KR20230072050A - High strength steel plate having excellent impact toughness after cold forming and high yield ratio and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high-strength steel and a manufacturing method thereof, and more specifically to the high-strength steel that has excellent impact resistance after cold forming and a high yield ratio, and the manufacturing method thereof. In the present invention, the high-strength steel includes, in weight %, 0.05 to 0.15% of C, 0.01 to 0.5% of Si, 1.0 to 2.0% of Mn, 0.01 to 0.1% of Al, 0.001 to 1.0% of Cr, 0.001 to 0.05% of P, 0.001 to 0.01% of S, 0.001 to 0.01% of N, 0.03 to 0.08% of Ti, 0.01 to 0.05% of Nb, and the balance Fe and other inevitable impurities.

Description

High yield ratio high-strength steel with excellent impact resistance after cold forming and its manufacturing method

The present invention relates to high-strength steel and a method for manufacturing the same, and more particularly, to a high-strength steel having excellent impact resistance after cold forming and a high yield ratio, and a method for manufacturing the same.

For conventional commercial vehicle chassis parts, steel with a thickness of 8 mm or more is mainly applied due to the characteristics of the vehicle, and high-strength hot-rolled steel with a tensile strength of 600 MPa is used for members and hot-rolled steel with a tensile strength of 440 MPa is used for wheel disks. However, recently, in order to reduce weight, high-strength steel sheets with a tensile strength of 700 MPa or more and a tensile strength of 590 MPa or more are applied to members and wheel disks, and the thickness of the steel sheet is reduced or the design of the parts is changed. In addition, the wheel has been conventionally manufactured by a press forming process, but recently, it is a trend to be manufactured by spinning and flow forming. Since such a forming process gives a greater amount of deformation to the hot-rolled steel sheet, a hot-rolled steel sheet having a higher elongation is required, and the molded part is required to secure durability and impact resistance during use.

However, when the conventional high-strength steel is applied to the above spinning and flow forming processes, the durability of the part should be equal to or higher than that of the existing one, but when forming the part, fine cracks occur on the shear surface, etc. It is difficult to apply, such as cancellation.

As in Patent Documents 1 and 2, the conventional high-strength steel is wound at high temperature after undergoing normal austenitic hot rolling to form ferrite as a base structure and fine precipitates, and as in Patent Document 3, coarse To prevent the formation of pearlite, the winding temperature is cooled to the temperature at which the bainite base structure is formed, and then the winding technology is applied. In addition, as in Patent Document 4, a technique of refining austenite crystal grains by reducing the size to 40% or more in the non-recrystallization region during hot rolling using Ti, Nb, etc. has also been proposed. Recently, as in Patent Document 5, a technique for improving the uniformity of the microstructure between the thick surface layer portion and the deep layer portion of the steel sheet and suppressing the formation of coarse carbides has been proposed, and as in Patent Document 6, pearlite and MA phases that adversely affect durability (Martensite and Austenite) and a technique for suppressing the formation of martensite at the same time have been proposed.

However, Patent Documents 1 to 4 do not consider the occurrence of cracks on the shear surface and its surroundings during shear forming of high-strength thick material materials, and in the case of thick material materials having a thickness of 8 mm or more, it is difficult to secure cooling rate conditions and large pressure made up of conditions. When using precipitate-forming elements such as Ti, Nb, and V to secure strength while miniaturizing the crystal grains of the material material, when coiled at a high temperature of 500~700℃ where precipitates are easily formed, ferrite grows excessively and the yield strength decreases. There is a problem of reduced and coarse pearlite formation. In addition, even when manufacturing at a low coiling temperature to utilize the bainite matrix structure, if the cooling rate in the width direction of the steel sheet is not uniformly controlled during cooling after hot rolling, a non-uniform microstructure is formed on the hot rolled steel sheet, making it difficult to have high elongation and stable resistance. It becomes difficult to secure composite ratio properties, and the sensitivity to forming cracks, such as cracks on the shear surface, increases during molding. Moreover, since these technologies target hot-rolled steel sheets with a thickness of less than 5 mm used for automobiles, the required cooling rate is too high, making them unsuitable for manufacturing thick materials. In addition, applying a large reduction of 40% in the non-recrystallization region deteriorates the shape quality of the rolled sheet and may cause a load on equipment, making it difficult to apply to a thick material having a thickness of 8 mm or more. Patent Literatures 5 and 6 are inventions for post-material materials. First, Patent Document 5 provides a technology for manufacturing to suppress the formation of MA phase and martensite by making the crystal grain shape in the deep-thickness portion (1/4t to 1/2t) have equiaxed and fine crystal grains in order to improve the durability of thick high-strength steel. suggested. Patent Document 6 proposes a technique for manufacturing by dividing a hot-rolled coil into three parts in the longitudinal direction through a relational expression derived for a specific component, cooling the head, mid, and tail under constant cooling rate conditions to different cooling end temperatures, respectively, and then winding it up. It became. These technologies are technologies that uniformly manufacture microstructures by controlling the cooling rate after hot rolling through a relational expression derived for a specific component in consideration of the quality of the shear surface of the part. There is an effective aspect to improve the durability of the applied commercial vehicle wheel, but the impact resistance after molding has not been considered. In addition, it is difficult to uniformly control the cooling of the steel sheet after hot rolling over the entire width, and in the case of a thick hot-rolled steel sheet having a thickness of 8 mm or more, it is difficult to control the actual cooling rate.

