KR101428375B1 - Ultra high strength cold rolled steel sheet, galvanized steel sheet having excellent surface property and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to an ultra-high strength cold-rolled steel sheet having excellent surface quality, a hot-dip galvanized steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet having improved surface quality, A cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a manufacturing method thereof.

Description

TECHNICAL FIELD [0001] The present invention relates to an ultra high strength cold rolled steel sheet having excellent surface quality, a hot dip galvanized steel sheet having excellent surface quality,

The present invention relates to an ultra-high strength cold-rolled steel sheet having excellent surface quality, a hot-dip galvanized steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet having improved surface quality, A cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a manufacturing method thereof.

Recently, steel plates for automobiles are increasing the adoption of ultra-high strength steels in order to regulate fuel consumption to preserve the global environment and to ensure crash stability of passengers. In order to manufacture such a high-strength steel, it is not easy to secure sufficient strength and ductility by using a steel material using general solidification reinforcement or a steel material using precipitation hardening.

So, what is developed is a metamorphic reinforcing steel that utilizes a metamorphic organization. Such a transformer-reinforced steel may include a dual phase steel (hereinafter also referred to as DP steel), a composite phase steel (hereinafter also referred to as CP steel), a Transformation Induced Plasticity Steel ).

DP steel is a type of steel which distributes finely homogeneous hard martensite in soft ferrite to ensure high strength and ductility. CP steel contains ferrite, martensite, bainite two-phase or three-phase, and is a steel grade containing precipitation hardening elements such as Ti and Nb for strength improvement. The TRIP steel is a steel grade that maintains strength and ductility by causing martensite transformation by processing finely homogeneously dispersed retained austenite at room temperature.

In general, when the steel is solidified, the solidification behavior varies depending on the content of C, and particularly when the C content is in the range of 0.08 to 0.17%, the solidification shrinkage due to the solidification reaction and the phase transformation from the delta ferrite to the austenite It is difficult to form a uniform solidification layer. When such uneven solidification proceeds, a deep oscillation mark tends to be generated on the surface of the cast steel, and such a deep oscillation mark causes a horizontal crack at the short side of the cast steel by the friction between the cast steel and the stress applied to the cast steel.

In addition, in the case of precipitation elements such as Al contained in steel and Nb added to secure strength, precipitation is likely to occur along the grain boundaries at the time of solidification, thereby deteriorating the high temperature ductility of the material. Particularly, in the region of 600 to 900 ° C., the embrittlement region due to such precipitates appears. This is a temperature of the bridge region of the casting casting, and there is a high possibility that a crack occurs in the corner portion of the casting due to low high temperature ductility.

The above-mentioned metamorphic structure phase used for producing ultra-high strength steels is contained in such heterogeneous solidification range due to a large amount of carbon and alloying elements, and contains precipitation strengthening elements such as Nb for increasing the strength, And thus, the surface quality of the cast steel is largely inferior to that of ordinary steel. Such surface quality problems of the cast steel may lead to quality problems in hot rolling or cold rolling in the production of ultra high strength steels, or in the worst case, post production may not be possible.

A representative prior art for a method of manufacturing an ultra high strength steel is disclosed in Japanese Unexamined Patent Application Publication No. 2008-304626, which discloses a method of manufacturing a DP steel having a tensile strength of 1100 MPa or more by controlling chemical components, 2008-068058 and 2007-235092 disclose a method for producing an ultra-high strength steel sheet having a tensile strength of 980 MPa or more using residual austenite. Further, Japanese Patent Application Laid-Open Nos. 2003-092208 and 2004-087296 disclose a method for producing TRIP steel having a tensile strength of 1180 MPa or more using residual austenite. However, most of the above-mentioned inventions have been proposed in order to obtain high strength and elongation, and the surface quality of the cast steel, which has an important influence on the productivity in commercial production, has not been examined.

An aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet, a hot-dip galvanized steel sheet and a method of manufacturing the same, which can improve the quality of the short side and corner of the cast steel by controlling the component system and the manufacturing conditions, .

