KR20240098897A - Hot rolled steel sheet and mehtod for the same - Google Patents

Abstract

A hot rolled steel sheet and a manufacturing method thereof are provided.
The hot rolled steel sheet of the present invention has, in weight percent, C: 0.05 to 0.10%, Si: 0.01 to 1.0%, Mn: 1.4 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 0.5%, Mo: 0.005 to 0.005%. 0.3%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.12%, contains remaining Fe and inevitable impurities, ferrite and bay It has a microstructure containing the sum of the nite phases: more than 90 area% and the sum of the remaining pearlite, martensite and MA phases: less than 10 area%.

Description

Hot rolled steel sheet and method of manufacturing the same {Hot rolled steel sheet and mehtod for the same}

The present invention mainly relates to a high-strength composite hot-rolled steel sheet with excellent formability, bake hardenability, and material uniformity that can be applied to components used in automobile chassis parts, lower arms, reinforcements, connectors, and frames. Secures a tensile strength of over 760 MPa, while the hole expandability (HER ₀ ) value is over 40%, the bake hardening amount (BH ₂ ) is over 30 MPa, and even after heat treatment at 300~600℃, the bake hardening amount (BH _h ) is over 30 MPa. It relates to a high-strength composite hot-rolled steel sheet with excellent material uniformity and a method of manufacturing the same.

High-strength hot-rolled steel sheets used for conventional automobile chassis and frames are being thinned for weight reduction, and at the same time, excellent formability is required considering the shape of the part. Furthermore, in order to maximize the durability of the part, the yield strength after painting heat treatment is increased. Bake hardenability, which means an increase, is required.

In addition, the use of hot-rolled plating materials has recently been increasing to increase the corrosion resistance of hot-rolled chassis parts, and making hot-rolled plating materials is only possible through additional heat treatment after hot rolling. This additional heat treatment can change the microstructure of the existing hot-rolled steel sheet, and it is very important to appropriately set the alloy composition, hot-rolled microstructure, heat treatment conditions, etc. in order to satisfy the desired final microstructure and physical properties.

Patent Document 1 is a technology related to composite steel with bainitic ferrite, a low-temperature region generated ferrite phase, and granular bainitic ferrite as the base structure, but because Cu must be used to secure additional strength, surface defects and high-temperature brittleness during hot rolling occur. This may occur, and there is a disadvantage that Ni must be added to prevent this.

Patent Document 2 is a technology that secures the strength of the weld damage area by adding Ti, Nb, Cr, Mo, etc. Specifically, during arc welding, the heat-affected zone adjacent to the molten welding material is heated to a temperature of over 600°C, and in particular, in some cases, it is heated to a temperature above the austenite range, adding Cr and Mo to increase the hardenability of the steel. This is a technology that secures strength by forming low-temperature phases such as bainite and martensite phases upon cooling. However, as in this patent, the concept based on maximizing hardenability has limitations when applied to automotive steel sheets that require high formability even after heat treatment as necessary after manufacturing the steel sheet.

Registered Patent KR 10-1114672 Registered Patent KR 10-0962745

The purpose of the present invention is to provide a high-strength composite hot-rolled steel sheet with excellent formability, bake hardenability, and material uniformity, and a method for manufacturing the same.

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

One aspect of the present invention is,

In weight percent, C: 0.05 to 0.10%, Si: 0.01 to 1.0%, Mn: 1.4 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 0.5%, Mo: 0.005 to 0.3%, P: 0.001 to 0.05%, S: 0.001-0.01%, N: 0.001-0.01%, Nb: 0.005-0.05%, Ti: 0.005-0.12%, including the balance Fe and inevitable impurities,

It has a microstructure comprising the sum of ferrite and bainite phases: more than 90 area% and the sum of the remaining pearlite, martensite and MA phases: less than 10 area%;

It relates to a hot rolled steel sheet that has a tensile strength of 760 MPa or more, hole expandability (HER ₀ ) of 40% or more, bake hardening (BH ₂ ) of 30 MPa or more, and △TS1 defined by the following relational equation 1 of 100 MPa or less.

[Relationship 1]

△TS1 = TS _max - TS _min

TS _max : Maximum TS value at 7 locations in the width direction, including the polar edge, within 30m of the rear end of the manufactured hot-rolled coil.

TS _min : Minimum TS value at 7 locations in the width direction, including the polar edge, within 30m of the rear end of the manufactured hot-rolled coil.

The hot rolled steel sheet of the present invention maintains a bake hardening amount (BH _h ) of 30 MPa or more after heat treatment at 300 to 600°C, and when defining △TS2 as shown in the relational equation 2 below, the absolute value of △TS2 × BH _h ^-1 is It may be less than 0.7.

[Relational Expression 2]

△TS2 = TS _h - TS ₀

TS _h : Tensile strength after heat treatment, TS ₀ : Tensile strength before heat treatment

The hot rolled sheet may additionally contain one or more components among V, Ni, and B in a total amount of 1.5% or less.

