KR102031453B1 - Hot-rolled steel sheet and method for manufacturing the same - Google Patents

Abstract

One preferred aspect of the present invention is wt%, carbon (C): 0.03 ~ 0.08%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 ~ 1.9%, aluminum (Al) : 0.1% or less (excluding 0%), phosphorus (P): 0.04% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), chromium (Cr): 0.5% or less ( Copper (Cu): 0.5% or less (excluding 0%), Nickel (Ni): 0.5% or less, Molybdenum (Mo): 0.3% or less (including 0%), Nitrogen (N): 0.02 % Or less (excluding 0%), niobium (Nb): 0.03 to 0.07%, titanium (Ti): 0.001 to 0.03%, and vanadium (V): 0.001 to 0.07%, the Nb, Ti and The sum of the contents of V satisfies the following relation 1 and includes the balance iron (Fe) and other unavoidable impurities;
[Relationship 1]
0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)
The microstructure comprises (Nb, Ti, V) (C, N) and (Fe in volume%, including at least 90% spherical polygonal ferrite and at least one of the remaining perlite and cementite and satisfying the following relation: , Mn, Cr) ₃ C precipitates; And
[Relationship 2]
Vc / Vtot ≥ 0.80 (where Vc is the volume fraction of precipitates greater than 20 nm in diameter, Vtot is the volume fraction of total precipitates)
Provided is a hot rolled steel sheet having a tensile strength (TS) of 400 MPa to 550 MPa and a method of manufacturing the same.

Description

Hot rolled steel sheet and manufacturing method {HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a hot rolled steel sheet having a tensile strength of 400MPa to 550MPa and a manufacturing method thereof.

Steel plate with high hardness is used for structure, steel pipe and various parts because of its excellent durability and load bearing ability.

However, the use of such high strength requires a strict alloy composition control at the same time, even if some deviation from the alloy component manufacturing standards, there is often no problem to apply to a lower strength grade applications.

In addition, high-strength slabs that do not meet the heavy-duty requirements required for their original demand are often scraped, despite their high quality, which results in losses in terms of productivity and socio-environment.

Accordingly, by securing a technology for producing a general-purpose hot rolled steel sheet having a lower strength from slabs containing an alloy component capable of obtaining high strength, a technology for satisfying various demands and increasing overall productivity is required.

Republic of Korea Patent Publication No. 2013-0125821

One preferred aspect of the present invention is to provide a hot rolled steel sheet having a tensile strength of 400MPa to 550MPa.

Another preferred aspect of the present invention is to provide a method for producing a hot rolled steel sheet having a tensile strength of 400MPa to 550MPa.

According to a preferred aspect of the present invention, in wt%, carbon (C): 0.03 ~ 0.08%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 ~ 1.9%, aluminum ( Al): 0.1% or less (excluding 0%), phosphorus (P): 0.04% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), chromium (Cr): 0.5% Or less (excluding 0%), copper (Cu): 0.5% or less (excluding 0%), nickel (Ni): 0.5% or less, molybdenum (Mo): 0.3% or less (including 0%), nitrogen (N) : 0.02% or less (excluding 0%), niobium (Nb): 0.03 to 0.07%, titanium (Ti): 0.001 to 0.03%, and vanadium (V): 0.001 to 0.07%, the Nb, The sum of the contents of Ti and V satisfies the following relational formula 1, and includes residual iron (Fe) and other unavoidable impurities;

[Relationship 1]

0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)

(Nb, Ti, V) (C, N) and (Fe) containing at least 90% spherical polygonal ferrite and at least one of the remaining pearlite and cementite, in volume percent, and satisfying Equation 2 below: , Mn, Cr) ₃ C precipitates; And

[Relationship 2]

Vc / Vtot ≥ 0.80 (where Vc is the volume fraction of precipitates greater than 20 nm in diameter, Vtot is the volume fraction of total precipitates)

A hot rolled steel sheet having a tensile strength TS of 400 MPa to 550 MPa is provided.

