KR100325542B1 - Ferritic stainless steel for welding structure and method thereof - Google Patents

Abstract

PURPOSE: A method for manufacturing ferritic stainless steel for welding structure is provided, which can produce a ferritic stainless steel for welding structure having equal quality of STS410L steel by the addition of Ni and Cu to conventional STS410L steel to widen possible temperature range for annealing. CONSTITUTION: The method for manufacturing ferritic stainless steel for welding structure includes step of continuous casting a steel comprising 0.02 wt.% or less of C, Si 0.2 to 0.4 wt.%, Mn 0.7 to 1.0 wt.%, 0.035 wt.% or less of P, 0.008 wt.% or less of S, Cr 11.0 to 11.6 wt.%, 0.020 wt.% or less of N, Cu 0.2 to 0.5 wt.%, Ni 0.5 to 1.0 wt.%, a balance of Fe and incidental impurities, followed by hot rolling; batch annealing the hot rolled steel strip at a temperature range of 640 to 680°C; cold rolling the steel strip; continuous annealing the steel strip under an atmosphere condition of 750 to 820°C; and pickling and temper rolling.

Description

Ferritic stainless steel for welding structure and manufacturing method

The present invention relates to a ferritic stainless steel for welding structure and a method of manufacturing the same, in particular, having excellent impact properties of the welded part and the base material, and can expand the cold-annealed temperature range to significantly reduce material defects during cold-rolled annealing It relates to a stainless steel and a method for manufacturing a steel sheet using the same.

Generally, stainless steels used for welded structures are representative steels of STS410L, and are widely used for framing and exterior of containers or disc brakes for motorcycles. Such STS410L steel is produced through the manufacturing process of melting → refining → casting → hot rolling → batch annealing → cold rolling → cold rolling annealing → temper rolling. In the above process, the cold rolling annealing process is a process of recrystallizing and softening the material by heat-treating the work hardened material during cold rolling at an arbitrary temperature higher than the recrystallization temperature and lower than the AC ₁ temperature. At this time, the annealing conditions are defined by the annealing temperature and time, and the wider the annealing temperature range, the easier the operation. However, in the case of conventional STS410L steel, the area between the temperature at which softening starts through recrystallization and the temperature of AC ₁ at which martensite formation starts is narrow, so that the annealing temperature is very difficult to control during cold rolling annealing, which causes a defect in material defects.

1 is a graph showing the material change according to the annealing temperature of the conventional STS410L steel, the conventional STS410L steel in weight% C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less , S: 0.03% or less, Cr: 11.0 to 13.5%, Ni: 0.6% or less, residual amount of Fe, and other inevitable impurities. STS410L steel is formed in the same way as the annealing after cold rolling, as shown in Figure 1 the temperature range that can be annealing is very narrow to 20 ~ 30 ℃ cause an inferior material material when the annealing temperature is not controlled within ± 10 ℃ range There is a problem. In addition, in the case of thermocouples used for industrial purposes in general, the temperature range of 800 to 1000 ℃ has a measurement error of ± 5 ~ ± 10 ℃, the temperature range of annealing of the STS410L steel has a range similar to the measurement error of the thermocouple. Therefore, there is a problem that the material defects due to the annealing temperature or more during cold rolling annealing, the material deviation between lots (LOT) is large.

The present invention is to solve the above problems, and to add an appropriate amount of Ni and Cu to the STS410L steel to expand the annealing temperature range, and to provide a ferritic stainless steel for welding structure equivalent to STS410L steel and its manufacturing method. do.

1 is a graph showing a change in material according to the annealing temperature of the conventional STS410L steel.

2 is a graph showing the effect of Ni on the DBTT (ductile-brittle transition temperature) of 25Cr steel.

3 is a graph illustrating hot annealing conditions;

Figure 4 is a graph showing the hardness change of the material according to the temperature change of the hot-annealed.

5 is a photograph showing a change in the annealing structure according to the temperature change of the hot-rolled annealing.

Figure 6 is a photograph showing the change in the annealing structure according to the temperature change of the hot-rolled annealing.

7 is a graph showing the change in hardness according to the cold rolling annealing temperature change.

8 is a graph showing the impact characteristics of the base material.

9 is a graph showing thermal cycle reproducing conditions.

10 is a graph showing weld impact properties.

11 is a graph comparing the impact characteristics of the base material and the weld.

