KR20190072321A - Method for manufacturing high copper stainless steel having good surface quality - Google Patents

Abstract

The present invention provides a method for manufacturing high-Cu stainless steel. According to an aspect of the present invention, the method for manufacturing high-Cu stainless steel has excellent surface quality and reduces manufacturing costs by using a twin-roll type thin film casting process to manufacture stainless steel by containing a large amount of Cu. The method for manufacturing high-Cu stainless steel includes: a step of manufacturing a strip; a step of obtaining a hot-rolled material; a step of annealing and pickling the hot-rolled material; and a step of cold-rolling the hot-rolled material annealed and pickled.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a high-Cu stainless steel having good surface quality,

The present invention relates to a method of manufacturing stainless steel, and more particularly, to a method of manufacturing high-Cu stainless steel having good quality.

Copper (Cu) is added in order to reduce the yield strength of stainless steel and improve the formability. When the yield strength of stainless steel is reduced by Cu addition, springback phenomenon is reduced and workability such as bending increases, It is possible to substitute for the use as a pipe of Cu material in an electronic product such as a pipe.

Such Cu-added stainless steels were conventionally produced by continuous casting and slabs were hot-rolled to produce thin plates with a thickness of 2 to 6 mm.

However, when a stainless steel containing Cu added by 3.0% by weight or more of Cu is produced by the continuous casting process, a large amount of Cu precipitates are produced in the inside of the slab due to a slow cooling and cooling rate, There is a problem that the rate of occurrence of water drops is reduced due to defects such as deteriorated hot workability and occurrence of edge cracks. In addition, when Cu is added in an amount of 4% or more, there is a problem that the steel itself is ineffective (see FIG. 2).

Therefore, in order to obtain the effect by the addition of Cu, the development of a manufacturing method capable of securing excellent surface quality without defects such as cracks in manufacturing a high Cu stainless steel containing a large amount of Cu This is a required situation.

An aspect of the present invention is to provide a method for producing a high-Cu stainless steel having a good surface quality while reducing manufacturing costs in the production of stainless steel containing a large amount of Cu.

The problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

One aspect of the present invention relates to a method for producing a strip, comprising: preparing a molten steel containing at least 4% by weight of copper (Cu), and injecting the molten steel into a pair of casting rolls rotating in opposite directions to form a strip; Hot rolling the strip to obtain a thermal laminate; Annealing and pickling the thermal expansion material; And a step of cold-rolling the annealed and pickled heat treated steel, wherein the superheated degree of the molten steel is 92 ° C or less, and the surface quality is good.

According to the present invention, manufacturing cost is reduced by manufacturing a high-Cu stainless steel by a twin roll thin plate casting method.

Further, the high-Cu stainless steel produced by the twin roll thin sheet casting method can have good surface quality without defects such as surface cracks.

1 is a schematic view showing a twin roll thin plate manufacturing process.
Fig. 2 is a photograph showing the quality of high Cu stainless steel produced under conventional conditions using a twin roll thin plate casting method.
FIG. 3 is a photograph of a surface unevenness of a casting roll according to an embodiment of the present invention.
4 is a photograph of a cast steel of Comparative Example 1 according to an embodiment of the present invention.
FIG. 5 is a photograph showing a cast of Inventive Example 1 according to an embodiment of the present invention.

The inventors of the present invention have found that it is effective to use a thin plate casting method using a twin roll type casting machine in order to solve the problems that occur in the production of stainless steel by the conventional continuous casting method.

A method of manufacturing a stainless steel using a twin roll type thin plate casting machine is a method of directly casting a cast steel having a thickness of 2 to 6 mm from a molten metal, and is a new process capable of omitting hot rolling in a slab playing method.

Therefore, it is advantageous in that the manufacturing cost is reduced as compared with the slab-making method, the precipitation phase is suppressed by rapid solidification, and the internal quality of the cast steel is also ensured by minification of inclusions.