The cooling process after hot rolling usually takes place within tens of seconds at a ROT (Run-Out Table) with a length of 100 to 120 m. difficult. Therefore, in the prior art, the hot-rolled steel sheet having a thickness of 8 mm or more is difficult to obtain an effect of inhibiting the formation of coarse carbides and is insufficient to secure a high level of impact resistance.

Korean Patent Publication No. 10-2010-0029138 (published on March 15, 2010) Japanese Unexamined Patent Publication No. 2007-262487 (published on October 11, 2007) Korean Registered Patent Publication No. 10-1528084 (2015.06.10. Publication) Japanese Unexamined Patent Publication No. 9-143570 (published on June 3, 1997) Korean Patent Publication No. 10-2020-0062422 (published on June 4, 2020) Korean Patent Publication No. 10-2021-0068808 (published on June 10, 2021)

According to one aspect of the present invention, it is intended to provide a high-strength steel having excellent impact resistance and a high yield ratio after cold forming, and a manufacturing method thereof.

The object of the present invention is not limited to the above. A person skilled in the art will have no difficulty understanding the further subject matter of the present invention from the general content of this specification.

One aspect of the present invention, in weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.001% 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, the balance including Fe and other unavoidable impurities, and the sum of Nb and Ti is 0.04 to 0.1 %ego,

The surface layer portion with a thickness of 50 μm from the surface contains 95 area% or more of equiaxed ferrite and 3 area% or less of pearlite as a microstructure, and contains 1 of bainitic ferrite, bainite, MA (Martensite-Austenite constituent) phase and martensite Containing more than 5 area% in total,

The central part in the range of 1/4 to 3/4 in thickness has a microstructure of bainitic ferrite by 80 to 95 area%, bainite by 10 area% or less, pearlite by 3 area% or less, MA (Martensite-Austenite constituent) phase and marten It is possible to provide a steel sheet containing 5 to 10 area% of one or two types of sites in total and containing equiaxed crystal ferrite as the remainder.

The steel sheet may have a thickness of 8 to 25 mm.

The steel sheet may have tensile strength of 590 MPa or more, elongation at break of 25% or more, yield ratio of 0.75 to 0.9, and impact toughness at -20° C. of 70 J or more after cold forming.

The steel sheet may have a ratio (E/YS) of impact toughness (E) at -20° C. after cold forming and yield strength (YS) before cold forming (E/YS) of 0.15 or more.

In the width direction of the steel sheet, the difference in tensile strength between the edge portion of 30% at both ends and the center portion of 40% excluding both edge portions is 10 MPa or less, the difference in elongation at break is 8% or less, and the difference in impact toughness at -20 ° C. It may be 20 J or less.

Another aspect of the present invention, in weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 ~0.05%, S: 0.001~0.01%, N: 0.001~0.01%, Ti: 0.03~0.08%, Nb: 0.01~0.05%, the balance including Fe and other unavoidable impurities, the sum of Nb and Ti being 0.04~ reheating the 0.1% steel slab;

hot rolling the reheated steel slab; and

Cooling and winding the hot-rolled steel sheet at an average cooling rate equal to or greater than the CR value defined in the following relational expression 1 within the range of 1 to 30 ° C / s to a temperature range of 500 to 650 ° C, and

In the cooling and winding step, the edge portion of 30% from both ends in the coil width direction has a temperature (TE) of 550 to 650 ° C, and the central portion of 40% of the center excluding both edge portions in the width direction has a temperature of 500 to 550 ° C. (TC) to cool,

It is possible to provide a steel sheet manufacturing method in which the average temperature difference between the edge portion and the central portion is 50 to 150 ° C.

[Relationship 1]

CR = 45-16.3x[C]-5.6x[Si]-16.3x[Mn]-2.9x[Cr]+15x[Ti]+23x[Nb]-0.9x(t-8)

(Where [C], [Si], [Mn], [Cr], [Ti], and [Nb] are the weight percent of each element, and t is the thickness (mm) of the steel sheet.)

The reheating temperature is 1100 ~ 1350 ℃,

The hot rolling temperature may be 800 ~ 1150 ℃.

Air-cooling the wound coil to a temperature range of 200° C. or less may be further included.

The steel sheet may have a thickness of 8 to 25 mm.

According to one aspect of the present invention, it is possible to provide a high-strength steel having excellent impact resistance after cold forming and a high yield ratio, and a manufacturing method thereof.

According to one aspect of the present invention, it is possible to provide a high-strength steel that can be applied as a steel material used for chassis members and wheels of medium-large commercial vehicles and a manufacturing method thereof.

1 is a photograph showing the microstructure of Inventive Example 2 according to an embodiment of the present invention observed with a scanning electron microscope (x3,000).
Figure 2 is a photograph showing the microstructure of Comparative Example 2 according to an embodiment of the present invention observed with a scanning electron microscope (x3,000).

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

The present inventors studied the change in impact resistance after cold forming according to the characteristics of the microstructure of the steel sheet in order to solve the above-mentioned conventional problems and to secure excellent formability and impact resistance. Accordingly, it was confirmed that target physical properties can be secured by controlling the microstructure along the thickness and width direction of the steel sheet by optimizing the alloy composition and manufacturing conditions, and the present invention has been completed.

