KR20240008059A - Thick-wall steel plate having different microstructure in a thickness direction, and method for the same - Google Patents

Abstract

A thick TMCP steel having an abnormal structure in the thickness direction and a method for manufacturing the same are provided.
The thick TMCP steel of the present invention has, in weight percent, C: 0.03 to 0.05%, Si: 0.2 to 0.3%, Mn: 1.20 to 1.30%, P: 0.008% or less, S: 0.0010% or less, Sol.Al: 0.02 ~0.05%, Nb: 0.035~0.050%, Ni: 0.05~0.30%, Cr: 0.05~0.25%, Mo: 0.10% or less, Ti: 0.007~0.017%, Ca: 5~40ppm, N: 50ppm or less, remainder The surface layer of the steel, which contains Fe and inevitable impurities, from the surface to a depth of 250㎛ in the thickness direction, contains polygonal ferrite: more than 90% by area, and residual acicular ferrite and bainite, and the inner layer of the steel excluding this contains bay. A mixture of nite and acicular ferrite: more than 90% by area, and containing residual polygonal ferrite.

Description

Thick TMCP steel having an abnormal structure in the thickness direction and its manufacturing method {THICK-WALL STEEL PLATE HAVING DIFFERENT MICROSTRUCTURE IN A THICKNESS DIRECTION, AND METHOD FOR THE SAME}

This relates to the production of thick TMCP steel having an abnormal structure in the thickness direction. More specifically, it relates to thick TMCP steel that can obtain a composite structure steel with a different microstructure in the thickness direction by controlling the cooling conditions after hot rolling and a manufacturing method thereof. It's about.

Conventional thick plate TMCP steel secured the desired steel temperature through single-shot cooling (ACR or ACC), and as a result, the steel surface and the steel structure at the 1/4t point were the same. This steel structure structure was not appropriate to obtain the ideal structure of soft surface and hard interior desired in the market.

In order to implement an abnormal structure with an abnormal distribution according to the thickness of the steel material, there was a method of utilizing a multi-cooling mode in a conventional cooler, but there were limitations in equipment. Specifically, the conventional accelerated cooler consists of a reduced pressure section (high flow rate, 8m) + a low pressure section (low flow rate, 16m), so when implementing a combined cooling mode of weak cooling + strong cooling, the strong cooling section must be performed in the low pressure section at the rear of the existing cooler. There was no choice but to have insufficient pressure and flow.

In addition, immediately after mild cooling, plate deformation occurred due to stress relief of the material, and as flatness was not secured, there was a limit to the concern of uneven cooling.

Public Notice KR10-2020-047926

The purpose of the present invention is to provide a thick TMCP steel in which a soft ferrite phase is formed in the surface layer in the thickness direction of the steel and a hard phase is formed in the inner layer in the thickness direction of the steel by controlling the cooling conditions after hot rolling, and a method for manufacturing the same.

In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by those skilled in the art from the description below. It could be.

One aspect of the present invention is,

In weight%, C: 0.03~0.05%, Si: 0.2~0.3%, Mn: 1.20~1.30%, P: 0.008% or less, S: 0.0010% or less, Sol.Al: 0.02~0.05%, Nb: 0.035~ 0.050%, Ni: 0.05-0.30%, Cr: 0.05-0.25%, Mo: 0.10% or less, Ti: 0.007-0.017%, Ca: 5-40ppm, N: 50ppm or less, including the balance of Fe and inevitable impurities,

The surface layer of the steel from the surface to a depth of 250㎛ in the thickness direction contains polygonal ferrite: 90% by area or more, and residual acicular ferrite and bainite, and the inner layer of the steel excluding this contains a mixture of bainite and acicular ferrite: 90% by area. It relates to hot-rolled TMCP steel having an area % or more and a structure in the thickness direction containing residual polygonal ferrite.

The remaining structure forming the inner layer of the steel may include one or more of pearlite, martensite, and residual austenite.

