KR100325714B1 - A bainitic steel with good low temperature toughness and a method of manufacturing thereof - Google Patents

Abstract

PURPOSE: A method for manufacturing a bainitic steel with good low temperature toughness is provided, which is characterized in that bainite matrix structure is minutely controlled by accelerated cooling after finishing hot rolling process at high temperatures, thus it is possible to produce a bainitic steel with a tensile strength of 60kgf/mm¬2. CONSTITUTION: The method for manufacturing a bainitic steel with good low temperature toughness includes the steps of reheating a slab comprising C 0.01 to 0.04 wt.%, Mn 1.2 to 2.0 wt.%, Si 0.1 to 0.3 wt.%, Al 0.02 to 0.06 wt.%, Ti 0.01 to 0.03 wt.%, Nb 0.04 to 0.06 wt.%, V 0.05 to 0.07 wt.%, Mo 0.15 to 0.25, B 0.001 to 0.002 wt.%, 0.02 wt.% or less of S, 0.02 wt.% or less of P, N 10 to 100 ppm, O 0.01 to 0.05 wt.%, Ca 10 to 100 ppm, a balance of Fe and incidental impurities in a temperature range of 1050 to 1300°C, followed by hot rolling; finish hot rolling the slab at a temperature higher than Ar3 transformation temperature by 100 to 200 deg.C; accelerated cooling the hot rolled steel sheet at a cooling rate of 7 to 15 deg.C.

Description

A bainitic steel with good low temperature toughness and a method of manufacturing

The present invention relates to a method for manufacturing bainite-based steel having excellent low temperature toughness. In particular, the present invention relates to a method for manufacturing a thick plate steel, which has a tensile strength of 62 kgf / mm ² or more and a low temperature toughness of ²⁰⁰ J / cm or more at -60 ° C., for high temperature rolling and accelerated cooling. It is related with the manufacturing method of the steel material which improved the production cost by manufacturing.

Conventionally, high strength structural steels requiring a tensile strength of 60kgf / mm ² or more contain a large amount of trace elements in addition to C and Mn to increase the strength of the steel, or to improve weldability during welding, which is an essential subsequent process of the steel. The method of reducing the content of the alloying element as much as possible and thereby reducing the strength of the alloying element through the control of the microstructure through accelerated cooling has been mainly used. On the other hand, in the case of ultra-low carbon type bainite alloy, a method of improving the strength and weldability by making the base structure into bainite through air cooling after hot rolling has been recently known. However, the low carbon steel produced by high temperature rolling and air cooling has the disadvantage that the bainite structure is relatively coarse and the low temperature toughness is weak. In particular, the finishing rolling temperature must be managed above 850 ° C. There is a disadvantage that causes.

In the present invention, instead of minimizing the content of C, which has a decisive influence on the strength of steel, to increase the hardenability of the steel by using Mo and B to make the base structure into bainite instead of ferrite, accelerated cooling after finishing rolling at a high temperature. By precisely controlling the bainite matrix structure to provide a method for producing a tensile strength of 60kgf / mm ² excellent in low temperature toughness.

1 is a graph showing the chaff impact energy values of the inventive steel and the comparative steel against the experimental temperature.

Figure 2 is a microstructure photograph of (a) the invention steel and (b) comparative steel (rolling end temperature = 950 ℃)

The present invention for achieving the above object is a method of producing a yield strength 60kgf / mm grade ² structural steel, the steel component in weight% C: 0.01 to 0.04%, Mn: 1.2 to 2.0%, Si: 0.1 to 0.3%, Al: 0.02 to 0.06%, Ti: 0.01 to 0.03%, Nb: 0.04 to 0.06%, V: 0.05 to 0.07%, Mo: 0.15 to 0.25%, B: 0.001 to 0.002%, S: 0.02% or less , P: 0.02% or less, N: 10 to 100 ppm, O: 0.01 to 0.05%, Ca: 10 to 100 ppm, the slab of steel consisting of the remaining Fe and other unavoidable impurities is heated in a temperature range of 1050 to 1300 ° C. Hot rolling in a method; Terminating the hot rolled pop at a temperature of 100 ° C. to 200 ° C. higher than a transformation temperature (Ar ₃ ); Accelerated cooling of the steel sheet after the rolling; provides a method for producing bainite-based steel having excellent low-temperature toughness, characterized in that it comprises a.