In a hot-rolled steel sheet usually manufactured in the form of a coil, coarse carbides and pearlite are likely to be formed when maintained in a high temperature region of about 500 to 700 ° C. for a long time. In particular, when the ferrite phase transformation initiated in the cooling process after the completion of hot rolling proceeds slowly, the solid solution of carbon increases in the non-transformed phase, making it easy to form coarse carbides or pearlite. Moreover, the cooling rate of the central part of the width of the coil is slower than that of the edge part, so that such a structure is further developed. Therefore, in order to suppress the formation of such coarse carbides and pearlite at the center of the coil width, it is necessary to cool the wound coil to room temperature through forced cooling such as water cooling. (Martensite and Austenite) phases are excessively formed, making the microstructure non-uniform, making it difficult to secure high elongation, and increasing shear cracks, which is undesirable. Therefore, the present invention proposes a method capable of suppressing the formation of coarse carbides and pearlite without forcibly cooling the coil.

Hereinafter, the present invention will be described in detail.

Hereinafter, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, % indicating the content of each element is based on weight.

Steel according to one aspect of the present invention, by weight%, C: 0.05 ~ 0.15%, Si: 0.01 ~ 0.5%, Mn: 1.0 ~ 2.0%, Al: 0.01 ~ 0.1%, Cr: 0.001 ~ 1.0%, P: 0.001 to 0.05% S: 0.001 to 0.01%, N: 0.001 to 0.01%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, the balance including Fe and other unavoidable impurities, the sum of Ti and Nb being 0.04 It can be -0.1%.

Carbon (C): 0.05 to 0.15%

Carbon (C) is the most economical and effective element for strengthening steel, and when the added amount increases, the precipitation hardening effect or the bainite phase fraction increases, making it easy to secure strength. However, when the thickness of the hot-rolled steel sheet increases, the cooling rate at the center of the thickness decreases during cooling after hot rolling, so that coarse carbides or pearlite are easily formed when the carbon (C) content is high. Therefore, if the carbon (C) content is less than 0.05%, it is difficult to obtain a sufficient strengthening effect, and if the content exceeds 0.15%, there is a problem in that impact resistance is lowered due to the formation of pearlite or coarse carbides in the center of the thickness, and the weldability is also poor. there is a risk of A more preferred lower limit may be 0.055%, and a more preferred upper limit may be 0.12%.

Silicon (Si): 0.01 to 0.5%

Silicon (Si) is an element that is effective in deoxidizing molten steel and strengthening steel by solid solution, and is also effective in improving formability by delaying the formation of coarse carbides. However, if the content is less than 0.01%, the effect of delaying solid solution strengthening and carbide formation cannot be maximized, and if the content exceeds 0.5%, red scale is formed on the surface of the steel sheet during hot rolling, resulting in very poor quality. In addition, there may be problems in that ductility and weldability are also deteriorated. More preferably, it may contain 0.05% or more, and more preferably, it may contain 0.3% or less.

Manganese (Mn): 1.0 to 2.0%

Manganese (Mn), like Si, is an element effective for solid solution strengthening of steel, and may facilitate the formation of bainite during cooling after hot rolling by increasing hardenability of steel. However, if the content is less than 1.0%, the above effects due to addition cannot be obtained, and if it exceeds 2.0%, the hardenability is greatly increased, so that martensitic phase transformation is likely to occur and segregation at the center of the thickness is greatly developed during slab casting in the casting process. There are concerns. More preferably, it may contain 1.3% or more, and more preferably, it may contain 1.8% or less.

Aluminum (Al): 0.01 to 0.1%

Aluminum (Al) is a component mainly added for deoxidation, and if the content thereof is less than 0.01%, the effect of addition may be insufficient. On the other hand, when the content exceeds 0.1%, AlN is formed by combining with nitrogen, and thus corner cracks easily occur in the slab during continuous casting, and defects due to inclusion formation easily occur. More preferably 0.015% or more, more preferably 0.06% or less.

Chromium (Cr): 0.001 to 1.0%

Chromium (Cr) strengthens steel by solid solution similarly to Mn, and serves to help bainite formation by delaying ferrite phase transformation during cooling. However, if the content is less than 0.001%, the effect of addition cannot be obtained, and if it exceeds 1.0%, ferrite transformation is excessively delayed, and elongation may be inferior due to excessive martensite formation. In addition, excessive addition of Cr greatly develops segregation at the center of the thickness, and makes the microstructure in the thickness direction non-uniform, resulting in inferior impact resistance. A more preferred lower limit may be 0.01%, and a more preferred upper limit may be 0.5%.

Phosphorus (P): 0.001 to 0.05%

Phosphorus (P) has the effect of strengthening solid solution and accelerating ferrite transformation at the same time as Si. However, in order to manufacture a phosphorus (P) content of less than 0.001%, a lot of manufacturing costs are required, which is economically unfavorable, and is insufficient to obtain strength. On the other hand, if the content exceeds 0.05%, brittleness occurs due to grain boundary segregation, and it is easy to generate fine cracks during molding, which can significantly deteriorate impact resistance.

Sulfur (S): 0.001 to 0.01%

S is an impurity present in steel, and when its content exceeds 0.01%, it combines with Mn to form non-metallic inclusions. Accordingly, there is a problem in that it is easy to generate fine cracks during molding and significantly lowers the impact resistance. However, in order to manufacture the content to less than 0.001%, a lot of time is required during steelmaking operation, resulting in a decrease in productivity.

