KR101412365B1 - High strength steel sheet and method of manufacturing the same - Google Patents

Abstract

A high strength steel sheet which can be upgraded to a strength of 1.8 to 2.0 GPa through heat treatment of a hot or cold rolled steel sheet in the form of a pipe followed by QT (Quenching & Tempering) heat treatment and a manufacturing method thereof.
The method of manufacturing a high strength steel sheet according to the present invention is characterized in that it comprises 0.30 to 0.38 wt% of carbon (C), more than 0 wt% to 0.5 wt% of silicon (Si), 1.0 to 1.5 wt% of manganese (Mn) : More than 0 wt% to 0.02 wt% or less, S (S): 0 wt% to 0.01 wt%, aluminum (Al): 0.01 to 0.05 wt%, titanium (Ti) A slab plate made of 0.1 to 1.0% by weight of Cr, 0.1 to 1.0% by weight of nitrogen (N): 0 to 0.01% by weight and boron (B) of 0.0005 to 0.0050% by weight and the balance of Fe and unavoidable impurities, A plate material manufacturing step for manufacturing a cold-rolled plate material; A gauging step of gauging the hot-rolled sheet material or the cold-rolled sheet material in the form of a pipe; And a Quenching & Tempering (QT) heat treatment step of annealing the quenched plate material at 800 to 1000 ° C. and quenching and then tempering at 150 to 550 ° C. for 1 to 30 minutes.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet,

The present invention relates to a steel sheet and a method of manufacturing the steel sheet, and more particularly, to a method of manufacturing a steel sheet and a method of manufacturing the steel sheet, And a method of manufacturing the same.

Generally, hot-rolled steel sheets are manufactured through a slab reheating process, a hot-rolling process, and a cooling / coiling process.

In the slab reheating process, the semi-finished slab plate is reheated.

In the hot rolling process, the slab plate is rolled to a final thickness at a high temperature using a rolling roll.

In the cooling / winding process, the rolled sheet is cooled to the coiling temperature (CT) and wound.

As a related prior art, there is Korean Patent Laid-Open Publication No. 2001-0062875 (published on July, 2001), which discloses a process for producing an oil for an oil having excellent hydrogen-organic cracking resistance.

It is an object of the present invention to provide a method for manufacturing a high strength steel sheet which can be upgraded to a strength of 1.8 to 2.0 GPa through a quenching & tempering (QT) heat treatment after hot rolling or cold rolling .

Another object of the present invention is to provide a method for producing a steel sheet having a tensile strength (TS) of 1,800 to 2,000 MPa, a yield point (YP) of 1,200 to 1,600 MPa, an elongation (El) of 7 to 10% The surface hardness of Hv can be satisfied.

In order to achieve the above object, a method of manufacturing a high strength steel sheet according to an embodiment of the present invention includes: 0.30 to 0.38 weight% of carbon; 0 to 0.5 weight% of silicon; 1.0 weight% of manganese (Mn) (Al): 0.01 to 0.05 wt.%, Ti (Ti): 0.1 wt.% Or more, (B): 0.0005 to 0.0050% by weight, and the balance of Fe and unavoidable impurities, in terms of Fe: 0.01 to 0.05 wt%, Cr (Cr): 0.1 to 1.0 wt% A step of producing a hot-rolled sheet or a cold-rolled sheet using the slab sheet; A gauging step of gauging the hot-rolled sheet material or the cold-rolled sheet material in the form of a pipe; And a Quenching & Tempering (QT) heat treatment step of annealing the quenched plate material at 800 to 1000 ° C. and quenching and then tempering at 150 to 550 ° C. for 1 to 30 minutes.

According to another aspect of the present invention, there is provided a high strength steel sheet comprising: 0.30 to 0.38 wt% of carbon; 0 to 0.5 wt% of silicon; (S): more than 0 wt% to 0.01 wt%, aluminum (Al): 0.01 to 0.05 wt%, titanium (Ti): 0.1 wt% (B): 0.0005 to 0.0050% by weight, and the balance of Fe and unavoidable impurities, in the range of 0.01 to 0.05 wt%, Cr (Cr): 0.1 to 1.0 wt% And has a tensile strength (TS) of 1,800 to 2,000 MPa, a yield point (YP) of 1,200 to 1,600 MPa and an elongation (El) of 7 to 10% by quenching and tempering (QT) heat treatment.