In the present invention, a hot-dip galvanized layer may be formed on the surface of the hot-rolled sheet.

In addition, another aspect of the present invention is,

Reheating the steel slab having the above-described alloy composition to a temperature range of 1100 to 1350°C;

Manufacturing a hot rolled steel sheet by hot rolling the reheated steel slab in the range of 850 to 1150°C;

Primary cooling the hot rolled steel sheet to a temperature in the range of 550 to 650° C. at an average cooling rate of 50 to 100° C./sec;

stopping cooling of the primary cooled steel sheet for 3 to 7 seconds; and

It relates to a method of manufacturing a hot rolled steel sheet including the step of secondary cooling the hot rolled steel sheet where the primary cooling has been stopped at an average cooling rate of 1 to 30°C/sec to a temperature in the range of 400 to 500°C and then winding the hot rolled steel sheet.

It may further include cooling the coiled hot-rolled steel sheet to a temperature in the range of room temperature to 200°C at an average cooling rate of 0.1 to 25°C/hr.

It may further include the step of oiling the wound hot-rolled steel sheet after pickling treatment.

After pickling the coiled hot-rolled steel sheet, it is heated to a temperature range of 450-750°C and then immersed in a plating bath containing 0.01-30% by weight of Mg, 0.01-50% of Al, and residual zinc, thereby forming a coating on the surface. It may further include forming a hot-dip galvanized layer.

The present invention, configured as described above, provides a tensile strength of 760 MPa or more, a hole expandability (HER ₀ ) value of 40% or more, a bake hardening amount (BH ₂ ) of 30 MPa or more, and a heat treatment temperature of 300 to 600°C. It can effectively provide steel plates with bake hardening (BH _h ) of 30 MPa or more and excellent material uniformity.

Accordingly, this steel plate can be effectively applied to parts used in automobile chassis components, lower arms, reinforcements, connectors, and frames.

Figure ¹ is a diagram showing the change in the absolute value of △ _TS2

Hereinafter, the present invention will be described.

In order to expand the applicability of hot-rolled chassis parts, the present inventors studied steels with different compositions and microstructures, and the control of microstructure due to precise temperature and time control in the hot-rolled manufacturing process determines the level of material deviation within the coil. It was concluded that this is the most important factor. From the results, the range of the main components of the steel was set so that the hot rolled steel sheet had excellent material uniformity, and the composition of the martensite, MA (martensite & austenite) phases and pearlite was less than 10%, and the remainder was the sum of ferrite and bainite phases of 90%. It was confirmed that it is possible to manufacture a high-strength composite hot-rolled steel sheet with a tensile strength of 760 MPa or more, a hole expandability (HER ₀ ) value of 40% or more, and a bake hardening amount (BH ₂ ) of 30 MPa or more, and the present invention is to present. The hot-rolled steel sheet of the present invention has no material deviation, and the bake hardening amount (BH _h ) can be maintained at 30 MPa or more even after heat treatment at 300 to 600 ° C, so it can be effectively used in the production of plated hot-rolled steel sheet using molten zinc, etc.

The hot rolled steel sheet of the present invention has, in weight percent, C: 0.05 to 0.10%, Si: 0.01 to 1.0%, Mn: 1.4 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 0.5%, Mo: 0.005. ∼0.3%, P: 0.001∼0.05%, S: 0.001∼0.01%, N: 0.001∼0.01%, Nb: 0.005∼0.05%, Ti: 0.005∼0.12%, contains the balance Fe and inevitable impurities, and contains ferrite and It has a microstructure containing the sum of the bainite phases: more than 90 area% and the sum of the remaining pearlite, martensite and MA phases: less than 10 area%, a tensile strength of more than 760 MPa, hole expandability of more than 40% (HER ₀ ), and bake hardening. The amount (BH ₂ ) is 30 MPa or more, and △TS1 defined by equation 1 satisfies 100 MPa or less.

Hereinafter, the composition of the steel sheet provided by the present invention will first be described in detail. At this time, unless otherwise specified, the content of each ingredient refers to weight percent.

C: 0.05∼0.10%

C is the most economical and effective element in strengthening steel. As the addition amount increases, the precipitation strengthening effect or low-temperature phase fraction increases, thereby increasing tensile strength. However, if the content is less than 0.05%, it is difficult to achieve sufficient precipitation strengthening effect and low-temperature phase formation, making it difficult to secure the target strength and bake hardenability. If the content exceeds 0.10%, excessive low-temperature phase and carbide are formed, resulting in poor formability and carbon equivalent. There is a disadvantage that generally the weldability becomes inferior. In addition, depending on the characteristics of the microstructure created when excessive C content is added, low-temperature phase deterioration and additional excess carbides are formed during additional heat treatment after hot rolling, which may significantly reduce the tensile strength after heat treatment. Therefore, in the present invention, it is preferable that the C content is included in the range of 0.05 to 0.10%.

