JP7063810B2 - High-strength duplex stainless steel with a minimum tensile strength of 600 MPa, hot-rolled, precipitation-strengthened, and finely divided crystal grains, and a method for manufacturing the same. - Google Patents

Description

The present invention relates to a method for producing a hot-rolled high-strength duplex stainless steel. Further, the present invention relates to hot-rolled high-strength duplex stainless steels having a tensile strength of over 600 MPa and a total elongation of 25%.

Vehicle fuel consumption and its emissions are one of the major causes of air pollution. Environmentally friendly lightweight vehicle design is required to address environmental pollution issues. The success of lightweight vehicles requires the use of advanced high-strength steel (AHSS) steel plates. However, since AHSS steel plate is inferior in formability, it cannot be easily applied to various automobile parts. Therefore, the demand for ductility and formability of AHSS steel sheets is increasing more and more. Therefore, addressing this trend required the development of hot-rolled steel sheets with high tensile strength combined with excellent uniform elongation, work hardening and total elongation for automotive parts such as wheel web applications. ..

Therefore, to replace existing steel grades used in automotive structures and wheel web applications, we have developed hot-rolled steel sheets that not only have a minimum tensile strength of 600 MPa, but also have good formability and good surface quality. There is a need to .

European Patent Application Publication No. 1398392 (A1) and US Patent No. 8337643 disclose methods for producing hot-rolled two-phase (ferrite + martensite) steels with a minimum tensile strength of 590 MPa. .. The proposed steel has strength but contains a large amount of Si (minimum 0.5% by weight (European patent application), 0.2% by weight (US patent)). The presence of Si will cause the generation of surface scales commonly referred to as tiger marks.

European Patent No. 2053139 (B1) discloses a method of forming a hot-rolled steel sheet and then performing a heat treatment so as to achieve a tensile strength varying in the range of 440 to 640 MPa. However, the heat treatment after molding, which is an essential configuration of this patent, tends to lead to an increase in processing cost and is not suitable for mass production.

European Patent Application Publication No. 2578714 (A1) discloses a method for producing a hot-rolled steel sheet having excellent quench curability and stretch flangeability and a tensile strength of at least 590 MPa. According to the proposed method, this steel must contain 1.7-2.5% by weight Mn. When Mn is added in such a large amount, Mn tends to segregate in the central portion in the thickness direction, not only cracks occur during press molding, but also it becomes difficult to obtain the desired stretch flangeability.

Understanding the wheels of an automobile is also important for the development of steel. Automotive wheels consist of discs and rims. The discs are press molded, but the rims are flared and rolled after butt flash welding. Therefore, the material required to form the disc must have good deep drawability, elongation formability and elongation, while the material required to form the rim has good formability after welding. There is a need. Wheel discs and rims are formed in each process and then assembled by spot welding or arc welding. Therefore, both materials used for rims and discs need to have good spot weldability. From the point of view of automobile wheel use, the most important functional requirement for automobile wheels is durability, which can be improved by increasing the fatigue strength of the wheel material.

Various studies conducted in recent years have shown that both precipitation hardened steels and two-phase (DP) steels are suitable for wheel disc applications. From the examination of fatigue strength, the upper limit of the tensile strength of steel for wheels is about 600 MPa (or 85 ksi) (Irie, Tsunoyama, Shinozaki, Kato, "SAE Paper", No. 880695, 1988 (T. Irie,). Tsunoyama, M. Shinozaki and T. Kato: SAE Paper No. 880695, 1988). This is because when the tensile strength exceeds 600 MPa, the notch sensitivity increases and the fatigue strength decreases. 600 MPa (or HR). Hot-rolled DP steel with a tensile strength of -DP600) has become a very popular choice for wheel disc applications due to its excellent strength and formability as well as good elongation (large n value) and spot weldability. However, it is difficult to manufacture the HR-DP600 on any rolling mill, because many process parameters, such as the finish rolling temperature, are needed to obtain the desired microstructure that determines the final mechanical properties. , Cooling rate etc. need to be optimized, runout table length, available water volume etc. need to be fine-tuned taking into account the rolling mill configuration. All existing patents and literature are steel fatigue. It contains a significant amount of Si to increase the strength of the ferrite for life.

