KR101630982B1 - High strength hot rolled steel sheet having excellent bendability andbake hardenability and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a high-strength hot rolled steel sheet usually used for an impact member and a reinforcement material of a vehicle. More specifically, the present invention relates to a manufacturing method of the high-strength hot rolled steel sheet, and the high-strength hot rolled steel sheet with excellent bendability and bake hardening property. The high-strength hot rolled steel sheet comprises: 0.1-0.25 wt% of carbon (C), 0.05-0.3 wt% of silicon (Si), 1.0-2.3 wt% of manganese (Mn), 0.01-0.1 wt% of aluminum (Al), 0.01-0.1 wt% of chromium (Cr), 0.005-0.05 wt% of phosphorus (P), 0.001-0.05 wt% of sulfur (S), 0.001-0.01 wt% of nitrogen (N), the remainder consisting of Fe and unavoidable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength hot-rolled steel sheet excellent in bending workability and bake hardenability, and a method of manufacturing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

More particularly, the present invention relates to a high strength hot-rolled steel sheet excellent in bending workability and bake hardenability, and a method for manufacturing the same.

Conventionally, multi-phase steel and Transformation Induced Plasticity Steel (TRIP steel) are mainly used as hot-rolled high-strength steel for application to automobile impact members and reinforcing materials.

These steels are steels containing martensite phase and retained austenite phase as ferrite or bainite phase at room temperature, and have improved strength and ductility at the same time. Double TRIP steels are steels that have been transformed into martensite by deformation of retained austenite, thereby further improving strength and elongation.

In order to achieve such an effect, it is necessary to increase the ingot strength of the steel and to improve the stability of the retained austenite, and thus it contains a large amount of various alloy components such as C, Mn, Cr, Si and Al.

However, the weldability of the steel is poor due to the large amount of alloy components, and particularly when the tensile strength is 1 GPa or more, the bending workability is poor.

Further, most of the steels having a composite structure as described above exhibit a bake hardenability (BH) upon heat treatment after coating, and the BH value at that time is comparatively low and there is a problem that the effect can not be utilized.

Therefore, there is a need for a method for improving bending workability and bake hardenability in a composite structure steel having high strength and ductility.

One aspect of the present invention is to provide a hot-rolled steel sheet which has high strength and high ductility as well as bending workability at the same time and which is superior in bake hardenability to existing high-strength hot- And the like.

An aspect of the present invention provides a method of manufacturing a semiconductor device, comprising 0.1 to 0.25% carbon, 0.05 to 0.3% silicon, 1.0 to 2.3% manganese, 0.01 to 0.1% aluminum, 0.001 to 0.05% of sulfur (S), 0.001 to 0.01% of nitrogen (N), the balance Fe and other unavoidable impurities Bending workability having a tensile strength (TS) to bending workability (R / t) ratio TS / (R / t) of 330 or more and a bake hardenability BH of 0.08 x YS (yield strength, MPa) A high-strength hot-rolled steel sheet excellent in bake hardenability is provided.

According to another aspect of the present invention, there is provided a method of manufacturing a steel slab, comprising: preparing a steel slab satisfying the above-described composition; Reheating the steel slab at 1150 to 1350 ° C; Subjecting the reheated steel slab to hot finish rolling at 850 to 1150 占 폚 and terminating the rolling at the austenite zone; After the rolling, primary cooling at an average cooling rate of from 10 to 200 DEG C / sec from 550 to 750 DEG C; Cooling in the range of 550 to 750 ° C after the primary cooling; Applying a strain of 0.2 to 5.0% in the range of 550 to 750 占 폚 after the air cooling; After the deformation, secondary cooling at an average cooling rate of 10 to 200 ° C / sec from 300 to 500 ° C; And a step of winding at 300 to 500 ° C after the secondary cooling, thereby producing a high strength hot rolled steel sheet excellent in bending workability and sintering hardening ability.

According to the present invention, it is possible to provide a hot-rolled steel sheet that not only has excellent strength and bending workability, but also greatly improves the bake hardenability.

