KR20110076431A

KR20110076431A - Hot rolled high strength steel sheet having excellent workability and method for manufacturing the same

Info

Publication number: KR20110076431A
Application number: KR1020090133133A
Authority: KR
Inventors: 한성호
Original assignee: 주식회사 포스코
Priority date: 2009-12-29
Filing date: 2009-12-29
Publication date: 2011-07-06

Abstract

PURPOSE: A high-strength hot-rolled steel sheet with excellent processing performance and manufacturing method thereof are provided to realize excellent elongation ratio and yield ratio by appropriately controlling ingredients and fine structure. CONSTITUTION: A high-strength hot-rolled steel sheet with excellent processing performance consists of 0.07-0.12weight% of C, 0.80-1.71weight% of Si, 1.5-2.5weight% of Mn, 0.5-1.0weight% of Cr, 0.1-0.3weight% of Al, 0.01-0.06weight% of Nb, below 0.005weight% of S, below 0.04weight% of P, Fe and other inevitable impurities. The elongation ratio of the hot-rolled steel sheet is over 12% and its yield ratio is below 0.750. The tensile strength of the hot-rolled steel sheet is over 980MPa. The fine structure of the hot-rolled steel sheet consists of ferrite, martensite and bainite.

Description

High-strength hot rolled steel sheet with excellent workability and manufacturing method {HOT ROLLED HIGH STRENGTH STEEL SHEET HAVING EXCELLENT WORKABILITY AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high-strength hot rolled steel sheet excellent in workability and a method for manufacturing the same, and more particularly, to a high-strength hot rolled steel sheet having low yield ratio and excellent workability and at the same time excellent in strength.

In general, the bumper reinforcement or the shock absorber in the door is a part directly related to passenger safety in the event of a collision of the vehicle, and an ultra high strength hot rolled steel sheet having a tensile strength of 780 MPa or more is mainly used, and it has to have high elongation and high elongation. In addition, the use rate of high-strength steel of higher strength parts is increasing in order to increase fuel efficiency in order to cope with the increasingly severe environmental pollution regulation, which is difficult to control the manufacturing conditions as the workability is low and the strength is increased. In order to be easy to manufacture together with workability, the yield ratio should be low, ferrite should be easily formed during cooling, and temperature control should be easy by coiling at high temperature. Recently, research on commercialization of high strength steel of more than 980 MPa has been increasing. Representative automotive high strength steels include multi-phase steel, Transformation Induced Plasticity (TRIP) and abnormal phase (DP: Dual Phase). There is a river.

Japanese Patent Application Laid-Open No. 2000-290749 has excellent press formability, characterized in that the average particle diameter of ferrite is less than 4 µm and the surface roughness is 0.8-1.5 µm, and the components are C: 0.01-0.2%, Si: 2.0% or less, and Mn. : 2.1% or less, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.1% or less, or a high strength hot rolled steel sheet containing one or two or more of Cr, Mo, Ni and V added thereto. Doing. The patent is a high-strength steel of 980 MPa class, and relates to a method for producing bainite single phase steel in which the final structure is precipitated and reinforced with (Ti, Mo) C. However, bainite steel has a uniform structure, which is advantageous for hole expansion, but has a problem in that it cannot be applied to parts requiring high workability due to high yield strength and high elongation.

In Japanese Patent Laid-Open No. 2001-234280, C: 0.12 to 0.3%, Si: 0.7% or less, Mn: 1.0 to 2.0%, P: 0.1% or less, S: 0.005% or less, sol.Al: 0.1% or less, N: 0.01 About hot-rolled hot-dip galvanized steel sheet containing% or less, and containing at least one of Cr: 0.05 to 0.8%, V: 0.005 to 0.15%, and microstructure consisting of ferrite and martensite. Disclosed is a method for producing an ultra high strength steel having excellent workability.