12 is a photograph showing a welded structure.

The present invention for achieving the above object, by weight% C: 0.02% or less, Si: 0.20 to 0.40%, Mn: 0.7 to 1.0%, P: 0.035% or less, S: 0.008% or less, Cr: 11.0 to 11.6 %, N: 0.020% or less, Cu: 0.2-0.5%, Ni: 0.5-1.0%, the residual amount of Fe and other inevitable impurities are achieved by the ferritic stainless steel for welding structure, of course, the above welding Performing continuous casting and hot rolling of the structural ferritic stainless steel at a temperature of 640 to 680 ° C .; Cold rolling the hot rolled steel strip and then cold rolling annealing at an ambient temperature of 750 to 820 ° C .; It is achieved by providing a method for producing a ferritic stainless steel for welded structure, which comprises the steps of pickling and temper rolling.

Hereinafter, the present invention will be described.

In the present invention, in order to solve the problem of causing material defects when the annealing temperature during annealing after cold rolling in the STS410L steel cannot be controlled within a range of ± 10 ° C, Cu: 0.2 to 0.5% and Ni: 0.5 to 1.0 in the STS410L steel. The present invention relates to a method for manufacturing a ferritic stainless steel for welded structure having excellent welded and base metal impact characteristics by greatly reducing cold-annealed material defects by expanding the cold-annealable temperature range by adding% to 50-70 ° C.

Hereinafter, the reason for limitation of the steel component of the present invention and the reason for addition thereof will be described.

C is a strong austenite-based stabilizing element that has the effect of increasing the strength of the product, but if the content is too high, the AC ₁ temperature is lowered during annealing to form martensite structure, and martensite at the heat affected zone during welding. Tissues are formed to embrittle the welds and the carbides are susceptible to deterioration in corrosion resistance. Therefore, the present invention is to limit the content of C to 0.02% or less.

Si acts as a deoxidizer and favors high temperature oxidation resistance, but its effect is insufficient when the addition amount is less than 0.20%, and when the addition amount is more than 0.40%, the weldability becomes poor and lamellar type silicate (Silicate) ) Because of occurrence of inclusions, cracking may occur during bending processing. Therefore, in the present invention, the content of Si is limited to 0.20 to 0.40%.

Mn not only acts as a deoxidizer, but also improves weldability, but when the added amount is less than 0.7%, Mn does not exert sufficient effect, and when more than 1.0% is added, austenite phase is generated. It is limited to 0.7 to 1.0%.

If the P content is large, workability and corrosion resistance are lowered, and P is segregated at the grain boundary during welding, causing deterioration of the welded part. Therefore, the P content is limited to 0.035% or less.

S is an element having a high affinity with Mn in the steel and acts as a starting point of corrosion when present as an inclusion in the form of MnS, and is limited to 0.008% or less in the present invention.

If the amount of Cr is too small, the corrosion resistance is lowered. If the amount of Cr is too high, the production cost is increased, so it is limited to 11.0 to 11.6% in the present invention.

N has the effect of improving the strength of the steel, but promotes the formation of austenite, in the present invention is limited to 0.020% or less.

Cu is a noble metal, which increases the natural corrosion potential of steel and decreases the cathode area by re-reduction, which not only improves the corrosion resistance but also improves the toughness of the weld by miniaturizing the structure of the weld. If the amount is less than%, the effect cannot be expected, and when the content is more than 0.5%, σ phase (Phase) is formed to embrittle the steel and inhibit the corrosion resistance, so the present invention is limited to 0.2 to 0.5%.

Ni is an austenite stabilizing element having a property of forming an austenite phase by itself, and the addition of Ni in ferritic stainless steel improves impact characteristics.

Figure 2 is a graph showing the effect of Ni on the DBTT (ductile-brittle transition temperature) of 25Cr steel, it can be seen that the DBTT is improved to -200 ℃ at room temperature by adding Ni from 2% to 5.8%. In terms of corrosion resistance, Ni stabilizes the passivation film and improves corrosion resistance and atmospheric corrosion resistance. In terms of stress corrosion cracking, the addition of Ni to ferrite stainless steel below 1.0% does not affect the stress corrosion sensitivity, but when Ni is added above 1.0%, it increases the stress corrosion sensitivity. Regarding the toughness of the welded part, a welded part having fine crystal grains is formed when Ni is added to 0.3% to 3.0% of 8-15Cr- <0.07C steel, so that the weld metal is strong, the toughness is improved, and the corrosion resistance of the welded part is improved. On the other hand, since the addition of Ni of the present invention promotes recrystallization softening at the time of cold rolling in 12Cr ferritic steel, Ni was limited to 0.5 to 1.0% in the present invention.

