EP4640900A1

EP4640900A1 - Hot rolled steel sheet and method for manufacturing same

Info

Publication number: EP4640900A1
Application number: EP23907636.7A
Authority: EP
Inventors: Je-Woong LEE; Sung-Il Kim; Il-Cheol YI
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-12-15
Publication date: 2025-10-29
Also published as: WO2024136351A1; EP4640900A4; KR20240098897A; JP2025541925A; MX2025007290A; CN120476223A

Abstract

Provided are a hot rolled steel sheet and a method for manufacturing same. This hot rolled steel sheet contains, in wt%, 0.050-0.100% of C, 0.01-1.00% of Si, 1.4-2.0% of Mn, 0.010-0.100% of Al, 0.005-0.500% of Cr, 0.005-0.300% of Mo, 0.001-0.050% of P, 0.001-0.010% of S, 0.001-0.010% of N, 0.005-0.050% of Nb, and 0.005-0.120% of Ti, with the remainder comprising Fe and inevitable impurities, and has a microstructure including at least 90 area% in total of ferrite and bainite phases, and less than 10 area% in total of residual pearlite, martensite, and MA phases.

Description

Technical Field

The present disclosure relates to a high-strength composite structure hot rolled steel sheet having excellent formability, hardenability and material uniformity, which may be applicable to members of automobile chassis parts and parts used in a lower arm, a reinforcement, a connecting material, and a frame, and a method for manufacturing the same.

Background Art

A general high-strength hot rolled steel sheet used for automobile chassis and frames may be required to have excellent formability considering the shape of parts while high-strength thinning has been carried out for weight reduction, and also, to maximize durability of parts, bake-hardenability indicating an increase in yield strength after painting heat treatment has been necessary.
Also, the use of hot-rolled plating materials has increased to increase corrosion resistance of a hot-rolled chassis part, and in order to manufacture a hot-rolled plating material, only additional heat treatment may be used after hot rolling. This additional heat treatment may change a microstructure of the hot rolled steel sheet, and in order to satisfy the desired final microstructure and properties, it may be important to appropriately determine an alloy component, a hot-rolled microstructure, and heat treatment conditions.
Cited document 1 relates to a composite structure steel having bainitic ferrite and granular bainitic ferrite, which are low-temperature zone-generated ferrite phases, as the base structure, but since Cu may need to be used to ensure additional strength, surface defects and high-temperature embrittlement may occur during hot rolling, and Ni may need to be added to prevent this.
Cited document 2 relates to ensuring the strength of a welded heat-affected zone by adding Ti, Nb, Cr, Mo, or the like. Specifically, during arc welding, a welding material melted by welding heat and a heat-affected zone adjacent to the welding material may be heated at a high temperature of 600°C or higher, and in particular, in some cases, the material may be heated to a temperature higher than that of the austenite region, such that the technique may be to secure strength by increasing hardenability of steel by adding Cr and Mo and forming low-temperature phases such as bainite and martensite phases during cooling. However, the concept based on maximizing hardenability as in this document may have limitations in being applied to an automotive steel sheet requiring high formability, even after heat treatment, if desired after manufacturing the steel sheet.

[Prior art]

[Patent Reference]

(Cited document 1) Korean Registered Patent Publication No. 10-1114672
(Cited document 2) Korean Registered Patent Publication No. 10-0962745

Detailed description of present disclosure

Technical problems to solve

An aspect in the present disclosure is to provide a high-strength composite structure hot rolled steel sheet having excellent hardenability and material uniformity and a method for manufacturing the same.
The technical problems to be achieved in the present disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by a person having ordinary skill in the technical field to which the present disclosure belongs from the description below.

Solution to Problem

An aspect in the present disclosure provides a hot rolled steel sheet comprising

by weight%, C:0.050-0.100%, Si:0.01-1.00%, Mn:1.4-2.0%, Al:0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P:0.001-0.050%, S:0.001-0.010%, N:0.001-0.010%, Nb: 0.005-0.050%, Ti: 0.005-0.120%, and a balance of Fe and inevitable impurities;
the hot rolled steel sheet having a microstructure comprising ferrite and bainite phases: 90 area% or more in total, and the remainder of pearlite, martensite and MA phases: less than 10 area% in total; and
the hot rolled steel sheet having a tensile strength of 760MPa or more, a hole expandability (HER₀) of 40% or more, and a bake hardening amount (BH₂) of 30MPa or more, and ΔTS1 defined by relational expression 1 satisfies 100MPa or lower:

$ΔTS 1 = {TS}_{\max} - {TS}_{\min}$
TS_max : maximum TS value in 7 positions in a width direction, including a pole edge, within 30m of a rear end of a manufactured hot-rolled coil
TS_min : minimum TS value in 7 positions in a width direction, including a pole edge, within 30m of a rear end of a manufactured hot-rolled coil
A bake hardening amount (BH_h) after heat treatment at 300-600°C may be maintained at 30 MPa or more, and when defining ΔTS2 as in relational expression 2 below, an absolute value of ΔTS2 × BH_h ^-1 is 0.7 or less.

