TWI640639B - Dual phase steel and method of forming the same - Google Patents

Abstract

The invention provides a duplex steel and a manufacturing method thereof, which are characterized in that a steel preform having a specific composition is subjected to a specific hot rolling process condition and combined with a specific laminar flow cooling process condition to obtain a ferrite phase having a specific volume ratio and Ma Tian loose iron phase, which in turn obtains duplex steel with high tensile strength and high elongation.

Description

Duplex steel and manufacturing method thereof

The present invention relates to a steel material and a method of manufacturing the same, and more particularly to a duplex steel material and a method of manufacturing the same.

A typical duplex steel is a composite structural steel that contains a soft ferrite iron phase and a hard granulated iron phase dispersed in the ferrite iron phase. Therefore, duplex steel has both soft and hard properties, which can meet the needs of the new generation of automotive component steels, because duplex steels not only provide sufficient safety, but also have good formability. In general, the smaller the ratio of the lower the ratio, the better the formability of the material. The drop ratio is defined as the ratio of the drop strength to the tensile strength, where the drop strength is the stress required to initiate plastic deformation, and the tensile strength is the maximum stress that can be withstood before the material breaks. In addition, at the time of processing, due to the characteristics of the two-phase structure, the interface of the soft and hard structure of the conventional duplex steel is easily broken by the force. Therefore, the elongation of the general duplex steel is generally low.

In recent years, due to the increasing importance of energy conservation and environmental protection, the goal of automotive steel is to combine high strength and thin size. However, duplex steel with high strength will cause cold in the laminar cooling process when the thickness is less than 3 mm. However, if the temperature and/or temperature holding time are not properly controlled, or the steel is too cold due to the edge, the difference in the ratio and properties of the steel at different positions may occur, and side waves and medium waves may be generated, resulting in poor shape and surface. The problem of red rust, which in turn makes the steel unable to meet the needs of subsequent applications.

In view of the above, it is not necessary to provide a duplex steel and a method of manufacturing the same to obtain a high tensile strength, a low drop ratio, and a high elongation, and there is no problem of poor sheet shape and uneven distribution of two phases.

One aspect of the present invention provides a method for producing a duplex steel obtained by subjecting a steel preform containing a specific component to a hot rolling pass including a two-stage hot rolling step, followed by cooling to obtain a duplex steel.

Another aspect of the present invention is to provide a duplex steel having a ferrite phase having a specific volume ratio of a ferrite phase and a granulated iron phase and having a specific grain size.

According to an aspect of the present invention, a method for manufacturing a duplex steel is provided, which comprises providing a steel blank, performing a hot rolling process on the steel blank to obtain a rolled steel, and performing a laminar cooling process on the finished steel. Obtained duplex steel. The steel germline comprises from 0.08 wt% to 0.20 wt% carbon, from 1.5 wt% to 3.0 wt% manganese, from 1.0 wt% to 1.5 wt% niobium, from 1.0 wt% to 2.0 wt% aluminum, greater than 0 wt% and less than Or equal to 0.5% by weight or less of molybdenum, more than 0% by weight and less than or equal to 0.20% by weight of titanium, more than 0% by weight and less than or equal to 0.20% by weight of vanadium, more than 0% by weight and less than or equal to 0.01% by weight The following nitrogen, less than or equal to 0.15 weight % of the bismuth, the balance is iron and inevitable impurities.

The hot rolling process includes performing a first hot rolling step on the steel blank to form a first hot rolled steel material, and then performing a second hot rolling step on the first hot rolled steel material to form a rolled steel material. The first rolling ratio of the first hot rolling step is 1.0 to 2.0, and the second rolling ratio of the second hot rolling step is 0 to 0.5. The above laminar cooling process cools the finished steel to below 450 ° C at a cooling rate of 10 ° C or more per second.

According to an embodiment of the present invention, the steel strip speed of the above hot rolling pass is from 7.0 m/s to 9.0 m/s.

