KR20160063168A

KR20160063168A - Steel and method of manufacturing the same

Info

Publication number: KR20160063168A
Application number: KR1020140166855A
Authority: KR
Inventors: 현영민; 김종호; 신경진
Original assignee: 현대제철 주식회사
Priority date: 2014-11-26
Filing date: 2014-11-26
Publication date: 2016-06-03

Abstract

Disclosed are a steel product for an architectural structure capable of securing strength and impact absorption energy at low temperatures in a balanced way through an adjustment of alloy substances and a control of process conditions, and a manufacturing method thereof. According to the present invention, the steel product comprises: 0.1-0.2 wt% of C; 0.4-0.6 wt% of Si; 1.4-1.6 wt% of Mn; 0.05 wt% or lower of P; 0.05 wt% or lower of S; 0.4-0.6 wt% of Ni; 0.05-0.15 wt% of Cu; 0.04-0.06 wt% of V; and the remaining consisting of iron (Fe) and unavoidable impurities, which has a complex tissue where the final minute tissue includes ferrite and pearlite. The average diameter of the ferrite tissue is 10-15 μm.

Description

STEEL AND METHOD OF MANUFACTURING THE SAME [0002]

TECHNICAL FIELD The present invention relates to a steel material and a method of manufacturing the same, and more particularly, to a steel material for building structure and a method of manufacturing the steel material, which can secure balance of strength and shock absorption energy at low temperatures through control of alloy components and process conditions will be.

Generally, the process for producing a high-strength steel is divided into a reheating process for reusing each component and a precipitate of the slab, a hot rolling process for rolling to a final thickness at a high temperature, and a cooling process .

At this time, to control the microstructure and mechanical properties by controlling the finish hot rolling temperature downward to control the high rolling steels, however, this control rolling eventually contributed to the deterioration of the sand carrier.

A related prior art document is Korean Patent Laid-Open Publication No. 10-2004-0075971 (published on Aug. 30, 2004), which discloses a high strength steel sheet and a manufacturing method thereof.

It is an object of the present invention to provide a method for manufacturing a structural steel for a building structure which can balance the strength and the shock absorption energy at low temperatures through the control of alloy components and process conditions.

Another object of the present invention is to provide a steel for building construction having a tensile strength (TS) of 600 to 720 MPa, a yield point (YP) of 460 MPa or more, and an elongation (EL) of 20% or more.

(A) 0.1 to 0.2% of C, 0.4 to 0.6% of Si, 1.4 to 1.6% of Mn, and 0.05% or less of P in weight%, based on the weight of the steel material, , A steel slab composed of S: not more than 0.05%, Ni: 0.4 to 0.6%, Cu: 0.05 to 0.15%, V: 0.04 to 0.06% and balance of iron (Fe) and unavoidable impurities was slab reheating temperature Lt; RTI ID = 0.0 > 1200 C; < / RTI > (b) subjecting the reheated steel slab to finishing hot rolling under the conditions of Finish Rolling Temperature (FRT): 880? 20 占 폚; (c) first cooling the finished hot rolled steel; (d) normalizing the primary cooled steel at 900 to 950 占 폚 for 1 to 3 hours; And (e) secondarily cooling the normalized heat-treated steel.

According to another aspect of the present invention, there is provided a steel according to an embodiment of the present invention, which comprises 0.1 to 0.2% of C, 0.4 to 0.6% of Si, 1.4 to 1.6% of Mn, (Fe) and unavoidable impurities, and the final microstructure is composed of ferrite and pealite. In the present invention, it is preferable that the ferrite and the ferrite be in a ratio of , Wherein the ferrite structure has an average diameter of 10 to 15 占 퐉.

The steel for building structure and its manufacturing method according to the present invention suppresses the growth of the structure after hot rolling and normalizing heat treatment through addition of vanadium (V) which is a precipitation hardening element and has a finer structure, N) precipitates can improve the strength of the ferrite, and slow cooling slower than air cooling during secondary cooling, so that the formation of low-temperature transformed structure is suppressed as much as possible, and the low-temperature impact toughness can be improved.