Nitrogen (N): 0.001 to 0.01%

Nitrogen (N) is a representative solid-solution strengthening element together with C, and forms coarse precipitates with Ti and Al. In general, the solid solution strengthening effect of nitrogen (N) is superior to that of C, but as the amount of nitrogen (N) in steel increases, the toughness decreases significantly. Therefore, the upper limit of the content may be limited to 0.01%. On the other hand, in order to manufacture the content to less than 0.001%, a lot of time is required during steelmaking operation, resulting in a decrease in productivity.

Titanium (Ti): 0.03 to 0.08%

Titanium (Ti) is a typical precipitation hardening element and forms coarse TiN in steel due to its strong affinity with N. TiN has an effect of suppressing the growth of crystal grains during the heating process for hot rolling. In addition, titanium (Ti) remaining after reacting with N is dissolved in steel and combined with C to form TiC precipitates, which is a useful component for improving the strength of steel. If the content of titanium (Ti) is less than 0.03%, the above effect cannot be obtained, and if the content exceeds 0.08%, there is a problem of inferior crash resistance during molding due to generation of coarse TiN and coarsening of precipitates. More preferably, it may contain 0.04% or more, and more preferably, it may contain 0.075% or less.

Niobium (Nb): 0.01 to 0.05%

Niobium (Nb) is a representative precipitation hardening element along with Ti, and is effective in improving the strength and impact toughness of steel by precipitating during hot rolling and refining grains by recrystallization delay. If the content of niobium (Nb) is less than 0.01%, the above effect cannot be obtained, and if the content exceeds 0.05%, formability is deteriorated due to the formation of elongated crystal grains and the formation of coarse complex precipitates due to excessive recrystallization delay during hot rolling. There is a problem with A more preferred lower limit may be 0.015%, and a more preferred upper limit may be 0.04%.

The steel of the present invention may include the remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing field, not all of them are specifically mentioned in this specification.

In the steel according to one aspect of the present invention, the sum of niobium (Nb) and titanium (Ti) may be 0.04 to 0.1%.

Niobium (Nb) and titanium (Ti) are precipitated as (Ti, Nb) (C, N)-based composite precipitates, and when precipitated during hot rolling, the crystal grain refinement effect due to recrystallization delay is greatly increased. However, if the formation of the complex precipitates is excessive, there is a problem in that the strength improvement effect is small and the formability is inferior because the coarse complex precipitates increase. When the sum of niobium (Nb) and titanium (Ti) is less than 0.04%, the effect of grain refinement and strength improvement may be small. On the other hand, if the sum exceeds 0.1%, formability is inferior and economically unfavorable. A more preferred lower limit may be 0.045%, and a more preferred upper limit may be 0.09%.

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, % representing the fraction of the microstructure is based on the area unless otherwise specified.

In the steel according to one aspect of the present invention, the surface layer having a thickness of 50 μm from the surface contains 95 area% or more of equiaxed ferrite and 3 area% or less of pearlite as a microstructure, and bainitic ferrite, bainite, MA (Martensite- It contains 5 area% or less of one or more of the austenite constituent phase and martensite in total, and the central part in the thickness range of 1/4 to 3/4 contains 80 to 95 area% of bainitic ferrite and bainite as a microstructure. 10 area% or less, 3 area% or less of pearlite, 5 to 10 area% of one or two of the MA (Martensite-Austenite constituent) phase and martensite in total, and the rest equiaxed ferrite may be included.

In the present invention, if the equiaxed crystal ferrite is less than 95% in the surface layer portion, ductility is insufficient during spinning and flow forming molding applied during commercial vehicle wheel manufacturing, and work hardening in the surface layer portion is severe, so that fine cracks may occur during molding. In particular, cracks easily propagate along the interface with the matrix phase when brittle pearlite is formed at 3% or more, or at least one of bainitic ferrite, bainite, MA phase, and martensite with high hardness is included in an amount exceeding 5%. There may be problems with being Therefore, in order to suppress the occurrence of fine cracks formed in the surface layer during molding and prevent the propagation of cracks, it is preferable to include 5% or less in total of one or more of bainitic ferrite, bainite, MA phase, and martensite. In the present invention, equiaxed crystal ferrite may be 100% as the microstructure of the surface layer portion, and the sum of pearlite, bainitic ferrite, bainite, MA phase, and martensite may be 0%.

In addition, if the bainitic ferrite is less than 80% in the center, there is a problem that cracks easily occur on the shear surface during punching and shear molding during wheel manufacturing, and after molding, there is a problem that the impact resistance is also inferior. In addition, in the process of cooling the steel sheet after hot rolling when manufacturing the steel sheet, bainitic ferrite, which is a matrix structure, is formed in the center of the thickness of the rolled sheet, and a high concentration of residual C remains in the untransformed austenite, so it is easy to form pearlite. can At this time, when the pearlite is formed in excess of 3%, cracks are severely generated on the shear surface during the molding process, and the impact resistance after molding is also inferior. When the pearlite fraction is 3% or less, there is no cracking caused by molding such as shearing, and the impact resistance at low temperatures may be excellent. In the present invention, carbides and nitrides having a diameter of 1 μm or more may be included as pearlite.

In contrast, when the MA phase or martensite is included in an amount of 5 to 10%, crack generation is not affected, and it may be advantageous to secure impact resistance and high strength after molding. The MA phase has an advantage in securing high strength by forming dislocation density around it, and when formed with a base structure composed of ferrite and bainite, it can have excellent impact resistance even if the dislocation density increases after cold forming. However, when the MA phase or martensite is less than 5%, yield strength and tensile strength are insufficient, and when it is included in excess of 10%, there is a problem in that ductility is insufficient and formability is inferior. When the bainite content also exceeds 10%, there may be a problem of insufficient ductility.