In the present invention, by weight%, C: 0.02% or less, Si: 0.2-0.4%, Mn: 0.7-1.0%, P: 0.035% or less, S: 0.008% or less, Cr: 11.0-11.6%, N: 0.02% Hereinafter, in order to manufacture the above stainless steel composed of 0.2% to 0.5% of Cu, 0.5% to 1.0% of Ni, residual Fe and other unavoidable impurities, first, continuous casting and hot rolling are carried out by a conventional method, and then 640 to 680. Hot-annealing at ° C .; Cold rolling the hot rolled steel sheet subjected to hot rolling annealing at an 750 to 820 ° C. ambient temperature; Pickling and temper rolling are provided.

This will be described in detail with reference to the following embodiments.

Example

Ferritic stainless steels of the inventive steels A, B, C and comparative steels 1 and 2 having the components shown in Table 1 were dissolved in a vacuum induction furnace to prepare 50 mm * 50 mm * 100 mm ingots. .

The ingot was heated at 1200 ° C. for 2 hours and hot rolled to produce a hot rolled steel sheet having a thickness of 6.4 mm. The hot rolled steel sheet was cut into a size of 20 mm * 20 mm * 6.4 mm and then heat cycle as shown in FIG. 3. Hot annealing was performed in a box furnace. The hot-rolled steel sheet was subjected to cold rolling to a thickness of 2.5 mm after removing the scale of the surface from the mixed solution of hydrofluoric acid and nitric acid. The cold rolled specimens were cut to a size of 20mm * 20mm * 2.5mm, and then heated to 750 ° C., 780 ° C., 820 ° C., 840 ° C., 860 ° C. for 30 seconds to confirm the cold range of possible annealing. After cooling with water. After the cold-annealed specimens were pickled in a mixed solution of hydrofluoric acid and nitric acid, mechanical properties and annealing structures were examined.

TABLE 1

* (Unit: weight%)

Figure 4 measures the change in hardness of the material according to the temperature change of the hot-annealed.

Comparative steels 1, 2 and inventive steels A and B having a Ni content of 1.0% or less show a gentle decrease in hardness when the annealing temperature increases from 640 ° C to 680 ° C. However, in the case of the inventive steel C having a Ni content of 1.47%, a high hardness of HRB 90 or more is measured in the entire annealing temperature range. This indicates that in the case of Invented Steel C containing 1.0% or more of Ni, fine martensite structure is generated during the hot annealing process in which the martensite structure generated during hot rolling must be decomposed into ferrite tissue, and thus the material is not softened sufficiently. .

This phenomenon is shown in Figure 5 and Figure 6 which shows the change in the annealing structure according to the temperature change of the hot annealing. As can be seen in the drawing, when the Ni content is less than 1.0%, the annealing temperature is increased from 640 ° C to 680 ° C, and the annealing structure is well formed, indicating a decrease in hardness. However, in the case of Ni content of 1.47%, the fine martensite is from 660 ° C. It can be seen that site organization occurs.

7 is a graph showing the change in hardness according to the cold rolling temperature change.

In comparison steels 1 and 2, the base region on the U curve showing the relationship between the annealing temperature and the hardness was very narrow, about 20 ° C. In the case of the inventive steels A and B, the base region on the U curve was 750 to 820 ° C. It can be seen that very wide in the range of 50 ~ 70 ℃. Here the base of the U curve corresponds to the annealing temperature range of the material.

Table 2 below shows the tensile test results after cold annealing of Comparative Steels 1,2 and Invented Steels A, B, and C.

TABLE 2

Ni content of 1.0% is easy to secure the ferrite single phase during annealing, showing a similar level of strength and ductility. However, 1.47% Ni-added steel (Invention Steel C) rapidly increased its strength and elongated rapidly due to the formation of fine martensite structure during annealing.

The impact characteristics of the base metal were first milled to a thickness of 4.0 mm of the hot rolled annealing material having a thickness of 6.4 mm, and then Charpy V-Notch specimens were prepared. And the impact test was performed in the temperature range of -100-100 degreeC, and DBTT (vTrs) was investigated.

