KR101634011B1 - Fire-resistant steel and method of manufacturing the same - Google Patents

Abstract

Disclosed are fire-resistant steel capable of ensuring excellent high temperature yield performance by controlling an alloy component and a process condition; and a method of manufacturing the same. According to the present invention, the fire-resistant steel is formed with at least one of 0.06-0.08 wt% of carbon (C), 0.1-0.3 wt% of silicon (Si), 1.1-1.3 wt% of manganese (Mn), equal to or less than 0.015 wt% of phosphorous (P), equal to or less than 0.003 wt% of sulfur (S), 0.3-0.4 wt% of chromium (Cr), 0.3-0.4 wt% of molybdenum (Mo), 0.02-0.03 wt% of niobium (Nb), 0.035-0.045 wt% of vanadium (V), and the remaining consisting of iron (Fe) and inevitable impurities. The final microstructure has a composite organization including ferrite, pearlite, and bainite. The bainite organization has a cross-sectional area rate equal to or less than 3%.

Description

FIRE-RESISTANT STEEL AND METHOD OF MANUFACTURING THE SAME [0001] FIELD OF THE INVENTION [0001]

The present invention relates to a refractory steel material and a method of manufacturing the same, and more particularly, to a refractory steel material capable of securing excellent high-temperature yield characteristics through control of alloy components and process conditions and a method of manufacturing the refractory steel material.

Recently, as the trend of the building structure changes gradually with the development of the industry, there is an increasing concern about collapse due to the decrease in the strength of the steel due to the heat generated by the fire.

These refractory steels are required to have a yield point not less than 2/3 of the room temperature standard value even at high temperature while maintaining general steel performance at room temperature without using the conventional fireproof coatings such as rock wool coverings, ceramic coatings and refractory coatings.

A related prior art is Korean Patent Registration No. 10-0415663 (published on Jan. 31, 2004), which discloses a method of manufacturing a hot-rolled steel sheet excellent in high-temperature strength.

It is an object of the present invention to provide a method for manufacturing a refractory steel for building structure capable of securing excellent high-temperature yielding properties through control of alloy components and process conditions.

Another object of the present invention is to provide a process for producing a polypropylene resin which is produced by the above process and has a tensile strength (TS) of 495 to 610 MPa, a yield point (YP) of 330 to 445 MPa and an elongation (EL) of 17 to 30% And an impact absorption energy at 0 DEG C: 110 J or more.

In order to accomplish the above object, the present invention provides a method of manufacturing a refractory steel comprising: (a) 0.06 to 0.08% of C, 0.1 to 0.3% of Si, 1.1 to 1.3% of Mn, 0.015% of P, (Fe) and unavoidable impurities (at least one of S: 0.003% or less, 0.3 to 0.4% of Cr and 0.3 to 0.4% of Mo, 0.02 to 0.03% of Nb and 0.035 to 0.045% Reheating the slab plate to be formed to a slab reheating temperature (SRT) of 1100 to 1200 ° C; (b) subjecting the reheated slab sheet to finishing hot rolling under the conditions of FRT (Finishing Rolling Temperature): 880 to 900 ° C; And (c) cooling the finished hot-rolled plate.

According to another aspect of the present invention, there is provided a refractory steel comprising: 0.06 to 0.08% of C, 0.1 to 0.3% of Si, 1.1 to 1.3% of Mn, 0.015% of P or less, (Fe) and unavoidable impurities, in the range of 0.003 to 0.003%, 0.3 to 0.4% of Cr and 0.3 to 0.4% of Mo, 0.02 to 0.03% of Nb and 0.035 to 0.045% of V, Wherein the microstructure has a composite structure including ferrite, pealite and bainite, wherein the bainite structure has a cross sectional area ratio of 3% or less.

The refractory steel according to the present invention and the method of manufacturing the same are capable of strictly controlling addition amounts of alloying elements such as Cr, Mo, Nb and V and reducing the temperature of the slab reheating and the finish hot rolling to minimize the initial austenite grain size, It is possible to secure an excellent high-temperature yielding property by reducing the fraction of the tissue.