According to an embodiment of the present invention, before the hot rolling process, the steel embryo is further subjected to a heating process to raise the steel embryo to 1200 ° C to 1250 ° C.

According to an embodiment of the invention, the rolling temperature of the hot rolling pass is greater than or equal to the Ar3 temperature.

According to an embodiment of the invention, after the laminar cooling process described above, the method further comprises the step of coiling the duplex steel.

According to an embodiment of the invention, the above cooling rate is from 10 ° C to 100 ° C per second.

According to an embodiment of the invention, the laminar cooling process includes a first cooling step, an air cooling step, and a second cooling process. First, a first cooling step is performed on the finished steel to form a first cooled steel, wherein the first cooling step cools the finished steel to 600 to 700 °C. Next, the first cooled steel material is subjected to an air cooling step to form a second cooled steel material. Then, the second cooling steel is subjected to a second cooling step to cool the second cooled steel material to Below 450 °C.

According to another aspect of the present invention, there is provided a duplex steel comprising 0.08 wt% to 0.20 wt% carbon, 1.5 wt% to 3.0 wt% manganese, 1.0 wt% to 1.5 wt% niobium, 1.0 wt. % to 2.0% by weight of aluminum, more than 0% by weight and less than or equal to 0.5% by weight of molybdenum, more than 0% by weight and less than or equal to 0.20% by weight of titanium, more than 0% by weight and less than or equal to 0.20% by weight or less Vanadium, more than 0% by weight and less than or equal to 0.01% by weight of nitrogen, less than or equal to 0.15% by weight of bismuth, the balance being iron and unavoidable impurities. Furthermore, the volume ratio of the ferrite phase of the duplex steel to the granulated iron phase is 1.2 to 1.9, and the average grain size of the ferrite phase is not more than 10 μm.

According to an embodiment of the present invention, the ferrite-iron phase has an average grain size of from 5 μm to 10 μm.

According to an embodiment of the invention, the duplex steel material does not substantially comprise a toughened iron phase.

The duplex steel of the present invention and a method for producing the same are obtained by using a steel preform having a specific composition to obtain a ferrite phase and a granulated iron phase having a specific volume ratio under a specific hot rolling condition. Dual phase steel with high tensile strength and high elongation.

100‧‧‧ method

110‧‧‧ Providing steel embryos

120‧‧‧First hot rolling step on steel blank to obtain first hot rolled steel

130‧‧‧Second hot rolling step for the first hot rolled steel to obtain finished steel

140‧‧‧Laminar cooling process for finished steel to obtain duplex steel

The above and other objects, features, advantages and embodiments of the present invention will become more apparent and understood.

Fig. 1 is a partial flow chart showing a method of manufacturing a duplex steel according to an embodiment of the present invention.

In view of the above, the present invention provides a duplex steel and a method for producing the same, which are obtained by subjecting a steel preform containing a specific component to a hot rolling process including a two-stage hot rolling step and then performing a laminar cooling process. A duplex steel having a specific two-phase volume ratio.

Please refer to FIG. 1 , which is a partial flow chart of a method 100 for manufacturing a duplex steel according to an embodiment of the invention. First, step 110 is performed to provide a steel blank. In an embodiment, the steel germ system comprises 0.08 wt% to 0.20 wt% carbon, 1.5 wt% to 3.0 wt% manganese, 1.0 wt% to 1.5 wt% niobium, 1.0 wt% to 2.0 wt% aluminum, More than 0% by weight and less than or equal to 0.5% by weight of molybdenum, more than 0% by weight and less than or equal to 0.20% by weight of titanium, more than 0% by weight and less than or equal to 0.20% by weight of vanadium, more than 0% by weight and Less than or equal to 0.01% by weight of nitrogen, less than or equal to 0.15% by weight of bismuth, and the balance being iron and unavoidable impurities. By having a specific composition and a specific content of the steel embryo, the starting temperature of the iron phase of the Worthite transformation into the ferrite phase can be greatly improved.

