KR20120087688A - Steel having enhanced strength and Charpy impact characteristic and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high-strength steel with excellent low-temperature impact characteristics and a manufacturing method thereof are provided to prevent the overload of a rolling machine by rolling at temperatures higher than 950°C. CONSTITUTION: A high-strength steel with excellent low-temperature impact characteristics is composed of 0.14-0.16wt.% of C, 0.25-0.35wt.% of Si, 1.25-1.35wt.% of Mn, less than 0.03wt.% of P, less than 0.015wt.% of S, 0.05-0.15wt.% of Ni, less than 0.15wt.% of Cr, 0.1-0.2wt.% of Cu, 0.04-0.06wt.% of Al, 0.008-0.012wt.% of N, 0.04-0.06wt.% of V, 0.035-0.045wt.% of Nb, and the rest of Fe.

Description

Steel having enhanced strength and Charpy impact characteristic and method for manufacturing the same

TECHNICAL FIELD The present invention relates to a steel manufacturing technology, and more particularly, to a high strength steel and a manufacturing method excellent in low temperature impact properties.

Structural steels used in buildings and structures require high strength properties. In addition, as the strength of the steel increases, the toughness characteristics may be lowered, and thus may be vulnerable to low temperature impact. There is a need for a high strength structural steel that can ensure and guarantee low temperature impact properties to resist brittle fracture at low temperatures. In addition, steel having excellent high-strength low temperature impact characteristics has been developed using a controlled rolling technology, but it is difficult to lower the rolling temperature due to the load of the rolling mill. Accordingly, there is a demand for the development of a method capable of realizing a high strength steel having excellent low temperature impact characteristics while using a general rolling equipment in which control rolling is practically impossible.

The technical structure described above is a background technique for assisting the understanding of the present invention, and does not mean the prior art widely known in the technical field to which the present invention belongs.

The present invention is to propose a steel material and a manufacturing method excellent in high strength and low temperature impact properties by adjusting the alloy composition of the steel material.

One aspect of the invention, 0.14 to 0.16 parts by weight (wt%) of carbon (C); 0.25 to 0.35 parts by weight of silicon (Si); 1.25 to 1.35 parts by weight manganese (Mn); Phosphorus (P) not more than 0.03 parts by weight; Sulfur (S) of 0.015 parts by weight or less; 0.05 to 0.15 parts by weight of nickel (Ni); 0.15 parts by weight or less of chromium (Cr); 0.1 to 0.2 parts by weight of copper (Cu); 0.04 to 0.06 parts by weight of aluminum (Al); 0.008 to 0.012 parts by weight of nitrogen (N); 0.04 to 0.06 parts by weight of vanadium (V); 0.035 to 0.045 parts by weight of niobium (Nb); And it provides a high-strength steel excellent in low-temperature impact properties including the residual amount of iron (Fe).

Another aspect of the invention, the step of reheating the slab (slab) of the steel in the temperature range of 1150 ℃ to 1250 ℃; And it provides a high-strength steel production method excellent in low-temperature impact properties including the step of rolling at a temperature range of 950 ℃ to 1050 ℃ immediately after the reheating to a rolling end temperature.

The carbon (C) is 0.15 parts by weight (wt%), the silicon (Si) 0.3 parts by weight, the manganese (Mn) is 1.3 parts by weight, the phosphorus (P) is 0.015 parts by weight, the sulfur (S) is 0.01 parts by weight, 0.1 parts by weight of nickel (Ni), 0.1 parts by weight of copper (Cu), 0.05 parts by weight of aluminum (Al), 0.01 parts by weight of nitrogen (N), and vanadium (V) 0.05 parts by weight and the niobium (Nb) is included 0.04 parts by weight, suggesting a high strength steel excellent in low-temperature impact properties having a yield strength of 450MPa or more, a tensile strength of 570MPa or more, and an impact value of 47J or more at -5 ℃.

The vanadium (V) may be included 0.045 to 0.055 parts by weight.

The vanadium (V) may be included 0.04 to 0.045 parts by weight.

The aluminum (Al) may be included 0.045 to 0.055 parts by weight.

The aluminum (Al) may be included 0.04 to 0.045 parts by weight.

According to the present invention, while reducing the content of vanadium (V), aluminum (Al) may be added to compensate for the strength due to the decrease in vanadium. Accordingly, the SM570 grade steel having a yield strength of 450 MPa or more, a tensile strength of 570 MPa or more, and an impact value of 46 J or more at -5 ° C can be presented. At this time, by performing the rolling at a temperature higher than 950 ℃, it is possible to solve the problem caused by the excessive load of the rolling mill by performing the rolling at a low temperature.

