KR102761144B1 - Hot rolled steel, products using the steel and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a product using hot-rolled steel, characterized in that the product comprises, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder is iron (Fe) and other unavoidable impurities, and the average density of MnS inclusions in a structure composed of ferrite and pearlite is 6/ ^mm2 or less and the maximum size of the MnS inclusions is 200㎛ or less. Provides.

Description

Hot rolled steel, products using the steel and method of manufacturing the same

The technical idea of the present invention relates to steel, products using the same, and a manufacturing method thereof, and more specifically, to hot-rolled steel, products using the same, and a manufacturing method thereof.

The stabilizer bar is installed on the front and rear wheels of the vehicle to reduce the tilt of the vehicle body. It does not work when the left and right wheels move up and down at the same time, but when the left and right wheels move up and down in opposite directions when the vehicle turns, it twists clockwise or counterclockwise around the center of the axle and reduces the tilt of the vehicle body due to the resulting twisting force.

To make a stabilizer bar, plate-shaped material is manufactured and then formed. Welding is performed through ERW welding during manufacturing, but if there is an extended inclusion inside, a hook crack may occur during manufacturing. Hook cracks are likely to occur along the inclusion when the non-metallic MnS inclusion inside the material is extended into a belt shape by hot rolling. In addition, if an extended inclusion exists, durability may be reduced. Therefore, it is important to control the density and size of the MnS inclusion.

Korean Patent Application No. 2009-0103205

The technical problem to be achieved by the technical idea of the present invention is to provide a hot-rolled steel having controlled density and size of MnS inclusions, a product using the same, and a manufacturing method thereof.

However, these tasks are exemplary and the technical idea of the present invention is not limited thereto.

According to one aspect of the present invention, a hot-rolled steel having controlled density and size of MnS inclusions, a product using the same, and a manufacturing method thereof are provided.

According to one embodiment of the present invention, the hot-rolled steel contains, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe) and other unavoidable impurities, and has a structure composed of ferrite and pearlite.

According to one embodiment of the present invention, the hot-rolled steel may further include niobium (Nb): more than 0% but less than 0.1%, copper (Cu): more than 0% but less than 0.1%, nickel (Ni): more than 0% but less than 0.1%, or vanadium (V): more than 0% but less than 0.01%.

According to one embodiment of the present invention, the hot-rolled steel can satisfy a yield strength (YS): 300 to 500 MPa, a tensile strength (TS): 500 to 700 MPa, and an elongation (EL): 25% or more.

According to one embodiment of the present invention, a product using hot-rolled steel contains, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe) and other unavoidable impurities, and an average density of MnS inclusions in a structure composed of ferrite and pearlite is 6/ ^mm2 or less, and a maximum size of the MnS inclusions is 200㎛ or less.

According to one embodiment of the present invention, a product using the hot-rolled steel can satisfy yield strength (YS): 1150 to 1280 MPa, tensile strength (TS): 1450 to 1550 MPa, and elongation (EL): 6 to 10%.

According to one embodiment of the present invention, a method for manufacturing a product using hot-rolled steel comprises the steps of: (a) preparing a hot-rolled steel including, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe) and other unavoidable impurities; (b) manufacturing the hot-rolled steel; (c) annealing and drawing the manufactured steel; (d) a step of forming the annealed and drawn steel material; and (e) a step of heat treating the formed steel material.

According to one embodiment of the present invention, the step (a) of the method for manufacturing a product using the hot-rolled steel may be performed under the conditions of a reheating temperature of 1100 to 1300°C, a finishing rolling end temperature of 800 to 1000°C, and a coiling temperature of 550 to 750°C.

According to one embodiment of the present invention, the step (c) of the method for manufacturing a product using the hot-rolled steel may be performed at a temperature of 700 to 900°C, the step (d) may be performed at a temperature of 800 to 900°C, and the step (e) may include a step of tempering at a temperature of 150 to 300°C after quenching.