European Patent Application Publication No. 1398392 US Pat. No. 8,376,43 European Patent No. 2053139 European Patent Application Publication No. 2578714

Irie, Tsunoyama, Shinozaki, Kato, "SAE Paper", No. 880695, 1988

In view of the above limitations inherent in the prior art, an object of the present invention is to propose a method for producing a hot-rolled precipitation-strengthened high-strength two-phase steel sheet having a low Si content and a tensile strength of more than 600 MPa. It is to be.
Another object of the present disclosure is to propose a method for producing a hot-rolled precipitation-strengthened high-strength two-phase steel sheet having a low Si content.
Another object of the present disclosure is to propose a hot-rolled precipitation-strengthened high-strength two-phase steel sheet having a low Si content and a tensile strength of more than 600 MPa.
Yet another object of the present disclosure is to propose a hot-rolled precipitation-strengthened high-strength two-phase steel sheet with a low Si content.

The present invention provides a method for manufacturing a two-phase steel sheet. This method
The chemical composition is by weight%, C: 0.03 to 0.12, Mn: 0.8 to 1.5, Si: <0.1, Cr: 0.3 to 0.7, S: maximum 0. 008, P: maximum 0.025, Al: 0.01 to 0.1, N: maximum 0.007, Nb: 0.005 to 0.035, V: maximum 0.06 step for producing molten steel,
Steps to continuously cast molten steel into slabs,
A step of hot rolling a slab onto a hot-rolled steel sheet at a finish rolling temperature (FRT) of 840 ± 30 ° C.
A step of cooling a hot-rolled steel sheet on a runout table at a cooling rate of 40 to 70 ° _{C./s until an intermediate temperature (TINT) 720 ≤ T INT ≤} 650 is reached.
A step that naturally cools a hot-rolled steel sheet for 5 to 7 seconds, and a carbon-enriched austenite that remains after quenching the hot-rolled steel sheet to a winding temperature of less than 400 ° C at a cooling rate of 40 to 70 ° C / s is transformed into martensite. Includes steps to make.

Various steps in the method of producing high-strength duplex stainless steel according to one embodiment of the present invention. FIG. 6 is a schematic diagram of a cooling curve for obtaining a high-strength duplex stainless steel according to a specific example of the present invention. Tensile stress-strain curve of steel plate 1 according to a specific example of the present invention. An optical micrograph (night game etching) of a steel plate 1 according to a specific example of the present invention. An optical microscope image of a repera-etched sample according to a specific example of the present invention (white: martensite (α), black: ferrite (α')). An optical microscope image of a repera-etched sample according to a specific example of the present invention (fine crystal grains of about 2 μm can be seen). A scanning electron microscope image of a steel plate 1 according to a specific example of the present invention. (A) TEM bright-field image of one precipitate in the ferrite matrix. (B) Dark field image of FIG. 8 (a). (C) Selected area diffraction pattern of Nb (C, N) precipitates. (D) Dark field image of Nb (C, N) precipitate. (E) EDS spectrum of precipitate. (F) Composition of precipitate.

Various embodiments of the present invention provide a method of manufacturing a two-phase steel sheet comprising the following steps. That is, this method
The chemical composition is C: 0.03 to 0.12, Mn: 0.8 to 1.5, Si: less than 0.1, Cr: 0.3 to 0.7, S: maximum 0. 008, P: maximum 0.025, Al: 0.01 to 0.1, N: maximum 0.007, Nb: 0.005 to 0.035, V: maximum 0.06 step for producing molten steel,
Steps to continuously cast molten steel into slabs,
A step of hot rolling a slab onto a hot-rolled steel sheet at a finish rolling temperature (FRT) of 840 ± 30 ° C.
A step of cooling a hot-rolled steel sheet on a runout table at a cooling rate of 40 to 70 ° _{C./s until an intermediate temperature (TINT) 720 ≤ T INT ≤} 650 is reached.
A step that naturally cools a hot-rolled steel sheet for 5 to 7 seconds, and a carbon-enriched austenite that remains after quenching the hot-rolled steel sheet to a winding temperature of less than 400 ° C at a cooling rate of 40 to 70 ° C / s is transformed into martensite. Includes steps to make.

According to another specific example of the present invention, in% by weight, C: 0.03 to 0.12, Mn: 0.8 to 1.5, Si: less than 0.1, Cr: 0.3 to 0. 7, S: maximum 0.008, P: maximum 0.025, Al: 0.01 to 0.1, N: maximum 0.007, Nb: 0.005 to 0.035, V: maximum 0.06 A two-phase steel plate having a chemical composition is provided.

FIG. 1 shows a method 100 for manufacturing a two-phase steel sheet. In step 104, molten steel is manufactured. The composition of the molten steel is C: 0.03 to 0.12, Mn: 0.8 to 1.5, Si: less than 0.1, Cr: 0.3 to 0.7, S: maximum in% by weight. 0.008, P: maximum 0.025, Al: 0.01 to 0.1, N: maximum 0.007, Nb: 0.005 to 0.035, V: maximum 0.06.