ΔTS 2 = {TS}_{h} - {TS}_{0}

_h

₀

One or more of components V, Ni, and B may be included in a total amount of 1.5% or less.
A hot dip galvanized plating layer is formed on a surface thereof.
An aspect in the present disclosure provides a method of manufacturing a hot rolled steel sheet including

reheating a steel slab having the alloy composition described above in a temperature range of 1100-1350°C;
manufacturing a hot rolled steel sheet by hot rolling the reheated steel slab in a range of 850-1150°C;
primary-cooling the hot rolled steel sheet to a temperature in a range of 550-650°C at an average cooling rate of 50-100°C/sec;
stopping the cooling of the primary-cooled steel sheet for 3-7 seconds; and
secondary-cooling the hot rolled steel sheet, on which the primary-cooling is stopped, to a temperature in a range of 400-500°C at an average cooling rate of 1-30°C/sec, and coiling the steel sheet.

The method may further include cooling the coiled coil to a temperature in a range of room temperature-200°C at an average cooling rate of 0.1-25°C/hr.
The method may further include pickling and oiling the coiled hot rolled steel sheet.
The method may further include pickling the coiled hot rolled steel sheet, heating the steel sheet in a temperature range of 450-750°C, and performing hot-dip galvanizing the steel sheet.

Advantageous Effects of Invention

According to an aspect in the present disclosure, a steel sheet providing a tensile strength of 760 MPa or more, a hole expandability (HER₀) value of 40% or more, a bake hardening amount (BH₂) of 30 MPa or more, and a bake hardening amount (BH_h) of 30 MPa or more after heat treatment at 300-600°C, and having excellent material uniformity may be effectively provided.
Accordingly, the steel sheet may be effectively applied to members of automobile chassis parts and parts used in a lower arm, a reinforcement, a connecting material, and a frame.

Brief Description of Drawings

FIG. 1 is an image indicating changes in an absolute value of ΔTS2 × BH_h ^-1 before and after a 520°C heat treatment for steel sheets of comparative examples 1-5 and inventive examples 6-10 according to an embodiment in the present disclosure.

Best Mode for Invention

Hereinafter, the present disclosure will be described.
The present disclosers studied various steels with various components and different microstructures to expand applicability of hot-rolled chassis parts, and deduced that the control over microstructure by precise temperature and time control in the hot-rolled manufacturing process may be the most important factor in determining the level of material deviation in the coil. From the results, the main component range of the steel was determined such that the hot rolled steel sheet had excellent material uniformity, and that, as for composition phase, martensite and MA (martensite & austenite) phases and pearlite were 10% or less in total, and as the remainder, ferrite and bainite phases were 90% or more in total. Accordingly, it was confirmed that a high-strength composite structure hot rolled steel sheet which ensures a tensile strength of 760 MPa or more, and having a hole expandability (HER₀) value of 40% or more and a bake hardening amount (BH₂) of 30 MPa or more, and the present disclosure was suggested. The hot rolled steel sheet in the present disclosure may have no material deviation and may maintain a bake hardening amount (BH_h) of 30MPa or more even after heat treatment at 300-600°C, such that the steel sheet may be effectively used in manufacturing a plated hot rolled steel sheet such as a hot dip galvanizing, or the like.
The hot rolled steel sheet in the present disclosure may comprise, by weight%, C:0.050-0.100%, Si:0.01-1.00%, Mn:1.4-2.0%, Al:0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P:0.001-0.050%, S:0.001-0.010%, N:0.001-0.010%, Nb: 0.005-0.050%, Ti: 0.005-0.120%, and a balance of Fe and inevitable impurities; the hot rolled steel sheet having a microstructure comprising ferrite and bainite phases: 90 area% or more in total, and the remainder of pearlite, martensite and MA phases: less than 10 area% in total; and the hot rolled steel sheet having a tensile strength of 760MPa or more, a hole expandability (HER₀) of 40% or more, and a bake hardening amount (BH₂) of 30MPa or more, and ΔTS1 defined by relational expression 1 satisfies 100MPa or lower.
In the description below, the composition of the steel sheet provided in the present disclosure may be described in detail. In this case, unless otherwise indicated, the content of each component may indicate weight%.

C: 0.050-0.100%

C may be the most economical and effective element for strengthening steel, and when the added amount increases, the precipitation strengthening effect or low-temperature phase fraction may increase, such that tensile strength may increase. However, when the content is less than 0.050%, a sufficient precipitation strengthening effect and low-temperature phase formation may be difficult, such that it may be difficult to ensure target strength and bake hardenability, and when the content exceeds 0.100%, excessive low-temperature phases and carbides may be formed, which may deteriorate formability, and may also increase the carbon equivalent, such that weldability may deteriorate. Also, depending on the characteristics of the microstructure generated when an excessive C content is added, tensile strength may decrease significantly after heat treatment due to the deterioration of the low-temperature phase and additional excess carbide during additional heat treatment after hot rolling. Accordingly, in the present disclosure, it may be preferable that the C content is included in the range of 0.05-0.10%, and more preferably, it may be included in the range of 0.050-0.080%.