According to one embodiment of the present invention, after step (e) of the method for manufacturing a product using the hot-rolled steel, the average density of MnS inclusions in a structure composed of ferrite and pearlite may be 6/ ^mm2 or less, and the maximum size of the MnS inclusions may be 200㎛ or less.

According to the technical idea of the present invention, a hot-rolled steel capable of controlling the density and size of MnS inclusions, a product using the same, and a manufacturing method thereof can be realized.

The effects of the present invention described above are described as examples, and the scope of the present invention is not limited by these effects.

Figure 1 is a process flow diagram schematically showing a method for manufacturing hot-rolled steel according to one embodiment of the present invention.
Figure 2 is a process flow diagram schematically showing a method for manufacturing a product using hot-rolled steel according to one embodiment of the present invention.
Figure 3 is a microscope photograph of the microstructure of a specimen according to Comparative Example 2 among the experimental examples of the present invention.
Figure 4 is a diagram schematically showing the distribution of the density and size of inclusions (MnS) in the microstructure of a specimen according to Comparative Example 2 among the experimental examples of the present invention.
Figure 5 is a microscope photograph of the microstructure of a specimen according to Example 3 among the experimental examples of the present invention.
Figure 6 is a diagram schematically showing the distribution of the density and size of inclusions (MnS) in the microstructure of a specimen according to Example 3 among experimental examples of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical idea of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

Hereinafter, hot-rolled steel, which is one aspect of the present invention, will be described.

hot rolled steel

A hot-rolled steel according to one aspect of the present invention contains, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the hot-rolled steel according to the present invention will be described as follows. In this case, the content of the component elements all refers to weight%.

Carbon (C): 0.20% ~ 0.25%,

Carbon is a representative element added to improve the strength of steel, and is added to improve tensile and yield strength and secure hardenability. If the carbon content is less than 0.20%, strength cannot be secured and hardenability may be insufficient. If the carbon content exceeds 0.25%, strength is improved, but formability required for forming parts decreases and weldability may deteriorate. Therefore, it is desirable to add carbon content of 0.20% to 0.25% of the total weight of hot-rolled steel.

Silicon (Si): 0.01% ~ 0.30%

Silicon is a key element that promotes pearlite and Mn segregation zone control and ferrite formation and finely disperses ferrite. When the silicon content is less than 0.01%, the effect of silicon addition is insignificant. When the silicon content exceeds 0.30%, since it has a strong affinity with oxygen, defects due to surface scale may occur, which may cause a deterioration of surface quality. Therefore, it is desirable to add silicon in an amount of 0.01% to 0.30% of the total weight of hot-rolled steel.

Manganese (Mn): 0.5% ~ 1.1%

Manganese inhibits ferrite formation, increases austenite stability, and facilitates the formation of low-temperature transformation phases, thereby increasing strength. As a substitutional element with an atomic diameter similar to that of iron (Fe), it is very effective in solid solution strengthening and is an effective element for securing strength after heat treatment by improving the hardenability of steel. When the manganese content is less than 0.5%, the effect of adding manganese is insignificant. When the manganese content exceeds 1.1%, it may form segregation zones and inclusions on the inside or outside or both of the cast slab and hot-rolled steel sheet, which may cause the occurrence and propagation of cracks, thereby deteriorating weldability and bending workability. Therefore, it is desirable to add manganese in an amount of 0.5 to 1.1% of the total weight of the hot-rolled steel.

Aluminum (Al): 0.01% ~ 0.08%

Aluminum is used as a deoxidizer, and at the same time, it suppresses cementite precipitation like silicon, stabilizes austenite, and improves strength. When the aluminum content is less than 0.01%, the effect of adding aluminum is insignificant. When the aluminum content exceeds 0.08%, it reacts with nitrogen during continuous casting to precipitate nitride (AlN), which may deteriorate the quality of slabs and hot-rolled products, such as corner cracks in slabs. Therefore, it is desirable to add aluminum to 0.01% to 0.08% of the total weight of hot-rolled steel.