Further, the hot-rolled steel sheet of the present invention can be suitably used for applications such as automobile collision members and reinforcing materials.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the ratio TS / R / t of the tensile strength TS and the bending workability R / t and the bake hardenability BH and MPa in Inventive and Comparative Examples according to an embodiment of the present invention. As shown in FIG.

The inventors of the present invention have conducted intensive studies to improve the low bending workability of the conventional composite steel and improve the hardening of the bake hardenability. As a result of optimizing the manufacturing conditions as well as the composition of the steel components, the present inventors have found that the bending workability And a hot-rolled steel sheet having a hardening ability of hardening can be provided. The present invention has been completed based on this finding.

Particularly, the present inventors have found that, in the process of manufacturing a hot-rolled steel sheet, when the rolled steel sheet is subjected to a specific deformation before the end of the ferrite phase transformation during cooling after the hot rolling, the baking hardenability can be greatly improved.

More specifically, it has been confirmed that as the dislocation density in the ferrite crystal grains increases while the ferrite phase transformation is promoted by the above specific modification, the density of the dislocation of the bainite phase and the ferrite phase at the subsequent secondary cooling increases. The dislocation density at this time is increased by about 1.2 times as compared with the case where the specific strain is not applied, and it is possible to exhibit very excellent curing properties due to the increase of the dislocation density.

Hereinafter, the present invention will be described in detail.

A high-strength hot-rolled steel sheet excellent in bending workability and bake hardenability according to one aspect of the present invention comprises 0.1 to 0.25% of carbon (C), 0.05 to 0.3% of silicon (Si) (P): 0.005 to 0.05%, sulfur (S): 0.001 to 0.05%, nitrogen (N): 0.001 To 0.01% by weight.

Hereinafter, the reason for controlling the alloy components of the hot-rolled steel sheet provided in the present invention will be described in detail. Herein, unless otherwise specified, the content of each component means weight%.

C: 0.1 to 0.25%

Carbon (C) is the most economical and effective element for strengthening the steel. When the amount of C added increases, the fraction of the bainite phase and the martensite phase of the composite steel having ferrite-bainite-martensite increases, .

When the content of C is less than 0.1%, the formation of the bainite phase and the martensite phase during cooling after hot rolling is not easy. On the other hand, when the content exceeds 0.25%, the steel strength is excessively increased, There is a problem that the properties, bending workability and toughness are lowered.

Therefore, the content of C in the present invention is preferably limited to 0.1 to 0.25%.

Si: 0.05 to 0.3%

Silicon (Si) is an element added not only for deoxidizing deoxidation of molten steel but also for strength improvement by solid solution strengthening. In addition, it has an effect of promoting ferrite transformation during cooling after hot rolling as a ferrite stabilizing element, and is an element effective for increasing the fraction of a ferrite phase which is a base structure in a ferrite-bainite-martensite composite structure steel.

If the content of Si is less than 0.05%, there is a problem that the effect of stabilizing ferrite is small and it is difficult to form the base structure into a ferrite structure. On the other hand, when the content exceeds 0.3%, the fraction of martensite phase becomes excessively large, There is a problem that a red color scale due to Si is formed on the surface of the steel sheet to deteriorate the surface quality of the steel sheet and deteriorate ductility and weldability.

Therefore, the content of Si in the present invention is preferably limited to 0.05 to 0.3%.

Mn: 1.0 to 2.3%

Manganese (Mn) is an element effective in strengthening the steel in the same manner as Si, and increases the hardenability of the steel to facilitate formation of a bainite phase and a martensite phase during cooling after hot rolling.

In order to sufficiently obtain such an effect in the present invention, it is preferable to contain Mn at 1.0% or more, but when the content exceeds 2.3%, the ferrite transformation is excessively delayed, making it difficult to secure a proper fraction of ferrite. In addition, there is a problem in that the segregation portion is greatly developed at the center of thickness during slab casting in the casting process, thereby deteriorating the weldability and mechanical properties of the final product.