Si: 0.01~1.0%

The Si deoxidizes the molten steel, has a solid solution strengthening effect, and is advantageous in improving formability by delaying the formation of coarse carbides. In addition, it has the effect of suppressing the formation of carbides during heat treatment in the range of 300 to 600°C. However, if the content is less than 0.01%, the effect of delaying carbide formation is small, making it difficult to improve formability and having little effect in improving strength. If it exceeds 1.0%, red scale due to Si is formed on the surface of the steel sheet during hot rolling, which not only deteriorates the surface quality of the steel sheet, but also reduces ductility and weldability, so it is desirable to limit the content to within 1.0%.

Mn: 1.4~2.0%

Like Si, Mn is an effective element in solid solution strengthening steel, and further improves the hardenability of steel, delaying ferrite transformation at the same cooling rate and facilitating the formation of low-temperature phases such as bainite and martensite. However, if the content is less than 1.4%, the effect of solid solution strengthening and hardenability improvement is small, and the desired strength increase effect cannot be obtained. On the other hand, if it exceeds 2.0%, the hardenability increases significantly and the martensite phase fraction exceeds the intended purpose. When casting slabs in the continuous casting process, a segregation zone develops greatly at the center of the thickness, resulting in poor formability and further deteriorating welding quality. It causes bad things. Therefore, in the present invention, it is preferable to limit the Mn content to 1.4 to 2.0%.

P: 0.001∼0.05%

Like Si, P has both solid solution strengthening and ferrite transformation promotion effects. However, if the content is less than 0.001%, the manufacturing cost is high, which is economically disadvantageous and insufficient to obtain strength. If the content exceeds 0.05%, brittleness occurs due to grain boundary segregation, microcracks are likely to occur during molding, and ductility is low. and greatly deteriorates the impact resistance characteristics. Therefore, it is desirable to limit the P content to 0.001 to 0.05%.

S: 0.001∼0.01%

The S is an impurity present in steel, and in order to manufacture it at a content of less than 0.001%, a lot of time is required during steelmaking operation, which reduces productivity. On the other hand, if it exceeds 0.01%, it combines with Mn, etc. to form non-metallic inclusions, and as a result, fine cracks are likely to occur during cutting and processing of steel. Therefore, it is desirable to limit the S content to 0.001 to 0.01%.

Sol.Al: 0.01∼0.1%

Sol.Al is an ingredient mainly added for deoxidation. If the content is less than 0.01%, the effect of the addition is insufficient. If the content exceeds 0.1%, it combines with nitrogen to form AlN, causing corner cracks in the slab during playing and casting. It is easy to do, and defects due to inclusions are likely to occur. Therefore, in the present invention, Sol. It is desirable to limit the Al content to 0.01-0.1%.

N: 0.001∼0.01%

N, together with C, is a representative solid solution strengthening element, and forms coarse precipitates together with Ti, Al, etc. In general, the solid solution strengthening effect of N is better than that of carbon, but as the amount of N in the steel increases, the toughness decreases significantly, so the upper limit is limited to 0.01%. On the other hand, manufacturing at less than 0.001% requires a lot of time during steelmaking operations, which may reduce productivity. Therefore, in the present invention, it is preferable to limit the N content to 0.001 to 0.01%.

Ti: 0.005∼0.12%

Ti is a representative precipitation strengthening element along with Nb and V, and forms coarse TiN in steel due to its strong affinity with N. TiN has the effect of suppressing grain growth during the heating process for hot rolling. In addition, TiC precipitates are formed by reacting with nitrogen and remaining Ti is dissolved in steel and combined with carbon, making it a useful ingredient in improving the strength of steel. However, if the Ti content is less than 0.005%, the above effect cannot be obtained, and if the Ti content exceeds 0.12%, there is a problem of poor formability due to the generation of coarse TiN and coarsening of TiC precipitates. Therefore, in the present invention, it is desirable to limit the Ti content to 0.005 to 0.12%.

Nb: 0.005∼0.05%

Nb is a representative precipitation strengthening element along with Ti and V, and is effective in improving the strength and impact toughness of steel through the grain refinement effect by precipitating during hot rolling and delaying recrystallization. However, if the Nb content is less than 0.005%, the above effect cannot be obtained, and if the Nb content exceeds 0.05%, the formability is inferior due to the formation of elongated grains and coarse composite precipitates due to excessive recrystallization delay during hot rolling. There is. Therefore, in the present invention, it is preferable to limit the Nb content to 0.005 to 0.05%.

Cr: 0.005∼0.5%

The Cr strengthens the steel by solid solution and delays the ferrite phase transformation upon cooling to help form bainite. However, if it is less than 0.005%, the above effect due to addition cannot be obtained, and if it exceeds 0.5%, ferrite transformation is excessively delayed and the elongation is inferior due to the formation of martensite phase. In addition, similar to Mn, the segregation zone at the center of the thickness is greatly developed, making the microstructure in the thickness direction non-uniform, resulting in poor elongation flangeability. Additionally, if the Cr content is too high, the corrosion resistance of the material may be inferior. Therefore, in the present invention, it is desirable to limit the Cr content to 0.005-0.5%.

