[Technical Field]
[0001] The present invention relates to a manufacturing method of a utility ferritic stainless
steel, More specifically, relates to a manufacturing method of a utility ferritic
stainless steel with improved slab hot workability through ferrite factor and δ-ferrite
phase fraction control through component control under hot rolling heating temperature
conditions of at least 1200°C before hot rolling.
[Background Art]
[0002] Utility ferritic stainless steel is a high-strength STS steel with dual phase (ferrite
base + tempered martensite) structure by controlling Ni, Mn content, etc. with a Cr
content of 11 to 12.5%. It is a steel type that is used as a substitute for carbon
steel in the field of structural materials requiring corrosion resistance / abrasion
resistance and weldability. This utility ferritic stainless steel is widely used as
a structural material requiring strength and corrosion resistance.
[0003] In some cases, austenitic 304 steel having excellent corrosion resistance is used
as a structural material, but a large amount of expensive Ni and Cr is included, which
causes economic problems. In addition, in the case of ferritic stainless steel containing
16% or more of Cr, especially in 430 steel, corrosion resistance is superior to carbon
steel, but workability is poor, and in particular, there is a limitation in the use
of a structural material that requires weldability due to problems such as deterioration
of toughness of the weld zone due to coarsening of the ferrite structure of the heat-affected
zone. And in the case of 409 steel containing relatively low Cr of about 11% or less,
corrosion resistance is similar to that of the existing 400-based STS, but due to
low impact toughness and yield strength, there are many limitations to apply as a
structural material.
[0004] In manufacturing such utility ferritic steel, it is desirable to perform slab heating
at high temperature for the purpose of removing the segregation inside the slab during
hot rolling and lowering the rolling load for smooth hot rolling. However, when the
slab is heated at a high temperature of 1300°C or higher, which is a temperature above
the ferrite single-phase region, it causes quality problems such as deterioration
in physical properties such as impact toughness due to grain boundary oxidation and
grain growth, and surface linear flaws. Therefore, when heating the slab, it is important
to heat at a temperature capable of forming a two-phase structure of austenite and
δ-ferrite. This is because grain coarsening during heat treatment can be suppressed
due to the austenite phase formed locally compared to the ferrite single phase. However,
the fraction of δ-ferrite in this two-phase structure not only changes depending on
the heating temperature, but also continuously changes the phase fraction when the
temperature of the initial heated slab decreases due to contact between the roll and
the material during hot rolling.
[Disclosure]
[Technical Problem]
[0005] The embodiments of the present disclosure, as the δ-ferrite fraction in the slab
structure is controlled by controlling the alloy component and phase fraction conditions,
when hot rolling of a wide slab under high temperature heat treatment conditions of
1200 to 1250°C, provide a utility ferritic stainless steel with excellent hot workability
that can prevent the occurrence of surface linear flaws and edge cracks, and a manufacturing
method thereof.
[Technical Solution]
[0006] In accordance with an aspect of the present disclosure, a manufacturing method of
a utility ferritic stainless steel with excellent hot workability includes: manufacturing
a slab including, in percent (%) by weight 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 of iron (Fe) and other inevitable impurities; and hot rolling the slab after
heating the slab, and the heating of the slab is performed in a temperature range
of 1200 to 1250°C so that the fraction of δ-ferrite phase in the internal structure
of the slab is 80 to 95%.
[0007] The heating time may be 3 hours or more.
[0008] The manufacturing method may further include: Cu: 0.2% or less and Ti: 0.03% or less.
[0009] In accordance with an aspect of the present disclosure, a utility ferritic stainless
steel with excellent hot workability includes, in percent (%) by weight 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 of iron (Fe) and other inevitable impurities, and a ferrite
factor represented by the following equation (1) satisfies the range of 10.5 to 12.0.

[0010] The ferritic stainless steel may further include: Cu: 0.2% or less and Ti: 0.03%
or less.
[0011] The reduction of area in the temperature range of 900 to 1200°C may be 70% or more.
[Advantageous Effects]
[0012] According to an embodiment of the present disclosure, it is possible to improve the
hot workability of the slab during hot rolling by controlling the ferrite factor and
the δ-ferrite phase fraction.
[0013] Accordingly, it is possible to prevent the occurrence of linear flaws and edge cracks
on the surface of the slab during hot rolling, and it is possible to improve the surface
and edge quality of the pickled coil annealed after hot rolling.
