KR101243259B1 - Martensitic stainless hot rolled steel strip and manufacturing method thereof - Google Patents

Abstract

The present invention is produced by a twin roll sheet metal casting process, consisting of by weight of C: 0.1 to 1.5%, Cr: 12 to 15%, Ni: 1% or less, balance Fe and other unavoidable impurities, grain boundary The martensitic stainless steel hot rolled sheet, characterized in that the average size of the chromium carbide deposited on the substrate is 5 µm or less, and 30 or more chromium carbides having a size of 5 µm or less, per 100 µm ² area, rotates in the opposite direction. Injecting molten steel between a pair of casting rolls to cast the martensitic stainless steel sheet, hot rolling the cast sheet in an in-line rolling mill, and using a cooling device immediately after the hot rolled sheet is blown into a winding machine. It provides a method for producing a martensitic stainless steel hot rolled sheet comprising the step of cooling and winding the thin plate injected into the winder.
According to the present invention, by applying a twin-roll thin sheet casting process to the production of martensitic stainless steel hot-rolled sheet, and rapid cooling during hot-rolled thin sheet winding to prevent the center segregation, cracks and breakage during casting to ensure casting stability, By forming uniformly distributed microstructure in the steel, it is possible to manufacture products with high hardness and edge quality when producing ceramics or tools.

Description

Martensitic stainless steel hot rolled sheet and its manufacturing method {MARTENSITIC STAINLESS HOT ROLLED STEEL STRIP AND MANUFACTURING METHOD THEREOF}

The present invention relates to a martensitic stainless steel hot rolled sheet manufactured by a twin roll type sheet casting process, and a method of manufacturing the same, and more particularly, to suppress center segregation, cracking, and sheet breakage during casting, thereby increasing casting stability, hardness, and edge quality. The martensitic stainless steel hot rolled sheet which can manufacture the product of this excellent ceramics, and its manufacturing method are related.

In general, martensitic stainless steel is used in the manufacture of various tools and ceramics because of its excellent corrosion resistance, hardness and abrasion resistance.

In the case of manufacturing such martensitic stainless steel by the continuous casting process, the higher the carbon content, the more severe coarse central segregation is formed in the center of the slab, and the solid-liquid coexistence area is wider, which is weak to castability. After the manufacture of re-heated and hot rolled to produce a hot rolled coil, after the pickling process through the annealing process is cold rolled to produce martensitic stainless steel.

However, even when the ingot casting method is used, coarse central segregation is formed in the slab due to the slow cooling rate, and the central segregation is hardly removed in a subsequent heat treatment process and remains in the hot-rolled steel sheet so that the lamination is performed during the cutting of the strip. It is accompanied by a defect, coarse chromium carbide (chromium carbide) is precipitated at the grain boundary and cracks or plate breakage occurs in the steel sheet during the post-treatment process.

In order to solve the above problems, techniques for increasing the annealing temperature and maintaining the annealing time for a long time in a batch annealing process (BAF) after hot rolling to employ carbide are known, There is a problem that the production cost is increased, the productivity decreases sharply.

In addition, low-temperature casting and low-speed casting methods have been proposed, but these methods form granular equiaxed structure at the center of cast steel and rapidly form cast solidification layer to reduce central segregation, but the nozzle is clogged during casting and operation is unstable and productivity is reduced. There is a problem.

Therefore, there has been a demand for development of a high quality martensitic stainless steel hot rolled sheet and a method of manufacturing the same, in which the segregation of the center is suppressed and the chromium carbide is finely deposited at the grain boundaries to ensure casting stability due to excellent crack resistance.

The present invention has been made to solve the above-mentioned problems, by applying a twin-roll thin sheet casting process to the production of martensitic stainless steel hot rolled sheet, by rapid cooling during hot rolled thin sheet winding, the center segregation, cracks and plate breaking during casting To ensure casting stability and form a uniformly distributed microstructure in steel to provide martensitic stainless steel hot rolled sheet with high hardness and excellent edge quality in the manufacture of ceramics and tools. It is done.

