KR20140010701A - Martensitic stainless steels by twin roll strip casting process and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a martensite-based stainless steel used for knifes or tools and, more specifically, to a martensite-based stainless steel manufactured by a twin roll type thin film casting process and a manufacturing method thereof. To this end, the present invention provides a martensite-based stainless steel which is manufactured by the twin roll type thin film casting process and consists of 0.1 to 1.2 wt% of carbon (C), 11 to 16 wt% of chrome (Cr), 0.01 to 2.0 wt% of nitrogen (N), 0.1 to 2.0 wt% of molybdenum (Mo), 0.1 to 2.0 wt% of tungsten (W), remainder iron (Fe), and other inevitable impurities. The composition is satisfied with 23<=Cr/(C+N)<=28 and has a PREN index of 17 or more.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel produced by a twin roll thin plate casting process and a method of manufacturing the same.

More particularly, the present invention relates to a martensitic stainless steel manufactured by a twin-roll type thin plate casting process and a method of manufacturing the same.

The martensitic stainless steel is excellent in corrosion resistance, hardness and abrasion resistance, and is used for various kinds of products such as razor blades, medical knives, general esophagus and scissors.

Generally, a martensitic stainless steel is produced by continuously casting molten steel or preparing a slab from an ingot, reheating and hot rolling to produce a hot-rolled sheet, and then performing a batch annealing process for annealing the hot-rolled sheet, And a pickling process is performed to remove a scale formed by the hot-rolled annealing. After pickling, the material can be subjected to cold rolling or product processing.

The steel structure after the hot rolling in the above-mentioned manufacturing process is present in a mixed state of martensite phase, tempered martensite phase, ferrite phase, retained austenite phase and the like, and after the hot-rolled annealing, it is transformed into ferrite and carbide and softened. As described above, the softened material (soft material) is transformed into martensitic steel through the final heat treatment process.

On the other hand, the higher the degree of hardness is required, the higher the hardness is required.

The martensite structure is a very hard microstructure that is produced when the hot austenite is rapidly cooled. The higher the carbon content in the austenite phase at high temperature, the more carbon in the martensite increases and the hardness of the martensite increases . Therefore, in order to produce a martensitic stainless steel having a high hardness, it is necessary to increase the amount of carbon added as much as possible.

However, the higher the carbon content of the steel, the more segregated and the higher the coexistence region becomes, the weaker the casting characteristic, and the martensitic stainless steel is produced mainly by the ingot casting method.

However, in the case of the ingot casting method, coarse precipitates and center segregation occur at the grain boundaries due to the slow cooling rate, which causes the quality degradation in the post-treatment process.

Therefore, if a strip casting process is applied instead of the ingot casting process, center segregation is suppressed and chromium carbide precipitation is reduced in the initial grain boundaries, which is a groundbreaking process for achieving the object of quality improvement.

As described above, when the martensitic stainless steel is manufactured by applying the strip casting process, it is important to optimize the steel component design while reducing the crack sensitivity and ensuring the casting stability when casting these steel types.

According to one aspect of the present invention, in the production of martensitic stainless steel using a twin roll type thin plate casting process, the steel component design is optimized and the casting condition is controlled according to the component design, And can improve the stability of casting. It is another object of the present invention to provide a method for producing the same.

An aspect of the present invention provides a method of manufacturing a steel sheet by a twin roll thin plate casting process, which comprises 0.1 to 1.2% of carbon (C), 11 to 16% of chromium (Cr), 0.01 to 2.0% of nitrogen (N) Cr / (C + N) &lt; / = 28, wherein the composition is composed of molybdenum (Mo) of 0.1 to 2.0%, tungsten (W) of 0.1 to 2.0%, balance Fe and other unavoidable impurities. Wherein the stainless steel has a PREN index of 17 or more. The martensitic stainless steel is excellent in corrosion resistance.