[Description of Drawings]
[0014]
FIG. 1 is a graph showing a correlation between a δ-ferrite fraction and hot workability
according to an embodiment of the present disclosure.
FIG. 2 is a picture for explaining the change in the microstructure during the high-temperature
slab heat treatment according to Examples and Comparative Examples of the present
disclosure.
FIG. 3 is a graph showing changes in hot workability when cooling slabs according
to Examples and Comparative Examples of the present disclosure.
[Best Mode]
[0015] A manufacturing method of a utility ferritic stainless steel with excellent hot workability
according to an embodiment of the present disclosure includes: manufacturing a slab
comprising, in percent (%) by weight 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
of iron (Fe) and other inevitable impurities; and hot rolling the slab after heating
the slab, and the heating of the slab is performed in a temperature range of 1200
to 1250°C so that the fraction of δ-ferrite phase in the internal structure of the
slab is 80 to 95%.
[Modes of the Invention]
[0016] Hereinafter, the embodiments of the present disclosure will be described in detail
with reference to the accompanying drawings. The following embodiments are provided
to transfer the technical concepts of the present disclosure to one 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, parts that are irrelevant to the
descriptions may be not shown in order to clarify the present disclosure, and also,
for easy understanding, the sizes of components are more or less exaggeratedly shown.
[0017] Also, when a part "includes" or "comprises" an element, unless there is a particular
description contrary thereto, the part may further include other elements, not excluding
the other elements.
[0018] An expression used in the singular encompasses the expression of the plural, unless
it has a clearly different meaning in the context.
[0019] Hereinafter, embodiments according to the present disclosure will be described in
detail with reference to the accompanying drawings. First, ferritic stainless steel
is described, and then a manufacturing method of ferrite stainless steel is described.
[0020] FIG. 1 is a graph showing the correlation between δ-ferrite fraction and hot workability
at 1000, 1100, and 1200°C.
[0021] The change in phase fraction of δ-ferrite during hot rolling causes a difference
in deformation resistance to processing between austenite and δ-ferrite structures
when processing materials at high temperatures. As a result, linear flaws and edge
cracks are generated. In particular, it is known that hot workability is the most
inferior as shown in FIG. 1 when the fraction of δ-ferrite in the range of 15 to 30%
at a material surface temperature of 1000 to 1200°C due to contact between the roll
and the material during hot rolling.
[0022] As a method of improving such hot workability, it is preferable to perform hot forming
while maintaining a δ-ferrite phase of 10% or less, but heat treatment at a low temperature
is essential when heating the slab. However, under low-temperature heat treatment
conditions, the heating load increases when the slab is heated, making it difficult
to produce a 5ft wide material.
[0023] Accordingly, it is required to derive utility ferritic stainless steel and its manufacturing
method that can increase the slab temperature and produce a wide slab, and at the
same time, ensure the excellent hot workability through the formation of an appropriate
phase fraction.
[0024] Inventors of the present disclosure have studied microstructures to improve hot workability
in ferritic stainless steel. As a result, they discovered that the fraction of δ-ferrite
formed in the tissue can be controlled by adjusting the temperature of the slab during
heating before hot rolling of the slab. In particular, in the case of utility ferritic
stainless steel, the fraction of delta-ferrite varies depending on the heating conditions,
and they discovered that a large amount of δ-ferrite structure is formed at higher
temperatures. Accordingly, alloy components, phase fraction and the temperature range
of the heating step was derived.
[0025] A utility ferritic stainless steel with excellent hot workability according to the
present disclosure includes, in percent (%) by weight 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 of iron (Fe) and other inevitable impurities.
[0026] Hereinafter, the reason for the numerical limitation of the alloy component content
in the embodiment of the present disclosure will be described. In the following, unless
otherwise specified, the unit is % by weight.
[0027] The content of C and N is 0.005 to 0.020%.
[0028] The lower the content of carbon (C) and nitrogen (N), the better the impact characteristics
of the weld zone and the ductility can be secured, so the upper limit is set to 0.02%
so that normal manufacturing is possible, and the sum of the two elements, C + N,
is set to be 0.04% or less. When the sum of the two elements exceeds 0.04%, there
is a problem that the ductility of the material decreases rapidly, and the toughness
of the martensite formed in the weld zone decreases rapidly.