Therefore, the content of Mn in the present invention is preferably limited to 1.0 to 2.3%.

Sol.Al: 0.01 to 0.1%

Aluminum (Sol.Al) is a component mainly added for deoxidation, and as an element stabilizing ferrite, there is an effect of assisting formation of a ferrite phase during cooling after hot rolling.

In order to sufficiently obtain the above-mentioned effect, the present invention preferably contains 0.01% or more of Sol.Al, but if the content exceeds 0.1%, defects tend to occur in the slab during continuous casting, There is a problem that defects are generated and the surface quality is lowered.

Therefore, in the present invention, the content of Sol.Al is preferably limited to 0.01 to 0.1%.

Cr: 0.05 to 1.0%

Chromium (Cr) is an effective element for strengthening the strength of steel, and it delays the phase transformation of ferrite during cooling and facilitates transformation of bainite phase and martensite phase.

In order to obtain the above-mentioned effect, it is preferable to contain Cr at 0.05% or more, but if the content exceeds 1.0%, the ferrite transformation is excessively delayed and the fraction of bainite phase and martensite phase becomes higher than necessary, There is a problem that ductility is lowered.

Therefore, the content of Cr in the present invention is preferably limited to 0.05 to 1.0%.

P: 0.005 to 0.05%

Phosphorus (P) has a solubility strengthening effect and ferrite transformation promoting effect like Si. If the content of P is less than 0.005%, it is insufficient to secure the intended strength. On the other hand, if the content exceeds 0.05%, ductility of steel is deteriorated due to band organization due to micro segregation.

Therefore, the content of P in the present invention is preferably limited to 0.005 to 0.05%.

S: 0.001 to 0.05%

Sulfur (S) is an impurity which is inevitably added during the steel making process, and forms a nonmetallic inclusion by bonding with Mn or the like, which causes a problem that the toughness of the steel is greatly lowered. Therefore, it is preferable that the S is controlled as low as possible.

In theory, it is advantageous to control the content of S to 0%, but it is important to manage the upper limit because the content of S is inevitably contained inevitably. In the present invention, if the content of S is 0.05% or less, the upper limit of S is preferably limited to 0.05%, since it does not affect the physical properties of the steel. However, in order to control the content of S to less than 0.001%, it takes much time for steelmaking and the productivity is lowered. Therefore, it is more preferable to control the content to 0.001 to 0.05%.

N: 0.001 to 0.01%

Nitrogen (N), together with carbon (C), is a typical solid solution strengthening element and forms coarse precipitates together with titanium (Ti), aluminum (Al) and the like. In general, the solid solution strengthening effect of nitrogen is superior to carbon, but the toughness of the steel is significantly lowered as the amount of nitrogen in the steel is increased.

In order to control the content of N to less than 0.001%, it takes a long time to perform the steelmaking operation and the productivity becomes poor. On the other hand, when the content exceeds 0.01%, the formation of coarse nitride easily occurs, There is a problem that it increases greatly.

Therefore, the content of N in the present invention is preferably limited to 0.001 to 0.01%.

In order to further improve the effect of the present invention, it is preferable to further include at least one element selected from the group consisting of niobium (Nb), titanium (Ti), and vanadium (V).

In this case, the content of the selected components is preferably 0.001 to 0.1% by weight.

The Ti, Nb and V are effective components for refining the crystal grains, and Ti exists as TiN in the steel to inhibit growth of crystal grains during the heating process for hot rolling. Also, it is a component useful for improving the strength of steel by forming TiC precipitate by reacting with nitrogen (N) and remaining Ti dissolved in the steel and bonding with carbon (C).

Nb and V are effective in grain refinement by forming carbides in the steel and form fine precipitates to improve not only the strength of the steel but also the toughness.

