showing a microstructure obtaining by magnifying the hot-rolled annealed strip shown in FIG. 6, in which the formation of the center segregation is suppressed.

FIG. 8 is a graph comparing Vickers hardness values at a carbide center segregation portion and a non-segregation portion.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments but may be implemented into different forms. These embodiments are provided only for illustrative purposes and for full understanding of the scope of the present invention by those skilled in the art. Throughout the drawings, like elements are designated by like reference numerals. In the drawings, the thickness or size of layers are exaggerated for clarity and not necessarily drawn to scale.

FIG. 1 is a schematic view showing a general strip-casting process. The strip-casting process is a process of directly producing a hot-rolled annealed strip of a thin material from a molten steel. The strip-casting process is a new steel production process capable of remarkably reducing production cost, facility investment cost, amount of energy used, amount of exhaust gas, and the like by omitting a hot rolling process. In a twin roll strip caster used in a general strip-casting process, as shown in FIG. 1, a molten steel is accommodated in a ladle 1 and then flowed in a tundish 2 along a nozzle. The molten steel flowed in the tundish 2 is supplied between edge dams 5 respectively provided to both end portions of casting rolls 6, i.e., between the casting rolls 6, through a molten steel injection nozzle 3 so that the solidification of the molten steel is started. In this case, a molten metal surface is protected with a meniscus shield 4 in a molten metal portion so as to prevent oxidation, and an appropriate gas is injected into the molten metal portion so as to form an appropriate atmosphere. A thin sheet 8 is produced while being extracted from a roll nip 7 formed between both the rolls, and rolled between rollers 9. Then, the rolled thin sheet goes through a cooling process, and is wound around a winding roll 10.

In this case, the important technique in a twin roll strip casting process of directly producing a thin sheet with a thickness of 10 mm or less from a molten steel is to produce a thin sheet with a desired thickness, which has no crack and an improved real yield by supplying the molten steel through an injection nozzle between internal air-cooled twin rolls rotating in opposite direction at a high speed.

The present inventors has found that center segregation difficult to be remove using the conventional casting method can be innovatively reduced using the strip-casting method. As a result, it can be seen that lamination defects are reduced in a process of sheering a strip, and the hardness in the thickness direction of a sheet is equalized.

Embodiments

Hereinafter, embodiments of the present invention will be described.

A base material used in the present invention is a martensitic stainless steel containing, as percentages by weight, 0.10 to 0.50% carbon (C) and 11 to 16% chromium (Cr). In case where the content of the carbon is set to 0.1% or less in the present invention, serious center segregation will not occur, but the hardness of the martensitic stainless steel is not preferable. In case where the content of the carbon is set to 0.5% or more, remaining austenite may excessively remain in a microstructure of the martensitic stainless steel in quenching heat treatment. Therefore, in the present invention, the martensitic stainless steel containing, as percentages by weight, 0.1 to 0.5% carbon (C) and 11 to 16% chromium (Cr) is proposed as the optical range of content.

The martensitic stainless steel according to the embodiments of the present invention is an alloy that contains compositions comprising, as percentages by weight, 0.1 to 1.0% silicon (Si), 0.1 to 1.0% manganese (Mn), over 0 to 0.1% nickel (Ni), over 0 to 0.04 sulfur (S), and over 0 to 0.05 phosphorus (P), and Fe and other unavoidable impurities as remnants.

In these embodiments, microstructural Characteristics of a hot-rolled annealed sheet produced using the conventional continuous casting method and a steel produced using the strip-casting method were compared.