Si: 0.01-1.00%

Si may deoxidize the molten steel, may have a solid-solution strengthening effect, and may be advantageous in improving formability by delaying formation of coarse carbide. Also, Si may also have an effect of suppressing formation of carbide during heat treatment in the range of 300-600°C. However, when the content is less than 0.01%, the effect of delaying the formation of carbides may be insignificant, such that it may be difficult to improve formability, and the effect of improving strength may be minimal. When the content exceeds 1.00%, red scale may be formed on the surface of the steel sheet during hot rolling due to Si, which may greatly deteriorate the surface quality of the steel sheet and may also reduce ductility and weldability, such that it may be preferable to limit the content to 1.00%. More preferably, the Si content may be controlled in the range of 0.02-0.08%.

Mn: 1.4-2.0%

Similarly to Si, Mn may be effective in solid-solution strengthening steel, and may further improve hardenability of steel, thereby delaying ferrite transformation at the same cooling rate and facilitating the formation of low-temperature phases such as bainite and martensite. However, when the content is less than 1.4%, the solid-solution strengthening and hardenability improvement effects may be insignificant, such that the intended strength increase effect may not be obtained. When the content exceeds 2.0%, hardenability may increase significantly, such that the martensite phase fraction may exceed the intended purpose, and accordingly, the segregation zone may develop significantly in the center in the thickness direction during slab casting in the continuous casting process, such that formability may degrade, and further, welding quality may deteriorate. Accordingly, in the present disclosure, it may be preferable to limit the content of the Mn to 1.4-2.0%, and more preferably, it may be limited to the range of 1.5-1.9%.

P : 0.001-0.050%

Similarly to Si, P may simultaneously have the effects of promoting solid-solution strengthening and ferrite transformation. However, in order to manufacture the steel sheet with P in an amount less than 0.001%, manufacturing costs may increase, which may be economically disadvantageous, and it may also be insufficient to obtain strength. When the content exceeds 0.050%, brittleness may occur due to grain boundary segregation, and fine cracks may be easily created during forming, which may significantly deteriorate ductility and impact resistance properties. Accordingly, it may be preferable to limit the content of P to 0.001-0.050%. More preferably, the content may be limited to the range of 0.002-0.004%.

S : 0.001-0.010%

S may be impurities present in steel, and when the content is less than 0.001%, it may take a great deal of time during steelmaking, which may reduce productivity. When the content exceeds 0.010%, S may be combined with Mn, and may form non-metallic inclusions, and accordingly, fine cracks may be easily created during cutting processing of steel. Accordingly, it may be preferable to limit the content of S to 0.001-0.01%. More preferably, the content may be limited to the range of 0.002-0.008%.

Sol.Al : 0.010-0.100%

Sol.Al may be mainly added for deoxidation, and when the content is less than 0.010%, the effect of addition may be insufficient, and when the content exceeds 0.100%, Sol.Al may be combined with nitrogen and may form AlN, such that corner cracks may be created in the slab during continuous casting and casting, and defects due to inclusion formation may occur. Accordingly, in the present disclosure, it may be preferable to limit the Sol.Al content to 0.010-0.100%.

N : 0.001-0.010%

N may be a representative solid-solution strengthening element along with C, and may form coarse precipitates along with Ti, Al, or the like. Generally, the solid-solution strengthening effect of N may be more excellent than that of carbon, but as the amount of N in steel increases, there is a problem that toughness may decrease significantly, and thus, an upper limit thereof may be limited to 0.010%. In order to manufacture the steel sheet with N in less than 0.001%, it may take a lot of time during the steelmaking process, which may lower productivity. Accordingly, in the present disclosure, it may be preferable to limit the N content to 0.001-0.010%. More preferably, the content may be limited to the range of 0.001-0.008%.

Ti: 0.005-0.120%

Ti may be a representative precipitation strengthening element along with Nb and V, and may form coarse TiN in steel due to strong affinity with N. TiN may have the effect of suppressing grain growth during the heating process for hot rolling. Also, the remaining Ti reacting with nitrogen may be solid solute in the steel and may combine with carbon, and TiC precipitate may be formed, such that Ti may be a useful component for improving strength of the steel. However, when the Ti content is less than 0.005%, the effect may not be obtained, and when the Ti content exceeds 0.120%, formability may degrade due to the generation of coarse TiN and coarsening of TiC precipitates. Accordingly, in the present disclosure, it may be preferable to limit the Ti content to 0.005-0.120%. More preferably, the content may be limited to the range of 0.010-0.100%.

Nb: 0.005-0.050%

Nb may be a representative precipitation strengthening element along with Ti and V, and may be effective in improving strength and impact toughness of steel due to grain refinement effect resulted from delay in recrystallization by precipitation during hot rolling. However, when the Nb content is less than 0.005%, the effect may not be obtained, and when the Nb content exceeds 0.050%, formability may degrade due to formation of elongated grains and coarse composite precipitate due to excessive recrystallization delay during hot rolling. Accordingly, in the present disclosure, it may be preferable to limit the Nb content to 0.005-0.050%. More preferably, the content may be limited to the range of 0.007-0.040%.