According to another preferred aspect of the present invention, in wt%, carbon (C): 0.03 ~ 0.08%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 ~ 1.9%, aluminum (Al): 0.1% or less (excluding 0%), phosphorus (P): 0.04% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), chromium (Cr): 0.5 % Or less (excluding 0%), copper (Cu): 0.5% or less (excluding 0%), nickel (Ni): 0.5% or less, molybdenum (Mo): 0.3% or less (including 0%), nitrogen (N ): 0.02% or less (excluding 0%), niobium (Nb): 0.03 to 0.07%, titanium (Ti): 0.001 to 0.03%, and vanadium (V): 0.001 to 0.07% A sum of the contents of Nb, Ti, and V satisfies the following relation 1, preparing a slab including residual iron (Fe) and other unavoidable impurities;

[Relationship 1]

0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)

Heating the slab to a temperature of 1100-1250 ° C .;

Hot rolling the heated slab to a hot finishing rolling temperature condition of (A ₃ + 30 ° C.) or more to obtain a hot rolled steel sheet;

Cooling the hot rolled steel sheet at an average cooling rate of 30 ° C./sec or less; And

Provided is a method for manufacturing a hot rolled steel sheet including winding the hot rolled steel sheet cooled as described above in a temperature range of 650 ° C. or higher.

According to another preferred aspect of the present invention, there is an effect that can obtain a tensile strength (TS) 400MPa grade hot rolled steel of tensile strength (TS) 400MPa to 550MPa.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The prior art has focused on obtaining high strength by utilizing alloy components to the maximum, but the present invention provides a low-strength hot-rolled steel sheet and a method of manufacturing the same by applying an appropriate hot-rolling process to a steel sheet sufficiently containing alloying elements.

In particular, the hot-rolled steel sheet of the present invention is a component system containing Nb, Ti, and V, which are elements that cause precipitation hardening, and a technique of sufficiently lowering the strength thereof has not been proposed until now.

First, a hot rolled steel sheet according to one preferred aspect of the present invention will be described.

Hot-rolled steel sheet according to a preferred aspect of the present invention is wt%, carbon (C): 0.03 ~ 0.08%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 ~ 1.9%, Aluminum (Al): 0.1% or less (excluding 0%), Phosphorus (P): 0.04% or less (excluding 0%), Sulfur (S): 0.02% or less (excluding 0%), Chromium (Cr): 0.5% or less (except 0%), copper (Cu): 0.5% or less (except 0%), nickel (Ni): 0.5% or less, nitrogen (N): 0.02% or less (excluding 0%), molybdenum (Mo): 0.3% or less (including 0%), niobium (Nb): 0.03-0.07%, titanium (Ti): 0.001-0.03%, and vanadium (V): 0.001-0.07% The sum of the contents of Nb, Ti, and V satisfies the following relational formula 1, and includes residual iron (Fe) and other unavoidable impurities.

[Relationship 1]

0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)

Carbon (C): 0.03-0.08 wt% (hereinafter also referred to as "%")

When the carbon content is less than 0.03%, it is difficult to obtain strength equivalent to TS 400MPa class even if precipitation hardening elements such as Nb, Ti, and V are added. This is because carbon is a basic element that supports the strength of steel through solid solution strengthening and precipitation strengthening. In addition, when the carbon content exceeds 0.08%, the tensile strength exceeds 550MPa is out of the management range of 400MPa grade steels.

Silicon (Si): 0.5% or less (except 0%)

Silicon is the preferred alloying element in terms of increasing the strength of the steel and deoxidizing. However, when too much Si is added, the surface properties and plating properties of the steel are deteriorated.

Therefore, the content of silicon (Si) is preferably limited to 0.5% or less (excluding 0%).

Manganese (Mn): 1.3-1.9%

Mn is similar to that of carbon, and if it falls outside the lower and upper limits of Mn, respectively, it is outside the target strength range. Therefore, the content of manganese (Mn) is preferably limited to 1.3 ~ 1.9%.

Aluminum (Al): 0.1% or less (except 0%)

Aluminum is added for the deoxidation of the steel, and has a function of reducing the grain size, while if added too much, the upper limit is limited to 0.1% because it deteriorates the castability of the steel.

Phosphorus (P): 0.04% or less (except 0%)

P is present as an impurity in the steel or may be added intentionally to increase the strength of the steel. However, in the present invention, the upper limit is limited to 0.04% to prevent deterioration of the toughness of the steel when added excessively as the level of impurities.

Sulfur (S): 0.02% or less (excluding 0%)

Sulfur (S) is also present as an impurity in the steel, and the upper limit thereof is preferably limited to 0.02% because it degrades the ductility and impact characteristics of the steel.

Chromium (Cr): 0.5% or less (excluding 0%), copper (Cu): 0.5% or less (excluding 0%), nickel (Ni): 0.5% or less,

Chromium (Cr), nickel (Ni), and copper (Cu) are elements that increase the strength of steel, and in the present invention, the upper limit of the content of each element is limited to 0.5%.

