KR102034448B1 - Steel and method for the same - Google Patents

Abstract

The steel according to the embodiment of the present invention is formed on the base and the surface of the base, and includes a surface layer portion containing 0.5 wt% or more of nickel (Ni).
Therefore, the surface layer part of the steel material which concerns on embodiment of this invention contains 0.5 wt% or more of nickel. Thus, compared with the related art, the occurrence of sticking defects at the base portion during hot rolling can be reduced, and the quality of the stainless steel can be improved.

Description

Steel and its manufacturing method {STEEL AND METHOD FOR THE SAME}

The present invention relates to a steel material and a method for manufacturing the same, and more particularly to a steel material and a method for producing the same is suppressed the occurrence of surface defects.

In general, steels such as slabs, blooms, and billets manufactured by continuous casting become steels of desired thickness through a hot rolling process. In addition, according to the composition of the steel produced by continuous casting, the product after hot rolling is divided into carbon steel, stainless steel, and the like.

In general, when a steel such as carbon steel is heated, the surface of the steel is phase transformed into an austenite phase. At this time, the austenite phase is not sticky in its tissue characteristics, and is easily broken. When rolling (hot rolling) is performed, the austenite phase is crushed while being scaled by the reduction force, and thus is scattered around the steel. And, during rolling, the scale functions as a lubrication between the rolling roll and the steel surface, so that the scale is scattered around without being attached to the rolling roll.

By the way, in the case of steel for manufacturing ferritic stainless steel, even if it is heated for hot rolling, the surface of the steel is not austenitized, the ferrite phase is maintained. The ferrite phase is sticky or sticky in tissue. Therefore, even if rolling (hot rolling) is performed, no scale is generated or the amount thereof is small, and due to the sticky property of the structure, it does not crumble and adheres to the rolling roll as it is, so that the surface of the steel material The tearing problem occurs, and the resulting defect is called a sticking defect.

Thus, in order to suppress sticking defects of ferritic stainless steel, in order to prevent the surface from falling off during hot rolling, the heating temperature for the hot rolling is adjusted to a low level, the rolling reduction rate is reduced, or oil is applied to the surface of the steel. Have been attempting to spray them. Alternatively, during the production of molten steel for the production of ferritic stainless steel, Nb was added to reduce the Si content of inhibiting phase transformation or scale formation into austenite, or to improve hot strength so that the steel surface does not adhere to the rolling roll.

However, the above-described methods do not secure a sufficient amount of reduction, or because the method of modifying the component, even if the sticking defect is reduced, there is a limit that the customer can not provide a product having the desired component or strength.

Korean Patent Publication 2003-0092911

The present invention provides a steel material and a method of manufacturing the same in which occurrence of surface defects is suppressed.

The present invention provides a steel material and a manufacturing method thereof in which defect generation due to hot rolling is suppressed.

The present invention is a steel for the production of ferritic stainless steel, the base portion; And a surface layer portion formed on the surface of the matrix portion and containing 0.5 wt% or more of nickel (Ni).

The nickel (Ni) content of the surface layer portion is 1.8 wt% or more.

The thickness of the surface layer portion is preferably 2% to 8% of the thickness of the steel.

It is preferable that the thickness of the surface layer portion is 2% to 5% of the thickness of the steel.

Method for producing a stainless steel according to an embodiment of the present invention comprises the steps of preparing a molten steel for the production of ferritic stainless steel; Solidifying the molten steel to cast steel; And a step of hot rolling the steel material. In casting the steel material, it is preferable to cast the nickel so that the nickel (Ni) content of the surface layer portion corresponding to the surface of the steel material is 0.5 wt% or more.

In casting the steel, it is preferable that the thickness of the surface layer portion is 2% to 8% of the thickness of the steel.

The steel material is hot rolled to separate the surface layer portion from the base portion inside the surface layer portion.

The surface layer part of the steel material which concerns on embodiment of this invention contains 0.5 wt% or more of nickel. Therefore, compared with the related art, the occurrence of sticking defects at the base portion during hot rolling can be reduced, and the quality of the stainless steel can be improved.

