KR20230095153A - Hot rolled steel with excellent cold bendability, steel tube, steel member after heat treatment, and method for manufacturing thereof - Google Patents

Abstract

Provided are a hot-rolled steel sheet, a steel pipe, and a steel member with excellent cold bendability after heating and quenching-tempering treatment, and a manufacturing method thereof. The hot-rolled steel sheet with excellent cold bendability after heating and quenching-tempering treatment, which has high strength and a maximum bending angle of 40° or more after heating and quenching-tempering treatment comprises, wt%, 0.20% or more and less than 0.3% of C, 0.5 to 1.3% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less of Cr, 0.1 to 0.4% of Ni, 0.05% (including 0%) of Ti, 0.0005 to 0.0050% of B, 0.01% or less (excluding 0%) of N, and the remainder of Fe and other impurities, and has a microstructure which satisfies the following relational expressions 1 to 3, has a hardness difference value of less than 15 between the surface layer of the steel sheet and a 1/4 position of the thickness, and includes, by vol%, 20 to 65% of ferrite and 35 to 80% of pearlite, wherein old austenite has an average grain size of 15 micrometres or more.

Description

Hot-rolled steel sheet, steel pipe, member with excellent cold bendability after heating and quenching-tempering heat treatment, and manufacturing method thereof

The present invention relates to a hot-rolled steel sheet used for automobile body components such as automobile suspension parts, steel pipes and members using the same, and a manufacturing method thereof, and more particularly, to a difference in hardness between the surface layer portion and the thickness/4 position portion of the steel sheet. It relates to a hot-rolled steel sheet having an excellent (ΔHv) value of less than 15 and exhibiting a high strength of 1300 MPa or more and a maximum bending angle of 40 degrees or more after heating and quenching-tempering heat treatment, a steel pipe using the same, a member, and a manufacturing method thereof.

Among automotive chassis parts, suspension parts are one of the parts that require fatigue durability, and high-strength thin hot-rolled steel sheets are preferred according to the trend of weight reduction.

On the other hand, it is common to manufacture such suspension parts through hot forming or cold forming and heat treatment of pipe-shaped materials, but after manufacturing ultra-high strength steel pipes subjected to heat treatment unlike conventional ones for eco-friendly manufacturing and cost reduction, cold forming New manufacturing methods for manufacturing parts are also proposed. This is to manufacture a steel pipe having a high strength of 1000MPa or more into a part of a desired shape by bending external force in a cold (≤25℃) or warm (≤550℃) state. It is required to exhibit high resistance to or high ductility.

On the other hand, as a method for improving the bending formability of the steel sheet itself, regardless of whether or not heat treatment is applied, the ferrite and pearlite microstructures of the final cold-rolled steel sheet are uniformly controlled, or the size of prior austenite and martensite in the martensite phase of 95% or more Studies on controlling the size distribution of carbides remaining in the interior have been conducted.

Patent Document 1 proposes a method of improving the bendability of a steel sheet for hot forming in order to solve the problem of declining bendability due to high strength of a hot press-formed product. This is because the microstructure of the cold-rolled steel sheet, in particular, the pearlite phase, is improved by controlling the Mn/Si ratio in the range of 0.05 to 2.0 and adding 0.5% or more of silicon (Si) element to the steel used for cold-rolled steel sheet and performing cold-rolled annealing heat treatment. It can be uniformly distributed, and it is proposed that the bendability is improved because the retained austenite phase can be formed in the martensitic structure when the cold-rolled steel sheet is subjected to painting heat treatment after hot press forming. In other words, this document suggests the addition of silicon (Si) with a content of 0.5% or more to steel used as cold-rolled steel sheets. In order to directly apply this to steel pipe-type chassis parts, butt welding is required during the electric arc welding process. Alternatively, a large amount of silicon oxide formed in the melting section must be effectively discharged, and if the oxide cannot be effectively discharged, welding defects may occur, which may deteriorate formability such as flattening or expansion characteristics of the steel pipe. Because of this, there were restrictions on its use.

In Patent Document 2, high-strength cold-rolled steel sheet of 1470 MPa or more by controlling the number of fine carbides with a size of 25 to 60 nm to 0 to 500,000 per 1 mm ² area in the martensite single-phase structure composed of fine-sized old austenite (PAGS) of 5 to 6 μm. A method for improving the bendability (limit bending radius (R)/thickness (t) ≤ 2.4) was proposed. On the other hand, the improvement in the bendability of the high-strength cold-rolled steel sheet or the very low R / t measured value seems to be mainly due to the fine old austenite size, which is caused by the bending external force at the old austenite grain boundary or the carbide / martensite interface It can be understood that the formation of crack sites or crack propagation is delayed. In particular, the tensile strength of cold-rolled steel sheet is as high as 1470 MPa or more due to the very fine martensitic structure, but it is expected to exhibit a low elongation of less than 6%, so it is considered that the formability is insufficient to manufacture parts with complex shapes. do. On the other hand, even in the case of the high-strength steel, a large amount of silicon (Si) content of 1.0% or more is added to control the size of carbides that precipitate and grow while the martensitic structure formed by rapid cooling of 100 ° C / sec or more undergoes tempering heat treatment. Difficulty in application to steel pipes or steel pipe parts can be expected.

Therefore, from the review of the steel plate and steel parts manufacturing process proposed in the above patent documents, the electric resistance welded steel pipe or drawn steel pipe previously subjected to heating and quenching-tempering heat treatment is subjected to bending external force in a cold (≤25 ° C) state. As a steel plate that can be manufactured into a stabilizer part of a desired shape, the hardness difference (ΔHv) value between the surface layer part and the thickness/4 position part of the steel plate is less than 15, and after heat treatment, high strength of 1300 MPa or more (elongation of 8% or more) and 40 ° There are no proposals for hot-rolled steel sheets, steel pipes, members, and manufacturing methods that exhibit the above maximum bending angle.

Republic of Korea Patent No. 10-1568549 Republic of Korea Patent Publication No. 10-2015-0105476

A preferred aspect of the present invention is that the difference in hardness between the surface layer of the steel sheet and the thickness/4 position is small (decarburization resistance is large), and after heating and quenching-tempering heat treatment, high strength and excellent bending with a maximum bending angle of 40° or more are obtained. It is intended to provide a hot-rolled steel sheet for cold-formed members and a manufacturing method thereof.

In addition, a preferred aspect of the present invention is that the difference in hardness between the surface layer of the steel sheet and the thickness / 4 position is small (decarburization resistance is large), and after heating and quenching-tempering heat treatment, it has high strength and a maximum bending angle of 40 ° or more. Steel pipe for cold-formed member manufactured using hot-rolled steel sheet showing bendability and its manufacturing method that you want to provide.

Furthermore, a preferred aspect of the present invention is that the hardness difference between the surface layer of the steel sheet and the thickness/4 position is small (decarburization resistance is high), and after heating and quenching-tempering heat treatment, having a high strength and a maximum bending angle of 40 degrees or more. It is intended to provide a cold-formed member manufactured using a steel pipe exhibiting excellent bendability and a manufacturing method thereof.

In addition, the technical problems to be achieved in the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned are clearly understood by those skilled in the art from the description below. It could be.

One aspect of the present invention, in weight%, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S : 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050% , N: 0.01% or less (excluding 0%), including the remaining Fe and other impurities, satisfying the following relational expression 1-3, and having a hardness difference value of less than 15 between the surface layer portion and the thickness / 4 position portion of the steel sheet After heating and quenching-tempering heat treatment, having a microstructure containing 20 to 65% of ferrite and 35 to 80% of pearlite in volume%, and having an average grain size of prior austenite of 15 μm or more It relates to a hot-rolled steel sheet for cold-formed members having high strength and excellent bendability having a maximum bending angle of 40° or more.

[Relationship 1]

(Mn/Si) ≥ 2 (weight ratio)

[Relationship 2]

(Ni)/(Mn) ≥ 0.05 (weight ratio)

[Relationship 3]

(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)

In addition, another aspect of the present invention,

Heating the steel slab having the above composition to a temperature range of 1150 to 1300 ° C;

Obtaining a hot-rolled steel sheet by hot-rolling the heated slab including rough rolling and finish rolling at an Ar3 temperature or higher; and

Cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750 ° C.; having a hardness difference value of less than 15 in the surface layer portion and thickness / 4 position portion of the steel sheet, including heating and quenching-tempering Provided is a method for manufacturing a hot-rolled steel sheet for cold-formed members having excellent bendability having high strength and a maximum bending angle of 40° or more after heat treatment.