The present invention relates to a ferritic stainless steel comprising 0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.01 to 0.10% 0.001 to 0.0030% of B, 0.01 to 0.1% of Ti, and 0.001 to 0.01% of N, and further contains 0.02 to 0.05% of Nb, 0.01 to 0.2% of Mo, 0.01 to 0.2% of V, % And W: 0.01 to 0.2%, and the balance of Fe and other unavoidable impurities, wherein Ti and N satisfy the relationships of 3.4 ≤ Ti / N ≤ 10, Nb, Mo, V and W is 0.02? Nb + 0.2 (Mo + V + W)? 0.05, C, Mn, Si and Cr are 0.17? C + 0.023Mn + 0.062Si-0.009Cr? 0.4 and Nb, C, A high-strength cold-rolled steel sheet satisfying the relationship of 0.2? 124 (Nb x C) +2900 (Al x N)? 1 and having a tensile strength of 1 GPa or more and excellent surface quality.

The microstructure of the cold-rolled steel sheet is preferably composed of 40 to 70% of bainite and residual ferrite and martensite in an area fraction.

The present invention relates to a ferritic stainless steel comprising 0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.01 to 0.10% 0.001 to 0.0030% of B, 0.01 to 0.1% of Ti, and 0.001 to 0.01% of N, and further contains 0.02 to 0.05% of Nb, 0.01 to 0.2% of Mo, 0.01 to 0.2% of V, % And W: 0.01 to 0.2%, and the balance of Fe and other unavoidable impurities, wherein Ti and N satisfy the relationships of 3.4 ≤ Ti / N ≤ 10, Nb, Mo, V and W is 0.02? Nb + 0.2 (Mo + V + W)? 0.05, C, Mn, Si and Cr are 0.17? C + 0.023Mn + 0.062Si-0.009Cr? 0.4 and Nb, C, 0.2 ≦ 124 (Nb × C) +2900 (Al × N) ≦ 1, and having excellent tensile strength of 1 GPa or more and excellent surface quality.

The microstructure of the hot-dip galvanized steel sheet is preferably composed of 40 to 70% bainite and residual ferrite and martensite in an area fraction.

The present invention relates to a ferritic stainless steel comprising 0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.01 to 0.10% 0.001 to 0.0030% of B, 0.01 to 0.1% of Ti, and 0.001 to 0.01% of N, and further contains 0.02 to 0.05% of Nb, 0.01 to 0.2% of Mo, 0.01 to 0.2% of V, % And W: 0.01 to 0.2%, and the balance of Fe and other unavoidable impurities, wherein Ti and N satisfy the relationships of 3.4 ≤ Ti / N ≤ 10, Nb, Mo, V and W is 0.02? Nb + 0.2 (Mo + V + W)? 0.05, C, Mn, Si and Cr are 0.17? C + 0.023Mn + 0.062Si-0.009Cr? 0.4 and Nb, C, A rolling step of subjecting a steel material satisfying the relationship of 0.2? 124 (Nb 占 폚) + 2900 (Al 占 N)? 1 to hot rolling and cold rolling; Annealing the rolled steel material at 770 to 850 캜; A quenching step of rapidly cooling the annealed steel to a range of the martensitic transformation starting temperature (Ms) to the bainite transformation starting temperature (Bs); And a slow cooling step of slowly cooling the quenched steel material at a rate of 10 to 50 ° C / minute. The present invention also provides a method of manufacturing an ultra high strength cold rolled steel sheet having excellent surface quality.

It is preferable that the annealing step is performed under the condition that the hydrogen concentration is 5 to 50% and the remainder is nitrogen, and the quenching step is performed at a cooling rate of 100 to 600 ° C / min.

The present invention relates to a ferritic stainless steel comprising 0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.01 to 0.10% 0.001 to 0.0030% of B, 0.01 to 0.1% of Ti, and 0.001 to 0.01% of N, and further contains 0.02 to 0.05% of Nb, 0.01 to 0.2% of Mo, 0.01 to 0.2% of V, % And W: 0.01 to 0.2%, and the balance of Fe and other unavoidable impurities, wherein Ti and N satisfy the relationships of 3.4 ≤ Ti / N ≤ 10, Nb, Mo, V and W is 0.02? Nb + 0.2 (Mo + V + W)? 0.05, C, Mn, Si and Cr are 0.17? C + 0.023Mn + 0.062Si-0.009Cr? 0.4 and Nb, C, A rolling step of subjecting a steel material satisfying the relationship of 0.2? 124 (Nb 占 폚) + 2900 (Al 占 N)? 1 to hot rolling and cold rolling; Annealing the rolled steel material at 770 to 850 캜; A quenching step of rapidly cooling the annealed steel to a range of the martensitic transformation starting temperature (Ms) to the bainite transformation starting temperature (Bs); And a step of immersing the quenched steel material in a molten zinc plating bath at 480 to 520 ° C to perform hot dip galvanizing, thereby providing a method of manufacturing an ultra high strength hot dip galvanized steel sheet having excellent surface quality.