In addition, by adjusting the thickness of the surface layer portion to 2% to 8% of the steel thickness, the surface layer portion can be removed so that the surface layer portion remains little or little on the base portion during hot rolling, and austenite is suppressed while suppressing sticking defects. It is possible to produce phaseless or almost free ferritic stainless steel.

1 is a flowchart illustrating a method of manufacturing stainless steel according to an embodiment of the present invention.
2 is a flow chart showing a method of manufacturing a steel according to an embodiment of the present invention.
3 is a view conceptually showing a steel according to an embodiment of the present invention.
4 is a photograph showing the surface layer portion and the base portion of the steel according to an embodiment of the present invention.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention in more detail. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various forms, and only the embodiments are intended to complete the disclosure of the present invention and to those skilled in the art to fully understand the scope of the invention. It is provided to inform you. Like numbers refer to like elements in the figures.

Embodiments of the present invention relate to stainless steel and its manufacturing method is suppressed the occurrence of surface defects. More specifically, an embodiment of the present invention relates to a ferritic stainless steel and a manufacturing method thereof in which the occurrence of defects due to hot rolling is suppressed.

1 is a flowchart illustrating a method of manufacturing stainless steel according to an embodiment of the present invention. 2 is a flow chart showing a method for manufacturing a steel according to an embodiment of the present invention. 3 is a view conceptually showing a steel according to an embodiment of the present invention. Figure 4 is a photograph showing the surface layer and the base portion of the steel according to an embodiment of the present invention.

Referring to FIG. 1, in the method of manufacturing stainless steel according to an exemplary embodiment of the present invention, a process of manufacturing the steel material 100 (S100) and hot rolling the manufactured steel material 100 (S200) may be performed to manufacture stainless steel. It includes.

In the process of manufacturing the steel material 100 (S100), a process of preparing molten steel for manufacturing stainless steel and injecting it into a mold (S110), a process of injecting a mold flux containing nickel oxide into the molten steel bath surface (S120), and the molten steel Solidifying and casting the steel (100) (S130).

In preparing molten steel for manufacturing stainless steel (S110), 11 wt% to 25 wt% of chromium (Cr), 0.05 wt% or less of carbon (C), and remainder include iron (Fe) and unavoidable impurities Provide a molten steel. And it is injected into the mold (S110).

In the embodiment, in manufacturing the steel 100, the nickel (Ni) content of the surface is prepared to contain a high concentration of 0.5 wt% or more. Herein, the nickel (Ni) content of the surface is 0.5 wt% or more, which means that the nickel (Ni) content of the surface is not less than 0.5 wt% based on the surface rather than the entire steel.

Hereinafter, for convenience of description, as shown in FIG. 3, the steel material 100 will be described as being composed of the base part 110 and the surface layer part 120 formed or provided on the base part 110. The surface layer part 120 according to the embodiment is a nickel rich layer having a nickel (Ni) content of 0.5 wt% or more, and more preferably, a nickel (Ni) content of 1.8 wt% or more. Here, the nickel (Ni) content of the surface layer portion 120 is 0.5 wt% or more or 1.8 wt% or more, the surface layer portion 120 is viewed as a whole, the nickel (Ni) content of the surface layer portion 120 is 0.5 wt% or more It means. The thickness B of the surface layer portion 120 is 2% or more, to 8% or less, more preferably 2% or more and less than 5% of the total thickness A of the steel material 100.

In order to manufacture the steel having the surface layer portion 120 having a nickel (Ni) content of 0.5 wt% or more as described above, in the embodiment, a mold flux including nickel oxide, for example, NiO, is prepared and injected into the molten steel. The mold flux according to the embodiment includes 5 wt% to 40 wt% of nickel oxide with respect to the entire mold flux, and includes at least one of SiO ₂ , CaO, MgO, Al ₂ O ₃ , Na ₂ O, and F.

On the other hand, when the nickel oxide in the mold flux is less than 5wt%, the nickel (Ni) content of the surface layer portion may be less than 0.5wt%, when the nickel oxide is more than 40wt%, the viscosity of the mold flux is high, the original of the mold flux The function of lubrication can be lost or reduced.