In addition, another aspect of the present invention is,

In weight%, C: 0.03~0.05%, Si: 0.2~0.3%, Mn: 1.20~1.30%, P: 0.008% or less, S: 0.0010% or less, Sol.Al: 0.02~0.05%, Nb: 0.035~ 0.050%, Ni: 0.05~0.30%, Cr: 0.05~0.25%, Mo: 0.10% or less, Ti: 0.007~0.017%, Ca: 5~40ppm, N: 50ppm or less, steel containing residual Fe and inevitable impurities Manufacturing hot rolled steel with a thickness of 12T to 32T by extracting the slab from a heating furnace and performing final hot rolling at a temperature range of 860 to 960°C;

Continuously primary cooling the manufactured hot rolled steel to a temperature of 750 to 840°C at a cooling rate of 40°C/s or less;

Air-cooling the primary cooled hot-rolled steel for 15 to 20 seconds and then flattening it; and

Secondary cooling the flattened hot rolled steel material to a cooling end temperature of 550°C or lower at a cooling rate of 24°C/s or higher,

Cooling start temperature in the secondary cooling step,

750~810℃ when the thickness of hot rolled steel is 12T~17T,

770~810℃ when the thickness of hot rolled steel exceeds 17T but does not exceed 25T, and

It relates to a method of manufacturing hot-rolled TMCP steel with an abnormal structure in the thickness direction, controlling the temperature to 790-810°C when the thickness of the hot-rolled steel is more than 25T and less than 32T.

The present invention, which has the structure described above, utilizes two coolers simultaneously to generate ferrite on the surface of the material by lightly cooling it to the extent that only the surface is cooled by primary cooling, and then air-cooling it to develop surface ferrite while sufficiently deforming it. , After correction in the pre-leveler, a hard second phase is formed inside the steel through secondary cooling, thereby providing TMCP steel with an abnormal structure in the thickness direction.

In addition, this can promote the development of new steel types, such as guaranteeing the hard spot of API and SOUR materials and improving the YR of low-temperature molten steel.

Figure 1 is a schematic diagram showing a cooling device capable of implementing an abnormal structure in the thickness direction of the present invention.
Figure 2 is a diagram showing the cooling profile for steel that has passed through the cooling device of Figure 1.
Figure 3 is an SEM tissue photo of the TOP part and the MIDDLE part of the 15T hot rolled steel in an embodiment of the present invention by thickness, the TOP part (PC988370-T) refers to the steel part 1 to 2M away from the tip of the hot rolled steel, and the MIDDLE part ( PC988370-M) refers to the steel part 12M away from the tip of the hot rolled steel.
Figure 4 is an SEM tissue photo of steel thickness according to the cooling start temperature before secondary cooling of 30T hot rolled steel in an embodiment of the present invention.
Figure 5 is a graph showing the effect of the secondary cooling start temperature for each thickness of the steel material on the yield strength and hardness of the steel material in an embodiment of the present invention. (a) is a graph showing the effect of the secondary cooling start temperature for each thickness of the steel material on the yield strength of the steel material ( (measured at the 1/4t point), and (b) is a graph showing the effect of the secondary cooling start temperature by steel thickness on the hardness of the steel surface layer.

Hereinafter, the present invention will be described.

The present invention is characterized by providing a composite structure TMCP steel with a different microstructure distribution in the steel thickness direction by controlling the cooling process of 12 to 35T thick material after hot rolling.

Figure 1 is a schematic diagram showing a cooling device capable of implementing an abnormal structure in the thickness direction of the present invention, and Figure 2 is a diagram showing a cooling profile for a steel material that has passed through the cooling device of Figure 1.

As shown in Figures 1-2, the cooling device of the present invention may be configured to include an adjacent rolling cooler (ACR), a pre-leveler, an accelerated cooler (ACC), and a hot leveler. That is, the ACR primary cooler and ACC acceleration cooler are separately arranged at the rear of the plate rolling mill, and the structure of the surface layer of the steel is formed into ferrite through mild cooling through the ACR cooler, followed by air cooling, and then the subsequent ACC cooler. TMCP steel with a different microstructure distribution in the steel thickness direction can be manufactured by forming a hard second phase inside the steel through quenching. In other words, it is a cooling technology that can control not only the structure of the material but also its shape through the process of correcting immediately after the first cooling and then calibrating again after the second cooling. The cooling rate and cooling temperature of the first and second cooling are variable. By controlling it, various types of complex tissues can be obtained.

