KR20090070150A - Hot rolled steel sheet having superior hot press forming property and high tensile strength, formed article using the steel sheet and method for manufacturing the steel sheet and the formed article - Google Patents

Abstract

A high strength hot rolled steel plate with a superior hot molding processability, a molded product using the same and a manufacturing method thereof are provided to improve the profitability compares with a cold rolled steel and to secure a hot molded structural member. A manufacturing method for a high strength hot rolled steel plate comprises: a step of hot-rolling steel at higher than Ar3 transformation point; and a step of winding at cooling speed over than 10°C/s under the Bs. W is added in order to improve a thermal process curing ability and heat resistance in the hot forming processing. Al removes the oxygen existing among the steel and prevents the formation of the inclusion in the coagulation in order to micronize the grain size by fixing nitrogen existing among the steel with AlN.

Description

Hot Rolled Steel Sheet Having Superior Hot Press Forming Property and High Tensile Strength, Formed Article Using the Steel Sheet and Method for Manufacturing the Steel Sheet and the Formed Article}

The present invention relates to a hot rolled steel sheet used in automobile structural materials and components and the like, and more particularly, to a high strength hot rolled steel sheet excellent in hot forming processability, a molded article using the same, and a manufacturing method thereof.

Recently, automobile structural materials and components have been lighter and stronger in order to secure environmental regulations and passenger safety.

In order to increase the strength of automotive steel sheets, research on the development of a method of manufacturing a high strength steel sheet having excellent formability has been conducted.

In order to simultaneously obtain high forming properties and high strength, hot pressing forming (Hot Press Forming) has been widely applied. The high strength steel sheet used in the hot forming method is a cold rolled steel sheet, which is heated and maintained at a high temperature, and then pressed into a mold of a certain shape to form an automobile structural member, and is rapidly cooled by a mold in which a coolant flows to form a high strength hot forming process. Produced in parts.

However, such a hot forming process is low in productivity and the cost of the cold rolled steel sheet used in the hot forming process is expensive, which is inevitably high.

In addition, since the hardenability of the cold rolled steel sheet is not sufficient, it has a different hardness value for each part of the part at a low cooling rate.

Conventional techniques for solving such a problem include the method proposed in Japanese Patent Laid-Open No. 2006-126733, Japanese Patent Laid-Open 2006-152427, Japanese Patent Laid-Open No. 2006-213959, and the like.

First, in Japanese Patent Laid-Open No. 2006-126733, C: 0.05 to 0.4%, Mn: 0.01 to 4.0%, Cr: 0.005 to 5.0%, Mo + Nb, one or two, 0.1 to 3.0% and Ti, V, W, B A method for producing a cold rolled steel sheet for high strength hot forming is proposed, in which a cold rolled steel sheet containing an alloy element such as Ni or Ni and N: 0.01% or less is hot pressed, so that the martensite structure is 60% or more.

In Japanese Laid-Open Patent Publication No. 2006-152427, C: 0.25 to 0.45%, Mn + Cr: 0.5 to 3.0% and alloy elements such as Mo, Nb, Ti, V, and B are added, and the steel sheet added to N 0.002% or less is subjected to high temperature. After maintaining at a cooling rate of 10 to 500 ° C / s, the cooling method below the Ms point is proposed a method for producing a cold rolled steel sheet.

Japanese Patent Laid-Open No. 2006-213959 also proposes a cold rolled steel sheet for hot forming processing similarly to the above two patents.

An object of the present invention is to provide a high strength hot rolled steel sheet having excellent hot forming processability, a molded article using the same, and a method of manufacturing the same.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, by weight% C: 0.1-0.5%, Mn: 1.0-3.0%, Si: 0.5% or less, W: 0.1% or less, N: 0.01-0.1%, Al: 0.01-0.1%, S: 0.03% P: 0.1% or less and B: 0.001% to 0.01%, including Ti: 0.001% to 0.1%, Nb: 0.001% to 0.1%, and V: 0.001% to 0.1%, and Mo and : 0.01 to 1.5%, Cr: 0.01 to 1.5%, Cu: 0.005 to 1.0%, and Ni: 0.005 to 2.0%, one or two or more of them are added, and the remaining Fe and other unavoidable impurity elements Hot-forming, characterized in that the tissue consists of less than 10% of the cornerstone ferrite, less than 10% pearlite, the remaining bainite, or less than 10% of the cornerstone ferrite, less than 10% pearlite, the remaining bainite and martensite It relates to a high strength hot rolled steel sheet excellent in workability.