Chromium (Cr): 0.10% ~ 0.50%

Chromium is an element that improves hardenability and carbide formation, and is an important element for manufacturing high-strength hot-formed structural parts. When the chromium content is less than 0.10%, the effect of adding chromium is insignificant. When the chromium content exceeds 0.50%, it is an expensive element, so it is not economically desirable, and the toughness and weld crack resistance may deteriorate. Therefore, it is desirable to add chromium in an amount of 0.10% to 0.50% of the total weight of the hot-rolled steel.

Titanium (Ti): 0.01% ~ 0.05%

Titanium forms carbides with carbon to enhance strength. It can also secure the hardenability effect of B through the formation of TiN. On the other hand, since titanium suppresses the formation of BN, boron is added to secure hardenability. That is, since boron has a high affinity for nitrogen, if boron and nitrogen combine to form nitrides, the intended effect of boron cannot be achieved. Therefore, if titanium is added to combine with nitrogen to form nitrides, the effect on boron can be further enhanced. When the titanium content is less than 0.01%, AIN or BN precipitates may be excessively precipitated. When the titanium content exceeds 0.05%, it is difficult to expect the grain refinement effect due to the formation of coarse TiN precipitates, and the manufacturing cost increases. Therefore, it is desirable to add titanium to 0.01% to 0.05% of the total weight of the hot-rolled steel.

Boron (B): 0.001% ~ 0.005%

Boron is an element that effectively contributes to improving the hardenability of steel, and even with a small amount of addition, it is an element that can effectively suppress transformation such as ferrite and pearlite during cooling after hot rolling. If the boron content is less than 0.001%, it is difficult to secure the desired hardenability. If the boron content exceeds 0.005%, it causes grain boundary embrittlement of the hard phase, which worsens formability and weldability. Therefore, it is desirable to add boron to 0.001% to 0.005% of the total weight of the hot-rolled steel.

P: More than 0% ~ less than 0.03%

Phosphorus contributes to some strength enhancement, but it can reduce weld toughness and low-temperature impact toughness, and can form not only center segregation but also micro segregation, which can have a negative effect on the material. Therefore, the lower the phosphorus content, the better. Therefore, it is desirable to limit the phosphorus (P) content to more than 0% and less than 0.03% of the total weight of hot-rolled steel.

Sulfur (S): More than 0% ~ less than 0.003%

Sulfur is an element that is inevitably included in the production of steel along with phosphorus, and forms non-metallic inclusions such as MnS. As a low-melting-point element, it has a high possibility of grain boundary segregation, which reduces toughness. Therefore, it is desirable to limit the sulfur content to more than 0% and less than 0.003% of the total weight of hot-rolled steel.

Meanwhile, a hot-rolled steel according to another embodiment of the present invention may optionally further include niobium (Nb): more than 0% but less than 0.1%, copper (Cu): more than 0% but less than 0.1%, nickel (Ni): more than 0% but less than 0.1%, or vanadium (V): more than 0% but less than 0.01%.

Niobium (Nb): more than 0% but less than 0.1%

Niobium is a precipitation-hardening element that can increase strength by combining with carbon and nitrogen to form carbides or nitrides, and contributes to making the grain size fine by delaying the recrystallization rate of austenite during hot rolling. When the niobium content is 0.1% or more, the yield strength and yield ratio increase and the elongation decreases, thereby lowering the hot-rolled workability and processability. Therefore, the niobium content may be added in an amount exceeding 0% and less than 0.1% of the total weight of the hot-rolled steel.

Vanadium (V): more than 0% but less than 0.01%

Vanadium can be added in amounts exceeding 0% and less than 0.01% of the total weight of the steel. Vanadium is an element that combines with carbon or nitrogen to form V(C, N) precipitates, contributing to the improvement of strength and improving hardenability. When vanadium is more than 0.01%, it causes the coarsening of precipitates, which in turn reduces the toughness.