The hot-rolled TMCP steel of the present invention is, in weight percent, C: 0.03 to 0.05%, Si: 0.2 to 0.3%, Mn: 1.20 to 1.30%, P: 0.008% or less, S: 0.0010% or less, Sol.Al: 0.02~0.05%, Nb: 0.035~0.050%, Ni: 0.05~0.30%, Cr: 0.05~0.25%, Mo: 0.10% or less, Ti: 0.007~0.017%, Ca: 5~40ppm, N: 50ppm or less, The surface layer of the steel, including the remaining Fe and inevitable impurities, from the surface to a depth of 250㎛ in the thickness direction, contains polygonal ferrite: 90% by area or more, and residual acicular ferrite and bainite, and the inner layer of the steel excluding this includes, Mixture of bainite and acicular ferrite: More than 90% by area, has an abnormal structure in the thickness direction including residual polygonal ferrite.

Hereinafter, the steel component composition of the hot-rolled TMCP steel of the present invention and the reasons for limiting its content will be described, where "%" means weight percent unless otherwise specified.

Carbon (C): 0.03~0.05%

C is the most important element in securing basic strength, so it needs to be contained in steel within an appropriate range. To obtain this added effect, C is preferably 0.03% or more. However, if the C content exceeds 0.05%, the strength and hardness of the base material may increase excessively when cooled with water.

Therefore, it is preferable that the C content ranges from 0.03 to 0.05%.

Silicon (Si): 0.2~0.3%

Si is a substitutional element that improves the strength of steel through solid solution strengthening and has a strong deoxidation effect, so it is an essential element in the production of clean steel, so it is preferable to add 0.2% or more. However, if it exceeds 0.3%, it may increase the strength of the matrix structure such as the hard phase in the center of the steel, causing deterioration in impact toughness, etc.

Therefore, it is preferable that the Si ranges from 0.2 to 0.3%.

Manganese (Mn): 1.20~1.30%

Mn is a useful element that improves strength through solid solution strengthening and improves hardenability to create a low-temperature transformation phase. Accordingly, in order to secure a yield strength of 460 MPa or more, it is desirable to add 1.20% or more. However, the Mn content may increase.

Rock Mn may reduce toughness by reacting with S to form MnS, an elongated non-metallic inclusion, so the upper limit of Mn is preferably 1.30% or less.

Therefore, the Mn content is preferably in the range of 1.20 to 1.30%.

Aluminum (Sol.Al): 0.02~0.05%

Al, along with Si, is one of the powerful deoxidizers in the steelmaking process, and to achieve this effect, it is preferable to add it in an amount of 0.02% or more. However, if the content exceeds 0.05%, the Al2O3 fraction among the oxidizing inclusions generated as a result of deoxidation increases excessively, becomes coarse in size, and may become difficult to remove during refining.

Therefore, it is preferable that the Al ranges from 0.02 to 0.05%.

Phosphorus (P): 0.008% or less

P is an element that causes embrittlement at grain boundaries or forms coarse inclusions, so it is desirable to control the P content to 0.008% or less.

Sulfur (S): 0.0010% or less

S is an element that causes embrittlement at grain boundaries or forms coarse inclusions, so it is desirable to control the S content to 0.0010% or less.

Niobium (Nb): 0.035~0.050%

Nb improves the strength of the base material by precipitating in the form of NbC or Nb(C,N). In addition, Nb dissolved in solid solution when reheated to a high temperature precipitates very finely in the form of NbC during rolling, which has the effect of suppressing recrystallization of austenite and refining the structure. For the above effect, it is preferable that 0.035% or more of Nb is added. However, if it exceeds 0.050%, undissolved Nb is generated in the form of Ti, Nb (C, N), which may be a factor in deteriorating strength characteristics.

Therefore, it is preferable that the Nb content ranges from 0.035 to 0.050%.

Titanium (Ti): 0.007~0.017%

Ti is a component that precipitates as TiN upon reheating and significantly improves low-temperature toughness by suppressing the growth of grains in the base metal and weld heat-affected zone. It is preferable to add 0.007% or more to achieve this addition effect. However, if Ti is added in excess of 0.017%, low-temperature toughness may be reduced due to clogging of the playing nozzle or crystallization in the center.