A step of pickling the hot-rolled steel sheet to obtain a hot-rolled pickled steel sheet may be further included.

In addition, another aspect of the present invention, in weight%, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%) ), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 ~0.0050%, including N: 0.01% or less (excluding 0%), including remaining Fe and other impurities, satisfying the following relational expression 1-3, and having a hardness difference of less than 15 between the surface layer of the steel pipe and the thickness/4 position Heating and quenching-tempering heat treatment having a microstructure containing 20 to 65% of ferrite and 35 to 80% of pearlite in volume%, and having an average grain size of prior austenite of 15 μm or more It relates to a steel pipe for a cold-formed member having excellent bendability having a high strength and a maximum bending angle of 40 ° or more.

[Relationship 1]

(Mn/Si) ≥ 2 (weight ratio)

[Relationship 2]

(Ni)/(Mn) ≥ 0.05 (weight ratio)

[Relationship 3]

(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)

In addition, another aspect of the present invention,

Heating the steel slab having the above composition to a temperature range of 1150 to 1300 ° C;

Obtaining a hot-rolled steel sheet by hot-rolling the heated slab including rough rolling and finish rolling at an Ar3 temperature or higher;

Cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750° C.;

Obtaining a steel pipe by welding the hot-rolled steel sheet; and

It relates to a method for manufacturing a steel pipe for a cold-formed member comprising the step of annealing the steel pipe at a temperature of Ac ₁ -50 ° C to Ac ₃ +150 ° C for 3 to 60 minutes.

A step of drawing the annealed heat-treated steel pipe may be further included.

A reheating step of heating the annealed or drawn steel pipe to a temperature of Ar ₃ ~ 970 ° C at a heating rate of 10 ° C / sec or more, and then maintaining it within 60 seconds; A quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350° C./sec; And after heating the quenched steel pipe to a temperature range of 150 ~ 350 ℃ at a heating rate of 2 ~ 20 ℃ / sec, tempering heat treatment step of maintaining at this temperature; may further include.

In addition, another aspect of the present invention, in weight%, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%) ), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 ~0.0050%, including N: 0.01% or less (excluding 0%), including the remaining Fe and other impurities, satisfying the following relational expression 1-3, in volume%, at least one of martensite and tempered martensite Fe ₃ C carbide having a microstructure containing 95% of retained austenite and the balance of 5% or less, an average old austenite grain size of 15 μm or more, and an average equivalent circle size of 300 nm or less It relates to a cold-formed member having 30 or less per ² ) and excellent in high strength and bendability.

[Relationship 1]

(Mn/Si) ≥ 2 (weight ratio)

[Relationship 2]

(Ni)/(Mn) ≥ 0.05 (weight ratio)

[Relationship 3]

(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)

Furthermore, another aspect of the present invention,

Heating the steel slab having the above composition to a temperature range of 1150 to 1300 ° C;

Obtaining a hot-rolled steel sheet by hot-rolling the heated slab including rough rolling and finish rolling at an Ar3 temperature or higher;

Cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750° C.;

Obtaining a steel pipe by welding the hot-rolled steel sheet;

Annealing heat treatment and drawing the steel pipe;

A reheating step of heating the annealed or drawn steel pipe to a temperature of Ar ₃ ~ 970 ° C at a heating rate of 10 ° C / sec or more, and then maintaining it within 60 seconds;

A quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350° C./sec;

A tempering heat treatment step of heating the quenched steel pipe to a temperature range of 150 to 350 ° C at a heating rate of 2 to 20 ° C / sec and maintaining it at this temperature; and

It relates to a cold-formed member manufacturing method comprising the step of cold-forming the tempered heat-treated steel pipe into a member.

According to the present invention having the above configuration, the hardness difference (ΔHv) value at the surface layer portion and the thickness / 4 position portion of the steel sheet is excellent at less than 15, and after heating and quenching-tempering heat treatment, high strength of 1300 MPa or more and maximum of 40 ° or more It is possible to provide a hot-rolled steel sheet and a steel pipe for a forming member capable of exhibiting a bending angle, and also, since parts can be manufactured by cold forming, there is an effect of reducing part manufacturing cost.

1 is a diagram showing a change curve of strength and maximum bending angle of a steel sheet after tempering heat treatment of Example 2 in one embodiment of the present invention.
Figure 2 is a photograph showing the shape after cold bending of the quenching-tempering heat treatment steel pipe of Example 2 in one embodiment of the present invention.
Figure 3 (ab) is a photograph showing whether cracks occur in the heat-treated steel pipe of one embodiment of the present invention, (a) shows Inventive Example 2 and (b) shows Comparative Example 2.

Hereinafter, the present invention will be described.

First, it has a hardness difference (ΔHv) value of less than 15 at the surface layer portion and the thickness / 4 position portion of the steel sheet according to a preferred aspect of the present invention, and after heating and quenching-tempering heat treatment, high strength and a maximum bending angle of 40 ° or more A hot-rolled steel sheet for cold-formed members having excellent bendability will be described.

Hot-rolled steel sheet for cold-formed members of the present invention, in weight%, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (0% including), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less (excluding 0%), including the remaining Fe and other impurities, satisfying the relational expression 1-3, in volume%, 20 to 65% ferrite and 35 to 80 % of pearlite, has a hardness difference value of less than 15 at the surface layer portion and thickness/4 position portion of the steel sheet, and has an average grain size of prior austenite of 15 μm or more.

First, the alloy composition of the steel sheet and steel pipe for cold forming members of the present invention and the reason for limiting the content thereof will be described, and hereinafter, "%" means weight% unless otherwise specified.

C: 0.20% or more and less than 0.35%

Carbon (C) is an element effective in increasing the strength of steel and increases strength after quenching-tempering heat treatment. If the content is less than 0.20%, it is difficult to secure sufficient strength of 1300 MPa or more after tempering heat treatment. On the other hand, if it is more than 0.35%, martensite with excessive hardness or strength is formed, and after heat treatment, when cold bending of steel plate material or steel pipe parts, It is difficult to secure the bending angle. If, in order to increase the bending angle, the tempering heating temperature can be increased to 350 ° C. or more, but there is a limitation in securing a strength of 1300 MPa or more. Therefore, it is preferable to limit the carbon (C) content to 0.20% or more and less than 0.35%.

·Mn: 0.5 ~ 1.3%

Manganese (Mn) is an essential element for increasing the strength of steel and increases the strength after quenching heat treatment of steel. If the content is less than 0.5%, it is difficult to secure sufficient strength of 1300 MPa or more after tempering heat treatment, whereas if it exceeds 1.3%, it is advantageous to secure strength, but it is difficult to secure a bending angle of 40 ° or more for steel sheets and steel pipes after heat treatment. In this case, since a relatively large amount of nickel (Ni) content may be required to appropriately control the prior austenite size or suppress carbide growth to improve cold bendability, manufacturing cost may increase. In addition, since a segregation zone may be formed inside and/or outside the cast slab and the hot-rolled steel sheet, a high frequency of processing defects may be caused during steel pipe manufacturing. Therefore, it is preferable to limit the manganese (Mn) content to 0.5 to 1.3%.

Si: 0.3% or less (excluding 0%)

Silicon (Si) is an element added to improve strength or ductility, and is added within a range where there is no problem in surface scale of the hot-rolled steel sheet and the hot-rolled pickling steel sheet. When the content exceeds 0.3% or more, surface defects are generated due to the generation of silicon oxide, which is not easy to remove by pickling. In addition, when the oxide is not smoothly discharged from the molten part of the steel pipe welded part during manufacturing of the steel pipe, the formability of the steel pipe may be deteriorated. Therefore, the silicon (Si) content is limited to 0.3% or less.

·Relational expression 1

The Mn and Si must satisfy the following relational expression 1.