Nickel (Ni): More than 0% but less than 0.1%

Nickel (Ni) plays a role in improving the hardenability and corrosion resistance of steel. In particular, nickel (Ni) is an effective element for improving low-temperature impact toughness. The nickel (Ni) may be added in a content ratio of more than 0% and less than 0.1% of the total weight of the hot-rolled steel according to the present invention. When the content of nickel (Ni) is 0.1% or more of the total weight of the hot-rolled steel, there is a problem of causing red heat embrittlement and increasing the manufacturing cost.

Copper (Cu): More than 0% but less than 0.1%

Copper (Cu) plays a role in improving the hardenability and corrosion resistance of steel together with nickel (Ni). The copper (Cu) may be added in an amount of more than 0% and less than 0.1% of the total weight of the hot-rolled steel according to the present invention. When the content of copper (Cu) is 0.1% or more of the total weight of the hot-rolled steel sheet, there is a problem of lowering the surface properties of the steel.

The remaining components of the above hot-rolled steel are iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during the normal manufacturing process, this cannot be ruled out. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of their contents are specifically mentioned in this specification.

By controlling the specific components of the alloy composition described above and the content ranges thereof, and manufacturing a hot-rolled steel through the manufacturing method of a hot-rolled steel described below, a hot-rolled steel satisfying a yield strength (YS): 300 to 500 MPa, a tensile strength (TS): 500 to 700 MPa, and an elongation (EL): 25% or more can be obtained.

The above hot-rolled steel may have a structure composed of ferrite and pearlite. The pearlite may have an area fraction of 10 to 20%, and the ferrite may have an area fraction of 80 to 90%.

Hereinafter, a method for manufacturing hot-rolled steel, which is one aspect of the present invention, will be described.

Manufacturing method of hot rolled steel

Figure 1 is a process flow diagram schematically showing a method for manufacturing hot-rolled steel according to one embodiment of the present invention.

Slabs in a semi-finished state can be obtained by obtaining molten steel of a certain composition through a steelmaking process and then through a continuous casting process.

The above hot-rolled steel contains, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe) and other unavoidable impurities. Furthermore, the hot-rolled steel may optionally further include niobium (Nb): more than 0% but less than 0.1%, copper (Cu): more than 0% but less than 0.1%, nickel (Ni): more than 0% but less than 0.1%, or vanadium (V): more than 0% but less than 0.01%.

Referring to FIG. 1, a method for manufacturing hot-rolled steel according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a cooling step (S130), and a coiling step (S140).

Reheating step (S110)

In the reheating step (S110), the steel having the above composition, for example, a slab plate, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of 1100 to 1300°C. Through this reheating, precipitates such as niobium precipitates or titanium precipitates segregated during casting can be re-dissolved. The reheating temperature requires a sufficiently high temperature to re-dissolve the precipitation hardening elements and form fine precipitates during cooling. Fine precipitates hinder the growth of grains, allowing fine grains to be obtained, and as a result, the effect of improving strength can be obtained. When the reheating temperature is less than 1100°C, there are problems in that niobium precipitates or titanium precipitates are not re-dissolved, components segregated during casting are not sufficiently evenly distributed, and the precipitation strengthening effect is minimal. If the above reheating temperature exceeds 1300℃, the austenite grains may become coarser and the strength may decrease.

Hot rolling stage (S120)

The above heated steel is first heated to adjust its shape, and then hot rolled. The above hot rolling can be performed sequentially as width rolling, rough rolling, and finish rolling. By the above hot rolling step, the steel can be formed into a steel plate.

The above hot rolling can be completed at a finish delivery temperature (FDT) of 800 to 1000°C. If the finish delivery temperature is less than 800°C, since the hot rolling temperature is completed in a low temperature range, that is, an ideal range of austenite and ferrite, grain mixing rapidly progresses, resulting in uneven deformability, which may result in a decrease in rollability. If the finish rolling temperature exceeds 1000°C, the strength of the steel may be reduced due to the growth of precipitates and grains.