It is preferable that the annealing step is performed under the condition that the hydrogen concentration is 5 to 50% and the remainder is nitrogen, and the quenching step is performed at a cooling rate of 100 to 600 ° C / min.

According to an aspect of the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet and a hot-dip galvanized steel sheet having an excellent surface quality having an ultra high strength of 1 GPa or more,

Fig. 1 is a photograph showing a short side of Example 3; Fig.
Fig. 2 is a photograph showing a corner portion of Comparative Example 10 of the embodiment. Fig.

In general, high strength steels for automobiles use various alloying elements such as C, Mn, Si, Cr, Nb and B in steels to satisfy various characteristics such as strength, bending workability and weldability. According to such an alloy element, the high-strength steel is subject to non-uniform solidification during the casting process for producing the cast steel, resulting in deterioration of the quality of the cast steel due to cracks or deep oscillation marks on the short side of the cast steel. A crack is generated at the corner of the cast steel due to the forming element. Cracks present on the surface of such a slab lead to defects such as edge cracks and edge scabs in the subsequent process, making it impossible to produce ultra-high-strength steels or causing a decrease in the rate of the slump.

Accordingly, the inventors of the present invention conducted studies for producing a cold-rolled steel sheet and a hot-dip galvanized steel sheet having an ultra-high strength and excellent surface quality of cast steel, while appropriately controlling the component system and the manufacturing conditions, It is possible to produce a cold-rolled steel sheet and a hot-dip galvanized steel sheet with improved quality at the short sides and corners of the piece, and the present invention has been completed through related experiments.

C (carbon): 0.1 to 0.3 wt%

C is an important element for securing strength in metamorphic steel. The content of C is preferably from 0.1 to 0.3%. When the carbon content is less than 0.1%, it is difficult to secure a tensile strength of 1 GPa or more. When the content is more than 0.3%, the ductility, bending workability and weldability are decreased, There is a drawback that it is bad.

Si (silicon): 0.1 to 1.5 wt%

Si is an element for improving the strength and elongation of the steel, and the content is preferably 0.1 to 1.5%. When the Si content is less than 0.1, it is difficult to obtain the above effect. When the Si content is more than 1.5%, surface scale defects are caused in relation to the surface quality. In addition, an oxide which causes non- plating of the coated steel sheet is formed on the surface, And the like.

Mn (manganese): 2.0 to 3.0 wt%

Mn is an element having a very strong solid solution strengthening effect, and its content is preferably 2.0 to 3.0%. When the Mn content is less than 2.0%, it is difficult to secure the desired strength in the present invention. When the Mn content exceeds 3.0%, there is a high possibility that problems such as weldability and cold rolling load increase occur, It may cause surface defects of the coated steel sheet.

P (phosphorus): 0.001 to 0.10 wt%

P is an element showing the effect of strengthening the steel sheet. If the P content is less than 0.001%, the above effect can not be ensured and the production cost is raised. On the other hand, when the P content is excessively added, the press formability is deteriorated and steel brittleness may be generated. To 0.10%.

S (sulfur): not more than 0.010%

S is an impurity element in steel and is an element that hinders ductility and weldability of a steel sheet. If the content of S is more than 0.01%, it is highly likely to deteriorate the ductility and weldability of the steel sheet, so the content of S is preferably limited to 0.01% or less.

Al (aluminum): 0.01 to 0.1 wt%

Al is a component effective to combine with oxygen in steel to deacidify and to distribute carbon in ferrite to austenite like Si to improve the hardenability of martensite. If the content of Al is less than 0.01%, the above effect can not be secured. On the other hand, when the content of Al exceeds 0.1%, the quality of the slab surface is lowered and the production cost is increased, so that the content of Al is preferably 0.01 to 0.1%.