In order to achieve the above object, the present invention is produced by a twin roll sheet metal casting process, by weight% C: 0.1 ~ 1.5%, Cr: 12 ~ 15%, Ni: 1% or less, the balance Fe and other unavoidably added Martensitic stainless steel hot-rolled sheet, which is composed of impurities and has an average size of chromium carbide deposited at grain boundaries of 5 µm or less and 30 or more chromium carbides having a size of 5 µm or less per 100 µm ² area. To provide.

At this time, the martensitic stainless steel hot rolled sheet is characterized in that the center pore is removed.

In addition, injecting molten steel between a pair of casting rolls rotating in the opposite direction to cast a martensitic stainless steel sheet, hot rolling the cast sheet in an inline rolling mill, and the hot rolled sheet is blown into the take-up It provides a method for producing a martensitic stainless steel hot rolled sheet comprising the step of cooling and winding the thin plate injected into the winder immediately after the use.

Here, the molten steel is characterized by being made of C: 0.1 to 1.5% by weight, Cr: 12 to 15%, Ni: 1% or less, balance Fe and other inevitable impurities.

In addition, the hot rolling step, the method of producing a martensitic stainless steel hot rolled sheet, characterized in that the rolling rate of the inline rolling mill is 5 to 50%.

In addition, the winding step is characterized by the rapid cooling of the temperature of the thin plate injected into the winder 200 ° C or more.

At this time, the winding step, it is also characterized in that the cooling device is controlled so that the temperature of the thin plate injected into the winder is 450 ~ 600 ℃.

According to the present invention, by applying a twin-roll thin sheet casting process to the production of martensitic stainless steel hot-rolled sheet, and rapid cooling during hot-rolled thin sheet winding to prevent the center segregation, cracks and breakage during casting to ensure casting stability, By forming uniformly distributed microstructure in the steel, it is possible to manufacture products with high hardness and edge quality when producing ceramics or tools.

1 is a configuration diagram of a twin roll thin plate casting process.
2 is a chromium carbide photograph deposited at the grain boundaries of martensitic stainless steel hot rolled sheet;
Figure 3 is a photograph of the crack generation of the martensitic stainless steel hot rolled sheet during casting.
Figure 4 is a photograph of the central pore in the cross-sectional structure of the martensitic stainless steel hot rolled sheet according to the hot rolling rate, (a) is a picture when not hot rolled, (b) is a picture when 25% hot rolling applied.
Figure 5 is a block diagram of a manufacturing process of martensitic stainless steel hot rolled sheet according to an embodiment of the present invention.
6 is a schematic diagram illustrating a rapid cooling process according to an embodiment of the present invention.
7 is a photograph of chromium carbide precipitated at the grain boundaries of martensitic stainless steel hot rolled sheet, (a) is a picture when cooling between the in-line rolling mill and the winder, (b) is a picture at the time of rapid cooling in the winder.
8 is a graph showing the maximum size of chromium carbide before and after rapid cooling by the cooling device of the invention steel 3.
9 is a flowchart illustrating a method of manufacturing a martensitic stainless steel hot rolled sheet according to the present invention.

Hereinafter, the martensitic stainless steel hot rolled sheet of the present invention and a method of manufacturing the same will be described in detail with reference to the drawings.

The martensitic stainless steel hot rolled sheet according to the present invention is composed of C: 0.1 to 1.5% by weight, Cr: 12 to 15%, Ni: 1% or less, balance Fe and other inevitable impurities.

The reason for limiting the composition range of the above-mentioned components is as follows.

The C is a very effective element for improving the hardness of stainless steel, when the content of C is less than 0.1% by weight, the hardness required for martensitic stainless steel cannot be secured, and when the content of C exceeds 1.5% by weight, it is relatively coarse. Since the formation of chromium carbide increases the crack sensitivity and reduces the corrosion resistance, the content of C is limited to 0.1 to 1.5% by weight.

