CN111448326B - General ferritic stainless steel having excellent hot workability and method for manufacturing same - Google Patents

General ferritic stainless steel having excellent hot workability and method for manufacturing same Download PDF

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CN111448326B
CN111448326B CN201880079341.9A CN201880079341A CN111448326B CN 111448326 B CN111448326 B CN 111448326B CN 201880079341 A CN201880079341 A CN 201880079341A CN 111448326 B CN111448326 B CN 111448326B
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slab
ferrite
stainless steel
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ferritic stainless
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CN111448326A (en
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李哉和
朴美男
曹圭珍
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Posco Holdings Co ltd
Posco Holdings Inc
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Abstract

Disclosed is a method for producing a general-purpose ferritic stainless steel having excellent hot workability. A method of manufacturing a ferritic stainless steel according to one embodiment of the present disclosure includes: manufacturing a slab, the slab comprising: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2% to 0.6%, P:0.035% or less (excluding 0), S:0.01% or less (excluding 0), with the balance being iron (Fe) and other unavoidable impurities; and hot rolling the slab after heating the slab, the heating of the slab being performed at a temperature ranging from 1200 ℃ to 1250 ℃ so that the fraction of the delta-ferrite phase in the internal structure of the slab is 80% to 95%.

Description

General ferritic stainless steel having excellent hot workability and method for manufacturing same
Technical Field
The present invention relates to a method of manufacturing a general-purpose ferritic stainless steel, and more particularly, to a method of manufacturing a general-purpose ferritic stainless steel having improved slab hot workability by controlling a delta-ferrite phase fraction and a ferrite factor through composition control under a hot rolling heating temperature condition of at least 1200 ℃ before hot rolling.
Background
General ferritic stainless steel is high strength STS steel having a dual phase (ferrite matrix + tempered martensite) structure by controlling Ni, mn content, etc., and Cr content of 11% to 12.5%. Which is a steel grade used as a substitute for carbon steel in the field of structural materials requiring corrosion/wear resistance and weldability. Such general-purpose ferritic stainless steel is widely used as a structural material requiring strength and corrosion resistance.
In some cases, austenitic 304 steel having excellent corrosion resistance is used as a structural material, but contains a large amount of expensive Ni and Cr, which causes economic problems. In addition, in the case of ferritic stainless steel containing 16% or more of Cr, particularly in the case of 430 steel, corrosion resistance is superior to carbon steel, but workability is poor, and in particular, use in structural materials requiring weldability is limited due to problems such as deterioration of toughness of a weld zone due to coarsening of ferrite structure of a heat affected zone. Also, in the case of 409 steel containing relatively low Cr of about 11% or less, the corrosion resistance is similar to that of the existing 400-series STS, but there are many limitations in using as a structural material due to low impact toughness and yield strength.
In the production of such general-purpose ferritic steel, it is desirable to heat the slab at a high temperature in order to remove segregation inside the slab during hot rolling and reduce a rolling load so as to smoothly perform hot rolling. However, when the slab is heated at a high temperature of 1300 ℃ or more, that is, a temperature higher than the ferrite single-phase region, quality problems, such as deterioration of physical properties, such as deterioration of impact toughness caused by grain boundary oxidation and grain growth, and surface linear defects, are caused. Therefore, when heating a slab, it is important to heat the slab at a temperature at which a two-phase structure of austenite and δ -ferrite is formed. This is because grain coarsening during heat treatment can be suppressed due to the locally formed austenite phase, as compared to the ferrite single phase. However, the fraction of delta-ferrite in the two-phase structure varies not only according to the heating temperature, but also continuously as the temperature of the initially heated slab decreases due to contact between the rolls and the material during hot rolling.
Disclosure of Invention
Technical problem
Since the fraction of delta-ferrite in the slab structure is controlled by controlling the alloy composition and phase fraction conditions, embodiments of the present disclosure provide general-purpose ferritic stainless steel having excellent hot workability that can prevent the occurrence of surface linear defects and edge cracks when a wide slab is hot-rolled under high-temperature heat treatment conditions of 1200 ℃ to 1250 ℃, and a method of manufacturing the same.