Cr (chrome): 0.3 to 1.0%

Cr is added to improve hardenability of steel and ensure high strength. In the present invention, the content of Cr is preferably 0.3 to 1.0% as an element which induces bainite formation through ferrite transformation delay. When the content of Cr is less than 0.3%, it is difficult to secure the above effect. When the Cr content exceeds 1.0%, the effect is saturated and the cold rolling load is increased and the manufacturing cost is greatly increased.

B (boron): 0.0010 to 0.0030 wt%

B is a component for retarding the transformation of austenite into pearlite in the process of cooling during annealing, and it is preferably 0.001 to 0.003% as an element for suppressing ferrite formation and promoting bainite formation. When the content of B is less than 0.001%, it is difficult to obtain the above effect. When the content of B exceeds 0.003%, the effect is saturated due to grain boundary segregation of B, and excessive formation of surface contaminants may lead to plating defects.

Ti (titanium): 0.01 to 0.1 wt%

Ti is an element added for increasing the strength of a steel sheet and for scavenging N present in steel. When the content of Ti is less than 0.01%, it is difficult to secure such effect. When the content exceeds 0.1%, continuous casting Which may cause process defects such as nozzle clogging.

N (nitrogen): 0.001 to 0.01 wt%

N is a solid solution strengthening element capable of raising the strength of a steel sheet, and is an element generally incorporated from the atmosphere. The content should be controlled by the steelmaking process degassing process. If the content of N is less than 0.001%, excessive degassing treatment is required, leading to an increase in the production cost. If the N content is more than 0.01%, the high temperature ductility is deteriorated due to excessive formation of precipitates such as AlN and TiN.

When the Ti / N ratio is less than 3.4, the Ti addition amount is insufficient compared to the amount of dissolved N, and NB and the like caused by the residual N The strength increase effect due to the addition of B may be lowered and the strength may be lowered. On the other hand, if it exceeds 10, the denitration treatment cost increases and the possibility of nozzle clogging or the like is increased in the performance process.

The cold-rolled steel sheet and hot-dip galvanized steel sheet of the present invention contain at least one selected from the group consisting of 0.02 to 0.05% of Nb, 0.01 to 0.2% of Mo, 0.01 to 0.2% of V and 0.01 to 0.2% of W desirable.

Nb (niobium): 0.02 to 0.05 wt%

Nb is an element added for increasing the strength of the steel sheet and refining the crystal grain, and its content is preferably 0.02 to 0.05%. When the content of Nb is less than 0.02%, it is difficult to exhibit the above effect. When the content of Nb is more than 0.05%, bending workability and ductility may be deteriorated due to an increase in manufacturing cost and excessive deposition.

Mo (molybdenum), V (vanadium), W (tungsten): 0.01 to 0.2 wt%

Mo, V and W are elements which act like Nb. When the Mo content is less than 0.01%, it is difficult to increase the strength and grain refinement effect. When the Mo content exceeds 0.2%, the manufacturing cost becomes excessively high in preparation for the strength effect.

It is preferable that the above Nb, Mo, V and W satisfy the relation of 0.02 Nb + 0.2 (Mo + V + W) 0.05. When the ratio is less than 0.02, it is difficult to expect grain refinement and precipitation strengthening effect. The manufacturing cost is excessively increased in preparation for the above effect.

The cold-rolled steel sheet and the hot-dip galvanized steel sheet proposed by the present invention preferably satisfy the relationship of 0.17? C + 0.023Mn + 0.062Si-0.009Cr? 0.4. The above relational expression is defined by experimentally deriving an appropriate range of components in which non-uniform solidification takes place. When the above condition is satisfied, the amount of coagulation shrinkage due to the entrapping reaction is reduced, and thus excellent quality of the short side portion of the cast steel can be secured. However, when the value of the above relation is less than 0.17, the quality of the short side of the cast steel becomes poor as a range where non-uniform solidification can occur during solidification. When the value exceeds 0.4, The probability of occurrence is increased.

