KR20160082398A - Lean duplex stainless steel manufacturing method - Google Patents

Abstract

The present invention relates to a method to manufacture a lean duplex stainless steel which is casted by a strip casting method and, more specifically, relates to a lean duplex stainless steel manufacturing method including a refining process of argon oxygen decarburization (AOD), a component adjustment process of ladle treatment (LT), and a strep casting process of twin roll strip casting. The refining process solely uses an Si with a deoxidizer and has 0.55-0.8 wt% of Si on an end.

Description

{LEAN DUPLEX STAINLESS STEEL MANUFACTURING METHOD}

The present invention relates to a method of manufacturing a lean duplex stainless steel, and more particularly, to a method of manufacturing a lean duplex stainless steel cast using a strip casting method.

In general, the strip casting process refers to a process of continuously supplying molten steel between a rotating pair of casting rolls to produce a thin plate product several mm thick directly from the molten steel. There has been known a method of manufacturing a lean duplex stainless steel using strip casting such as a high ductility duplex stainless steel and a method of manufacturing the same (Korean Patent Laid-open Publication No. 10-2014-0069945 (2014.06.10)).

1, a conventional strip casting apparatus 100 for performing a strip casting process mainly comprises a casting roll 110, a ladle 120, a tundish 130, a meniscus shield 140, and an edge dam (150).

The molten steel M loaded in the ladle 120 is supplied to the tundish 130 through the first nozzle 130 and the molten steel M is supplied to the tundish 130 Is supplied to the space formed by the pair of casting rolls 110 and the edge dam 170 provided at both ends in the axial direction of the casting roll 110 through the second nozzle 131, Lt; / RTI > At this time, to protect the molten steel between the casting rolls 110, the meniscus shield 150 protects the molten metal surface, and when the atmosphere is adjusted by injecting appropriate gas, the molten steel is solidified, The strip S is manufactured.

In a strip casting process for directly manufacturing a thin plate having a thickness of 10 mm or less from the molten steel M, molten steel M supplied between the casting rolls 110 rotating at a high speed in the opposite direction to each other is made into a thin plate having a desired thickness It is important to prevent the occurrence of cracks and to improve the rate of water loss.

The lean duplex stainless steel produced through the strip casting apparatus 100 is characterized in that it rapidly solidifies and produces minute inclusions. If such an inclusion remains on the surface of the product, it may cause surface damage or cracking, and may act as a site that causes the corrosion resistance to become poor. The level of adverse effects of inclusions on the steel is strongly influenced by the size and fraction of the inclusions contained in the steel. In particular, nonmetallic inclusions are inevitably generated through processes such as the deoxidation of molten steel and the addition of ferroalloys for temperature control, so that the generation of inclusions can not be prevented, so the generation of inclusions must be minimized.

However, since it is not easy to control the size of inclusions in the strip casting process, a more fundamental inclusion size control technique is required.

Korean Patent Publication No. 10-2014-0069945 (June 10, 2014)

SUMMARY OF THE INVENTION The present invention has been made in order to solve such a problem, and an object of the present invention is to provide a method of manufacturing a lean duplex stainless steel which can improve the corrosion resistance by controlling the inclusion size.

In order to accomplish the above object, a method of manufacturing a lean duplex stainless steel according to an embodiment of the present invention includes: AOD (Argon Oxygen Decarburization); LT (Ladle Treatment); and a twin roll strip casting ) Characterized in that Si is used alone as a deoxidizing agent and the content of Si is 0.55 to 0.8% by weight at the end of the refining process.

The refining process is characterized in that the basicity of the slag is controlled to 2.2 to 2.5.

The refining step is characterized in that the temperature of the hot water is controlled to 1680 to 1710 占 폚.

Wherein the stainless steel contains 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, 0.5 to 1.5% of Ni, , Cu: 0.3 to 1.2%, P: 0.035% or less (excluding 0), S: 0.003% or less (excluding 0), the balance Fe and other unavoidable impurities.

The stainless steel is characterized in that the formula potential is 300 mV or more.

The stainless steel is characterized in that the number of inclusions having a size of 10 m or more is 80 / cm < ² > or less.

The method of manufacturing a lean duplex stainless steel according to the present invention has the following effects.

First, improvement in corrosion resistance of steel can be expected by controlling the number of inclusions having a predetermined size or more.

Second, it is easy to apply, and it is possible to effectively manage inclusions without increasing costs.