The addition of each alloying element and the limitation of each element are essential to achieve the target microstructure and properties.

C: 0.03 to 0.12%
Carbon is one of the most effective and economical fortifying elements. Carbon combines with Nb or V to form carbides or carbonitrides, which causes precipitation strengthening. This requires 0.03% or more C in the steel. However, in order to obtain good weldability, the carbon content must be limited to less than 0.12%.

Mn: 0.8-1.5%:
Manganese not only strengthens the solid solution of ferrite, but also lowers the transformation temperature from austenite to ferrite, thereby reducing the ferrite grain size. However, the Mn content cannot exceed 1.5%. Such a high content increases the occurrence of central segregation during continuous casting.

Si <0.1% by weight
Silicon is a very effective solid solution strengthening element like Mn. However, Si leads to surface scale problems in hot rolling. It should be limited to less than 0.1% to prevent the formation of surface scales.

Nb: Maximum 0.035%
Niobium is the most potent trace element for grain refinement, even when added in trace amounts. Niobium lowers the austenite-to-ferrite transformation temperature in a solid solution, not only reducing the ferrite grain size, but also promoting the formation of low temperature transformation products such as bainite. However, in order to guarantee the effectiveness of Nb, Nb must not be precipitated before reaching the transformation temperature. The maximum Nb content is limited to 0.035% to ensure that all Nb components remain in the solid solution before rolling is initiated and to be added alone.

V: Up to 0.06%
Vanadium microalloying also results in precipitation strengthening and grain refinement. Since the solubility of vanadium in austenite is higher than that of other microalloy elements, it is likely to remain in the solid solution before transformation. During the phase transformation, vanadium precipitates at the grain boundaries as carbides and / or nitrides, depending on the relative carbon and nitrogen content, resulting in precipitation strengthening and grain refinement. It is necessary to add either Nb or V to achieve the desired enhancement. Both can be added. When only V is added, a maximum of 0.06% by weight is required.

P: Maximum 0.025%
A high phosphorus content causes a decrease in toughness and weldability due to segregation of P into grain boundaries, and therefore needs to be limited to a maximum of 0.025%.

S: Maximum 0.008%
Sulfur content should be limited. Otherwise, the inclusions will be very large and the formability will be deteriorated.

N: <0.007
If the N content is too high, the melting temperature of Nb (C, N) will rise and the effectiveness of Nb will decrease. Reducing the nitrogen content also has a positive effect on aging stability, toughness and resistance to intergranular stress corrosion cracking in the heat-affected zone of the weld. Therefore, it is preferable that the nitrogen content should be kept below 0.007.

Al: 0.01-0.1
Al is used to remove unwanted oxygen from molten steel. Therefore, the steel contains a certain amount of Al. The amount may be up to 0.05% by weight. Excessive (high) Al in steelmaking is a major problem because it reduces thermal deformation of the cast slab as well as nozzle clogging during casting. Therefore, Al needs to be limited to 0.1% by weight.

In step 108, molten steel is continuously cast into a slab.
The molten steel of the specified composition is first continuously cast by either a conventional continuous casting machine or a thin slab casting machine. When casting in a thin slab casting machine, it is not permissible for the temperature of the casting slab to drop below 950 ° C. This is because when the thin slab temperature is lower than 950 ° C., precipitation of Nb occurs. This is because it is difficult to completely dissolve the precipitate in the subsequent reheating step, and the effect of strengthening the precipitate is lost.

Reheating After casting a slab with the composition specified above, the slab is reheated at a temperature of 1100 ° C to 1200 ° C for 20 minutes to 2 hours. The reheating temperature should be above 1100 ° C. to ensure that the Nb and / or V precipitates formed in the previous processing steps are completely dissolved. A reheating temperature above 1200 ° C. is not desirable as it causes austenite coarsening and / or loss due to excessive scale.

In step 112, the slab is hot-rolled into a hot-rolled steel plate at a finish rolling temperature (FRT) of 840 ± 30 ° C.

Hot rolling is composed of a roughing process performed at a temperature higher than the recrystallization temperature and a finishing process performed at a temperature lower than the recrystallization temperature when rolling with a conventional hot rolling machine. When the steel plate is manufactured using a continuous processing process, there is no separate rough rolling machine and the rolling schedule is designed to break the cast structure at the first stand and perform finish rolling below the recrystallization temperature. More specifically, in any setting, the finish rolling needs to be performed at a temperature of 840 ± 30 ° C. in TFRT.