When the mold flux according to the embodiment as described above is provided, it is injected into the molten steel in the mold.

Thereafter, the molten steel is first solidified in the mold, and casting is performed to secondly solidify the steel drawn out to the lower side of the mold (S130), thereby manufacturing the stainless steel 100. Nickel (Ni) is a metal having a low oxygen affinity, and when injected into molten steel through a mold flux, the solidification behavior of molten steel is controlled so that an overstable reaction with less solidification shrinkage occurs on the steel surface. Thus, when the mold flux containing nickel oxide is introduced into the molten steel, the nickel oxide is reduced to metallic nickel (Ni) on the surface of the steel, and as a predetermined depth diffuses from the surface, as shown in Figure 4, A high Ni concentration layer is formed on the steel surface.

More specifically, molten steel is in a state where there is almost no free oxygen, and when nickel oxide is introduced into molten steel, the molten steel is reacted with calcium (Ca), aluminum (Al), silicon (Si), iron (Fe), etc. in the molten steel. Nickel (Ni) is reduced and exists in an ionic state. At this time, by the generated heat of reaction, the warm surface insulation effect of molten steel can be obtained, and nickel in an ion state diffuses to the surface part of steel materials, and forms Ni high concentration layer in a surface part.

On the other hand, when the content of nickel oxide in the mold flux is less than 5 wt%, the amount of metal nickel (Ni) reduced to the surface side of the steel is small, the nickel (Ni) concentration on the surface of the steel 100 is less than 0.5 wt%, In comparison, the incidence of sticking cannot be reduced. On the contrary, when the content of nickel oxide in the mold flux exceeds 40 wt%, the nickel (Ni) concentration on the surface of the steel 100 becomes 0.5 wt% or more, which can suppress the occurrence of sticking compared to the conventional one, but the melting point and the total There is a problem that the amount of heat increases, the viscosity increases, and the lubricating power, which is a function of the mold flux, is lost.

Therefore, in the embodiment, a mold flux containing 5 wt% or more and 40 wt% or less nickel oxide is added to the molten steel bath surface to produce steel materials.

When the mold flux according to the embodiment as described above is added, the steel 100 having a nickel (Ni) content of 0.5 wt% or more on the surface is manufactured. That is, as shown in Figure 2, the steel according to the embodiment includes a base portion 110 and the surface layer portion 120 formed on the base portion 110, the surface layer portion 120 has a nickel (Ni) content of 0.5 wt% or more, and the base portion 110 has little nickel (Ni) to converge at 0 ppm.

And the thickness B of the surface layer part 120 is 2%-8% of the thickness A of steel materials. The thickness B of the surface layer portion can be adjusted by controlling at least one of casting time and casting speed.

In the embodiment, as described above, the nickel (Ni) content of the surface of the steel 100, that is, the surface layer portion 120 is adjusted to 0.5 wt% or more, preferably 1.8 wt% or more. Here, the surface layer part 120 means the area | region in the inward direction from the surface of the steel material 100 by the thickness corresponding to 2%-8% of the total thickness A of the steel material 100.

When the steel material 100 is manufactured, the steel material 100 is heated to 900 ° C to 1200 ° C, and then hot rolling is performed using a rolling roll (S200). When the steel material 100 is heat-treated, the surface layer portion 120 containing nickel (Ni) of 0.5 wt% or more is transformed into an austenite phase and scaled up. When rolling is performed, the surface layer 120 is removed from the base 110 while the scale collapses due to the rolling force. In this application, the base 110 having the surface layer removed by rolling is made of stainless steel. Name it. According to the steel material 100 according to the embodiment, the phenomenon in which the surface layer portion 120 adheres to the rolling rolls at the time of rolling is suppressed as compared with the prior art, thereby reducing the occurrence of sticking defects due to hot rolling.

Hereinafter, with reference to Table 1, the austenite phase fraction (%) and sticking defect occurrence rate (%) of the surface layer portion 120 according to the nickel (Ni) content of the surface layer portion 120 will be described. Here, the austenite phase fraction of the surface layer part 120 means the ratio of the austenite phase in a predetermined area. This is a sample having a predetermined area after heating the cast steel to a temperature of 900 ℃ to 1200 ℃, or a sample sampled a portion of the steel material before the heat treatment, or a sample having a predetermined area after heating to a temperature of 900 ℃ to 1200 ℃ The austenite phase ratio of the surface of is computed. In Examples, the ratio of the austenite phase was calculated using the current value different according to the structure (ie, phase).

