KR101412230B1 - Hot coil and method for manufacturing the same - Google Patents

Abstract

A hot-rolled coil having a small material deviation between the inner and outer portions through precipitation control and a method of manufacturing the same are disclosed.
A method for manufacturing a hot-rolled coil according to the present invention comprises the steps of: (a) mixing 0.02 to 0.08% carbon (C), 0.1 to 0.5% silicon (Si), 1.3-1.7% manganese (Mn) : 0.03% or less, sulfur (S): 0.005% or less, aluminum (Al): 0.01 to 0.03%, niobium (Nb): 0.03 to 0.09%, titanium (Ti): 0.02 to 0.08%, molybdenum Hot rolled at a finishing rolling temperature of 810 to 870 캜 at a slab plate made up of 0.3% to 0.3%, nitrogen (N): 0.005% or less, and the remaining iron (Fe) and unavoidable impurities; (b) subjecting the hot-rolled plate to primary cooling to (A1 transformation point + A2 transformation point) / 2 +/- 10 deg. C, and then maintaining or air cooling for 5 to 7 seconds; And (c) secondarily cooling the held or air-cooled plate to a temperature not higher than the A1 transformation point, and then winding.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled coil manufacturing technique, and more particularly, to a hot-rolled coil having a small variation in material between an outer portion and an inner portion through precipitation control and a manufacturing method thereof.

The hot-rolled coil includes an outer coil portion to which one side of the sheet material is exposed to the outside and an inner side portion to which one side of the sheet material is not exposed to the outside. The outer part can be divided into the outer part and the inner part again.

When the hot-rolled coil is made of precipitated hardened steel whose strength is increased in the precipitate, active precipitation occurs near the coiling temperature.

However, in the case of the outer coil portion of the hot-rolled coil, one side is exposed to the outside, and therefore, it is cooled more rapidly than the inner side portion. In this case, further growth of the precipitate does not occur at the outer periphery, while precipitation growth at the inner periphery continues for a certain period of time. As a result, the material of the inner portion of the hot-rolled coil deteriorates.

Background Art related to the present invention is an ultra-high strength steel sheet having excellent strength and strength as disclosed in Korean Patent Publication No. 10-2009-0068989 (published on Jun. 29, 2009) and a method for manufacturing the same.

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot-rolled coil having a small material deviation between an outer portion and an inner portion through control of an alloy component and a process condition, and a method of manufacturing the same.

In order to achieve the above object, a hot-rolled coil manufacturing method according to an embodiment of the present invention includes the steps of (a) 0.02 to 0.08% of carbon, 0.1 to 0.5% of silicon, manganese (Mn) (S): more than 0% to 0.005%, aluminum (Al): 0.01 to 0.03%, niobium (Nb): 0.03 to 0.09% A slab plate composed of 0.02 to 0.08% of titanium (Ti), 0.1 to 0.3% of molybdenum (Mo), more than 0 to 0.005% of nitrogen (N), and the balance of iron and unavoidable impurities, Hot rolling at a finishing rolling temperature condition; (b) subjecting the hot-rolled plate to primary cooling to (A1 transformation point + A2 transformation point) / 2 +/- 10 deg. C, and then maintaining it for 5 to 7 seconds; And (c) secondarily cooling the held or air-cooled plate to a temperature not higher than the A1 transformation point, and then winding.

At this time, the primary cooling may be performed at an average cooling rate of 50 to 100 ° C / sec, and the secondary cooling may be performed at an average cooling rate of 50 to 100 ° C / sec.

It is more preferable that the winding is performed at 530 to 570 캜.

In order to achieve the above object, the hot-rolled coil according to an embodiment of the present invention includes 0.02 to 0.08% of carbon (C), 0.1 to 0.5% of silicon (Si), 1.3 to 1.7% of manganese (Mn) (P): more than 0% to 0.03%, sulfur (S): more than 0% to 0.005%, aluminum (Al): 0.01 to 0.03%, niobium (Nb): 0.03 to 0.09% : 0.02 to 0.08%, molybdenum (Mo): 0.1 to 0.3%, nitrogen (N): more than 0% to 0.005%, and the balance of iron (Fe) and unavoidable impurities, The tensile strength of the outer portion exposed to the outside is 790 to 890 MPa and the difference in tensile strength between the inner surface of the plate and the inner surface of the plate is not more than 10 MPa.

At this time, the outer portion of the hot-rolled coil may have a yield strength of 730 to 730 MPa, an elongation of 17 to 25%, and a hole expansion ratio of 60% or more.