As described above, by improving the surface quality of the cast steel, it is possible to manufacture the cold-rolled steel sheet by using the cast steel without throwing away or recycling the cast steel, thereby reducing manufacturing cost and productivity.

The cold-rolled steel sheet and the hot-dip galvanized steel sheet of the present invention preferably satisfy the relationship of 0.2? 124 (Nb x C) +2900 (Al x N)? 1. The above relational expression defines the behavior of high-temperature embrittlement due to the formation of NbC and AlN precipitates as respective components. When the above condition is satisfied, cracking of the corner portion of the slab can be suppressed by weakening the high-temperature embrittlement phenomenon of the material. On the other hand, when the value of the above formula is less than 0.2, the strengthening effect by the precipitate is insufficient and it becomes difficult to secure the strength of the final material to 1 GPa or more.

1, the brittleness of the material is severely generated in the vicinity of 600 to 900 DEG C, where the major part is corrected by precipitation of a large amount of NbC and AlN, so that the occurrence of cracks at the corner of the bill increases.

The microstructure of the cold-rolled steel sheet and hot-dip galvanized steel sheet of the present invention is preferably composed of 40 to 70% of bainite and residual ferrite and martensite in an area fraction. If the fraction of bainite is less than 40%, the bending workability is deteriorated, and when it exceeds 70%, it is difficult to realize a high tensile strength of 1 GPa or more. The fraction of the ferrite and martensite structure is not particularly limited. However, in order to secure excellent strength, ductility and bending workability, it is preferable that the ferrite structure and the martensite structure have a range of 10 to 40% by area and 15 to 30% Do.

The cold-rolled steel sheet and the hot-dip galvanized steel sheet of the present invention have a tensile strength of 1 GPa or more.

Hereinafter, the production method of the present invention will be described.

The steel material satisfying the composition components and ranges of the present invention is hot-rolled and cold-rolled under ordinary conditions.

When the annealing temperature is lower than 770 ° C, the ferrite structure fraction exceeds 40%, the strength is hardly secured and the bending workability is lowered. When the annealing temperature exceeds 850 ° C The bending workability is improved, but the amount of the surface grains such as Si, Mn, and B generated at the high temperature annealing is greatly increased, resulting in a large amount of unplated defects.

It is preferable that the annealing step is performed under an atmospheric condition that the hydrogen concentration is 5 to 50% and the balance is nitrogen. When the hydrogen concentration is less than 5%, the elements having large oxygen affinity such as Si, Mn, Surface agglomeration is easy to occur and causes dent and plating defects. On the other hand, if it exceeds 50%, the increase in the effect is insignificant compared with the manufacturing cost. Nitrogen used as a remainder material prevents formation of a surface agglomeration of a steel sheet and is low in manufacturing cost and can be suitably used as an atmospheric gas.

Thereafter, the annealed steel material is quenched to the cold / cold zone, that is, the cooling stop temperature range. Quot; means the range from the start temperature (Ms) to the bainite transformation start temperature (Bs) of the martensitic transformation starting temperature, and the cooling rate can ensure an appropriate range of microstructure fractions. At this time, it is preferable that the quenching is performed at a rate of 100 to 600 ° C / min. When the quenching rate is less than 100 ° C / min, the target strength in the present invention is secured due to the formation of ferrite and pearlite If the heating temperature is higher than 600 ° C / min, the elongation rate is lowered due to the formation of an excessive hard phase, and problems such as defective shape may occur.

In the cold-rolled steel sheet manufacturing method of the present invention, after the above-described steps, the quenched steel material is subjected to a slow cooling step of slowly cooling the steel material at a rate of 10 to 50 ° C / minute. If the slow cooling rate is less than 10 ° C / minute, it is difficult to secure a proper martensite fraction and it is difficult to obtain the desired strength in the present invention. When the cooling rate is more than 50 ° C / minute, There is a problem that the bending workability is deteriorated.

The method of manufacturing a hot-dip galvanized steel sheet according to the present invention includes a step of dipping a quenched steel material in a hot-dip galvanizing bath at 480 to 520 ° C. after the quenching step and performing hot-dip galvanizing. If the temperature of the plating bath is less than 480 DEG C, the formation of the alloying inhibition layer may be insufficient and the plating may be peeled off. If the temperature exceeds 520 DEG C, a problem arises that the generation of dross increases.