The high-strength steel sheet according to the present invention can satisfy the elongation of 7 to 10% while being upgraded to the strength of 1.8 to 2.0 GPa through the quenching & tempering treatment by subjecting the hot-rolled or cold rolled steel sheet to pipe- have.

Accordingly, when the high-strength steel sheet produced by the method according to the present invention is used as a steel sheet for automobiles, since it has a strength of 1.8 to 2.0 GPa, it can actively cope with weight reduction through thickness reduction, . Further, since the steel sheet according to the present invention has a high strength and an appropriate elongation, the safety of a passenger can be ensured when the steel sheet is used as a collision structural member.

FIG. 1 is a process flow chart showing a method of manufacturing a high-strength steel sheet according to an embodiment of the present invention.
Fig. 2 is a photograph showing the final microstructure of the specimen prepared according to Example 1. Fig.
3 is a photograph showing the final microstructure of the specimen prepared according to Comparative Example 1. Fig.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with 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 high strength steel sheet 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.

High strength steel plate

The high strength steel sheet according to the present invention has a tensile strength (TS) of 1,800 to 2,000 MPa, a yield point (YP) of 1,200 to 1,600 MPa and an elongation (El) of 7 to 10% by QT (Quenching & .

For this, the high strength steel sheet according to the present invention comprises 0.30 to 0.38 wt% of carbon (C), more than 0 wt% to 0.5 wt% of silicon (Si), 1.0 to 1.5 wt% of manganese (Mn) (Al): 0.01 to 0.05% by weight, titanium (Ti): 0.01 to 0.05% by weight, chromium (Cr) (Cr): 0.1 to 1.0 wt%, nitrogen (N): more than 0 wt% to 0.01 wt%, boron (B): 0.0005 to 0.0050 wt%, and the balance Fe and unavoidable impurities.

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

Carbon (C)

Carbon (C) is added in order to secure strength, and it is preferable to add carbon (C) or less in order to secure strength after quenching and tempering (QT) heat treatment.

The carbon (C) is preferably added in a content ratio of 0.30 to 0.38% by weight based on the total weight of the steel sheet according to the present invention. When the content of carbon is less than 0.30 wt%, it may be difficult to secure sufficient strength before or after the heat treatment. On the other hand, when the content of carbon is more than 0.38% by weight, it is difficult to secure the aimed elongation due to the reduction in toughness, and the moldability is deteriorated.

Silicon (Si)

In the case of silicon (Si), when added in an amount exceeding 0.5% by weight, the weldability of steel is deteriorated and a red scale is generated during reheating and hot rolling, which may cause problems in surface quality. Can be inhibited. This is because the amount of silicon and manganese contained in the ERW is within a certain range of 1: 4 ~ 1: 10, so that cracking of welds is significantly reduced.

Therefore, silicon is preferably added in a content ratio of more than 0 wt% to 0.5 wt% or less of the total weight of the steel sheet according to the present invention.

Manganese (Mn)

Manganese (Mn) inhibits the formation of pearlite phase and promotes austenite formation and carbon enrichment in the interior to contribute to the formation of retained austenite. It is an element effective for enhancing the hardenability of the steel sheet and securing strength after quenching.

The content of manganese is preferably 1.0 to 1.5% by weight based on the total weight of the steel sheet according to the present invention. If the content of manganese is less than 1.0 wt%, it may be difficult to secure a sufficient strength even if the content of carbon is high. On the contrary, when the content of manganese exceeds 1.5% by weight, the amount of nonmetal inclusions increases, and the surface characteristics are lowered, and defects such as cracks may occur. Further, segregation phenomenon such as center segregation and micro-segregation becomes serious, and the formability may be deteriorated.

In the high-strength steel sheet according to the present invention, it is preferable to add silicon and manganese in a ratio of 1: 4 to 1:10. This is because the addition ratio of silicon and manganese is 1: 4 to 1:10 The occurrence of cracks in welds is significantly reduced. When the content ratio of silicon and manganese is less than 1: 4 or more than 1: 10, excess oxide of MnO and SiO _{2 is formed} at high temperature, thereby causing hook crack in electric resistance welding, .