Figure 1 shows a typical strip casting device,
2 is a flow chart showing a refining-component adjustment-strip casting process,
3 is a graph showing the change in the formal potential according to the number of inclusions having a size of 10 탆 or more,
4 is a graph showing changes in the number of inclusions having a size of 10 탆 or more according to the basicity,
5 is a graph showing changes in the number of inclusions having a diameter of 10 탆 or more,
FIG. 6 is a photograph showing a pinhole generated in the cylindrical sampling,
FIG. 7 is a photograph showing a state in which pinholes are not generated in the disk-shaped sampling.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not expressly express the opposite meaning thereto. Means that a particular feature, region, integer, step, operation, element and / or component is specified, and that other specific features, regions, integers, steps, operations, elements, components, and / And the like.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

Hereinafter, a method of manufacturing a lean-duplex stainless steel according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

The present invention relates to a process for producing a duplex stainless steel sheet through an AOD (Argon Oxygen Decarburization) process, a LT (Ladle Treatment) process and a twin roll strip casting process , And controlling the conditions in the refining process (AOD) to reduce the size and number of inclusions to a certain level or less. Particularly, the present invention relates to a method for controlling the Si content at the end of the refining process to 0.55 to 0.8 wt% . All percentages below refer to% by weight.

In the refining process (AOD), desulfurization and deoxidation are performed through decarburization and slag production. Since the desulfurization reaction of Ca and S must be actively performed, the reduction of S becomes easy. Therefore, in order to reduce the amount of oxygen in the steel, deoxidation of steel was performed by injecting Si and Al. Al is a useful element as a deoxidizer, thereby to form a non-metallic inclusion such as Al ₂ O ₃ to promote a high melting point of the inclusions. The thus formed high melting point inclusions remain in the steel because the separation of the impurities into slag and the floatation separation is not easy. This is because these inclusions are easily incorporated into the steel because of their low basicity. As a result, the addition of Al increases the number and size of inclusions formed in the steel.

In the present invention, this problem is solved by excluding the introduction of Al during the deoxidation process and deoxidizing molten steel only by injecting Si. It is necessary to increase the input amount of Si compared to the prior art in consideration of deoxidation ability which is insufficient by excluding the input of Al. It is preferable to inject Si into the molten steel in the refining process in the form of ferroalloy, and the Si content is checked by analyzing the composition after considering the dissolution rate of Si and the purity of the Si- To adjust the content of Si at the end point of the process.

The above-mentioned content of Si does not mean the content of the final product but means the content at the time when the refining process is finished in the refining furnace. Only when the content of Si present in the molten steel at the end of the refining process is 0.55 to 0.80%, a sufficient deoxidation reaction occurs and the amount of oxygen in the steel can be reduced. Si can be added to the component adjustment process after the refining process but Si added in the component adjustment process can not be added simply to adjust the content component of the final product after the deoxidation reaction is completed. It has no influence on the reduction of inclusions.

If the Si content at the end of the refining process is not controlled within the above-mentioned range, the following problems may occur. If the content of Si is less than 0.55%, the oxygen in the steel can not be effectively removed and the metal oxide is increased. This causes the mechanical properties of the final material, in particular, the elongation ratio to be lowered, and the workability to be degraded. If the content of Si exceeds 0.8%, oxygen in the steel can be effectively removed, but due to the characteristics of Si, the material becomes excessively strong, so that the possibility of plate breakage in the strip casting process is increased. Therefore, the content of Si should be controlled to 0.55 to 0.8%.

In the refining process, it is preferable to control the basicity of the slag to 2.2 to 2.5.

The basicity of the slag is CaO / SiO ₂ ratio, SiO ₂ is controlled by the amount of Si contained in the Si alloy iron supplied during the refining process, CaO is adjusted according to the amount of quicklime introduced for controlling the basicity, The ratio of the basicity of the slag, CaO / SiO ₂ , changes. As the basicity increases, the viscosity and melting point of the slag increase, and the slag absorbed and absorbed in the molten steel increases. This becomes an inclusion in the subsequent process and adversely affects the physical properties of the steel.

The basicity of the slag is determined from the concentration of dissolved oxygen and Si and Al in the steel. The higher the Si concentration, the higher the basicity and the lower the oxygen in the steel. If the basicity is too low, the equilibrium oxygen concentration in the steel increases and the amount of inclusions increases. If the basicity is too high, the oxygen concentration in the steel decreases and the inclusion production decreases. However, AL ₂ O ₃ among the impurities and ladle refractories Thereby increasing aluminum in the steel and causing surface defects. If the basicity of the slag is less than 2.2, there is a problem that the amount of inclusions generated increases due to an increase in the equilibrium oxygen concentration in the steel. If the slag has a basicity of more than 2.5, the reactivity between the slag and the ladle refractory becomes better, There is a problem that alumina (Al ₂ O ₃ ) of the surface is introduced and causes surface defects.