In order to sufficiently obtain the above-mentioned effects from the addition of at least one of Ti, Nb and V, it is preferable that the content of the above components is included in an amount of 0.001% or more, but if the content exceeds 0.1% (C) and (N) in the steel are remarkably decreased and the hardening ability of hardening (BH) is rapidly lowered, which is not preferable.

The remaining components other than the above-mentioned components are iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

It is preferable that the hot-rolled steel sheet of the present invention satisfying all of the above-mentioned composition and composition relationships has a microstructure having a composite structure of ferrite, bainite and martensite, wherein each phase contains 10% , And it is preferable that these microstructures are all contained in a total amount of 95% or more.

If the bainite fraction and the martensite fraction are less than 10%, respectively, the movable dislocation density of the present invention can not be ensured and the yield strength is increased. have. On the other hand, when the fraction of ferrite is less than 10%, the fraction of the bainite phase and the martensite phase increases relatively, resulting in an excessive increase in tensile strength and a significant decrease in elongation.

By including the ferrite-bainite-martensite composite structure in this way, not only the tensile strength of the steel is increased but also the moving dislocation density is increased and the yield strength is lowered.

Therefore, the hot-rolled steel sheet of the present invention has a tensile strength (TS) / bending workability (R / t) ratio TS / (R / t) of 330 or more and a bake hardenability BH of 0.08 x YS , MPa). In addition, the hot-rolled steel sheet of the present invention has a tensile strength of 780 MPa or more, which not only has a high strength, but also has excellent bending workability and bake hardenability.

Hereinafter, a method for producing a high-strength hot-rolled steel sheet excellent in bending workability and bake hardenability, which is another aspect of the present invention, will be described in detail.

[Reheating step]

First, after preparing a steel slab satisfying the above-described composition and composition, it is preferable to reheat the steel slab at 1150 to 1350 ° C.

If the temperature for reheating is less than 1150 DEG C, the precipitates are not sufficiently reused, and the content of solid solution C and N is decreased in the process after hot rolling. On the other hand, when the temperature exceeds 1350 DEG C, there is a problem that the strength is lowered due to abnormal grain growth of the austenite grains.

Therefore, in the present invention, it is preferable that the steel slab is heated at a temperature of 1150 to 1350 ° C. At this time, it is preferable to heat to the above temperature range at a temperature raising rate of 0.1 to 10 ° C / s.

[Hot rolling step]

It is preferable that the steel slab reheated according to the above is hot finished and rolled to produce a hot-rolled steel sheet.

If the hot finish rolling temperature is less than 850 DEG C, the rolling load is greatly increased. On the other hand, when the temperature exceeds 1150 DEG C, the hot rolling is performed at a temperature of 850 to 1150 DEG C, The steel structure becomes coarse, the steel becomes weak, the scale becomes thick, and there is a problem that surface quality degradation such as high-temperature rolling-scale defects occurs.

Therefore, in the present invention, the hot rolling is preferably performed at a temperature of 850 to 1150 ° C, and the hot finish rolling preferably finishes at austenite, that is, Ar 3 or more before the ferrite phase transformation occurs in the steel.

[Primary cooling step]

It is preferable to cool the hot rolled steel sheet obtained by the hot finish rolling, and it is preferable to perform the multi-step cooling at this time.

It is preferable that the hot-rolled steel sheet is first cooled, wherein the primary cooling is preferably performed at a cooling rate of 10 to 200 ° C / sec until the temperature reaches 550 to 750 ° C from the hot rolling temperature .

If the temperature at which the primary cooling is terminated is less than 550 ° C, most of the steel microstructure contains a bainite phase and a martensite phase, so that the microstructure to be obtained in the present invention can not be obtained. On the other hand, The coarse ferrite and pearlite structure are formed to reduce the strength of the steel.

If the cooling rate is less than 10 DEG C / sec during the primary cooling, coarsening of the ferrite crystal grains occurs, and the precipitates are also coarsened, so that the strength to be obtained in the present invention can not be secured. On the other hand, Exceeds 200 占 폚 / sec, the temperature of the hot-rolled steel sheet becomes uneven due to excessive cooling, making it difficult to control the ferrite phase fraction and the shape of the hot-rolled steel sheet becomes poor.