In the present invention, bainite and pearlite may each be 0% as the microstructure of the center, and may inevitably include equiaxed ferrite in addition to bainitic ferrite, bainite, pearlite, MA phase, and martensite.

In the present invention, the area fraction of the microstructure can be analyzed using an optical microscope and a scanning electron microscope (SEM), and the area fraction of the image obtained from the image observed at 3,000 magnification at a location corresponding to the center of the thickness of the rolled section can be measured

Hereinafter, the steel manufacturing method of the present invention will be described in detail.

Steel according to one aspect of the present invention can be produced by reheating, rolling, cooling and winding a steel slab satisfying the above-described alloy composition.

reheat

A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1100 to 1350 ° C.

If the reheating temperature is less than 1100 ° C., the formation of precipitates may be reduced in the process after hot rolling because the precipitates are not sufficiently re-dissolved, and coarse TiN may remain. On the other hand, when the temperature exceeds 1350 ° C., strength may be reduced due to abnormal grain growth of austenite grains.

hot rolled

The reheated steel slab may be hot-rolled in a temperature range of 800 to 1150 ° C.

When the hot-rolling temperature exceeds 1150° C., the temperature of the hot-rolled steel sheet increases, resulting in coarse grain size and poor surface quality of the hot-rolled steel sheet. On the other hand, if the temperature is less than 800 ° C, elongated crystal grains may develop due to recrystallization retardation, resulting in severe anisotropy and poor formability. can

cooling and winding

The hot-rolled steel sheet may be cooled and wound at an average cooling rate greater than or equal to the CR value defined in the following relational expression 1 within the range of 1 to 30 °C/s up to a temperature range of 500 to 650 °C. During the cooling, the edge portion, which is 30% from both ends in the coil width direction, has a temperature (TE) of 550 to 650 ° C, and the central portion, which is 40% of the center excluding both edge portions in the width direction, has a temperature (TC) of 500 to 550 ° C. can be cooled with At this time, the average temperature difference between the edge portion and the central portion may be 50 to 150°C.

In the present invention, relational expression 1 was derived in order to induce an appropriate level of ferrite phase transformation, form a fine and uniform MA phase, and suppress excessive pearlite formation during cooling of the steel sheet. When the cooling rate is less than the CR value of relational expression 1, ferrite in the center of the thickness becomes coarse, pearlite is excessively formed, and cracks on the shear surface become severe, and impact resistance after molding may be inferior. In addition, when the cooling rate exceeds 30 °C / s, bainite, MA phase, and martensite are excessively formed, resulting in insufficient ductility and poor shear quality.

[Relationship 1]

CR = 45-16.3x[C]-5.6x[Si]-16.3x[Mn]-2.9x[Cr]+15x[Ti]+23x[Nb]-0.9x(t-8)

(Where [C], [Si], [Mn], [Cr], [Ti], and [Nb] are the weight percent of each element, and t is the thickness (mm) of the steel sheet.)

In order to suppress excessive carbide and pearlite formation, the cooling end temperature should be lowered during cooling after hot rolling, but there is a concern that it is difficult to secure the target elongation due to ferrite reduction due to excessive formation of bainite or excessive formation of MA phase and martensite. .

Therefore, in the present invention, in order to increase the cooling rate at the central portion in the width direction of the coil and reduce the time the coil is maintained at a high temperature after winding, during cooling after hot rolling, the cooling end temperature of the central portion in the width direction and the edge portion may be set differently. there is. However, at this time, the average temperature difference between the edge portion and the central portion may be 50 to 150°C. If the average temperature difference is less than 50 ° C., it may be difficult to obtain the above effect. On the other hand, if the temperature exceeds 150 °C, the above effect does not increase any more, but it may be difficult to control the temperature of each section of the coil.

In the present invention, a method of differently controlling the cooling end temperature of the edge portion and the center portion during winding is not particularly limited. For example, when cooling a hot-rolled steel sheet, the cooling water poured into the edge portion is blocked before reaching the steel sheet. Alternatively, a method of differently adjusting the amount of cooling water injected can be applied. Alternatively, both methods may be used in parallel.

In the present invention, it is preferable to satisfy both the relational expression 1 and the cooling end temperature condition in order to secure the desired strength, moldability and impact resistance. When all of the above cooling conditions are satisfied, bainitic ferrite is used as a base structure in the center of the thickness direction to have a uniform and fine microstructure, and coarse carbides or pearlite are reduced in the inner winding part and the center of the thickness of the coil with a slow cooling rate. The non-uniform structure of the hot-rolled steel sheet can be eliminated. In addition, non-uniform formation of the MA phase and formation of coarse martensite can be suppressed in the outer winding part and the edge part of the coil where the cooling rate is relatively fast.

Cooling

The wound coil may be air-cooled to a temperature range of 200° C. or less.

In the present invention, the wound coil can be air-cooled to a temperature range of 200° C. or less. Air cooling of the coil means cooling in the air at room temperature at a cooling rate of 0.001 to 10 °C/h. At this time, if the cooling rate exceeds 10 ° C / h, some of the non-transformed phase of the steel in the outer winding portion of the coil is easily transformed to the MA phase, and the shear formability and punching formability of the steel may be inferior. On the other hand, in order to control the cooling rate to less than 0.001 ° C / h, separate heating and heat preservation facilities are required, which may be economically disadvantageous. A more preferred lower limit may be 0.01 °C/h, and a more preferred upper limit may be 1 °C/h.