As a result of the study by the inventors of the present invention, it has been found that when stainless steels containing a large amount of Cu are produced by using the twin roll type thin plate casting method, high-quality Cu stainless steels having a good quality compared with the continuous casting method can be obtained. (Liquid-Cu) in the molten steel was generated, thereby deteriorating the surface quality.

Specifically, it was found that Cu components exceeding the solubility limit in the molten steel generate surface liquid cracking along the weakened grain boundaries as a result of the formation of some liquid-Cu during the solidification process and concentration in the grain boundaries (see FIG. 2) . Fig. 2 is a mapping result of the cross-sectional components of the casting with surface cracks, and it can be seen that a liquid-Cu was formed along the grain boundaries around the cracks (the lighter the color, the greater the degree of segregation Lt; / RTI >

Accordingly, the inventors of the present invention have found that, even when copper (Cu) is contained in an amount of 4.0 wt% or more, stainless steel having a low yield strength can be produced without occurrence of edge cracks and surface cracks .

As a result, it was confirmed that high Cu stainless steels having a desired quality can be produced by optimizing the process conditions at the time of manufacturing high Cu stainless steels by using the twin roll type thin plate casting method. I have come to completion.

Hereinafter, the present invention will be described in detail.

A method for producing a high Cu stainless steel having good surface quality according to an aspect of the present invention includes preparing a molten steel containing copper (Cu) in an amount of not less than 4% by weight, To form a strip; Hot rolling the strip to obtain a thermal laminate; Annealing and pickling the thermal expansion material; And cold rolling the annealed and pickled heat treated strips.

First, in producing a strip containing molybdenum containing copper (Cu) in an amount of not less than 4% by weight using a twin roll type casting machine, in order to uniformly solidify over a full width so as to avoid defects such as surface cracks, It is possible to use a meniscus shield 150 which can completely cover the molten bath surface and can supply gas.

At this time, the atmospheric gas in the meniscus shield 150 is important, and the oxidation of the molten metal from the atmospheric gas is prevented, and the interface energy between the molten steel meniscus and the casting roll 110 at the time of solidification is changed, .

The atmospheric gas is not particularly limited as long as it can effectively prevent the oxidation of the molten metal. As an example, nitrogen (N ₂ ) can be used. By sufficiently supplying nitrogen, the oxygen concentration can be controlled to 0.1% There is a need.

On the other hand, molten steel containing copper (Cu) in an amount of 4 wt% or more is supplied into the casting rolls 110 for the production of the cast steel, and the superheating degree of the molten steel may be 92 ° C or less.

Since copper (Cu) has a melting point of about 1085 ° C, it is generally present in a liquid phase in molten steel, but when it is contained within the solubility limit range, it may exist in an employment state during the solidification process. If Cu is contained in an amount exceeding the solubility limit of the copper, the Cu is not dissolved in the steel, and Cu segregated at the dendrite boundary during solidification generates a liquid-Cu to induce grain boundary embrittlement, This is a starting point of cracking on the surface of the strip, and in severe cases, it also causes plate breakage.

Accordingly, in the present invention, when copper (Cu) is contained in an amount of 4 wt% or more, it is preferable to control the superheat degree of the molten steel to 92 캜 or less in order to suppress the formation of liquid-Cu during solidification.

That is, by reducing the degree of superheat of molten steel, the holding time to the solidification temperature is minimized, and the solidification rate can be further increased, thereby suppressing the formation of liquid-Cu.

The temperature of the molten steel is based on the value measured in the tundish, and the temperature of the molten steel can be set based on the degree of superheat. The superheat degree means a temperature at which the molten steel begins to solidify at the tundish temperature, that is, a value obtained by subtracting the solidification temperature.

In another aspect of the present invention, the surface of the casting roll 110 may have irregularities. Specifically, it is preferable that the surface roughness Ra of the casting roll 110 is 20 μm or more 3). The surface roughness of the casting roll can be controlled from the shot blasting process. More advantageously, the casting roll can have a surface roughness (Ra) of 20 to 25 占 퐉 based on the widthwise center portion.