According to the method of manufacturing a hot-rolled coil according to the present invention, after cooling to a temperature at which as much precipitates as possible are produced, that is, a middle temperature between the transformation start temperature of austenite to ferrite and the transformation end temperature, To keep it up for a certain period of time or by cooling after air cooling so that the precipitate no longer grows after winding.

As a result, during the production of the hot-rolled coil according to the present invention, precipitates are generated as much as possible during the holding or air-cooling time after the first cooling, and the strength can be improved. Since the coils are wound after sufficient cooling, , The size and amount of the total precipitate of the coil become the same, and the material can be stabilized as a whole.

1 is a flowchart showing a method for manufacturing a hot-rolled coil according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those 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.

Hereinafter, a hot-rolled coil according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.

Hot-rolled coils

The hot-rolled coil according to the present invention is formed by winding a hot-rolled steel sheet. The hot-rolled coil according to the present invention contains 0.02 to 0.08% of carbon (C), 0.1 to 0.5% of silicon (Si), 1.3 to 1.7% of manganese (Mn) (Al), 0.03 to 0.09% of niobium (Nb), 0.02 to 0.08% of titanium (Ti), 0.02 to 0.08% of molybdenum (Mo) ): 0.1 to 0.3%, and nitrogen (N): more than 0 to 0.005%.

In addition to the above components, the remainder is composed of iron (Fe) and impurities inevitably included in the steelmaking process.

Hereinafter, the role and content of each component included in the hot-rolled coil or hot-rolled steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) is an element contributing to the increase in strength of steel.

The carbon is preferably added in an amount of 0.02 to 0.08% by weight based on the total weight of the coil. If the addition amount of carbon is less than 0.02% by weight, securing strength may be insufficient. On the other hand, when the addition amount of carbon exceeds 0.08% by weight, the weldability may deteriorate, and it is difficult to exhibit a hole expansion ratio of 60% or more.

Silicon (Si)

Silicon (Si) contributes to securing strength and also acts as a deoxidizer to remove oxygen in the steel.

The silicon is preferably added in an amount of 0.1 to 0.5% by weight based on the total weight of the coil. When the addition amount of silicon is less than 0.1% by weight, the deoxidation effect and the strength improving effect according to the addition of silicon can not be properly exhibited. On the other hand, when the addition amount of silicon exceeds 0.5% by weight, the weldability is lowered and a red scale can be generated during hot rolling.

Manganese (Mn)

Manganese (Mn) is an element that increases the strength and toughness of steel and increases the ingotability of steel. Addition of manganese causes less deterioration of ductility when strength is increased than that of carbon.

The manganese is preferably added in an amount of 1.3 to 1.7% by weight based on the total weight of the coil. When the addition amount of manganese is less than 1.3% by weight, the effect of addition is insufficient. On the other hand, when the addition amount of manganese exceeds 1.7% by weight, MnS-based nonmetallic inclusions are excessively generated and the weldability may be lowered.

In (P)

The phosphorus (P) contributes to the strength improvement, but if it is included excessively, the center segregation as well as the micro segregation is formed, which adversely affects the steel material.

Therefore, in the present invention, the content of phosphorus is limited to more than 0 wt% and not more than 0.03 wt% of the total weight of the coils.

Sulfur (S)

Sulfur forms emulsion based inclusions (MnS), which deteriorates impact toughness at low temperature and deteriorates weldability.

Therefore, in the present invention, the content of sulfur is limited to over 0 wt% to 0.005 wt% of the total weight of the coil.

Aluminum (Al)

Aluminum (Al) reacts with nitrogen (N) to form fine AlN precipitates, thereby contributing to improvement of strength by precipitation strengthening as well as grain refinement.

The aluminum is preferably added in an amount of 0.01 to 0.03% by weight based on the total weight of the coil. If the content of aluminum is less than 0.01% by weight, the amount of AlN precipitates may be reduced and it may be difficult to secure sufficient strength. On the contrary, when the content of aluminum exceeds 0.03% by weight, toughness is deteriorated and the yield strength is excessively increased.

Niobium (Nb)

Niobium (Nb) contributes to precipitation hardening of the steel produced by forming niobium precipitates in the steel.

The niobium is preferably added in an amount of 0.03 to 0.09% by weight based on the total weight of the coil. When the addition amount of niobium is less than 0.03% by weight, the effect of addition is insufficient. On the contrary, when the addition amount of niobium exceeds 0.09% by weight, the workability is lowered.

Titanium (Ti)

Titanium (Ti) contributes to grain refinement and strength enhancement.

The titanium is preferably added in an amount of 0.02 to 0.08% by weight based on the total weight of the coil. When the addition amount of titanium is less than 0.02% by weight, the effect of addition is insufficient. On the contrary, when the amount of titanium added exceeds 0.08% by weight, surface defects of the steel sheet to be produced may be caused.