In (P)

Phosphorous (P) is added to inhibit cementite formation and increase strength.

However, phosphorus deteriorates the weldability and causes the final material deviation by slab center segregation. Therefore, in the present invention, the content of phosphorus (P) is limited to not less than 0 wt% and not more than 0.02 wt% of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) inhibits the toughness and weldability of steel, and forms an MnS non-metallic inclusion by binding with manganese, thereby generating cracks during steel processing.

Therefore, in the present invention, the content of sulfur (S) is limited to more than 0 wt% to 0.01 wt% or less of the total weight of the steel sheet.

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 0.05% by weight based on the total weight of the steel sheet according to the present invention. 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 0.05% by weight, difficulties are encountered in the performance process, resulting in deterioration of productivity and excessively increasing the yield strength.

Titanium (Ti)

Titanium (Ti) is a strong carbonitride-forming element that precipitates solid carbon and solid nitrogen to improve non-vitrification and processability. In particular, titanium (Ti) prevents Boron (B) from being precipitated as a nitride precipitate, and boron is present in a solid state in the steel so that boron serves to improve the hardenability of the steel.

The titanium is preferably added in an amount of 0.01 to 0.05% by weight based on the total weight of the steel sheet according to the present invention. When the content of titanium is less than 0.01% by weight, there arises a problem that aging hardening occurs due to the remaining solid carbon and nitrogen employed without precipitation. On the contrary, when the content of titanium exceeds 0.05% by weight, TiC precipitates and thus the toughness of the weld heat affected zone (HAZ) is lowered.

Chromium (Cr)

Chromium (Cr) is a ferrite stabilizing element and contributes to strength improvement. In addition, chromium inhibits decarburization and microstructural coarsening by reducing carbon activity and boundary mobility to prevent conversion to spheroidization.

The chromium is preferably added in an amount of 0.1 to 1.0% by weight based on the total weight of the steel sheet according to the present invention. If the content of chromium is less than 0.1% by weight, the above effect can not be exhibited properly. On the contrary, when the content of chromium exceeds 1.0% by weight, not only the toughness of the weld heat affected zone (HAZ) deteriorates but also the workability is deteriorated.

Nitrogen (N)

Nitrogen (N) is an element to be controlled inevitably added to the blast furnace or electric furnace process. Further, when nitrogen is excessively contained in the steel, it is difficult to secure the hygroscopicity at room temperature, and BN (boron nitrogen compound) is formed, which causes degradation of heat treatment curability. Therefore, in the present invention, the content of nitrogen is limited to not less than 0 wt% and not more than 0.010 wt% of the total weight of the steel sheet.

Boron (B)

Boron (B) serves to improve the hardenability of the hot-rolled steel sheet by delaying the transformation of austenite into ferrite during the continuous cooling transformation. In addition, it is an element that further enhances the effect of ensuring stable strength after quenching.

The boron is preferably added in an amount of 0.0005 to 0.0050% by weight based on the total weight of the steel sheet according to the present invention. When the content of boron is less than 0.0005 wt%, the addition amount is insignificant, and the above effect can not be exhibited properly. On the other hand, if the boron content exceeds 0.0050 wt% and is added in excess, the formation of boron oxide may cause a problem of deteriorating the surface quality of the steel sheet.

High strength steel plate manufacturing method

FIG. 1 is a process flow chart showing a method of manufacturing a high-strength steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, a method of manufacturing a high strength steel sheet according to an embodiment of the present invention includes a plate material manufacturing step (S110), a tube making step (S120), and a QT heat treatment step (S130).

Sheet material manufacturing

In the sheet material manufacturing step (S110), 0.30 to 0.38 wt% of carbon (C), more than 0 wt% to 0.5 wt% of silicon (Si), 1.0 to 1.5 wt% of manganese (Mn) 0.01 to 0.05% by weight of aluminum (Al), 0.01 to 0.05% by weight of titanium (Ti), 0.01 to 0.05% by weight of chromium (Cr) : 0.1 to 1.0 wt%, nitrogen (N): more than 0 wt% to 0.01 wt%, boron (B): 0.0005 to 0.0050 wt%, and the balance of Fe and unavoidable impurities is used as the hot- .