If the niobium content in the steel embryo is less than 1.0% by weight, the effect of solid solution strengthening cannot be effectively achieved; however, if too much niobium is added to the steel embryo, for example, more than 1.5% by weight, it is possible to make the subsequently obtained steel along the crystal. Snow-bound carbon iron is produced in the world. Since the ferritic carbon-iron is a component of the hard and brittle phase, if the ferritic carbon iron is produced in the grain boundary, the steel is easily broken during the process. Steel embryo A fixed amount of aluminum helps the steel to transform the Worth iron phase into a ferrite iron phase during subsequent hot rolling. However, in the conventional method for manufacturing a duplex steel, the ferrite phase is formed only during the cooling process. Therefore, if the aluminum content of the steel slab used is less than 1.0% by weight, a high carbon phase tends to be formed in the structure, and if the aluminum content of the steel slab used is more than 2.0% by weight, serious casting problems may occur, and thus good results cannot be obtained. Duplex steel properties.

The addition of manganese in the steel embryo can reduce the phase transition temperature of the iron phase of the Worthfield, which is beneficial to the residual of the iron phase of some Vostian and delays the formation of the iron phase. However, if the steel embryo has too high content of manganese (for example, more than 3.0% by weight), the stability of the residual Wolster iron phase will be greatly improved, which is not conducive to the improvement of the ductility of the steel, and will also increase the band structure in the steel. And then gather hard and brittle phase structure, it is not conducive to the overall performance of steel.

The addition of specific amounts of molybdenum, niobium and vanadium to the steel embryo contributes to the effect of refining the grains. In addition, when a specific content of titanium, vanadium, and nitrogen is added to the steel, the effect of precipitation strengthening can be achieved, and the strength of the subsequently obtained steel can be increased.

Next, the steel blank is subjected to a hot rolling process. In one embodiment, the steel blank may be selectively heated prior to the hot rolling to heat the steel to 1200 ° C to 1250 ° C. This heating process contributes to the solid solution of the alloy components of the steel. The hot rolling process first proceeds to step 120 where a first hot rolling step is performed on the steel blank to obtain a first hot rolled steel. In an embodiment, the rolling ratio of the first hot rolling step is from 1.0 to 2.0. It is to be noted that the rolling ratio used in the specification of the present invention is defined as the ratio of the difference between the initial steel blank thickness and the post-rolling steel blank thickness to the thickness of the rolled steel after the rolling, as in the formula (1). Show. If the rolling ratio of the first hot rolling step is greater than 2.0, the deformation amount of the steel material is too large, or the plate type of the steel is not suitable for the demand, and the rolling ratio of more than 2.0 may exceed the load of the machine. If the rolling ratio is too small, for example, less than 1.0, the subsequently obtained steel may have a problem of uneven distribution of the two-phase structure, and there is also an insufficient iron phase of the precipitated precipitate, which may affect the subsequent hot rolling step. Yanbi.

In the formula (1), r represents the rolling ratio, h represents the thickness of the steel after rolling, and h ₀ represents the thickness of the steel before rolling.

Next, in step 130, a second hot rolling step is performed on the first hot rolled steel material to obtain a rolled steel material. In one embodiment, the second hot rolling step has a rolling ratio of from 0 to 0.5. If the rolling ratio is more than 0.5, it means that the rolling ratio of the first hot rolling step is too small to achieve the effect of the hot rolling pass of the present invention. In one example, the rolling ratio of the first hot rolling step is greater than the rolling ratio of the second hot rolling step. In a specific example, if the hot rolling process includes seven hot rolling mills, the first four hot rolling mills may be selected for the first hot rolling step, and the last three hot rolling mills are subjected to the second hot rolling step, but the invention is not limited thereto. this. Furthermore, by controlling the rolling ratio of the first hot rolling step and the second hot rolling step, the average grain size of the Wostian iron phase in the microstructure can be reduced, thereby reducing the two-phase structure of the subsequently obtained steel. Average grain size.