The Cr is added to improve the corrosion resistance, when the content of Cr is less than 12% by weight, the effect of improving the corrosion resistance is insignificant, and when the content of Cr is more than 15% by weight, the corrosion resistance is improved, but the strength is high and the elongation is low, the workability is reduced. And because the cost is relatively high, the Cr content is limited to 12 to 15% by weight.

When Ni is added as a gamma (γ) phase generating element, the γ phase increases, and when the coil is air-cooled after hot rolling, martensite phase is promoted to increase strength and hardness, while elongation decreases. It is preferable to limit it to 1 weight% or less.

As described above, the martensitic stainless steel hot rolled sheet according to the present invention is manufactured using a twin roll thin sheet casting process. As shown in FIG. 1, the twin roll thin sheet casting process is a molten steel between a pair of rotating casting rolls 3. (4) is a facility to continuously manufacture thin products of several mm thickness directly from the molten steel.

That is, the molten steel 4 having a predetermined component injected through the nozzle 2 between the casting rolls 3 cooled while rotating in the opposite direction is solidified to form a solidification shell, which is pressed down on the roll chuck to form the thin plate 7. Cast. The thin plate 7 thus cast is guided by a pinch roll 10 and hot rolled by a rolling roll in an inline rolling mill (IRM) 11 and wound by a winder 12 to produce a martensitic stainless steel sheet. Will be.

When manufacturing martensitic stainless steel hot rolled sheet by conventional continuous casting process or ingot casting method, central segregation is formed, linear defects and plate separation occur, coarse chromium carbide precipitates at grain boundary, There was a problem that cracks or plate breakage occurred, but in the case of manufacturing the martensitic stainless steel hot rolled sheet by the twin roll type sheet casting process, a squeezing flow is generated while the molten steel near the roll nip is pressed, so that As the molten steel in the section where the concentration occurs is squeezed out, not only the central segregation is removed but also the cooling speed of solidifying the molten steel is faster, so that the grain size of the grain boundary is reduced and the precipitates of chromium carbide are reduced. By suppressing the casting stability can be secured.

As such, the martensitic stainless steel hot rolled sheet produced by the twin-roll thin plate casting process is removed from the center segregation as compared with the conventional casting method, and as shown in FIG. And plate break is suppressed, but as shown in the photo of Figure 3, the chromium carbide may act as a cause of cracks and plate break during casting should be suppressed as much as possible.

To this end, the present invention first performs a step (S10) of casting a martensitic stainless steel sheet by injecting molten steel between a pair of casting rolls rotating in the opposite direction.

The cast thin plate 7 performs the step S20 of hot rolling in the in-line rolling mill 11, that is, the cast thin plate 7 is guided by the pinch roll 10 to the in-line rolling mill 11. It is hot rolled by the rolling rolls inside, and it is manufactured from a martensitic stainless steel hot rolled sheet. At this time, when the hot rolling is not applied, since the elongation is not secured and is so brittle that it is difficult to perform the post-process, the hot rolling is essentially performed in the manufacturing method according to the present invention.

At this time, it is preferable to roll at a rolling rate of 5 to 50%, but when the rolling rate is less than 5%, pores are formed in the center, and the thin plate is embrittled by the pores, and there is a problem that the elongation is low, and the rolling rate exceeds 50%. This is because the cost increases in the construction of the equipment.

4 is a comparison of the central pore picture in the cross-sectional structure according to the rolling rate of hot rolling of martensitic stainless steel, pores are generated in the equiaxed crystal region when the hot rolling is not applied as shown in FIG. Figure 4 (b) applying a rolling rate of 25 can be confirmed that all pores are removed in the equiaxed crystal region.

After performing the hot rolling step, performing the step (S30) of winding the hot rolled thin plate by a winding machine. Here, the winding step is blown into the winder 12 using the cooling device 13 immediately after the hot-rolled thin plate is blown into the winder 12, as shown in FIG. Cool the sheet.