Cr: 0.005-0.500%

Cr may allow solid-solution strengthening of steel and may delay ferrite phase transformation during cooling, thereby contributing to form bainite. However, when the content is less than 0.005%, the effect of addition may not be obtained, and when the content exceeds 0.500%, ferrite transformation may be excessively delayed, such that elongation may be deteriorated due to martensite phase formation. Also, similarly to Mn, a segregation zone in the center of thickness may be greatly developed, and a microstructure in the thickness direction may become nonuniform, such that stretch-flangeability may degrade. Also, when the Cr content is excessively high, corrosion resistance of the material may be deteriorated. Accordingly, in the present disclosure, it may be preferable to limit the Cr content to 0.005-0.500%. More preferably, the content may be limited to the range of 0.010-0.400%.

Mo: 0.005-0.300%

Mo may increase hardenability of steel and may facilitate formation of a bainite structure. However, when the content is less than 0.005%, the effect of addition may not be obtained, and when the content exceeds 0.300%, the martensite phase may be formed due to an excessive increase in hardenability, which may rapidly deteriorate formability, which may be economically disadvantageous and detrimental to weldability. Accordingly, in the present disclosure, it may be preferable to limit the content of Mo to 0.005-0.3%. More preferably, the content may be limited to the range of 0.007-0.250%.
Also, in the present disclosure, one or more components of V, Ni, and B may be included in some cases, and the total content may be within 1.5%. Other components and the remainder may include iron and inevitable impurities.
Ni and B may effectively improve hardenability of a steel material. When these elements are added, the ferrite transformation may be delayed during cooling, such that it may be easy to ensure a low-temperature transformation phase (bainite, martensite, or the like), and thus, there may be an advantage of increasing the final strength of the material by increasing the low-temperature transformation phase fraction. Also, similarly to Ni and Ti, V may be added as a precipitate element combined with C, forming precipitates and increasing the strength of the material using the precipitation strengthening effect.
Also, the hot rolled steel sheet in the present disclosure may have a steel sheet microstructure comprising ferrite and bainite phases: 90% or more in total, and the remainder of pearlite, martensite, and MA phases: less than 10% in total. When the combined phase fraction of ferrite and bainite is less than 90 area%, the remaining pearlite or martensite and MA phases may exceed 10%, which may degrade hole expandability. Hole expandability may be greatly affected by the microstructure composition of the steel sheet, and especially when soft and hard phases are complexly formed in the steel sheet, deterioration may occur due to the difference in hardness between the phases. In particular, as the fraction of pearlite or martensite, which are hard microstructures, increases, cracks may be easily created at the interfacial surface between phases during hole expansion, such that hole expandability may be deteriorated, such that it may be necessary to limit the phase fraction.
The hot rolled steel sheet having the microstructure described above in the present disclosure may have a tensile strength of 760 MPa or more, a hole expandability (HER₀) of 40% or more, and a bake hardening amount (BH₂) of 30 MPa or more.
Also, an excellent hot rolled steel sheet without material deviation, in which ΔTS1 defined by relational expression 1 as below satisfies 100MPa or lower may be provided. $ΔTS 1 = {TS}_{\max} - {TS}_{\min}$

TS_max : maximum TS value in 7 positions in a width direction, including a pole edge, within 30m of a rear end of a manufactured hot-rolled coil
TS_min : minimum TS value in 7 positions in a width direction, including a pole edge, within 30m of a rear end of a manufactured hot-rolled coil

Also, the hot rolled steel sheet in the present disclosure may have high-temperature bake hardening properties in which the bake hardening amount (BH_h) maintains 30 MPa or more after heat treatment at 300-600°C, and when defining ΔTS2 as in relational expression 2 below, the absolute value of ΔTS2 × BH_h ^-1 satisfies 0.7 or less. That is, since the hot rolled steel sheet in the present disclosure has a bake hardening amount (BH_h) of 30MPa or more even after a heat treatment at 300-600°C, it may effectively manufacture a plated steel sheet in a subsequent hot-dip galvanizing process, or the like. $ΔTS 2 = {TS}_{h} - {TS}_{0}$
TS_h : tensile strength after heat treatment, TS₀: tensile strength before heat treatment
Thereafter, the method of manufacturing a hot rolled steel sheet according to the preferable embodiment in the present disclosure may be described in detail.
The method of manufacturing a hot rolled steel sheet in the present disclosure may comprise include reheating a steel slab having the alloy composition described above in a temperature range of 1100-1350°C; manufacturing a hot rolled steel sheet by hot rolling the reheated steel slab in a range of 850-1150°C; primary-cooling the hot rolled steel sheet to a temperature in a range of 550-650°C at an average cooling rate of 50-100°C/sec; stopping the cooling of the primary-cooled steel sheet for 3-7 seconds; and secondary-cooling the hot rolled steel sheet, to which the primary-cooling is stopped, to a temperature in a range of 400-500°C at an average cooling rate of 1-30°C/sec, and coiling the steel sheet.

Reheating

First, in the present disclosure, the steel slab having the alloy composition described above may be reheated in the temperature range of 1100-1350°C. In this case, when the reheating temperature is lower than 1100°C, the re-solid-solution rate of precipitates including Ti, Nb, Mo and V may decrease, such that the formation of fine precipitates may be reduced in the process after hot rolling. When the content exceeds 1350°C, strength may decrease due to coarsening of austenite grains, and accordingly, it may be preferable to limit the reheating temperature to 1100-1350°C.