Technical scheme
According to one aspect of the present disclosure, a method of manufacturing a general-purpose ferritic stainless steel having excellent hot workability includes: manufacturing a slab, the slab comprising: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2% to 0.6%, P:0.035% or less (excluding 0), S:0.01% or less (excluding 0), the remainder being iron (Fe) and other unavoidable impurities; and hot rolling the slab after heating the slab, wherein the heating of the slab is performed at a temperature ranging from 1200 ℃ to 1250 ℃ so that the fraction of the delta-ferrite phase in the internal structure of the slab is 80% to 95%.
The heating time may be 3 hours or more.
The manufacturing method may further include: cu:0.2% or less and Ti:0.03% or less.
According to one aspect of the present disclosure, a general-purpose ferritic stainless steel having excellent hot workability includes: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2% to 0.6%, P:0.035% or less (excluding 0), S:0.01% or less (excluding 0), the balance being iron (Fe) and other unavoidable impurities, and the ferrite factor represented by the following equation (1) satisfies the range of 10.5 to 12.0.
Equation (1): ferrite factor = [ Cr +6Si ] - [2Mn +4Ni +40 (C + N) ]
The ferritic stainless steel may further comprise: cu:0.2% or less and Ti:0.03% or less.
A reduction of area (reduction of area) in a temperature range of 900 to 1200 ℃ may be 70% or more.
Advantageous effects
According to an embodiment of the present disclosure, hot workability of a slab during hot rolling may be improved by controlling a ferrite factor and a delta-ferrite phase fraction.
Therefore, linear defects and edge cracks can be prevented from occurring on the surface of the slab during hot rolling, and the surface and edge qualities of the pickled coil annealed after hot rolling can be improved.
Drawings
Fig. 1 is a graph illustrating a correlation between a delta-ferrite fraction and hot workability according to an embodiment of the present disclosure.
Fig. 2 is a diagram for explaining a change in a microstructure during heat treatment of a high-temperature slab according to an example and a comparative example of the present disclosure.
Fig. 3 is a graph showing a change in hot workability when cooling slabs according to the examples and comparative examples of the present disclosure.
Detailed Description
A method of manufacturing a general-purpose ferritic stainless steel having excellent hot workability according to one embodiment of the present disclosure includes: manufacturing a slab, the slab comprising: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2 to 0.6%, P:0.035% or less (excluding 0), S:0.01% or less (excluding 0), the remainder being iron (Fe) and other unavoidable impurities; and hot rolling the slab after heating the slab, the heating of the slab being performed at a temperature ranging from 1200 ℃ to 1250 ℃ so that the fraction of the delta-ferrite phase in the internal structure of the slab is 80% to 95%.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to convey the technical concept of the present disclosure to those of ordinary skill in the art. However, the present disclosure is not limited to these embodiments, and may be embodied in another form. In the drawings, portions irrelevant to the description may not be shown in order to clarify the present disclosure, and sizes of components are more or less exaggeratedly shown for easy understanding.
In addition, when a portion "includes" or "includes" an element, unless there is a particular description to the contrary, the portion may include other elements, not excluding other elements.
Unless the context clearly dictates otherwise, expressions used in the singular include plural expressions.
Hereinafter, embodiments according to the present disclosure will be described in detail with reference to the accompanying drawings. First, a ferritic stainless steel is described, and then a method of manufacturing the ferritic stainless steel is described.
Fig. 1 is a graph showing the correlation between the delta-ferrite fraction and the hot workability at 1000 ℃, 1100 ℃ and 1200 ℃.
The change in the delta-ferrite phase fraction during hot rolling results in a difference in deformation resistance to working between austenite and delta-ferrite structures when the material is worked at high temperatures. Thus, linear defects and edge cracks are generated. In particular, it is known that hot workability is the worst when the fraction of δ -ferrite is in the range of 15% to 30% at a material surface temperature of 1000 ℃ to 1200 ℃ due to contact between rolls and materials during hot rolling, as shown in fig. 1.
As a method for improving such hot workability, it is preferable to perform hot forming while keeping the δ -ferrite phase at 10% or less, but in heating a slab, heat treatment at a low temperature is necessary. However, under the low temperature heat treatment condition, when the slab is heated, the heat load increases, making it difficult to produce a 5-foot wide material.
Therefore, there is a need to obtain a general-purpose ferritic stainless steel and a manufacturing method thereof, which can increase a slab temperature and produce a wide slab while ensuring excellent hot workability by forming a suitable phase fraction.