In the twin roll type sheet casting process for manufacturing a stainless steel in general, as shown in Figure 1, after injecting molten steel between a pair of casting rolls rotating in the opposite direction to cast the martensitic stainless steel sheet (7), the inline rolling mill It is rolled through (11) and then wound on the winding machine 12 at a high temperature of 800 ° C. or higher. The inline rolling mill 11 and the winding machine 12 are used to control troubles of subsequent line equipment and to prevent winding shape defects. Cooling device (not shown) is installed between the) and by the cooling device performs a step of winding the hot rolled thin plate to 600 ~ 700 ℃ by winding the winder (12).

However, in the case of the martensitic stainless steel hot rolled sheet according to the present invention containing high carbon, when cooled by a cooling device (not shown) provided between the inline rolling mill 11 and the winder 12, the edge portion of the hot rolled sheet Since the temperature falls below the martensite transformation start temperature (Ms) and becomes brittle, the fracture occurs. In the present invention, the cooling water other than the minimum cooling water for protecting the equipment between the in-line rolling mill 11 and the winding machine 12 is used. Do not use

However, in the case of the martensitic stainless steel hot rolled sheet having a high carbon blown into the winder 12, the cooling time is taken for a long time during winding, so that the chromium carbide (chromium carbide) is excessively formed or grown and coarsened to make the chromium carbide It is not expected that the refinement of the present invention, therefore, the present invention, as shown in Figures 5 and 6, the cooling installed on the winder 12 side immediately after the hot-rolled thin plate is blown into the winder 12 The apparatus 13 is used to cool the hot rolled sheet blown into the winder 12.

At this time, the cooling device 13 is arranged in a plurality of spray pipes with a plurality of nozzles to cool the hot rolled sheet wound by spraying the cooling water, in order to prevent the further generation and coarsening of chromium carbide winding machine 12 It is preferable to rapidly cool the temperature of the hot rolled sheet in the high temperature state of 800 degreeC or more blown into 200 degreeC or more.

As such, by controlling the cooling water injection speed and the injection amount of the cooling device 13, the hot rolled sheet in a high temperature state blown into the winder is rapidly cooled to have a temperature of 450 to 600 ° C. at which the temperature is directly above the martensite transformation start temperature (Ms). Control as possible. If the temperature of the hot-rolled sheet is less than 450 ℃ cracks and plate breakage may occur due to the further generation and coarsening of chromium carbide, if the temperature of the hot-rolled sheet exceeds 600 ℃ causes trouble or winding of the subsequent line This is because there is a problem of poor shape.

The hot rolled sheet thus manufactured is martensitic stainless steel by performing an annealing (BAF) process in a reducing gas atmosphere to stabilize the solid solution of chromium carbide, and performing cold rolling after pickling after finishing the annealing process. Transformed into

On the other hand, the martensitic stainless steel hot-rolled sheet produced by the manufacturing method of the present invention is characterized in that the chromium carbide precipitated at the grain boundary is fine, as shown in the photograph of FIG. 7, by the cooling device 13 The chromium carbide precipitated at the grain boundaries of the rapidly cooled martensitic stainless steel hot-rolled sheet has a band shape, and the grain size is finely distributed so that the grain boundary is strengthened so that cracks and plate breaks are suppressed during casting, thereby improving completion rate.

At this time, the chromium carbide is mainly chromium carbide, as shown in Figure 8, the size is 5㎛ or less, most of the diameter is uniformly distributed in the size of 0.1 ~ 3.0㎛. In addition, the chromium carbide having a size of 5 μm or less has a high hardness of 100 to 300 Hv by forming 30 or more microstructures per 100 μm ² , and martensitic stainless steel having excellent edge quality when manufacturing tools or ceramics. Steel can be produced.