Therefore, in the present invention, the primary cooling is preferably performed at a cooling rate of 10 to 200 ° C / sec from 550 to 750 ° C.

[Air cooling step]

It is preferable to carry out the step of air cooling prior to the secondary cooling of the primary cooled hot-rolled steel sheet.

At this time, it is preferable that the air cooling is performed at a temperature range in which the primary cooling is completed, that is, at 550 to 750 ° C for 4 to 10 seconds. If the air cooling time is less than 4 seconds, the ferrite structure in the steel can not be sufficiently formed, There is a problem of deterioration. On the other hand, when the air cooling time exceeds 10 seconds, the ferrite fraction increases and the bainite phase and the martensite phase fraction can not be sufficiently secured, so that the strength and bending workability can not be ensured.

Therefore, in the present invention, it is preferable that the air cooling is performed for 4 to 10 seconds.

[Deformation step]

In the step of air-cooling the hot-rolled steel sheet in accordance with the above, it is preferable that deformation is applied to the hot-rolled steel sheet before the end of the ferrite phase change.

This promotes the ferrite phase transformation and increases the dislocation density in the ferrite crystal grains, thereby enhancing the density of the moving dislocation between the bainite phase and the ferrite phase interface formed during the subsequent cooling. This has a very advantageous effect for increasing the hardening ability (BH) of the steel.

Since the deformation is carried out before the end of the ferrite phase change during air cooling, the deformation is carried out at a temperature of 550 to 750 ° C, and the amount of deformation is preferably 0.2 to 5.0%. If the deformation amount is less than 0.2%, it is difficult to obtain the above effect. On the other hand, if the deformation amount is more than 5.0%, the quality of the shape becomes poor and the dislocation density of the steel becomes too high to increase the yield strength, thereby lowering the bending workability and elongation have.

This modification can be performed, for example, through a rolling roll provided on a cooling stand for cooling of the present invention, but is not limited thereto.

[Second cooling step]

It is preferable that the hot rolled steel sheet after the deformation is cooled secondarily.

At this time, it is preferable that the secondary cooling is performed at a cooling rate of 10 to 200 ° C / sec until the temperature reaches 300 to 500 ° C.

When the temperature at which the secondary cooling is terminated is less than 300 ° C, a martensite phase is formed and ductility and bending workability are poor. On the other hand, when the temperature exceeds 500 ° C, a bainite phase and a martensite phase are not sufficiently formed There is a problem in that the high strength to be secured in the present invention can not be achieved.

If the cooling rate is less than 10 캜 / sec during the secondary cooling, the formation of bainite phase and martensite phase becomes difficult, and the strength to be obtained in the present invention can not be secured. On the other hand, ° C / sec, there is a problem that the shape of the hot-rolled steel sheet becomes poor due to excessive cooling.

Therefore, in the present invention, it is preferable that the secondary cooling is performed at a cooling rate of 10 to 200 ° C / sec from 300 to 500 ° C.

The hot-rolled steel sheet after completion of the secondary cooling may be further subjected to a winding step so as to facilitate storage and movement. The winding temperature at this time is not particularly limited, and it is preferable that the hot-rolled steel sheet is wound in a cooled state. Therefore, in the present invention, it can be performed in a temperature range of 300 to 500 ° C.

Further, the hot-rolled steel sheet produced by the above-mentioned method may be naturally cooled, pickled, removed from the surface layer scale, and further roughened to obtain a pickled steel sheet.

The produced pickled steel sheet may be reheated at 450 to 480 ° C and then passed through a hot dip galvanizing bath to produce a hot-dip galvanized steel sheet. If the reheating temperature is less than 450 ° C, unplating tends to occur. If the reheating temperature is higher than 480 ° C, plating defects may occur or the thickness of the plating layer may not be uniform.

Therefore, the reheating temperature is preferably limited to 450 to 480 캜.