Mo: 0.005~0.3%

Mo increases the hardenability of steel and facilitates the formation of bainite structure. However, if it is less than 0.005%, the above effect due to addition cannot be obtained, and if it exceeds 0.3%, a martensite phase is formed due to an excessive increase in hardenability, which sharply deteriorates formability. It is also economically disadvantageous and detrimental to weldability. Therefore, in the present invention, it is preferable to limit the Mo content to 0.005 to 0.3%.

Additionally, in the present invention, in some cases, one or more of V, Ni, and B may be included, and the total content is within 1.5%. Other ingredients and the remainder consist of iron and inevitable impurities.

In addition, the hot rolled steel sheet of the present invention may have a steel sheet microstructure including the sum of ferrite and bainite phases: 90% or more and the sum of remaining pearlite, martensite, and MA phases: less than 10%. If the phase fraction of the sum of the ferrite and bainite phases is less than 90 area%, which means that the sum of the remaining pearlite or martensite and MA phases exceeds 10%, there may be a problem of poor hole expandability. Hole expandability is greatly affected by the microstructural composition of the steel sheet. In particular, when the steel sheet is composed of a composite of soft and hard phases, it is greatly inferior due to the difference in hardness between each constituent phase. In particular, as the fraction of pearlite or martensite, which is a very hard microstructure, increases, it becomes easier for cracks to occur at the interface between phases during hole expansion, which deteriorates hole expandability, so it is necessary to limit the phase fraction.

The hot rolled steel sheet of the present invention having the microstructure as described above may have a tensile strength of more than 760 MPa, hole expandability (HER ₀ ) of more than 40%, and bake hardening (BH ₂ ) of more than 30 MPa.

In addition, it is possible to provide an excellent hot-rolled steel sheet without material deviation that satisfies △TS1, defined by the following relational equation 1, of 100 MPa or less.

[Relational Expression 1]

△TS1 = TS _max - TS _min

TS _max : Maximum TS value at 7 locations in the width direction, including the polar edge, within 30m of the rear end of the manufactured hot-rolled coil.

TS _min : Minimum TS value at 7 locations in the width direction, including the polar edge, within 30m of the rear end of the manufactured hot-rolled coil.

In addition, the hot rolled steel sheet of the present invention maintains a bake hardening amount (BH _h ) of 30 MPa or more after heat treatment at 300 to 600°C, and when defining △TS2 as in the following relational equation 2, the absolute value of △TS2 × BH _h ^-1 It can have high temperature bake hardening characteristics that satisfy this value of 0.7 or less. That is, the hot-rolled steel sheet of the present invention has a bake hardening amount (BH _h ) of 30 MPa or more even after heat treatment at 300 to 600°C, so the plated steel sheet can be effectively manufactured in the subsequent hot-dip galvanizing process, etc.

[Relational Expression 2]

△TS2 = TS _h - TS ₀

TS _h : Tensile strength after heat treatment, TS ₀ : Tensile strength before heat treatment

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

The method for manufacturing a hot rolled steel sheet of the present invention includes the steps of reheating a steel slab having the above-described alloy composition to a temperature range of 1100 to 1350°C; Manufacturing a hot rolled steel sheet by hot rolling the reheated steel slab in the range of 850 to 1150°C; Primary cooling the hot rolled steel sheet to a temperature in the range of 550 to 650° C. at an average cooling rate of 50 to 100° C./sec; stopping cooling of the primary cooled steel sheet for 3 to 7 seconds; And a step of secondary cooling the hot rolled steel sheet for which the primary cooling has been stopped at an average cooling rate of 1 to 30°C/sec to a temperature in the range of 400 to 500°C and then winding it.

reheat

First, in the present invention, a steel slab containing the above-described alloy composition is reheated to a temperature range of 1100 to 1350°C. At this time, if the reheating temperature is less than 1100°C, the resolubility rate of precipitates including Ti, Nb, Mo, and V decreases, thereby reducing the formation of fine precipitates in the process after hot rolling. If it exceeds 1350°C, the strength decreases due to coarsening of austenite grains, so it is desirable to limit the reheating temperature to 1100-1350°C.

hot rolling

Next, in the present invention, a hot rolled steel sheet is manufactured by hot rolling the reheated steel slab in the range of 850 to 1150°C. At this time, if hot rolling is started at a temperature higher than 1150°C, the temperature of the hot rolled steel sheet increases, the grain size becomes coarse, and the surface quality of the hot rolled steel sheet deteriorates. Additionally, if the hot rolling is completed at a temperature lower than 850°C, the elongated crystal grains develop due to excessive recrystallization delay, resulting in severe anisotropy and poor formability.