The inventors of the present disclosure have studied the microstructure to improve hot workability of the ferritic stainless steel. As a result, it was found that the fraction of delta-ferrite formed in the structure can be controlled by adjusting the temperature of the slab during heating before hot rolling of the slab. In particular, in the case of general-purpose ferritic stainless steel, the fraction of δ -ferrite varies depending on heating conditions, and it is found that a large amount of δ -ferrite structure is formed at a higher temperature. Thus, the alloy composition, phase fraction and temperature range of the heating step are obtained.
The general ferritic stainless steel having excellent hot workability according to the present disclosure includes: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2% to 0.6%, P:0.035% or less (excluding 0), S:0.01% or less (excluding 0), with the remainder being iron (Fe) and other unavoidable impurities.
Hereinafter, the reason for the numerical limit of the content of the alloy component in the embodiment of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is weight%.
The content of C and N is 0.005% to 0.020%.
The lower the contents of carbon (C) and nitrogen (N), the better ductility and impact characteristics of the weld zone can be secured, so the upper limit is set to 0.02% so that normal manufacturing can be performed, and the sum C + N of the two elements is set to 0.04% or less. When the sum of these two elements exceeds 0.04%, there are problems that the ductility of the material is drastically reduced and the toughness of martensite formed in the weld zone is drastically reduced.
The content of Si is 0.5% to 0.8%.
Silicon (Si) is generally added as a deoxidizer to reduce inclusions in steel, and when high strength is required, 0.5% or more is preferably added because it prevents excessive generation of delta-ferrite, which may reduce strength. However, when the content is too large, there is a problem of deteriorating toughness of the weld zone, and in particular, the upper limit may be limited to 0.8%.
The Mn content is 0.5% to 1.5%.
Manganese (Mn) is an austenite forming element, and is effective in improving toughness because it controls ferrite grain size growth. Therefore, it is preferable to add 0.5% or more to improve toughness and processability of the material. However, if the content is too large, the workability and toughness of the steel material are drastically reduced, and the upper limit may be limited to 1.5%.
The content of Cr is 11.0 to 12.5%.
Chromium (Cr) is the highest content of corrosion resistance enhancing elements of stainless steel, and is preferably added by 11% or more to exhibit corrosion resistance. However, when the content is too large, since a large amount of austenite forming elements such as Ni, mn, and Cu must be added, there is a problem that it is difficult to secure toughness of the weld zone and workability of the material, and the upper limit may be limited to 12.5%.
The content of Ni is 0.2% to 0.6%.
Nickel (Ni) is an austenite forming element and contributes to improving the toughness of the base material. In addition, since it is an element that improves toughness of a weld zone by refinement of ferrite grains caused by residual austenite during welding and refinement of martensite transformation grains during cooling, it is preferable to add 0.2% or more. However, if the content is too large, the effect is saturated, leading to an increase in cost, and the upper limit may be limited to 0.6%.
The content of P is 0.035% or less.
Phosphorus (P) is an impurity inevitably contained, and its content is preferably controlled to be as low as possible. Theoretically, it is advantageous to control the content of phosphorus to 0% by weight, but inevitably, it is necessarily included in the manufacturing process. Therefore, it is important to control the upper limit, and in the present disclosure, the upper limit is controlled to 0.035%.
The content of S is 0.01% or less.
Sulfur (S) is an impurity inevitably contained, and the content is preferably controlled to be as low as possible. Theoretically, it is advantageous to control the content of phosphorus to 0% by weight, but inevitably, it is necessarily included in the manufacturing process. Therefore, it is important to control the upper limit, and in the present disclosure, the upper limit is controlled to 0.01%.
In addition, the general-purpose ferritic stainless steel having excellent hot workability according to an embodiment of the present disclosure may further include Cu:0.2% or less and Ti:0.03% or less.
The Cu content is 0.2% or less.
Copper (Cu) is an austenite forming element similar to Ni, which contributes to improving the toughness of the base material. In addition, when a certain amount of Cu is added, there is an effect of improving ductility. However, the content is limited to 0.2% or less in view of cost.
The content of Ti is 0.03% or less.
Titanium (Ti) is an element that fixes carbon and nitrogen and forms precipitates to reduce the content of solid solution C and solid solution N to improve the corrosion resistance of steel. However, if the content is too large, surface defects may be generated due to coarse Ti inclusions and present a problem of increasing costs, and the upper limit may be limited to 0.03%.
