KR20220089397A - High strength hot rolled steel sheet having low yield ratio and method of manufacturing the same - Google Patents

Abstract

One embodiment of the present invention is by weight%, C: 0.13 to 0.20%, Si: 0.3% or less (excluding 0%), Mn: 1.0 to 2.0%, Al: 0.1% or less (excluding 0%), Nb : 0.02 to 1.0%, Ti: 0.03% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (0%) excluding), B: 0.005% or less (excluding 0%), the remainder contains Fe and other unavoidable impurities, and the microstructure is an area%, containing 50% or more ferrite and 50% or less bainite, and average size Provided are a high-strength hot-rolled steel sheet having a yield ratio of 5×10 ⁶ pieces/mm 2 or more containing precipitates of 3 to 200 nm, and a method for manufacturing the same.

Description

High-strength hot-rolled steel sheet having a resistance yield ratio and manufacturing method thereof

The present invention relates to a high-strength hot-rolled steel sheet having a resistive yield ratio and a method for manufacturing the same, and more particularly, to a high-strength hot-rolled steel sheet having a resistive yield ratio and a method for manufacturing the same it's about

Structural high-strength steel with excellent earthquake resistance has a low yield ratio when used for construction and civil engineering, and can absorb a lot of energy when an external stress such as an earthquake occurs, so it is not easily broken. For this purpose, thick material of 6mm or more is mainly used. As the thickness increases, it is not easy to satisfy materials such as strength and elongation and at the same time satisfy a yield ratio of 80% or less.

However, methods for manufacturing high-strength structural steel with excellent earthquake resistance that satisfy these requirements have been developed through accurate design of the hot-rolled manufacturing process and adjustment of components according to thickness, and Patent Document 1 is a representative example. In Patent Document 1, a ferrite having relatively low strength is formed in primary cooling through two-stage cooling, and a hard phase bainite is generated in secondary cooling to achieve a resistive yield ratio. However, since this method is difficult to secure the yield strength of the center of the plate as the thickness increases, there is a limitation in that it can only be used in a thin steel sheet that is relatively easy to refine the structure of the center. In addition, if a lot of hard phase bainite is made in secondary cooling, it may be difficult to secure elongation.

Korean Patent Publication No. 10-0545959

One aspect of the present invention is to provide a high-strength hot-rolled steel sheet having a resistance yield ratio and a method for manufacturing the same.

One embodiment of the present invention is by weight%, C: 0.13 to 0.20%, Si: 0.3% or less (excluding 0%), Mn: 1.0 to 2.0%, Al: 0.1% or less (excluding 0%), Nb : 0.02 to 1.0%, Ti: 0.03% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (0%) excluding), B: 0.005% or less (excluding 0%), the remainder contains Fe and other unavoidable impurities, and the microstructure is an area%, containing 50% or more ferrite and 50% or less bainite, and average size Provided is a high-strength hot-rolled steel sheet having a yield ratio of 5×10 ⁶ pieces/mm 2 or more containing precipitates of 3 to 200 nm.

Another embodiment of the present invention is by weight, C: 0.13 to 0.20%, Si: 0.3% or less (excluding 0%), Mn: 1.0 to 2.0%, Al: 0.1% or less (excluding 0%), Nb : 0.02 to 1.0%, Ti: 0.03% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (0%) Except for), B: 0.005% or less (excluding 0%), heating the slab containing the remainder Fe and other unavoidable impurities at 1100 ~ 1300 ℃; Finishing hot rolling the heated slab at 750 ~ 880 ℃ to obtain a hot rolled steel sheet; shear cooling the hot-rolled steel sheet at a cooling rate of 20° C./s or more and less than 50° C./s so that the surface temperature thereof becomes 500 to 540° C.; and reheating the shear-cooled hot-rolled steel sheet to a surface temperature of 540 to 600° C. and then winding it.

According to one aspect of the present invention, it is possible to provide a high-strength hot-rolled steel sheet having a resistance yield ratio and a method for manufacturing the same.

1 is a photograph of Inventive Example 2 according to an embodiment of the present invention observed under an optical microscope.
2 is a photograph of Inventive Example 2 according to an embodiment of the present invention observed with a transmission electron microscope.
3 is a photograph of Comparative Example 3 according to an embodiment of the present invention observed under an optical microscope.

Hereinafter, a high-strength hot-rolled steel sheet having a resistive yield ratio according to an embodiment of the present invention will be described. First, the alloy composition of the hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, the content of the alloy composition described below means wt%.