Hereinafter, the present invention will be described in more detail by way of examples. It should be noted, however, that the embodiments described below are for illustrating and embodying the present invention, and not for limiting the scope of the present invention. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

Steel slabs satisfying the composition shown in the following Table 1 were reheated to 1250 占 폚 and subjected to hot rolling at the hot finishing temperature (FDT) shown in Table 2 below to prepare respective hot-rolled steel sheets. Then, each of the hot-rolled steel sheets prepared above was subjected to primary cooling at a cooling rate of 70 DEG C / sec to 680 DEG C, followed by air cooling for 6 seconds. Thereafter, the steel sheet was subjected to secondary cooling at a cooling rate of 70 DEG C / sec up to the winding temperature (CT) shown in Table 2, and then wound at the winding temperature. At this time, for some hot-rolled steel sheets, after air cooling, deformation was applied at a strain rate shown in Table 2 below, followed by secondary cooling and winding.

(YS), tensile strength (TS), fracture elongation (T-El), bending workability (R / t) and bake hardenability (BH) of each final hot-rolled steel sheet obtained through the above- And the results are shown in Table 2 below.

In Table 2, the strength was 0.2% offset, and the yield strength, tensile strength and elongation were evaluated by fabricating a tensile test piece (JIS No. 5) of the JIS standard in the direction perpendicular to the rolling direction.

The tensile hardening capacity (BH) was also evaluated by preparing a tensile test piece of JIS standard (JIS No. 5) in the direction perpendicular to the rolling direction. To this end, the tensile test piece was heat treated at 170 ° C for 20 minutes The difference between the measured lower yield strength or 0.2% offset yield strength in the tensile test and the measured value in the 2% tensile test.

The bending workability (R / t) of each hot rolled plate was evaluated by using a V-bending tester. At this time, the minimum radius of curvature (R), at which bending of the plate did not occur during 90 ° bending, .

Each final hot-rolled steel sheet was etched with a lepera etchant and observed with an optical microscope at 500 magnification and analyzed with an image analyzer to determine the fraction of ferrite phase, bainite phase and martensite phase, The values are shown in Table 3 below.

division Component composition (% by weight) C Si Mn Cr Al P S N Ti Nb V Inventive Steel 1 0.18 0.15 1.62 0.27 0.03 0.012 0.003 0.005 0.02 0.03 0.002 Invention river 2 0.17 0.24 1.48 0.10 0.03 0.011 0.003 0.005 0.04 0.02 0.004 Invention steel 3 0.14 0.15 1.80 0.20 0.025 0.006 0.004 0.003 0.03 0.025 0.001 Comparative River 1 0.08 0.18 1.4 0.35 0.035 0.015 0.003 0.004 0.015 0.01 0.001 Comparative River 2 0.14 0.25 0.85 0.25 0.05 0.02 0.006 0.005 0.028 0.03 0.001 Comparative Steel 3 0.13 0.10 2.5 0.50 0.03 0.01 0.004 0.004 0.015 0.03 0.001 Comparative Steel 4 0.13 0.10 1.97 1.20 0.035 0.01 0.003 0.005 0.01 0.04 0.001 Comparative Steel 5 0.18 0.28 1.88 0.05 0.027 0.009 0.003 0.004 0.045 0.04 0.05 Comparative Steel 6 0.30 0.18 1.25 0.65 0.045 0.01 0.003 0.005 0.02 0.03 0.003 Inventive Steel 4 0.14 0.18 1.65 0.30 0.025 0.01 0.002 0.004 0.035 0.025 0.001 Invention steel 5 0.18 0.30 1.5 0.55 0.027 0.008 0.003 0.003 0.025 0.035 0.002 Invention steel 6 0.21 0.15 1.7 0.25 0.025 0.006 0.003 0.005 0.04 0.025 0.001 Invention steel 7 0.11 0.16 1.3 0.40 0.025 0.01 0.003 0.004 0.02 0.01 0.003 Inventive Steel 8 0.13 0.20 1.8 0.75 0.028 0.006 0.003 0.003 0.045 0.05 0.003 Invention river 9 0.16 0.20 2.0 0.10 0.032 0.01 0.002 0.004 0.03 0.05 0.004