Hereinafter, the present invention will be described in detail with reference to Examples. The following examples are intended to further illustrate the present invention and do not limit the scope of the present invention.

(Example)

After the slabs having the compositional components shown in Tables 1 and 2 were prepared, the level of cracks observed on both sides and four corners of the 10-m cast steel was evaluated in four stages (very good, good, normal, and poor) The results are shown in Table 4 below. More detailed surface quality classification criteria of the cast steel are shown in Table 5 below. Then, the produced slab was subjected to hot rolling and cold rolling under ordinary conditions, followed by annealing and cooling treatment under the conditions shown in Table 3 below. At this time, the atmosphere in the annealing treatment was hydrogen and nitrogen.

division Chemical composition (% by weight) C Si Mn P S Al Cr B
(ppm) Inventory 1 0.12 0.1 2.7 0.01 0.003 0.02 1.0 20 Inventory 2 0.15 0.1 2.6 0.01 0.003 0.02 0.7 20 Inventory 3 0.17 0.1 2.5 0.01 0.003 0.02 0.5 20 Honorable 4 0.14 0.1 2.5 0.01 0.003 0.02 0.7 20 Inventory 5 0.1 0.5 2.8 0.01 0.003 0.02 0.5 20 Inventory 6 0.1 0.5 2.8 0.01 0.003 0.025 0.7 20 Honorable 7 0.12 0.5 3.0 0.01 0.002 0.02 0.5 20 Honors 8 0.2 0.1 2.3 0.01 0.002 0.03 0.7 20 Proposition 9 0.14 0.5 2.5 0.01 0.002 0.02 0.7 20 Comparative Example 1 0.13 0.5 2.5 0.01 0.03 0.03 1.0 0 Comparative Example 2 0.1 0.5 2.8 0.01 0.002 0.04 0.5 20 Comparative Example 3 0.1 0.1 2.7 0.01 0.003 0.02 1.0 20 Comparative Example 4 0.08 0.1 2.8 0.01 0.003 0.025 1.0 20 Comparative Example 5 0.11 0.1 2.6 0.01 0.003 0.02 1.0 20 Comparative Example 6 0.1 0.01 2.8 0.01 0.004 0.03 1.2 20 Comparative Example 7 0.15 0.1 2.7 0.01 0.003 0.04 0.7 20 Comparative Example 8 0.1 0.5 2.8 0.01 0.002 0.04 0.5 20 Comparative Example 9 0.2 0.1 2.3 0.01 0.002 0.03 0.7 20 Comparative Example 10 0.17 0.1 2.5 0.01 0.003 0.05 0.5 20 Comparative Example 11 0.15 0.1 2.7 0.01 0.003 0.03 0.5 20

division Chemical composition (% by weight) Ti N
(ppm) Nb Mo V W Ti / N Relationship 1 Relation 2 Inventory 1 0.02 40 0.05 - - - 5 0.179 0.98 Inventory 2 0.02 40 0.04 - - - 5 0.210 0.98 Inventory 3 0.02 50 0.03 - - - 4 0.229 0.92 Honorable 4 0.02 50 0.025 - 0.03 - 4 0.197 0.72 Inventory 5 0.025 50 0.04 - - - 5 0.195 0.79 Inventory 6 0.03 40 0.05 - - - 7.5 0.189 0.91 Honorable 7 0.03 50 0.04 - - - 6 0.216 0.89 Honors 8 0.02 40 0.02 0.03 - - 5 0.253 0.84 Proposition 9 0.02 40 - 0.05 0.03 0.03 5 0.222 0.23 Comparative Example 1 0.02 50 0.03 - - - 4 0.210 0.92 Comparative Example 2 0.01 50 0.03 - - - 2 0.191 0.95 Comparative Example 3 0.02 40 0.05 - - - 5 0.159 0.85 Comparative Example 4 0.02 40 0.05 - - - 5 0.142 0.79 Comparative Example 5 0.02 50 0.05 - - - 4 0.167 0.97 Comparative Example 6 0.02 40 0.05 - - - 5 0.154 0.92 Comparative Example 7 0.02 40 0.06 - - - 5 0.212 4.58 Comparative Example 8 0.02 60 0.03 - - - 3.33 0.191 1.07 Comparative Example 9 0.02 50 0.06 - - - 4 0.253 1.92 Comparative Example 10 0.02 100 0.05 - - - 2 0.229 1.87 Comparative Example 11 0.02 40 0.1 - - - 5 0.214 1.54 However, the relational expression 1 is C + 0.023Mn + 0.062Si-0.009Cr, and the relational expression 2 is 124 (Nb × C) +2900 (Al × N).