The steel of the present invention thus prepared has a thickness of 8 to 25 mm, a tensile strength of 590 MPa or more, a breaking elongation of 25% or more, a yield ratio of 0.75 to 0.9, and an impact toughness at -20 ° C of 70 J after cold forming. In the above, the ratio of the impact toughness after cold forming and the yield strength before cold forming is 0.15 or more, so that while having a high yield ratio, it is possible to have excellent impact toughness characteristics. In addition, the difference between the central portion and the edge portion in tensile strength may be 10 MPa or less, the difference in elongation at break may be 8% or less, and the difference in impact toughness at -20 ° C may be 20 J or less.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention.

(Example)

A steel slab having the alloy composition shown in Table 1 below was prepared as a hot-rolled steel sheet under the conditions shown in Table 2 below. At this time, the steel slab was reheated to a temperature of 1100 ~ 1350 ℃ and then hot rolled. Table 2 shows the cooling rate applied during manufacturing and the CR value of relational expression 1, and the cooling end temperature is a temperature (TC) in the range of 40% in the center of the width direction and a temperature (TE) in the range of 30% in both edge parts in the width direction, respectively. indicated respectively. In addition, the temperature difference between the temperature of the central part and the temperature of the edge part was shown.

steel grade Alloy composition (wt%) C Si Mn Cr Al P S N Ti Nb Ti+Nb A 0.11 0.11 1.2 0.5 0.03 0.01 0.003 0.004 0.05 0.03 0.08 B 0.06 0.21 1.2 0.9 0.03 0.01 0.003 0.005 0.04 0.04 0.08 C 0.08 0.18 1.7 0.5 0.03 0.01 0.003 0.005 0.05 0.04 0.09 D 0.07 0.08 1.6 0.7 0.03 0.008 0.003 0.005 0.05 0.02 0.07 E 0.06 0.25 1.7 0.5 0.03 0.005 0.002 0.004 0.07 0.04 0.11 F 0.07 0.08 1.6 0.7 0.03 0.008 0.003 0.005 0.05 0.02 0.07 G 0.07 0.08 1.6 0.7 0.03 0.008 0.003 0.005 0.05 0.02 0.07 H 0.11 0.11 1.2 0.5 0.03 0.01 0.003 0.004 0.05 0.03 0.08 I 0.06 0.21 1.2 0.9 0.03 0.01 0.003 0.005 0.04 0.04 0.08 J 0.08 0.18 1.7 0.5 0.03 0.01 0.003 0.005 0.05 0.04 0.09 K 0.06 0.11 1.7 0.5 0.03 0.01 0.003 0.004 0.02 0.015 0.035

Psalter
number steel grade thickness
(mm) hot rolled Cooling Temperature (℃) speed
(℃/s) Relation 1
(CR) end temperature TE(℃) TC(℃) |TE-TC| One A 10 900 30 21 605 520 85 2 B 12 870 25 19 590 515 75 3 C 20 830 15 4 610 535 75 4 D 15 850 25 10 580 525 55 5 E 18 840 15 6 600 520 80 6 F 11 880 5 14 560 510 50 7 G 20 840 15 6 580 700 120 8 H 21 840 15 11 700 520 180 9 I 11 850 25 19 600 400 200 10 J 18 850 15 6 300 510 210 11 D 6 870 25 18 590 520 70 12 K 18 885 8 6 570 510 60 13 F 11 890 40 14 580 510 70

[Relationship 1]

CR = 45-16.3x[C]-5.6x[Si]-16.3x[Mn]-2.9x[Cr]+15x[Ti]+23x[Nb]-0.9x(t-8)

(Where [C], [Si], [Mn], [Cr], [Ti], and [Nb] are the weight percent of each element, and t is the thickness (mm) of the steel sheet.)

In Table 3 below, the microstructure of the manufactured steel sheet was measured and described. The microstructure was expressed by measuring the surface layer portion and the center portion in the thickness direction, respectively, and measuring the fractions of the edge portion and the center portion in the width direction, respectively. The microstructure of the surface layer part was observed from the surface to the thickness of 50 μm, and the center part was observed for the 1/4 ~ 3/4t (t is thickness (mm)) part. In addition, the microstructure of the portion corresponding to 30% of the edge portion was observed at both ends in the width direction, and the central portion was observed based on the portion corresponding to the central 40% excluding the edge portion. The area fraction of the MA phase and martensite was measured using an optical microscope and an image analyzer after etching with the Lepera etching method, and was analyzed at 1000 magnification. In addition, the fractions of equiaxed ferrite (PF), bainitic ferrite (BF), bainite (B) and pearlite (P) were measured using a scanning electron microscope (SEM) at 3000x and 5000x magnifications. did Here, PF is polygonal ferrite having an equiaxed crystal shape, and BF may include ferrite observed in a low temperature region such as acicular ferrite and bainitic ferrite. In addition, P included pearlite and coarse carbides and nitrides having a diameter of 1 μm or more.