Steel grade Manufacturing conditions Mechanical division FDT
(° C) CT
(° C) Strain
(%) YS
(MPa) TS
(MPa) T-EL
(%) R / t TS / (R / t) BH
(MPa) 0.08 x YS Inventive Steel 1 872 379 0 932 1132 11.5 3.3 343 64 75 Comparative Example 1 Invention river 2 876 419 0 760 986 12.4 2.7 365 52 61 Comparative Example 2 Invention steel 3 893 422 0 781 977 14.1 2.8 349 48 62 Comparative Example 3 Comparative River 1 884 446 0 588 722 20.0 1.0 722 38 47 Comparative Example 4 Comparative River 2 888 438 0 612 684 23.5 1.0 684 22 49 Comparative Example 5 Comparative Steel 3 878 383 0.5 1045 1289 8.6 5.5 234 85 84 Comparative Example 6 Comparative Steel 4 896 403 0.5 1010 1245 9.0 4.9 254 82 81 Comparative Example 7 Comparative Steel 5 887 378 2.0 1012 1155 10.6 4.0 289 78 81 Comparative Example 8 Comparative Steel 6 898 369 2.0 1170 1420 5.4 7.1 200 102 94 Comparative Example 9 Inventive Steel 4 897 356 0.4 745 986 13.2 2.8 352 66 60 Inventory 1 Invention steel 5 904 413 0.5 966 1159 11.0 3.0 386 79 77 Inventory 2 Invention steel 6 890 357 1.0 1040 1244 9.5 3.5 355 86 83 Inventory 3 Invention steel 7 885 398 2.0 652 835 18.0 1.0 835 62 52 Honorable 4 Inventive Steel 8 899 379 3.0 998 1154 10.5 3.2 361 87 80 Inventory 5 Invention river 9 879 349 4.0 905 1071 14.2 2.6 412 78 72 Inventory 6

division Microstructure fraction (%) Bay knight Martensite ferrite total Comparative Example 1 49 20 26 95 Comparative Example 2 45 13 39 97 Comparative Example 3 48 12 36 96 Comparative Example 4 28 4 65 97 Comparative Example 5 12 0 85 97 Comparative Example 6 38 51 9 98 Comparative Example 7 35 53 8 96 Comparative Example 8 55 20 20 95 Comparative Example 9 30 56 9 95 Inventory 1 50 14 35 99 Inventory 2 56 22 18 96 Inventory 3 55 28 14 97 Honorable 4 28 10 59 97 Inventory 5 38 46 12 96 Inventory 6 40 34 23 97

(The remainder excluding the total sum in Table 3 includes other carbides and nitrides.)

As shown in Tables 1 to 3, Inventive Examples 1 to 6, which satisfy both the composition and the manufacturing conditions of the present invention, formed a ferrite-bainite-martensite composite structure having a proper fraction and exhibited a tensile strength of 780 MPa or more , It can be confirmed that a hot rolled steel sheet having a bending workability of TS / (R / t) of 330 or more and a bending hardenability (BH) of higher than 0.08 x YS and a hardness of 60 MPa or more can be obtained .

On the other hand, in Comparative Examples 1 to 3, it can be confirmed that although the steel having the steel composition satisfies the present invention, the sintering ability of the sintering is improved as the specific deformation is not performed after air cooling.

Also, in Comparative Examples 4 and 5, not only the super-curing ability was deviated due to no specific deformation after air cooling, but the content of C was inadequate in Comparative Example 4, and the fraction of the ferrite phase became excessive, %, The strength was lowered. In the case of Comparative Example 5, the content of ferrite phase became excessive due to insufficient Mn content, and the martensite phase was not formed at all.