division Annealing temperature
(° C) Annealing atmosphere
(Hydrogen concentration,%) Quenching rate
(° C / minute) Rust
(° C) Slow cooling rate
(° C / minute) Inventory 1 840 40 300 460 20 Inventory 2 840 40 300 460 20 Inventory 3 840 40 300 460 20 Honorable 4 840 40 300 460 20 Inventory 5 820 30 270 460 30 Inventory 6 820 30 270 460 30 Honorable 7 820 40 270 460 30 Honors 8 800 40 240 440 20 Proposition 9 800 40 240 440 20 Comparative Example 1 840 30 300 460 20 Comparative Example 2 840 30 300 460 20 Comparative Example 3 820 30 270 460 30 Comparative Example 4 820 40 270 460 30 Comparative Example 5 820 25 270 460 30 Comparative Example 6 840 30 300 480 30 Comparative Example 7 840 30 330 430 30 Comparative Example 8 840 30 330 430 20 Comparative Example 9 800 40 240 480 20 Comparative Example 10 800 40 240 480 20 Comparative Example 11 800 40 240 480 20 However, the quenching zone means the quenching stop temperature range (martensitic transformation starting temperature (Ms) to bainite transformation starting temperature (Bs)).

The cold-rolled steel sheets produced under the above conditions were measured for microstructure and physical properties. The results are shown in Table 4 below.

division Slab
Short side
crack Slab
corner
crack B Organization
(area%) M Organization
(area%) F Organization
(area%) surrender
burglar
(MPa) Seal
burglar
(MPa) Elongation
(%) Inventory 1 ○ ○ 55 20 25 912 1255 6 Inventory 2 ◎ ○ 65 14 21 860 1195 8 Inventory 3 ◎ ○ 58 18 24 901 1230 7 Honorable 4 ○ ◎ 62 13 25 863 1186 8 Inventory 5 ○ ◎ 59 19 22 760 1203 9 Inventory 6 ○ ○ 60 21 19 885 1210 7 Honorable 7 ◎ ○ 52 25 23 785 1245 7 Honors 8 ◎ ○ 48 29 23 912 1270 7 Proposition 9 ◎ ◎ 61 20 19 862 1197 8 Comparative Example 1 ○ ○ 81 5 14 703 975 10 Comparative Example 2 ○ ○ 74 8 18 730 994 11 Comparative Example 3 × ○ 58 16 26 875 1203 7 Comparative Example 4 × ○ 63 9 28 840 1165 8 Comparative Example 5 △ ○ 55 13 32 877 1199 8 Comparative Example 6 × ○ 62 17 21 888 1205 7 Comparative Example 7 ○ × 60 22 18 905 1220 7 Comparative Example 8 ○ × 59 22 19 894 1210 7 Comparative Example 9 ○ × 48 29 23 945 1305 6 Comparative Example 10 ◎ × 52 27 21 921 1276 6 Comparative Example 11 ○ × 50 24 26 907 1233 7

Quality level Short side crack (based on 10m of both sides of cast steel) Corner Crack (Based on 10m corner of cast steel) Very good (◎) No cracks No cracks Good (O) 2 or less cracks of 5 mm or less 2 or less cracks of 2 mm or less Usually (△) Intermediate level of good and bad Intermediate level of good and bad Bad (×) More than 5 cracks of 10mm or more 5 or more cracks of 5 mm or more

As can be seen from Table 4, Examples 1 to 9 in accordance with the composition system and the production conditions of the present invention show that the surface quality in the cast state shows excellent results in both the short sides and the corner portions, and the tensile strength is also 1 GPa or more It has high strength.