Therefore, the Ti content is preferably in the range of 0.007 to 0.017%.

Chromium (Cr): 0.05~0.25%

Chromium (Cr) plays a role in increasing yield and tensile strength by increasing hardenability and forming a low-temperature transformation structure. For this purpose, it is preferable to add Cr in an amount of 0.05% or more. However, if the Cr content exceeds 0.25%, Cr-Rich coarse carbides such as M23C6 may be formed and impact toughness may decrease.

Therefore, the Cr content is preferably in the range of 0.05 to 0.25%.

Molybdenum (Mo): 0.10% or less

Mo, like Cr, is an element effective in preventing a decrease in strength during post-process tempering or post-weld heat treatment (PWHT), and is effective in preventing a decrease in toughness due to grain boundary segregation of impurities such as P. In addition, it increases hardenability and increases the fraction of low-temperature phases such as martensite and bainite, thereby increasing the strength of the matrix phase. However, Mo is an expensive element and if added excessively, the manufacturing cost can increase significantly, so it is preferable to add it in an amount of 0.10% or less.

Nickel (Ni): 0.05~0.30%

Ni is an important element that improves impact toughness by increasing stacking faults at low temperatures, making it easier to cross slip dislocations, and increasing strength by improving hardenability. To achieve this effect, it must be added at least 0.05%. desirable. However, if Ni is added in excess of 0.3%, the manufacturing cost may increase due to the high cost compared to other hardenability elements, so the Ni content is preferably in the range of 0.05 to 0.30%.

Calcium (Ca): 5~40ppm

When Ca is added after deoxidation by Al, it combines with S forming MnS inclusions to suppress the production of MnS, and at the same time forms spherical CaS, which has the effect of suppressing the occurrence of cracks. In the present invention, it is preferable to add 5 ppm or more of Ca in order to sufficiently form S contained as an impurity into CaS. However, if it exceeds 40ppm, Ca remaining after forming CaS combines with O to generate coarse oxidative inclusions, which are stretched and broken during rolling, causing a problem of acting as the starting point of cracks. Therefore, the Ca content is preferably in the range of 5 to 40 ppm.

Nitrogen (N): 50ppm or less

Since it is difficult to completely remove nitrogen (N) from steel industrially, the upper limit is 50ppm, which is the allowable range in the manufacturing process. Meanwhile, N reacts with Al, Ti, Nb, V, etc. in steel to form nitrides, thereby suppressing the growth of austenite grains, which has a beneficial effect on improving the toughness and strength of the material, but its content exceeds 50 ppm. Therefore, if excessively added, N exists in a solid solution, which may have a negative effect on low-temperature toughness.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all of them are not specifically mentioned in this specification.

Meanwhile, in the hot rolled TMCP steel of the present invention, the surface layer of the steel from the surface to a depth of 250㎛ in the thickness direction contains polygonal ferrite: 90% by area or more, and residual acicular ferrite and bainite, excluding this, the inner layer of the steel may include a mixture of bainite and acicular ferrite: 90% by area or more, with residual polygonal ferrite.

The remaining structure forming the inner layer of the steel may include one or more of pearlite, martensite, and residual austenite.

Next, the manufacturing method of hot-rolled TMCP steel of the present invention will be described.

The hot-rolled TMCP steel manufacturing method of the present invention includes the steps of extracting a steel slab of the above-described composition from a heating furnace and then performing final hot rolling at a temperature range of 860 to 960° C. to produce hot-rolled steel with a thickness of 12T to 32T; Continuously primary cooling the manufactured hot rolled steel to a temperature of 750 to 840°C at a cooling rate of 40°C/s or less; Air-cooling the primary cooled hot-rolled steel for 15 to 20 seconds and then flattening it; And secondary cooling of the flattened hot-rolled steel to a cooling end temperature of 550°C or lower at a cooling rate of 24°C/s or higher, wherein the cooling start temperature in the secondary cooling step is set to the thickness of the hot-rolled steel. It is controlled at 750~810℃ when it is 12T~17T, at 770~810℃ when the thickness of hot rolled steel is over 17T and below 25T, and at 790~810℃ when the thickness of hot rolled steel is between 25T and below 32T.