[Example]

The molten steel component injected between the two rolls contains C: 0.67% and Cr: 13.2% by weight, and an appropriate amount of Si and Mn is included therein, and the P and S components were managed to minimize segregation. Then, the casting width was 1,300mm, the molten steel was cast by 100 tons by a twin roll type sheet casting machine to produce martensitic stainless steel sheet, and the sheet was rolled by an inline rolling mill immediately after casting to continuously produce a hot rolled sheet, At this time, the rolling conditions and the winding conditions shown in Table 1, after winding the winding machine, the average size of chromium carbide precipitated at the grain boundary of the manufactured hot rolled coil, the number of chromium carbides of 5㎛ or less and the crack generation state again Table 1 Shown in

[Table 1]

As shown in Table 1, in the case of cooling only by the cooling device installed between the in-line rolling mill and the winder as in Comparative Steels 1 and 2, the temperature of the thin plate injected into the winder was high, and chromium carbide was coarsened at the grain boundaries. Therefore, the crack generation state was bad.

In addition, the comparative steel 3, which was not applied with the rolling rate, could not easily secure the elongation, so that the thin plate was easily embrittled and cracks propagated along the grain boundary, resulting in a poor state of cracking. The comparative steels 4 and 5 produced chromium carbide at the grain boundary and became coarse, resulting in poor cracking. Cooling immediately after being blown into the winder, however, when the temperature of the sheet was relatively high at 650 ° C, many cracks were not generated. However, it was confirmed that the condition is difficult to apply due to the problem of poor winding shape.

On the other hand, when the coolant is not used between the in-line rolling mill and the winding machine and is rapidly cooled under the temperature conditions corresponding to the scope of the present invention immediately after being blown into the winding machine, the average size of chromium carbide precipitated at grain boundaries is 5 µm or less. The number of chromium carbides less than 5㎛ per 100㎛ ² area was found to form more than 30 microstructures, due to the good crack generation state is suppressed plate breakage, martensite excellent edge quality It was confirmed that the stainless steel can be produced.

As a result, in the case of manufacturing martensitic stainless steel hot rolled sheet according to the present invention, rapid cooling during hot rolled thin sheet winding prevents segregation and cracking and plate breakage during casting to ensure casting stability and uniformly distributed in steel. By forming a structure, it is possible to manufacture a product having high hardness and edge quality when manufacturing ceramics or tools.

1: tundish 2: nozzle
3: casting roll 4: molten steel
5: edge dam 6: brush roll
7: thin sheet 8: loop fit
9: meniscus shield 10: pinch roll
11: in-line rolling mill (IRM) 12: winding machine
13: chiller

Claims (7)

A twin roll type sheet casting machine having a pair of casting rolls and casting a thin plate from molten steel for manufacturing martensitic stainless steel injected between the casting rolls; And
An inline rolling mill for hot rolling the thin plate;
A winder for winding the hot rolled sheet; And
Apparatus for producing a martensitic stainless steel hot rolled sheet, characterized in that it comprises a cooling device for cooling the thin plate injected into the winder. The method of claim 1,
The rolling rate of the rolling mill is 5 to 50%,
The cooling device is an apparatus for producing martensitic stainless steel hot rolled sheet, characterized in that for cooling the thin plate so that the temperature of the thin plate injected into the winder is 450 ~ 600 ℃. Casting molten steel between a pair of casting rolls rotating in opposite directions to cast martensitic stainless steel sheets;
Hot rolling the cast thin plate in an in-line rolling mill;
Winding the hot rolled thin plate through a winder, and cooling the thin plate blown into the winder. The method of claim 3,
The molten steel is C: 0.1 to 1.5% by weight, Cr: 12 to 15%, Ni: 1% or less, the balance of Fe and other unavoidably added impurities manufacturing method of martensitic stainless steel hot rolled sheet . The method of claim 3,
The hot rolling step, the method for producing a martensitic stainless steel hot rolled sheet, characterized in that the rolling rate of the inline rolling mill is 5 to 50%. The method of claim 3,
The winding step is a method for producing a martensitic stainless steel hot rolled sheet, characterized in that the cooling of the temperature of the thin plate injected into the winder 200 ° C or more. The method of claim 3,
The winding step, the martensitic stainless steel hot rolled sheet, characterized in that for controlling the cooling device so that the temperature of the thin plate injected into the winder is 450 ~ 600 ℃.

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