[Relationship 1]

(Mn/Si) ≥ 2 (weight ratio)

The Mn/Si ratio is an important parameter for determining the quality of welded parts of steel pipes. When the Mn/Si ratio is less than 2, the Si content is relatively high, so silicon oxide is formed in the molten metal of the welded part, and if it is not forcibly discharged, defects may be formed at the welded part, resulting in poor steel pipe production. limit

P: 0.03% or less (including 0%)

The phosphorus (P) may cause brittleness by being segregated at austenite grain boundaries and/or interphase grain boundaries. Therefore, the content of phosphorus (P) is kept as low as possible, and the upper limit is limited to 0.03%. A preferable phosphorus (P) content is 0.02% or less.

S: 0.004% or less (including 0%)

The sulfur (S) may cause high-temperature cracks by being segregated in MnS non-metallic inclusions in steel or during casting solidification. In addition, it is necessary to control it as low as possible because it can deteriorate the impact toughness of the heat-treated steel sheet or steel pipe. Therefore, in the present invention, the sulfur (S) content is kept as low as possible, and the upper limit is preferably limited to 0.004%.

Al: 0.04% or less (excluding 0%)

The aluminum (Al) is an element added as a deoxidizing agent. On the other hand, AlN is precipitated by reacting with nitrogen (N) in the steel, and when manufacturing slabs, these precipitates may cause slab cracks in the cast steel cooling condition in which precipitates precipitate, thereby reducing the quality of the cast steel or hot-rolled steel sheet. In addition, since the presence of Al-rich inclusions or oxides inside the steel sheet or steel pipe may deteriorate the fatigue durability of the final part, it is necessary to keep the content thereof as low as possible. Therefore, it is preferable to limit the content of aluminum (Al) to 0.04% or less (excluding 0%).

Cr: 0.3% or less (excluding 0%)

The chromium (Cr) is an element that delays the ferrite transformation of austenite to increase hardenability and improve heat treatment strength during quenching heat treatment of steel. When chromium (Cr) is added in excess of 0.3% to steel containing more than 0.30% carbon (C), it may cause excessive hardenability of the steel, so the content is limited to 0.3% or less (excluding 0%).

·Ni: 0.1 ~ 0.4%

The nickel (Ni) is an element that simultaneously increases hardenability and toughness of steel. On the other hand, in the present invention, when the tensile properties and the maximum bending angle are evaluated after quenching-tempering heat treatment of a steel plate or steel pipe having an increased nickel (Ni) content in the basic component, the yield strength after heat treatment decreases as the Ni content increases . This promotes the movement of dislocations introduced into martensite by the nickel (Ni) element, or suppresses excessive coarsening of prior austenite to an average of 26 μm or more during heat treatment, and furthermore, martensite during tempering heat treatment It is considered that the size of the carbide precipitated in the structure is controlled to be less than 300 nm. However, if the content is less than 0.1%, the effect of lowering the yield strength and increasing the maximum bending angle is insufficient, while if the content exceeds 0.4%, despite the above advantages, the manufacturing cost of the steel sheet can be rapidly increased. . Therefore, the content is limited to the range of 0.1 to 0.4%.

·Relationship 2

The Mn and Ni must satisfy the following relational expression 2.

[Relationship 2]

(Ni/Mn)≥ 0.05 (weight ratio)

The (Ni/Mn) ratio is a condition required to secure a maximum bending angle of 40° or more while securing strength of 1300 MPa or more after quenching-tempering heat treatment. If the (Ni/Mn) ratio is less than 0.05, it is out of the range of manganese (Mn) or nickel (Ni) content suggested in the present invention. In addition, when the silicon (Si) content is low or the manganese (Mn) content is high, a band structure with a high manganese (Mn) content can be easily formed in the microstructure of the hot-rolled steel sheet, and the heating and quenching-tempering heat treatment Later, the bendability may be deteriorated. On the other hand, in the heat treatment process, the nickel (Ni) element decomposes the band structure or segregates at the grain boundaries of the carbide to prevent the complete decomposition or dissolution of the carbide to prevent excessive growth of the old austenite size. . In addition, in the tempering process, carbides that precipitate in martensite are segregated in the vicinity of carbides, so that the growth of carbides can be suppressed so that they exist in a fine size. Therefore, as a method of improving the cold bendability of a steel sheet or steel pipe after heating and quenching-tempering heat treatment, the (Ni/Mn) ratio is limited to 0.05 or more in the present invention.

·Relational expression 3

The C, Mn, Si, and Ni must satisfy the following relational expression 3.

[Relationship 3]

(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)

The (Si+Ni)/(C+Mn) ratio is a condition required to secure a maximum bending angle of 40° or more while securing strength of 1300 MPa or more after quenching-tempering heat treatment. In general, the cold bendability of a steel sheet or pipe after quenching-tempering heat treatment is shown to be in inverse proportion to the tensile strength. Meanwhile, in the present invention, relational expression 3 was devised in order to simultaneously satisfy high strength and high formability after heat treatment. If the (Si+Ni)/(C+Mn) ratio is less than 0.2, it corresponds to the case where the (C+Mn) content is high compared to the (Si+Ni) content. In this case, the hardenability is high, so the strength after heat treatment is very high. Since the maximum bending angle is very low, bending cracks may occur frequently during the bending process. In addition, when a precipitate forming element is added, since the size of prior austenite (PAGS) is fine less than 15 μm during heat treatment, the tensile strength may be high and the bendability may be inferior.

Ti: 0.05% or less (excluding 0%)

Titanium (Ti) is an element that forms precipitates (TiC, TiCN, TiNbCN) in the hot-rolled steel sheet, and increases the strength of the hot-rolled steel sheet by suppressing the growth of austenite grains.

If the content exceeds 0.05%, it can be effective in increasing the strength of quenching-tempering heat-treated steel, but if it exists in the form of coarse crystallization rather than fine precipitates in the hot-rolled steel sheet, it deteriorates toughness or deteriorates in the cold bending process. It acts as a starting point for cracks, and can reduce the cold formability of heat-treated steel sheets and steel pipe parts or reduce the fatigue durability of final parts. Therefore, the content is limited to 0.05% or less (excluding 0%).

B: 0.0005 to 0.005% or less (excluding 0%)

The boron (B) is a beneficial element that greatly increases the hardenability of steel even at a low content. When an appropriate amount is added, it is effective in increasing hardenability by suppressing ferrite formation, but when it is excessively contained, the austenite recrystallization temperature is increased and weldability is deteriorated. If the boron (B) content is less than 0.0005%, it is difficult to secure the above effect of the steel, and if it exceeds 0.005%, the effect is saturated or it is difficult to secure appropriate strength and toughness. Therefore, the content is limited to 0.0005 to 0.005% or less. More preferably, limiting the content to 0.003% or less is effective in securing strength and formability of the heat-treated steel at the same time.

N: 0.01% or less (excluding 0%)

The nitrogen (N) is an austenite stabilizing and nitride forming element. If the nitrogen (N) content exceeds 0.01%, coarse AlN nitride may be formed, which may deteriorate fatigue durability by acting as a starting point for generating fatigue cracks when evaluating durability in heat-treated steel sheets or steel pipe parts. Therefore, its content is limited to 0.01% or less (excluding 0%). More preferably, it is desirable to limit the content to 0.006% or less.

In addition, when boron (B) elements are added together, it is necessary to control the nitrogen (N) content as low as possible in order to increase the effective boron (B) content.

The steel of the present invention basically contains the above components, the balance being essentially Fe and other impurities, but the following allowable components may be optionally added within a range not impairing the present invention.

·Mo, Cu, Nb and V

Optionally, in the present invention, Mo: 0.01 ~ 0.2%, Cu: 0.05 ~ 0.2%, Nb: 0.005 ~ 0.02%, and V: may include one or more of 0.01 ~ 0.05%. These elements increase the hardenability of the steel or refine the grain size of prior austenite to refine the lath size constituting the martensite or tempered martensite structure of the final part. Therefore, this can contribute to increasing the tensile strength of the steel or, furthermore, improving the bending angle or bendability.