Table 1 shows compositions of the steels respectively produced using the continuous casting method and the strip-casting method. A hundred tons of casting slab with a thickness of 200 mm was produced with a 420J2 stainless steel using the conventional continuous casting method. This is shown as a comparative example in #1 of Table 1. Then, the slab was reheated in a heating furnace for the purpose of hot rolling, and the reheated slab was hot-rolled to a final thickness of 3 mm. The steel of the present invention, which has components similar to those of the steel in #1 of Table 1, were produced in the form of a hot-rolled coil using a twin roll strip caster. The twin roll strip caster supplies a molten steel between twin-drum rolls rotating in opposite directions and between side dams, and casts the molten steel while discharging a large amount of heat through surfaces of the water-cooled rolls. In this case, a solidified cell was formed on the surfaces of the rolls at a high cooling speed, and a hot-rolled thin sheet with a thickness of about 1 to 5 mm was finally produced through in-line rolling continuously performed at a high temperature after the casting. In these embodiments, the hot-rolled thin sheet having a thickness of 3.0 mm was cast with the 420J2 stainless steel, and the in-line rolling was performed just after the casting, thereby producing a hot-rolled coil with a thickness of 2 mm. Batch annealing under the same condition was performed on the hot-rolled sheet with the thickness of 3 mm, produced using the continuous casting method, and the hot-rolled sheet with the thickness of 2 mm, produced using the strip-casting method.

TABLE 1
Compositions of steel produced by means of continuous
casting method and strip-casting method
ID	C	Si	Mn	P	S	Cr	Ni	N	□Hv	Remark
#1	0.29	0.43	0.45	0.021	0.004	13.2	0.1	0.03	98	Continuous casting
										(Comparative
										example)
#2	0.13	0.43	0.50	0.023	0.001	12.3	0.25	0.02	4	Strip-casting
										(Embodiment)
#3	0.30	0.45	0.49	0.021	0.001	13.3	0.1	0.03	6	Strip-casting
										(Embodiment)
#4	0.32	0.46	0.50	0.019	0.001	13.4	0.4	0.03	9	Strip-casting
										(Embodiment)
#5	0.45	0.35	0.40	0.018	0.002	14.0	0.15	0.02	8	Strip-casting
										(Embodiment)
#6	0.48	0.52	0.45	0.018	0.002	14.8	0.3	0.02	10	Strip-casting
										(Embodiment)

FIG. 2 is a scanning electron microscope (SEM) photograph showing a sectional microstructure of a slab with a thickness of 200 mm, which is the comparative example, produced with a steel having components in #1 of Table 1 using the conventional casting method. In FIG. 2, a carbide center segregation portion etched black exits in a central portion of the slab. On the other hand, in the steel #3 in Table 1 as a hot-rolled sheet with a thickness of 2 mm, cast using the strip-casting method, only the existence of equiaxed crystals was identified at a central portion in the thickness direction of the sheet, but marks of segregation were not identified at the central portion on the SEM photograph. This can be seen in detail in FIG. 3. FIGS. 4 and 5 SEM photographs respectively taken in magnification of ×50 and ×1000, showing a microstructure of the central portion of the steel with a thickness of 3 mm as the comparative example, softened through batch annealing using the conventional casting method. It can be seen that a center segregation portion in which carbide with a thickness of 20 μm is coarsely formed in a band shape is formed at a central portion of the sectional structure in the thickness direction of the steel. However, such a center segregation portion was not observed at a central portion of the sectional structure of #3 in Table 1, which is a steel with a thickness of 2 mm, softened through batch annealing using the strip-casting method. This can be seen through the structural photographs of FIGS. 6 and 7.

FIG. 8 shows a result obtained by measuring Vickers hardness values at the carbide center segregation portion identified in FIG. 5 and the non-segregation portion. The Vickers hardness values of the portions were measured, and the Vickers hardness value of each of the portions was measured with a load of 100 g ten times. The measured result was shown in the shape of a box in FIG. 8. The average hardness of the carbide center segregation portion was 288 Hv, and the average hardness of the non-segregation portion was 193 Hv. Here, it can be seen that the difference in hardness value between the carbide center segregation portion and the non-segregation portion is about 95 Hv. The result obtained by measuring sectional hardness values of martensitic stainless-steel hot-rolled annealed sheets having various compositions, produced using the strip-casting method, was shown in Table 1. The difference □Hv in Vickers hardness value, shown in Table 1, is a result obtained by measuring the difference in hardness value between the carbide center segregation portion and the non-segregation portion, and the differences in sectional hardness in all the embodiments were measured as 10 Hv or less, except #1 in Table, which is the steel produced using the continuous casting method. The result described above means that the carbide center segregation is removed by producing the steel using the strip-casting method, so that the uniformity of hardness is improved.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.