<Relationship>

PREN index (official resistance index) = Cr + 3.3 (Mo + 0.5W) + 16N

In another aspect of the present invention, there is provided a method for producing a casting roll, which comprises, by weight, 0.1 to 1.2% of carbon, 11 to 16% of chromium, 0.01 to 2.0% of nitrogen, molybdenum (Mo) : 0.1 to 2.0%, tungsten (W): 0.1 to 2.0%, the balance Fe and other unavoidable impurities, and the composition satisfies 23? Cr / (C + N)? 28, ; And a step of rolling the thin plate in a rolling mill to produce a thermal laminate, wherein the surface roughness distribution of the casting roll has an edge roughness of 0.75 to 0.95 times that of the middle portion, wherein the roughness in the edge portion linearly increases from the end of the roll toward the middle portion of the roll.

According to the present invention, when the martensitic stainless steel is manufactured by applying the twin roll type thin plate casting process, the surface roughness of the casting roll is controlled at the time of casting together with the control of the constituent elements, Stainless steel can be obtained.

1 shows a schematic diagram of a twin roll thin plate casting process.
FIG. 2 is a view showing the edge defects of the martensitic stainless steel produced by the conventional twin roll thin plate casting process, wherein (A) shows an edge crack when the edge roughness gradient is large, (B) And the edge temperature is increased due to the edge coagulation delay when the roughness gradient is gentle.
3 is a graph showing the results of observing the state of a steel plate edge portion according to the surface roughness distribution at the edge of the casting roll edge when a steel satisfying the composition of the present invention is cast by a twin roll type thin plate casting process (A) shows a state in which the edge state of the cast steel is in a bad state when the surface roughness distribution does not satisfy the range proposed by the present invention, (B) shows the case where the surface roughness distribution proposed in the present invention is satisfied, And a good state is shown.
FIG. 4 is a graph showing the state of a steel sheet edge portion according to the surface roughness distribution at the edge of a casting roll edge when a steel satisfying the composition of the present invention is cast by a twin roll type thin plate casting process .

Hereinafter, the martensitic stainless steel of the present invention will be described in detail.

The martensitic stainless steel excellent in corrosion resistance according to the present invention is produced by a twin roll thin plate casting process.

1, the molten steel supplied to a pair of casting rolls 6 rotating in opposite directions to each other through an injection nozzle 3 attached to the tundish 2 is supplied to the casting roll 3, (IRM) 9 is formed by forming a molten pool surrounded by an edge dam (6) and an edge dam (5) and forming a thin plate (8) in contact with the surface of the casting roll (6) And then wound by the coil winding apparatus 10. [0052] As shown in Fig. Here, reference numeral 1 denotes a ladle, 4 denotes a meniscus shield, and 7 denotes a roll nip.

That is, the molten steel having a predetermined component injected through the nozzle between the casting rolls which are rotated while being rotated in the opposite direction is solidified to form a solidified shell, and is rolled in the roll nip to produce a thin plate. The thus produced thin plate is guided by a pinch roll and rolled by a rolling roll in IRM to be made of martensitic stainless steel.

As described above, in the case of producing martensitic stainless steel by the conventional ingot casting method, center segregation is formed and linear defects and plate-like separation are generated, and chromium (Cr) carbide is precipitated in the grain boundaries, There is a problem that cracks or plate breakage occur in the steel sheet in the process.

However, in the case of manufacturing by the above-described twin roll type thin plate casting process, the molten steel in the vicinity of the roll nip is squeezed out due to the squeezing flow, and the molten steel in the zone where the solute concentration in the central portion is generated squeezes out, Not only the segregation is eliminated but also the cooling rate at which molten steel is solidified is accelerated so that the crystal grains of the grain boundaries are made finer and the precipitates are reduced so that the center segregation and cracking during casting can be suppressed and the casting stability can be ensured.

As described above, the martensitic stainless steel produced by the above-described twin roll thin plate casting process has lower center segregation than that of the conventional casting method and can secure the casting stability by reducing the occurrence of cracking and plate fracture, There is a problem that the solidification speed at the edge portion is nonuniform and an edge crack is generated in the final product as shown in Fig. Therefore, in manufacturing the martensitic stainless steel by the twin roll thin plate casting process, it is necessary to secure the corrosion resistance for application and minimize the edge defects.