Primary cooling and maintenance

Then, the hot-rolled steel sheet is first cooled to a temperature in the range of 550-650°C at an average cooling rate of 50-100°C/sec, and then cooling of the primarily cooled steel sheet is stopped for 3-7 seconds.

That is, in the present invention, the hot rolled steel sheet is first cooled to a temperature in the range of 500 to 650° C. at an average cooling rate of 50 to 100° C./sec. More preferably, primary cooling is performed to a temperature in the range of 550 to 600°C. If first cooling is performed below 500℃, the amount of ferrite and precipitates within the ferrite may decrease when forming the final microstructure, resulting in a decrease in strength. On the other hand, if the primary cooling exceeds 650℃, some pearlite transformation may occur in the final microstructure, resulting in poor formability.

Additionally, in the present invention, it is preferable to control the average cooling rate at 50 to 100°C/sec during the primary cooling. If the cooling rate is less than 50℃/sec, the ferrite phase fraction may be formed too much, which is disadvantageous in securing strength. If the cooling rate exceeds 100℃/sec, the ferrite phase fraction may be formed in the area where the first cooling end temperature is low. It may decrease significantly, resulting in insufficient elongation.

Next, cooling of the primary cooled steel sheet is stopped for 3 to 7 seconds to ensure that the temperature of the hot rolled steel sheet satisfies a temperature range of 550 to 700°C, taking into account internal latent heat and transformation heat. At this time, if the cooling interruption time is less than 3 seconds, the effect of ferrite phase transformation and precipitation is minimal, and if it exceeds 7 seconds, the ferrite phase fraction in the microstructure increases significantly and the hard bainite and bainitic ferrite phases decrease, ultimately resulting in the desired microstructure. Organization cannot be obtained.

Secondary cooling and winding

Next, in the present invention, the hot rolled steel sheet whose primary cooling has been stopped is secondary cooled to a temperature in the range of 400 to 500°C at an average cooling rate of 1 to 30°C/sec and then coiled.

The cooling end temperature during the secondary cooling is preferably in the range of 400 to 500°C, and more preferably 430 to 470°C. If the secondary cooling end temperature is too high, bainite is not formed sufficiently, making it difficult to secure strength. On the other hand, if it is too low, bainite, martensite, and MA phases are formed in larger quantities than necessary, reducing the ductility and elongation of the steel. All intelligence becomes inferior.

In addition, it is preferable that the cooling rate during the secondary cooling is set to an average of 1 to 30°C/sec. If the cooling rate is too high, the MA phase is easily formed and an excessive bainite phase is formed, resulting in a decrease in elongation. There is no particular limitation on the lower limit of the cooling rate. However, in order to control the cooling rate slowly to less than 1°C/sec, separate cooling and heat insulation facilities are required, which may be economically disadvantageous. Taking this into consideration, the lower limit is set to 1°C/sec. It can be limited to .

room temperature cooling

Subsequently, in the present invention, the final hot rolled steel sheet can be manufactured by cooling the wound coil to a temperature ranging from room temperature to 200°C at an average cooling rate of 0.1 to 25°C/hr. At this time, if the cooling rate exceeds 25℃/hr, the MA phase is likely to be formed in the steel, which deteriorates the elongation flangeability of the steel. In order to control the cooling rate to less than 0.1℃/hr, separate heating equipment is required, making it economically disadvantageous. It's disadvantageous. Preferably, it is good to cool at 1 to 10°C/hr.

Additionally, if necessary, the present invention may further include the step of oiling the coiled hot-rolled steel sheet after pickling treatment.

Additionally, if necessary, the coiled hot-rolled steel sheet is pickled, heated to a temperature range of 450 to 750°C, and then placed in a plating bath containing 0.01 to 30% by weight of Mg, 0.01 to 50% of Al, and residual zinc. It may additionally include the step of forming a hot-dip galvanized layer on the surface by dipping.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for illustrating the implementation of the present invention and the present invention is not limited by the description of these examples.

(Example)

After preparing steel slabs having the alloy composition shown in Table 1 below, these slabs were reheated at 1200°C. Next, the reheated slabs were hot rolled, primary cooled and maintained, secondary cooled and coiled under the conditions shown in Table 2 below, and finally cooled to room temperature at a cooling rate of 8°C/hr to produce the final hot rolled steel product.

The microstructure composition and fraction of the hot rolled steel sheets manufactured as above were measured, and the results are shown in Table 3 below.

Specifically, the fractions of ferrite phase (F), bainite phase (B), martensite phase (M), and pearlite phase (P) were measured from the results of analysis at ×3000 and ×5000 magnifications using SEM. To read martensite and MA phases, analysis was performed at ×1000 magnification using an optical microscope and an image analyzer after Nital and Lepera etching.

In addition, the tensile strength (TS ₀ ), bake hardening (BH ₂ ), and hole expandability (HER ₀ ) of the manufactured hot rolled steel sheets were measured, and the results are shown in Table 3.

The tensile material and bake hardening amount were tested by collecting DIN standard test pieces in the rolling direction, and the tensile evaluation was conducted at room temperature.