In Comparative Examples 6, 7 and 9, it was confirmed that the Mn, Cr and C contents were excessively added, respectively, and that the ferrite phase was not sufficiently formed and that a relatively large amount of martensite phase was formed, have.

However, in this case, when the deformation was applied, the sintering property was found to be at an appropriate level due to the deformation.

In Comparative Example 8, when the precipitate-forming elements were excessively added, the yield strength was greatly increased by precipitation hardening, and the value of TS / (R / t) was extremely low as the bending workability was favorable.

However, in this case, when the deformation was applied, the sintering property was found to be at an appropriate level due to the deformation.

1 shows the tensile strength (TS) and the bending workability (R / t) ratio (Ts / (R / t)) and the hardening hardening properties (BH, MPa ). It can be seen that only the inventive examples satisfying the present invention can secure excellent bending workability and bake hardenability at the same time.

That is, as shown in FIG. 1, the values of the BH values of all the steels in the inventive and comparative examples, the ratio of the tensile strength (TS) and the bending workability (R / t) (Ts / (R / t) The effect of the present invention can be confirmed.

Claims (8)

(Al): 0.01 to 0.1%, chromium (Cr): 0.05%, and the like, in terms of% by weight, carbon (C): 0.1 to 0.25%, silicon (Si): 0.05 to 0.3%, manganese 0.001 to 0.05% of phosphorus (S), 0.001 to 0.01% of nitrogen (N), the balance Fe and other unavoidable impurities,
Wherein the microstructure comprises 10% or more of each of bainite, martensite and ferrite in an area fraction, the sum of the microstructures is 95%
Wherein the bending workability and the bending workability in which the ratio TS / R / t of the tensile strength TS to the R / t is 330 or more and the bake hardenability BH is 0.08 x YS (yield strength, MPa) High strength hot-rolled steel with excellent hardenability.
The method according to claim 1,
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet further contains 0.001 to 0.1 wt% of at least one selected from the group consisting of niobium (Nb), titanium (Ti) and vanadium (V).
delete The method according to claim 1,
The hot-rolled steel sheet has a tensile strength (TS) of 780 MPa or more and excellent bending workability and bake hardenability.
(Al): 0.01 to 0.1%, chromium (Cr): 0.05%, and the like, in terms of% by weight, carbon (C): 0.1 to 0.25%, silicon (Si): 0.05 to 0.3%, manganese Preparing a steel slab containing 0.001 to 1.0% phosphorus, 0.005 to 0.05% sulfur, 0.001 to 0.05% sulfur, 0.001 to 0.01% nitrogen, and the balance Fe and other unavoidable impurities;
Reheating the steel slab at 1150 to 1350 ° C;
Subjecting the reheated steel slab to hot finish rolling at 850 to 1150 占 폚 and terminating the rolling at the austenite zone;
After the rolling, primary cooling at an average cooling rate of from 10 to 200 DEG C / sec from 550 to 750 DEG C;
Cooling in the range of 550 to 750 ° C after the primary cooling;
Applying a strain of 0.2 to 5.0% in the range of 550 to 750 占 폚 after the air cooling;
After the deformation, secondary cooling at an average cooling rate of 10 to 200 ° C / sec from 300 to 500 ° C; And
After the secondary cooling, a step of winding at 300 to 500 ° C
Which is excellent in bending workability and bake hardenability.
6. The method of claim 5,
Wherein said steel slab comprises 0.001 to 0.1% by weight of at least one selected from the group consisting of niobium (Nb), titanium (Ti), and vanadium (V) Way.
6. The method of claim 5,
Wherein the air cooling is performed for 4 to 10 seconds, and the bending workability and the bake hardenability are excellent.
6. The method of claim 5,
Pickling the hot-rolled steel sheet; And
Further comprising the step of reheating the hot-rolled steel sheet subjected to the pickling treatment to a temperature in the range of 450 to 480 캜, followed by hot-dip galvanizing, and a method of producing a high-strength hot-rolled steel sheet excellent in bending workability and bake hardenability.

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