In the present invention, C: 0.1 to 0.5%, Mn: 1.0 to 3.0%, Si: 0.5% or less, W: 0.1% or less, N: 0.01 to 0.1%, Al: 0.01 to 0.1%, S: 0.03% or less, P: 0.1% or less, and B: 0.001-0.01%, wherein Ti: 0.001-0.1%, Nb: 0.001-0.1%, and V: 0.001-0.1% And Mo: 0.01% to 1.5%, Cr: 0.01% to 1.5%, Cu: 0.005% to 1.0%, and Ni: 0.005% to 2.0%, and one or two or more kinds thereof are added, and the remaining Fe and other unavoidable impurities are formed. The structure relates to a molded article containing at least 80% martensite and at least 20% of bainite, perlite and ferrite in an area fraction and at least 1470 MPa in tensile strength.

In the present invention, C: 0.1 to 0.5%, Mn: 1.0 to 3.0%, Si: 0.5% or less, W: 0.1% or less, N: 0.01 to 0.1%, Al: 0.01 to 0.1%, S: 0.03% or less, P: 0.1% or less, and B: 0.001-0.01%, wherein Ti: 0.001-0.1%, Nb: 0.001-0.1%, and V: 0.001-0.1% And steel: 0.01 to 1.5% of Mo, 0.01 to 1.5% of Cr, 0.005 to 1.0% of Cu, 0.005 to 1.0% of Ni, and 0.005 to 2.0% of Ni, and the remaining Fe and other unavoidable impurities. The method relates to a method for producing a high-strength hot rolled steel sheet having excellent hot forming processability, characterized in that hot rolling is finished at an Ar3 transformation point and cooled to a cooling rate of 10 ° C / s or more and wound at a Bs (beginite transformation start temperature) or less. .

In addition, the present invention by using the hot-rolled steel sheet prepared as described above to maintain at a temperature of Ac3 point or more, performing hot forming process, and quenching at a cooling rate of 1 ℃ / s or more to Ms (martensite transformation start temperature) or more It relates to a method for producing a molded article characterized by.

According to the present invention, a hot formed structural member having superior strength can be obtained even when cooling at a lower cooling rate during hot forming than a conventional hot rolled cold rolled steel sheet, and a high strength hot rolled steel sheet having excellent economical efficiency than a cold rolled steel sheet and a molded article using the same. Can be provided.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The steel sheet of the present invention is a weight% C: 0.1-0.5%, Mn: 1.0-3.0%, Si: 0.5% or less, W: 0.1% or less, N: 0.01-0.1%, Al: 0.01-0.1%, S: 0.03% or less, P: 0.1% or less, and B: 0.001-0.01%, wherein Ti: 0.001-0.1%, Nb: 0.001-0.1%, and V: 0.001-0.1% And Mo: 0.01-1.5%, Cr: 0.01-1.5%, Cu: 0.005-1.0% and Ni: 0.005-2.0% are added one or two or more of the remaining Fe and other unavoidable impurities, The reason for numerical limitation of each component is as follows.

C is limited to 0.1-0.5%. Carbon is an essential element for increasing the strength of the steel sheet, and the lower limit of the C content is 0.1% in order to obtain a high strength martensite phase in a hot formed part. If carbon is added in excess of 0.5%, weldability may be degraded, which may cause defects in assembling automobiles. Also, the upper limit is 0.5% because the strength of the steel sheet becomes too high in the plating process, which makes it difficult for the plate to be plated. Restrict.

Mn is an element having a very high solid solution strengthening effect in steel, and at the same time, it delays the transformation of austenite to ferrite and lowers the temperature of Ar3. If the amount is too small, hot forming is difficult in the austenitic single-phase zone during hot forming. If the amount is too high, problems in manufacturing hot rolled coil due to the increase in weldability and rolling load during hot rolling are increased. Limited to 1.0-3.0%.

The Si is an element having the effect of improving the ferrite strength by solid solution strengthening, but when added in a large amount, the surface defects are lowered due to the increase in scale defects and the plating property is lowered, so the upper limit thereof is limited to 0.5%.