C: 0.13 to 0.20%

Carbon (C) is the most effective element for securing strength, and is preferably added in an amount of 0.13% or more in order to obtain excellent hardness. In addition, when the content of C is less than 0.13%, as the alloy composition of the apocrystalline region is designed, there is a high possibility that cracks may occur on the surface of the slab due to non-uniform solidification during continuous casting. On the other hand, when the content of C exceeds 0.20%, not only can the weldability be impaired, but also it is difficult to secure the yield strength. Therefore, the C content is preferably in the range of 0.13 to 0.20%. The lower limit of the C content is more preferably 0.135%, even more preferably 0.14%. The upper limit of the C content is more preferably 0.19%, even more preferably 0.185%.

Si: 0.3% or less (excluding 0%)

Silicon (Si) is an element mainly used as a deoxidizer. When the content of Si exceeds 0.3%, it is possible to cause grain boundary oxidation and red scale on the surface. Therefore, the Si content is preferably in the range of 0.3% or less. The Si content is more preferably 0.28% or less, and even more preferably 0.25% or less.

Mn: 1.0~2.0%

Manganese (Mn), together with C, is the most commonly used element to improve the strength of steel. When the content of Mn is less than 1.0%, it may cause grain boundary embrittlement at high temperature due to the formation of FeS, which is disadvantageous in securing yield strength. On the other hand, if it exceeds 2.0%, center segregation, inclusion formation, and grain boundary oxidation may occur, which may adversely affect not only the quality of the steel material but also the weldability. Accordingly, the Mn content is preferably in the range of 1.0 to 2.0%. The lower limit of the Mn content is more preferably 1.1%, even more preferably 1.2%. The upper limit of the Mn content is more preferably 1.9%, and even more preferably 1.8%.

Al: 0.1% or less (excluding 0%)

Aluminum (Al) is an element mainly used as a deoxidizer, and is sometimes added for solid solution strengthening effect. When the Al content exceeds 0.1%, cracks may be induced in the slab during continuous casting, and grain boundary oxidation may occur in the final product. Accordingly, the Al content is preferably in the range of 0.1% or less. The Al content is more preferably 0.08% or less, and even more preferably 0.05% or less.

Nb: 0.02~1.0%

Niobium (Nb) is an element used for precipitation strengthening and grain refinement, and for the above effect, it is preferably added in an amount of 0.02% or more. However, since Nb is an expensive element, it is preferable to add it in an amount of 1.0% or less in consideration of economic feasibility. Accordingly, the content of Nb is preferably in the range of 0.02 to 1.0%. The lower limit of the Nb content is more preferably 0.025%, and even more preferably 0.03%. The upper limit of the Nb content is more preferably 0.9%, and even more preferably 0.8%.

Ti: 0.03% or less (excluding 0%)

Titanium (Ti) is added to solve the decrease in the hardenability of B due to BN precipitation in B-added steel, and by forming TiN at a high temperature, the hardenability of B is increased. However, when the content of Ti exceeds 0.03%, not only the manufacturing cost increases, but also when Ti is combined with Nb to form a lot of complex elements, the solid solution of Nb is not good, so that the Nb precipitation strengthening effect can be reduced. . Accordingly, the Ti content is preferably in the range of 0.03% or less. The Ti content is more preferably 0.025% or less, and even more preferably 0.02% or less.

P: 0.03% or less (excluding 0%)

Phosphorus (P) is an impurity that is unavoidably contained in steelmaking, and it is preferable to include as little amount as possible because it causes brittleness by segregation. Accordingly, in the present invention, the content of P is limited to 0.03% or less. The P content is more preferably 0.025% or less, and even more preferably 0.02% or less.

S: 0.02% or less (excluding 0%)

Sulfur (S) is an impurity unavoidably contained in steelmaking, and is an element that may form inclusions or form FeS compounds with a low melting point, thereby causing grain boundary embrittlement during hot rolling. Therefore, it is preferable to include the S in an amount as small as possible, and in the present invention, the content of S is limited to 0.02% or less. The S content is more preferably 0.01% or less, and even more preferably 0.005% or less.

N: 0.01% or less (excluding 0%)

Although nitrogen (N) is an unavoidably contained impurity, it is an element that contributes to an increase in strength through solid solution strengthening. However, when the content of N exceeds 0.01%, a problem of lowering the impact characteristics may occur. Therefore, the content of N is preferably in the range of 0.01% or less. The N is more preferably 0.09% or less, and even more preferably 0.08% or less.