Accordingly, the refractory steel according to the present invention and its manufacturing method can contribute not only to the fire resistance of the building, the construction workability and the appearance of the building but also the cost of the construction through securing excellent high temperature yielding characteristics.

1 is a process flow diagram illustrating a refractory steel manufacturing method according to an embodiment of the present invention.
Fig. 2 is a photograph showing the final microstructure of the specimen according to Comparative Example 1. Fig.
3 is a photograph showing the final microstructure of the specimen according to Example 1. Fig.
4 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, refractory steel according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Refractory steel

The refractory steel according to the present invention has a tensile strength (TS) of 495 to 610 MPa, a yield point (YP) of 330 to 445 MPa, an elongation (EL) of 17 to 30%, a yield ratio (YR) of 75% Shock absorption energy: aiming to show 110J or more.

For this, the refractory steel according to the present invention contains 0.06 to 0.08% of C, 0.1 to 0.3% of Si, 1.1 to 1.3% of Mn, 0.015% or less of P, 0.003% or less of S, (Fe) and unavoidable impurities of at least one of Nb: 0.02 to 0.03% and V: 0.035 to 0.045%, and 0.4 to 0.4% of Mo and 0.3 to 0.4% of Mo.

At this time, the refractory steel has a composite structure including ferrite, pealite, and bainite, and the bainite structure has a sectional area ratio of 3% or less.

Hereinafter, the role and content of each component contained in the refractory 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 0.06 to 0.08% by weight of the total weight of the refractory steel according to the present invention. When the content of carbon (C) is less than 0.06% 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 0.08% by weight, the strength of the steel increases, but the impact resistance and weldability at low temperatures are deteriorated.

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 0.1 to 0.3% by weight based on the total weight of the refractory steel according to the present invention. If the content of silicon (Si) is less than 0.1% by weight, it may be difficult to exhibit the above effect properly. On the contrary, when the content of silicon (Si) is more than 0.3% by weight, a large amount of nonmetallic inclusions are formed on the surface of the steel to deteriorate 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 1.1 to 1.3% by weight based on the total weight of the refractory steel according to the present invention. When the content of manganese (Mn) is less than 1.1% 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 1.3% by weight, sulfur precipitated 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 0.015% by weight or less based on the total weight of the refractory steel.

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 0.003% by weight or less based on the total weight of the refractory steel.

Chromium (Cr)

Chromium (Cr) has an effect of suppressing center segregation by increasing the diffusion of manganese (Mn) in the manufacture of slabs, and is an element that forms a low-temperature transformation phase upon cooling to have excellent strength and corrosion resistance.

It is preferable that the chromium (Cr) is added at a content ratio of 0.3 to 0.4 wt% of the total weight of the refractory steel according to the present invention. When the content of chromium (Cr) is less than 0.3% by weight, the effect of the addition can not be exhibited properly. On the other hand, when the content of chromium (Cr) exceeds 0.4% by weight, the weld heat affected zone (HAZ) tends to deteriorate toughness.

Molybdenum (Mo)

Molybdenum (Mo) contributes to the improvement of strength and toughness, and also contributes to securing stable strength at high temperature by increasing the temperature of the low temperature transformed structure.

The molybdenum (Mo) is preferably added in an amount of 0.3 to 0.4% by weight based on the total weight of the refractory steel according to the present invention. When the content of molybdenum (Mo) is less than 0.3% by weight, the effect of adding molybdenum is insufficient. On the other hand, when the content of molybdenum (Mo) exceeds 0.4% by weight, the weldability is deteriorated.

Niobium (Nb)

Niobium (Nb) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. Niobium-based carbides or nitrides improve grain strength and low-temperature toughness by suppressing grain growth during rolling and making crystal grains finer.