W is the only element capable of improving the heat treatment hardenability and improving heat resistance during hot forming processing. W is also an important element in the present invention because the effect of suppressing grain growth of the grains and reducing the particle size in reheating the steel sheet can be expected. However, when the amount of W added exceeds 0.1%, not only this effect is saturated, but the manufacturing cost is increased due to high price, so the upper limit thereof is limited to 0.1%.

The Al is added for two purposes, one of which is intended to prevent the formation of nonmetallic inclusions during solidification by removing oxygen present in the steel, and the other is to fix the grain size by fixing the nitrogen present in the steel with AlN. This is for miniaturization. Therefore, Al should also be added in an appropriate range. If the content is too low, the purpose of the addition cannot be achieved. On the contrary, if Al is too high, there is a problem of increasing the strength of the steel and raising of the steelmaking unit. Limit to%

Since S is an impurity that increases the amount of precipitates by precipitation in the form of MnS, it is necessary to manage the amount of S low, the upper limit is limited to 0.03%. The lower limit was not defined because the lower the amount of S, the better the moldability for the same reason as described above.

Excessive addition of P adversely affects weldability and hot roll production. In addition, since the workability is deteriorated, the upper limit thereof is limited to 0.1%.

N is a very important element in the present invention. N is a solid solution strengthening element and at the same time is an element which combines with Ti, Nb, V, Al and the like to form a nitride. In the present invention, sufficient N is added for heat treatment property and strength increase. If the amount of N added is less than 0.01%, such an effect cannot be expected, and if the amount of N added exceeds 0.1%, problems may occur in the steelmaking and casting process.

Therefore, the content of N is limited to 0.01 to 0.1%.

B plays a role of suppressing the transformation of austenite into ferrite or bainite by segregating at the grain boundary and lowering the grain boundary energy. When the amount is too small, the addition effect cannot be sufficiently secured. Since there is a fear that the deterioration of toughness and quenchability due to the grain boundary precipitation of a large amount of B precipitates, the content of B is limited to 0.001 to 0.01%.

The hot rolled steel sheet of the present invention contains one or two or more of Ti, Nb, and V.

The Ti, Nb, and V are elements that are effective in promoting the deposition of carbonitrides to increase the strength of the steel sheet and improving the toughness of the hot formed structural parts through grain refinement. If the amount of each addition is less than 0.001%, such an effect cannot be obtained. If the amount of each addition is more than 0.1%, it may cause problems in the hot rolled manufacturing process due to the increase in manufacturing cost and excessive carbonitride precipitation.

In addition, the hot rolled steel sheet of the present invention contains one or two or more of Mo: 0.01 to 1.5%, Cr: 0.01 to 1.5%, Cu: ℃ 0.005 to 1.0% and Ni: 0.005 to 2.0%.

The Mo greatly improves the hardenability to allow the manufacture of high strength hot formed parts due to the smooth securing of martensite structure after hot forming. In addition, the precipitation of fine carbides is promoted, thereby increasing the strength and minimizing the particle diameter, thereby affecting the improvement of toughness. In order to obtain such an effect, the lower limit is limited to 0.01%, and when the content reaches 1.5%, the effect is saturated, and there is a problem of rising steel manufacturing cost, so the upper limit is limited to 1.5%.

Cr is an element that promotes hardenability improvement and carbide generation, and is an important element for producing high strength hot formed structural parts. In order to obtain the effect, the lower limit is limited to 0.01%, and when the content reaches 1.5%, the effect is saturated, and there is a problem of rising steel manufacturing cost, and the upper limit is limited to 1.5%.

Cu is an element effective for increasing the strength by encouraging fine precipitates. If the effect is not obtained at less than 0.005%, and the workability is deteriorated at more than 1.0%, the Cu content is limited to 0.005 to 1.0%.

Ni is an element effective in improving the heat treatment property by improving the hardenability with solid solution strengthening. If the content is less than 0.005%, the effect cannot be obtained. If the content is higher than 2.0%, problems of workability deterioration, scale formation during hot rolling, and increase in manufacturing cost occur, so that the Ni content is limited to 0.005 to 2.0%.

1 schematically shows the change in the continuous cooling phase transformation due to the addition of alloying elements.