1 is a view presented to explain a high strength steel and a manufacturing method excellent in the low-temperature impact characteristics of the present invention.
Figure 2 is a measurement results presented to explain the effect of the high-strength steel and the manufacturing method excellent in the low-temperature impact characteristics of the present invention.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of a high-strength steel excellent in low temperature impact properties and a manufacturing method according to the present invention. In this process, the thicknesses of the lines and the sizes of the components shown in the drawings may be exaggerated for clarity and convenience of explanation. In addition, terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to a user's or operator's intention or custom. Therefore, definitions of these terms should be made based on the contents throughout this specification.

1 is a view presented to explain a high strength steel and a manufacturing method excellent in the low-temperature impact characteristics of the present invention, Figure 2 is a measurement presented to explain the effect of the high strength steel and a manufacturing method excellent in the low-temperature impact properties of the present invention The results are.

The present invention has a high content of aluminum (Al) in the component system of the steel to 0.05 parts by weight (wt%), leading to compensating the strength by the generation of supercooled structure by the manganese segregation of aluminum. Accordingly, an SM570 grade steel having a yield strength of 450 MPa or more, a tensile strength of 570 MPa or more, and an impact value of 46 J or more at −5 ° C. is provided.

Hereinafter, the limited range of the steel material component system of this invention and its reason are demonstrated.

A) carbon (C): 0.14 to 0.16 parts by weight (wt%)

Carbon (C) is added to secure the strength and toughness of the steel. In the present invention, the carbon content is high from 0.14 wt% to 0.16 wt%, thereby allowing the addition of silicon (Si), vanadium (V) and niobium (Nb). If the content of carbon is added higher than 0.16 wt% can lower the low temperature toughness, if lower than 0.14wt% can cause a drop in strength. More preferably, it may be contained 0.15wt%.

B) silicon (Si): 0.25-0.35 wt%

Silicon (Si) may be added as a deoxidizer to remove oxygen in the steel during the steelmaking process. In addition, silicon (Si) increases the strength due to the solid solution strengthening effect. The higher the content, the higher the strength, but when added to more than 0.35wt% may deteriorate the toughness, if less than 0.25wt% may cause a decrease in the deoxidation effect. It is effective to add about 0.3wt%.

C) manganese (Mn): 1.25-1.35 wt%

Manganese (Mn) may be added as an element to enhance strength by solid solution strengthening. By rolling, the grains can be refined to contribute to improving the strength and toughness. When manganese is added below 1.25wt%, the effect of contributing to the improvement of strength may be drastically reduced, and when added to more than 1.35wt%, toughness may be greatly weakened when steel is used. Therefore, manganese (Mn) is added 1.25 to 1.35 wt%, preferably 1.3 wt%.

Phosphorus (P): greater than 0wt% and less than 0.03wt%

Phosphorus (P) is an ingredient that is excellent for excellent solid solution effect and corrosion resistance, but when phosphorus (P) is added in a large amount, phosphorus (P) may be segregated at grain boundaries and cause secondary processing brittleness. Thus, phosphorus is added above 0 wt% and below 0.03 wt%. More preferably, it may be added at about 0.023 wt%.

ㅁ) sulfur (S): more than 0wt% 0.015wt% or less

Sulfur (S) may cause the formation of sulfide-based inclusions such as manganese sulfide (MnS), and may act as an element to lower the impact value by lowering the Charpy impact absorption energy. Since it is advantageous to keep it as low as possible, the upper limit is made into 0.015 wt%. It is effective to contain 0.012wt%.

Iii) nickel (Ni): 0.05wt% to 0.15wt%

Nickel (Ni) can refine the grains and be dissolved in austenite and ferrite to strengthen the matrix. Thereby, the strength and toughness of steel materials can be improved. Nickel is effective in improving toughness when added at 0.05 wt% or more, but is limited to 0.15 wt% because it is an expensive element and may cause brittleness when excessively added. More preferably 0.11 wt%.

Chromium (Cr): more than 0wt% and within 0.15wt%

Chromium (Cr) is effective in increasing strength by increasing the hardenability, but it is effective to add within 0.15wt%. It is more effective to contain 0.1wt%.

ㅇ) Copper (Cu): 0.1wt% to 0.2wt%

Copper (Cu) is a tramp element that may remain in the steel material and is an impurity that cannot be completely removed in the steelmaking process. Although copper (Cu) mainly serves as an element to increase strength, it acts as a factor for lowering elongation and surface quality of steel, and therefore it is preferably limited to 0.1 wt% to 0.2 wt%. More preferably, 0.1 wt% is added.