On the other hand, in the case of Comparative Examples 1 and 2, the surface quality of the slab was satisfactory, but it was not satisfied with the B content range and the Ti / N range defined in the present invention, (C + 0.023Mn + 0.062Si-0.009Cr) was not satisfied in the case of the casting surface of the cast steel, the cracks occurred at the short side of the cast steel due to non-uniform solidification during casting. FIG. 1 shows a photograph of a short side crack of Comparative Example 3, and it can be seen that cracks were considerably generated in the short side of the strip as shown in FIG.

Also in the case of Comparative Examples 7 to 11, the relation (124 (Nb x C) +2900 (Al x N)) concerning the surface quality of the cast steel was not satisfied, and corner cracks of the cast steel due to deterioration of high temperature ductility of the material occurred. Fig. 2 is a photograph showing a corner crack in Comparative Example 10, and a considerable amount of cracks was observed in the corner portion of the casting of Comparative Example 10. Fig.

Claims (5)

0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.010% or less of S, 0.01 to 0.1% of Al, 0.3 to 1.0 0.001 to 0.0030% of B, 0.01 to 0.1% of Ti, 0.001 to 0.01% of N and 0.02 to 0.05% of Nb, 0.01 to 0.2% of Mo, 0.01 to 0.2% of V, 0.01 to 0.2% of V, : 0.01 to 0.2%, and the balance of Fe and other unavoidable impurities,
Ti and N satisfy the relationship of 3.4 < = Ti / N < = 10,
Wherein Nb, Mo, V and W satisfy the following relationship: 0.02? Nb + 0.2 (Mo + V + W)? 0.05,
C, Mn, Si and Cr satisfy the following equations: 0.17? C + 0.023Mn + 0.062Si-0.009Cr? 0.4
Wherein the Nb, C, Al, and N satisfy a relationship of 0.2? 124 (NbxC) +2900 (AlxN)? 1; hot rolling and cold rolling a steel material;
Annealing the rolled steel material at 770 to 850 캜;
A quenching step of rapidly cooling the annealed steel to a range of the martensitic transformation starting temperature (Ms) to the bainite transformation starting temperature (Bs); And
And a slow cooling step of slowly cooling the quenched steel material at a rate of 10 to 50 ° C / min. The method of manufacturing an ultra-high strength cold-rolled steel sheet having excellent surface quality.
The method of manufacturing an ultra-high strength cold rolled steel sheet according to claim 1, wherein the annealing step is performed at a hydrogen concentration of 5 to 50% and the remainder is nitrogen.
The method of manufacturing an ultra-high strength cold rolled steel sheet according to claim 1, wherein the quenching step is performed at a cooling rate of 100 to 600 ° C / min.
0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.010% or less of S, 0.01 to 0.1% of Al, 0.3 to 1.0 0.001 to 0.0030% of B, 0.01 to 0.1% of Ti, 0.001 to 0.01% of N and 0.02 to 0.05% of Nb, 0.01 to 0.2% of Mo, 0.01 to 0.2% of V, 0.01 to 0.2% of V, : 0.01 to 0.2%, and the balance of Fe and other unavoidable impurities,
Ti and N satisfy the relationship of 3.4 < = Ti / N < = 10,
Wherein Nb, Mo, V and W satisfy the following relationship: 0.02? Nb + 0.2 (Mo + V + W)? 0.05,
C, Mn, Si and Cr satisfy the following equations: 0.17? C + 0.023Mn + 0.062Si-0.009Cr? 0.4
Wherein the Nb, C, Al, and N satisfy a relationship of 0.2? 124 (NbxC) +2900 (AlxN)? 1; hot rolling and cold rolling a steel material;
Annealing the rolled steel material at 770 to 850 캜;
A quenching step of rapidly cooling the annealed steel to a range of the martensitic transformation starting temperature (Ms) to the bainite transformation starting temperature (Bs); And
And dipping the quenched steel material in a molten zinc plating bath at 480 to 520 ° C. and performing hot dip galvanizing, wherein the annealing step is performed under a condition that the hydrogen concentration is 5 to 50% and the remainder is nitrogen Wherein the hot dip galvanized steel sheet has an excellent surface quality.
5. The method of manufacturing an ultra-high strength hot-dip galvanized steel sheet according to claim 4, wherein the quenching step is performed at a cooling rate of 100 to 600 ° C / min.

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