Hot rolling

Thereafter, in the present disclosure, the hot rolled steel sheet may be manufactured by hot rolling the reheated steel slab in the range of 850-1150°C. In this case, when hot rolling is started at a temperature higher than 1150°C, the temperature of the hot rolled steel sheet may increase, such that the grain size may become coarse and the surface quality of the hot rolled steel sheet may deteriorate. Also, when the hot rolling is terminated at a temperature lower than 850°C, elongated grains may develop due to excessive recrystallization delay, such that anisotropy may become severe and formability may deteriorate.

Primary-cooling and holding

Thereafter, the hot rolled steel sheet may be primary-cooled to a temperature in the range of 550-650°C at an average cooling rate of 50-100°C/sec, and thereafter, the primary-cooled steel sheet may be stopped from cooling for 3-7 seconds.
That is, in the present disclosure, the hot rolled steel sheet may be primary-cooled to a temperature in the range of 500-650°C at an average cooling rate of 50-100°C/sec. More preferably, the primary-cooling may be performed to a temperature in the range of 550-600°C. When the primary-cooling is performed below 500°C, the amount of ferrite and precipitate in ferrite may decrease when forming the final microstructure, such that strength may decrease. When the primary-cooling is performed above 650°C, some pearlite transformation may occur when forming the final microstructure, such that formability may degrade.
Also, in the present disclosure, it may be preferable to control the average cooling rate during the primary-cooling to 50-100°C/sec. When the cooling rate is less than 50°C/sec, the ferrite phase fraction may be formed excessively, which is disadvantageous for ensuring strength, and when the cooling rate exceeds 100°C/sec, the ferrite phase fraction may be greatly reduced in the area in which the primary-cooling end temperature is low, such that elongation may be insufficient.
Thereafter, by stopping the cooling of the primary-cooled steel sheet for 3-7 seconds, the temperature of the hot rolled steel sheet may be allowed to satisfy the temperature range of 550-700°C considering the internal latent heat and transformation heat. When the cooling stop time is less than 3 seconds, the effect of ferrite phase transformation and precipitation may be minimal, and when the content exceeds 7 seconds, the ferrite phase fraction in the microstructure may increase significantly and the hard phases, bainite and bainitic ferrite phases, may decrease, such that the desired microstructure may not be obtained.

Secondary-cooling and coiling

Thereafter, in the present disclosure, the hot rolled steel sheet to which the primary cooling is stopped may be cooled down to a temperature range of 400-500°C at an average cooling rate of 1-30°C/sec, and may be coiled.
It may be preferable that the secondary-cooling end temperature is in the range of 400-500°C, and 430-470°C may be more preferable. When the secondary-cooling end temperature is too high, bainite may not be sufficiently formed, and it may be difficult to ensure strength. When the temperature is too low, the bainite phase, martensite phase, and MA phase may be formed in excessive amounts, ductility and stretch-flangeability of the steel may deteriorate.
Also, it may be preferable that the secondary-cooling cooling rate is determined to an average of 1-30°C/sec. When the cooling rate is too high, the MA phase may be easily formed, and excessive bainite phase may be formed, which may reduce the elongation. a lower limit of the cooling rate may not be limited to any particular rate, but in order to control the cooling rate to less than 1°C/sec, separate cooling and heat preservation equipment, or the like, may be necessary, which may be economically disadvantageous. Thus, the lower limit may be limited to 1°C/sec in consideration of this.

Room temperature cooling

Subsequently, in the present disclosure, the coiled coil may be cooled to a temperature in the range of room temperature - 200°C at an average cooling rate of 0.1-25°C/hr and the final hot rolled steel sheet may be manufactured. In this case, when the cooling rate exceeds 25°C/hr, the MA phase may be easily formed in the steel, such that stretch-flangeability of the steel may deteriorate, and in order to control the cooling rate to less than 0.1°C/hr, separate heating equipment, or the like, may be necessary, which may be economically disadvantageous. Preferably, the cooling may be performed at 1-10°C/hr.
Also, the present disclosure may additionally include pickling and oiling the coiled hot rolled steel sheet if desired.
Also, in the present disclosure, after pickling the coiled hot rolled steel sheet, the method may further include heating the steel sheet to a temperature range of 450-750°C, immersing the steel sheet in a plating bath containing 0.01-30% Mg, 0.01-50% Al, and the remainder zinc by weight% and form a hot-dip galvanizing layer on the surface.

Mode for Invention

Hereinafter, the present disclosure may be described more specifically through examples. However, it should be noted that the examples as below are only intended to describe the present disclosure in greater detail, and are not intended to limit the scope of the rights in the present disclosure. It may be noted that the scope of the rights in the present disclosure is determined by the subject matters described in the claims and reasonably inferred therefrom.