Aluminum: Al: 0.04 wt% to 0.06 wt%

Aluminum (Al) is added as a deoxidizer component and lowers the amount of dissolved oxygen in the steel to keep the amount of dissolved oxygen low. In addition, aluminum (Al) is added to compensate for the strength due to the decrease in the amount of vanadium, instead of reducing the content of vanadium (V) added to increase the strength by increasing the precipitate fraction. Aluminum induces grain reduction and some manganese segregation due to aluminum nitride (AlN), compensating for strength by increasing the low temperature supercooled tissue created around it. When aluminum is added in an amount greater than 0.06 wt%, the strength increases due to the increase of the supercooled structure due to Al segregation, but the impact value decreases. Therefore, 0.06 wt% is added as the upper limit. In addition, when aluminum is added at less than 0.04 wt%, since the effect of the addition is insignificant, aluminum is added by setting 0.04 wt% to the lower limit. Aluminum (Al) is effective to increase the impact value when added 0.045 to 0.055wt%, more preferably 0.04 to 0.045wt% may be added.

Nitrogen: N: 0.008wt% to 0.012wt%

Since nitrogen (N) increases strength while greatly reducing toughness, the content is limited to 0.012 wt% or less, and the lower limit is limited to 0.008 wt% for segregation of aluminum nitride (AlN). Nitrogen is more effective to add 0.01wt%.

Niobium (Nb): 0.035 wt% to 0.045 wt%

Niobium (Nb) is added as an element which precipitates in the form of NbC or NbCN and greatly improves the strength of the base metal. By inhibiting grain growth during rolling, the grains are refined, thereby inducing toughness improvement and precipitation strengthening effect after rolling cooling. When added below 0.035 wt%, the effect is negligible, and when added above 0.045 wt%, brittle cracks can be induced. Effectively, about 0.04 wt% may be added.

Iii) Vanadium (V): 0.04 wt% to 0.06 wt%

Vanadium (V) combines with carbon during cooling to form VC carbides, contributing to strengthening precipitation and suppressing grain growth. In addition, the temperature employed is lower than other additive elements, which may lead to an effect of preventing a drop in strength. Increasing the vanadium content is the main effect of increasing the strength of the precipitate fraction, but the vanadium price fluctuates severely, making it difficult to use stably as an additive element. Therefore, aluminum (Al) is added to minimize the vanadium content to compensate for the strength. Therefore, vanadium is added at 0.04 wt% to 0.06 wt%. In view of the impact value improvement effect, vanadium (V) is effective when added at 0.045 wt% to 0.055 wt%, and more effective than when added at 0.04 wt% to 0.045 wt%.

The steel of the present invention contains the above-mentioned components, and contains iron (Fe) in the remainder. In addition, molybdenum (Mo) may be added in about 0.01 to 1.0wt%. Molybdenum has an effect of greatly improving the hardenability to suppress ferrite production, and may be added in an amount of 0.01 wt% or more, but is limited to 1.0 wt% or less because toughness is inhibited due to an increase in hardness. More preferably, it may be added in the order of 0.02 wt% to 0.2 wt%, substantially 0.03 wt%. In addition, although elements that are inevitably included may be mixed, these elements are elements that are inevitably mixed in raw materials, materials, manufacturing facilities, and the like.

The steel material of the present invention having the above-mentioned compositional component system has a yield strength of 450 MPa or more and 570 MPa as a compensation for strength by addition of aluminum (Al) while reducing the content of vanadium (V) despite introducing a rolling temperature of 950 ° C. or higher. The above tensile strength and impact value of 46J or more at -5 ° C can be realized.

The rolling process according to the invention can be carried out as follows.

In order to re-use the steel having the above-described composition, a temperature control and rolling process as shown in FIG. 1 is introduced. That is, the slab of the steel having the composition of the present invention is reheated to a temperature of 1150 ° C to 1250 ° C to re-use segregated components during casting. At this time, it is reheated about 1.5 to 3.5 hours, More preferably, about 2 hours. When the reheating temperature is lower than 1150 ° C, the rolling load may be increased, and when the reheating temperature is higher than 1250 ° C, the austenite grains may be coarsened, thereby making it difficult to secure strength.

After reheating, for example, a cumulative reduction of 80%, that is, a rolling reduction of 80% or more relative to the initial thickness. At this time, the rolling end temperature at which rolling is completed is set to 950 ° C to 1050 ° C. That is, 1000 degreeC is set to rolling completion temperature, and a temperature margin is 50 degreeC up and down, and rolling is performed. Since the controlled rolling is carried out at 950 ℃ or less when rolling is excluded, it is possible to suppress the generation of the load of the rolling mill, it is possible to implement a steel having a high strength and excellent low-temperature impact characteristics in a general rolling equipment.

Rolling is carried out in a rolling process immediately after extraction of the reheated slab material in the furnace. In other words, the rolling start temperature is substantially set in a temperature range of 1150 ° C to 1250 ° C. Therefore, in rolling, since it is made in the temperature range above recrystallization temperature, grain size is implemented small.

SM570 class structural steel can be provided by adjusting the component content of the steel, in particular, the content of vanadium (V) and aluminum (Al).

