KR101193756B1 - Cold-rolled steel sheet having excellent surface quality, and method for producing the same - Google Patents

Abstract

The present invention relates to a high strength high strength steel sheet having excellent surface properties and a manufacturing method thereof. In the present invention, C: 0.002-0.01 wt%, Si: 0.05-0.25 wt%, Mn: greater than 0 wt% or less, P: greater than 0 wt% or less, S: greater than 0 wt% or less, 0.01 wt% or less, Al: 0.01 to less than or equal to 0.06wt%, Ti: 0.02 ~ 0.07wt%, Nb: 0.01 ~ 0.05wt%, Sb: 0.01 ~ 0.2wt%, Mo: 0.01 ~ 0.2wt%, Cu: 0.05 ~ 0.25wt%, Sn: 0.008 ~ 0.025wt %, N: 0.002-0.01wt% and the rest is composed of Fe and inevitable impurities, the content of Si, Mn, Sb, Mo satisfies the formula 2≤ (Si + Mn) / (Sb + Mo) ≤22 .

The present invention satisfies the tensile strength of 440MPa or more, elongation of 40% or more and r value of 1.5 or more, and the plating surface grade also satisfies the second or more grades. Therefore, there is an advantage that can be widely used in automotive steel sheets that require excellent surface properties.

Cold rolled steel, surface quality

Description

High-strength high-form steel sheet with excellent surface properties and its manufacturing method {Cold-rolled steel sheet having excellent surface quality, and method for producing the same}

The present invention relates to a high strength high strength steel sheet, and more particularly, to a high strength high strength steel sheet having excellent surface properties having a tensile strength of 440 MPa or more and excellent elongation and excellent plating surface quality.

Recently, the research interests of the steel industry and the automobile industry are focused on environmental pollution, high strength and light weight, and as the automobile design is complicated and the needs of consumers are diversified, demands for high strength, processability and formability are required.

In particular, in the case of cold rolled steel used as exterior materials of automobiles, shape maintenance, safety, and energy saving are important through the improvement of strength according to the trend of the times, and sufficient moldability as the customer's demand for design is diversified and complicated There is a demand for high quality steel sheets with basic surface quality.

Steel sheets requiring formability require excellent deep drawing, and automotive materials or general structural materials are required to secure deep drawing since they are judged to be defective due to high deep drawing.

In order to secure deep drawing property, it is necessary to secure excellent r value basically. .

Therefore, the object of the present invention is to ensure high elongation and r-value by developing the texture in the ultra low carbon steel, and to control the formation of the scale to suppress the formation of scale, high surface strength, excellent surface properties to improve the surface quality It is to provide a steel sheet and a method of manufacturing the same.

According to a feature of the present invention for achieving the above object, the present invention is C: 0.002 ~ 0.01wt%, Si: 0.05 ~ 0.25wt%, Mn: more than 0wt 1.0wt%, P: more than 0wt 0.05wt % Or less, S: more than 0 and 0.01 wt% or less, Al: 0.01 to 0.06 wt%, Ti: 0.02 to 0.07 wt%, Nb: 0.01 to 0.05 wt%, Sb: 0.01 to 0.2 wt%, Mo: 0.01 to 0.2 wt %, Cu: 0.05 ~ 0.25wt%, Sn: 0.008 ~ 0.025wt%, N: 0.002 ~ 0.01wt% and the rest are composed of Fe and inevitable impurities, tensile strength of 440MPa or more, elongation of 40% or more, r value This is over 1.5.

The content of Si, Mn, Sb, and Mo satisfies Equation 2 ≦ (Si + Mn) / (Sb + Mo) ≦ 22.

C: 0.002 to 0.01 wt%, Si: 0.05 to 0.25 wt%, Mn: greater than 0 wt% and less than 1.0, P: greater than 0 wt% and 0.05 wt% or less, S: greater than 0 wt% and 0.01 wt% or less, Al: 0.01 to 0.06 wt% , Ti: 0.02 ~ 0.07wt%, Nb: 0.01 ~ 0.05wt%, Sb: 0.01 ~ 0.2wt%, Mo: 0.01 ~ 0.2wt%, Cu: 0.05 ~ 0.25wt%, Sn: 0.008 ~ 0.025wt%, N : 0.002 ~ 0.01wt% and the remaining slab composed of Fe and unavoidable impurities are reheated to 1100 ~ 1200 ℃, followed by finishing hot rolling at 880 ~ 930 ℃ and wound at 500 ~ 700 ℃ to manufacture hot rolled steel sheet. Making; Pickling the hot rolled steel sheet and cold rolling at a reduction ratio of 30 to 80% to produce a cold rolled steel sheet; And annealing and hot dip galvanizing the cold rolled steel sheet to develop a (111) aggregate structure.