Claims

1. A production method for a martensitic stainless steel, wherein, in a strip-casting device comprising a pair of rolls rotating in opposite directions, edge dams respectively provided to both sides of the rolls so as to form a molten steel pool, and a meniscus shield for supplying inert nitrogen gas to the upper surface of the molten steel pool, a stainless-steel thin sheet is produced by supplying a molten stainless steel containing, as percentages by weight, 1.10 to 0.50% carbon and 11 to 16% chromium, to the molten steel pool via a nozzle from a tundish, and a hot-rolled annealed strip is produced to a rolling reduction of 5 to 40% using in-line rollers.

2. The production method of claim 1, wherein the martensitic stainless steel contains, as percentages by weight, 0.1 to 1.0% silicon (Si), 0.1 to 1.0% manganese (Mn), over 0 to 0.1% nickel (Ni), over 0 to 0.04 sulfur (S), and over 0 to 0.05 phosphorus (P), and Fe and other unavoidable impurities as remnants.

3. The production method of claim 1, wherein a hot-rolled annealed sheet is produced by performing batch annealing on the hot-rolled annealed strip at a temperature of 700 to 950° C. under a reducing gas atmosphere.

4. The production method of claim 3, wherein the hot-rolled annealed strip has a hardness difference of 90 Hv or less between a carbide segregation portion and a non-segregation portion at a sectional portion in the thickness direction thereof when measuring their Vickers hardness values with a load of 100 g.

5. The production method of claim 3, wherein the thickness of the carbide segregation portion is 20 μm or less at the sectional portion in the thickness direction of the hot-rolled annealed strip.

6. A martensitic stainless steel produced by means of a production method, wherein, in a strip-casting device comprising a pair of rolls rotating in opposite directions, edge dams respectively provided to both sides of the rolls so as to form a molten steel pool, and a meniscus shield for supplying inert nitrogen gas to the upper surface of the molten steel pool, a stainless-steel thin sheet is produced by supplying a molten stainless steel containing, as percentages by weight, 1.10 to 0.50% carbon and 11 to 16% chromium, to the molten steel pool via a nozzle from a tundish, and a hot-rolled annealed strip is produced to a rolling reduction of 5 to 40% using in-line rollers.

7. The martensitic stainless steel of claim 6, wherein the martensitic stainless steel contains, as percentages by weight, 0.1 to 1.0% silicon (Si), 0.1 to 1.0% manganese (Mn), over 0 to 0.1% nickel (Ni), over 0 to 0.04 sulfur (S), and over 0 to 0.05 phosphorus (P), and Fe and other unavoidable impurities as remnants.

8. The martensitic stainless steel of claim 6, wherein a hot-rolled annealed sheet is produced by performing batch annealing on the hot-rolled annealed strip at a temperature of 700 to 950° C. under a reducing gas atmosphere.

9. The martensitic stainless steel of claim 8, wherein the hot-rolled annealed strip has a hardness difference of 90 Hv or less between a carbide segregation portion and a non-segregation portion at a sectional portion in the thickness direction thereof when measuring their Vickers hardness values with a load of 100 g.

10. The martensitic stainless steel of claim 8, wherein the thickness of the carbide segregation portion is 20 μm or less at the sectional portion in the thickness direction of the hot-rolled annealed strip.

11. The martensitic stainless steel of claim 6, wherein the stainless steel is a thin sheet of which thickness is 1 to 5 mm.

12. The martensitic stainless steel of claim 7, wherein the stainless steel is a thin sheet of which thickness is 1 to 5 mm.

13. The martensitic stainless steel of claim 8, wherein the stainless steel is a thin sheet of which thickness is 1 to 5 mm.

14. The martensitic stainless steel of claim 9, wherein the stainless steel is a thin sheet of which thickness is 1 to 5 mm.

15. The martensitic stainless steel of claim 10, wherein the stainless steel is a thin sheet of which thickness is 1 to 5 mm.