In addition, the sticking defect incidence rate calculates the number of steel materials in which sticking occurs among the total number of manufactured steels when a plurality of steels are manufactured and hot rolled. The incidence of sticking defects was calculated according to the nickel content in the surface layer portions, respectively. In addition, the surface layer part according to the comparative example and the embodiment means an area in the inward direction from the surface of the steel material 100 by a thickness corresponding to 5% of the total thickness A of the steel material 100, regardless of the nickel content. In addition, the austenite phase fraction in the surface layer portion was measured.

division Nickel content in surface layer (wt%) Austenite phase percentage (%) Sticking defect rate (%) Comparative example 0 0 100 First embodiment 0.5 0 88 Second embodiment One 0 81 Third embodiment 1.5 0 70 Fourth embodiment 1.7 0 71 Fifth Embodiment 1.8 2.8 49 Sixth embodiment 2 10.2 28 Seventh embodiment 3 30.5 4 Eighth embodiment 5 74.8 0 9th embodiment 6 92.3 0 10th embodiment 6.5 99 One Eleventh embodiment 7 100 0 12th embodiment 8 100 0

In the comparative example of Table 1, nickel is not included in the surface layer portion of the steel, and 100% of sticking occurred during hot rolling.

However, in the case of the embodiment containing more than 0.5wt% nickel in the surface layer portion 120, it can be seen that the occurrence of sticking during hot rolling is less than 100% compared to the comparative example. When the nickel content in the surface portion 120 is 0.5 wt% or more and 1.7 wt% or less, the sticking occurrence rate is 70% or more and 88% or less, and the sticking defect occurrence rate gradually decreases in the range of 10% to 30%. Can be.

It can be seen that the sticking incidence decreases to 50% or less when the nickel includes 1.8 wt% or more of the surface layer 120. This is because when the nickel content in the surface portion 120 is greater than or equal to 1.8 wt%, an austenite phase having a brittle characteristic begins to precipitate in the surface layer portion 120. As the nickel content increases to 1.8 wt% or more, the austenite phase is increased. As the fraction is rapidly increased, when the nickel content is 3%, almost no sticking occurs, and from 5% or more, the sticking occurrence rate is 0%.

On the other hand, if the surface layer portion 120 does not contain nickel (Ni) as in the prior art, even if the heat treatment, the surface layer portion is not transformed into the austenite phase, when the surface layer portion is attached to the rolling roll during rolling, King is raised.

However, in an embodiment in which nickel (Ni) is 0.5 wt% or more in the surface layer portion 120, at least a part of the ferrite phase of the surface layer portion 120 is transformed into an austenite phase by heat treatment, so that the scale is broken during rolling and the base portion 110 is formed. ) Is dispersed to the outside. That is, in removing the surface layer part 120 by hot rolling, it does not adhere to a rolling roll but the scale is broken by a rolling force, and it spread | disperses to the base part 110 outside. At this time, since the scale is torn off from the base 110, that is, it is not sticked, but naturally flows in a broken state on the base 110, the incidence of sticking of the base 110 is reduced compared to the comparative example. Can be.

In addition, the scaled surface layer portion 120 acts as a lubrication layer between the rolling roll and the base 110, thereby suppressing the surface contact between the rolling roll and the base 110, and thus, the rolling roll and the base 110. It is possible to reduce the occurrence of sticking due to direct contact between them. In other words, as the surface layer portion 120 is transformed into an austenite phase, the contact between the rolling roll and the base portion 110 at the time of hot rolling can be reduced as compared with the comparative example, so that the occurrence of sticking can be further suppressed. .

In particular, when 1.8 wt% or more of nickel is contained in the surface layer portion 120, the austenite phase is precipitated in the surface layer portion 120 at the hot rolling temperature, thereby making it easier to form scales compared to the ferrite phase, and thus the rolling roll and the base portion ( As the contact rate between 110) decreases, the occurrence of sticking begins to decrease rapidly.