Cooling stage (S130)

The above hot-rolled steel is cooled to 550 to 750°C at a cooling rate of 70°C/sec or higher. The cooling is performed by shear quenching. A cooling rate of 70°C/sec or higher is required to achieve the grain refinement effect due to the cooling rate and to avoid the pearlite region during cooling. The faster the cooling rate, the greater the grain refinement effect, which improves the strength; however, it is desirable to set an appropriate cooling rate considering the performance of the equipment.

Winding stage (S140)

After the above cooling is completed, the steel is coiled at a coiling temperature (CT) of 550 to 750°C. If coiling is performed below 550°C, the strength increases due to grain refinement, which reduces the forming properties and weldability. If coiling is performed above 750°C, there is a disadvantage in that it is difficult to secure sufficient strength.

Below, a product using hot-rolled steel, which is another aspect of the present invention, is described.

Products using hot rolled steel

Another aspect of the present invention is a product using hot-rolled steel, which contains, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe) and other unavoidable impurities. Furthermore, a product using the hot-rolled steel may optionally further include niobium (Nb): more than 0% but less than 0.1%, copper (Cu): more than 0% but less than 0.1%, nickel (Ni): more than 0% but less than 0.1%, or vanadium (V): more than 0% but less than 0.01%.

The role and content of each component included in a product using hot-rolled steel have already been explained in the hot-rolled steel described above, so they are omitted here.

A product using hot-rolled steel manufactured by controlling the specific components of the alloy composition described above and the content range thereof and using the method for manufacturing a product using hot-rolled steel described below may have an average density of MnS inclusions in a structure composed of ferrite and pearlite of 6/mm2 or ^less and a maximum size of MnS inclusions of 200㎛ or less.

Products using the above hot-rolled steel can satisfy yield strength (YS): 1150 to 1280 MPa, tensile strength (TS): 1450 to 1550 MPa, and elongation (EL): 6 to 10%.

Below, a method for manufacturing a product using hot-rolled steel, which is another aspect of the present invention, will be described.

Manufacturing method of products using hot rolled steel

Figure 2 is a process flow diagram schematically showing a method for manufacturing a product using hot-rolled steel according to one embodiment of the present invention.

Referring to FIG. 2, a method for manufacturing a product using hot-rolled steel according to one embodiment of the present invention includes a step of preparing hot-rolled steel (S210); a step of forming the hot-rolled steel (S220); a step of annealing and drawing the formed steel (S230); a step of forming the annealed and drawn steel (S240); and a step of heat-treating the formed steel (S250).

Hot rolled steel preparation (S210)

The above hot-rolled steel is manufactured using the manufacturing method described with reference to Fig. 1.

Hot rolled steel pipe (S220)

The above hot rolled steel in plate shape is manufactured. During the manufacturing process, welding is performed through ERW welding. However, if there is an extended inclusion inside, a hook crack may occur during the manufacturing process. The hook crack is likely to occur along the inclusion when the non-metallic MnS inclusion inside the material is extended into a belt shape by hot rolling. In addition, if an extended inclusion exists, durability may be reduced. Therefore, it is important to control the shape and size of the MnS inclusion.

Annealing and drawing (S230)

The above-mentioned steel can be annealed and drawn. The annealing and drawing processes are carried out simultaneously, and are carried out at a temperature of 700 to 900℃ through a box furnace and can be transported through a conveyor.

Plastic surgery (S240)

The above annealed and drawn steel is formed. The forming process can be carried out in a box furnace at a temperature of 800 to 900° C for 1 minute. Through the forming step, a product can be formed, and for example, a stabilizer bar can be formed.

Heat treatment (S250)

The above-mentioned formed steel may be heat treated. The heat treatment may include a step of heating to a temperature of 800 to 1000°C, then quenching, and then continuously tempering at a temperature of 150 to 300°C. This heat treatment may be performed to release the martensite structure.