Molybdenum (Mo)

Molybdenum (Mo) contributes to strength improvement through hardenability and also contributes to improvement of toughness.

The molybdenum is preferably added in an amount of 0.1 to 0.3 wt% based on the total weight of the coil. When the addition amount of molybdenum is less than 0.1% by weight, the effect of adding molybdenum is insufficient. On the other hand, when the content of molybdenum exceeds 0.3% by weight, the weldability is lowered and the yield ratio is increased.

Nitrogen (N)

Nitrogen (N), if added in excess, inhibits weldability, plating ability, and the like.

Therefore, in the present invention, the content of nitrogen is limited to over 0 wt% to 0.005 wt% of the total weight of the coil.

The hot-rolled coil according to the present invention can exhibit a tensile strength of 790 to 890 MPa through the above composition and the steps described later. In particular, the hot-rolled coil according to the present invention is characterized in that the difference in tensile strength between the outer and inner parts is within 10 MPa. This is because in the hot rolled coil according to the present invention, a sufficient amount of precipitate is formed at a midpoint of the temperature range of the cooled ferrite after the hot rolling and the cooling is maintained for a certain period of time or air cooling, and thereafter the rolled coil is cooled sufficiently below the phase transformation point. And further precipitates do not grow.

In addition, the hot-rolled coil according to the present invention, more specifically, the outer periphery of the hot-rolled coil can exhibit a yield strength of 730 to 830 MPa, an elongation of 17 to 25%, and a hole expansion ratio of 60% or more.

Hot-rolled coil manufacturing method

1 is a flowchart showing a method for manufacturing a hot-rolled coil according to the present invention.

Referring to FIG. 1, a hot-rolled coil manufacturing method according to the present invention includes a hot rolling step (S110), a primary cooling / holding step (S120), and a secondary cooling / winding step (S130).

Hot rolling

In the hot rolling step (S110), the slab plate having the above composition is hot-rolled. At this time, the slab plate may be reheated at about 1150 to 1250 ° C for 1 to 3 hours.

The hot rolling is preferably carried out at a finishing rolling temperature of 810 to 870 캜. In the present invention, the temperature at which the phase transformation from the austenite to the ferrite starts and the temperature below the static and dynamic recrystallization temperature is defined as the finishing rolling temperature. When the finish rolling temperature exceeds 870 캜, the austenite grains are coarsened, and after the transformation, the ferrite grain refinement can not be sufficiently performed and the strength can hardly be secured. On the other hand, if the finish rolling temperature is lower than 810 占 폚, a problem such as occurrence of blistering due to abnormal reverse rolling may occur.

Primary Cooling / Maintaining

In the primary cooling / holding step (S120), the hot-rolled plate is first cooled and then maintained or air-cooled for a certain period of time.

The end of the primary cooling and the holding temperature are preferably (A1 transformation point + A2 transformation point) / 2 10 deg. This temperature range is near the middle temperature in the ferrite temperature range and can be about 610 to 630 캜 according to the composition of the hot-rolled coil according to the present invention. As a result of the experiment, it was found that the precipitation amount of the precipitate was the most preferable in the above-mentioned temperature range, and the precipitation of the precipitate could be more stable in the above-mentioned temperature range.

The holding or air cooling time is preferably 5 to 7 seconds. When the holding or air cooling time is less than 5 seconds, the amount of precipitation of the precipitate is insufficient. Conversely, when the holding or air cooling time exceeds 7 seconds, the coil material may be deteriorated by precipitate growth without further precipitation.

The primary cooling is preferably carried out at an average cooling rate of 50-100 DEG C / sec. If the cooling rate of the primary cooling is less than 50 ° C / sec, the steel sheet structure may become coarse and it may become difficult to secure sufficient tensile strength. On the other hand, when the cooling rate of the primary cooling exceeds 100 캜 / sec, the process control may become difficult due to an excessively short cooling time.

Secondary Cooling / Winding

In the secondary cooling / winding step (S130), the plate material which has been maintained for a predetermined time after the primary cooling or the air-cooled plate material is secondarily cooled to the A1 transformation point or lower and then wound.

The reason why the termination temperature of the secondary cooling is made to be lower than the A1 transformation point is to minimize the variation of material between the inner and outer portions of the coil so that no more precipitates grow in the wound state as described above.

The secondary cooling is preferably carried out at an average cooling rate of 50-100 DEG C / sec. If the cooling rate of the secondary cooling is less than 50 DEG C / sec, it may become difficult to secure strength. On the contrary, when the cooling rate of the secondary cooling is more than 100 ° C / sec, the structure of the steel sheet may become weak and the toughness may be rapidly lowered, and the process control may become difficult due to an excessively short cooling time.