As a result, the steel material produced by the method according to the present invention has a composite structure in which the final microstructure includes ferrite and pealite, wherein the average diameter of the ferrite structure is 10 to 15 mu m, and the tensile strength ( (YR): 75% or less, and an impact absorption energy at 40 deg. C: 40 J or more. The tensile strength (TS) is 600 to 720 MPa, YP is 460 MPa or more, elongation is 20%

Further, the steel material produced by the method according to the present invention further comprises at least one low-temperature transformation structure among bainite and bainitic ferrite, wherein the low-temperature transformation structure has a cross-sectional area ratio of 1% or less, This is because the formation of the low-temperature transformed structure is suppressed as much as possible through slow cooling.

FIG. 1 is a flowchart showing a method of manufacturing a steel material according to an embodiment of the present invention.
Fig. 2 is a photograph showing the final microstructure of the specimen according to Comparative Example 2. Fig.
3 is a photograph showing the final microstructure of the specimen according to Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and how to accomplish them, will become apparent by reference to the embodiments described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various other forms, and it should be understood that the present embodiment is intended to be illustrative only and is not intended to be exhaustive or to limit the invention to the precise form disclosed, To fully disclose the scope of the invention to a person skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a steel material according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Steel

The steel according to the present invention has a tensile strength (TS) of 600 to 720 MPa, a yield point (YP) of not less than 460 MPa, an elongation (EL) of not less than 20%, a yield ratio (YR) of not more than 75% Energy: 40J or more.

The steels according to the present invention may contain 0.1 to 0.2% of C, 0.4 to 0.6% of Si, 1.4 to 1.6% of Mn, 0.05% or less of P, 0.05% or less of S, 0.6%, Cu: 0.05 to 0.15%, V: 0.04 to 0.06%, and the balance of iron (Fe) and unavoidable impurities.

At this time, it is preferable that the steel has a composite structure including a ferrite and a pealite, and the average diameter of the ferrite structure is 10 to 15 mu m.

Further, the steel material may further include at least one low-temperature transformed structure selected from the group consisting of bainite and bainitic ferrite, and the low-temperature transformed structure has a cross-sectional area ratio of 1% or less.

Hereinafter, the role and content of each component contained in the steel according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added to secure the strength of the steel.

The carbon (C) is preferably added in an amount of 0.1 to 0.2% by weight based on the total weight of the steel according to the present invention. When the content of carbon (C) is less than 0.1% by weight, the fraction of the second phase structure is lowered and the strength is lowered. On the contrary, when the content of carbon (C) exceeds 0.2% by weight, the strength of the steel increases, but the low-temperature impact toughness and weldability deteriorate.

Silicon (Si)

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel in the steelmaking process. In addition, silicon has a solubility enhancing effect.

The silicon (Si) is preferably added in an amount of 0.4 to 0.6% by weight based on the total weight of the steel material according to the present invention. If the content of silicon (Si) is less than 0.4% by weight, it may be difficult to exhibit the above effects properly. On the contrary, when the content of silicon (Si) is more than 0.6% by weight, a large amount of nonmetallic inclusions are formed on the surface of the steel to lower the toughness.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element and serves to improve the strength and toughness by reducing the Ar ₃ point to expand the control rolling temperature range, thereby finer crystal grains by rolling.

The manganese (Mn) is preferably added in an amount of 1.4 to 1.6% by weight based on the total weight of the steel according to the present invention. When the content of manganese (Mn) is less than 1.4% by weight, the fraction of the second phase structure is lowered and it may be difficult to secure the strength. On the other hand, when the content of manganese (Mn) exceeds 1.6% by weight, the sulfur dissolved in the steel precipitates into MnS, which lowers impact toughness at low temperatures.

Phosphorus (P), sulfur (S)

Phosphorus (P) contributes partly to the strength improvement, but it is a representative element that lowers impact toughness at low temperatures. The lower the content is, the better. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.05% by weight or less based on the total weight of the steel material.