The remaining component of the present disclosure is iron (Fe). However, in a normal manufacturing process, undesirable impurities from raw materials or the surrounding environment may be inevitably mixed in, and thus cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.
According to one embodiment of the present disclosure, the general-purpose ferritic stainless steel having excellent hot workability satisfying the above alloy composition may satisfy the ferrite factor represented by the following equation (1) in the range of 10.5 to 12.0.
Equation (1): ferrite factor = [ Cr +6Si ] - [2Mn +4Ni +40 (C + N) ]
In the above equation, cr and Si are ferrite-forming elements that suppress austenite formation at high temperatures, and Mn, ni, C, and N are austenite-forming elements that promote austenite formation at high temperatures. That is, the larger the ferrite factor, the more difficult it is to form austenite at high temperature.
For example, when the ferrite factor exceeds 12, a δ -ferrite single-phase structure is formed during heat treatment, and deterioration of hot workability due to grain coarsening may occur. When the ferrite factor is less than 10.5, the fraction of delta-ferrite falls within the range of 15% to 30% due to the decrease in the temperature of the material during hot rolling, so that there is a problem of poor hot workability. Therefore, it is preferable that the ferrite factor satisfies the range of 10.5 to 12.
According to an embodiment of the present disclosure, a general-purpose ferritic stainless steel having excellent hot workability satisfying the above alloy composition may have a fraction of a delta-ferrite phase when heated before hot rolling of 80% to 95%.
Therefore, even considering that the temperature of the heated slab is lowered due to contact with the rolls during hot rolling, a relatively high reduction of area of 70% or more may be exhibited. Therefore, the problems of linear defects and edge cracks occurring during the production of products can be solved.
Next, a method of manufacturing a general-purpose ferritic stainless steel having excellent hot workability according to another aspect of the present disclosure will be described.
A method of manufacturing a general-purpose ferritic stainless steel having excellent hot workability according to one embodiment of the present disclosure includes: manufacturing a slab, the slab comprising: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2% to 0.6%, P:0.035% or less (excluding 0), S:0.01% or less (excluding 0), with the balance being iron (Fe) and other unavoidable impurities; and hot rolling the slab after heating the slab, the heating of the slab may be performed at a temperature ranging from 1200 ℃ to 1250 ℃ such that the fraction of the delta-ferrite phase in the internal structure of the slab is 80% to 95%.
The reasons for the numerical limits of the content of the alloying elements are as described above.
After the molten steel containing the above composition is cast into a slab in a continuous casting machine, the cooled slab is heated and then hot rolled to produce a hot rolled product.
The produced slab undergoes a heating process before hot rolling.
The present disclosure adjusts the heating temperature of the slab to control the fraction of the delta-ferrite phase in the internal structure of the slab during the heating process to 80% to 95%.
Fig. 2 is a diagram for explaining a change in a microstructure during heat treatment of a high-temperature slab according to an example and a comparative example of the present disclosure.
The delta-ferrite measured in the present disclosure refers to the delta-ferrite content present during heating of the slab prior to hot rolling. To infer the delta-ferrite content in this state, samples of different alloy compositions heat treated at 1250 ℃ were quenched and quantified by observing the microstructure at room temperature as shown in fig. 2.
Referring to fig. 2, in examples 1 and 2, it was confirmed that the tempered martensite structure was distributed along the grain boundary of the ferrite matrix. On the other hand, in the case of the comparative example, it can be seen that the fraction of martensite is higher than that of ferrite, and it can be seen that the phase fractions of austenite and δ -ferrite constituting the microstructure vary according to the variation in the alloy composition.
The difference in phase fraction of the initial slab state greatly affects the hot workability of the material, and the result is shown in fig. 3.
Fig. 3 is a graph showing the results of area reduction (%) measured at different hot rolling temperatures of 900 to 1200 ℃ by a high temperature gleeble tensile test after holding at a temperature of 1250 ℃ for 3 hours using different alloy compositions. The measured reduction of area means that the higher the value, the better the hot workability.
As described above, as the heating temperature of the slab increases, the fraction of the δ -ferrite phase in the internal structure of the slab increases, and thus in order to control the fraction of the δ -ferrite phase to 80% to 95%, the heating temperature of the slab is set to 1200 ℃ to 1250 ℃. This is achieved by charging the slab into the furnace interior and then maintaining the furnace interior at 1200 to 1250 ℃ for at least 3 hours.