8 is a graph showing the impact characteristics of the base material, the Ni content of 0.72% DBTT (vTrs) is about -65 ~ -70 ℃ It can be seen that the impact characteristics are similar, but the invention steel B DBTT is about -90 ℃ As a result, the impact characteristic of -20 ° C or more is improved. However, invented steel C to which Ni content was added 1.47% had DBTT (vTrs) of -60 ° C, and the impact characteristic was rather deteriorated. This is thought to be due to the formation of martensite during annealing of 1.47% Ni-added steel.

In order to investigate the impact characteristics of the weld, the 6.4mm thick hot rolled annealing material was milled to a thickness of 4.0mm, and then the weld cooling time (△ T _12/8 ) was measured using a metal thermal cycle simulator. Second control was used to reproduce the weld heat affected zone.

9 is a graph showing the thermal cycle reproducing conditions, the maximum temperature is 1350 ℃ to be the δ phase, the 1200 to 800 ℃ section is considering the transformation in the region of r + α or more during cooling at the maximum temperature. On the other hand, the 800 to 500 ° C section reproduces the temperature gradient under practical welding conditions. After applying the welding heat cycle to each test member as described above, a sub-size impact test piece having a thickness of 4 mm was manufactured to be easily compared with the impact characteristics of the base material, and the impact test was conducted at a temperature range of 50 to -80 ° C. The impact characteristics of each Ni content were investigated.

Figure 10 is a graph showing the impact characteristics of the weld, the impact characteristics of each test specimen was the most deteriorated comparative steel 1, Comparative steel 2, the invention steel A, B having a Ni content of more than 0.4% showed similar impact characteristics.

FIG. 11 is a graph comparing the impact characteristics of the base material and the weld part. In Comparative Steel 1, the impact properties of the weld part are significantly lower than that of the base material part.

12 is a photograph showing a welded structure, in which the welded structure is observed to determine the cause of the change in the impact characteristics of each test material. Comparative steel 1, which has the lowest impact characteristics, is an ideal structure containing martensite and a coarse ferrite phase of about 14%, whereas comparative steel 2 and invented steels A, B, and C containing 0.4% or more are fine martensite. Represents single-phase tissue.

In view of the above results, it can be seen that the amount of Ni added to the Cu-added STS410L steel affects the amount of weld martensite, and the amount and shape of martensite structure generated at this time affect the weld impact properties. Welded impact characteristics of Comparative Steel 2 and Invented Steels A, B, and C with 0.4 to 1.47% Ni were excellent, but the base steel impact characteristics of Comparative Steel 1 were relatively low.

As described above, the present invention was conducted to test the hot rolling, cold rolling annealing conditions and mechanical properties, impact characteristics by adding a small amount of Cu and Ni to the existing STS410L steel as shown in Table 3a and Table 3b.

TABLE 3a

TABLE 3b

In view of the above results, by adding 0.2 to 0.5% of Cu and 0.5 to 1.0% of Ni to the existing STS410L steel, the welded and base metal parts have better impact characteristics than the STS410L steel, and the cold rolling annealing temperature range is wider. It is possible to manufacture stainless steel for welding structure of 12Cr system which can greatly reduce defects.

Claims (2)

By weight% C: 0.02% or less, Si: 0.2-0.4%, Mn: 0.7-1.0%, P: 0.035% or less, S: 0.008% or less, Cr: 11.0-11.6%, N: 0.020% or less, Cu: A ferritic stainless steel for welded structure, which is composed of 0.2 to 0.5%, Ni: 0.5 to 1.0%, residual amount of Fe and other unavoidable impurities. In the manufacturing method of ferritic stainless steel for welded structures, in weight% C: 0.02% or less, Si: 0.2 to 0.4%, Mn: 0.7 to 1.0%, P: 0.035% or less, S: 0.008% or less, Cr: 11.0 to 11.6%, N: 0.020% or less, Cu: 0.2-0.5%, Ni: 0.5-1.0%, residual casting Fe and other unavoidable impurities after continuous casting and hot rolling, hot-annealed at 640-680 ° C Wow; Cold rolling the hot rolled steel sheet subjected to hot rolling annealing at an 750 to 820 ° C. ambient temperature; Pickling and temper rolling; Method for producing a ferritic stainless steel for welded structure, characterized in that consisting of.

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