When the molten steel is supplied into the casting rolls having the unevenness (roughness) formed on the surface by the shot blasting process, the molten steel is first brought into contact with the molten acid first, so that the rough acid becomes the nucleation site of the molten steel solidification. Therefore, by forming a roughening acid so as to have a surface roughness (Ra) of 20 m or more on the center of the casting roll in the width direction center, the solidifying cell can be sufficiently transferred to the roughing of the casting roll, Uniformity and uniform coagulation is caused, thereby making it possible to manufacture a defect-free strip.

The solidification rate by the above-described casting roll may be 100 ° C / s or higher. By applying such a rapid cooling rate, the segregation can be reduced and the generation of Cu precipitates can be suppressed. That is, even if Cu in the molten steel is contained in a large amount, a strip having good surface quality can be obtained.

The strip prepared as described above can be made into a hot laminate by a hot rolling mill. At this time, the hot rolling is preferably performed at a reduction rate of 10 to 50%.

If the reduction rate is less than 10% in the hot rolling, the core pores formed in the casting structure are not completely removed and cause defects in the final product. On the other hand, if the reduction ratio exceeds 50%, surface defects due to hot rolling may occur It is not preferable.

The thermal product thus produced can be obtained in a coil state by winding in a winder. The coil product is a black coil product having a scale on its surface, which is subjected to a subsequent annealing and pickling process, A coil can be obtained.

As described above, the annealing and the pickling process for removing the scale of the surface of the black coil product can be carried out according to ordinary conditions and can be carried out without difficulty by a person skilled in the art, but the conditions are not particularly limited.

Thereafter, the produced white coil can be cold-rolled to produce a cold rolled steel sheet.

The cold rolling can be performed at a reduction ratio of 75 to 85%. If the reduction rate exceeds 85%, there is a problem that the rolling load is increased and the strength is lowered due to coarsening of the grain size. On the other hand, if it is less than 75%, non-uniform rolling occurs, and an excessive time is required for the rolling process, resulting in a problem of deteriorating productivity.

After the cold rolling, a annealing heat treatment step may be further performed. This annealing may also be performed under ordinary conditions, for example, in a temperature range of 1100 to 1200 ° C.

On the other hand, the high-Cu stainless steel of the present invention, which is manufactured through the above-described processes, contains 0.08% or less (including 0%) carbon (C), 1.0% (Mn): not more than 1.5% (including 0%), chromium (Cr): 16 to 18%, nickel (Ni): 7 to 9%, copper (Cu): not less than 4% , The balance Fe and other unavoidable impurities.

In particular, the high-Cu stainless steel of the present invention can contain up to 6% of copper (Cu), and even if it contains such a large amount of Cu, it can have a good surface quality.

In the case of producing high Cu stainless steel from a conventional casting process, liquid-Cu is produced in the reheating process after casting, and cracking occurs at hot rolling due to embrittlement due to grain boundary segregation, resulting in very low productivity due to loss of execution rate . There is a problem that cracking occurs and plate breakage occurs during hot rolling and production of the product becomes impossible. In addition, even if the product is produced, the high-concentration Cu precipitates are present in a large amount, and the physical properties are poor. When these products are welded, the Cu precipitate is reused and regenerated as a liquid-Cu to weaken the weld heat affected zone (HAZ) There is a problem of making.

On the other hand, if the high-Cu stainless steel is produced by the twin-roll thin plate casting method according to the method proposed in the present invention, the generation of the liquid-Cu is minimized because the rapid cooling cools the liquid- , The time for diffusion and fusion of Cu atoms in the material is reduced, so that the amount of Cu segregation can be lowered to suppress Cu precipitates. In addition, since the degree of concentration of Cu is small, there is an effect of minimizing a problem caused by the produced Cu precipitate.

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 )

Each of the strips was produced by casting molten steel having the composition shown in the following Table 1 under the conditions shown in Table 2 using a twin roll type casting machine. At this time, the casting roll width A casting roll having a surface roughness of 20 mu m was used.