Hereinafter, the present disclosure will be described in more detail by preferred embodiments thereof.
Examples
[ Table 1]
Figure BDA0002528943020000081
As shown in table 1, after the slab manufactured while varying the contents of the respective components was heat-treated at a temperature of 1250 ℃ for 3 hours, hot rolling was performed, and thus, the δ -ferrite fraction, the austenite fraction, and the reduction of area, the linear defects, and the edge cracks are shown in table 2.
[ Table 2]
Figure BDA0002528943020000091
Referring to fig. 2 and table 2, it can be seen that the phase fractions of austenite and δ -ferrite constituting the microstructure of the slab are changed according to the change in the alloy composition when heated before hot rolling. Specifically, in table 2, it can be seen that the δ -ferrite phase is more than the austenite phase in the inventive example, and the austenite phase is more than the δ -ferrite phase in the comparative example.
Referring to tables 2 and 3, in the case of the inventive examples, ferrite fractions of about 80% or more were shown under the initial heat treatment condition of 1250 deg.c, compared to the comparative examples. It shows a level of reduction of area of 98% similar to the former at a high temperature state during hot working, and it can be seen that the reduction of area is reduced to about 70% as the temperature is lowered. That is, linear defects and edge cracks did not occur due to a relatively high reduction of area at low temperature, as compared to the comparative example.
On the other hand, in comparative examples 1 and 2, the Si contents were 0.31% and 0.44%, which were less than 0.5%, and exhibited ferrite fractions of about 5% or less under the initial heat treatment conditions of 1250 ℃. In a high temperature state during hot working, a high reduction of area of about 98% is exhibited, but it can be seen that the reduction of area is reduced to about 55% as the temperature is lowered. That is, linear defects and edge cracks occur due to low reduction of area at relatively low temperatures.
Further, in the case of comparative example 3, the entire composition range of the present disclosure was satisfied except that the ferrite factor was 10.4 (less than 10.5), and the ferrite fraction of about 35% was shown under the initial heat treatment condition of 1250 ℃. It showed a high reduction of area of about 98% in a high temperature state during hot working, but it can be seen that the reduction of area is reduced to about 52% as the temperature is reduced, compared to comparative examples 1 and 2. Since a low reduction of area at low temperatures is obtained, linear defects and edge cracks occur.
When hot rolling is performed after heating a slab in a temperature range of 1200 to 1250 ℃ such that the delta-ferrite content satisfies a range of 80 to 95%, a general-purpose ferritic stainless steel manufactured according to one embodiment of the present disclosure can produce a wide range of materials while minimizing the occurrence of linear defects and edge cracks.
While the present disclosure has been particularly described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the disclosure.
Industrial applicability
The ferritic stainless steel according to the present disclosure has improved durability, and may be used as a material for bus structures.

Claims (2)

1. A manufacturing method of a general-purpose ferritic stainless steel having excellent hot workability, the manufacturing method comprising:
manufacturing a slab, the slab comprising: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2% to 0.6%, P:0.035% or less excluding 0, S:0.01% or less excluding 0, with the balance being iron (Fe) and other unavoidable impurities; and
hot rolling the slab after heating the slab,
wherein the heating of the slab is performed at a temperature in the range of 1200 ℃ to 1250 ℃ such that the fraction of delta-ferrite phase in the internal structure of the slab is 80% to 95%,
wherein the heating time is 3 hours or more, an
The slab further comprises: cu:0.2% or less and Ti:0.03% or less.
2. A general-purpose ferritic stainless steel having excellent hot workability, the ferritic stainless steel comprising: in weight percent (%) of the entire composition, C:0.005% to 0.020%, N:0.005% to 0.020%, si:0.5 to 0.8%, mn:0.5 to 1.5%, cr:11.0% to 12.5%, ni:0.2% to 0.6%, P:0.035% or less excluding 0, S:0.01% or less excluding 0, and the balance being iron (Fe) and other unavoidable impurities,
wherein the ferrite factor represented by the following equation (1) satisfies the range of 10.5 to 12.0,
wherein the reduction of area in a temperature range of 900 ℃ to 1200 ℃ is 70% or more by controlling the delta-ferrite fraction, and
the ferritic stainless steel further comprises: 0.2% or less of Cu and 0.03% or less of Ti,
equation (1): ferrite factor = [ Cr +6Si ] - [2Mn +4Ni +40 (C + N) ].
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