Hereinafter, the present invention will be described in detail.

The martensitic stainless steel according to the present invention contains 0.1 to 1.2% of carbon (C), 11 to 16% of chromium (Cr), 0.01 to 2.0% of nitrogen, 0.01 to 2.0% of molybdenum (Mo) To 2.0%, tungsten (W): 0.1 to 2.0%, the balance Fe and other unavoidable impurities.

Hereinafter, the reasons for limiting the constituent conditions of the steel of the martensitic stainless steel of the present invention will be described in detail.

C: 0.1 to 1.2%

Carbon (C) is a very effective element for improving the hardness of stainless steel. When the content of C is less than 0.1%, it is difficult to apply to the distribution furnace even if the hardness is low, while when it exceeds 1.2% there is a problem.

Cr: 11 to 16%

Chromium (Cr) is an element added to improve the corrosion resistance of steel. When the content of Cr is less than 11%, it is difficult to secure the corrosion resistance of stainless steel. On the other hand, if it exceeds 16% .

N: 0.01 to 2.0%

Nitrogen (N) is an element added to improve hardness and corrosion resistance of steel. When the content is less than 0.01%, it is difficult to obtain the above effect. On the other hand, when the content exceeds 2.0%, there is no problem in securing high hardness. There is a problem that the main composition is deteriorated.

Mo and W: 0.1 to 2.0%

Molybdenum (Mo) and tungsten (W) are important constituent elements for improving the corrosion resistance of steel. When the content of these elements is less than 0.1%, it is difficult to obtain the above effect. On the other hand, The hardness is excessively increased and the brittleness is strengthened, so that the casting composition is deteriorated.

In martensitic stainless steels, in order to obtain excellent corrosion resistance, it is necessary to control the distribution of carbides in addition to the above-mentioned component compositions. That is, since the distribution of carbide in the steel is fine and uniform, the carbide dissolves well as a base material in the heat treatment process of the final product, and the final hardness can be secured.

In order to control the carbide distribution in the present invention, it is preferable to control the Cr content in accordance with the content of C and the content of N in the steel.

That is, the degree of cohesion of carbide can be controlled by controlling the Cr / (C + N) ratio. At this time, the ratio of Cr / (C + N) satisfies a value of 23 or more to 28 or less. When the value is less than 23, aggregation of carbide is seriously observed. On the other hand, It is difficult to obtain high quality logistics.

In addition, the fact that the martensitic stainless steel of the present invention produced by the twin roll thin sheet casting process exhibits excellent corrosion resistance can be expressed as an official resistance index (PREN index) representing the corrosion resistance.

That is, the martensitic stainless steel according to the present invention preferably has a PREN index value of 17 or more expressed by the following relational expression (1). When the PREN index value satisfies 17 or more, excellent corrosion resistance can be secured.

<Relationship>

PREN index (official resistance index) = Cr + 3.3 (Mo + 0.5W) + 16N

Hereinafter, a method for producing a martensitic stainless steel according to the present invention will be described in detail.

A method for producing a martensitic stainless steel, which is one aspect of the present invention, includes the steps of forming a thin plate by injecting molten steel satisfying the above composition into the space between the casting rolls, rolling the thin plate by a rolling roll to produce a thermal laminate .

In the present invention, when the above-described twin roll type thin sheet casting process is applied, a casting roll is first processed to give roughness to the surface of the casting roll. At this time, a shot blast is used for processing.

Shot blast generally refers to a processing method in which steel particles or silica particles are collided against a subject to process the surface. The impact surface is not limited to the shape and size of the projection surface, It is a method that can also process the inner surface.

When the surface of the casting roll is processed using the above-described method, it is preferable to control the distribution of the surface roughness of the casting rolls in a differential manner.