In the case of manufacturing a normal abnormal tissue steel, the temperature control at the time of cooling after hot rolling is difficult to control as the winding temperature is low. This is because the transformation is completed before winding when the coiling temperature is low, especially in the section of 100 ℃ or more 300 ℃ less cooling unevenness due to the transformation heat is very difficult to control the proper winding temperature. On the other hand, if the coiling temperature is lowered to room temperature, martensite can be easily produced without the occurrence of transformation heat, and thus it may be easier to manufacture abnormal tissue steel. desirable.

Therefore, in order to consider the shape and to suppress the transformation heat during cooling, it is preferable to wind the work at medium temperature.In particular, in the case of ultra-high strength steel with a tensile strength of 980 MPa or more, as described in the present invention, the increase in the winding temperature is very high in terms of the shape of the coil. desirable. However, in order to form martensite even after winding at a medium temperature, it is important to stabilize austenite to a low temperature through the addition of chromium or the like. In other words, it is desirable to suppress the transformation of austenite to bainite to a temperature as low as possible during winding of the steel, thereby securing a sufficient amount of austenite necessary for the transformation of martensite.

The present invention is to solve the above-described problems, to provide a high-strength hot-rolled steel sheet excellent in workability having excellent elongation and yield ratio by appropriately controlling the component system and microstructure and a method of manufacturing the same.

In the present invention, in weight%, C: 0.07 to 0.12%, Si: 0.80 to 1.7%, Mn: 1.5 to 2.5%, Cr: 0.5 to 1.0%, Al: 0.1 to 0.3%, Nb: 0.01 to 0.06%, S : 0.005% or less, P: 0.04% or less Provides a high-strength hot rolled steel sheet having excellent workability including the remaining Fe and other unavoidable impurities, an elongation of 12% or more, and a yield ratio (YR) of 0.750 or less.

Yield Ratio (YR) = 0.924-0.00622 * M Volume%-0.00114 * F Volume%

(However, M: Martensite, F: Ferrite)

At this time, the hot rolled steel sheet preferably has a tensile strength of 980 MPa or more. In addition, the microstructure of the hot rolled steel sheet is preferably made of ferrite, martensite and bainite. The microstructure preferably satisfies that the ferrite content is 60 vol% or more, the martensite content is 15 vol% or more, and the bainite content is 15 vol% or less.

In addition, the present invention is a weight%, C: 0.07-0.12%, Si: 0.8-1.7%, Mn: 1.5-2.5%, Cr: 0.5-1.0%, Al: 0.1-0.3%, Nb: 0.01-0.06% S: 0.005% or less, P: 0.04% or less, reheating the slab containing the remaining Fe and other unavoidable impurities in the temperature range of 1150 ~ 1250 ℃; Finishing hot rolling of the reheated slab at a temperature of Ar3 + 60 ° C. or higher; Cooling the finished hot-rolled steel sheet and maintaining it for 5 to 8 seconds at a temperature range of 680 to 720 ° C .; It provides a method for producing a high strength hot rolled steel sheet having excellent workability including the step of winding the heat-treated steel sheet in the temperature range of 450 ~ 550 ℃. At this time, the reheating is preferably performed for 60 to 180 minutes.

According to the present invention, by adding an appropriate amount of Si and Al to secure the appropriate ferrite fraction, and also by maintaining the coiling temperature of more than 450 ℃ the elongation of more than 12% and the yield ratio of less than 0.750 tensile strength of 980MPa grade or more excellent workability steel sheet and The manufacturing method can be provided.

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

As the ratio of martensite increases, the ductility increases as the ratio of martensite increases, and the ductility increases as the ferrite ratio increases. However, when the martensite ratio becomes too large to increase the strength, the ferrite ratio decreases and the ductility decreases. Therefore, the present inventors have been focused on the component design to facilitate the formation of ferrite during cooling, and when the ferrite stabilizing elements such as silicon, aluminum, etc. are added, the present inventors have come to the present invention.