Meanwhile, the above-described hot-rolled steel sheet and steel pipe of the present invention have a microstructure including 20 to 65% of ferrite and 35 to 80% of ferrite in volume%. When the fraction of ferrite is less than 20%, it is difficult to secure high strength and bending angle according to the development of a band structure because the pearlite content is too high. Therefore, it is preferable to limit the fraction of the ferrite to 20% or more. A preferred fraction of ferrite is 20 to 60%. On the other hand, if the ferrite fraction exceeds 65%, the total amount of hardenable elements added to the hot-rolled steel sheet may be insufficient, and in this case, it is difficult to secure sufficient strength after heating and quenching-tempering heat treatment.

In addition, it may have a hardness difference (ΔHv) value of less than 15 between the surface layer portion and the thickness/4 position portion of the hot-rolled steel sheet of the present invention, and may have a tensile strength of 320 to 950 MPa.

In addition, when the hot-rolled steel sheet is heated and quenched-tempered, high strength and high strength having a microstructure including 95% of at least one of martensite and quenched martensite and the remaining 5% or less of retained austenite are obtained by volume%. A molded member having a maximum bending angle of 40° or more can be obtained.

Next, a method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention will be described.

The hot-rolled steel sheet manufacturing method of the present invention includes the steps of heating a steel slab having the above composition in a temperature range of 1150 to 1300 ° C; Obtaining a hot-rolled steel sheet by hot-rolling the heated slab including rough rolling and finish rolling at a temperature of Ar ₃ or higher; and winding the hot-rolled steel sheet at a temperature of 550 to 750° C. by cooling the hot-rolled steel sheet on a run-out table.

Heating stage of steel slab

First, in the present invention, the steel slab prepared as described above is heated to a temperature range of 1150 to 1300 ° C.

Heating the steel slab to a temperature range of 1150 to 1300 ° C is to have a uniform structure and distribution of components within the slab. cannot obtain On the other hand, when the slab heating temperature exceeds 1300 ° C., it is difficult to secure the target material and surface quality of the hot-rolled steel sheet because excessive increase in decarburization depth and grain growth occur. Therefore, the slab heating temperature is limited to the range of 1150 to 1300 ° C.

Steps to obtain hot-rolled steel sheet

Then, in the present invention, the heated slab is hot-rolled at a temperature of Ar ₃ or higher, including rough rolling and finish rolling, to obtain a hot-rolled steel sheet.

The hot rolling is preferably hot finish rolling at Ar ₃ or higher. When the hot rolling is performed at a temperature of less than Ar ₃ , some of the austenite is transformed into ferrite, and the deformation resistance of the material for hot rolling becomes non-uniform, resulting in poor sheet permeability including straightness of the steel sheet, resulting in poor operation such as sheet breakage. Chances are high. However, if the finish-rolling temperature exceeds 950°C, scale defects or the like occur, so the hot finish-rolling temperature is preferably limited to 950°C or less.

winding step

And, in the present invention, the hot-rolled steel sheet obtained through hot rolling as described above is cooled on a run-out table and wound at a temperature of 550 to 750 ° C.

Cooling on the run-out table after the hot rolling and winding at a temperature range of 550 to 750 ° C is to secure a uniform material of the hot-rolled steel sheet. If the coiling temperature is too low, below 550 ° C, bainite or A low-temperature transformation phase such as martensite is introduced, so that the strength of the steel sheet may rapidly increase, and the variation in hot-rolled strength in the width direction increases.

On the other hand, when the coiling temperature exceeds 750 ° C., internal oxidation is promoted on the surface layer of the steel sheet, and surface defects such as cracks or surface irregularities may occur on the surface after hot rolling pickling. In addition, coarsening of pearlite may cause variations in surface hardness of the steel sheet. Therefore, the coiling temperature after cooling the hot-rolled steel sheet is preferably limited to 550 ~ 750 ℃.

In the present invention, the hot-rolled steel sheet manufactured as described above may be further pickled to produce a hot-rolled pickling steel sheet. The pickling treatment method is not limited to a specific method because any pickling treatment method generally used in a hot rolling pickling process can be used.

On the other hand, in the present invention, in the winding process, the winding temperature in the range of 550 to 750 ° C is controlled to be as low as possible to minimize the occurrence of decarburization, or the winding coil is charged in a water cooling bath rather than naturally cooled, so that the winding coil is at a high temperature for a long time. It is preferable to selectively use one of the processes of removing the decarburization region of the surface of the coil by preventing it from being maintained at or by over-acidifying after cooling the winding coil to 200 to 250 ° C. By selectively using these processes, the hardness difference (ΔHv) between the surface layer portion of the steel sheet and the position t/4 can be effectively reduced to less than 15.

The hot-rolled steel sheet of the present invention manufactured by the manufacturing process as described above may have a microstructure including 20 to 65% of ferrite and 35 to 80% of ferrite in volume%.

In addition, it may have a hardness difference (ΔHv) value of less than 15 at the surface layer portion of the hot-rolled steel sheet and the t/4 position portion, and may have a tensile strength of 320 to 950 MPa.

In addition, the hot-rolled steel sheet of the present invention manufactured by the manufacturing process as described above may have a microstructure in which the prior-austenite average grain size is 15 μm or more.

When a hot-rolled steel sheet having an average prior austenite grain size of 15 μm or more is subjected to heating and quenching-tempering heat treatment for manufacturing a molding member described later, the formed martensitic structure is relatively coarse, so that the onset of crack generation during bending is delayed. Therefore, the bendability may be improved or the bending angle may be increased.

In the case of a hot-rolled steel sheet satisfying the above-described relational expression 2-3 in the present invention, the prior austenite average grain size can be effectively controlled to 15 μm or more, and as a result, after quenching-tempering, after heat treatment, appropriate yield strength or tensile strength and A high bending angle can be secured at the same time.

If the heating and quenching-tempering heat treatment for manufacturing the above-described molded member using a hot-rolled steel sheet or steel pipe containing a prior austenite structure having an average grain size of less than 15 μm, the martensitic structure is finely formed Therefore, the strength after heat treatment is increased, and as a result, cracks may occur in a short time during bending, resulting in a decrease in bendability or a decrease in bending angle. Therefore, in this case, there may be restrictions on shape implementation when manufacturing parts by cold forming.

Hereinafter, a method for manufacturing a steel pipe according to an embodiment of the present invention will be described.

A preferred method for manufacturing a steel pipe according to the present invention includes the steps of obtaining a steel pipe by welding a hot-rolled steel sheet manufactured according to the method for manufacturing a hot-rolled steel sheet according to the present invention; and annealing and heat-treating the steel pipe.

Steps to get a steel pipe

A steel pipe is obtained by welding the hot-rolled steel sheet manufactured according to the manufacturing method of the hot-rolled steel sheet of the present invention described above.

A steel pipe is obtained by using the hot-rolled steel sheet or the hot-rolled pickling steel sheet, for example, by electric resistance welding or induction heating welding.

Annealing heat treatment step of steel pipe

The steel pipe obtained by manufacturing the pipe as described above is annealed and heat treated.

In the present invention, using the hot-rolled steel sheet or hot-rolled pickling steel sheet, for example, by using a conventional cold forming method including the process of forming, annealing, heating and cold drawing a steel pipe through electric resistance welding or induction heating welding, Steel tubes can be manufactured.

The annealing heat treatment of the steel pipe is preferably carried out for 3 to 60 minutes at a temperature of Ac ₁ -50 ℃ ~ Ac ₃ +150 ℃.

The annealing heat treatment may include furnace cooling and air cooling.

At this time, the present invention may further include a step of drawing the steel pipe subjected to the annealing heat treatment. The diameter of the steel pipe can be reduced by cold drawing the steel pipe. A cold drawing method is mentioned as the said drawing method.

The steel pipe of the present invention manufactured as described above has a microstructure containing 20 to 65% of ferrite and 35 to 80% of pearlite by volume%, and preferably, the microstructure of the steel pipe is 20 to 65% by volume%. It may contain 50% ferrite.

Then, in the present invention, the reheating step of heating the cooled steel pipe or the drawn steel pipe to a temperature of Ar ₃ ~ 970 ° C at a heating rate of 10 ° C / sec or more, and then maintaining it within 60 seconds; A quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350° C./sec; And after heating the quenched steel pipe to a temperature range of 150 ~ 350 ℃ at a heating rate of 2 ~ 20 ℃ / sec, tempering heat treatment step of maintaining at this temperature; may further include. A description of this is described in detail below.