B: 0.005% or less (excluding 0%)

Boron (B) is an element that increases hardenability with only a small amount of addition, and it is easy to secure hardenability in high-strength steel without adding an expensive hardenability improving element. However, when the content of B exceeds 0.005%, the precipitation of Fe ₂₃ (C,B) ₆ may be excessive and hardenability may rather decrease. Therefore, the content of B is preferably in the range of 0.005% or less. The B content is more preferably 0.004% or less, and even more preferably 0.0035% or less.

In addition to the above-described steel composition, the remainder may include Fe and unavoidable impurities. Inevitable impurities may be unintentionally mixed in a typical steel manufacturing process, and this cannot be entirely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. In addition, the present invention does not entirely exclude the addition of compositions other than the above-mentioned steel composition.

On the other hand, in the hot-rolled steel sheet of the present invention, it is preferable that Ceq expressed by the following formula 1 is 0.44 or less. When the value of Equation 1 exceeds 0.44, it may be difficult to secure weldability.

[Equation 1] Ceq = C + Mn/6 + (Ni+Cu)/15 + (Cr+Mo+V)/5)

In addition, the hot-rolled steel sheet preferably has a Pcm of 0.26 or less expressed by the following formula (2). When the value of Equation 2 exceeds 0.26, it may be difficult to secure weldability.

[Equation 2] Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 +5B

Hereinafter, the microstructure and precipitates of the hot-rolled steel sheet of the present invention will be described.

The microstructure of the hot-rolled steel sheet of the present invention preferably contains 50% or more of ferrite and 50% or less of bainite in terms of area%. In general, it is difficult to secure a high yield strength in a steel with a high yield ratio, but by appropriately controlling the precipitation-strengthened ferrite matrix, bainite, and low-temperature transformation structure, it is possible to secure high yield strength and high yield strength at the same time. When the fraction of ferrite is less than 50% or the fraction of bainite exceeds 50%, high yield strength and tensile strength may be secured, but it may be difficult to achieve a resistive yield ratio. In the present invention, the upper limit of the ferrite fraction and the lower limit of the bainite fraction are not particularly limited, but may be, for example, 90% and 10%, respectively.

The hot-rolled steel sheet of the present invention preferably contains 5×10 ⁶ pieces/mm 2 or more of precipitates having an average size of 3 to 200 nm. By including a large amount of fine precipitates in this way to obtain a precipitation strengthening effect, the strength of the center can be improved. One feature of the present invention is to achieve the object of the present invention by including the precipitates in the average size range as the fraction, but it is not excluded that the precipitates having an average size of less than 3 nm are further included. Meanwhile, in the present invention, the type of precipitate is not particularly limited, but may include, for example, at least one of NbC, Nb(C,N), and (Ti,Nb)(C,N).

The hot-rolled steel sheet according to an embodiment of the present invention provided as described above may have a yield strength of 460 MPa or more and a yield ratio of 80% or less. In addition, the hot-rolled steel sheet of the present invention does not generate cracks during continuous casting, and can have good plate shape and bending properties. In addition, the hot-rolled steel sheet of the present invention may have a thickness of 2 to 24 mm, more preferably 4 to 22 mm, still more preferably 5 to 18 mm, most preferably 6 to 16 mm.

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

First, the slab satisfying the above alloy composition is heated at 1100 ~ 1300 ℃. In general, steel to which a precipitation strengthening element such as Ti or Nb is added is used for precipitation strengthening by dissolving the precipitation element in a heating furnace. However, since it is difficult to dissolve the precipitates such as TiN in a heating furnace, it is preferable to set the slab heating temperature high. Therefore, in the present invention, it is preferable that the heating temperature of the slab is in the range of 1100 to 1300 °C. If the slab temperature is less than 1100 ℃, it may be difficult to sufficiently obtain the effect, and if it exceeds 1300 ℃, the amount of Fe lost to scale is excessively large, so that the manufacturing cost may be lowered, and the alloy at the interface between the scale and the base material Due to the concentration of the components, the descalability of the scale is not good, and defects may occur in the rolling plate. The slab heating temperature is more preferably in the range of 1200 ~ 1300 ℃.

Then, finish hot rolling the heated slab at 750 ~ 880 ℃ to obtain a hot rolled steel sheet. The thicker the material, the more important it is to secure the strength of the center in the thickness direction in order to secure the yield strength. Accordingly, in the present invention, by setting the finish hot rolling temperature range as described above, the crystal grains at the center of the thickness direction are refined. If the finish hot rolling temperature is less than 750 ℃, the rolling load is severe, which is unfavorable to shape control of the rolled plate, and non-uniform structure may occur due to abnormal rolling, and if it exceeds 850 ℃, austenite grains are coarsened or The ferrite grains may be coarsened and the impact properties may be inferior. On the other hand, in the case of a thick material having a thickness of 10 mm or more, it is more preferable that the finish rolling temperature is in the range of 750 to 850 °C.