In addition, it is important to control the cooling conditions after hot rolling in order to obtain excellent elongation along with the strength.In order to obtain ferrite through air cooling during cooling, it is necessary to optimize the cooling pattern and optimize the components. Since it is important to maintain, it was also considered that the grains should be refined by adding silicon, manganese, and the like, and adding trace elements niobium and titanium.

The present inventors controlled the component system as follows in order to obtain a steel sheet having high strength and at the same time excellent in elongation. (Hereinafter,% is% by weight).

C: 0.07 ~ 0.12%

Among steels, C is the most important component in steel materials. It is closely related to all physical and chemical properties such as strength, toughness and corrosion resistance, and has the greatest influence on the properties of steel. In particular, if the amount of carbon is too small, the fraction of the second phase is reduced and the strength is reduced, so that the tensile strength required by the present patent cannot be manufactured at least 980 MPa grade, so it should be at least 0.07%. However, when added in excess of 0.12%, there is a disadvantage such as a decrease in weldability, a sharp decrease in workability due to a sharp increase in the second phase fraction, etc., the content is limited to 0.12% or less.

Si: 0.80-1.7%

Silicon is a solid solution of ferrite as a ferrite stabilizing element and contributes to strength, and is usually added as a deoxidizer. Silicon is an element that increases the ferrite fraction by promoting austenite-ferrite transformation upon cooling in composite steel. If the silicon is 0.8% or less, the ferrite strength is reduced, the carbide inhibiting effect is reduced, as well as 0.80% or more should be added to secure an appropriate level of ductility in the manufacture of ultra high strength steel such as the present patent. It is also effective for the flotation separation of MnS inclusions in steels with a large amount of manganese as an element to increase the fluidity of molten steel, and improves the fresh butt weldability in the range of 5-30 manganese / silicon ratio, but not only causes hot rolled scale. Since there is a problem that the weldability is deteriorated, it is preferable to limit it to 1.7% or less.

Mn: 1.5 ~ 2.5%

Manganese is an austenite stabilizing element as an element that facilitates the formation of low-temperature transformation phases such as martensite and bainite, that is, increases hardening ability and increases strength. When carbon and silicon are added in the range of 0.07 to 0.12% and 0.80 to 1.7%, respectively, it is preferable to add at least 1.5% to have a strength of 980 MPa or more. When excessively added, weldability is lowered, hydrogen organic brittleness is caused by inclusion formation, and a segregation zone is formed at the center of the sheet during hot rolling, so the upper limit of Mn is limited to 2.5% or less.

Nb: 0.01 ~ 0.06%

In general, nionium is an element which solidifies ferrite grains by reducing the grains of austenite by forming a solid solution or solid solution in austenite during hot rolling. It is preferable to add at least 0.01% for the solid solution and precipitation effect. When excessively added, it is limited to 0.06% or less because it causes cracks during continuous casting and increases the manufacturing cost.

Al: 0.1 ~ 0.3%

Aluminum, like silicon, is a ferrite stabilizing element in solid solution, which contributes to strength, and is commonly added as a deoxidizer. In particular, in order to produce ultra high strength steel with a tensile strength of 980 MPa or more, as in the present invention, a ferrite stabilizing element such as aluminum is necessary and an element that must be added to a certain amount or more in order to distinguish it from other inventions. Aluminum is an element that promotes austenite-ferrite transformation when hot-rolled and then increases ferrite fraction. If the aluminum is less than 0.1%, the ultra-high strength steel such as the present invention steel has a low ferrite formation effect to secure proper ductility, and carbide inhibiting effect is reduced, so 0.1% or more should be added. However, if excessively added, there is a problem that the weldability is lowered and deteriorated, so the content thereof is preferably limited to 0.3% or less.

Cr: 0.5 ~ 1.0%

Chromium increases austenite stability so that austenite does not transform into bainite and forms martensite even when wound at high temperature. In the present invention, it is preferable to add 0.5% or more of chromium for strength and austenite stability. However, when excessively added, the weldability is lowered, the ferrite fraction is lowered to inhibit the elongation, so the content is limited to 1.0% or less.