Figure 1 (a) is a schematic diagram showing the microstructure obtained by cooling the ordinary steel from a high temperature (e.g. finishing finishing temperature) to a different cooling rate (cooling rate 1> 2> 3) at room temperature Figure 1 (b) is a schematic diagram showing the microstructure as a continuous cooling state when adding an alloying element to improve the hardenability to the general steel.

In Fig. 1 (a), martensite single phase is obtained when cooling at a cooling rate of 1, and ferrite + bainite + martensite structure is obtained when cooling at a cooling rate of 2, and ferrite is cooled at a cooling rate of 3. The structure of + pearlite + bainite is obtained.

As shown in Figure 1 (b), it can be seen that the ferrite, pearlite bainite transformation curve is shifted to the right in the time axis relative to the position of Figure 1 (a), which means that the effect of delayed transformation occurs will be. The effect of this alloying element is to obtain a different microstructure from that of ordinary steel for the same cooling rate, i.e., martensite is obtained at the cooling rate of 1 in FIG. At a cooling rate of 3, microstructures of bainite and martensite are obtained. That is, there is an advantage in that the cooling rate is obtained without the enhancement of the cooling rate.

In the present invention, by adding an alloying element to improve the hardenability to the general steel to obtain the effect that the cooling rate is enhanced without enhancement of the cooling rate.

The hot-rolled steel sheet of the present invention is composed of 10% or less of salt-free ferrite, 10% or less of pearlite, remaining bainite, or 10% or less of salt-free ferrite, 10% or less of pearlite, remaining bainite and martensite. Is done.

In the case where the tissue contains martensite, bainite is preferably at least 50% and martensite at 30% or less.

Hereinafter, a method for producing a steel sheet according to the present invention will be described.

In the present invention, the steel of the composition is reheated in a conventional manner and hot rolled, but the hot finish rolling temperature is finished by hot rolling at an Ar3 transformation point or higher, and cooled to a cooling rate of 10 ° C / s or more, thereby reducing Bs (bainite transformation). The high temperature hot rolled steel sheet with excellent hot forming processability is produced by winding up to a starting temperature) or less (typically 600? Or less).

The reason for defining the hot rolling finish temperature above the Ar3 transformation point is to prevent the two-phase reverse rolling, but in the case of the present invention steel, when the two-phase rolling is performed, a large amount of the cornerstone ferrite without carbide is generated, It is not possible to obtain bainite tissue over the entire tissue sought in the present invention.

In addition, the cooling rate after hot rolling is limited to 10 ° C./s or more. At a cooling rate below that, a large amount of ferrite and pearlite precipitates, and thus hot rolled bainite or bainite and martensite intended by the present invention. It is limited because it is impossible to obtain a mixed structure of, and as there is no upper limit, as shown in FIG. 1, the faster the cooling rate, the more difficult it is to obtain bainite and martensite structures.

In addition, the winding temperature is limited to the winding temperature below Bs (Bainite transformation start temperature), because the winding at a higher temperature causes pearlite transformation to obtain a low temperature structure intended by the present invention. As long as the performance of the actual hot rolled winding machine is excellent and the winding is possible, the present invention is not limited because it is advantageous for obtaining the fine bainite or martensite structure intended by the present invention.

Using the hot rolled steel sheet prepared as described above, after maintaining at a temperature of Ac3 or more, hot forming is performed, and in the case of quenching at a cooling rate of 1 ° C / s or more to Ms (martensite transformation start temperature), the martensite area fraction This molded article having 80% or more and tensile strength of 1470 MPa or more can be obtained.

The structure of the molded article contains at least 80% martensite and at least 20% of bainite, perlite and ferrite in area fraction.

The hot rolled steel sheet of the present invention can obtain a molded article having a martensite area fraction of 80% or more and a tensile strength of 1470 MPa or more even at a relatively low cooling rate of 1 to 30 ° C / s.