Hereinafter, the present invention will be described in detail.

In the present invention, instead of minimizing the content of C, C: 0.01 to 0.04% and Mo and B are added in the range of Mo: 0.15 to 0.25% and B: 0.001 to 0.002%, respectively, to increase the hardenability of the steel, In making slab base structure into bainite instead of ferrite, it is heated in the temperature range of 1050 to 1300 ° C, finish rolling at a high temperature of 100 to 200 ° C higher than transformation temperature (Ar ₃ ), and then accelerated cooling to bainite base By precisely controlling the structure, it is possible to produce a tensile strength 60kgf / mm grade ² steel excellent in low temperature toughness.

Hereinafter, the reason for component limitation of this invention is demonstrated.

In general, when the content of the alloying elements of steel is small, the fraction of the second phase structure is lowered, and the strength is decreased. Therefore, the present invention manages the C content to 0.04% or less, to make the base structure after rolling to bainite and to suppress the formation of the second phase, if the content of C is less than 0.01%, the self strength of the bainite matrix is reduced, so The content is limited to the range of 0.01% to 0.04%.

Si is added as a deoxidizer during steelmaking and also has a solid solution effect. However, when the impact transition temperature is increased and more than 0.3% is added, weldability is deteriorated, and an oxide film is severely formed on the surface of the steel sheet so that the content is limited to 0.1 to 0.3%. .

Since Mn forms MnS, a non-metallic inclusion that is elongated with S, and lowers room temperature elongation and low temperature toughness, it is preferable to manage it at 2.0% or less. However, when Mn is less than 1.2%, it decreases the steel's pressure resistance and forms benite at air cooling. Since it is difficult to secure strength, it is limited to 1.2 ~ 2.0%.

Al is added as a deoxidizer during steelmaking, but when it is added more than 0.06%, Al ₂ O ₃ , a nonmetallic oxide, forms the impact toughness, and when it is less than 0.02%, desulfurization is difficult due to the oxidizing atmosphere during steelmaking. It is preferable to limit the range.

Ti is a major element that forms a TiN precipitate during the solidification process of steel and suppresses grain growth during heating of the ingot, and suppresses growth of recrystallized grains during hot rolling, thereby playing a major role in grain refinement of steel. The amount of Ti added varies depending on the content of N. When the amount of Ti is relatively low compared to the amount of nitrogen, i.e., less than 0.01%, the amount of TiN formed is disadvantageous due to the small amount of TiN formed, whereas 0.03% When added in excess, TiN coarsens during heating, and the grain growth inhibiting effect is reduced. Therefore, Ti addition amount is limited to 0.01-0.03.

Nb is an important element that increases the strength of the steel by solid solution of austenite to increase the hardenability of austenite and to precipitate as carbonitride (Nb (C, N)) to match the matrix (Martrix). Since the characteristics of the present invention are to produce a steel having excellent strength and low temperature toughness by forming the bainite matrix by accelerated cooling, it is important to contain a large amount of Nb in order to enhance the hardenability, but may be added even when 0.06% or more is added. It does not increase the effect of, and exists in the solid state in the ferrite, there is a risk of lowering the impact toughness, may also reduce the weldability, when added to less than 0.04%, the effect of improving the hardenability is reduced 0.04% to 0.06% Will be limited.

V forms VC during cooling, contributing to strengthening precipitation and suppressing grain boundary growth.It is preferable to contain 0.05% or more because fine VC precipitates during cooling to increase strength, but when it exceeds 0.07%, coarse VC causes brittleness. Since weldability is impaired, it limits to 0.05%-0.07%.

B is known not only to increase the hardenability but also to segregate at the grain boundaries to increase the binding force of the grain boundaries. It is necessary to contain B not less than 0.001% because B contributes to the low temperature toughness in the steel exhibiting the toughness fracture property that breaks along the grain boundary. However, if it contains more than 0.002%, the grain boundary tends to be embrittled, so it is limited to the range of 0.001% to 0.002%.