At this time, the hot-rolled sheet material is subjected to a process of reheating the slab sheet having the above composition at a slab reheating temperature (SRT) of 1150 to 1250 ° C and a finishing hot-rolling process of the reheated sheet material at a finishing delivery temperature (FDT) And a step of cooling the hot rolled plate to a CT (Coiling Temperature) of 500 to 700 ° C and winding. Here, the slab reheating process is not necessarily performed, but it is more preferable to carry out the process to derive effects such as reuse of precipitates.

In the high-strength steel sheet according to the present invention, it is preferable to add silicon and manganese in a ratio of 1: 4 to 1:10, because the addition ratio of silicon and manganese is 1: 4 to 1:10 This is because the generation of weld cracks is significantly reduced. When the content ratio of silicon and manganese is less than 1: 4 or more than 1: 10, excess oxide of MnO and SiO _{2 is formed} at high temperature, thereby causing hook crack in electric resistance welding, .

On the other hand, when the slab reheating temperature (SRT) is less than 1150 ° C, there is a problem that the segregated components are not sufficiently reused in casting. On the contrary, when the SRT exceeds 1250 ° C, the austenite grain size increases and the ferrite of the final microstructure is coarsened, so that it is difficult to secure the strength, and the manufacturing cost of the steel sheet is raised only by the excessive heating process can do.

When the finishing rolling temperature (FDT) is lower than 800 ° C, an abnormal reverse rolling occurs and an uneven structure is formed, which can considerably lower the impact resistance at low temperature. On the other hand, when the finish rolling temperature (FDT) exceeds 950 DEG C, the ductility and toughness are excellent, but the strength is rapidly lowered.

When the coiling temperature (CT) is less than 500 ° C, the strength in the hot rolled state is increased and the workability is lowered, which makes it difficult to secure toughness. On the other hand, when the coiling temperature (CT) exceeds 700 캜, hot oxide is generated on the surface of the hot-rolled coil, which may be difficult.

On the other hand, the cold-rolled sheet may be manufactured by subjecting the hot-rolled sheet manufactured by the above process to a pickling process, a cold rolling process, a annealing heat treatment process, or the like.

Procurement

In the pipe making step S120, the hot rolled plate or the cold rolled plate is formed to have a predetermined shape. By carrying out such tube making, the hot-rolled sheet or the inner-sheet can be produced in the form of a pipe.

QT heat treatment

In the QT heat treatment step (S130), a plate material having a desired shape is annealed at 800 to 1000 占 폚 to form austenite, followed by quenching to transform it into martensite. Then, for the purpose of improving the toughness, a tempering process is performed at 150 to 550 ° C for 1 to 30 minutes.

If the annealing temperature is less than 800 占 폚 at this stage, there is a problem that the austenite is insufficiently formed and is not sufficiently transformed into martensite so that the strength is lowered. On the contrary, when the annealing temperature exceeds 1000 캜, the desired strength can not be secured due to the increase of the production cost and the coarsening of the austenite.

In particular, quenching is preferably carried out at an average cooling rate of 30 to 400 DEG C / sec. If the quenching rate is less than 30 DEG C / sec, it may be difficult to secure sufficient strength. On the other hand, when the quenching rate is higher than 400 ° C / sec, it is advantageous in securing the strength but there is no way to realize the quenching rate and the problem that the toughness drops sharply may follow.

On the other hand, when the tempering temperature is less than 150 ° C, the effect is insignificant. On the contrary, when the tempering temperature exceeds 550 ° C, excessive tempering is performed to deteriorate the mechanical properties of the heat treated steel sheet.

The steel sheet manufactured in the above steps S110 to S130 is subjected to QT (quenching & tempering) heat treatment after the steel making to have a tensile strength TS of 1,800 to 2,000 MPa, a yield point YP of 1,200 to 1,600 MPa, (El): 7 to 10%.

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

Specimens according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared with the compositions shown in Tables 1 and 2 and the process conditions shown in Table 3. At this time, in the case of the specimens produced according to Examples 1 to 3 and Comparative Examples 1 and 2, ingots having respective compositions were prepared and subjected to hot rolling such as heating, hot rolling, cooling (water cooling) And charged them into the winding path. Thereafter, the specimens prepared according to Examples 1 to 3 charged into the winding path were subjected to annealing, quenching, and tempering. On the other hand, only specimens prepared according to Comparative Examples 1 and 2 were subjected to quenching after annealing. That is, the specimens prepared according to Comparative Examples 1 and 2 were not tempered.