Psalter
number steel grade microstructure (% area) location surface layer center PF bf B P MA Mart PF bf B P MA Mart One A central part 98 0 0 One One 0 6 82 7 0 5 0 edge part 99 0 0 One 0 0 4 85 5 0 6 0 2 B central part 98 0 0 One One 0 4 81 7 2 6 0 edge part 99 0 0 One 0 0 5 82 8 0 5 0 3 C central part 98 0 0 One One 0 7 83 3 0 7 0 edge part 98 0 0 2 0 0 5 85 4 0 6 0 4 D central part 97 0 One One One 0 5 83 4 One 7 0 edge part 98 0 0 One One 0 7 84 3 0 6 0 5 E central part 97 0 0 2 One 0 7 81 4 2 6 0 edge part 98 0 0 One One 0 6 82 5 2 5 0 6 F central part 92 0 0 8 0 0 13 75 2 7 3 0 edge part 93 0 0 7 0 0 12 79 2 5 2 0 7 G central part 93 0 0 5 2 0 15 72 0 11 2 0 edge part 95 0 0 3 2 0 23 68 0 6 3 0 8 H central part 98 0 0 2 0 0 5 83 4 One 7 0 edge part 90 3 0 5 2 0 15 68 8 8 One 0 9 I central part 12 69 0 6 8 5 18 61 12 0 7 2 edge part 6 82 5 2 5 0 12 82 0 0 5 One 10 J central part 5 86 8 0 One 0 9 78 5 One 7 0 edge part 10 65 5 2 8 10 12 62 19 One 6 0 11 D central part 75 23 0 2 0 0 12 70 5 5 8 0 edge part 87 12 0 One 0 0 8 83 2 5 2 0 12 K central part 97 2 0 One 0 0 10 82 0 2 6 0 edge part 95 3 0 2 0 0 11 83 2 2 2 0 13 F central part 85 12 0 0 3 0 5 77 13 2 3 0 edge part 82 10 5 0 3 0 8 82 7 One 2 0

In Table 4 below, the physical property values for each of the manufactured specimens are measured and shown for the central portion and the edge portion in the width direction. Yield strength (YS), tensile strength (TS), yield ratio (YR), and breaking elongation (T-El) were evaluated by taking a JIS5 standard test piece in a direction perpendicular to the rolling direction and performing a tensile test. In addition, the impact absorption energy (E) at -20 ℃ after cold forming was measured, and the ratio (E / YS) of impact absorption energy and yield strength at -20 ℃ after cold forming was shown. The impact absorption energy was tested by using Charpy V-notch specimens manufactured in accordance with the ASTM standard (ASTM A370), sampled in a direction perpendicular to the rolling direction.

city
side
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bell Properties division width direction center Edge part in the width direction YS
(MPa) TS
(MPa) YR U-
El
(%) T-
El
(%) E
(J) E/YS YS
(MPa) TS
(MPa) YR U-
El
(%) T-
El
(%) E
(J) E/YS One A 557 655 0.85 8 27 102 0.18 552 648 0.85 8 27 109 0.20 Invention example 1 2 B 524 624 0.84 8 28 107 0.20 527 630 0.84 8 28 115 0.22 Invention Example 2 3 C 496 605 0.82 9 35 113 0.23 491 608 0.81 9 34 125 0.25 Invention example 3 4 D 536 631 0.85 8 30 108 0.20 528 637 0.83 8 31 112 0.21 Invention Example 4 5 E 541 615 0.88 8 34 67 0.12 532 603 0.88 8 33 72 0.13 Comparative Example 1 6 F 525 632 0.83 8 27 58 0.11 520 636 0.82 8 26 65 0.13 Comparative Example 2 7 G 503 609 0.83 7 31 54 0.11 518 613 0.84 7 29 62 0.12 Comparative Example 3 8 H 489 598 0.82 8 32 85 0.17 507 611 0.83 8 33 56 0.11 Comparative Example 4 9 I 544 635 0.86 7 15 118 0.22 530 623 0.85 8 25 108 0.20 Comparative Example 5 10 J 515 621 0.83 7 27 72 0.14 516 653 0.79 6 24 111 0.22 Comparative Example 6 11 D 552 645 0.86 7 18 115 0.21 542 640 0.85 7 18 128 0.24 Comparative Example 7 12 K 493 572 0.83 9 36 45 0.09 490 577 0.85 8 35 59 0.12 Comparative Example 8 13 F 534 643 0.83 7 24 125 0.23 536 649 0.83 7 23 130 0.24 Comparative Example 9

As shown in Tables 3 and 4, in the case of inventive examples satisfying the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the desired physical properties were secured in the present invention. 1 is a photograph showing the microstructure of a hot-rolled steel sheet of Inventive Example 2 according to one aspect of the present invention observed with a scanning electron microscope (x3,000).

On the other hand, in Comparative Example 1, when the sum of Ti and Nb contents exceeded the scope of the present invention, impact resistance was poor due to the formation of coarse precipitates and TiN due to excessive precipitates in ferrite grains.

Comparative Example 2 is a case where the cooling rate did not meet the cooling rate standard proposed in relational expression 1, and as shown in FIG. 2, excessive pearlite was formed in the microstructure.

As shown in FIG. 2, due to this, the strength did not decrease significantly, but the target impact resistance could not be secured.

In Comparative Example 3, the coiling temperature of the center portion in the width direction exceeded the range proposed in the present invention. In particular, excessive pearlite was formed in the center portion in the thickness direction in both the center portion and the edge portion, and the target impact resistance was not secured. .

Comparative Example 4 is a case where the winding temperature of the edge portion in the width direction exceeds the range proposed in the present invention. Accordingly, the impact resistance was inferior due to the formation of excessive pearlite at the center of the thickness direction of the edge portion. This is because the temperature of the edge portion is high and the heat transfer of the winding coil proceeds slowly at the edge portion.