Next, a method for manufacturing a cold formed member according to an embodiment of the present invention will be described.

The cold-formed member manufacturing method of the present invention includes the steps of reheating the steel pipe obtained according to the steel pipe manufacturing method; quenching-tempering heat treatment of the reheated steel pipe; and manufacturing a member by cold forming the quenching-tempered steel pipe.

The steel pipe is first subjected to the following heat treatment before being manufactured into a chassis component such as a stabilizer by cold bending.

Steel pipe reheating stage

In the present invention, the annealed heat-treated steel pipe or drawn steel pipe is reheated to produce a member for cold forming.

This reheating temperature may be Ar ₃ ~ 970 ℃. That is, while passing a high-frequency induction heating furnace at a moving speed of less than 100 mpm, a steel pipe of a specific length is heated at a heating rate of 10 ° C / sec or more to a temperature in the target range and maintained under the condition of less than 60 seconds. At this time, the moving speed can be variously changed under the condition of less than 100 mpm so that the thickness of the inner wall of the steel pipe can have a uniform temperature of 3 to 6 mm. In the case of rapidly heating to the target temperature at a heating rate of 10 ° C / sec or more, the depth of the decarburization layer that may occur on the outer or inner wall of the steel pipe can be minimized, so the durability of the final part can be improved. .

Stages of quenching-tempering heat treatment of steel pipe

Subsequently, in the present invention, the reheated steel pipe is subjected to quenching-tempering heat treatment.

In the case of the quenching cooling, water or oil is sprayed to a specific length of the steel pipe heated to a temperature of less than 970 ° C. to cool it to room temperature. The cooling rate of the entire steel pipe can be variously applied from 20 to 350 ° C / sec, and is not limited to a specific range if the entire inner wall of the steel pipe can have a martensitic structure. In this case, the cooling rate is carried out by properly controlling the amount of water or oil to be sprayed and the moving speed of the steel pipe.

The heated and quenched steel pipe is subjected to tempering heat treatment to impart toughness.

In the tempering heat treatment temperature condition, the microstructure of the steel may form a tempered martensitic structure corresponding to the grain size of prior austenite of 15 μm or more as a main phase. Accordingly, during bending, the resistance to grain boundary cracking along the old austenite grain boundary increases or it can be sufficiently plastically deformed without cracking, so that a bending angle of 40° or more can be exhibited.

If the heating temperature during the tempering is less than 150° C., the bending angle may be low because the formation of the tempered martensite structure is insufficient or the strength after heat treatment is relatively high due to the high dislocation density in the martensite. On the other hand, when the heating temperature during tempering exceeds 350 ° C., a high bending angle can be secured due to the excessive tempering effect of the martensitic structure, but it is difficult to secure a strength of 1300 MPa or more. Therefore, the tempering heat treatment temperature is preferably limited to a range of 150 to 350 ° C. More preferably, heat treatment is performed in a temperature range of 200 to 250° C. in which temper brittleness can be avoided.

On the other hand, in the heat treatment heating rate condition, the steel microstructure has Fe ₃ C carbides of various sizes in the tempered martensite crystal grains and grain boundaries. If the tempering heat treatment heating rate is less than 2° C./sec, and the heating rate is too slow, excessive Fe ₃ C growth occurs due to excessive tempering softening effect, and the strength after heat treatment tends to be low. In addition, since the heating rate is slow under the above conditions, the productivity of the steel pipe is low, so it is not economical. On the other hand, if the heating rate exceeds 20 °C, Fe ₃ C growth in martensite is inhibited, and the strength after heat treatment tends to be too high, so it may be difficult to secure appropriate strength and a high bending angle at the same time. That is, when the size of Fe ₃ C is fine, there are few crack sites including grain boundaries between Fe ₃ C and tempered martensite, but the strength tends to be too high to secure a high bending angle. Therefore, the heating rate of the tempering heat treatment is limited to the range of 2 to 20° C./sec, but the heating rate is preferably selected in consideration of the heating temperature range. Considering the above circumstances, in the present invention, it is preferable to limit the number of Fe ₃ C carbides having an average equivalent circle size of 300 nm or less to 30 or less per unit area (μm ² ).

cold forming step

A member is manufactured by cold forming the steel pipe subjected to the reheating and quenching-tempering heat treatment as described above.

The steel pipe is formed by a cold forming method. For example, the heat-treated steel pipe may be formed by a room temperature forming method using a cold forming machine having molds having various bending radii (R, Radius). An example of the member is a suspension part such as a stabilizer.

In the cold forming of the steel pipe, it is preferable to obtain a member by inserting a steel pipe of a specific length into a mold having a bending radius of 30 to 60R and performing minimum to maximum bending. In the present invention, the bending radius means the degree of bending (curvature) of a straight steel pipe, the radius of a curve formed by a circle, and the radius of curvature. Therefore, a bending radius of 30R indicates a small curvature radius but a large curvature or bending angle, and a bending radius of 60R indicates that the radius of curvature is large, but the curvature or bending angle is small. Incidentally, a bending radius of 60R means relatively gentle bending. On the other hand, in the present invention, if the member can be manufactured by adjusting the bending speed in the range where bending cracks do not occur during the member manufacturing process and the friction coefficient between the mold and the steel pipe, the specific ranges for the bending speed and friction coefficient are not limited. don't

On the other hand, after heat treatment, the bending angle can be measured even for a flat plate other than a steel pipe, and in the present invention, according to the VDA 238-100 standard test of a three-point bending test, the maximum bending of Angles were evaluated.

According to the manufacturing method of the member of the present invention, it has a tensile strength of 1300 MPa or more after heat treatment, a bending angle of 40 ° or more, or no cracking even at a bending radius of less than R50, and has high strength and excellent cold bending formability at the same time after heat treatment It is possible to manufacture members with

As described above, in the present invention, a member having a tensile strength of 1300 MPa or more after heat treatment, a bending angle of 40 ° or more, or a member having high strength and excellent cold bending formability after heat treatment without cracking even at a bending radius of less than R50 In order to manufacture, it is preferable to limit the number of Fe ₃ C carbides having an average old austenite grain size of 15 μm or more and an average equivalent circle size of 300 nm or less to 30 or less per unit area (μm ² ). As a result, the appropriate strength, toughness or plastic deformation characteristics of the tempered martensitic steel having carbides of an appropriate size are secured in the tempered martensitic structure, thereby preventing the occurrence of grain boundary cracks along the old austenite grain boundary during bending of the steel plate or steel pipe. This is because plastic deformation can be sufficiently induced until the resistance is increased or cracks occur.

As described above, the present invention secures strength of 1300 to 1800 MPa by performing heating and quenching-tempering heat treatment on the drawn steel pipe, and this ultra-high strength steel pipe can be made into a complex shape such as a stabilizer by cold forming. Compared to parts manufactured by the conventional hot forming process, these cold-formed parts do not have a big difference in shape / performance of cold-formed parts of ultra-high strength steel pipes, but the shape of ultra-high strength steel pipes can be relatively simple, and furthermore, the parts manufacturing cost is low. can be drastically reduced.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

steel grade C Si Mn P S S. Al Cr Mo Ni invention steel 1 0.200 0.200 0.500 0.01 0.002 0.010 0.300 - 0.150 invention steel 2 0.335 0.167 0.998 0.0073 0.0007 0.035 0.090 0.10 0.180 invention steel 3 0.345 0.103 1.020 0.014 0.005 0.037 0.120 - 0.211 comparative steel 1 0.335 0.098 0.990 0.014 0.005 0.025 - - 0.001 comparative steel 2 0.348 0.105 1.020 0.014 0.006 0.03 - - 0.009 Invention Steel 4 0.337 0.103 1.010 0.014 0.005 0.037 0.150 - 0.207 comparative lecture 3 0.337 0.300 1.000 0.015 0.005 0.035 - - 0.001 comparative lecture 4 0.340 0.303 0.900 0.014 0.005 0.025 - - 0.001 comparative steel 5 0.345 0.150 1.400 0.01 0.002 0.030 0.140 0.10 0.001 comparative steel 6 0.385 0.200 1.400 0.01 0.002 0.030 0.150 - 0.001 comparative steel 7 0.263 0.258 1.311 0.0071 0.0009 0.038 0.150 - 0.001 comparative river 8 0.338 0.164 1.28 0.0064 0.0008 0.036 0.140 0.10 0.001 comparative steel 9 0.386 0.2 1.28 0.0063 0.0012 0.039 0.130 - 0.002 comparative steel 10 0.041 0.099 1.3 0.01 0.002 0.03 0.190 0.09 0.001