The results of evaluating the composition and the physical properties of the inventive examples and comparative examples prepared as described above are shown in FIG. 2. Figure 2 shows the composition of the invention examples and comparative examples in parts by weight (wt%), yield strength (YS: MPa), tensile strength (TS: MPa) and after rolling to 80% reduction (%) and The measurement result of Charpy impact value (J) at -5 degreeC is shown.

Considering Inventive Examples 1 to 5 and Comparative 1 to 3 of FIG. 2, when the content of vanadium (V) and aluminum (Al) is exceeded as in Comparative 1, the strength is increased due to the increase of vanadium precipitates and the Al segregation is caused. Increasing the supercooled tissue increases the strength but decreases the impact value. When the content of vanadium is insufficient as in Comparative 2, the strength is insufficient due to the sharp drop in vanadium precipitates. If the vanadium content is less than that of Comparative 3, and the aluminum content is exceeded, a partial increase in strength may appear due to an increase in the amount of aluminum added, but it does not compensate for the insufficient strength due to the reduction of precipitates. In view of the results of Inventive Examples 1 to 5, the impact value can be improved while securing the strength by reducing the grains caused by vanadium precipitates and aluminum nitride (AlN) and by the low temperature structure generated around some manganese segregation zones. have. Accordingly, in the present invention, it is possible to provide a SM570-class structural steel by controlling the content of vanadium and aluminum.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and those skilled in the art to which the art belongs can make various modifications and other equivalent embodiments therefrom. I will understand.

Therefore, the true technical protection scope of the present invention will be defined by the claims below.

Claims (7)

0.14 to 0.16 parts by weight (wt%) of carbon (C); 0.25 to 0.35 parts by weight of silicon (Si); 1.25 to 1.35 parts by weight manganese (Mn); Phosphorus (P) not more than 0.03 parts by weight; Sulfur (S) of 0.015 parts by weight or less; 0.05 to 0.15 parts by weight of nickel (Ni); 0.15 parts by weight or less of chromium (Cr); 0.1 to 0.2 parts by weight of copper (Cu); 0.04 to 0.06 parts by weight of aluminum (Al); 0.008 to 0.012 parts by weight of nitrogen (N); 0.04 to 0.06 parts by weight of vanadium (V); 0.035 to 0.045 parts by weight of niobium (Nb); And a high strength steel having excellent low temperature impact properties including a residual amount of iron (Fe).
The method of claim 1,
The carbon (C) is 0.15 parts by weight (wt%), the silicon (Si) 0.3 parts by weight, the manganese (Mn) is 1.3 parts by weight, the phosphorus (P) is 0.015 parts by weight, the sulfur (S) is 0.01 parts by weight, 0.1 parts by weight of nickel (Ni), 0.1 parts by weight of copper (Cu), 0.05 parts by weight of aluminum (Al), 0.01 parts by weight of nitrogen (N), and vanadium (V) 0.05 parts by weight and the niobium (Nb) is contained 0.04 parts by weight
A high strength steel having excellent low temperature impact characteristics having a yield strength of 450 MPa or more, a tensile strength of 570 MPa or more, and an impact value of 47 J or more at -5 ° C.
The method of claim 1,
The vanadium (V) is a high strength steel excellent in low-temperature impact properties containing 0.045 to 0.055 parts by weight.
The method of claim 1,
The vanadium (V) is a high-strength steel excellent in low-temperature shock properties containing 0.04 to 0.045 parts by weight.
The method of claim 1,
The aluminum (Al) is a high strength steel excellent in low-temperature impact properties contained 0.045 to 0.055 parts by weight.
The method of claim 1,
The aluminum (Al) is a high-strength steel excellent in low-temperature impact properties containing 0.04 to 0.045 parts by weight.
0.14 to 0.16 parts by weight (wt%) of carbon (C); 0.25 to 0.35 parts by weight of silicon (Si); 1.25 to 1.35 parts by weight manganese (Mn); Phosphorus (P) not more than 0.03 parts by weight; Sulfur (S) of 0.015 parts by weight or less; 0.05 to 0.15 parts by weight of nickel (Ni); 0.15 parts by weight or less of chromium (Cr); 0.1 to 0.2 parts by weight of copper (Cu); 0.04 to 0.06 parts by weight of aluminum (Al); 0.008 to 0.012 parts by weight of nitrogen (N); 0.04 to 0.06 parts by weight of vanadium (V); 0.035 to 0.045 parts by weight of niobium (Nb); And reheating the slab of the steel including the balance of iron (Fe) in a temperature range of 1150 ° C. to 1250 ° C .; And
A method for producing high strength steel having excellent low temperature impact characteristics, including rolling at a temperature range of 950 ° C to 1050 ° C as a rolling end temperature immediately after the reheating.

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