(Example 1)

After preparing steel slabs having the alloy compositions listed in Table 1 below, these slabs were reheated at 1200°C. Thereafter, the reheated slabs were hot rolled, primary-cooled, held, secondary-cooled and coiled under the conditions listed in Table 2 below, and finally cooled to room temperature at a cooling rate of 8°C/hr, thereby manufacturing the final hot rolled steel sheet product.
The microstructure composition and fraction of the hot rolled steel sheets manufactured as described above were measured, and the results are listed in Table 3 below.
Specifically, the ferrite phase (F), bainite phase (B), martensite phase (M), and pearlite phase (P) fractions were measured from the results of analysis using SEM at ×3000 and ×5000 magnifications. To identify martensite and MA phase, after etching with Nital and Lepera, analysis was performed at ×1000 magnification using an optical microscope and an image analyzer.
Also, the tensile strength (TS₀), bake hardening amount (BH₂), and hole expandability (HER₀) of the manufactured hot rolled steel sheet were measured, and the results are listed in Table 3.
The tensile strength and a bake hardening amount are the results of tests by collecting DIN standard test samples in the rolling direction, and the tensile evaluation was performed at room temperature.
The bake hardening amount was measured as the difference between the strength value in 2% pre-strain at room temperature and the strength value after heat treatment at 170°C for 20 minutes after 2% pre-strain and cooling to room temperature. In particular, when measuring the bake hardening amount, the strength was measured as the lower yield strength after heat treatment at 170°C for 20 minutes after pre-strain, and the lower bake hardening amount was measured.
The hole expandability was the average value of the results of a total of three evaluations at room temperature. Specifically, the test was stopped when a crack was created visually in each test, and the long axis length of the crack was measured, thereby evaluating the hole expandability, and this was measured regardless of the rolling direction of the sample.

Also, in order to measure the degree of material deviation in the same hot rolled steel sheet coil, ΔTS1 was calculated by relational expression 1, and the results are also listed in Table 3 below. one or more of components V, Ni, and B are included in a total amount of 1.5% or less.

[Table 1]

Classi ficati on	Alloy composition (weight%)
Classi ficati on	C	Si	Mn	Al	P	S	N	Ti	Nb	Cr	Mo
Compar ative steel1	0.123	0.33	1.72	0.032	0.008	0.002	0.004	0.091	0.012	0.005	0.001
Compar ative steel2	0.018	0.34	1.61	0.032	0.009	0.002	0.004	0.122	0.036	0.031	0.008
Compar ative steel3	0.075	1.62	1.75	0.023	0.011	0.002	0.003	0.095	0.016	0.026	0.004
Compar ative steel4	0.075	0.31	2.16	0.028	0.012	0.001	0.003	0.033	0.042	0.009	0.007
Compar ative steel5	0.082	0.55	0.61	0.031	0.011	0.002	0.003	0.042	0.041	0.008	0.009
Invent ive steel1	0.081	0.21	1.52	0.032	0.009	0.001	0.003	0.092	0.026	0.012	0.007
Invent ive steel2	0.069	0.32	1.66	0.025	0.012	0.002	0.003	0.101	0.036	0.015	0.015
Invent ive steel3	0.075	0.43	1.75	0.022	0.011	0.002	0.003	0.011	0.031	0.032	0.032
Invent ive steel4	0.095	0.35	1.82	0.021	0.012	0.003	0.004	0.089	0.018	0.021	0.125
Invent ive steel5	0.065	0.35	1.94	0.032	0.011	0.002	0.003	0.098	0.021	0.027	0.221

[Table 2]

	FDT(°C)	primary-cooling		Holding time (sec)	secondary-cooling		Notes
	FDT(°C)	Cooling temperatur e (°C)	Cooling rate (°C/s)	Holding time (sec)	Cooling stop temperatur e (°C)	Cooling rate (°C/s)	Notes
Compara tive steel1	882	520	65	4.2	441	20	Compara tive example 1
Compara tive steel2	892	552		5.1	432		Compara tive example 2
Compara tive steel3	902	621		3.9	492		Compara tive example 3
Compara tive steel4	903	631		6.2	482		Compara tive example 4
Compara tive steel5	895	582		5.5	452		Compara tive example 5
Inventi ve steel1	896	623		4.1	446		Inventi ve example 6
Inventi ve steel2	906	635		5.9	458		Inventi ve example 7
Inventi ve steel3	882	605		6.4	472		Inventi ve example 8
Inventi ve steel4	907	534		4.9	429		Inventi ve example 9
Inventi ve steel5	896	579		3.5	439		Inventi ve example 10
Inventi ve steel3	905	736		4.2	465		Compara tive example 11
Inventi ve steel3	905	640		8. 8	454		Compara tive example 12
Inventi ve steel4	895	607		5.2	621		Compara tive example 13
Inventi ve steel4	888	584		6.1	318		Compara tive example 14
Inventi ve steel5	885	599		2.1	419		Compara tive example 15

*In Table 2, FDT is the final hot rolling temperature

[Table 3]