steel grade Cu Ti Nb V B N relational expression 1 relational expression 2 Relational expression 3 invention steel 1 0.100 0.0200 - - 0.0025 0.0040 2.5 0.300 0.500 invention steel 2 - 0.0300 - - 0.0019 0.0032 6.0 0.180 0.260 invention steel 3 - 0.0054 - - 0.0024 0.0042 9.9 0.207 0.230 comparative steel 1 - 0.0052 - 0.011 0.0021 0.0038 10.1 0.001 0.075 comparative steel 2 - 0.0052 0.0085 0.012 0.0023 0.0016 9.7 0.009 0.083 Invention Steel 4 - 0.0054 0.0088 0.012 0.0021 0.0018 9.8 0.205 0.230 comparative lecture 3 - 0.0056 - 0.011 - 0.0020 3.3 0.001 0.225 comparative lecture 4 - 0.0049 - 0.011 - 0.0024 3.0 0.001 0.245 comparative steel 5 - 0.0300 - - 0.0020 0.0060 9.3 0.001 0.087 comparative steel 6 - 0.0300 - - 0.0022 0.0070 7.0 0.001 0.113 comparative steel 7 0.012 0.0370 0.0012 - 0.0031 0.0044 5.1 0.001 0.165 comparative river 8 0.024 0.0290 0.0020 - 0.0017 0.0037 7.8 0.001 0.102 comparative steel 9 - 0.0300 - - 0.0023 0.0047 6.4 0.002 0.121 comparative steel 10 0.01 0.0290 0.0010 - 0.0019 0.0041 13.1 0.001 0.075

Hot-rolled steel sheet having a thickness of 3.6 mm was prepared by performing hot rolling under the conditions of Table 3 below using the steel composed as shown in Table 1-2, and then pickling was performed. Specifically, the slab or lap manufacturing ingot having the composition of Table 1-2 was homogenized by heating in the range of 1200 ± 20 ° C. for 200 minutes, and subsequently, after performing rough rolling and finishing rolling on individual slabs or ingots , a hot-rolled steel sheet having a thickness of 3.6 mm was prepared by winding at a temperature of 550 to 750 ° C.

steel grade finishing pressure
Annual temperature (℃) winding temperature
(℃) YS(MPa) TS(MPa) EL(%) MeanΔHv perlite fraction
(volume%) note invention steel 1 880 640 332 524 31.2 8 35 Invention example 1 invention steel 2 880 610 389 607 26.4 10 40 bill honor 2 invention steel 3 880 640 339 545 30.7 13 44 Invention Example 3 invention steel 3 880 580 369 594 29.3 14 57 Invention example 3-1 comparative steel 1 880 620 338 534 31.7 8 39 Comparative Example 1 comparative steel 2 880 660 387 597 27.2 22 47 Comparative Example 2 Invention Steel 4 880 640 377 575 28.4 7 42 Invention example 4 Invention Steel 4 870 580 440 657 26.1 8 76 Invention example 4-1 comparative lecture 3 880 580 462 665 28.0 14 72 Comparative Example 3 comparative lecture 4 880 570 459 672 25.9 18 73 Comparative Example 4 comparative steel 5 880 640 377 625 22.7 16 40 Comparative Example 5 comparative steel 6 880 640 367 620 23.1 15 38 Comparative Example 6 comparative steel 7 880 700 436 631 24.0 17 41 Comparative Example 7 comparative river 8 860 640 521 756 23.1 16 47 Comparative Example 8 comparative steel 9 860 640 443 690 21.3 15 56 Comparative Example 9 comparative steel 10 880 650 458 700 22.0 16 63 Comparative Example 10

With respect to the hot-rolled steel sheet manufactured as described above, yield strength (YS), tensile strength (TS), elongation (EL), Vickers hardness difference between the surface layer and t/4, and microstructure fraction were measured, and the results were reported as described above. Table 3 shows. The microstructure other than pearlite is ferrite. Meanwhile, in this embodiment, the yield strength (YS), tensile strength (TS), and elongation (EL) of the hot-rolled steel sheet were measured using the JIS 5 standard for specimens taken in a direction parallel to the rolling direction, and the pearlite fraction was Nital The etched specimen was measured using an image analysis program under the condition of an optical microscope at X500 magnification.

In addition, a steel pipe having a diameter of 28 mm was manufactured by using the hot-rolled steel sheets by electric resistance welding, followed by annealing heat treatment and cold drawing to manufacture a drawn steel pipe having a diameter of 22.2 mm. In addition, the steel pipe was subjected to heating-quenching-tempering heat treatment under the conditions of Table 4, and then a member was manufactured through cold forming.

At this time, in the quenching heat treatment, the steel pipe is heated to a temperature of 930 ~ 970 ° C, and the temperature of the steel pipe is cooled to 200 ° C or less and cooled by charging or spraying water or oil so that it is completely cooled to room temperature. was carried out. In addition, the tempering heat treatment was performed after heating the steel pipe to a temperature of 200 ~ 300 ℃ at a heating rate in the range of 2 ~ 20 ℃ / s, followed by cooling.

steel grade Heating temperature (℃) Cooling rate (℃/s) Tempering temperature (℃) Tempering rate (℃/s) note invention steel 1 970 50 300 2 Invention example 1 invention steel 2 950 40 220 3 Invention example 2 invention steel 3 950 45 250 3 Invention example 3 invention steel 3 950 40 220 15 Invention example 3-1 comparative steel 1 930 40 230 3 Comparative Example 1 comparative steel 2 930 40 230 3 Comparative Example 2 Invention Steel 4 930 50 230 3 Invention example 4 Invention Steel 4 950 50 200 5 Invention example 4-1 comparative lecture 3 930 40 230 3 Comparative Example 3 comparative lecture 4 930 40 230 3 Comparative Example 4 comparative steel 5 930 51 230 3 Comparative Example 5 comparative steel 6 930 50 230 3 Comparative Example 6 comparative steel 7 950 30 350 5 Comparative Example 7 comparative river 8 930 42 300 7 Comparative Example 8 comparative steel 9 930 43 220 5 Comparative Example 9 comparative steel 10 930 40 250 5 Comparative Example 10

After the quenching-tempering heat treatment, tensile properties and a three-point bending test were performed on the steel pipe, and the microstructure was observed, and the results are shown in Table 5 below. Specifically, after heating and quenching-heat treatment of the steel pipe, the tensile properties of the steel sheet and the maximum bending angle were measured through a 3-point bending test.

old austenite grains For the average size, the cross section of the same specimen whose optical microstructure was observed was polished and etched with picric acid, and then the size of at least 10 grains was measured at X500 magnification, and the average of the results was calculated. The number of Fe ₃ C carbides per unit area was measured by measuring the number of Fe ₃ C carbides present within an area of 2.9 um in width X 3.1 um in height at X5,000 to X10,000 magnification using a scanning electron microscope using the same specimen. , and the result was taken as the number of carbides per unit area. Specifically, the number of Fe ₃ C carbides present in possible tempered martensite grains was measured. After heating and quenching-tempering heat treatment, the microstructure was measured in detail as described above to determine the correlation between the microstructure change and the bending angle.

