KR102492030B1 - 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, in 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), B: 0.005% or less (excluding 0%), the balance includes Fe and other unavoidable impurities, and the microstructure, by area%, includes 50% or more ferrite and 50% or less bainite, and average size Provided is a high-strength hot-rolled steel sheet having a low yield ratio including 5×10 ⁶ pieces/mm 2 or more of precipitates having a range of 3 to 200 nm and a method for manufacturing the same.

Description

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

The present invention relates to a high-strength hot-rolled steel sheet having a low yield ratio and a method for manufacturing the same, and more particularly, to a high-strength hot-rolled steel sheet having a low yield ratio that can be used in various construction fields, such as for construction, bridges, and ground support, 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, so it can absorb a lot of energy when external stress such as an earthquake occurs, so it is not easily broken. Accordingly, it can be used to secure the stability of buildings. To this end, thick materials of 6 mm or more are mainly used. As the thickness becomes thicker, it is not easy to satisfy materials such as strength and elongation and at the same time satisfy the yield ratio of 80% or less.

However, through accurate design of the hot-rolling manufacturing process and component adjustment according to the thickness, methods for manufacturing high-strength structural steel with excellent earthquake resistance have been developed, and Patent Document 1 is representative. Patent Document 1 achieves a low yield ratio by forming ferrite having a relatively low strength in the first cooling through two-stage cooling and producing hard bainite in the second cooling. However, since this method is difficult to secure the yield strength at the center of the plate as the thickness increases, there is a limitation that it can only be used for thin steel plates that are relatively easy to refine the center structure. In addition, if a lot of bainite, a hard phase, is made in the secondary cooling, it may be difficult to secure the elongation.

Korean Registered Patent Publication No. 10-0545959

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

One embodiment of the present invention, in 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), B: 0.005% or less (excluding 0%), the balance includes Fe and other unavoidable impurities, and the microstructure, by area%, includes 50% or more ferrite and 50% or less bainite, and average size Provided is a high-strength hot-rolled steel sheet having a low yield ratio including 5×10 ⁶ pieces/mm 2 or more of precipitates having a range of 3 to 200 nm.

Another embodiment of the present invention, in weight percent, 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: heating a slab containing 0.005% or less (excluding 0%), the balance of Fe and other unavoidable impurities at 1100 to 1300 ° C; Obtaining a hot-rolled steel sheet by finish hot-rolling the heated slab at 750 to 880° C.; 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 is 500 to 540° C.; and reheating the shear-cooled hot-rolled steel sheet so that the surface temperature thereof is 540 to 600° C., and then winding the hot-rolled steel sheet.

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

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

Hereinafter, a high-strength hot-rolled steel sheet having a low 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. The content of the alloy composition described below refers to % by weight unless otherwise specified.

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 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 risk of cracking 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%, weldability may be impaired, and it is difficult to secure yield strength. Accordingly, the content of C is preferably in the range of 0.13 to 0.20%. The lower limit of the C content is more preferably 0.135%, and even more preferably 0.14%. The upper limit of the C content is more preferably 0.19%, and even more preferably 0.185%.

Si: 0.3% or less (excluding 0%)

Silicon (Si) is an element mainly used as a deoxidizer. When the Si content exceeds 0.3%, oxidation of grain boundaries and red scale may occur 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 to 2.0%

Manganese (Mn), along with C, is the most commonly used element to improve the strength of steel. When the Mn content is less than 1.0%, grain boundary brittleness may occur at high temperatures due to FeS formation, and it is unfavorable to secure yield strength. On the other hand, if it exceeds 2.0%, it may cause center segregation, formation of inclusions, and grain boundary oxidation, which may adversely affect not only the quality of steel materials, but also weldability. Therefore, 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%, and 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 also added for a 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. Therefore, 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 crystal grain refinement, and is preferably added in an amount of 0.02% or more for the above effects. However, since Nb is an expensive element, it is preferable to add 1.0% or less in consideration of economic feasibility. Therefore, the Nb content 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 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 does the manufacturing cost increase, but when Ti is combined with Nb to form a large number of complex elements, Nb is not dissolved well, thereby reducing the effect of strengthening Nb precipitation. . Therefore, 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 since it causes brittleness by segregation, it is preferable to include as little amount as possible. 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 that is unavoidably contained in steelmaking, and is an element that can cause grain boundary brittleness during hot rolling by forming inclusions or forming FeS compounds with a low melting point. Therefore, it is preferable to include as little amount of S 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%)

Nitrogen (N) is an unavoidable impurity, but it is an element that contributes to the increase in strength through solid solution strengthening. However, when the N content exceeds 0.01%, a problem of deteriorating impact characteristics may occur. Therefore, the N content 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 even with the addition of a small amount, 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 is excessive, and the 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 rest may include Fe and unavoidable impurities. Inevitable impurities can be unintentionally mixed in the normal steel manufacturing process, and cannot be completely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. Further, the present invention does not entirely exclude the addition of other compositions than the aforementioned steel composition.