Classi ficati on	Microstructure (area%)					Mechanical properties					Notes
Classi ficati on	F	B	M	MA	P	TS_o	BH₂	TS_o*BH₂	HER_o	ΔTS1	Notes
Compar ative steel1	42	43	9	6	0	892	56	49952	21	152	Compara tive example 1
Compar ative steel2	98	2	0	0	0	722	20	14440	46	85	Compara tive example 2
Compar ative steel3	82	10	7	1	0	796	21	16716	26	96	Compara tive example 3
Compar ative steel4	73	8	15	4	0	916	49	44884	32	122	Compara tive example 4
Compar ative steel5	95	4	1	0	0	731	22	16082	46	84	Compara tive example 5
Invent ive steel1	71	24	3	2	0	796	52	41392	45	75	Inventi ve example 6
Invent ive steel2	82	15	2	1	0	812	65	52780	52	66	Inventi ve example 7
Invent ive steel3	69	28	2	1	0	825	71	58575	59	89	Inventi ve example 8
Invent ive steel4	62	34	3	1	0	865	63	54495	43	52	Inventi ve example 9
Invent ive steel5	63	34	2	1	0	851	55	46805	58	63	Inventi ve example 10
Invent ive steel3	87	2	5	1	5	775	27	20925	26	139	Compara tive example 11
Invent ive steel3	93	3	0	1	0	741	35	25935	22	142	Compara tive example 12
Invent ive steel4	89	10	0	1	0	745	28	20860	43	91	Compara tive example 13
Invent ive steel4	52	24	21	3	0	935	49	45815	18	151	Compara tive example 14
Invent ive steel5	62	30	6	2	0	781	32	24992	35	109	Compara tive example 15

* In Table 3, F represents ferrite, B represents bainite, M represents martensite, MA represents martensite and austenite Constituents, and P represents pearlite. ΔTS1 represents the tensile strength difference in accordance with relational expression 1.

As listed in tables 1-3, in inventive examples 6-10 satisfying the component range and manufacturing conditions suggested in the present disclosure, the target material was ensured. Specifically, the hot rolled steel sheets in inventive example 6-10 exhibited a tensile strength of 760 MPa or more, a hole expandability (HER₀) of 40% or more, and a bake hardening amount (BH₂) of 30 MPa or more, and the ΔTS1 value defined by relational expression 1 satisfied 100 MPa or lower, such that excellent hot rolled steel sheets without material deviation was obtained.
Comparative examples 1-5 exceeded the component range presented in the present disclosure. In particular, the comparative examples did not satisfy the appropriate content range of C, Si, and Mn, which have the greatest influence on determining the microstructure and mechanical properties among the steel components, and thus, the martensite phase and MA phase were often formed unnecessarily, and the appropriate precipitation strengthening effect was not obtained, such that the strength or hole expandability and bake hardenability of the steel sheet were deteriorated. Specifically, in comparative examples 2 and 5, when the C and Mn content were insufficient, sufficient low-temperature phase fraction was not ensured, thereby decreasing hardenability, such that the strength of the steel sheet did not satisfy 760 MPa. Also, in comparative examples 1 and 4, when the C and Mn content were exceeded, the low-temperature phase fraction was higher than the target due to excessive hardenability, such that strength may excessively increase, and accordingly, hole expandability was deteriorated.
Comparative examples 11-15 satisfied the component range suggested in the present disclosure, but did not satisfy the manufacturing conditions.
Specifically, in comparative example 11, it may be confirmed that pearlite was formed in the microstructure because the primary-cooling temperature was excessively high. When a pearlite structure was formed, the hardness difference between microstructures may become severe and non-uniformity may occur, such that hole expandability and formability may be deteriorated. The non-uniformity in the microstructure may also cause material deviation in the manufacturing coil, which may be problematic.
In comparative examples 12 and 15, the holding time after primary-cooling was excessive or insufficient. When the holding time after primary cooling was excessively long, the bainite transformation was reduced due to excessive phase transformation of the soft ferrite phase, and the precipitate was coarsened, so that the strength was deteriorated. When the holding time is too short, the low-temperature transformation phase fraction was excessively increased, the precipitate formation was reduced, and the hole expandability was deteriorated due to the increase in the hardness difference between the microstructure phases.
Also, it may be confirmed that, in comparative example 13, the coiling temperature was higher than the appropriate manufacturing conditions suggested in the present disclosure, such that the target tensile properties were not ensured.

(Example 2)

Additional heat treatment was performed on the steel sheets of comparative example 1-5 and inventive example 6-10 of embodiment 2 under the conditions listed in Table 4 below. That is, hot rolled steel sheets were heat-treated at a heat treatment temperature of 520°C for 8 minutes, and air-cooled to room temperature. Thereafter, after performing the heat treatment, the mechanical properties of the steel sheets before and after the heat treatment were evaluated, and also listed in Table 4 below. Specifically, the tensile strength and a bake hardening amount of the steel sheets after the heat treatment were measured, and the results were compared with the tensile strength and a bake hardening amount of the steel sheets before the heat treatment of Example 1. In this case, the method for measuring the tensile strength and a bake hardening amount after the heat treatment may be the same as in Example 1described above.

[Table 4]

Classific ation	Additional heat treatment conditions		Material before additional heat treatment		Material after additional heat treatment		Absolute value of ΔTS2 × BH_h ^-1
Classific ation	Temper ature (°C)	Time (min)	TS_o (MPa )	BH₂ (MPa )	TS_h (MPa)	BH_h (MPa)	Absolute value of ΔTS2 × BH_h ^-1
Comparati ve example 1	520	8	892	56	831	25	2.44
Comparati ve example 2	520	8	722	20	705	4	4.25
Comparati ve example 3	520	8	796	21	762	33	1.03
Comparati ve example 4	520	8	916	49	853	30	2.10
Comparati ve example 5	520	8	731	22	726	2	2.50
Inventive example 6	520	8	796	52	790	42	0.14
Inventive example 7	520	8	812	65	801	38	0.29
Inventive example 8	520	8	825	71	799	42	0.62
Inventive example 9	520	8	865	63	849	37	0.43
Inventive example 10	520	8	851	55	841	49	0.20

* In Table 4, TS₀ and BH₂ represent the tensile strength and a bake hardening amount before heat treatment, respectively.