That is, in controlling the roughness of the middle portion and the edge portion of the casting roll differentially, the middle portion is controlled to have an illuminance of 15 to 20 μm on the basis of Ra on the basis of the width direction of the roll Preferably, the roughness of the edge of the edge portion is controlled to be 0.75 to 0.95 times as great as the roughness of the middle portion, and the roughness of the edge portion is controlled to be linear .

The edge portion refers to a section of 30 to 250 mm from the end of the roll toward the middle portion and the middle portion refers to a section between 0 to 30 mm of both ends of the roll and an edge of 30 to 250 mm from both ends of the roll. (edge) portion. At this time, the roughness of the edge portion of the roll, 0 to 30 mm, has the same roughness value as the roughness of the edge of the edge portion.

The reason why the surface roughness of the casting roll is differentially applied as described above is to prevent edge cracking or edge wave in the final product after the twin roll type thin plate casting process, which can be confirmed from FIG. 2 to FIG.

Figs. 2 and 3 are graphs showing the results obtained when casting a steel satisfying the composition of the present invention (the invention of Table 1 below) in the twin roll type thin sheet casting process, the edge of the casting roll edge (located at 30 mm from the roll end (A part of the surface roughness distribution).

First, as shown in FIG. 2A, when the illuminance of the edge of the edge portion is less than 0.75 times as much as the middle portion and the illuminance gradient is large (surface roughness less than about 12.5, see FIG. 4) It was confirmed that edge coagulation of the edge portion was excessive and edge cracking or breakage occurred on the surface of the steel sheet. On the other hand, as shown in FIG. 2B, when the illuminance gradient is more than 0.95 times (surface roughness exceeding about 16.5, see FIG. 4), a temperature difference occurs between the middle portion and the edge portion And the solidification of the edge portion is delayed, thereby causing a problem of impairing the safety of casting such as edge bulging.

As shown in Fig. 3, when the edge roughness of the edge portion is (A) not satisfactory to the present invention, it can be visually confirmed that the edge portion is in poor condition. On the other hand, In the case of (B), it can be seen that the edge condition is good.

Therefore, when the surface roughness of the casting roll is differently applied, it is possible to suppress defects due to edge coagulation unevenness such as edge cracks or edge waves which have been caused by the existing thin plate casting process.

The molten steel satisfying the composition according to the present invention is injected through a nozzle between a pair of casting rolls surface-worked by the above-described method, and rolled into rolls to produce a thin plate. The thus produced thin plate is hot-rolled by a rolling roll in an inline mill (IRM) to produce a martensitic stainless steel hot-rolled steel sheet having a thickness of 1 to 5 mm.

In order to produce the hot-rolled steel sheet as a final product, a hot rolling process or a cold rolling process may be applied.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example  One)

Hot rolled steel sheets were continuously produced by casting molten steel having the composition shown in Table 1 (weight%) in a twin roll type thin plate casting process. The width of the cast thin plate was 1300 mm and the thickness was 2.9 mm.

Then, the degree of carbide distribution and the hardness were observed with respect to the hot-rolled sheet, and the results are shown in Table 1 below.