Therefore, by controlling the basicity of the slag to 2.2 to 2.5, the interfacial reaction between the molten steel and the slag is induced to reduce the amount of equilibrium oxygen in the molten steel, thereby reducing the size and number of inclusions.

It is also preferable that the temperature of the hot water, which is the temperature of the molten steel discharged after the refining process, is in the range of 1680 to 1710 ° C.

The higher the hot water temperature is, the longer the workable time in the subsequent component adjusting process (LT) becomes. Thus, molten steel heated to a temperature at which stable sampling can be stably performed several times has been conventionally discharged. Such a temperature is usually 1750 DEG C or more, and in this case, it takes several tens of minutes to confirm the components of the molten steel. However, this has caused a problem of increasing the process cost due to the delay of the steelmaking time. In order to solve such a problem, the present invention uses a disk-shaped sampler to reduce the defective sampling rate, thereby reducing the time required to identify components of the molten steel and reducing the tapping temperature of molten steel.

The tapping temperature and the equilibrium oxygen concentration in the steel are proportional to each other. Therefore, as the molten steel temperature decreases, the oxygen concentration in the steel decreases. When the oxygen concentration in the steel is reduced, the amount of oxygen forming the oxide decreases and the size and number of inclusions decrease. The change in the number of inclusions according to the hot water temperature is shown in Fig. According to Fig. 5, it can be seen that the number of inclusions is 80 ea / cm ² or less when the hot water temperature is in the above range.

If the tapping temperature is set to less than 1680 ° C, the amount of inclusions produced can be further reduced as the equilibrium oxygen concentration in the steel decreases. However, in the process of reaching the strip casting process, the bath surface may become coagulated and the molten steel may become stagnant, . In contrast becomes hot water temperature exceeds the 1710 ℃ has increased the equilibrium oxygen concentration in the steel as the temperature of the molten steel increases the amount of inclusions increases as 80ea / cm ² or more.

Therefore, it is preferable that the temperature of the molten steel to be sprayed is in the range of 1680 to 1710 DEG C after the refining process is completed.

The lean duplex stainless steel according to the present invention has a composition of 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, 1.5%, Cu: 0.3 to 1.2%, P: 0.035% or less (excluding 0), S: 0.003% or less (excluding 0), the balance Fe and other unavoidable impurities.

The above-mentioned composition is a component of a lean duplex stainless steel in which the addition amount of Mo and Ni, which are expensive components in the conventional duplex stainless steel sheet, is increased and the mechanical properties are increased by increasing the contents of Mn and N and the corrosion resistance is secured by adding Cu and N, to be.

Such a range of alloy compositions can provide a linseed duplex stainless steel sheet containing microstructure including ferrite and austenite and having excellent physical properties while reducing manufacturing cost.

It is an object of the present invention to reduce the size and the number of inclusions which are factors affecting corrosion resistance in a linseed duplex stainless steel which is a corrosion resistant steel, and to adjust the process conditions in the above-mentioned refining process . Therefore, although the above-mentioned component system is a condition necessary for manufacturing a lean-duplex stainless steel, it is not possible to reduce the size and number of inclusions through such a component system

Hereinafter, the reason for limiting the numerical value of each element will be described.

C: C is an element for forming an austenite phase, and is an effective element for increasing the strength of a material by solid solution strengthening. The effective content to expect such an effect is 0.02% or more. However, when C is excessively added, it easily bonds with a carbide forming element such as Cr which is effective for corrosion resistance at the ferrite-austenite phase boundary, thereby reducing the corrosion resistance by lowering the Cr content around the grain boundary. Therefore, in order to maximize the corrosion resistance, By weight.

Si: Si is added for the deoxidation effect, and the content is further increased because Al is not used as a deoxidizer in the present invention. Si is a ferrite phase forming element and is an element which is concentrated in ferrite during annealing heat treatment. Therefore, it is necessary to add 0.55% or more for deoxidation and securing the ferrite phase fraction. However, when 0.8% or more is added, the hardness of the ferrite phase is rapidly increased to lower the elongation and decrease the fraction of the austenite phase. Further, the slag fluidity is lowered in the steelmaking process, and inclusions are formed by binding with oxygen, thereby lowering the corrosion resistance. Therefore, the Si content is preferably limited to 0.55 to 0.8%.