Molybdenum (Mo): 0.3% or less (including 0%)

Molybdenum (Mo) is an element that greatly increases the hardenability of the steel material, but when added in an excessively large amount may exceed the target strength grade and is an expensive element, so the economic upper limit of the content is limited to 0.3%.

Nitrogen (N): 0.02% or less (excluding 0%)

Nitrogen (N) increases the strength, when excessively added to form a large amount of AlN precipitate adversely affects the rollability and toughness. Therefore, the content of nitrogen (N) is limited to 0.02% or less (excluding 0%).

Niobium (Nb): 0.03 to 0.07%, Titanium (Ti): 0.001 to 0.03% and Vanadium (V): 0.001 to 0.07%

Steel of the present invention comprises niobium (Nb): 0.03 to 0.07%, titanium (Ti): 0.001 to 0.03%, vanadium (V): 0.001 to 0.07%, the content of the Nb, Ti and V The sum satisfies the following relational expression 1.

[Relationship 1]

0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)

 Precipitation hardening elements such as Nb, Ti, and V are combined with C and N to form carbonitride precipitates, which are necessarily added to obtain appropriate strength.

If they fall outside of each of the content range and the range of the above equation 1 it is impossible to obtain a tensile strength of 400MPa to 550MPa. In other words, if the sum of Nb + Ti + V is less than 0.05, the amount of precipitation is small, so that it may not have a high strength of 550 MPa or more at first, or the change according to the winding temperature is small. If the sum of Nb + Ti + V exceeds 0.13, there is a high possibility that the strength of the 400MPa class will be exceeded even if the coiling temperature is 700 ° C or higher.

The steel sheet contains the remaining iron (Fe) and other unavoidable impurities.

Hot rolled steel sheet according to a preferred aspect of the present invention is a volume percent, having a microstructure comprising 90% or more of spherical polygonal ferrite and one or more of the remaining pearlite and cementite, and satisfies the following relation 2 (Nb , Ti, V) (C, N) and (Fe, Mn, Cr) ₃ C precipitates.

[Relationship 2]

Vc / Vtot ≥ 0.80, where Vc: volume fraction of precipitates greater than 20 nm in diameter, Vtot: volume fraction of total precipitates

The spherical polygonal ferrite is a phase necessary to ensure a relatively low strength, when the fraction is less than 90% by volume, it may be difficult to secure the target strength of the present invention. Therefore, the spherical polygonal ferrite is preferably limited to 90% by volume.

The fraction of more preferable spherical polygonal ferrite is 95% or more by volume.

As for the particle size of the said spherical polygonal ferrite, 5-100 micrometers is preferable.

When the particle diameter of the spherical polygonal ferrite is less than 5㎛, the strength is increased, and when it exceeds 100㎛, the curing ability may be increased, rather the strength may be increased, and the impact toughness may be worsened.

The average spacing of the perlite band structure preferably has a value of t [steel thickness (mm)] / 50 to t [steel thickness (mm)] / 150.

As described above, by controlling the average spacing of the pearlite band structure, carbon is consumed as the band structure, and the amount of carbide precipitates in the ferrite is reduced, and the carbon content in the ferrite, which is the majority of the structure, is reduced, so that the overall strength can be lowered.

When the value of Vc / Vtot is less than 0.8, the volume fraction of the fine precipitates may be increased to obtain high strength.

Therefore, the fraction of precipitate should satisfy the following relation.

[Relationship 2]

Vc / Vtot ≥ 0.80, where Vc: volume fraction of precipitates greater than 20 nm in diameter, Vtot: volume fraction of total precipitates

More preferable value of Vc / Vtot is 0.90 or more.

It is preferable that the coarse precipitate of 20 nm or more of effective diameter distribute | distributes at least 1 average per 1 micrometer <^2> . As such, when the fraction of the coarse precipitate is above a certain level, the volume fraction of the fine precipitate is limited, so that a low precipitation hardening amount, that is, low strength can be obtained.

Hot rolled steel sheet according to a preferred aspect of the present invention has a tensile strength (TS) [400MPa class tensile strength] of 400MPa to 550MPa.

Hereinafter, a method of manufacturing a hot rolled steel sheet according to another preferred aspect of the present invention.