Therefore, in the embodiment, in manufacturing the steel 100, the nickel (Ni) content of the surface layer portion 120 is to be 0.5% or more, more preferably 1.8% or more.

After the hot rolling, the more the surface layer 120 remains on the base 110, the higher quality ferrite stainless steel can be delivered to the customer, which depends on the thickness of the surface layer 120.

Hereinafter, with reference to Table 2, after the hot rolling according to the thickness of the surface layer portion 120 having a nickel content of 0.5 wt% or more, the thickness of the remaining surface layer portion on the matrix and whether or not sticking defects will be described. For the experiment, steel specimens were prepared such that the nickel content of the surface layer 120 was equal to 6.5 wt%.

division After hot rolling,
Residual Surface Thickness (μm) Sticking occurs First Comparative Example 0 ○ First embodiment 0 × Second embodiment 0 × Third embodiment 0 × Fourth embodiment Less than 1㎛ × 2nd comparative example 101 μm × Third Comparative Example 213 μm ×

Referring to Table 2, in the case of the first comparative example in which the thickness B of the surface layer portion 120 is less than 2% of the thickness A of the steel material, the thickness of the surface layer portion 120 was thin and sticking occurred. However, since the first comparative example contains 6.5 wt% of nickel (Ni), the incidence of sticking is lower than in the prior art in which nickel (Ni) is not contained.

In addition, in the case of the second and third comparative examples and the first to fourth examples, no sticking occurred (sticking occurrence rate 0%). However, in the first to fourth embodiments, the surface layer portion 120 does not remain (thickness 0 占 퐉), or less than 1 占 퐉, and remains in a very thin thickness without any problem in quality.

In the case of the second and third comparative examples, the thickness of the surface layer portion 120 remains thicker than 10 μm. When the surface layer portion 120 remains thick on the base portion 110 after hot rolling as in the second and third comparative examples, there is a problem in shipping it as a ferritic stainless steel to a customer.

Therefore, in the embodiment of the present invention, the thickness B of the surface layer portion 120 containing 0.5 wt% or more of nickel (Ni) is 2% to 8% of the thickness A of the base portion 110, and the surface layer portion ( The thickness of 120 may be adjusted by adjusting the casting speed and the casting time.

As described above, in the embodiment of the present invention, in manufacturing the steel material 100 for producing stainless steel, the nickel content is manufactured to have the surface portion 120 having 0.5 wt% or more. As a result, the occurrence of sticking defects of the base 110 may be reduced during hot rolling, and the quality of the stainless steel may be improved.

In addition, by adjusting the thickness B of the surface layer portion 120 to 2% to 8% of the thickness A of the steel material 100, the surface layer portion 120 does not remain on the base portion 110 during hot rolling, The surface layer portion 120 can be removed so that the ferrite stainless steel can be produced with little or no austenite phase while suppressing sticking defects.

110: base 120: surface layer

Claims (7)

Steel for manufacturing ferritic stainless steel,
Base; And
A surface layer portion formed on the surface of the matrix portion and containing 0.5 wt% or more of nickel (Ni);
Including,
The thickness of the surface layer portion containing 0.5 wt% or more of nickel (Ni) is 2% to 8% of the thickness of the steel. The method according to claim 1,
The nickel (Ni) content of the surface layer portion is more than 1.8 wt% steel. delete The method according to claim 1,
The thickness of the surface layer portion is 2% to 5% of the thickness of the steel. Preparing molten steel for manufacturing ferritic stainless steel;
Solidifying the molten steel to cast steel; And
Hot rolling the steel;
Including,
In casting the steel, the nickel (Ni) content of the surface layer portion corresponding to the surface of the steel is 0.5 wt% or more,
A method for producing stainless steel, such that the thickness of the surface layer portion having a nickel (Ni) content of 0.5 wt% or more is 2% to 8% of the thickness of the steel. delete The method according to claim 5,
A method for producing stainless steel that hot-rolls the steel material to separate the surface layer portion from the base portion inside the surface layer portion.

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