The annealing heat treatment is carried out by heating the cold-rolled steel sheet to 800 ~ 860 ℃ 30 to 100 seconds, and then cooled to 400 ~ 500 ℃ at a cooling rate of 10 ~ 20 ℃ / sec and maintained for 180 to 300 seconds.

The content of Si, Mn, Sb, and Mo satisfies Equation 2 ≦ (Si + Mn) / (Sb + Mo) ≦ 22.

The present invention is an ultra-low carbon steel containing less than 100ppm of C and N to increase the Si content and lower the Mn content, to suppress the formation of scale in the addition of Sb, Mo and to form an even coating layer to improve the surface properties of the steel sheet Let's do it.

In the present invention, even if an impurity element is inevitably included in the mini-mill process, sufficient r-values are secured by adding Ti and Nb to sufficiently remove the solid-solution elements C and N present in the steel and to generate the texture. do.

Therefore, in addition to the tensile strength of 440MPa or more, elongation of 40% or more, and r value of 1.5 or more, it is possible to produce a steel sheet having excellent surface properties satisfying all of the plating surface grades. Therefore, there is an effect that can be widely used in the outer plate material of the automobile.

Hereinafter, the preferred embodiment of the high strength high strength steel sheet excellent in surface characteristics according to the present invention and a method of manufacturing the same will be described in detail.

In the present invention, C: 0.002-0.01 wt%, Si: 0.05-0.25 wt%, Mn: greater than 0 wt% or less, P: greater than 0 wt% or less, S: greater than 0 wt% or less, 0.01 wt% or less, Al: 0.01 to less than or equal to 0.06wt%, Ti: 0.02 ~ 0.07wt%, Nb: 0.01 ~ 0.05wt%, Sb: 0.01 ~ 0.2wt%, Mo: 0.01 ~ 0.2wt%, Cu: 0.05 ~ 0.25wt%, Sn: 0.008 ~ 0.025wt %, N: 0.002-0.01 wt% and the rest are composed of Fe and inevitable impurities.

In the manufacturing method, the slab having the alloy composition described above is homogenized at a temperature of Ac3 or higher, and then hot-rolled at an Ar3 or higher temperature and wound in a steel sheet coil at 500 to 700 ° C.

The hot rolled steel sheet coil is pickled and cold rolled at a reduction ratio of 30 to 80%. After the cold rolled steel sheet is subjected to annealing, hot dip galvanizing and alloying heat treatment.

The present invention increases the Si content to suppress surface scale formation. Increasing the Si content, the FeSiO ₂ layer is preferentially formed on the surface of the hot-rolled steel sheet to suppress scale formation.

Increasing the Si content increases the strength, so the content of Mn and P having a strength improving effect is minimized.

In addition, Sb and Mo are added to form an even plating layer.

In addition, in order to ensure the excellent moldability in the mini-mill process containing an impurity element, r value is made 1.5 or more. In order to secure the r value, the content of impurity elements, especially Sn, is regulated to 0.025wt% or less, and Ti and Nb are added to sufficiently remove the solid solutions C and N present in the steel. Sufficient removal of elements C and N ensures a sufficient value of r.

Hereinafter, the reason for limitation of the function and content of the alloying elements of the present invention will be described.

C: 0.002 ~ 0.01wt%

C is the most effective element to improve the strength of steel. However, C exists as a solid solution element in the steel and inhibits the formation of (111) aggregate structure, which is advantageous for processability, during the formation of the aggregate structure of the steel sheet during cold rolling and annealing, thereby degrading the workability and formability.

If C is less than 0.002wt%, it is difficult to secure the target strength and grain boundary embrittlement may occur. On the contrary, when C exceeds 0.01 wt%, C exists as a solid element in the steel sheet, thereby degrading workability and formability. In addition, the C present in the steel causes aging problems, causing the strainer strain (Strecher Strain) problem, there is a risk of cracking when playing the slab. In this case, the addition amount of the carbide forming elements Ti and Nb should be increased. However, when the content of Ti and Nb is increased, not only the cost of steel is increased but also the addition of a large amount of Ti and Nb reduces the material and surface properties of the steel sheet.

Therefore, C limits the content to 0.002 to 0.01 wt%.

More precisely, ultra low carbon steels containing less than 100 ppm of C and N are applied to improve workability.

Si: 0.05 ~ 0.25wt%

Si is present as a solid element in the steel to increase the strength of the steel sheet. In the present invention, Si is added for the purpose of suppressing scale formation on the surface of the steel sheet. Si suppresses scale formation by forming a thin 2FeSiO ₂ on the surface of the steel sheet to prevent contact between the steel sheet and oxygen.

Since Si does not form 2FeSiO ₂ below 0.05 wt%, there is no effect of inhibiting scale formation. And, the higher the content of Si, the more effective in increasing the strength. However, if the content exceeds 0.25wt%, the plating property of the post process is lowered.

Therefore, the content of Si is set in the range 0.05 ~ 0.25 wt%.