The bake hardening amount was measured as the difference between the strength value when prestrained by 2% at room temperature and the strength value after heat treatment at 170°C for 20 minutes after prestraining by 2% and cooling to room temperature. In particular, when measuring the bake hardening amount, the strength after pre-straining and heat treatment at 170°C for 20 minutes was measured as the lower yield strength and the lower bake hardening amount.

And the hole expandability was expressed as the average value of the results evaluated three times at room temperature. Specifically, in each test, when a crack occurred visually, the test was stopped and the major axis length of the crack occurred was measured to evaluate hole expandability, which was measured regardless of the rolling direction of the specimen.

In addition, in order to measure the degree of material deviation within the same hot rolled steel coil, △TS1 was calculated using Equation 1, and the results are also shown in Table 3 below.

division Alloy composition (% by weight) C Si Mn Al P S N Ti Nb Cr Mo Comparison lecture 1 0.123 0.33 1.72 0.032 0.008 0.002 0.004 0.091 0.012 0.005 0.001 Comparison lecture 2 0.018 0.34 1.61 0.032 0.009 0.002 0.004 0.122 0.036 0.031 0.008 Comparison lecture 3 0.075 1.62 1.75 0.023 0.011 0.002 0.003 0.095 0.016 0.026 0.004 Comparison lecture 4 0.075 0.31 2.16 0.028 0.012 0.001 0.003 0.033 0.042 0.009 0.007 Comparison lecture 5 0.082 0.55 0.61 0.031 0.011 0.002 0.003 0.042 0.041 0.008 0.009 Invention Lecture 1 0.081 0.21 1.52 0.032 0.009 0.001 0.003 0.092 0.026 0.012 0.007 Invention Lecture 2 0.069 0.32 1.66 0.025 0.012 0.002 0.003 0.101 0.036 0.015 0.015 Invention Lecture 3 0.075 0.43 1.75 0.022 0.011 0.002 0.003 0.011 0.031 0.032 0.032 Invention Lecture 4 0.095 0.35 1.82 0.021 0.012 0.003 0.004 0.089 0.018 0.021 0.125 Invention Lecture 5 0.065 0.35 1.94 0.032 0.011 0.002 0.003 0.098 0.021 0.027 0.221

FDT(℃) Primary cooling
Holding time (sec) secondary cooling note cooling temperature
(℃) Cooling speed
(℃/s) Cooling end temperature (℃) Cooling speed
(℃/s) Comparison lecture 1 882 520 65 4.2 441 20 Comparative Example 1 Comparison lecture 2 892 552 5.1 432 Comparative example 2 Comparison lecture 3 902 621 3.9 492 Comparative example 3 Comparison lecture 4 903 631 6.2 482 Comparative example 4 Comparison lecture 5 895 582 5.5 452 Comparative Example 5 Invention Lecture 1 896 623 4.1 446 Invention Example 6 Invention Lecture 2 906 635 5.9 458 Invention Example 7 Invention Lecture 3 882 605 6.4 472 Invention Example 8 Bilmyeong River 4 907 534 4.9 429 Invention Example 9 Invention Lecture 5 896 579 3.5 439 Invention Example 10 Invention Lecture 3 905 736 4.2 465 Comparative Example 11 Invention Lecture 3 905 640 8.8 454 Comparative Example 12 Invention Lecture 4 895 607 5.2 621 Comparative Example 13 Invention Lecture 4 888 584 6.1 318 Comparative Example 14 Invention Lecture 5 885 599 2.1 419 Comparative Example 15

*In Table 2, FDT is the finishing hot rolling temperature.

division Microstructure (area%) mechanical properties note F B M M.A. P TS _o BH ₂ TS _o *BH ₂ HER _o △TS1 Comparison lecture 1 42 43 9 6 0 892 56 49952 21 152 Comparative Example 1 Comparison lecture 2 98 2 0 0 0 722 20 14440 46 85 Comparative example 2 Comparison lecture 3 82 10 7 One 0 796 21 16716 26 96 Comparative example 3 Comparison lecture 4 73 8 15 4 0 916 49 44884 32 122 Comparative example 4 Comparison lecture 5 95 4 One 0 0 731 22 16082 46 84 Comparative Example 5 Invention Lecture 1 71 24 3 2 0 796 52 41392 45 75 Invention Example 6 Invention Lecture 2 82 15 2 One 0 812 65 52780 52 66 Invention Example 7 Invention Lecture 3 69 28 2 One 0 825 71 58575 59 89 Invention Example 8 Invention Lecture 4 62 34 3 One 0 865 63 54495 43 52 Invention Example 9 Invention Lecture 5 63 34 2 One 0 851 55 46805 58 63 Invention Example 10 Invention Lecture 3 87 2 5 One 5 775 27 20925 26 139 Comparative Example 11 Invention Lecture 3 93 3 0 One 0 741 35 25935 22 142 Comparative Example 12 Invention Lecture 4 89 10 0 One 0 745 28 20860 43 91 Comparative Example 13 Invention Lecture 4 52 24 21 3 0 935 49 45815 18 151 Comparative Example 14 Invention Lecture 5 62 30 6 2 0 781 32 24992 35 109 Comparative Example 15

*In Table 3, F stands for ferrite, B stands for bainite, M stands for martensite, MA stands for island martensite, and P stands for pearlite. And △TS1 represents the difference in tensile strength according to equation 1.