The invention utilizes the addition of a specific content of alloy and a hot rolling process to manufacture a duplex steel, so that the growth time of the two-phase structure is short, and therefore, the ferrite-iron phase begins to grow earlier in the hot rolling process, rather than during the cooling process. The beginning of the growth of the ferrite iron phase is more conducive to the subsequent generation of duplex steel with good strength and ductility.

In one embodiment, the hot strip rolling speed (including the first hot rolling step and the second hot rolling step) is from 7.0 m/s to 9.0 m/s. Since the present invention is required to produce a ferrite-grained iron phase at the beginning of the hot rolling process, and the strip speed affects the air-cooling time of the steel, the speed of the strip will affect the time of growing the ferrite phase. If the speed of the steel strip is too fast, for example, higher than 9.0 m/s, the air cooling time of the steel is too short, and there is not enough time to grow the ferrite iron phase; conversely, if the steel strip speed is lower than 7.0 m/s, the air cooling time of the steel If it is too long, it may form an excessively large grain of ferrite, which cannot satisfy the grain size of the iron phase of the ferrite of the present invention, thereby affecting the strength of the subsequent steel.

In one embodiment, the finishing temperature of the hot rolling pass must be controlled to be greater than or equal to the Ar3 temperature. The Ar3 temperature system is defined as the temperature at which the steel phase of the Vostian begins to metamorphose into the ferrite phase during the cooling process. Therefore, if the rolling temperature is not controlled at a temperature greater than or equal to Ar3, and the steel may contain both the Worthfield iron phase and the ferrite iron phase, this is not the microstructure of the duplex steel of the present invention (ie, ferrite iron). It is also impossible to achieve the strength of the duplex steel of the present invention.

Then, in step 140, a laminar cooling process is performed on the finished steel to obtain a duplex steel. In one embodiment, the laminar cooling process comprises a first cooling step, an air cooling step, and a second cooling step. First, a first cooling step is performed on the obtained rolled steel material to form a first cooled steel material. In one embodiment, the first cooling step cools the finished steel to 650 ° C to 700 ° C. Next, the first cooled steel material is subjected to an air cooling step to form a second cooled steel material. Then, the second cooling steel is subjected to a second cooling step to cool the second cooled steel to 450 ° C or lower. If the laminar cooling process is cold However, if the temperature is higher than 450 ° C, the microstructure of the formed duplex steel may include the ferrite iron phase and the tough iron phase. However, the duplex steel of the present invention comprises the ferrite iron phase and the Ma Tian iron phase, which can be substantially Improve steel strength and elongation, and reduce the ratio of fall and fall.

In one embodiment, the laminar cooling process has a cooling rate of 10 ° C or more per second, preferably 10 ° C to 100 ° C per second, more preferably 60 ° C to 90 ° C per second. If the cooling rate is lower than 10 ° C per second, because the cooling rate is too slow, the time of the first cooling step is increased, the time of the air cooling step is reduced, which is not conducive to the formation of the ferrite phase of the smaller grains, and the strength of the steel cannot be made. There is a significant improvement.

In an embodiment, after step 140, the duplexing steel may be selectively subjected to a coiling step to obtain a steel coil, wherein the coiling temperature of the coiling step is cooling of the second cooling step in the laminar cooling process. temperature. Next, in another embodiment, the steel coil may be selectively subjected to a pickling step to remove the surface rust plate and subjected to reprocessing.

The duplex steel obtained by the method 100 for producing a duplex steel includes a ferrite phase iron phase and a 麻田散铁 phase. In one embodiment, the biphasic steel produced does not substantially contain a toughened iron phase. In one embodiment, the volume ratio of the fermented iron phase to the 麻田 loose iron phase is from 1.2 to 1.9. If the volume ratio is greater than 1.9, indicating that the iron phase of the fertilizer is too much, the strength of the obtained steel is low; however, if the volume ratio is less than 1.2, indicating that the iron phase of the field is too much, the elongation of the steel is low. In one embodiment, the ferrite iron phase has an average grain size of no greater than 10 μm, preferably 5 μm to 10 μm. If the average grain size of the ferrite grain iron phase is larger than 10 μm, there will be a problem of uneven distribution of the ferrite grain phase and the granule iron phase, and the coarser The tissue tends to cause cracks in the steel during shaping or other applications. In another embodiment, since the duplex steel has a uniformly distributed ferrite phase and a granulated iron phase, the average grain size of the granulated iron phase is not more than 10 μm.