Mn: Mn is an element that increases deoxidizing agent and nitrogen solubility, and is added in place of expensive Ni as an austenite forming element. As the Mn content increases, it has an effect of improving the nitrogen solubility, but it has a disadvantage that it forms MnS by binding with S in the steel and degrades the corrosion resistance. Further, if the content of Mn is decreased, it is difficult to secure a proper austenite phase fraction even if the austenite forming elements such as Ni, Cu, and N are controlled, and the solubility of N to be added is low and sufficient employment of nitrogen at normal pressure can not be obtained . Therefore, the content of Mn is preferably limited to 2.8 to 3.2%.

N: N is an element contributing to the stabilization of the austenite phase together with Ni in duplex stainless steel, which is one of the elements which is concentrated in the austenite phase during annealing. Therefore, by increasing the N content, it is possible to enhance the corrosion resistance and the strength, but it is necessary to control the content because the solubility of N is changed according to the content of Mn added. If the N content exceeds 0.28% in the Mn range of the present invention, blowholes and pinholes are generated during casting due to exceeding nitrogen solubility, thereby causing surface defects of the product. On the other hand, if the N content is too low, it becomes difficult to secure a proper phase fraction of the lean duplex stainless steel together with the decrease of the strength. Therefore, the content of N is preferably limited to 0.20 to 0.28%.

Cr: Cr is an element stabilizing ferrite together with Si, which plays a major role in securing the ferrite phase of duplex stainless steel and is an essential element for securing corrosion resistance. As the Cr content increases, the corrosion resistance increases. However, since the ferrite phase is excessively stabilized, there is a disadvantage that the content of expensive Ni and other austenite stabilizing elements must be increased in order to maintain the phase fraction of the austenite phase. Therefore, it is preferable to limit the content of Cr to 19 to 21% in order to secure sufficient corrosion resistance while maintaining the phase fraction of ferrite-austenite.

Ni: Ni is an element which stabilizes the austenite phase together with Mn, Cu and N, plays a main role in securing the austenite phase of the duplex stainless steel and is one of the main elements securing the elongation in cold working. Therefore, the stability of the austenite phase should be ensured by adding at least 0.5% in order to avoid the phenomenon of rapid curing due to the occurrence of excessive fired organic martensite and deterioration of workability. On the other hand, when a large amount of Ni is added, the percentage of austenite phase is increased, which makes it difficult to secure an appropriate ferrite phase fraction. Moreover, it is difficult to secure sufficient work hardening property because the austenite phase is too stabilized to suppress the generation of calcined organic martensite during cold working. It is difficult to secure competitiveness due to an increase in the manufacturing cost of products due to Ni. Therefore, in order to reduce the cost, the amount of expensive Ni is reduced as much as possible, and the proportion of austenite phase can be maintained by increasing Mn and N to replace it. Considering these conditions, it is preferable to limit the content of Ni to 0.5 to 1.5%.

Cu: It is desirable to reduce the content of Cu, which plays a role similar to Ni, to a minimum to reduce the cost. However, in order to ensure the stability of the austenite phase, it is necessary to add at least 0.3% in order to suppress the formation of excessive calcined organic martensite which occurs during cold working. On the other hand, if the Cu content is 1.2% or more, the product is difficult to process due to hot brittleness, and the Cu content is preferably limited to 0.3 to 1.2%.

P and S are inevitable impurities and it is preferable to remove them as much as possible. However, since the process cost is increased in order to lower the content, it is preferable to limit P to 0.035% or less and S to 0.003% or less.

It is preferable that the stainless steel produced according to the present invention has an average electric potential of 300 mV or more and an inclusion of 10 탆 or more in diameter of 80 / cm ² or less.

In order to improve the corrosion resistance of steel, control of the size of inclusions is very important. When large inclusions are large, the corrosion resistance of the steel is rapidly lowered, which causes defects such as surface cracks in subsequent steps. As a result of investigating the correlation between the corrosion resistance and the inclusion size of the linseed duplex stainless steel, it is known that inclusions having a diameter of 10 탆 or more act as sites of action for lowering the corrosion resistance. As the method of measuring the inclusion size, the hot-rolled steel sheet is polished in the rolling direction, and then the number of inclusions having a diameter of 10 占 퐉 or more is measured with a microscope after the mirror polishing. We use image analyzer which can analyze the size of inclusions during measurement.