Comparative Example 5 is a case where the winding temperature of the central part in the width direction does not reach the range of the present invention, and bainite is excessively formed in the central part in the thickness direction of the central part, and pearlite, MA phase, martensite, etc. are at the level proposed in the present invention It was excessively formed and the elongation was inferior. On the other hand, the edge portion satisfying the coiling temperature range had relatively good elongation and impact resistance, but the ferrite in the surface layer portion fell short of the range proposed in the present invention. It is believed that this is because the temperature of the edge portion of the coil after winding also rapidly decreased due to the low cooling end temperature of the central portion in the width direction.

In Comparative Example 6, when the winding temperature of the edge portion in the width direction did not reach the range of the present invention, bainite was excessively formed at the center in the thickness direction, and it was confirmed that the elongation rate was inferior. In addition, the surface layer of the central part lacked ferrite, and the central part lacked bainitic ferrite, and therefore, the ratio of impact resistance and yield strength did not satisfy the level proposed by the present invention.

Comparative Example 7 is a case where the thickness of the steel is less than 8 mm, and since the cooling rate is excessively applied for a given steel component, ferrite is insufficient in the surface layer portion, and bainitic ferrite is reduced in the center in the thickness direction. At the same time, pearlite is excessively formed. . This is considered to be that the untransformed phase increased during the initial cooling process, and pearlite was formed in a region with a relatively high C content. As a result, the target elongation was inferior.

In Comparative Example 8, the sum of Ti and Nb contents did not fall within the scope of the present invention, and the fraction of phases at each position in the microstructure satisfied the range suggested by the present invention, but due to the decrease in precipitation during hot rolling, the microstructure It became coarse and fine precipitates also decreased after cooling and winding, resulting in insufficient strength and greatly reduced impact resistance.

Comparative Example 9 is a case where the cooling rate exceeds the CR value of relational expression 1 and satisfies the range proposed in the present invention, but exceeds 30 ° C / s. As a result, the polygonal ferrite in the surface layer was insufficient, and bainite was excessively formed in the center in the thickness direction, so that the desired level of elongation could not be secured.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (9)

In weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.001% 0.01%, N: 0.001-0.01%, Ti: 0.03-0.08%, Nb: 0.01-0.05%, the balance including Fe and other unavoidable impurities, the sum of Nb and Ti is 0.04-0.1%,
The surface layer portion with a thickness of 50 μm from the surface contains 95 area% or more of equiaxed ferrite and 3 area% or less of pearlite as a microstructure, and contains 1 of bainitic ferrite, bainite, MA (Martensite-Austenite constituent) phase and martensite Containing more than 5 area% in total,
The central part in the range of 1/4 to 3/4 in thickness has a microstructure of bainitic ferrite by 80 to 95 area%, bainite by 10 area% or less, pearlite by 3 area% or less, MA (Martensite-Austenite constituent) phase and marten A steel sheet containing 5 to 10% by area of one or two types of sites in total and the remainder containing equiaxed ferrite.
According to claim 1,
The steel plate is a steel plate having a thickness of 8 to 25 mm.
According to claim 1,
The steel sheet has a tensile strength of 590 MPa or more, a breaking elongation of 25% or more, a yield ratio of 0.75 to 0.9, and an impact toughness at -20 ° C. of 70 J or more after cold forming.
According to claim 1,
The steel sheet has a ratio (E / YS) of impact toughness (E) at -20 ° C after cold forming and yield strength (YS) before cold forming is 0.15 or more.
According to claim 1,
In the width direction of the steel sheet, the difference in tensile strength between the edge portion of 30% at both ends and the center portion of 40% excluding both edge portions is 10 MPa or less, the difference in elongation at break is 8% or less, and the difference in impact toughness at -20 ° C. Steel plate with 20J or less.
In weight percent, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.01 to 0.1%, Cr: 0.001 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.001% 0.01%, N: 0.001-0.01%, Ti: 0.03-0.08%, Nb: 0.01-0.05%, balance Fe and other unavoidable impurities, and the sum of Nb and Ti is 0.04-0.1%. step;
hot rolling the reheated steel slab; and
Cooling and winding the hot-rolled steel sheet at an average cooling rate equal to or greater than the CR value defined in the following relational expression 1 within the range of 1 to 30 ° C / s to a temperature range of 500 to 650 ° C, and
In the cooling and winding step, the edge portion of 30% from both ends in the coil width direction has a temperature (TE) of 550 to 650 ° C, and the central portion of 40% of the center excluding both edge portions in the width direction has a temperature of 500 to 550 ° C. (TC) to cool,
A method for manufacturing a steel plate in which the average temperature difference between the edge portion and the center portion is 50 to 150 ° C.
[Relationship 1]
CR = 45-16.3x[C]-5.6x[Si]-16.3x[Mn]-2.9x[Cr]+15x[Ti]+23x[Nb]-0.9x(t-8)
(Where [C], [Si], [Mn], [Cr], [Ti], and [Nb] are the weight percent of each element, and t is the thickness (mm) of the steel sheet.)
According to claim 6,
The reheating temperature is 1100 ~ 1350 ℃,
The hot rolling temperature is 800 ~ 1150 ℃ steel sheet manufacturing method.
According to claim 6,
Steel sheet manufacturing method further comprising the step of air-cooling the wound coil to a temperature range of 200 ℃ or less.
According to claim 6,
The steel plate is a steel plate manufacturing method having a thickness of 8 ~ 25mm.

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