On the other hand, the winding is preferably carried out at 530 to 570 캜. If the coiling temperature is less than 530 캜, it is advantageous in securing strength but the formability of the steel sheet to be produced is deteriorated. On the other hand, when the coiling temperature exceeds 570 占 폚, the precipitate may grow after winding.

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

Hot-rolled coils according to Examples 1 to 3 and Comparative Examples 1 to 3 were produced with the composition shown in Table 1 and the process conditions shown in Table 2. The primary cooling and the secondary cooling were carried out at an average cooling rate of 50 DEG C / sec.

 [Table 1] (unit:% by weight)

 [Table 2]

2. Evaluation of mechanical properties

Table 3 shows the tensile test results of the coils produced according to Examples 1 to 3 and Comparative Examples 1 to 3, respectively.

In Table 3, tensile strength (TS), yield strength (YS) and elongation (EL) were measured by a tensile test according to JIS 5 test specimens.

The burring property was determined by forming a perforation hole having an initial diameter (d ₀ : 10 mm), then expanding it with a 60-pound cone punch to measure a hole expansion ratio ((d)) obtained from the hole diameter d at the time when the crack penetrated the plate dd ₀ ) / d ₀ X 100).

 [Table 3]

Tables 1 to 3 show that tensile strength (TS), yield strength (YS), elongation (EL), and burrability of the specimens prepared according to Examples 1 to 3 satisfy all of the target values, It can be confirmed that the difference in tensile strength between the outer diameter portion and the inner diameter portion is within 10 MPa.

On the other hand, the coil according to Comparative Example 1 in which the holding process was not applied and cooled to the winding temperature (620 ° C) immediately showed a lower strength characteristic than that in Example 1, and in particular, the difference in tensile strength between the outer diameter portion and the inner diameter portion Respectively.

Further, in the case of the coil of Comparative Example 2 in which the holding temperature was slightly out of the range of the present invention, a somewhat lower strength characteristic was exhibited as compared with Example 1. It can be seen that precipitation of precipitates is most actively occurring at (A1 + A3) / 2 ± 10 ° C even in the ferrite temperature region.

Also, the coil according to Comparative Example 3 having a holding time of only 3 seconds was also somewhat lower in strength characteristics than Example 1. This is because the retention time is insufficient and precipitation of the precipitate is insufficient.

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: Hot rolling step
S120: primary cooling / maintenance phase
S130: Second cooling / winding step

Claims (6)

(a) from 0.02 to 0.08% carbon (C), from 0.1 to 0.5% silicon (Si), from 1.3 to 1.7% manganese (Mn) (S): more than 0% to 0.005%, aluminum (Al): 0.01 to 0.03%, niobium (Nb): 0.03 to 0.09%, titanium (Ti): 0.02 to 0.08%, molybdenum Hot rolled at a finishing rolling temperature of 810 to 870 캜 at a slab plate made of 0.3%, nitrogen (N): 0% to 0.005% and balance of iron (Fe) and unavoidable impurities;
(b) subjecting the hot-rolled plate to primary cooling to (A1 transformation point + A2 transformation point) / 2 +/- 10 deg. C, and then maintaining or air cooling for 5 to 7 seconds; And
(c) secondarily cooling the held or air-cooled plate to a temperature not higher than the A1 transformation point, and winding the hot-rolled coil.
The method according to claim 1,
The primary cooling
Is carried out at an average cooling rate of 50 to 100 占 폚 / sec.
The method according to claim 1,
The secondary cooling
Is carried out at an average cooling rate of 50 to 100 占 폚 / sec.
The method according to claim 1,
The winding
Wherein the heat treatment is performed at 530 to 570 占 폚.
(P): more than 0% to 0.03%, sulfur (S): 0.1 to 0.5%, manganese (Mn): 1.3 to 1.7% ): More than 0% to 0.005%, aluminum (Al): 0.01 to 0.03%, niobium (Nb): 0.03 to 0.09%, titanium (Ti): 0.02 to 0.08%, molybdenum (Mo) (N): not less than 0% and not more than 0.005%, and a balance of iron (Fe) and unavoidable impurities,
Wherein a tensile strength of the outer surface of one side of the plate exposed to the outside is 790 to 890 MPa and a difference in tensile strength between the inner surface of the plate and the inner side of the plate is 10 MPa or less.
6. The method of claim 5,
The outer portion of the hot-
A yield strength of 730 to 830 MPa, an elongation of 17 to 25%, and a hole expansion ratio of 60% or more.

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