[0029] The content of Si is 0.5 to 0.8%.
[0030] Silicon (Si) is usually added as a deoxidizer to reduce inclusions in steel, and
when high strength is required, it is preferable to add 0.5% or more since it prevents
excessive generation of delta ferrite that can lower strength. However, when the content
is excessive, there is a problem of deteriorating the toughness of the weld zone,
in particular, the upper limit can be limited to 0.8%.
[0031] The content of Mn is 0.5 to 1.5%.
[0032] 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 workability of the material. However, if the
content is excessive, the workability and toughness of the steel material rapidly
decreases, and the upper limit can be limited to 1.5%.
[0033] The content of Cr is 11.0 to 12.5%.
[0034] Chromium (Cr) is the most contained element of the corrosion resistance enhancing
element of stainless steel, and it is preferable to add 11% or more to express corrosion
resistance. However, when the content is excessive, 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 the toughness of the weld zone and the workability of the material,
and the upper limit can be limited to 12.5%.
[0035] The content of Ni is 0.2 to 0.6%.
[0036] Nickel (Ni) is an austenite-forming element and contributes to the improvement of
the toughness of the base material. In addition, since it is an element that improves
weld zone toughness by refinement of ferrite grains due to austenite residue during
welding and refinement of martensite transformation grains during cooling, it is preferable
to add 0.2% or more. However, if the content is excessive, the effect is saturated,
causing an increase in cost, and the upper limit can be limited to 0.6%.
[0037] The content of P is 0.035% or less.
[0038] Phosphorus (P) is an inevitably contained impurity, and its content is preferably
managed as low as possible. Theoretically, it is advantageous to control the content
of phosphorus to 0% by weight, but inevitably, it must be contained in the manufacturing
process. Therefore, it is important to manage the upper limit, and in the present
disclosure, the upper limit is managed as 0.035%.
[0039] The content of S is 0.01% or less.
[0040] Sulfur (S) is an inevitably contained impurity, and it is preferable to manage the
content as low as possible. Theoretically, it is advantageous to control the content
of phosphorus to 0% by weight, but inevitably, it must be contained in the manufacturing
process. Therefore, it is important to manage the upper limit, and in the present
disclosure, the upper limit is managed as 0.01%.
[0041] In addition, utility ferritic stainless steel with 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.
[0042] The content of Cu is 0.2% or less.
[0043] Copper (Cu) is an austenite-forming element similar to Ni, which contributes to the
improvement of the toughness of the base material. In addition, there is an effect
of improving the ductility when adding a certain amount of Cu. However, considering
the cost aspect, the content is limited to 0.2% or less.
[0044] The content of Ti is 0.03% or less.
[0045] Titanium (Ti) is an element that fixes carbon and nitrogen, and forms a precipitate
to lower the content of solid solution C and solid solution N to improve corrosion
resistance of steel. However, if the content is excessive, surface defects may occur
due to coarse Ti inclusions, and there is a problem in that manufacturing costs increase,
and the upper limit may be limited to 0.03%.
[0046] The remaining component of the present disclosure is iron (Fe). However, in the normal
manufacturing process, impurities that are not intended from the raw material or the
surrounding environment can be inevitably mixed, and therefore cannot be excluded.
Since these impurities are known to anyone skilled in the ordinary manufacturing process,
they are not specifically mentioned in this specification.
[0047] According to one embodiment of the present disclosure, the utility ferritic stainless
steel with excellent hot workability that satisfies the aforementioned alloy composition
may satisfy a range of 10.5 to 12.0 in a ferrite factor represented by the following
equation (1).

[0048] In the above equation, Cr and Si are ferrite forming elements, which inhibit the
formation of austenite at high temperatures, and Mn, Ni, C, and N are austenite forming
elements, which promote the formation of austenite at high temperatures. That is,
the larger the ferrite factor, the more difficult it is to form austenite at high
temperatures.
[0049] For example, when the ferrite factor exceeds 12, the formation of δ-ferrite single-phase
structure during heat treatment may cause hot workability deterioration due to grain
coarsening. When the ferrite factor is less than 10.5, the δ-ferrite fraction falls
within a range of 15 to 30% due to a decrease in the material temperature during hot
rolling, and thus there is a problem of inferior hot workability. Therefore, it is
preferable that the ferrite factor satisfies the range of 10.5 to 12.