A product using hot-rolled steel that has undergone the above-described forming step (S220); annealing and drawing step (S230); forming step (S240); and heat treatment step (S250) has an average density of MnS inclusions in a structure composed of ferrite and pearlite of 6/ ^mm2 or less, a maximum size of the MnS inclusions may be 200㎛ or less, and may satisfy yield strength (YS): 1150 to 1280 MPa, tensile strength (TS): 1450 to 1550 MPa, and elongation (EL): 6 to 10%.

Experimental example

Hereinafter, preferred experimental examples are presented to help understand the present invention. However, the following experimental examples are provided only to help understand the present invention, and the present invention is not limited to the following experimental examples. Since the contents not described herein can be sufficiently technically inferred by those skilled in the art, their description will be omitted.

Hot rolled steel having the composition shown in Table 1 below was manufactured. The remainder consists of iron (Fe) and impurities inevitably contained during the steelmaking process, etc. In addition to the components shown in Table 1, aluminum was controlled to 0.05%, phosphorus to 0.02 wt% or less, and sulfur to 0.003 wt% or less.

C Si Mn Cr Ti B Comparative Example 1 0.23 0.24 1.2 0.19 0.033 0.003 Comparative Example 2 0.24 0.25 1.3 0.15 0.035 0.002 Example 1 0.26 0.21 1.1 0.25 0.032 0.002 Example 2 0.26 0.19 1.1 0.25 0.033 0.002 Example 3 0.26 0.20 0.9 0.25 0.038 0.002 Example 4 0.28 0.20 1.0 0.25 0.035 0.002

Referring to Table 1, Examples 1, 2, 3, and 4 satisfy the composition range of, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe).

In contrast, Comparative Examples 1 and 2 are not satisfactory as they exceed the manganese (Mn) range of 0.5% to 1.1%.

Table 2 shows the properties of a product using hot-rolled steel having the composition of Table 1 but implemented by applying the same manufacturing method described with reference to FIGS. 1 and 2. Specifically, the conditions applied were reheating temperature: 1200°C, finishing rolling end temperature: 900°C, coiling temperature: 600°C, annealing temperature: 800°C, forming temperature: 850°C, heat treatment temperature: 950°C, and tempering temperature: 200°C.

MnS
density
(pcs/ ^mm2 ) MnS
Maximum size
(㎛) YS
(MPa) TS
(MPa) EL
(%) Comparative Example 1 13.9 532 1219 1478 8.9 Comparative Example 2 8.5 410 1318 1545 7.9 Example 1 4.9 181 1188 1421 9.1 Example 2 4.7 176 1200 1482 7.7 Example 3 4.2 144 1258 1531 7.3 Example 4 5.1 195 1272 1508 8.1

Referring to Table 2, it can be confirmed that Examples 1, 2, 3, and 4 satisfy yield strength (YS): 1150 to 1280 MPa, tensile strength (TS): 1450 to 1550 MPa, and elongation (EL): 6 to 10%.

In contrast, Comparative Example 2 is not satisfactory as it exceeds the range of yield strength (YS): 1150 to 1280 MPa.

Meanwhile, the average density of MnS inclusions and the maximum size of MnS inclusions in the structure composed of ferrite and pearlite were measured from microscopic photographs of the microstructure within the product implemented by the experimental example of the present invention.

For example, FIG. 3 is a micrograph showing the microstructure of a specimen according to Comparative Example 2 among the experimental examples of the present invention, FIG. 4 is a drawing schematically showing the distribution of the density and size of inclusions (MnS) in the microstructure of a specimen according to Comparative Example 2 among the experimental examples of the present invention, FIG. 5 is a micrograph showing the microstructure of a specimen according to Example 3 among the experimental examples of the present invention, and FIG. 6 is a drawing schematically showing the distribution of the density and size of inclusions (MnS) in the microstructure of a specimen according to Example 3 among the experimental examples of the present invention.