The niobium (Nb) is preferably added in an amount of 0.02 to 0.03% by weight based on the total weight of the refractory steel according to the present invention. When the content of niobium (Nb) is less than 0.02% by weight, the effect of adding niobium can not be exhibited properly. On the contrary, when the content of niobium (Nb) exceeds 0.03% by weight, the weldability of steel is deteriorated. If the content of niobium (Nb) exceeds 0.03% by weight, the strength and low temperature toughness due to the increase in niobium content are not improved any more, but are present in a solid state in ferrite, which may lower the impact toughness.

Vanadium (V)

Vanadium (V) plays a role in improving the strength of the steel sheet through precipitation strengthening effect by precipitate formation.

The vanadium (V) is preferably added in an amount of 0.035 to 0.045% by weight of the total weight of the refractory steel according to the present invention. If the content of vanadium (V) is less than 0.035% by weight, it may be difficult to exhibit the precipitation strengthening effect properly. On the contrary, when the content of vanadium (V) is more than 0.045 wt% and added in a large amount, there is a problem that the low-temperature impact toughness is lowered.

Manufacturing method of refractory steel

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

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

In the refractory steelmaking method according to the present invention, the semi-finished slab plate to be subjected to the hot rolling process is composed of 0.06 to 0.08% of C, 0.1 to 0.3% of Si, 1.1 to 1.3% of Mn, 0.015% of P, (Fe) and unavoidable impurities (at least one of S: 0.003% or less, 0.3 to 0.4% of Cr and 0.3 to 0.4% of Mo, 0.02 to 0.03% of Nb and 0.035 to 0.045% .

Reheating slabs

In the slab reheating step S110, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1100 to 1200 ° C. Here, the slab plate 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 slab plate obtained through the continuous casting process is reheated to reuse the segregated components during casting.

In particular, in the present invention, it is preferable to control the slab reheating temperature as low as 1100 to 1200 ° C in order to minimize the grain size of the initial austenite, thereby reducing the fraction of the final low-temperature transformed structure after the cooling step.

When the slab reheating temperature (SRT) is less than 1100 ° C, the Nb-based precipitate NbC can not reach the solid-solution temperature and can not be precipitated as a fine precipitate during hot rolling so that the austenite grain growth can not be suppressed and the austenite grains sharply There is a problem of coarsening. 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 slab plate is subjected to finish hot rolling under the conditions of FRT (Finishing Rolling Temperature): 880 to 900 ° C.

At this time, in the present invention, since the rolling finish temperature is lowered to about 50 캜 or lower in comparison with the conventional one, the hot rolling can be performed at a temperature of 880 to 900 캜 with a single stage rolling sufficiently.

If the rolling finish temperature (FRT) is less than 880 ° 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 rolling finish temperature FRT exceeds 900 캜, the ductility and toughness are excellent, but the strength is rapidly lowered.

At this time, it is preferable that the hot rolling is finish-rolled so 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.

Cooling

In the cooling step (S130), the finished hot rolled plate is cooled.

At this time, it is preferable to perform air cooling to a normal temperature at a rate of 0.8 ° C / sec or less. As the cooling rate slows down, the low-temperature transformed structure almost disappears and the final microstructure becomes complex structure of ferrite, pearlite and bainite It is possible to secure an appropriate mechanical property at room temperature while securing an excellent high-temperature yielding property.

At this time, when the cooling rate exceeds 0.8 ° C / sec, although the strength is increased at room temperature due to the formation of the low-temperature transformation texture, there is a problem that the low-temperature toughness is lowered.

The refractory steel produced in the above-described processes (S110 to S130) strictly controls the amount of alloying elements such as Cr, Mo, Nb, and V, and reduces the temperature of the slab reheating and finish hot rolling to minimize the initial austenite grain size Thereby reducing the fraction of the low-temperature transformed structure and securing excellent high-temperature yielding characteristics.

Accordingly, the refractory steel according to the present invention and its manufacturing method can contribute not only to the fire resistance of the building, the construction workability and the appearance of the building but also the cost of the construction through securing excellent high temperature yielding characteristics.