Mo is an essential element to make the base structure into bainite structure by increasing the hardenability and suppressing ferrite transformation. If it contains more than 0.25%, Mo-C precipitates are formed to embrittle the grain boundary, and the cooling ability by C is also lowered. If the content is less than 0.15%, it is difficult to secure hardenability, so it is preferably limited to 0.15 to 0.25%.

P is a particularly bad element of impact toughness, the lower the content is better but the inevitable element in the steelmaking process, so the content is limited to 0.02% or less so as not to adversely affect the physical properties.

S is present as a non-metallic inclusion of MnS and is elongated by hot rolling to promote anisotropy of steel sheet properties and to lower impact toughness. Therefore, its content is controlled to 0.002% or less.

As other alloy components in addition to the alloy components, N, O and Ca are commonly contained in the case of general structural steel, the content of which is also in the usual range of N: 10 to 100 ppm, O: 0.01% to 0.05%, and Ca: 10 to 100 ppm.

Hereinafter, the hot rolling and cooling conditions will be described in more detail.

In the present invention, after heating the slab of the steel composition as described above in the temperature range of 1050 ~ 1300 ℃ hot rolling in the recrystallization region, and hot rolling in the unrecrystallized region below the recrystallization temperature to 60 ~ 80% rolling reduction range transformation temperature of (Ar ₃₎ + by the rolled sheet subjected to hot rolling terminating at a temperature of 100~200 ℃ using water cooled speed 7 ~ 15 ℃ / sec accelerated cooling.

Before the hot rolled slab of the low-alloy steel formed as described above is heated to a temperature range of 1050 ~ 1300 ℃, the reason is as follows. In the present invention, Nb is present in a dissolved state in austenite, and the refinement of the ferrite to the drop in the transformation temperature caused by the increase in the hardenability by the Nb is a major point for improving the strength and low temperature toughness. In the slab state, Nb is combined with C to exist as carbide NbC. Therefore, before hot rolling, the slab must be heated to 1050 ° C or higher to dissolve NbC so that Nb exists in an atomic state, provided that the heating temperature exceeds 1300 ° C. In this case, the slab heating temperature is limited to the range of 1050 to 1300 ° C. because the austenite particles are too coarse and delta-ferrite is partially generated in the steel to degrade the properties of the steel sheet.

The ingot heated to the heating temperature is hot-rolled in the recrystallization zone and then unrecrystallized rolled so that the amount of reduction in the unrecrystallized zone below the recrystallization temperature is 60 to 80%. If the rolling reduction is less than 60%, the ferrite particles may not be miniaturized by recrystallization rolling. If the rolling reduction exceeds 80%, the rolling reduction may be controlled in the range of 60 to 80% because of severe material deviation in the plate after rolling.

At this time, the uncrystallized reduction start temperature (T4) is preferably about 50 ℃ lower than the recrystallization temperature, the recrystallization temperature (R) is changed according to the composition of the steel, it can be calculated by the following equation (1) by the empirical formula.

Here, it can be seen that the recrystallization temperature according to the present invention is about 1030 ° C. in terms of Equation (1).

The rolling end temperature (T5) is closely related to low temperature toughness as well as strength, which is an indicator to be strictly controlled. In the case of conventional accelerated cooling steel, rolling is finished at about ± 30 ° C based on the transformation temperature (Ar ₃ ). . If the rolling end temperature (T5) is too high, the ductility and low temperature toughness are excellent, but the strength is lowered. If the rolling end temperature is too low, abnormal reverse rolling occurs, resulting in stretched ferrite and perlite, and the formation of a perlite band. Since the ductility and low-temperature toughness is very low, it is a conventional technique to manage the rolling end temperature within ± 30 ℃ based on the transformation temperature (Ar3) of the following formula (2).

That is, when the transformation temperature (Ar ₃ ) is converted by Equation 2, the transformation temperature (Ar ₃ ) is about 750 ° C. In the related art, grain size may be reduced by ending rolling at 720 to 780 ° C lower or higher than the temperature.

The most important feature of the ultra-low carbon type bainite steel of the present invention is to provide a method for manufacturing a steel material which improves productivity by about 25% by requiring rolling at 900 ° C. ± 50 ° C., which is relatively hotter than 750 ° C., which is after air cooling. This is possible because the microstructure can consist of bainite. When the structure after air cooling becomes bainite, the rolling end temperature dependency of strength and toughness becomes very small, so that rolling can be terminated at a higher temperature than in the prior art. In the present invention, the temperature is 100 ° C. to 200 ° C. above the transformation temperature (Ar ₃ ). It is limited to finishing rolling in.