fair C Cr Mo W V N PREN Cr / (C + N) Carbide
Distribution Hardness Conventional example
(DIN 1.4116) CC 0.55 14.3 0.6 0 0.15 0.03 16.76 24.7 ○ ○ Comparative Example 1 SC 0.40 13.0 0.5 0.5 - 0.03 15.96 30.2 ○ X Comparative Example 2 SC 0.40 14.0 0.5 0.5 - 0.03 16.96 32.6 ○ X Comparative Example 3 SC 0.40 15.0 0.5 0.5 - 0.03 17.96 34.9 ○ X Inventory 1 SC 0.40 13.0 0.5 0.5 - 0.10 17.08 26.0 ○ ○ Inventory 2 SC 0.40 14.0 0.5 0.5 - 0.10 18.08 28.0 ○ ○ Comparative Example 4 SC 0.40 15.0 0.5 0.5 - 0.10 19.08 30.0 ○ X Comparative Example 5 SC 0.45 13.0 0.5 0.5 - 0.03 15.96 27.1 ○ ○ Comparative Example 6 SC 0.45 14.0 0.5 0.5 - 0.03 16.96 29.2 ○ X Comparative Example 7 SC 0.45 15.0 0.5 0.5 - 0.03 17.96 31.3 ○ X Inventory 3 SC 0.45 13.0 0.5 0.5 - 0.10 17.08 23.6 ○ ○ Honorable 4 SC 0.45 14.0 0.5 0.5 - 0.10 18.08 25.5 ○ ○ Inventory 5 SC 0.45 15.0 0.5 0.5 - 0.10 19.08 27.3 ○ ○ Inventory 6 SC 0.45 13.0 0.5 1.0 - 0.10 17.90 23.6 ○ ○ Honorable 7 SC 0.45 14.0 0.5 1.0 - 0.10 18.90 25.5 ○ ○ Honors 8 SC 0.45 15.0 0.5 1.0 - 0.10 19.90 27.3 ○ ○ Comparative Example 8 SC 0.50 13.0 0.5 1.0 - 0.03 16.78 24.5 ○ ○ Proposition 9 SC 0.50 14.0 0.5 1.0 - 0.03 17.78 26.4 ○ ○ Comparative Example 9 SC 0.50 15.0 0.5 1.0 - 0.03 18.78 28.3 ○ X Comparative Example 10 SC 0.50 13.0 0.5 0.5 - 0.10 17.08 21.7 X ○ Inventory 10 SC 0.50 14.0 0.5 0.5 - 0.10 18.08 23.3 ○ ○ Exhibit 11 SC 0.50 15.0 0.5 0.5 - 0.10 19.08 25.0 ○ ○ Comparative Example 11 SC 0.50 13.0 0.5 1.0 - 0.10 17.90 21.7 X ○ Inventory 12 SC 0.50 14.0 0.5 1.0 - 0.10 18.90 23.3 ○ ○ Inventory 13 SC 0.50 15.0 0.5 1.0 - 0.10 19.90 25.0 ○ ○ Comparative Example 12 SC 0.55 14.0 0.5 0.5 - 0.03 16.96 24.1 ○ ○ Inventory 14 SC 0.55 15.0 0.5 0.5 - 0.03 17.96 25.9 ○ ○ Comparative Example 13 SC 0.55 13.0 0.5 1.0 - 0.03 16.78 22.4 X ○ Honorable Mention 15 SC 0.55 14.0 0.5 1.0 - 0.03 17.78 24.1 ○ ○ Inventory 16 SC 0.55 15.0 0.5 1.0 - 0.03 18.78 25.9 ○ ○ Comparative Example 14 SC 0.55 13.0 0.5 0.5 - 0.10 17.08 20.0 X ○ Comparative Example 15 SC 0.55 14.0 0.5 0.5 - 0.10 18.08 21.5 X ○ Inventory 17 SC 0.55 15.0 0.5 0.5 - 0.10 19.08 23.1 ○ ○ Comparative Example 16 SC 0.55 13.0 0.5 1.0 - 0.10 17.90 20.0 X ○ Comparative Example 17 SC 0.55 14.0 0.5 1.0 - 0.10 18.90 21.5 X ○ Inventory 18 SC 0.55 15.0 0.5 1.0 - 0.10 19.90 23.1 ○ ○ Comparative Example 18 SC 0.64 13.2 0.5 1.0 - 0.03 16.98 19.7 X ○ Comparative Example 19 SC 1.00 17 0.5 1.0 - 0.03 20.78 16.5 X ○

* Carbide distribution: Good (good), Good (poor)

* Hardness: Good (Good), Good (Bad)

As shown in Table 1, it can be confirmed that it is difficult to secure corrosion resistance in the conventional example in which the hot-rolled sheet was produced by the conventional ingot casting method.