In addition, it is preferable that S, P, and nitrogen are added at 0.005% or less, 0.04% or less, and 0.006% or less, respectively, as impurities. When the S exceeds 0.005%, coarse TiS and MnS are generated in the hot rolled sheet, thereby reducing workability and toughness, and thus, reducing the S is effective, and P and N are advantageously limited to the range generally included in steel.

The slabs formed as described above are obtained through ingot or continuous casting process after obtaining molten steel through a steelmaking process. The slabs are manufactured by reprocessing the segregated components during the casting in the furnace, hot rolling process to control the thickness of the plate, securing the material, and cooling and winding process to cool the plate to room temperature. The manufacturing process for each star is explained in detail.

The process of heating the slab having the composition and the range is for reusing the segregated components during casting, and the segregated components are not reusable when the reheating temperature is 1150 ° C. or lower, and when the re heating temperature is 1250 ° C. or higher, the austenite grain size is increased. As the ferrite grain size is coarsened, the strength is reduced. Therefore, the slab reheating temperature is preferably limited to 1150 to 1250 ° C, and preferably reheated at about 1200 ° C.

In addition, in the present invention, it is preferable to reheat 60 to 180 minutes in the temperature range, which is not effective removal of segregation present in the ingot when the reheating in less than 60 minutes, recrystallization when reheated for more than 180 minutes This makes the tissue coarse and causes a problem of a decrease in strength. Therefore, it is preferable to perform the said reheating for 60 to 180 minutes.

There is the finishing hot rolling at a slab for hot rolling, and more than Ar ₃ + 60 ℃ reheating as described above, if the finish rolling at the Ar ₃ immediately above the difficulty of the task increases the rolling roll load increased significantly, and less than Ar ₃ of Rolling at temperature introduces many dislocations into the ferrite formed during hot rolling, and these ferrites grow during cooling or winding to form surface coarse grains. However, in the case of finishing rolling at an excessively high temperature, the ferrite grain size is increased and the strength is decreased, so Ar ₃ + 60 ° C. or more is preferable.

Since the hot rolled hot rolled sheet should be secured simultaneously with strength and elongation, it is preferable to control the fraction of bainite and martensite for improving the strength, and mainly to control the ferrite fraction for elongation. In order to secure an appropriate ferrite fraction, the present invention applies a step cooling method. The intermediate cooling method is a method of securing a certain amount of ferrite by maximizing the transformation of ferrite by maintaining it for a predetermined time in the temperature range where the transformation of austenite into ferrite after hot rolling is maximized.

The inventors have investigated the temperature at which the transformation of the ferrite is the maximum for the slab satisfying the above-described component range, it was found that the most preferred temperature range for the intermediate cooling is 680 ~ 720 ℃. It is found that the intermediate cooling temperature is more preferably 700 ° C., and in addition, it is also necessary to maintain a predetermined time at 680 to 720 ° C. in order to maximize ferrite transformation. The holding time is preferably 5 to 8 seconds, and in 5 seconds or less, a sufficient ferrite fraction necessary for an elongation of 12% or more is not obtained, and in 8 seconds or more, a decrease in strength occurs due to coarsening of the tissue.

As described above, the winding work is performed at a temperature of 450 ° C. or higher after cooling by using the abnormal structure of ferrite + austenite obtained through cooling at an intermediate temperature. If the coiling temperature is less than 450 ℃ as described above most of the austenite is transformed to bainite can not satisfy the yield strength and yield ratio of less than 0.750 required by the present invention. On the other hand, when the work temperature is higher than 550 ℃ ferrite amount is too high to secure sufficient tensile strength required by the present invention steel. Therefore, preferably the winding temperature is preferably controlled in a temperature range of 450 ~ 550 ℃.

The steel sheet that satisfies the component system and the manufacturing method proposed by the present invention will have a yield ratio (YR) of 0.750 or less represented by the following relational expression.