Thereafter, the hot-rolled steel sheet is shear-cooled at a cooling rate of 20° C./s or more and less than 50° C./s so that the surface temperature thereof is 500 to 540° C. Through the cooling process, it is possible to secure the microstructure to be obtained by the present invention. If the cooling end temperature is less than 500 ° C, recuperation is not properly performed, so it may be difficult to obtain ferrite with adequate precipitation strengthening, and when it exceeds 540 ° C, it is difficult to form a hard bainite. Difficult-to-reach problems may arise. If the cooling rate is less than 20 ℃ / s, the ferrite may be coarsened during cooling, whereas if it is 50 ℃ / s or more, the plate shape may be poor, which may cause local material deviation. The cooling rate more preferably has a range of 20 ~ 40 ℃ / s. On the other hand, the aforementioned shear cooling means starting cooling within 3 seconds after the end of rolling.

Thereafter, the shear-cooled hot-rolled steel sheet is reheated to a surface temperature of 540 to 600° C. and then wound up. As described above, it is possible to form precipitates on the soft and fine ferrite through the recuperative process, and through this, not only high yield strength but also a resistance yield ratio can be realized. On the other hand, when the winding temperature is less than 540 ℃, there is a disadvantage in that the ratio of the low-temperature transformation structure is high and it is difficult to obtain a resistive yield ratio, and when it exceeds 600 ℃, it may be difficult to secure strength due to ferrite coarsening.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only for illustrating the present invention in more detail, and do not limit the scope of the present invention.

(Example)

After the slab having the alloy composition shown in Table 1 was heated at 1250° C., a hot-rolled steel sheet was manufactured under the conditions shown in Table 2 below. The microstructure, mechanical properties, cracks during continuous casting, and plate shape were measured for the thus-prepared hot-rolled steel sheet, and the results are shown in Table 3 below.

The microstructure was measured using an optical microscope.

Precipitation was measured using a transmission electron microscope.

Tensile strength and yield strength were measured in the same way as specified in KS B 0802 standard.

The occurrence of cracks during continuous casting was confirmed by visual inspection, and in some cases, the occurrence of cracks was determined by visual observation after removal of surface scale through surface hot grinding and hot scarfing.

The plate shape was judged as defective when the edge wave height was 5 mm or more.

Bending characteristics were measured by observing the surface with the naked eye or a microscope to see if cracks occurred in the bent part of the outer surface by bending 90 degrees at the R value specified in the standard of KS D 3864. If cracks are observed, the depth of the cracks And it was judged to be defective regardless of size.

steel grade
No. Alloy composition (wt%) C Mn Si Al P S Nb Ti B N Ceq Pcm One 0.12 1.4 0.07 0.02 0.01 0.003 0.05 0.02 0.002 0.005 0.353 0.202 2 0.14 1.4 0.07 0.02 0.01 0.003 0.05 0.02 0.002 0.005 0.373 0.222 3 0.17 1.4 0.07 0.02 0.01 0.003 0.05 0.02 0.002 0.005 0.403 0.252 4 0.17 1.4 0.07 0.02 0.01 0.003 0.05 0 0 0.005 0.403 0.242 Ceq = C + Mn/6 + (Ni+Cu)/15 + (Cr+Mo+V)/5)
Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 +5B

division Kang type No. Finishing hot rolling temperature
(℃) cooling rate
(℃/s) Based on the steel plate surface temperature
Cooling end temperature (℃) Based on the steel plate surface temperature
Coiling temperature (℃) Invention Example 1 2 780 30 500 554 Comparative Example 1 3 780 10 554 569 Invention example 2 3 780 20 533 564 Invention example 3 3 780 30 521 582 Invention Example 4 3 780 30 503 554 Comparative Example 2 3 780 30 480 531 Comparative Example 3 3 780 30 400 430 Comparative Example 4 3 780 50 450 480 Comparative Example 5 4 780 30 523 569 Comparative Example 6 One 780 30 526 578

division microstructure
(area%)
Precipitates with an average size of 200 nm or less
(×10 ⁶ pieces/㎟) surrender
burglar
(MPa) Seal
burglar
(MPa) yield ratio
(%) crack when playing
Occur
Whether plate shape bend
characteristic ferrite bainite Invention Example 1 65 35 5.2 478 662 72 non-occurring Good Good Comparative Example 1 91 9 4.2 447 632 71 non-occurring Good Good Invention example 2 54 46 7.2 482 672 72 non-occurring Good Good Invention example 3 66 34 6.8 496 683 73 non-occurring Good Good Invention Example 4 55 45 5.9 483 669 72 non-occurring Good Good Comparative Example 2 0 100 3.1 584 701 83 non-occurring Good Good Comparative Example 3 0 100 1.3 635 753 84 non-occurring error error Comparative Example 4 0 100 2.4 598 722 83 non-occurring error error Comparative Example 5 98 8 3.4 444 624 71 non-occurring Good Good Comparative Example 6 72 28 6.4 466 662 70 Occur Good Good