Layered cooling on the run-out table (ROT) In step 116, the hot-rolled steel plate is cooled on the run-out table at a cooling rate of 40 ° C. to 70 ° C./s. This cooling rate is maintained until the intermediate temperature (T _INT ) 720 ≤ T _INT ≤ 650 is achieved.

The cooling rate should be greater than 40 ° C./sec to prevent the formation of pearlite. The formation of any pearlite or pseudo-pearlite results in deterioration of both tensile strength and stretch flangeability. When the cooling rate is high, the ferrite starting temperature is lowered, and the ferrite grain size is also made finer. In addition, the growth of ferrite is also inhibited. By increasing the cooling rate and controlling the rolling schedule, the desired crystal grain size of 2 to 6 μm can be achieved. The cooling rate should not exceed 70 ° C./s because the desired amount of ferrite is not formed. This large cooling rate continues to intermediate temperatures. The intermediate temperature (T _INT ) should be 650 ° C ≤ T _INT ≤ 720 ° C.

In step 120, the steel plate is naturally cooled while being conveyed on the RoT. Air cooling time is very important, 5-7 seconds. If the air cooling time of the steel plate is less than 5 seconds, a sufficient amount of ferrite will not be formed. On the other hand, if the air cooling time of the steel plate exceeds 7 seconds, the amount of martensite becomes insufficient.

During this time, austenite transforms into ferrite. However, the entire austenite does not transform into ferrite because there is not enough time to completely transform it. As a result, the austenite remaining at the end of natural cooling is carbon-rich. This is because ferrite cannot contain the average carbon content of steel.

After natural cooling in step 120, the steel plate is further rapidly cooled in step 124. This ensures that the remaining carbon-enriched austenite is transformed into martensite. The cooling rate during this period is 40 ° C. to 70 ° C./s until a winding temperature of less than 400 ° C. is reached. The cooling temperature can be as low as about 100 ° C.

The contribution of reinforcement from solid solution elements and trace alloying elements is limited. Further, the degree of grain refinement possible by controlling rolling and cooling is limited to 2 μm. As a result, high-strength duplex stainless steel can be obtained.

The resulting microstructure has martensitic particles / phase in the ferrite matrix. The microstructure is uniform. In other words, the martensite phase is uniformly distributed throughout the ferrite matrix. In addition, bainite or pseudo-pearlite / pearlite and grain boundary cementite are avoided, and high-strength two-phase steel sheets achieve good work hardening, low yield strength and continuous yield. The contribution of each phase of the microstructure is shown below.

a) Ferrite The hot-rolled steel sheet according to the present invention has 75 to 90% by volume of ferrite. Ferrite is strengthened by solid solution strengthening due to contribution from Mn. With appropriate processing conditions, the crystal grain size is limited to 2-5 μm. Aluminization of ferrite enhances ferrite by a Hall-Petch relationship. Further, precipitation is strengthened by the formation of fine Nb and V (CN) precipitates.

b) Martensite The amount of martensite in the microstructure is 10-25% by volume. Martensite reinforcement is due to its structure, carbon content and high dislocation density.

c) The amount of martensite in the bainite microstructure is less than 5% by volume.

The high-strength two-phase steel sheet has an improved fatigue life because fine precipitates are present in the ferrite matrix bonded to martensite as the second phase.

The yield strength of the obtained high-strength two-phase steel sheet is 350 to 500 MPa. The obtained tensile strength is at least 600 MPa. The minimum uniform elongation and total elongation are 16% and 22%, respectively.

Further, the work hardening index (n) of the high-strength two-phase steel sheet is 0.15 to 0.16. The ratio of the yield strength of the high-strength two-phase steel sheet to the tensile strength is 0.6 to 0.8, and the hole expansion rate of the punched hole is about 40%.

For the purpose of illustration only, a slab having a composition (shown in Table 1) according to the method 100 (steel plate 1) was continuously cast by a CSP mill, and the slab was hot-rolled. The mechanical properties of the steel plate are shown in Table 2, Table 3 and Table 4. The microstructure of the steel plate is shown in FIGS. 4, 5, 6 and 7. From the mechanical properties and microstructure obtained, it is clear that the desired properties can be achieved if the chemical composition and ROT cooling parameters meet the requirements of the present disclosure.