Steel grade Alloy composition (% by weight) C Si Cr Ni Cu N One 0.025 0.55 17.0 7.56 4.91 0.048 2 0.029 0.57 17.1 7.61 4.95 0.050 3 0.031 0.55 17.2 7.62 4.99 0.051 4 0.023 0.56 17.1 7.60 4.95 0.050 5 0.029 0.57 17.0 7.62 4.93 0.051 6 0.028 0.55 17.0 7.59 5.01 0.050

Steel grade Tundish molten steel temperature
(° C) Solidification temperature
(° C) Superheat of molten steel
(° C) Surface crack Index * (1 to 5) division One 1517 1403 114 4 Comparative Example 1 2 1513 1400 113 4 Comparative Example 2 3 1510 1399 111 3 Comparative Example 3 4 1491 1402 89 0 Inventory 1 5 1489 1401 88 0 Inventory 2 6 1492 1400 92 0 Inventory 3 * In surface crack index, 0 is best when surface cracking does not occur, and 5 is when surface cracking is very severe.

The occurrence of surface cracks was evaluated for the strips prepared according to each condition, and the results are shown in Table 2 above.

In this case, the surface crack index is measured by observing the surface of the cast material with a naked eye and a magnifying glass while setting the casting material coil on the inspection table and setting the Index value according to the number of cracks. In the present invention, the surface cracking index for the casting material length of 4 m was confirmed based on the following table.

Surface crack Index Surface crack number (EA) 0 0 One <3 2 <5 3 <7 4 <10 5 ≥10 The number of surface cracks is represented by the number of cracks having a size of 500 μm or more in the longitudinal direction

Comparative Examples 1 to 3 are examples in which casting conditions corresponding to the conventional superheating degree of molten steel were used. Examples 1 to 3 show examples of casting conditions satisfying the molten steel superheating degree proposed in the present invention.

As shown in Table 2, in the case where the superheating degree of molten steel containing copper (Cu) at 4% by weight or more is 111 ° C or more (Comparative Examples 1 to 3), cracks are generated on the surface of the cast material, When the temperature is lowered to 92 占 폚 or less (Examples 1 to 3), it can be confirmed that cracks do not occur on the surface of the cast material.

Fig. 4 is a photograph of the cast steel of Comparative Example 1, showing that surface cracks occurred in several places. On the other hand, according to Fig. 5 showing the cast steel of the inventive example 1, it can be confirmed that the surface crack did not occur.

100 ... twin roll casting machine
110 ... casting roll 120 ... ladle
130 ... tundish 140 ... immersion nozzle
150 ... Meniscus Shield 160 ... Brush Roll
170 ... edge dam 180 ... slope

Claims (7)

Preparing a molten steel containing at least 4% by weight of copper (Cu), and injecting the molten steel into a pair of casting rolls rotating in opposite directions to produce a strip;
Hot rolling the strip to obtain a thermal laminate;
Annealing and pickling the thermal expansion material; And
And cold rolling the annealed and pickled heat treated strips,
Wherein the superheating degree of the molten steel is 92 占 폚 or less.
The method according to claim 1,
Wherein the casting roll has a surface roughness of 20 mu m or more by a shot blasting process.
The method according to claim 1,
Wherein a soluble gas is supplied into the molten steel through a manifold shield when molten steel is injected into the casting rolls, and the soluble gas has a good surface quality in a nitrogen atmosphere.
The method according to claim 1,
Wherein the coagulation speed of the casting roll is 100 DEG C / s or more.
The method according to claim 1,
Wherein the hot rolling is performed at a reduction rate of 10 to 50%.
The method according to claim 1,
Further comprising a step of annealing the steel sheet after the cold rolling.
The method according to claim 1,
Wherein the stainless steel contains 0.08% or less of carbon (C), 1.0% or less of silicon (Si), 1.5% or less of manganese (Mn), 16-18% of chromium (Cr) To 9%, copper (Cu): 4% or more, nitrogen (N): 0.1% or less, balance Fe and other unavoidable impurities.

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