As can be seen from Tables 1 to 3, in the case of Examples 1 to 4 that satisfy the alloy composition and manufacturing conditions proposed by the present invention, excellent mechanical properties are secured by obtaining the microstructure and precipitates targeted by the present invention In addition, it can be seen that cracks do not occur during continuous casting and have a good plate shape.

On the other hand, in the case of Comparative Examples 1 to 5 that the alloy composition proposed by the present invention is satisfactory but does not satisfy the manufacturing conditions, the microstructure or precipitates targeted by the present invention cannot be obtained, so the mechanical properties are inferior, and some have poor plate shape. it can be seen that

In the case of Comparative Example 6, it can be seen that cracks occurred during continuous casting due to insufficient C content.

1 is a photograph of Inventive Example 2 according to an embodiment of the present invention observed with an optical microscope, and FIG. 2 is a photograph of Inventive Example 2 according to an embodiment of the present invention observed with a transmission electron microscope. As can be seen from FIGS. 1 and 2, in the case of Inventive Example 2, it can be seen that ferrite and bainite in the fractions proposed by the present invention are formed, and it can be confirmed that a large amount of fine precipitates are precipitated.

3 is a photograph of Comparative Example 3 according to an embodiment of the present invention observed under an optical microscope. As can be seen from Figure 3, in the case of Comparative Example 3, it can be confirmed that the hard phase consisting of a bainite structure.

Claims (7)

By weight%, C: 0.13 to 0.20%, Si: 0.3% or less (excluding 0%), Mn: 1.0 to 2.0%, Al: 0.1% or less (excluding 0%), Nb: 0.02 to 1.0%, Ti : 0.03% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (excluding 0%), B: 0.005 % or less (excluding 0%), including the remainder Fe and other unavoidable impurities,
The microstructure is an area%, containing 50% or more of ferrite and 50% or less of bainite,
A high-strength hot-rolled steel sheet having a resistive yield ratio containing more than 5×10 ⁶ pieces/mm2 of precipitates with an average size of 3 to 200 nm.
The method according to claim 1,
The hot-rolled steel sheet is a high-strength hot-rolled steel sheet having a resistance yield ratio of 0.44 or less Ceq expressed by the following formula (1).
[Equation 1] Ceq = C + Mn/6 + (Ni+Cu)/15 + (Cr+Mo+V)/5)
The method according to claim 1,
The hot-rolled steel sheet is a high-strength hot-rolled steel sheet having a resistive yield ratio of 0.26 or less Pcm expressed by the following formula (2).
[Equation 2] Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 +5B
The method according to claim 1,
The hot-rolled steel sheet is a high-strength hot-rolled steel sheet having a yield strength of 460 MPa or more and a resistive yield ratio having a yield ratio of 80% or less.
By weight%, C: 0.13 to 0.20%, Si: 0.3% or less (excluding 0%), Mn: 1.0 to 2.0%, Al: 0.1% or less (excluding 0%), Nb: 0.02 to 1.0%, Ti : 0.03% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.01% or less (excluding 0%), B: 0.005 % or less (excluding 0%), heating the slab containing the remainder Fe and other unavoidable impurities at 1100 ~ 1300 ℃;
Finishing hot rolling the heated slab at 750 ~ 880 ℃ to obtain a hot rolled steel sheet;
shear cooling the hot-rolled steel sheet at a cooling rate of 20° C./s or more and less than 50° C./s so that the surface temperature thereof becomes 500 to 540° C.; and
A method of manufacturing a high-strength hot-rolled steel sheet having a resistance yield ratio, comprising the step of reheating the shear-cooled hot-rolled steel sheet to a surface temperature of 540 to 600° C. and then winding it.
6. The method of claim 5,
The slab is a method of manufacturing a high-strength hot-rolled steel sheet having a resistive yield ratio of 0.26 or less Pcm expressed by the following formula (2).
[Equation 2] Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 +5B
6. The method of claim 5,
The slab is a method of manufacturing a high-strength hot-rolled steel sheet having a resistive yield ratio of 0.26 or less Pcm expressed by the following formula (2).
[Equation 2] Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 +5B

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