As shown in Table 1-3 above, Inventive Examples 6-10 that satisfied the ingredient range and manufacturing conditions proposed in the present invention were all able to secure the target material. Specifically, the hot rolled steel sheets of Invention Example 6-10 all exhibit a tensile strength of more than 760 MPa, hole expandability (HER ₀ ) of more than 40%, and bake hardening amount (BH ₂ ) of more than 30 MPa, and the △TS1 value defined by relational equation 1 is It can be confirmed that excellent hot-rolled steel sheets without material deviation can be obtained by satisfying 100 MPa or less.

On the other hand, Comparative Examples 1-5 are cases where the component ranges suggested in the present invention were exceeded. In particular, the above comparative examples did not satisfy the appropriate content ranges of C, Si, and Mn, which have the greatest influence on microstructure and mechanical properties among steel components, and martensite and MA phases were often formed unnecessarily. , the appropriate precipitation strengthening effect was not obtained, resulting in the strength of the steel sheet being deteriorated or the hole expandability and bake hardenability being deteriorated. Specifically, in Comparative Examples 2 and 5, the C and Mn contents were insufficient, respectively, and a sufficient low-temperature phase fraction was not secured due to a decrease in hardenability, so the strength of the steel sheet did not satisfy 760 MPa. In addition, Comparative Examples 1 and 4 were cases in which the C and Mn contents were exceeded, respectively, and the low-temperature phase fraction was higher than the target due to excessive hardenability, resulting in excessively high strength and poor hole expandability.

Meanwhile, Comparative Examples 11-15 are cases where the manufacturing conditions are not satisfied even though the ingredient range proposed in the present invention is satisfied.

Specifically, in the case of Comparative Example 11, it can be confirmed that the first cooling temperature was too high and pearlite was generated in the microstructure. When a pearlite structure is created, the difference in hardness between microstructure phases intensifies and causes non-uniformity, resulting in inferior hole expandability and formability. This non-uniformity in microstructure also has the problem of causing material variation within the manufactured coil.

In Comparative Examples 12 and 15, the holding time after primary cooling was exceeded or shortened, respectively. If the holding time after primary cooling is excessively long, the bainite transformation is reduced due to excessive phase transformation of ferrite, which is a soft phase, and strength inferiority occurs due to coarsening of precipitates. Conversely, if the holding time is too short, the low-temperature transformation phase fraction is reduced. This was excessively increased and the production of precipitates was reduced, resulting in inferior hole expansion due to an increase in the hardness difference between microstructure phases.

In addition, it can be confirmed that in Comparative Example 13, the target tensile properties were not secured because the coiling temperature was higher than the appropriate manufacturing conditions proposed in the present invention.

(Example 2)

Additional heat treatment was performed on the steel sheets of Comparative Examples 1-5 and Inventive Examples 6-10 of Example 2 under the conditions shown in Table 4 below. That is, the hot rolled steel sheets were heat treated at the heat treatment temperature of 520°C for 8 minutes and then air cooled to room temperature. After performing this heat treatment, the mechanical properties of the steel sheet before and after heat treatment were evaluated and are shown in Table 4 below. Specifically, the tensile strength and bake hardening amount of the steel sheet after heat treatment were measured, and the results were compared with the tensile strength and bake hardening amount of the steel sheet before heat treatment in Example 1. At this time, the method of measuring the tensile strength and bake hardening amount after heat treatment is the same as in Example 1 described above.

division Additional heat treatment conditions Material before additional heat treatment Material after additional heat treatment △Absolute value of TS2 × BH _h ^-1 Temperature (℃) Time (min) TS _o (MPa) BH ₂ (MPa) TS _h (MPa) BH _h (MPa) Comparative Example 1 520 8 892 56 831 25 2.44 Comparative Example 2 520 8 722 20 705 4 4.25 Comparative Example 3 520 8 796 21 762 33 1.03 Comparative Example 4 520 8 916 49 853 30 2.10 Comparative Example 5 520 8 731 22 726 2 2.50 Invention Example 6 520 8 796 52 790 42 0.14 Invention Example 7 520 8 812 65 801 38 0.29 Invention Example 8 520 8 825 71 799 42 0.62 Invention Example 9 520 8 865 63 849 37 0.43 Invention Example 10 520 8 851 55 841 49 0.20

*In Table 4, TS ₀ and BH ₂ represent the tensile strength and bake hardening amount before heat treatment, respectively.