On the other hand, the hot-rolled steel sheet of the present invention preferably has a Ceq of 0.44 or less represented by Equation 1 below. When the value of Equation 1 below 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 represented by Equation 2 below. When the value of Equation 2 below exceeds 0.26, it may be difficult to secure weldability.

[Formula 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 according to the present invention will be described.

The microstructure of the hot-rolled steel sheet according to the present invention preferably contains 50% or more of ferrite and 50% or less of bainite, in terms of area%. Generally, it is difficult to secure a high yield strength in a low yield steel material, but it is possible to secure a high yield strength and a low yield ratio at the same time by appropriately controlling the precipitation-hardened ferrite base, bainite, and low-temperature transformation structure. When the fraction of ferrite is less than 50% or the fraction of bainite exceeds 50%, it is possible to secure high yield strength and tensile strength, but it may be difficult to achieve a low 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. In this way, by including a large amount of fine precipitates to obtain a precipitation strengthening effect, the strength of the center portion can be improved. One feature of the present invention is to achieve the object of the present invention by including precipitates of the average size range in the above fraction, but does not exclude the further including precipitates having an average size of less than 3 nm. Meanwhile, in the present invention, the type of precipitate is not particularly limited, but for example, one or more of NbC, Nb(C,N), and (Ti,Nb)(C,N) may be included.

The hot-rolled steel sheet according to one 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 may have good plate shape and bending characteristics. 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, even more preferably 5 to 18 mm, and 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, a slab satisfying the above-described alloy composition is heated at 1100 to 1300 ° C. In general, steels to which precipitation hardening elements such as Ti and Nb are added are used for precipitation hardening by dissolving the precipitation elements in a heating furnace. However, since it is difficult to dissolve 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 slab heating temperature has a range of 1100 to 1300 ° C. If the slab temperature is less than 1100 ° C., it may be difficult to sufficiently obtain the above effect, and if it exceeds 1300 ° C., the amount of Fe lost to the scale may be excessively large, reducing manufacturing cost, and alloy at the interface between the scale and the base material Due to the concentration of the components, the descalability of the scale may be deteriorated and defects may occur in the rolled sheet. The slab heating temperature is more preferably in the range of 1200 ~ 1300 ℃.

Thereafter, the heated slab is finished hot-rolled at 750 to 880° C. to obtain a hot-rolled steel sheet. It is important to secure the strength of the center in the thickness direction in order to secure the yield strength of thicker materials. Accordingly, in the present invention, by setting the finish hot rolling temperature range as described above, it is intended to refine the crystal grains in the central portion in the thickness direction. When the finish hot rolling temperature is less than 750 ° C, the rolling load is severe, which is disadvantageous to the shape control of the rolled sheet, and an uneven structure may occur due to abnormal reverse rolling, and when the temperature exceeds 850 ° C, the austenite grains are coarsened Ferrite crystal grains may be coarsened, resulting in inferior impact properties. On the other hand, in the case of thick material having a thickness of 10 mm or more, it is more preferable that the finish rolling temperature has a 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 in the present invention. If the cooling end temperature is less than 500 ° C, it may be difficult to obtain ferrite with adequate precipitation strengthening because recuperation is not performed properly, and if it exceeds 540 ° C, it is difficult to form hard phase bainite, thereby increasing the low yield ratio through an increase in tensile strength. There may be problems that are difficult to obtain. If the cooling rate is less than 20 ° C / s, ferrite may be coarsened during cooling, whereas if it is 50 ° C / s or more, the plate shape may be poor, which may cause local material deviation. The cooling rate is more preferably in the range of 20 ~ 40 ℃ / s. On the other hand, the aforementioned shear cooling means starting cooling within 3 seconds after completion of rolling.