Method for producing a hot rolled steel sheet according to another preferred aspect of the present invention is wt%, carbon (C): 0.03 ~ 0.08%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 ~ 1.9%, Aluminum (Al): 0.1% or less (excluding 0%), Phosphorus (P): 0.04% or less (excluding 0%), Sulfur (S): 0.02% or less (excluding 0%), Chromium (Cr): 0.5% or less (excluding 0%), Copper (Cu): 0.5% or less (excluding 0%), Nickel (Ni): 0.5% or less, Molybdenum (Mo): 0.3% or less (including 0%) ), Nitrogen (N): 0.02% or less (excluding 0%), Niobium (Nb): 0.03 to 0.07%, Titanium (Ti): 0.001 to 0.03%, and Vanadium (V): 0.001 to 0.07% To include, wherein the sum of the contents of Nb, Ti and V satisfies the following relation 1, preparing a slab containing the balance iron (Fe) and other unavoidable impurities;

[Relationship 1]

0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)

Heating the slab to a temperature of 1100-1250 ° C .;

Hot rolling the heated slab to a hot finishing rolling temperature condition of (A ₃ + 30 ° C.) or more to obtain a hot rolled steel sheet;

Cooling the hot rolled steel sheet at an average cooling rate of 30 ° C./sec or less; And

Winding the hot rolled steel sheet cooled as described above in a temperature range of 650 ° C. or more.

Slab heating stage

The slab formed as described above is heated to a temperature of 1100 ~ 1250 ℃.

If the slab temperature is low, it is difficult to perform hot rolling and the winding temperature after cooling is lowered. If it exceeds 1250 ℃, a lot of energy is unnecessary to increase the temperature, and Ti, Nb, V, etc. are employed so that strength increases. do. Therefore, the heating temperature of the slab is preferably limited to 1100 ~ 1250 ℃.

Obtaining Hot Rolled Steel Sheets

The slab heated as described above is hot rolled under hot finishing rolling temperature conditions of (A ₃ + 30 ° C.) or more to obtain a hot rolled steel sheet.

The higher the hot finish rolling temperature is, the lower the final strength is, and the higher the hot finish rolling temperature is A ₃ + 30 ° C. or more, the ferrite grains have an average size of 5 μm or more, and coarse precipitates are formed during rolling. The increase in strength can also be avoided. The upper limit of the preferred hot finish rolling temperature is 1000 DEG C, and higher than this results in inferior surface quality.

Cooling step

The hot rolled steel sheet is cooled at an average cooling rate of 30 ° C./sec or less.

When the cooling rate exceeds 30 ° C / sec, the ferrite structure is changed into a needle and the strength is increased by the phase transformation at a low temperature. The lower limit of the cooling rate is set to a range in which the actual hot rolling operation is possible, for example, may be 5 ° C./sec, and it may be difficult to perform continuous hot rolling at a slow cooling rate of less than 5 ° C./sec.

Coiled  step

The hot rolled steel sheet cooled as described above is wound in a temperature range of 650 ° C or higher.

If the coiling temperature is high, the precipitates can be made coarse and the ferrite grains can be made coarse, which is advantageous for lowering the strength. If the coiling temperature is lower than 650 ° C, precipitation becomes active and a high strength can be obtained. If the coiling temperature is lower than 700 ℃ crystal grains are smaller and the precipitates are finer to increase the strength. The upper limit of the coiling temperature is about 800 ° C. If the upper limit of the coiling temperature is exceeded, an operation difficulty may be greatly increased, such as an increase in the temperature of the coiler and a fire.

Therefore, the coiling temperature may be 650 ° C or higher, preferably 650-800 ° C. More preferable winding temperature is 700-800 degreeC.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, these examples are only for illustrating the present invention, the present invention is not limited thereto.

(Example)

A slab having a thickness of 250 mm having a composition of Table 1 below was heated to a slab under the conditions of Table 2, hot rolled with a hot rolled steel sheet, cooled, and then wound up to prepare a hot rolled steel sheet having a thickness of Table 2 below. It was. The component content units in Table 1 below are by weight. In Table 2, FDT represents the finish rolling temperature, and CT represents the coiling temperature.

With respect to the hot rolled steel sheet manufactured as described above, yield strength (YS), tensile strength (TS), elongation (EL), tensile strength when manufactured at a coiling temperature of 550 ° C (TS @ 550 ° C CT), ferrite fraction (Vferrite) ) And the precipitate fraction (Vc / Vtot) of (Nb, Ti, V) (C, N) and (Fe, Mn, Cr) ₃ C were measured and the results are shown in Table 3 below.