Yield Ratio (YR) = 0.924-0.00622 * M Volume%-0.00114 * F Volume%

(However, M: Martensite, F: Ferrite)

In addition, the steel sheet of the present invention is a composite tissue steel, and has a ferrite, martensite and bainite structure, in order to have a strength of 980 MPa or more and an elongation of 12% or more, the ferrite content of the microstructure is 60 vol% or more, and martensite It is preferable to satisfy that content is 15 volume% or more and bainite content is 15 volume% or less. This is because when the fraction of ferrite in the microstructure is less than 60% by volume, the fraction of martensite is less than 15%, and the fraction of bainite exceeds 15% by volume, the yield ratio exceeds 0.750. to be.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following examples.

Example 1

Microstructure was observed for 21 specimens according to the present invention, and the microstructure fractions are shown in Table 1 below.

Steel grade Martensite
Fraction (% by volume) Bainite
Fraction (% by volume) ferrite
Fraction (% by volume) Yield Ratio (YR) Remarks One 17.4 12 70.6 0.735 normal 2 16.8 13.6 71.6 0.738 normal 3 18.9 10.8 70.3 0.726 normal 4 13.3 25.3 61.4 0.771 Bad 5 22.2 10.2 67.6 0.709 normal 6 17.6 8 74.4 0.730 normal 7 15.1 16.4 68.5 0.752 Bad 8 15.7 3.8 80.5 0.735 normal 9 20.1 13.9 66 0.724 normal 10 6 25 69 0.808 Bad 11 25.3 12.6 62.1 0.696 normal 12 10.8 42.4 46.8 0.803 Bad 13 13.6 21.3 65.1 0.765 Bad 14 21.6 13.1 65.3 0.715 normal 15 18.1 13.8 68.1 0.734 normal 16 16.4 22.2 61.4 0.752 Bad 17 18.4 11.4 70.2 0.730 normal 18 8 34.7 57.3 0.809 Bad 19 18.5 14.4 67.1 0.732 normal 20 12.2 36.2 51.6 0.789 Bad 21 20 14.4 65.6 0.725 normal

As a result of the study based on Table 1, the yield ratio was found to be very sensitively dependent on the fraction of metamorphic tissue present in the river, the present inventors investigated the correlation between the yield ratio and the fraction of metamorphic tissue, Yield ratio could be expressed as [Relative] Yield Ratio (YR) = 0.924-0.00622 * M Volume%-0.00114 * F Volume%.

Among the steel grades listed in Table 1, the yield ratio was 0.750 or less, satisfactory, and exceeding 0.750 was expressed as bad. In the case of steel grade having a bainite fraction of 15% by volume or less, the yield ratio is 0.750 or more.

[Example 2]

The slab that satisfies the component system described in Table 2 below is heated at 1200 ° C. for 1 hour, and then hot-rolled at 860 ° C., and then maintained at about 700 seconds at an intermediate cooling process temperature of 700 ° C. as shown in Table 2 below. Air-cooled and then wound up. Strength and elongation were measured using the specimens prepared after final cooling, and the results are shown in Table 2 below. In addition, the microstructure fraction of the specimens was measured, yield ratio (YR) was derived, and the results are shown in Table 3 below.

In addition, the yield strength and the yield ratio change according to the winding temperature change of the invention steel 2 of Table 2 were investigated, and the yield strength change is shown in FIG. 1, and the yield ratio change is shown in FIG. 2. In addition, the microstructure according to the change of the coiling temperature was observed with respect to the invention steel 2 of Table 2 below, in FIG. 3 for the coiling temperature of 400 ° C (comparative material 1), and in FIG. And the winding temperature of 500 degreeC (invention material 3) is shown in FIG.