In Examples 1, 2, 3, and 4, it can be confirmed that the average density of MnS inclusions in the structure composed of ferrite and pearlite is 6/ ^mm2 or less, and the maximum size of the MnS inclusions is 200㎛ or less. In contrast, it can be confirmed that Comparative Examples 1 and 2 are not satisfactory because the average density of MnS inclusions exceeds the range of 6/ ^mm2 or less, and the maximum size of the MnS inclusions exceeds the range of 200㎛ or less.

The present invention aims to develop a steel having improved welding characteristics and durability by controlling inclusions by controlling the composition of alloy elements in a material for application to a conventional stabilizer bar. Even when manufactured under the same process conditions, the density and size of MnS inclusions were reduced through alloy element control, and thus improved cleanliness and reduced defect rate during pipe manufacturing could be expected. According to the present invention, it was confirmed that the defects occurring during welding and forming were reduced and durability was improved by controlling inclusions compared to the conventional 1.5 GPa material, and thus the possibility of application to parts using steel pipes such as stabilizers is confirmed.

It will be apparent to a person skilled in the art that the technical idea of the present invention described above is not limited to the above-described embodiments and the attached drawings, and that various substitutions, modifications, and changes are possible within a scope that does not depart from the technical idea of the present invention.

Claims (9)

In weight %, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities.
It has a structure composed of ferrite and pearlite, with the area fraction of ferrite being 80 to 90% and the area fraction of pearlite being 10 to 20%.
Yield strength (YS): 300 to 500 MPa, tensile strength (TS): 500 to 700 MPa, and elongation (EL): 25% or more,
Hot rolled steel. In paragraph 1,
Niobium (Nb): more than 0% but less than 0.1%, Copper (Cu): more than 0% but less than 0.1%, Nickel (Ni): more than 0% but less than 0.1%, or Vanadium (V): more than 0% but less than 0.01%,
Hot rolled steel. delete In weight %, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities.
The average density of MnS inclusions in the structure composed of ferrite and pearlite is 6/ ^mm2 or less, and the maximum size of the MnS inclusions is 200㎛ or less.
Yield strength (YS): 1150 ~ 1280 MPa, tensile strength (TS): 1450 ~ 1550 MPa and elongation (EL): 6 ~ 10% are satisfied.
Products made from hot rolled steel. delete (a) a step for preparing a hot-rolled steel including, in wt%, carbon (C): 0.20% to 0.25%, silicon (Si): 0.01% to 0.30%, manganese (Mn): 0.5% to 1.1%, aluminum (Al): 0.01% to 0.08%, chromium (Cr): 0.10% to 0.50%, titanium (Ti): 0.01% to 0.05%, boron (B): 0.001% to 0.005%, phosphorus (P): more than 0% to 0.03% or less, sulfur (S): more than 0% to 0.003% or less, and the remainder being iron (Fe) and other unavoidable impurities;
(b) a step of manufacturing the hot-rolled steel;
(c) a step of annealing and drawing the above-mentioned steel;
(d) a step of forming the annealed and drawn steel material; and
(e) a step of heat treating the formed steel; including,
The step (a) is characterized in that it is performed under the conditions of a reheating temperature of 1100 to 1300℃, a finishing rolling end temperature of 800 to 1000℃, cooling to 550 to 750℃ at a cooling rate of 70℃ or more after finishing rolling, and a coiling temperature of 550 to 750℃, and in the step (a), the hot rolled steel has a structure composed of ferrite and pearlite, wherein the area fraction of ferrite is 80 to 90% and the area fraction of pearlite is 10 to 20%.
The above step (e) includes a step of tempering at a temperature of 150 to 300°C after quenching,
After the step (e) above, the average density of MnS inclusions in the tissue consisting of ferrite and pearlite is 6/ ^mm2 or less, and the maximum size of the MnS inclusions is 200㎛ or less.
A method for manufacturing a product using hot-rolled steel. delete In paragraph 6,
The above step (c) is performed at a temperature of 700 to 900℃.
The above step (d) is performed at a temperature of 800 to 900℃.
A method for manufacturing a product using hot-rolled steel. delete

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