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 2 and Comparative Example 1 were produced with the composition shown in Table 1 and the process conditions shown in Table 2. At this time, in the case of the specimens according to Examples 1 and 2 and Comparative Example 1, the ingots having the respective compositions were prepared, heated and hot-rolled using a rolling simulation tester, and then cooled. Thereafter, tensile tests were performed at room temperature (20 캜) and high temperature (600 캜) on the specimens prepared according to Examples 1 and 2 and Comparative Example 1.

[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 and 2 and Comparative Example 1. [

[Table 3]

(YP): 330 to 445 MPa and elongation (EL): 17 to 30 MPa, which correspond to the target values, of the specimens according to Examples 1 and 2, %, Yield ratio (YR): 75% or less, and impact absorption energy at 0 캜: 110J or more.

Particularly, in the case of the specimens according to Examples 1 and 2, the YP value at 600 ° C is 2/3 or more of the YP value at 20 ° C, Respectively.

On the other hand, in the case of the specimen according to Comparative Example 1, the elongation (EL) and the yield ratio (YR) satisfied the target value, but the tensile strength TS and the yield point YP exceeded the target value, It can be seen that the impact absorption energy (CVN) is only 22J.

In particular, in the case of the test piece according to Comparative Example 1, it can be seen that the yield point (YP) at 600 ° C. is only half the yield point (YP) at 20 ° C., Respectively.

Fig. 2 is a photograph showing the final microstructure of the specimen according to Comparative Example 1, and Figs. 3 and 4 are photographs showing the final microstructure of the specimen according to Examples 1 and 2. Fig.

As shown in FIG. 2, in the case of the test piece according to Comparative Example 1, it can be seen that the final microstructure has a composite structure including ferrite, pealite and bainite, The tensile strength (TS) value increased due to the large fraction of the tissue fraction at the cross sectional area of 7.5%, but the low temperature impact toughness decreased.

On the other hand, as shown in Figs. 3 and 4,

In the case of the specimens according to Examples 1 and 2, the final microstructure had a structure in which the final microstructure had a composite structure including ferrite, pealite and bainite, And 2.1% and 2.4%, respectively.

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: cooling step

Claims (6)

(a) 0.3 to 0.4% of Cr, 0.3 to 0.4% of Mo, 0.3 to 0.4% of Cr, 0.1 to 0.3% of Si, 0.1 to 0.3% of Si, (Slab reheating temperature) of 1100 to 1200 占 폚, which is composed of at least one of Nb: 0.02 to 0.03% and V: 0.035 to 0.045%, and the balance of Fe and unavoidable impurities, ;
(b) subjecting the reheated slab sheet to finishing hot rolling under the conditions of FRT (Finishing Rolling Temperature): 880 to 900 ° C; And
(c) cooling the finished hot rolled plate,
Wherein the cooling in the step (c) is performed at a rate of 0.8 ° C / sec or less to a normal temperature.
delete 0.1 to 0.3% of Si, 1.1 to 1.3% of Mn, 0.015% or less of P, 0.003% or less of S, 0.3 to 0.4% of Cr and 0.3 to 0.4% of Mo, , At least one of Nb: 0.02 to 0.03% and V: 0.035 to 0.045%, and the balance of iron (Fe) and unavoidable impurities,
Wherein the final microstructure has a composite structure including ferrite, pealite and bainite, wherein the bainite structure has a cross sectional area ratio of 3% or less.
The method of claim 3,
The steel
, A tensile strength (TS) of 495 to 610 MPa, a yield point (YP) of 330 to 445 MPa, and an elongation (EL) of 17 to 30%.
The method of claim 3,
The steel
A yield ratio (YR) of 75% or less and an impact absorption energy at 0 캜 of 110J or more.
The method of claim 3,
The steel
(YP) value at 600 占 폚 has 2/3 or more of the yield point (YP) value at 20 占 폚.

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