TS _h and BH _h represent the tensile strength and bake hardening amount after heat treatment.

△TS2 = TS _h - TS ₀

As shown in Table 4, in the case of the hot rolled steel sheet of Example 6-10 of the present invention, the bake hardening amount (BH _h ) after additional heat treatment is maintained at 30 MPa or more, and the absolute value of △TS2 × BH _h ^-1 described above is 0.7. It can be confirmed that the hot rolled steel sheet can be effectively applied to various plating processes by satisfying the following.

On the other hand, in Comparative Example 1-5, as the target microstructure was not obtained, it can be seen that the changes in strength and bake hardenability after heat treatment were relatively large compared to Invention Example 6-10. In particular, when the hardenability element content is too high, as in the case of Comparative Examples 1 and 4, the fraction of low-temperature transformation phases in the microstructure increases, and these phases lose strength due to the tempering effect during additional heat treatment (300 to 600°C). It can be seen that the degradation occurs significantly. Therefore, in the case of hot rolled steel sheets having these microstructure characteristics, it may be difficult to use them for additional heat treatment for plating.

Figure ¹ is a diagram showing the change in absolute value of △ _TS2 As shown in Figure 1, it can be confirmed that the hot rolled steel sheet of the present invention has excellent properties with an absolute value of △TS2 × BH _h ^-1 of 0.7 or less.

Although the description has been made with reference to the above embodiments, various modifications and changes can be made to the present invention within the scope of the basic idea of the present invention by those skilled in the art, and the scope of the rights of the present invention is limited to the scope of the patent claims. It is specified that it should be interpreted based on

Claims (9)

In weight percent, C: 0.05 to 0.10%, Si: 0.01 to 1.0%, Mn: 1.4 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 0.5%, Mo: 0.005 to 0.3%, P: 0.001 to 0.05%, S: 0.001-0.01%, N: 0.001-0.01%, Nb: 0.005-0.05%, Ti: 0.005-0.12%, including the balance Fe and inevitable impurities,
It has a microstructure comprising the sum of ferrite and bainite phases: more than 90 area% and the sum of the remaining pearlite, martensite and MA phases: less than 10 area%;
A hot-rolled steel sheet that has a tensile strength of 760 MPa or more, hole expandability (HER ₀ ) of 40% or more, bake hardening (BH ₂ ) of 30 MPa or more, and △TS1 defined by the following relational equation 1 of 100 MPa or less.
[Relational Expression 1]
△TS1 = TS _max - TS _min
TS _max : Maximum TS value at 7 locations in the width direction, including the polar edge, within 30m of the rear end of the manufactured hot-rolled coil.
TS _min : Minimum TS value at 7 locations in the width direction, including the polar edge, within 30m of the rear end of the manufactured hot-rolled coil.
According to claim 1, after heat treatment at 300 to 600°C, the bake hardening amount (BH _h ) is maintained at 30 MPa or more, and when defining △TS2 as shown in equation 2 below, the absolute value of △TS2 × BH _h ^-1 is Hot rolled steel sheet with a thickness of 0.7 or less.
[Relational Expression 2]
△TS2 = TS _h - TS ₀
TS _h : Tensile strength after heat treatment, TS ₀ : Tensile strength before heat treatment
The hot rolled steel sheet according to claim 1, comprising at least one of V, Ni, and B in a total amount of 1.5% or less.
The hot-rolled steel sheet according to claim 1, wherein a hot-dip galvanized layer is formed on the surface.
In weight percent, C: 0.05 to 0.10%, Si: 0.01 to 1.0%, Mn: 1.4 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.005 to 0.5%, Mo: 0.005 to 0.3%, P: 0.001 to Steel slabs containing 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.12%, the balance Fe and inevitable impurities are heated to a temperature range of 1100 to 1350°C. reheating;
Manufacturing a hot rolled steel sheet by hot rolling the reheated steel slab in the range of 850 to 1150°C;
Primary cooling the hot rolled steel sheet to a temperature in the range of 550 to 650° C. at an average cooling rate of 50 to 100° C./sec;
stopping cooling of the primary cooled steel sheet for 3 to 7 seconds; and
Secondary cooling the hot rolled steel sheet for which the primary cooling has been stopped at an average cooling rate of 1 to 30°C/sec to a temperature in the range of 400 to 500°C and then winding it; A hot rolled steel sheet manufacturing method comprising.
The method of claim 5, wherein the total content of at least one of V, Ni, and B is 1.5% or less.
The method of claim 5, further comprising cooling the wound coil to a temperature in the range of room temperature to 200°C at an average cooling rate of 0.1 to 25°C/hr.
The method of claim 5, further comprising the step of oiling the coiled hot-rolled steel sheet after pickling treatment.
The method of claim 5, further comprising the step of pickling the wound hot-rolled steel sheet, heating the coiled hot-rolled steel sheet to a temperature range of 450 to 750° C., and then hot-dip galvanizing the coiled steel sheet.

Images