Steel grade C Mn Si Cr Al Nb S P Inventive Steel 1 0.092 1.53 1.53 0.71 0.21 0.032 0.0036 0.029 Inventive Steel 2 0.081 1.97 1.50 0.70 0.20 0.032 0.0036 0.030 Inventive Steel 3 0.087 2.07 1.57 0.69 0.23 0.030 0.0010 0.013 Inventive Steel 4 0.102 1.78 1.70 0.82 0.28 0.021 0.0031 0.013 Inventive Steel 5 0.098 1.59 1.51 0.69 0.19 0.033 0.0030 0.029 Inventive Steel 6 0.117 1.51 1.51 0.69 0.19 0.033 0.0040 0.029 Inventive Steel 7 0.089 1.68 0.98 0.96 0.28 0.023 0.0035 0.029 Comparative Steel 1 0.079 3.00 1.21 0.68 0.25 0.021 0.0028 0.031 Comparative Steel 2 0.141 1.79 0.99 1.12 0.25 0.015 0.0019 0.012 Comparative Steel 3 0.081 1.95 1.43 0.78 0.43 0.036 0.0029 0.031 Comparative Steel 4 0.091 1.97 1.63 0.81 0.21 0.120 0.0035 0.024 Comparative Steel 5 0.086 2.81 1.75 0.71 0.26 0.076 0.0024 0.021

division middle
maintain
Temperature
(℃) Winding
Temperature
(℃) YS
(MPa) TS
(MPa) Uni.
El
(%) Tot.
El
(%) YR TS * El
(MPa *%) Steel grade Steel Inventive Steel 1 Invention 1 700 450 656.7 995.0 7.9 14.2 0.66 14129.0 Inventive Steel 2 Comparative Material 1 700 400 726.2 954.2 7.7 13.0 0.76 12404.6 Invention 2 450 642.2 994.0 8.3 15.1 0.65 15009.4 Invention 3 500 643.1 989.4 7.7 13.6 0.65 13455.8 Invention Steel 3 Invention 4 700 450 687.0 999.0 8.9 16.9 0.69 16883.1 Comparative Material 2 600 450 805.0 1070.2 6.7 11.0 0.76 11772.2 Inventive Steel 4 Invention 5 700 450 642.1 1021.3 8.6 16.4 0.63 16749.3 Inventive Steel 5 Invention 6 700 450 594.7 991.2 10.8 17.5 0.60 17346.0 Inventive Steel 6 Invention Material7 700 450 596.2 1027.9 10.3 18.6 0.58 19118.9 Inventive Steel 7 Invention Material 8 700 450 582.0 1003.4 9.8 16.7 0.58 16756.8 Comparative Steel 1 Comparative Material 3 700 450 1013.7 1221.3 4.2 7.9 0.83 9648.3 Comparative Steel 2 Comparative Material 4 700 450 761.1 1150.7 7.1 11.2 0.66 12887.8 Comparative Steel 3 Comparative Material 5 700 450 771.1 993.2 5.9 10.9 0.77 10825.9 Comparative Steel 4 Comparative Material 6 700 450 830.2 1260.2 4.3 7.5 0.66 9451.5 Comparative Steel 5 Comparative Material7 700 450 912.5 1261.0 3.6 6.9 0.72 8700.9

As shown in Table 3, Invention 1 to 8 in accordance with the present invention has a yield strength (YS) of 582 ~ 687MPa, a tensile strength (TS) of 989.4 ~ 1027.9 MPa, elongation of 13.6 ~ 18.6% (Tot.El ), It has a yield ratio (YR) of 0.692 ~ 0.748. As such, Inventive Materials 1 to 8 show that the tensile strength of 980 MPa or more, the elongation of 12% or more, and the yield ratio of 0.750 or less are satisfied.

Meanwhile, as can be seen in Table 3 and FIGS. 1 and 2, Comparative Material 1 wound at 400 ° C., which is lower than Inventive Materials 2 and 3, has a higher yield strength than Inventive Materials 2 and 3, and the present invention. Yield ratio required under 0.750 was not satisfied. In addition, as shown in Figures 3 to 5, Comparative Material 1 (FIG. 3) is different from Inventive Material 2 (FIG. 4) and Inventive Material 3 (FIG. 5) that the steel structure is transformed from austenite to bainite. Able to know.