Sulfur (S), together with phosphorus (P), is an element that is inevitably contained in the production of steel, and forms MnS to lower impact toughness at low temperatures. Therefore, in the present invention, the content of sulfur (S) is limited to 0.05% by weight or less based on the total weight of the steel material.

Nickel (Ni)

Nickel (Ni) is an element effective for improving toughness while improving toughness.

The nickel (Ni) is preferably added in an amount of 0.4 to 0.6% by weight based on the total weight of the steel material according to the present invention. When the content of nickel (Ni) is less than 0.4% by weight, the addition effect is insignificant. On the contrary, when the content of nickel (Ni) exceeds 0.6% by weight, the workability of the steel sheet is lowered and the manufacturing cost is increased.

Copper (Cu)

Copper (Cu) together with nickel (Ni) serves to improve the hardenability of the steel and the impact resistance at low temperatures.

The copper (Cu) is preferably added in an amount of 0.05 to 0.15% by weight based on the total weight of the steel according to the present invention. When the content of copper (Cu) is less than 0.05% by weight, the effect of adding copper can not be exhibited properly. On the contrary, when the content of copper (Cu) exceeds 0.15% by weight, it exceeds the solubility limit and does not contribute to the increase in strength.

Vanadium (V)

Vanadium (V) improves the strength of steel through precipitation strengthening effect by formation of V (C, N) precipitate, and contributes to texture refinement after normalizing heat treatment.

The vanadium (V) is preferably added in an amount of 0.04 to 0.06% by weight based on the total weight of the steel according to the present invention. When the content of vanadium (V) is less than 0.04% by weight, it may be difficult to exhibit the above effect properly. On the contrary, when the content of vanadium (V) exceeds 0.06% by weight, the low-temperature impact toughness deteriorates.

Steel manufacturing method

1 is a process flow diagram illustrating a method of manufacturing a steel material according to an embodiment of the present invention

Referring to FIG. 1, a method of manufacturing a steel material according to an embodiment of the present invention includes a slab reheating step S110, a hot rolling step S120, a first cooling step S130, a normalizing heat treatment step S140, And a cooling step (S150). At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to carry out the reheating step to obtain effects such as reuse of precipitates.

At this time, in the steel product manufacturing method according to the present invention, the semi-finished steel slab in the semi-finished product state is composed of 0.1 to 0.2% of C, 0.4 to 0.6% of Si, 1.4 to 1.6% of Mn, (Fe) and unavoidable impurities. The amount of S is not more than 0.05%, the content of S is not more than 0.05%, the content of Ni is 0.4 to 0.6%, the content of Cu is 0.05 to 0.15%, the content of V is 0.04 to 0.06%

Reheating slabs

In the slab reheating step S110, the steel slab having the above composition is reheated to a slab reheating temperature (SRT) of 1100 to 1200 ° C. Here, the steel slab can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. At this time, in the slab reheating step (S110), the steel slabs obtained through the continuous casting process are reheated to reuse the segregated components during casting.

At this stage, when the slab reheating temperature (SRT) is less than 1100 ° C, there is a problem that the reheating temperature is low and the rolling load becomes large. Further, since the V-based precipitates VC and VN can not reach the solid solution temperature, they can not be precipitated as fine precipitates during hot rolling, and the austenite grain growth can not be suppressed, and the austenite grains are rapidly coarsened. On the other hand, when the slab reheating temperature exceeds 1200 ° C, the austenite grains are rapidly coarsened and it is difficult to secure the strength and low temperature toughness of the steel to be produced.

Hot rolling

In the hot rolling step (S120), the reheated steel slab is finely hot-rolled under FRT (Finish Rolling Temperature): 880? 20 占 폚.