In one embodiment, the duplex steel obtained by the method 100 for producing a duplex steel has a tensile strength of 900 MPa or more, a drop ratio of 0.50 to 0.65, and an elongation of more than 15%.

The following examples are used to illustrate the application of the present invention, and are not intended to limit the present invention. Those skilled in the art can make various changes without departing from the spirit and scope of the present invention. Retouching.

Using the steel embryo components listed in Table 1 below, the steel materials were produced in accordance with the process conditions listed in Table 2 in conjunction with the above-described manufacturing method of the duplex steel, and the microstructure and mechanical properties of the obtained steel materials are listed in Table 3 below. The "-" in Table 1 indicates the unadded ingredients. In the microstructure of Table 3, "F" indicates the ferrite phase, "B" indicates the toughened iron phase, and "M" indicates the Matian iron phase. The mechanical properties of Table 3 were measured according to the tensile test of Japanese Industrial Standards (JIS), wherein the ratio of the drop is the ratio of the strength of the drop to the tensile strength, and the elongation measured by the energy is indicated by "-".

Steel embryo C and steel embryo D can not only achieve the effect of solid solution strengthening due to the high content of cerium (between 1.0% and 1.5% by weight), but also help to avoid the formation of schmöcarbon in the grain boundary. Avoid steel breakage. Furthermore, the manganese content of the steel embryo C and the steel embryo D is between 2.0% by weight and 3.0% by weight and the aluminum content is between 1.0% and 2.0% by weight, so that the steel embryo can be started in the hot rolling stage. Produces a ferrite grain iron phase and delays the formation of the Borne iron phase.

According to the results in Table 3 above, the microstructure of the steel produced by the steel embryo C and the steel embryo D in the condition three or four conditions is the ferrite iron phase and the granitic iron phase, and the ferrite phase and the granitic iron The volume fraction of the phase is between 1.28 and 1.85 and has an iron phase of the ferrite having an average particle diameter of from 7 μm to 9 μm. Therefore, the aforementioned steel materials each have a tensile strength of 900 MPa or more, a fall ratio of 0.50 to 0.65, and an elongation of more than 15%.

However, the average grain size of the iron phase of the steel grain A to steel embryo D obtained by the process of Condition 1 and Condition 2 is larger (greater than 10 μm), mainly due to the heating of Condition 1 and Condition 2 The lower temperature (all 1150 ° C) causes the added Nb, Ti, V, and Mo to be insufficiently dissolved, and thus the effect of grain refinement cannot be achieved. Furthermore, the first rolling of Condition 1 and Condition 2 is relatively small (less than 1), resulting in uneven dispersion of the two-phase structure of the obtained steel, and insufficient precipitation of the ferrite-iron phase during the hot rolling.

Condition 1 The hot rolling speed of the steel strip is small (less than 7.0 m / s) and the laminar cooling process has a slower cooling rate (less than 10 ° C / s), resulting in excessive ferrite iron phase formation and average crystal The iron grain phase of the grain size is larger, and the strength of the obtained steel is insufficient (the tensile strength is less than 900 MPa).

Condition 1 and Condition 2, because of the higher cooling temperature (greater than 450 ° C), it is easier to form a two-phase structure of the ferrite phase and the tough iron phase, and the obtained steel has lower tensile strength and higher fluctuation. Not conducive to the production of high-strength steel.