Comparative material 2 had a low ferrite fraction due to insufficient transformation from austenite to ferrite because the intermediate temperature was lowered to 600 ° C, resulting in a very high yield strength of 805 MPa and an elongation of 11%. It did not satisfy the required elongation.

Comparative material 3 has a very high Mn content of 3%, which causes a significant amount of austenite to be converted to bainite upon cooling. Therefore, the elongation was very low (7.9%) compared to the high strength. In addition, Comparative Material 3 increased the carbon equivalent (Ceq = C + Mn / 6 + (Cr + Mo + V) / 3 + (Cu + Ni) / 152) due to the excessive addition of Mn, resulting in deterioration of weldability. .

Comparative material 4 had a carbon content of 0.141%, far exceeding 0.12% of the range of the present invention steel, and the elongation was reduced, and the weldability was very deteriorated due to the addition of excessive carbon.

Comparative material 5 is an excessive addition of Al, a ferrite stabilizing element, it can be seen that a large amount of oxides are generated in the steel, the ductility deteriorated.

Comparative material 6 is an excessive addition of Nb, resulting in the formation of fine NbC precipitates in the steel due to excessive addition, resulting in a significant increase in yield strength and decrease in elongation due to precipitation strengthening.

Comparative material 7 is an excessively added Mn and Nb, the yield strength and tensile strength was very increased, but it can be seen that the elongation is very low as 6.9%.

1 is a graph showing the change in yield strength according to the change in winding temperature for the inventive steel 2.

2 is a graph showing the change in yield ratio according to the change in winding temperature for the inventive steel 2.

3 is a microstructure photograph of Comparative Material 1 wound around Inventive Steel 2 at 400 ° C.

4 is a microstructure photograph of Inventive Material 2 wound around Inventive Steel 2 at 450 ° C.

5 is a microstructure photograph of Inventive Material 3 wound around Inventive Steel 2 at 500 ° C.

Claims

By weight%, C: 0.07 to 0.12%, Si: 0.80 to 1.71%, Mn: 1.5 to 2.5%, Cr: 0.5 to 1.0%, Al: 0.1 to 0.3%, Nb: 0.01 to 0.06%, S: 0.005% Or less, P: 0.04% or less, high strength hot rolled steel sheet having excellent workability including Fe and other unavoidable impurities, an elongation of 12% or more, and a yield ratio (YR) of 0.750 or less.

Yield Ratio (YR) = 0.924-0.00622 * M Volume%-0.00114 * F Volume%

(However, M: Martensite, F: Ferrite)

The high strength hot rolled steel sheet according to claim 1, wherein the hot rolled steel sheet has excellent tensile workability of 980 MPa or more.

The high-strength hot-rolled steel sheet of claim 1, wherein the microstructure of the hot-rolled steel sheet includes ferrite, martensite, and bainite.

The high-strength hot rolled steel sheet according to claim 3, wherein the microstructure has a ferrite content of 60 vol% or more, a martensite content of 15 vol% or more, and a bainite content of 15 vol% or less.

By weight%, C: 0.07 to 0.12%, Si: 0.80 to 1.71%, Mn: 1.5 to 2.5%, Cr: 0.5 to 1.0%, Al: 0.1 to 0.3%, Nb: 0.01 to 0.06%, S: 0.005% P: 0.04% or less Reheating the slab containing the remaining Fe and other unavoidable impurities in the temperature range of 1150 ~ 1250 ℃;

Finishing hot rolling of the reheated slab at a temperature of Ar3 + 60 ° C. or higher;

Cooling the finished hot-rolled steel sheet and maintaining it for 5 to 8 seconds at a temperature range of 680 to 720 ° C .;

The method of manufacturing a high strength hot rolled steel sheet having excellent workability, comprising the step of winding the heat-treated steel sheet in a temperature range of 450 ~ 550 ° C.

The method of manufacturing a high strength hot rolled steel sheet having excellent workability according to claim 5, wherein the reheating is performed for 60 to 180 minutes.