Thereafter, the shear-cooled hot-rolled steel sheet is reheated so that the surface temperature thereof is 540 to 600° C., and then wound. In this way, it is possible to form precipitates on the soft fine ferrite through the recuperation process, and through this, it is possible to realize a high yield strength as well as a low yield ratio. On the other hand, when the winding temperature is less than 540 ℃, the ratio of the low-temperature transformation structure is high, there is a disadvantage that it is difficult to obtain a low 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 explaining the present invention in more detail and do not limit the scope of the present invention.

(Example)

After heating the slabs having the alloy compositions shown in Table 1 at 1250 ° C., hot-rolled steel sheets were manufactured under the conditions shown in Table 2 below. After measuring the microstructure, mechanical properties, crack generation during continuous casting, and plate shape of the hot-rolled steel sheet prepared as described above, the results are shown in Table 3 below.

The microstructure was measured using an optical microscope.

Precipitates were measured using a transmission electron microscope.

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

The occurrence of cracks during continuous casting was confirmed by visual inspection, and some of them were determined by visual observation after removing surface layer scale through surface hot grinding and hot scarfing.

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

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

steel grade
No. Alloy composition (% by weight) 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 Steel grade No. Finish hot rolling temperature
(℃) cooling rate
(℃/s) Steel plate surface temperature standard
Cooling end temperature (℃) Steel plate surface temperature standard
Winding 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 less than 200 nm
(×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-occurrence Good Good Comparative Example 1 91 9 4.2 447 632 71 non-occurrence Good Good Invention example 2 54 46 7.2 482 672 72 non-occurrence Good Good Invention Example 3 66 34 6.8 496 683 73 non-occurrence Good Good Invention Example 4 55 45 5.9 483 669 72 non-occurrence Good Good Comparative Example 2 0 100 3.1 584 701 83 non-occurrence Good Good Comparative Example 3 0 100 1.3 635 753 84 non-occurrence error error Comparative Example 4 0 100 2.4 598 722 83 non-occurrence error error Comparative Example 5 98 8 3.4 444 624 71 non-occurrence 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 Room Examples 1 to 4 satisfying the alloy composition and manufacturing conditions proposed by the present invention, excellent mechanical properties are secured by obtaining the microstructure and precipitate 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, which satisfy the alloy composition proposed by the present invention but do not satisfy the manufacturing conditions, the microstructure or precipitate targeted by the present invention is not obtained, so the mechanical properties are inferior, and some have poor plate shapes. can know 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 are formed in the proportion proposed by the present invention, 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 with an optical microscope. As can be seen from FIG. 3, in the case of Comparative Example 3, it can be confirmed that the hard phase consists of a bainite structure.

Claims (7)

In % by weight, C: 0.17 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 balance Fe and other unavoidable impurities,
The microstructure, by area%, contains 50% or more ferrite and 50% or less bainite,
A high-strength hot-rolled steel sheet having a low yield ratio including 5×10 ⁶ pieces/mm 2 or more of precipitates having an average size of 3 to 200 nm.
The method of claim 1,
The hot-rolled steel sheet is a high-strength hot-rolled steel sheet having a low yield ratio of 0.44 or less, represented by Equation 1 below.
[Equation 1] Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5)
The method of claim 1,
The hot-rolled steel sheet is a high-strength hot-rolled steel sheet having a low yield ratio of 0.26 or less Pcm represented by Equation 2 below.
[Formula 2] Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 +5B
The method of 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 low yield ratio having a yield ratio of 80% or less.
In % by weight, C: 0.17 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 balance Fe and other unavoidable impurities at 1100 to 1300 ° C;
Obtaining a hot-rolled steel sheet by finish hot-rolling the heated slab at 750 to 880° C.;
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 is 500 to 540° C.; and
Reheating the shear-cooled hot-rolled steel sheet so that the surface temperature thereof is 540 to 600 ° C. and then winding the high-strength hot-rolled steel sheet having a low yield ratio.
The method of claim 5,
The slab is a method for producing a high-strength hot-rolled steel sheet having a low yield ratio of 0.44 or less with Ceq represented by Equation 1 below.
[Equation 1] Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5)
The method of claim 5,
The slab is a method of manufacturing a high-strength hot-rolled steel sheet having a low yield ratio of 0.26 or less Pcm represented by Equation 2 below.
[Formula 2] Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 +5B

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