First, in the present invention, hot-rolled steel of 12T to 32T (mmt) is manufactured by extracting steel slabs of the above-described composition from a heating furnace and then finishing hot rolling them in a temperature range of 860 to 960 ° C. In the present invention, the temperature range of the heating furnace is general and is not limited thereto. For example, it may have a temperature range of 1050 to 1200°C.

And through the finishing hot rolling, hot rolled steel with a desired thickness of 12T to 32T can be manufactured. At this time, if the finish rolling temperature is lower than 860°C, the cooling start temperature may drop below the Ar3 temperature, resulting in abnormal cooling, which may cause a problem of insufficient strength. On the other hand, if the temperature exceeds 960℃, the austenite grain size is large, so even if sufficiently cooled later, strength and impact toughness are weak.

Considering this, in the present invention, it is preferable to limit the finish rolling temperature range to 860 to 960°C.

Subsequently, in the present invention, the finished hot-rolled hot-rolled steel is continuously cooled to a temperature of 750 to 840°C for the first time at a cooling rate of 40°C/s or less, and through this, the thickness of the hot-rolled steel is reduced to 250㎛ from the surface of the hot-rolled steel. A ferrite structure may form in the surface layer.

In the present invention, it is preferable to limit the primary cooling end temperature to the range of 750 to 840°C. If the cooling end temperature is less than 750°C, transformation has already begun at a certain depth in the surface layer of the hot rolled steel sheet before secondary cooling, which will be described later, and there is a possibility that strength will decrease during secondary cooling. On the other hand, if the cooling end temperature exceeds 840°C, reverse transformation due to excessive reheating occurs on the surface of the hot rolled steel during the ACR to ACC transfer of about 30 m, and all ferrite generated on the surface is converted to austenite, and then the subsequent There is a risk that it will be transformed into bainite by secondary cooling and the surface hardness will increase again.

Additionally, in the present invention, it is preferable to limit the primary cooling rate to 40°C/s or less.

In the present invention, the primary cooled hot-rolled steel is air-cooled for 15 to 20 seconds and then flattened.

First, the primary cooled hot-rolled steel is air-cooled for 10 to 30 seconds. In the present invention, through this air-cooling, the ferrite structure formed in the surface layer of the hot-rolled steel is transformed to develop a ferrite structure.

In the present invention, it is preferable to limit the air cooling time to the range of 10 to 30 seconds in consideration of the distance between the ACR and ACC cooling periods of about 30 m, the transfer speed of the steel material, and the degree of natural cooling due to air cooling.

And in the present invention, the air-cooled hot-rolled steel is subjected to a flattening treatment to correct its shape using a pre-leverler.

Subsequently, in the present invention, the flattened hot rolled steel is secondarily cooled to a cooling end temperature of 550°C or lower at a cooling rate of 24°C/s or higher.

First, in the present invention, it is preferable to control the secondary cooling end temperature to 550°C or lower. If the secondary cooling end temperature exceeds 550°C, a sufficiently hard second phase structure cannot be obtained inside the hot rolled steel, and the strength of the steel may decrease.

Additionally, if the secondary cooling rate is less than 24°C/s, hot rolled steel having the desired strength cannot be obtained. Meanwhile, although the present invention does not suggest an upper limit for the secondary cooling rate, it is better to limit it to 100°C/s in consideration of problems such as cooling equipment constraints and plate deformation due to cooling.

Meanwhile, in the present invention, the cooling start temperature in the secondary cooling step is 750 to 810 ℃ when the thickness of the hot rolled steel is 12T to 17T, 770 to 810 ℃ when the thickness of the hot rolled steel is more than 17T and 25T or less, and the thickness of the hot rolled steel When is over 25T but below 32T, it is required to control it to 790~810℃.