According to the above embodiment, the present invention adopts a steel embryo having a specific composition and content, and is matched with a heating temperature, a rolling ratio of a hot rolling pass, and a steel strip speed. And the cooling rate and temperature of the laminar cooling process, so that the obtained steel has a fine and uniform ferrite grain iron phase, and the ferrite grain iron phase and the mai field iron phase have a specific volume ratio, thereby obtaining high tensile strength, Duplex steel with high elongation and specific drop ratio.

While the invention has been described above in terms of several embodiments, it is not intended to limit the scope of the invention, and the invention may be practiced in various embodiments without departing from the spirit and scope of the invention. The scope of protection of the present invention is defined by the scope of the appended claims.

Claims (10)

A method for manufacturing a duplex steel, comprising: providing a steel embryo, wherein the steel embryo comprises: 0.08 wt% to 0.20 wt% carbon; 1.5 wt% to 3.0 wt% manganese; 1.0 wt% to 1.5 wt% niobium 1.0% by weight to 2.0% by weight of aluminum; more than 0% by weight and less than or equal to 0.5% by weight of molybdenum; more than 0% by weight and less than or equal to 0.20% by weight of titanium; more than 0% by weight and less than or equal to 0.20 Less than 0% by weight of vanadium; more than 0% by weight and less than or equal to 0.01% by weight of nitrogen; less than or equal to 0.15% by weight of bismuth; and the balance being iron and unavoidable impurities; a hot rolling process for the steel embryo Obtaining a rolled steel, wherein the hot rolling process comprises: performing a first hot rolling step on the steel blank to form a first hot rolled steel, wherein the first rolling ratio is one of the first hot rolling steps a 1.0 to 2.0; and a second hot rolling step of the first hot rolled steel to form the finished steel, wherein one of the second hot rolling steps has a second rolling ratio of 0 to 0.5; Finishing the rolled steel for a one-stage cooling process to cool the finished steel to 450 ° C Next, the duplex steel is obtained, wherein one of the laminar cooling processes has a cooling rate of 10 ° C or more per second. The manufacturing method according to claim 1, wherein the steel strip speed of one of the hot rolling passes is from 7.0 m/s to 9.0 m/s. The manufacturing method of claim 1, further comprising, prior to the hot rolling process, performing a heating process on the steel embryo to raise the steel embryo to 1200 ° C to 1250 ° C. The manufacturing method of claim 1, wherein one of the hot rolling cycles is greater than or equal to the Ar3 temperature. The manufacturing method according to claim 1, wherein after the laminar cooling process, the manufacturing method further comprises: performing a coiling step on the duplex steel to obtain a steel coil. The manufacturing method according to claim 1, wherein the cooling rate is from 10 ° C to 100 ° C per second. The manufacturing method of claim 1, wherein the laminar cooling process comprises: performing a first cooling step on the rolled steel to form a first cooled steel, wherein the first cooling step is The rolled steel is cooled to 600 to 700 ° C; an air cooling step is performed on the first cooled steel to form a second cooled steel; and a second cooling step is performed on the second cooled steel to make the second cooled steel Cool to below 450 °C. A duplex steel comprising: 0.08 wt% to 0.20 wt% carbon; 1.5 wt% to 3.0 wt% manganese; 1.0 wt% to 1.5 wt% bismuth; 1.0 wt% to 2.0 wt% aluminum; greater than 0 wt% % and less than or equal to 0.5% by weight of molybdenum; more than 0% by weight and less than or equal to 0.20% by weight of titanium; more than 0% by weight and less than or equal to 0.20% by weight of vanadium; greater than 0% by weight and less than or equal to 0.01% by weight or less of nitrogen; less than or equal to 0.15% by weight of bismuth; and the balance being iron and unavoidable impurities, wherein one volume ratio of one of the duplex steels to the granulated iron phase is 1.2. To 1.9, and one of the ferrite phases has an average grain size of not more than 10 μm. The duplex steel according to claim 8, wherein the average grain size of the ferrite phase is from 5 μm to 10 μm. The duplex steel of claim 8, wherein the duplex steel does not substantially comprise a toughened iron phase.