If the finishing rolling finish temperature (FRT) is less than 860 ° C, an abnormal reverse rolling occurs to form an uneven structure, which may significantly reduce the low temperature impact toughness. On the other hand, when the finish rolling finish temperature (FRT) exceeds 900 캜, the ductility and toughness are excellent, but the strength is rapidly lowered.

In this step, it is preferable that hot rolling is performed in such a manner that the cumulative rolling reduction in the non-recrystallized region is 60 to 70%. When the cumulative rolling reduction of the hot rolling is less than 60%, it is difficult to obtain a uniform but fine structure, so that the deviation of the strength and the impact toughness may occur severely. On the other hand, when the cumulative rolling reduction of the hot rolling exceeds 70%, there is a problem that the rolling process time is prolonged and the fishy property is deteriorated.

Primary cooling

In the primary cooling step (S130), the finished hot-rolled steel is first cooled. Here, the primary cooling may be air cooling, which is performed in a natural cooling manner up to room temperature. At this time, the normal temperature may be 1 to 40 ° C, but is not limited thereto.

In this step, the primary cooling rate may be 1 to 5 占 폚 / sec, but is not limited thereto. When the primary cooling rate is less than 1 캜 / sec, it is difficult to secure sufficient strength and toughness. On the other hand, if the primary cooling rate exceeds 5 DEG C / sec, cooling control is difficult, and excessive cooling may lower the economical efficiency.

Normalizing heat treatment

In the normalizing heat treatment step (S140), the primary cooled plate is subjected to a normalizing heat treatment at 900 to 950 DEG C for 1 to 3 hours.

If the normalizing heat treatment temperature is less than 900 ° C, it is difficult to reuse the solid solute elements, thereby making it difficult to secure sufficient strength. On the contrary, when the normalizing heat treatment temperature exceeds 950 DEG C, crystal grains grow, which lowers the low temperature toughness.

In this step, when the normalizing heat treatment time is out of the range of 1 to 3 hours, it is not easy to remove the residual stress. Therefore, it is appropriate that the normalizing heat treatment time is strictly limited to the range of 1 to 3 hours.

Secondary cooling

In the second cooling step (S150), the normalized heat-treated steel is secondarily cooled.

In this step, it is preferable to perform the secondary cooling rate at a rate of 0.25 to 0.40 DEG C / sec, which is slower than air cooling, in order to adjust the transformation, size and shape of the final microstructure after the normalizing heat treatment step (S140). If the secondary cooling rate is out of the range of 0.40 ° C / sec, the low-temperature transformed structure may be formed on the grain boundaries of the ferrite, or the shape of the low-temperature transformed structure may be sharply formed, have.

The steel material produced in the above steps S110 to S150 suppresses the growth of the structure after the hot rolling and the normalizing heat treatment through addition of the vanadium (V) which is the precipitation hardening element and has a finer structure, , N) precipitates can improve the strength of ferrite, and slow cooling is slower than air cooling during secondary cooling, so that the formation of low-temperature transformed structure is suppressed as much as possible and the low-temperature impact toughness can be improved.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

The specimens according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared with the composition shown in Table 1 and the process conditions shown in Table 2.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Table 3 shows the results of evaluation of mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 and 2.

[Table 3]

The specimens according to Examples 1 to 3 have tensile strength (TS) of 600 to 720 MPa, yield point (YP) of 460 MPa or more, elongation (EL) of 20% or more, Yield ratio (YR): 75% or less.

Particularly, all of the specimens according to Examples 1 to 3 had an impact absorption energy of -40 J or more at -20 캜, and the impact absorption energy at -40 캜 was also measured at 40 to 48 J.

On the other hand, in the case of the specimen according to the comparative example 1, most of the physical properties satisfied the target values set forth in the present invention, but no nickel, copper (Cu), and vanadium (V) It can be seen that tensile strength (TS) and yield point (YP) are below the target value due to the speed of 2.0 ° C / sec.

In addition, most of the alloy components were added in a similar amount as in Example 1, but in the case of the specimen according to Comparative Example 2 in which the secondary cooling rate was 1.5 ° C / sec, the tensile strength was as high as 735 MPa , But the yield point was below the target value and the shock absorption energy at -20 ℃ was only 23J, which is below the target value.