In hot rolled steel for each thickness range above, if the secondary cooling start temperature exceeds the upper limit of 810°C, reverse transformation from F (ferrite) to A (austenite) occurs during air cooling due to excessively weak cooling in the above-mentioned primary cooling, resulting in primary cooling. Cooling can have the effect of being nullified, and during secondary quenching, bainite is generated entirely from the surface to the center, making it possible to manufacture steel with the same microstructure as ordinary water-cooled materials.

On the other hand, in hot rolled steel for each thickness range, if the secondary cooling start temperature is less than the respective lower limits, 750°C, 770°C, and 790°C, ferrite is generated to the depth inside the steel due to subcooling during the first cooling, and secondary cooling Bainite is generated and the surface hardness is satisfactory, but YS is 444Mpa, which may fall short of the target of 460Mpa.

The hot-rolled steel of the present invention manufactured through the manufacturing process described above has a ferrite structure in the surface layer of the steel from the surface to a depth of 250㎛ in the thickness direction, and the inner layer of the steel excluding this contains bainite, martensite, and residual auste. It contains one or more hard phase structures of anite and pearlite, and therefore it is possible to manufacture a biphasic composite steel having different microstructure distributions in the thickness direction.

Hereinafter, the present invention will be described in detail through examples.

(Example)

Steel slabs with compositions shown in Table 1 below were prepared. Next, the prepared slabs were manufactured into hot-rolled TMCP steels through finishing hot rolling, primary cooling, air cooling, and secondary cooling. At this time, specific manufacturing process conditions are shown in Table 2 below. Meanwhile, under the conditions in Table 2, the hot-rolled steel was kept in air cooling for 12 seconds after primary cooling, and then secondary cooling was performed.

Meanwhile, as shown in Table 2 below, hot-rolled steel having thicknesses of 15T, 19.5T and 30T was prepared through finish hot rolling, and then hot-rolled TMCP steel was manufactured using this under the conditions in Table 2 below.

In Table 2 below, TO, T4, T5, RR4, SCT1, FCT1, CR1, SCR2, FCT2, and CR2 are the heating furnace extraction temperature (℃), finishing rolling start temperature (℃), finishing rolling finishing temperature (℃), and Finishing rolling cumulative reduction rate (%), first cooling start temperature (°C), first cooling end temperature (°C), first cooling rate (°C/s), second cooling start temperature (°C), second cooling end Indicates the temperature (℃) and the second cooling rate (℃/s).

Steel composition (% by weight) ingredient C Si Mn P S Sol. Al Nb Ni Cr Mo Ti Ca N content 0.04 0.25 1.25 0.005 0.0006 0.03 0.045 0.25 0.2 0.08 0.012 20 45

*In Table 1, the remaining components are Fe and inevitable impurities, and the unit of content of Ca and N is ppm.

thickness TO T4 T5 RR4 SCT1 FCT1 CR1 SCT2 FCT2 CR2 15mmt 1140 1050 960 75 840 800 20 760 550 40 19.5mmt 1140 1010 940 75 870 800 30 770 500 50 30mmt 1120 990 940 75 890 790~830 24~40 770~810 450 24~29

First, the microstructure according to the steel thickness in the TOP and MIDDLE parts of the manufactured 15T hot rolled steel was observed and shown in FIG. 3. As shown in Figure 3, it can be seen that the ferrat structure increases in the surface layer of the steel, and the bainite fraction, a low-temperature structure, increases toward the center of the steel.

Figure 4 is an SEM tissue photo of steel thickness (surface layer 250㎛, t/4) according to the cooling start temperature (760℃, 804℃, and 819℃) before secondary cooling of 30T hot rolled steel in an embodiment of the present invention.

Figure 4(b) is an invention example of manufacturing a 30T hot-rolled steel using the conditions of the present invention. Ferrite was generated in the surface layer thickness range of 150 to 200㎛ by the first mild cooling, and bainite was formed in the center of the steel by the second quenching. It can be confirmed that an F+B mixed structure is formed at the 250㎛ position, thereby satisfying both the target yield strength of 460MPa and hardness of 220Hv.

On the other hand, Figure 4(a) is Comparative Example 1 in which the secondary cooling start temperature exceeds the range of the present invention, and the F → A reverse transformation occurs during air cooling due to excessively weak cooling in the primary cooling, which nullifies the primary cooling. It was shown that during secondary quenching, bainite was generated throughout the entire surface from the surface to the center, indicating a steel material with the same microstructure as normal quenching.