[Table 1] (unit:% by weight)

[Table 2] (unit:% by weight)

[Table 3]

2. Evaluation of mechanical properties

Table 4 shows the evaluation results of the mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 and 2.

[Table 4]

Tensile Strength (TS): 1,800 to 2,000 MPa, Yield Point (YP): 1,200 to 1,600 MPa, Elongation Rate (YP) corresponding to the target value for the specimens prepared according to Examples 1 to 3, EL) of 7 to 10% and a surface hardness of 550 to 600 Hv.

On the other hand, compared with Example 1, most of the alloy components are added in a similar amount, but niobium (Nb) is further added and titanium (Ti), chromium (Cr) and boron (B) The tensile strength TS, the yield point YP and the surface hardness satisfied the target value, but it can be seen that the elongation percentage EL did not meet the target value.

Compared with Example 1, most of the alloy components were added in similar amounts, but niobium (Nb) was added, chromium (Cr) and boron (B) were not added, and Comparative Example 2 The tensile strength TS, the yield point YP and the surface hardness satisfied the target values, but it can be seen that the elongation percentage EL is less than the target value.

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

As shown in Fig. 2, in the case of the specimen produced according to Example 1, it can be confirmed that the final microstructure has a composite structure including ferrite and martensite.

On the other hand, as shown in FIG. 3, in the case of the specimen produced according to Comparative Example 1, it can be confirmed that the final microstructure has a composite structure including ferrite, pearlite and bainite.

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: plate material manufacturing step
S120:
S130: QT heat treatment step

Claims (7)

(C): 0.30 to 0.38 wt%, silicon (Si): 0 to 0.5 wt%, manganese (Mn): 1.0 to 1.5 wt%, phosphorus (P) (Al): 0.01 to 0.05 wt%, titanium (Ti): 0.01 to 0.05 wt%, chromium (Cr): 0.1 to 1.0 wt% A step of producing a hot-rolled plate or a cold-rolled plate using a slab plate made of nitrogen (N): more than 0 wt% to 0.01 wt%, boron (B): 0.0005 to 0.0050 wt%, and balance of Fe and unavoidable impurities;
A gauging step of gauging the hot-rolled sheet material or the cold-rolled sheet material in the form of a pipe; And
And annealing the quenched plate material at 800 to 1000 ° C. and quenching the quenched and tempered plate material, followed by tempering at 150 to 550 ° C. for 1 to 30 minutes.
The method according to claim 1,
The slab plate
Wherein the silicon (Si) and manganese (Mn) are added in a ratio of 1: 4 to 1:10.
The method according to claim 1,
The hot-rolled sheet manufacturing step
Reheating the slab plate at a slab reheating temperature (SRT) of 1150 to 1250 ° C,
Subjecting the reheated plate to finishing hot rolling at a finishing delivery temperature (FDT) of 800 to 950 占 폚,
And cooling the hot-rolled plate material to a CT (Coiling Temperature): 500 to 700 占 폚 and winding the hot-rolled plate material.
The method according to claim 1,
In the QT heat treatment step,
Wherein the quenching is performed at a rate of 30 to 400 DEG C / sec.
(C): 0.30 to 0.38 wt%, silicon (Si): 0 to 0.5 wt%, manganese (Mn): 1.0 to 1.5 wt%, phosphorus (P) (Al): 0.01 to 0.05 wt%, titanium (Ti): 0.01 to 0.05 wt%, chromium (Cr): 0.1 to 1.0 wt% (N): more than 0 wt% to 0.01 wt%, boron (B): 0.0005 to 0.0050 wt%, and the balance Fe and unavoidable impurities,
(YT): 1,200 to 1,600 MPa and elongation (El): 7 to 10% by quenching and tempering (QT) heat treatment.
6. The method of claim 5,
The steel sheet
Wherein said silicon (Si) and manganese (Mn) are added in a ratio of 1: 4 to 1:10.
6. The method of claim 5,
The steel sheet
And a surface hardness of 550 to 600 Hv.

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