[0050] According to one embodiment of the present disclosure, the fraction of the δ-ferrite
phase upon heating before hot rolling of utility ferritic stainless steel with excellent
hot workability satisfying the aforementioned alloy composition may be 80 to 95%.
[0051] Accordingly, even if it is considered that the heated slab decreases in temperature
due to contact with the roll during hot rolling, a relatively high reduction of area
of 70% or more can be exhibited. Therefore, it is possible to solve the problem of
linear flaws and edge cracks occurring in the production process of the product.
[0052] Next, a manufacturing method of a utility ferritic stainless steel with excellent
hot workability according to another aspect of the present disclosure will be described.
[0053] A manufacturing method of a utility ferritic stainless steel with excellent hot workability
according to an embodiment of the present disclosure includes a manufacturing a slab
comprising, in percent (%) by weight 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
of iron (Fe) and other inevitable impurities; and a hot rolling the slab after heating
the slab, and the heating of the slab may be performed in a temperature range of 1200
to 1250°C so that the fraction of δ-ferrite phase in the internal structure of the
slab is 80 to 95%.
[0054] The reason for the numerical limitation of the alloying element content is as described
above.
[0055] 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.
[0056] The produced slab is subjected to a heating process before hot rolling.
[0057] The present disclosure adjusts the heating temperature of the slab to control the
fraction of δ-ferrite phase in the internal structure of the slab to be 80 to 95%
during the heating process.
[0058] FIG. 2 is a picture for explaining the change in the microstructure during the high-temperature
slab heat treatment according to Examples and Comparative Examples of the present
disclosure.
[0059] The δ-ferrite measured in the present disclosure refers to the δ-ferrite content
present during slab heating before hot rolling. In order to infer the δ-ferrite content
in this state, the specimens heat-treated at 1250°C for various alloying components
were quenched and quantified through observation of microstructures at room temperature
as shown in FIG. 2.
[0060] Referring to FIG. 2, in Examples 1 and 2, it can be confirmed that the tempered-martensite
structure is distributed along the grain boundary of the ferrite matrix. On the other
hand, in the case of the comparative examples, it can be seen that the fraction of
martensite is higher than that of ferrite, and it can be seen that the phase fraction
of austenite and δ-ferrite constituting the microstructure changes according to the
change in alloy composition.
[0061] The difference in phase fraction of the initial slab state greatly affects the hot
workability of the material, and the results are shown in FIG. 3.
[0062] FIG. 3 is a result showing the reduction of area (%) measured through a high temperature
gleeble tensile test at various hot rolling temperatures of 900 to 1200°C after maintaining
for 3 hours at a temperature of 1250°C using various alloy components. The measured
reduction of area means that the higher the value, the better the hot workability.
[0063] As described above, the fraction of δ-ferrite phase in the internal structure of
the slab increases as the heating temperature of the slab increases, and thus in order
to control the fraction of the δ-ferrite phase to be 80 to 95%, the heating temperature
of the slab is set to 1200 to 1250°C. To this end, it is achieved by charging the
slab into the interior of the furnace and then maintaining the interior of the furnace
at 1200 to 1250°C for at least 3 hours.
[0064] Hereinafter, it will be described in more detail through a preferred embodiment of
the present disclosure.
Example
[0065]
[Table 1]
|
C |
N |
Si |
Mn |
Cr |
Ni |
P |
S |
Inventive Example 1 |
0.017 |
0.017 |
0.79 |
1.21 |
12.4 |
0.44 |
0.02 |
> 0.001 |
Inventive Example 2 |
0.012 |
0.010 |
0.6 |
1.0 |
11.8 |
0.4 |
0.018 |
0.001 |
Comparative Example 1 |
0.011 |
0.013 |
0.31 |
1.4 |
11.3 |
0.41 |
0.018 |
> 0.001 |
Comparative Example 2 |
0.019 |
0.013 |
0.44 |
0.6 |
11.0 |
0.42 |
0.02 |
> 0.001 |
Comparative Example 3 |
0.016 |
0.014 |
0.5 |
0.56 |
11.3 |
0.4 |
0.018 |
> 0.001 |
Comparative Example 4 |
0.021 |
0.018 |
0.6 |
1.079 |
11.48 |
0.44 |
0.026 |
0.008 |
[0066] As shown in Table 1, after heat treatment was performed for 3 hours at a temperature
of 1250°C for the slabs produced while changing the content of each component, hot
rolling was performed, and accordingly, the δ-ferrite fraction, austenite fraction,
and Reduction of Area, linear flaw and edge cracks are shown in Table 2.