After rolling, accelerated cooling is performed using water. At this time, the accelerated cooling rate is limited to 7 ~ 15 ℃ / second, but below 7 ℃, before the bainite transformation occurs, the precipitated cornerstone ferrite precipitates at the austenite grain boundary, significantly lowering the strength and toughness. Since it becomes a martensitic structure, the low-temperature toughness due to carbide precipitation is remarkably lowered, or in order to prevent it, it is preferable to limit it to 7 to 15 ° C./sec.

Hereinafter, the present invention will be described in detail through examples.

Slabs of alloy steel were prepared as shown in Table 1 below, and the same slabs were divided into several parts and rolled by controlling the finish temperature of rolling to 950 ° C, 850 ° C, etc., and the rolled sheet was accelerated or air-cooled, where air-cooled The ash is a comparative steel (B), and the accelerated coolant is from the inventive steel (A), and the rolling and cooling conditions are shown in Table 2 below.

As can be seen in Table 2, the invention steel and the comparative steel are made of slabs after the same vacuum melting, and then cut and rolled, respectively, to be accelerated-cooled or air-cooled steels. It can be seen that the tensile strength increased by about 2kg / mm ² at the same finish rolling temperature.

(Example 2)

1 shows the Charpy impact toughness values of the inventive steel and the comparative steel having a rolling end temperature of 850 ° C. and 950 ° C. in the rolled sheet material with respect to the test temperature. Although the toughness of the invention steel exceeds 200J / cm at -60 ° C regardless of the rolling end temperature, when the rolling end temperature of the comparative steel is 950 ° C, the toughness decreases rapidly at low temperatures. This means that when the accelerated cooling is not applied, the rolling end temperature must be managed at 850 ° C. or lower, which leads to a decrease in productivity. In addition, when the rolling finish temperature is not maintained below 850 ° C. during the production of steel sheet, the rolled steel sheet has a disadvantage of poor temperature toughness treatment. On the other hand, in the case of the invention steel to which the accelerated cooling method is applied, the end temperature of the rolling can be increased to 950 ° C., thus eliminating the need for air-cooling waiting time during rolling, resulting in improved productivity. As it is secured, there is an advantage that can greatly reduce the defective rate.

(Example 3)

As can be seen in Examples 1 and 2, the invention steel was superior in strength and low temperature toughness as compared to the comparative steel, which showed the same bainite structure as the invention steel and the comparative steel, but the invention steel as shown in FIG. This is because it has a finer and more uniform microstructure than the comparative steel.

As described above, the present invention satisfies the tensile strength of 60kgf / mm ² , has excellent low-temperature toughness, and the steel can be manufactured by using high-temperature rolling + acceleration cooling method, thereby improving productivity.

Claims (2)

In the present invention, C: 0.01 to 0.04%, Mn: 1.2 to 2.0%, Si: 0.1 to 0.3%, Al: 0.02 to 0.06%, Ti: 0.01 to 0.03%, Nb: 0.04 to 0.06%, V: 0.05 to 0.07%, Mo: 0.15 to 0.25, B: 0.001 to 0.002%, S: 0.02% or less, P: 0.02% or less, N: 10 to 100 ppm, O: 0.01 to 0.05%, Ca: 10 to 100 ppm, remaining In the method for producing a structural steel material of 60kgf / ㎜ class tensile strength slab of steel consisting of Fe and other unavoidable impurities, Heating the slab of the steel in a temperature range of 1050 to 1300 ° C. and then hot rolling the steel slab in a conventional manner; Terminating the hot rolled plate at a temperature of 100 to 200 ° C. higher than the transformation temperature (Ar ₃ ); Accelerated cooling the steel sheet after the rolling; Method for producing bainite-based steel excellent in low temperature toughness, characterized in that it comprises a. The method of claim 1, A method for producing bainite steel having excellent low temperature toughness, wherein the cooling rate is controlled in a range of 7 ° C. to 15 ° C./sec when the cold rolling is accelerated.

Images