FIG. 2 is a photograph showing the final microstructure of the specimen according to Comparative Example 2, and FIG. 3 is a photograph showing the final microstructure of the specimen according to Example 1. FIG.

As shown in FIG. 2 and FIG. 3, it can be confirmed that both of the specimens according to Comparative Example 1 and Example 1 were composed mainly of ferrite and pearlite, and that some low-temperature transformation textures were produced. At this time, it was confirmed that the low-temperature transformed structure was at least one of bainite and bainitic ferrite.

However, in the case of the specimen according to Comparative Example 2, it was confirmed that the mean diameter of the ferrite was 17.6 μm due to the second cooling rate of 1.5 ° C./sec, and the low temperature transformation texture was observed at the grain boundaries of the ferrite It can be confirmed that a large amount is formed. Thus, it is understood that the impact absorption energy value tends to be lowered when a large amount of low-temperature transformed structure is formed in the grain boundaries of the ferrite, or when the shape is sharp.

On the other hand, in the case of the specimen according to Example 1, after the hot rolling and the normalizing heat treatment through the addition of vanadium, the growth of the structure was suppressed to have a finer structure, and the V (C, N) precipitate in the ferrite increased the strength of the ferrite It is believed that it has brought about the improvement effect.

Particularly, in the case of the specimen according to Example 1, since the slow cooling at a secondary cooling rate of 0.25 deg. C / sec is carried out, the formation of the low temperature transformation structure is suppressed as much as possible, Value, and it was confirmed that the average diameter of the ferrite structure was 12.6 mu m.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Slab reheating step
S120: Hot rolling step
S130: primary cooling step
S140: Normalizing heat treatment step
S150: Secondary cooling step

Claims

(a) 0.1 to 0.2% of C, 0.4 to 0.6% of Si, 1.4 to 1.6% of Mn, 0.05% or less of P, 0.05% or less of S, 0.4 to 0.6% of Ni, To about 0.15%, V: about 0.04% to about 0.06%, and the balance of iron (Fe) and unavoidable impurities to a slab reheating temperature (SRT) of 1100 to 1200 ° C.
(b) subjecting the reheated steel slab to finishing hot rolling under the conditions of Finish Rolling Temperature (FRT): 880? 20 占 폚;
(c) first cooling the finished hot rolled steel;
(d) normalizing the primary cooled steel at 900 to 950 占 폚 for 1 to 3 hours; And
(e) secondarily cooling the normalized heat-treated steel.

The method according to claim 1,
In the step (c)
The primary cooling
And cooling the steel sheet to a normal temperature by a natural cooling method.

The method according to claim 1,
In the step (e)
The secondary cooling
And then slowly cooled to room temperature at a cooling rate of 0.25 to 0.40 DEG C / sec.

0.1 to 0.2% of C, 0.4 to 0.6% of Si, 1.4 to 1.6% of Mn, 0.05% or less of P, 0.05% or less of S, 0.4 to 0.6% of Ni, 0.05 to 0.15% of Cu, , V: 0.04 to 0.06%, and the balance of iron (Fe) and unavoidable impurities,
Wherein the final microstructure has a composite structure including ferrite and pealite, wherein the average diameter of the ferrite structure is 10 to 15 占 퐉.

5. The method of claim 4,
The steel
, A tensile strength (TS) of 600 to 720 MPa, a yield point (YP) of 460 MPa or more, and an elongation (EL) of 20% or more.

5. The method of claim 4,
The steel
A yield ratio (YR) of 75% or less and an impact absorption energy at -20 캜 of 40 J or more.

5. The method of claim 4,
The steel
Wherein the low-temperature transformed structure further comprises at least one low-temperature transformed structure selected from the group consisting of bainite, bainite and bainitic ferrite, wherein the low-temperature transformed structure has a cross-sectional area ratio of 1% or less.