[Table 2]
|
Ferrite factor |
δ-ferrite fraction(%) |
Austenite fraction (%) |
Reduction of Area (%) |
Linear flaw |
Edge crack |
Inventive Example 1 |
11.6 |
91 |
9 |
≥ 70 |
x |
X |
Inventive Example 2 |
10.9 |
82 |
18 |
≥ 70 |
X |
X |
Comparative Example 1 |
7.8 |
5 |
95 |
≥ 55 |
O |
O |
Comparative Example 2 |
9.5 |
5 |
95 |
≥ 55 |
O |
O |
Comparative Example 3 |
10.4 |
35 |
65 |
≥ 52 |
O |
O |
Comparative Example 4 |
9.6 |
15 |
85 |
≥ 48 |
O |
O |
[0067] Referring to FIG. 2 and Table 2, when heating before hot rolling according to the
change of alloy component, it can be seen that the phase fraction of austenite and
δ-ferrite constituting the microstructure of the slab changes. Specifically, in Table
2, it can be seen that, in the inventive examples, the δ-ferrite phase is more than
the austenite phase, whereas in the comparative examples, the austenite phase is more
than the δ-ferrite phase.
[0068] Referring to Table 2 and Table 3, in the case of inventive examples, compared to
comparative examples, it shows a ferrite fraction of about 80% or more at an initial
heat treatment condition of 1250°C. In the high temperature state during hot working,
it showed a 98% level reduction of area similar to the previous one, and it can be
seen that the reduction of area decreases to about 70% as the temperature decreases.
That is, linear flaw and edge cracking did not occur due to a relatively high reduction
of area at a low temperature compared to the comparative example.
[0069] On the other hand, in Comparative Examples 1 and 2, the Si content was 0.31% and
0.44%, which was less than 0.5%, and as the ferrite factor was derived low, the ferrite
fraction of about 5% or less was exhibited under the initial heat treatment conditions
of 1250°C. In the high temperature state during hot working, a high reduction of area
of about 98% was shown, but it can be seen that the reduction of area decreases to
about 55% as the temperature decreases. That is, linear flaw and edge cracks occurred
due to the low reduction of area at relatively low temperatures.
[0070] In addition, in the case of Comparative Examples 3, all the component ranges of the
present disclosure were satisfied, but the ferrite factor was 10.4, which was less
than 10.5, indicating a ferrite fraction of about 35% at the initial heat treatment
condition of 1250°C. In the high temperature state during hot working, it showed a
high reduction of area of about 98%, but it can be seen that the reduction of area
decreases by about 52% compared to Comparative Examples 1 and 2 as the temperature
decreases. Linear flaw and edge cracks occurred due to securing low reduction of area
at low temperature.
[0071] In addition, in the case of Comparative Examples 4, as the carbon content was 0.021%,
which exceeded 0.2% and as the ferrite factor was derived low, it showed a ferrite
fraction of about 15% at the initial heat treatment condition of 1250°C. In the high
temperature state during hot working, it showed a high reduction of area of about
98%, but it can be confirmed that the reduction of area decreases by about 48% compared
to Comparative Examples 1 and 2 as the temperature decreases. Linear flaw and edge
cracks occurred due to securing low reduction of area at low temperature.
[0072] When hot rolling is performed after heating the slab in a temperature range of 1200
to 1250°C so that the δ-ferrite content satisfies the range of 80 to 95%, the utility
ferritic stainless steel manufactured according to an embodiment of the present disclosure
is capable of producing wide materials, while minimizing the occurrence of linear
flaws and edge cracks.
[0073] While the present disclosure has been particularly described with reference to exemplary
embodiments, it should be understood by those of skilled in the art that various changes
in form and details may be made without departing from the spirit and scope of the
present disclosure.
[Industrial Applicability]
[0074] The ferritic stainless steel according to the present disclosure has improved durability
and can be used as a material for bus structures.