If the cooling rate is slower than 1 ℃ / s after hot forming, austenite is transformed into ferrite, pearlite, etc., which is a high-temperature phase during cooling, and thus it is difficult to obtain a sufficient martensite fraction, and thus it is difficult to obtain a tensile strength of 1470 MPa or more. It is preferable to limit the lower limit of the cooling rate to 1 ° C./s, and the upper limit is not limited because the faster the cooling rate, the better the martensite structure.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The steel ingot having the composition shown in Table 1 below was prepared in a thickness of 60 mm and a width of 175 mm by vacuum induction melting, and re-heated for 1 hour at 1200 ° C., followed by hot rolling to obtain a hot roll thickness of 1.6 mm. The hot rolling finish temperature was above Ar3 transformation point, and the ROT cooling rate was cooled to 5 ℃ / s and 50 ℃ / s, cooled to the target hot-rolling temperature, and then maintained in a furnace preheated to 400 ~ 650 ℃ for 1 hour. The hot rolling was simulated by Sikkim. The hot forming simulation was cooled to 20 ° C./s using a dilator, and the hardness was measured and converted into tensile strength. The results are shown in Table 2 below.

Steel grades 1, 2, and 3 among the steel grades in Table 1 belong to the scope of the invention, but steel grades 4 and 5 are steel grades which deviate from the component conditions of the inventive steel. Table 2 shows the manufacturing conditions for the steel grades in Table 1, that is, the cooling rate after finishing rolling (ROT cooling rate), the presence of cornerstone ferrite according to the coiling temperature, and the converted tensile strength after the final hot forming process simulation.

In the following, the criteria of the cornerstone ferrite / pearlite oil (free), no (free) shown in Table 2 is expressed in euros when the amount of the cornerstone ferrite / pearlite is more than 10%, respectively, expressed as nothing when less than 10%, cornerstone ferrite Only when the pearlite is free of the present invention steel.

Steel grade Chemical composition (wt%) C Mn Si Al S P N W B Ti Etc One 0.22 2.2 0.25 0.032 0.002 0.012 0.012 0.03 0.0055 0.025 Ni 0.2, Mo 0.1 2 0.21 2.16 0.29 0.024 0.003 0.01 0.016 0.03 0.0028 0.023 Cr 0.2, Nb 0.01 3 0.21 2.31 0.23 0.042 0.002 0.01 0.011 0.028 0.0037 0.022 Cu 0.03, V 0.01 4 0.07 0.76 0.20 0.030 0.0026 0.011 0.0014 - 0.0018 0.021 Cr 0.21, Mo 0.1 5 0.23 0.41 0.20 0.037 0.0031 0.012 0.0050 0.25 0.0021 0.017 Nb 0.011, V 0.098

Steel grade Steel grade-manufacturing conditions ROT cooling rate (℃ / s) Winding temperature (℃) Ferrite / Pearlite With / Without Tensile strength after hot forming simulation (MPa) Remarks One 1-1 5 450 U 1270 Comparative Steel 1 1-2 50 450 radish 1610 Inventive Steel 1 2 2-1 50 550 radish 1580 Inventive Steel 2 2-2 50 450 radish 1670 Invention Steel 3 2-3 50 650 radish 1320 Comparative Steel 2 3 3-1 50 550 radish 1540 Inventive Steel 4 3-2 5 450 U 1280 Comparative Steel 3 3-3 5 650 U 1140 Comparative Steel 4 4 4-1 5 550 U 1210 Comparative Steel 5 4-2 50 550 U 1270 Comparative Steel 6 5 5-1 50 500 U 1260 Comparative Steel 7 5-2 50 650 U 1220 Comparative Steel 8

As shown in Table 2, bainite structure containing fine carbide in the case of the invention steel subjected to quenching (cooling rate 50 ° C./s) during ROT cooling of steel grades 1, 2, and 3 and wound at a temperature of Bs or less After hot forming simulation, martensite was well formed at relatively low cooling rate (20 ℃ / s), and the tensile strength was over 1470MPa.

On the other hand, even after adjusting ROT quenching and winding temperature, steel grades 4 and 5 were obtained bainite structure mixed with cornerstone ferrite / pearlite, and based on this, cooling was performed at low cooling rate of 20 ℃ / s after hot forming simulation. In this case, tensile strength of 1470 MPa could not be obtained.

In case of ROT cooling using steel grades 1, 2, and 3, quenching was performed. However, when the coiling temperature was over Bs, the final hot rolled structure showed ferrite + pearlite structure, and the tensile strength after hot forming simulation showed a low level of 1200MPa.

In addition, although the coiling temperature was maintained below Bs by using steel grades 1, 2, and 3, when the cooling rate was lower than the range of the present invention, the composite structure of ferrite / pearlite + bainite was shown as a whole, and the tensile strength after hot forming. Did not reach 1470 MPa.