The tensile strength (TS @ 550 ° C.) when manufactured at the winding temperature (CT) of 550 ° C. represents the tensile strength when produced at an appropriate winding temperature (550 ° C.) capable of sufficiently exhibiting the potential of the alloying component.

River C Mn Ti Nb V Si Cr Mo Cu Ni Al P S N A 0.0437 1.598 0.012 0.081 0.050 0.245 0.24 0.08 0.203 0.19 0.035 0.0053 0.0007 0.0046 B 0.05 1.6 0.01 0.06 0.05 0.25 0.25 0.08 0.2 0.2 0.026 0.0052 0.0007 0.0041 C 0.0460 1.781 0.010 0.061 0.047 0.253 0.24 0.07 0.191 0.17 0.032 0.0043 0.0007 0.0045 D 0.0491 1.587 0.011 0.059 0.052 0.252 0.25 0.08 0.187 0.19 0.039 0.0042 0.0009 0.0047 E 0.0761 1.445 0.016 0.050 0.040 0.257 0.18 0.00 0.016 0.01 0.031 0.0057 0.0010 0.0028 F 0.0591 1.384 0.008 0.048 0.001 0.249 0.02 0 0.016 0.01 0.033 0.0111 0.0035 0.0046 G 0.0344 1.300 0.012 0.050 0.042 0.239 0.18 0.00 0.013 0.01 0.034 0.0047 0.0008 0.0043 H 0.0668 1.287 0.014 0.036 0.035 0.191 0.02 0.00 0.014 0.01 0.034 0.0075 0.0013 0.0045 I 0.029 1.35 0.01 0.045 0.025 0.25 0 0 0 0 0.027 0.0132 0.0042 0.0044 J 0.0718 0.994 0.001 0.021 0.002 0.192 0.02 0 0.015 0.01 0.031 0.0052 0.0012 0.0033 K 0.07 One 0.002 0.02 0.001 0.2 0 0 0 0 0.029 0.0127 0.0021 0.0035 L 0.035 0.9 0.01 0.04 0.002 0.2 0.1 0 0 0 0.011 0.0075 0.0023 0.0036

River Example No. Nb + Ti + V thickness Heating temperature
(℃) FDT
(℃) Cooling rate
(° C / sec) CT
(℃) A Comparative Example 1 0.143 5.8 1214 930 12.7 775 B Inventive Example 1 0.120 9.8 1185 887 6.6 758 C Inventive Example 2 0.118 9.8 1210 890 6.2 770 D Comparative Example 2 0.122 8.8 1193 902 15.6 642 E Inventive Example 3 0.106 9.8 1203 908 8.1 750 F Inventive Example 4 0.057 5.8 1184 995 19.4 768 G Inventive Example 5 0.104 5.7 1195 908 13.1 748 H Inventive Example 6 0.085 8.8 1197 904 9.4 750 I Comparative Example 3 0.080 5.8 1187 917 20.2 646 J Comparative Example 4 0.024 3.9 1211 901 19.6 711 K Comparative Example 5 0.020 5.8 1193 895 16.9 673 L Comparative Example 6 0.050 2.9 1211 908 20.3 708

River Example No. YS
(MPa) TS
(MPa) EL
(%) TS (MPa)
@ 550 ℃ CT Vferrite
(%) Vc / Vtot A Comparative Example 1 486 572 24 680 93 0.89 B Inventive Example 1 446 530 27 679 95 0.87 C Inventive Example 2 465 537 25 668 96 0.93 D Comparative Example 2 391 570 27 676 86 0.78 E Inventive Example 3 422 529 19 646 94 0.89 F Inventive Example 4 342 441 43 600 96 0.91 G Inventive Example 5 404 482 26 585 94 0.88 H Inventive Example 6 386 488 25 579 95 0.92 I Comparative Example 3 468 537 24 549 87 0.79 J Comparative Example 4 410 489 35 536 93 0.83 K Comparative Example 5 401 484 26 525 89 0.81 L Comparative Example 6 443 504 31 511 92 0.82

As shown in Tables 1 to 3, Comparative Example 1 shows that the sum of Ti + Nb + V exceeds 0.143, so that the strength exceeding TS 550 MPa is obtained even if the CT is 700 ° C or higher. Can be. In Comparative Example 2, the alloy component satisfies the scope of the invention, but exhibited strength exceeding TS 550 MPa by CT of less than 650 ° C. As such, when the coiling temperature is lowered, the phase ratio of polygonal ferrite falls below 90%, and the proportion of fine precipitates is increased so that the Vc / Vtot ratio is also less than 0.80. Comparative Examples 3 to 6 are cases in which the alloy component is out of the range of the present invention, and the tensile strength at CT 550 ° C. does not reach 550 MPa. The present invention is to obtain a low strength to the TS 400 grade (400 ~ 550MPa) from the alloy components that can easily obtain a high strength of 550 ~ 700MPa level.