Microstructures by light microscope (etched with Nital and Repeller) and SEM are shown in FIGS. 4, 5, 6 and 7. Its microstructure consists of ferrite and martensite. Tensile test samples with a measuring section length of 50 mm were prepared in accordance with ASTM E8 standards. A typical tensile curve is shown in FIG. As is clear from the chart, the newly developed steel has a tensile strength of 600 MPa or more, a uniform elongation of 16%, a total elongation of 22% or more, a work hardening index of 0.15, and a yield ratio (tensile). The yield strength with respect to the strength) is 0.6 to 0.8. In steel, fine precipitates are dispersed in the ferrite matrix. Identification of these precipitates is confirmed using energy dispersive spectroscopy (EDS) and selective region diffraction (SAD) techniques in TEM. As shown in FIGS. 8a-f, the precipitate is mainly Nb (C, N). Further, the steel has a very fine average crystal grain size of less than 3 μm.

Claims (17)

It is a manufacturing method of two-phase steel sheet.
The chemical composition is C: 0.03 to 0.12, Mn: 0.8 to 1.5, Si: less than 0.1, Cr: 0.3 to 0.7, S: maximum 0. 008, P: maximum 0.025, Al: 0.01 to 0.1, N: maximum 0.007, Nb: 0.005 to 0.035, V: maximum 0.06, the balance is iron and unavoidable Steps to manufacture molten steel, which is an impurity,
The step of continuously casting the molten steel into a slab,
A step of hot rolling the slab onto a hot-rolled steel sheet at a finish rolling temperature (FRT) of 840 ± 30 ° C.
A step of cooling the hot-rolled steel sheet on a run-out table at a cooling rate of 40 to 70 ° C./s until an intermediate temperature (T _INT ) of 650 ° C ≤ T _INT ≤ 720 ° C is reached.
Martensite is a step that naturally cools the hot-rolled steel sheet for 5 to 7 seconds, and martensite that remains after quenching the hot-rolled steel sheet to a winding temperature of less than 400 ° C. at a cooling rate of 40 to 70 ° C./s . Including the step of transforming into
The production method, wherein the two-phase steel sheet has 75 to 90% by volume of ferrite, 10 to 25% by volume of martensite, and less than 5% by volume of bainite, and has a crystal grain size of 2 to 5 μm. The production method according to claim 1, wherein the continuously cast slab is reheated in a temperature range of 1100 ° C. to 1200 ° C. for 20 minutes to 2 hours in order to dissolve the precipitate. The manufacturing method according to claim 1, wherein the two-phase steel sheet has a yield strength of 350 to 500 MPa. The manufacturing method according to claim 1, wherein the two-phase steel sheet has a tensile strength of 600 MPa or more. The manufacturing method according to claim 1, wherein the two-phase steel sheet has a uniform elongation of 16% or more. The manufacturing method according to claim 1, wherein the two-phase steel sheet has a total elongation of 22% or more. The manufacturing method according to claim 1, wherein the work hardening index (n) of the two-phase steel sheet is 0.15 to 0.16. The manufacturing method according to claim 1, wherein the ratio of the yield strength of the two-phase steel sheet to the tensile strength is 0.6 to 0.8. The manufacturing method according to claim 1, wherein the two-phase steel sheet has a hole expansion rate of 38.5% to 40% for punched holes. By weight%, C: 0.03 to 0.12, Mn: 0.8 to 1.5, Si: less than 0.1, Cr: 0.3 to 0.7, S: maximum 0.008, P: Chemistry containing up to 0.025, Al: 0.01-0.1, N: up to 0.007, Nb: 0.005 to 0.035, V: up to 0.06, with the balance being iron and unavoidable impurities A duplex stainless steel having a composition of 75-90% by volume of ferrite, 10 to 25% by volume of martensite, less than 5% by volume of bainite, and a crystalline grain size of 2 to 5 μm. The duplex stainless steel sheet according to claim 10, wherein the duplex stainless steel has a yield strength of 350 to 500 MPa. The duplex stainless steel sheet according to claim 10, wherein the duplex stainless steel sheet has a tensile strength of 600 MPa or more. The duplex stainless steel sheet according to claim 10, wherein the duplex stainless steel sheet has a uniform elongation of 16% or more. The duplex stainless steel sheet according to claim 10, wherein the duplex stainless steel sheet has a total elongation of 22% or more. The duplex stainless steel plate according to claim 10, wherein the work hardening index (n) of the duplex stainless steel sheet is 0.15 to 0.16. The duplex stainless steel sheet according to claim 10, wherein the ratio of the yield strength of the duplex stainless steel to the tensile strength is 0.6 to 0.8. The duplex stainless steel sheet according to claim 10, wherein the duplex stainless steel sheet has a hole expansion ratio of 38.5% to 40%.

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