The presence or absence of the cornerstone ferrite depends on the case where the last hot rolling is operated below the Ar3 transformation point, and also on the cooling rate after finishing rolling (ROT cooling rate), and the winding temperature. Depends on

That is, the Ar3 transformation temperature mainly depends on the cooling rate after starting cooling in the austenite region, but rolling below the Ar3 transformation point means the formation of the cornerstone ferrite, which causes microstructure heterogeneity.

And, the slower the ROT cooling rate is promoted ferrite and pearlite transformation, the faster the cooling rate can be seen that the bainite and martensite transformation occurs in Figure 1.

In addition, the lower the winding temperature at which the hot roll transformation is finished, the lower the probability of the cornerstone ferrite is present. This is consistent with the formation of more cornerstone ferrite as the winding temperature is higher, even in the same composition and cooling conditions, as shown in Table 2 above.

In addition, the present invention is intended to secure a high strength of 1470MPa or more tensile strength even after cooling at a low cooling rate after hot forming, by suppressing the cornerstone ferrite, pearlite in the hot rolled steel sheet to obtain bainite or martensite structure at a temperature of Ac3 or more When heating and holding, carbides dissolve faster and more uniformly, and it is possible to achieve the above target by preventing fine segregation inside the hot rolled sheet.

1 is a schematic diagram showing the control of continuous phase transformation by addition of alloying elements, (a) shows a case of ordinary steel, and (b) shows a case of adding an alloying element.

Claims (4)

By weight% C: 0.1-0.5%, Mn: 1.0-3.0%, Si: 0.5% or less, W: 0.1% or less, N: 0.01-0.1%, Al: 0.01-0.1%, S: 0.03% or less, P : 0.1% or less and B: 0.001 to 0.01%, wherein Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1% and V: 0.001 to 0.1% of one or two or more and Mo: 0.01 to 1.5%, Cr: 0.01-1.5%, Cu: 0.005-1.0%, and Ni: 0.005-2.0% are added one or two or more of the remaining Fe and other unavoidable impurities, and the structure is 10 Excellent hot forming processability, consisting of up to 10% or less of salt-free ferrite, less than 10% pearlite, remaining bainite, or less than 10% of salt-free ferrite, less than 10% pearlite, remaining bainite and martensite. High strength hot rolled steel sheet By weight% C: 0.1-0.5%, Mn: 1.0-3.0%, Si: 0.5% or less, W: 0.1% or less, N: 0.01-0.1%, Al: 0.01-0.1%, S: 0.03% or less, P : 0.1% or less and B: 0.001 to 0.01%, wherein Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1% and V: 0.001 to 0.1% of one or two or more and Mo: 0.01 to 1.5%, Cr: 0.01 ~ 1.5%, Cu: 0.005 ~ 1.0% and Ni: 0.005 ~ 2.0%, one or two or more are added, and the remaining Fe and other unavoidable impurities are made up, and the structure is composed of an area fraction. A molded article comprising at least one of martensite at least 80% and at least 20% of bainite, perlite and ferrite and having a tensile strength of at least 1470 MPa. By weight% C: 0.1-0.5%, Mn: 1.0-3.0%, Si: 0.5% or less, W: 0.1% or less, N: 0.01-0.1%, Al: 0.01-0.1%, S: 0.03% or less, P : 0.1% or less and B: 0.001 to 0.01%, wherein Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1% and V: 0.001 to 0.1% of one or two or more and Mo: 0.01 to One or two or more of 1.5%, Cr: 0.01 to 1.5%, Cu: 0.005 to 1.0%, and Ni: 0.005 to 2.0% are added, and the steel made of the remaining Fe and other unavoidable impurity elements is added at or above Ar3 transformation point. The method of manufacturing high strength hot rolled steel sheet having excellent hot forming processability, which is finished by hot rolling and cooled at a cooling rate of 10 ° C / s or more and wound at a Bs (beginite transformation start temperature) or less. After the hot rolled steel sheet manufactured by the method of claim 3 is maintained at a temperature of at least Ac3 point, hot forming is performed, and quenching is performed at a cooling rate of 1 ° C / s or more to Ms (martensite transformation start temperature). Manufacturing method of molded article

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