Meanwhile, the steel pipe was simply Zig-Zag formed using molds having various bending radii or cold formed using a cold forming machine. After bending the steel pipe under various conditions with a cold forming machine, whether or not cracks occurred and the minimum bending radius without cracks were investigated, and the results are shown in Table 6 below. Here, the mechanical property values of the heat-treated steel sheet are values measured by processing and heat-treating specimens taken in a direction parallel to the rolling direction according to the JIS 5 standard, and the maximum bending test value is the specimen edge property while following the VDA 238-100 standard test In order to exclude this, both edges of the long side of the heat-treated specimen were treated with surface grinding (grinding-off).

steel grade YS
(MPa) TS
(MPa) EL(%) YR Maximum bending angle α (°) TS×α Average size of old austenite
(μm) Number of Fe ₃ C carbides
(/㎛ ² ) note invention steel 1 1210 1327 9.8 0.91 63 83601 19 24 Invention example 1 invention steel 2 1439 1733 8.8 0.83 61 104847 21 26 Invention Example 2 invention steel 3 1330 1626 9.4 0.82 49 79837 21 26 Invention example 3 invention steel 3 1371 1667 8.4 0.82 53 88351 21 11 Invention example 3-1 comparative steel 1 1361 1647 9.1 0.83 38 61763 18 35 Comparative Example 1 comparative steel 2 1436 1726 8.9 0.83 38 65588 13 29 Comparative Example 2 Invention Steel 4 1396 1685 9.3 0.83 46 77510 15 27 Invention example 4 Invention Steel 4 1404 1684 9.1 0.83 51 85884 15 12 Invention example 4-1 comparative lecture 3 1371 1672 6.2 0.82 38 63703 17 38 Comparative Example 3 comparative lecture 4 1397 1691 6.3 0.83 39 65780 16 29 Comparative Example 4 comparative steel 5 1464 1819 8.7 0.80 37 67303 15 21 Comparative Example 5 comparative steel 6 1432 1805 8.6 0.79 36 64980 16 22 Comparative Example 6 comparative steel 7 1291 1365 9.3 0.95 53 72345 14 29 Comparative Example 7 comparative river 8 1458 1616 8.8 0.90 43 69488 14 29 Comparative Example 8 comparative steel 9 1451 1889 10.0 0.77 39 73681 14 36 Comparative Example 9 comparative steel 10 1480 1829 8.6 0.82 38 69502 13 32 Comparative Example 10

steel grade steel pipe minimum bending radius pass judgment note invention steel 1 45 (45R) ○ Invention example 1 invention steel 2 48 (48R) ○ Invention example 2 invention steel 3 48 (48R) ○ Invention Example 3 invention steel 3 48 (48R) ○ Invention example 3-1 comparative steel 1 55 (55R) X crack occurrence Comparative Example 1 comparative steel 2 55 (55R) X crack occurrence Comparative Example 2 Invention Steel 4 50 (50R) ○ Invention example 4 Invention Steel 4 50 (50R) ○ Invention example 4-1 comparative lecture 3 55 (55R) X crack occurrence Comparative Example 3 comparative lecture 4 55 (55R) X crack occurrence Comparative Example 4 comparative steel 5 55 (55R) X crack occurrence Comparative Example 5 comparative steel 6 55 (55R) X crack occurrence Comparative Example 6 comparative steel 7 50 (50R) X crack occurrence Comparative Example 7 comparative river 8 50 (50R) X crack occurrence Comparative Example 8 comparative steel 9 50 (50R) X crack occurrence Comparative Example 9 comparative steel 10 50 (50R) X crack occurrence Comparative Example 10

As shown in Table 1-6, Inventive Example 1-4, Inventive Example 3-1 and Inventive Example 4-1 manufactured using Inventive Steel 1-4 satisfying the steel composition and relational expression 1-3 are the maximum It can be seen that bending cracks did not occur even when the bending angle exceeded 40° or the bending radius was 50R or less.

In addition, Inventive Example 1-4, Inventive Example 3-1, and Inventive Example 4-1 all have a yield strength of 1200 to 1400 MPa, a tensile strength of 1300 to 1700 MPa, a yield ratio of 0.8 or more, and a maximum bending angle of 40° or more, which is excellent in bending formability. can know

In addition, in the case of the hot-rolled steel sheet before heat treatment, it can be seen that the examples of the present invention have a relatively small hardness difference value of less than 15 between the surface layer and t/4 compared to Comparative Examples 1-6. This is understood to be that the difference in hardness according to the position in the thickness direction of the hot-rolled steel sheet is small, or that the occurrence of decarburization in the surface layer is small.

In contrast, in Comparative Examples 1-10 prepared using Comparative Steel 1-10 that does not satisfy at least one of the alloy components and relational expressions 1 to 3 of the present invention, the maximum bending angle of the steel sheet after heat treatment is relatively less than 40 ° or Or, it was the case where the bending radius in which cracks did not occur in the steel pipe was 55R or more.

On the other hand, Figure 1 is a diagram showing the strength and maximum bending angle change curve of the steel sheet after tempering heat treatment of Example 2 in one embodiment of the present invention, Figure 2 is the quenching of Example 2 in one embodiment of the present invention -A photograph showing the shape of a tempered heat-treated steel pipe after cold bending, and FIG. 3 (a-b) is a photograph showing whether or not cracks occur in the heat-treated steel pipe according to an embodiment of the present invention, (a) shows invention example 2 (b) represents Comparative Example 2.

That is, the strength and maximum bending angle change of the heat-treated steel sheets of the inventive examples and comparative examples of various examples were investigated, and the strength and maximum bending angle results of the steel sheet after tempering heat treatment of the inventive example 2 are shown in FIG. 1 as a representative example.

In addition, bending tests were performed on heat-treated steel pipes of various embodiments of inventive steel and comparative steel using a cold forming machine having a 30 ~ 60R bending radius mold, and as a representative example, the result of the final shape of the steel pipe according to the bending test of Inventive Example 2 is shown in Figure 2.

In addition, it was found that cracks occurred in the surface layer of the heat-treated steel pipe when the bending radius was small during cold bending of various steel pipes, and inventive example 2 and comparative example 2 were shown in FIG.

As shown in FIG. 1, it can be seen that a tensile strength of 1300 MPa or more and a high bending angle of 40 ° or more are obtained at a tempering temperature of 200 to 250 ° C.

In addition, as shown in FIGS. 2-3, in the case of a heat-treated steel pipe manufactured as an inventive example that satisfies the conditions of the present invention under the condition of a bending radius of 50R or less, it can be confirmed that the member can be manufactured without cracking during cold forming. .

As described above, the reason why the steel pipe bending (bending) crack does not occur after the QT heat treatment in the present invention or the flat plate 3-point bending angle is measured high is that the steel type of the present invention has relatively coarse old austenite crystal grains. Therefore, it is considered that the start of crack generation is delayed for the bending external force, or the Fe ₃ C carbide size is not sufficiently grown in the coarse tempered martensite grains due to the application of a fast heating rate during tempering heating. In addition, it is believed that the retardation effect of the Fe ₃ C growth is further promoted when nickel (Ni) elements are segregated at the Fe ₃ C adjacent interface during tempering heating.

As described above, the detailed description of the present invention has been described with respect to the preferred embodiments of the present invention, but those skilled in the art to which the present invention belongs can make various modifications without departing from the scope of the present invention. Of course this is possible. Therefore, the scope of the present invention should not be limited to the described embodiments and should not be defined, and should be defined by not only the claims described later, but also those equivalent thereto.