TS_h and BH_h represent the tensile strength and a bake hardening amount after heat treatment. $ΔTS 2 = {TS}_{h} - {TS}_{0}$
As indicated in Table 4, in the case of the hot rolled steel sheet in inventive example 6-10, the bake hardening amount (BH_h) was maintained at 30 MPa or more after additional heat treatment, and the absolute value of ΔTS2 × BHh-1 satisfied 0.7 or less, such that the hot rolled steel sheet may be effectively applied to various plating processes.
In comparative example 1-5, it is indicated that, as the target microstructure was not obtained, the change in strength and bake hardenability after heat treatment was relatively large as compared to inventive example 6-10. In particular, as in comparative examples 1 and 4, when hardenability element content was excessively high, the fraction of the low-temperature transformation phase in the microstructure increased, and these phases may significantly decrease in strength due to the tempering effect during additional heat treatment (300-600°C). Accordingly, in the case of a hot rolled steel sheet having such microstructure characteristics, it may be difficult to use the steel sheet for additional heat treatment for plating.
FIG. 1 may be an image indicating changes in the absolute value of ΔTS2 × BH_h ^-1 before and after a 520°C heat treatment for the steel sheets of comparative example 1-5 and inventive example 6-10 in an Example of the present disclosure. As indicated in FIG. 1, it may be confirmed that the hot rolled steel sheet in the present disclosure had an absolute value of ΔTS2 × BH_h ^-1 of 0.7 or less, having excellent characteristics.
Although the present disclosure has been described in detail through embodiments above, other forms of embodiments may also be possible. Accordingly, the technical spirit and scope of the claims described below are not limited to the embodiments.

Claims

A hot rolled steel sheet, comprising:
by weight%, C:0.050-0.100%, Si:0.01-1.00%, Mn:1.4-2.0%, Al:0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P:0.001-0.050%, S:0.001-0.010%, N:0.001-0.010%, Nb: 0.005-0.050%, Ti: 0.005-0.120%, and a balance of Fe and inevitable impurities; and

the hot rolled steel sheet having a microstructure comprising ferrite and bainite phases: 90 area% or more in total, and the remainder of pearlite, martensite and MA phases: less than 10 area% in total; and

the hot rolled steel sheet having a tensile strength of 760MPa or more, a hole expandability (HER₀) of 40% or more, and a bake hardening amount (BH₂) of 30MPa or more, and ΔTS1 defined by relational expression 1 satisfies 100MPa or lower: $ΔTS 1 = {TS}_{\max} - {TS}_{\min}$

TS_max : maximum TS value in 7 positions in a width direction, including a pole edge, within 30m of a rear end of a manufactured hot-rolled coil

TS_min : minimum TS value in 7 positions in a width direction, including a pole edge, within 30m of a rear end of a manufactured hot-rolled coil.
The hot rolled steel sheet of claim 1, wherein a bake hardening amount (BH_h) after heat treatment at 300-600°C is maintained at 30 MPa or more, and when defining ΔTS2 as in relational expression 2 below, an absolute value of ΔTS2 × BH_h ^-1 is 0.7 or less. $ΔTS 2 = {TS}_{h} - {TS}_{0}$ TS_h : tensile strength after heat treatment, TS₀: tensile strength before heat treatment.
The hot rolled steel sheet of claim 1, wherein one or more of components V, Ni, and B are included in a total amount of 1.5% or less.
The hot rolled steel sheet of claim 1, wherein a hot dip galvanized plating layer is formed on a surface thereof.
A method of manufacturing a hot rolled steel sheet, the method comprising:
reheating a steel slab comprising, by weight%, C:0.050-0.100%, Si:0.01-1.00%, Mn:1.4-2.0%, Al:0.010-0.100%, Cr: 0.005-0.500%, Mo: 0.005-0.300%, P:0.001-0.050%, S:0.001-0.010%, N:0.001-0.010%, Nb: 0.005-0.050%, Ti: 0.005-0.120%, and a balance of Fe and inevitable impurities, in a temperature range of 1100-1350°C;

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

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

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

secondary-cooling the hot rolled steel sheet, on which the primary-cooling is stopped, to a temperature in a range of 400-500°C at an average cooling rate of 1-30°C/sec, and coiling the steel sheet.
The method of claim 5, wherein one or more of components V, Ni, and B are included in a total amount of 1.5% or less.
The method of claim 5, further comprising:
cooling the coiled coil to a temperature in a range of room temperature-200°C at an average cooling rate of 0.1-25°C/hr.
The method of claim 5, further comprising:
pickling and oiling the coiled hot rolled steel sheet.
The method of claim 5, further comprising:
pickling the coiled hot rolled steel sheet, heating the steel sheet in a temperature range of 450-750°C, and performing hot-dip galvanizing the steel sheet.