As shown in Fig. 3, as the number of inclusions per unit area decreases, the corrosion resistance of the hot-rolled steel sheet is increased. However, when the number of subranges is less than 300mV, there is no significant change at the level of 300mV or more, which means that further reduction of the number of inclusions per unit area is ineffective. Considering that the typical potential of the 304 hot-rolled steel sheet is about 300 mV, sufficient corrosion resistance can be obtained by controlling the number of inclusions having a diameter of 10 탆 or more per unit area to 80 or less.

Hereinafter, practical embodiments of the present invention will be described.

Since the nitrogen content of the present invention is relatively high, there is a high possibility that pin holes are formed in the molten steel due to nitrogen gas.

Figs. 6 and 7 show photographs showing the occurrence of pinhole defects due to nitrogen gas according to the shape of the sampler for sampling molten steel during the sampling for confirming the molten steel composition during the component adjusting process.

In the conventional sampler (cylindrical type) shown in Fig. 6, a molten steel sample is taken in a cylindrical shape and solidified, and after cutting horizontally, the cross section is polished to analyze the components. However, this method requires repeated sampling and component analysis several times because the rate of occurrence of pinhole due to supersaturated nitrogen gas inside the molten steel is high up to 60%. As a result, the refining process is completed so that the component analysis can be performed several times in the component adjusting process, and the temperature of the hot water at the time of tapping is increased to make the process time longer.

As shown in FIG. 7, in order to solve the above problem, a sampler in the form of a disk was introduced to reduce the height of the molten steel sample. The use of a low-height mold minimizes nitrogen gas trapped in the inside, which can prevent the occurrence of pinholes, eliminating the horizontal cutting process, and allowing the surface to be polished immediately after analysis. . In addition, even if there is a pinhole defect in the inner space of the sample, the sample is not exposed to the outside, thereby causing no problem in the analysis process. By introducing the disk type sampler, it is possible to minimize the repetition of the component analysis process, thereby preventing the lack of spare time by lowering the tapping temperature of the molten steel. In fact, since the disk type sampler has been introduced, the defect rate of the sample has been reduced to 7% or less, which is significant.

division Casting conditions Number of inclusions
(ea / cm2) Si
(weight %) Hot water temperature
(° C) basicity
(CaO / SiO ₂₎ Comparative Example 1 0.43 1738 1.95 266 Comparative Example 2 0.47 1733 2.02 228 Comparative Example 3 0.41 1744 2.05 193 Comparative Example 4 0.41 1749 1.87 209 Example 1 0.58 1682 2.27 31 Example 2 0.65 1698 2.3 28 Example 3 0.6 1703 2.24 79 Example 4 0.62 1710 2.31 71 Example 5 0.64 1701 2.33 22

Table 1 and Figs. 4 and 5 show changes in the number of inclusions depending on Si content, hot water temperature and basicity. (Ea / cm < 2 >) increases sharply when the content of Si is less than 0.55 at the end of the refining process and the temperature of the hot water exceeds 1710 degrees and the basicity is out of the range of 2.2 to 2.5.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

1: Refiner (AOD) 2: Component regulator
100: strip casting apparatus 110: casting roll
120: ladle 121: first nozzle
130: Tundish 131: Second nozzle
140: Meniscus shield 150: Edge dam
200: Coil
M: Molten steel S: Strip (thin plate)

Claims (6)

A method of manufacturing a lean duplex stainless steel including AOD, Argon Oxygen Decarburization (AOD), Ladle Treatment (LT), and Twin roll strip casting,
Wherein the refining step comprises using Si as a deoxidizing agent alone and a Si content of 0.55 to 0.8% by weight at the time of finishing.
The method according to claim 1,
Wherein the refining step comprises controlling the basicity of the slag to 2.2 to 2.5.
The method according to claim 1,
Wherein the refining step controls the temperature of the hot water to be 1680 to 1710 占 폚.
The method according to any one of claims 1 to 3,
Wherein the stainless steel contains 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, 0.5 to 1.5% of Ni, , Cu: 0.3 to 1.2%, P: 0.035% or less (excluding 0), S: 0.003% or less (excluding 0), the balance Fe and other unavoidable impurities.
The method of claim 4,
Wherein the stainless steel is characterized in that the formula potential is 300 mV or more.
The method of claim 4,
Wherein the stainless steel has an inclusions having a size of 10 mu m or more of 80 / cm < ² > or less.

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