Inventive examples (1 to 6) according to the present invention satisfy all of the alloying components, strength and microstructure of the present invention.

Although the present invention has been described above, it will be readily understood by those skilled in the art that various modifications and variations can be made without departing from the spirit and scope of the claims set out below.

Claims (8)

wt%, carbon (C): 0.03 to 0.08%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 to 1.9%, aluminum (Al): 0.1% or less (0% ), Phosphorus (P): 0.04% or less (except 0%), sulfur (S): 0.02% or less (except 0%), chromium (Cr): 0.5% or less (except 0%), copper (Cu): 0.5% or less (excluding 0%), nickel (Ni): 0.5% or less, molybdenum (Mo): 0.3% or less (including 0%), nitrogen (N): 0.02% or less (excluding 0%) ), Niobium (Nb): 0.03 to 0.07%, titanium (Ti): 0.001 to 0.03% and vanadium (V): 0.001 to 0.07%, the sum of the contents of the Nb, Ti and V is Satisfies relation 1 and includes balance iron (Fe) and other unavoidable impurities;
[Relationship 1]
0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)
The microstructure is 95% or more by volume, and includes spherical polygonal ferrite having an average particle diameter of 5 to 100 µm and at least one of the remaining pearlite and cementite and satisfying the following relation (2). ) (C, N) and (Fe, Mn, Cr) ₃ C precipitates; And
[Relationship 2]
Vc / Vtot ≥ 0.80 (where Vc is the volume fraction of precipitates greater than 20 nm in diameter, Vtot is the volume fraction of total precipitates)
Hot rolled steel sheet having a tensile strength (TS) of 400MPa to 550MPa.
delete The hot rolled steel sheet according to claim 1, wherein the value of Vc / Vtot is 0.90 or more.
delete The hot rolled steel sheet according to claim 1, wherein the average spacing of the perlite band structure has a value of t [steel thickness (mm)] / 50 to t [steel thickness (mm)] / 150.
The hot rolled steel sheet according to claim 1, wherein at least one coarse precipitate having an effective diameter of 20 nm or more is distributed per 1 µm ² .
wt%, carbon (C): 0.03 to 0.08%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 to 1.9%, aluminum (Al): 0.1% or less (0% ), Phosphorus (P): 0.04% or less (except 0%), sulfur (S): 0.02% or less (except 0%), chromium (Cr): 0.5% or less (except 0%), copper (Cu): 0.5% or less (excluding 0%), nickel (Ni): 0.5% or less, molybdenum (Mo): 0.3% or less (including 0%), nitrogen (N): 0.02% or less (excluding 0%) ), Niobium (Nb): 0.03 to 0.07%, titanium (Ti): 0.001 to 0.03%, and vanadium (V): 0.001 to 0.07%, and the sum of the contents of Nb, Ti, and V is Preparing a slab that satisfies the following relation 1 and includes balance iron (Fe) and other unavoidable impurities;
[Relationship 1]
0.05 ≤ Nb + Ti + V ≤ 0.13 (wt.%)
Heating the slab to a temperature of 1100-1250 ° C .;
Hot rolling the heated slab to a hot finishing rolling temperature condition of (A ₃ + 30 ° C.) or more to obtain a hot rolled steel sheet;
Cooling the hot rolled steel sheet at an average cooling rate of 30 ° C./sec or less; And
Winding the hot rolled steel sheet cooled as described above in a temperature range of 650 to 800 ° C.,
The hot rolled steel sheet,
Volumetric microstructure comprises 95% or more, spherical polygonal ferrite having an average particle diameter of 5 to 100 µm, and at least one of the remaining pearlite and cementite, and satisfying the following relational formula (Nb, Ti, V) ) (C, N) and (Fe, Mn, Cr) ₃ C precipitates; And
[Relationship 2]
Vc / Vtot ≥ 0.80 (where Vc is the volume fraction of precipitates greater than 20 nm in diameter, Vtot is the volume fraction of total precipitates)
Tensile strength (TS) is a method for producing a hot rolled steel sheet, characterized in that 400MPa to 550MPa.
delete