Claims (19)

In % by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (0% including), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less ( 0%), contains the remaining Fe and other impurities, satisfies the following relational expression 1-3, has a hardness difference value of less than 15 between the surface layer portion and the thickness / 4 position portion of the steel sheet, in volume%, It has a microstructure containing 20 to 65% of ferrite and 35 to 80% of pearlite, and has a high austenite average grain size of 15㎛ or more, high strength after heating and quenching-tempering heat treatment, and a maximum temperature of 40° or more. Hot-rolled steel sheet for cold-formed members with excellent bendability and bending angle.
[Relationship 1]
(Mn/Si) ≥ 2 (weight ratio)
[Relationship 2]
(Ni)/(Mn) ≥ 0.05 (weight ratio)
[Relationship 3]
(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)
According to claim 1, Mo: 0.01 ~ 0.2%, Cu: 0.05 ~ 0.2%, Nb: 0.005 ~ 0.02% and V: heating and ?? A hot-rolled steel sheet for cold-formed members with excellent bendability having high strength and a maximum bending angle of 40° or more after quenching-tempering heat treatment.
According to claim 1, wherein the hot-rolled steel sheet has a tensile strength of 320 ~ 950 MPa, characterized in that after heating and quenching-tempering heat treatment for cold-formed members with excellent bendability having a high strength and a maximum bending angle of 40 ° or more hot-rolled steel sheet.
In % by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (0% including), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less ( 0%), including the remaining Fe and other impurities, and heating a slab satisfying the following relational expression 1-3 to a temperature range of 1150 to 1300 ° C.;
Obtaining a hot-rolled steel sheet by hot-rolling the heated slab including rough rolling and finish rolling at an Ar3 temperature or higher; and
Cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750 ° C; including,
Manufacture of hot-rolled steel sheet for cold-formed parts with excellent bendability, having a hardness difference of less than 15 at the surface layer of the steel sheet and the thickness/4 position, and having high strength and a maximum bending angle of 40° or more after heating and quenching-tempering heat treatment method.
[Relationship 1]
(Mn/Si) ≥ 2 (weight ratio)
[Relationship 2]
(Ni)/(Mn) ≥ 0.05 (weight ratio)
[Relationship 3]
(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)
According to claim 4, Mo: 0.01 ~ 0.2%, Cu: 0.05 ~ 0.2%, Nb: 0.005 ~ 0.02% and V: heating and ?? A method for manufacturing a hot-rolled steel sheet for cold-formed members having high strength and excellent bendability having a maximum bending angle of 40° or more after quenching-tempering heat treatment.
In % by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (0% including), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less ( except for 0%), contains the remaining Fe and other impurities, satisfies the following relational expression 1-3, has a hardness difference value of less than 15 at the surface layer of the steel pipe and the thickness/4 position, in volume%, 20 to It has a microstructure containing 65% of ferrite and 35 to 80% of pearlite, and has a prior austenite average grain size of 15㎛ or more, high strength after heating and quenching-tempering heat treatment, and a maximum bending angle of 40° or more. Steel pipe for cold-formed members having excellent bendability.
[Relationship 1]
(Mn/Si) ≥ 2 (weight ratio)
[Relationship 2]
(Ni)/(Mn) ≥ 0.05 (weight ratio)
[Relationship 3]
(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)
According to claim 6, Mo: 0.01 ~ 0.2%, Cu: 0.05 ~ 0.2%, Nb: 0.005 ~ 0.02% and V: heating and ?? A steel pipe for cold-formed members with high strength and excellent bendability with a maximum bending angle of 40° or more after quenching-tempering heat treatment.
In % by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (0% including), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less ( 0%), including the remaining Fe and other impurities, and heating a slab satisfying the following relational expression 1-3 to a temperature range of 1150 to 1300 ° C.;
Obtaining a hot-rolled steel sheet by hot-rolling the heated slab including rough rolling and finish rolling at an Ar3 temperature or higher;
Cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750° C.;
Obtaining a steel pipe by welding the hot-rolled steel sheet; and
Annealing the steel pipe at a temperature of Ac ₁ -50 ° C to Ac ₃ +150 ° C for 3 to 60 minutes; after heating and quenching-tempering heat treatment comprising a bending having a high strength and a maximum bending angle of 40 ° or more A method for manufacturing steel pipes for cold-formed members with excellent properties.
[Relationship 1]
(Mn/Si) ≥ 2 (weight ratio)
[Relationship 2]
(Ni)/(Mn) ≥ 0.05 (weight ratio)
[Relationship 3]
(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)
According to claim 8, Mo: 0.01 ~ 0.2%, Cu: 0.05 ~ 0.2%, Nb: 0.005 ~ 0.02% and V: heating and ?? A method of manufacturing a steel pipe for cold-formed members having excellent bendability having high strength and a maximum bending angle of 40° or more after quenching-tempering heat treatment.
According to claim 8, Heating and quenching further comprising the step of drawing the steel pipe subjected to the annealing heat treatment - Manufacturing of a steel pipe for cold-formed members with excellent bendability having a maximum bending angle of 40 ° or more and high strength after heat treatment method.
The method of claim 8 or 9,
A reheating step of heating the annealed or drawn steel pipe to a temperature of Ar ₃ ~ 970 ° C at a heating rate of 10 ° C / sec or more, and then maintaining it within 60 seconds;
A quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350° C./sec; and
After heating the quenched steel pipe at a heating rate of 2 to 20 ° C / sec to a temperature range of 150 to 350 ° C, and then maintaining the tempering heat treatment step at this temperature; A method of manufacturing a steel pipe for a cold-formed member having excellent bendability having high strength and a maximum bending angle of 40° or more.
In % by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (0% including), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less ( except for 0%), including the rest of Fe and other impurities, satisfying the following relational expression 1-3, in volume%, at least one of martensite and iron martensite at 95%, the balance remaining at 5% or less Has a microstructure containing austenite, has a prior austenite average grain size of 15 μm or more, and has an average equivalent circle size of 300 nm or less Fe ₃ C carbide per unit area (μm ² ) of 30 or less, high strength and cold-formed member with excellent bendability.
[Relationship 1]
(Mn/Si) ≥ 2 (weight ratio)
[Relationship 2]
(Ni)/(Mn) ≥ 0.05 (weight ratio)
[Relationship 3]
(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)
The method of claim 12, Mo: 0.01 ~ 0.2%, Cu: 0.05 ~ 0.2%, Nb: 0.005 ~ 0.02%, and V: 0.01 ~ 0.05% High strength and bendability characterized in that it further comprises one or more of This excellent cold-formed member.
[Claim 13] The cold-formed member with excellent high strength and bendability according to claim 12, wherein the forming member has a tensile strength of 1300 MPa or more and a maximum bending angle of 40 ° or more.
In % by weight, C: 0.20% or more and less than 0.35%, Mn: 0.5 to 1.3%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (0% including), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less, Ni: 0.1 to 0.4%, Ti: 0.05% (including 0%), B: 0.0005 to 0.0050%, N: 0.01% or less ( 0%), including the remaining Fe and other impurities, and heating a slab satisfying the following relational expression 1-3 to a temperature range of 1150 to 1300 ° C.;
Obtaining a hot-rolled steel sheet by hot-rolling the heated slab including rough rolling and finish rolling at an Ar3 temperature or higher;
Cooling the hot-rolled steel sheet on a run-out table and winding it at a temperature of 550 to 750° C.;
Obtaining a steel pipe by welding the hot-rolled steel sheet;
Annealing heat treatment and drawing the steel pipe;
A reheating step of heating the annealed or drawn steel pipe to a temperature of Ar ₃ ~ 970 ° C at a heating rate of 10 ° C / sec or more, and then maintaining it within 60 seconds;
A quenching step of cooling the reheated steel pipe to room temperature at a cooling rate of 20 to 350° C./sec or more;
A tempering heat treatment step of heating the quenched steel pipe to a temperature range of 150 to 350 ° C at a heating rate of 2 to 20 ° C / sec and maintaining it at this temperature; and
Method for producing a cold-formed member having excellent high strength and bendability, including the step of cold forming the tempered heat-treated steel pipe into a member.
[Relationship 1]
(Mn/Si) ≥ 2 (weight ratio)
[Relationship 2]
(Ni)/(Mn) ≥ 0.05 (weight ratio)
[Relationship 3]
(Si+Ni)/(C+Mn)≥ 0.2 (weight ratio)
The method of claim 15, wherein Mo: 0.01 ~ 0.2%, Cu: 0.05 ~ 0.2%, Nb: 0.005 ~ 0.02%, and V: 0.01 ~ 0.05% High strength and bendability characterized in that it further comprises at least one member. A method for producing this excellent cold-formed member.
The method of claim 15, wherein the forming member has a tensile strength of 1300 MPa or more and a maximum bending angle of 40 ° or more.
The method of claim 15, wherein the steel pipe is subjected to annealing heat treatment at a temperature of Ac ₁ -50 ° C to Ac ₃ + 150 ° C for 3 to 60 minutes.
[Claim 16] The method of claim 15, wherein the heat-treated steel pipe is cold-formed in a bending radius of 30 to 60R during cold-forming.

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