Further, in the case where the content of N does not satisfy the requirements of the present invention and the ratio of Cr, C and N exceeds 28 (Comparative Examples 1 to 3, 4, 6, 7 and 9) But the hardness was poor. In the case where the ratio of Cr, C and N was less than 23 (Comparative Examples 10, 11 and 13 to 19), hardness was good but distribution of carbide was poor. Also, in Comparative Examples 5, 8, and 12, the PREN index was measured to be less than 17, making it difficult to secure the corrosion resistance required for the martensitic stainless steel.

On the other hand, Inventive Examples 1 to 18 satisfied all of the component ranges proposed in the present invention, and it was possible not only to secure excellent corrosion resistance, but also to ensure that the carbide distribution was fine and uniform and the final hardness was high.

( Example  2)

In order to observe the state of the edge of the cast strip according to the casting roll surface roughness distribution when the cast steel was produced by the twin roll type thin plate casting process, casting roll surface roughness distribution was measured for molten steel according to the inventive example 1 The casting process was applied differently. At this time, as shown in Fig. 4, the roughness distribution on the casting roll surface was set to a range of 12 to 17 mu m on the basis of Ra on the basis of Ra at the edge of the edge of the casting roll.

After the casting process, the state of the edges of the produced cast steel was observed and shown in Figs. 2 and 3. Fig.

When the hot rolled steel sheet is manufactured by casting the molten steel of Inventive Example 1 of Table 1 in the twin roll type thin plate casting process, when the roughness of the edge of the edge of the casting roll is about 12 μm, as shown in FIG. It can be confirmed that edge cracking or rupture occurs on the surface of the hot rolled plate. Further, when the same molten steel is cast to produce a hot rolled plate, when the roughness of the edge of the edge of the casting roll is about 16.5 占 퐉, as shown in Fig. 2 (B) it was confirmed that the temperature difference of the edge part occurred and the solidification at the edge part was relatively delayed.

That is, when the surface roughness distribution of the casting roll in the twin roll thin plate casting process is processed to satisfy the present invention, no edge defect of the steel sheet occurs as shown in FIG. 3 (B) , And if the present invention is not satisfied, a defect in a steel sheet edge portion occurs as shown in Fig. 3A.

The above results show that in the case of producing martensitic stainless steel through the twin roll thin plate casting process, in order to prevent not only excellent corrosion resistance but also defects at the edge of the steel sheet, the composition of the steel sheet as well as the casting process conditions are controlled To provide a martensitic stainless steel suitable for application to high grade logistics or air detention.

1: Ladle;
2: tundish;
3: injection nozzle;
4: Maniscus shield;
5: edge dam;
6: casting roll;
7: roll nip;
8: Lamination;
9: rolling mill (IRM);
10: Coil winding facility.

Claims (3)

It is manufactured by twin roll sheet metal casting process,
By weight%, carbon (C): 0.1-1.2%, chromium (Cr): 11-16%, nitrogen (N): 0.01-2.0%, molybdenum (Mo): 0.1-2.0%, tungsten (W): 0.1 ˜2.0%, remainder Fe and other unavoidable impurities, the composition satisfies 23 ≦ Cr / (C + N) ≦ 28,
Martensitic stainless steel characterized by the above-mentioned PREN index of 17 or more.
<Relationship>
PREN index (official resistance index) = Cr + 3.3 (Mo + 0.5W) + 16N
The casting rolls may contain 0.1 to 1.2% of carbon (C), 11 to 16% of chromium (Cr), 0.01 to 2.0% of nitrogen (N), 0.1 to 2.0% of molybdenum (Mo) W): 0.1 to 2.0%, the balance Fe and other unavoidable impurities, and the composition satisfies 23? Cr / (C + N)? 28; And rolling the sheet in a rolling mill to produce a thermal laminate,
The surface roughness distribution of the casting roll is 0.75 ~ 0.95 times the roughness of the end of the edge (edge) compared to the middle (middle),
Wherein the roughness in the edge portion linearly increases from the end of the roll toward the middle portion.
3. The method of claim 2,
Wherein the edge portion is 30 to 250 mm from the end of the roll.

Images