In addition, Figure 4(c) shows Comparative Example 2 in which the secondary cooling start temperature is below the range of the present invention. During primary cooling, ferrite is generated to some internal depth by supercooling, and bainite is generated by secondary quenching, so the surface hardness is satisfactory. One, YS was 444Mpa, falling short of the target of 460Mpa.

Meanwhile, Figure 5 is a graph showing the effect of the secondary cooling start temperature for each thickness of the manufactured hot rolled steel on the yield strength and hardness of the steel. (a) shows that the secondary cooling start temperature for each thickness of the steel is the yield strength of the steel ( (measured at the 1/4t point), and (b) is a graph showing the effect of the secondary cooling start temperature by steel thickness on the hardness of the steel surface layer.

As shown in Figure 3, as a result of the 30T hot rolled steel test, in order to secure hardness of 220Hv or less at a depth of 250㎛ in the thickness direction, the upper limit of the secondary cooling start temperature is about 810℃, and since this is a mechanism limited to the surface layer of the material, the thickness Regardless, it is believed to be constant. However, the lower limit of the secondary cooling start temperature to secure the lower yield strength limit of 460Mpa for each thickness is different. Specifically, it can be seen that 15T is judged to be 750℃, 19.5T is 770℃, and 30T is judged to be 790℃.

As described above, the detailed description of the present invention has described preferred embodiments of the present invention, but those skilled in the art can make various modifications without departing from the scope of the present invention. Of course this is possible. Therefore, the scope of rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described later, but also by their equivalents.

Claims (3)

In weight%, C: 0.03~0.05%, Si: 0.2~0.3%, Mn: 1.20~1.30%, P: 0.008% or less, S: 0.0010% or less, Sol.Al: 0.02~0.05%, Nb: 0.035~ 0.050%, Ni: 0.05-0.30%, Cr: 0.05-0.25%, Mo: 0.10% or less, Ti: 0.007-0.017%, Ca: 5-40ppm, N: 50ppm or less, including the balance of Fe and inevitable impurities,
The surface layer of the steel from the surface to a depth of 250㎛ in the thickness direction contains polygonal ferrite: 90% by area or more, and residual acicular ferrite and bainite, and the inner layer of the steel excluding this contains a mixture of bainite and acicular ferrite: 90% by area. Hot-rolled TMCP steel having an area % or more and a structure in the thickness direction containing residual polygonal ferrite.
The hot-rolled TMCP steel according to claim 1, wherein the residual structure forming the inner layer of the steel has an abnormal structure in the thickness direction including at least one of pearlite, martensite, and retained austenite.
In weight%, C: 0.03~0.05%, Si: 0.2~0.3%, Mn: 1.20~1.30%, P: 0.008% or less, S: 0.0010% or less, Sol.Al: 0.02~0.05%, Nb: 0.035~ 0.050%, Ni: 0.05~0.30%, Cr: 0.05~0.25%, Mo: 0.10% or less, Ti: 0.007~0.017%, Ca: 5~40ppm, N: 50ppm or less, steel containing residual Fe and inevitable impurities Manufacturing hot rolled steel with a thickness of 12T to 32T by extracting the slab from a heating furnace and performing final hot rolling at a temperature range of 860 to 960°C;
Continuously primary cooling the manufactured hot rolled steel to a temperature of 750 to 840°C at a cooling rate of 40°C/s or less;
Air-cooling the primary cooled hot-rolled steel for 15 to 20 seconds and then flattening it; and
Secondary cooling the flattened hot rolled steel material to a cooling end temperature of 550°C or lower at a cooling rate of 24°C/s or higher,
Cooling start temperature in the secondary cooling step,
750~810℃ when the thickness of hot rolled steel is 12T~17T,
770~810℃ when the thickness of hot rolled steel exceeds 17T but does not exceed 25T, and
A method of manufacturing hot-rolled TMCP steel with an abnormal structure in the thickness direction, controlling the temperature to 790~810℃ when the thickness of the hot-rolled steel exceeds 25T and is less than 32T.