Mn: greater than 0 and less than 1.0wt%

Mn is a component added to prevent the occurrence of cracks by forming MnS together with securing strength to a certain extent.

However, in the present invention, the Si content and the impurity elements of the mini-mill process contribute to the increase in strength, and the Mn content is minimized because the workability should be improved rather than the strength increase effect. In addition, since Mn exists in a grain boundary and causes plating property to fall, it is preferable to add Mn minimum.

An upper limit is set to 1.0 wt% or less. This is because when the content of Mn exceeds 1.0 wt%, workability and moldability are lowered due to grain boundary segregation.

P: greater than 0 and less than 0.05wt%

P is a component having excellent solid solution strengthening effect, and is added a lot for the purpose of strength increase, and the effect by small amount addition is considerable.

The reason for setting the upper limit is that in the case of the present invention, the purpose is to improve the workability rather than the effect of increasing the strength. Induction of secondary processing brittleness can be suppressed by the addition of trace amounts of B.

Therefore, the content of P is limited to greater than 0 and less than 0.05wt%.

S: greater than 0 and less than 0.01wt%

S is an element that is inevitably contained in the production of steel, inhibits the toughness and weldability of the steel, forms an emulsion-based (MnS, etc.) inclusions, forms FeS to cause the occurrence of edge cracks. In particular, S is limited to the range of 0.01wt% or less because it increases the coarse inclusions in the case of over addition, deteriorates the fatigue characteristics and lowers the low temperature impact toughness of the steel.

Al: 0.01 ~ 0.06wt%

Al is a component that serves as a deoxidizer and keeps the dissolved oxygen content in the steel sufficiently low. As the role of the deoxidizer, Al may cause problems in performance when added above the upper limit. In addition, Al reacts with solid solution N to form AlN precipitates to remove solid solution elements. In addition, when applying the continuous annealing method to maintain a high coiling temperature to precipitate AlN in advance in the hot rolling step.

If Al is less than 0.01wt%, the effect is insignificant, and if it exceeds 0.06wt%, problems may occur when playing as described above.

Therefore, the content of Al is set in the range of 0.01 ~ 0.06wt%.

Ti: 0.02 ~ 0.07wt%

As the amount of Ti added, the value of r increases and exhibits non-aging characteristics.

Ti improves the value of r by removing C and N present as solid elements in the steel in the form of precipitates such as TiN and TiC to remove solid elements in the steel.

Therefore, the content of Ti should be taken into consideration so as to sufficiently remove the solid solution element in the steel.

The lower limit of Ti is set to an amount capable of stoichiometric precipitation of solid solution. The stoichiometric amount of solid solution can be calculated as 4C + 3.4N + 1.5S. 4C + 3.4N + 1.5S is an equation calculated by a plurality of experiments. Ti combines with N or S prior to Al or Nb at high temperature to remove solid solution N and prevents brittleness by S.

Therefore, the r value increases as the Ti content increases. However, if the Ti content increases, the cost rises by that amount, so it is regulated as an upper limit.

Nb: 0.01 ~ 0.05wt%

Nb, like Ti, improves the value of r by removing C and N, which are solid solutions in the steel, in the form of precipitates of NbC and NbN. This removes C in the steel and exhibits no aging characteristics.

Nb lowers the value of r when added above an amount capable of stoichiometric precipitation of solid solution elements. In addition, the cost rises, thereby increasing the manufacturing cost. However, when Nb is added, it shows excellent plating characteristics.

The amount of Nb that can yield stoichiometrically the employment element is calculated as 0.5C * 93/12. 0.5C * 93/12 is an equation calculated by a plurality of experiments. Nb is removed by Ti to precipitate C in the remaining steel in the form of NbC during hot rolling.

Nb does not have a removal effect of solid solution C at less than 0.01 wt%, and if it exceeds 0.05 wt%, the r value is lowered.

Sb: 0.01 ~ 0.2wt%

Sb does not form an oxide film itself at high temperatures, but is concentrated at the surface and grain interfaces to control oxide formation by suppressing diffusion of Mn and Si in the steel to the surface.

Such Sb improves plating properties by suppressing oxide generation during the annealing process and suppresses dent defects on the surface of the plating material.

Sb is difficult to exert its effect at less than 0.01wt%, and if it exceeds 0.2wt%, the ductility is lowered and the material properties deteriorate.

Therefore, the content of Sb is set in the range of 0.01 ~ 0.2wt%.

Mo: 0.01 ~ 0.2wt%

Mo, like Ti and Nb, precipitates C, which is a solid solution element in the steel, in the form of a MoC precipitate to remove the solid solution C in the steel, thereby increasing the value of r. In addition, the MoC precipitates evenly plated during hot dip galvanizing of the steel sheet. In addition, the removal of the employment element C exhibits non-aging characteristics.

Mo is difficult to secure the strength value and excellent plating characteristics less than 0.01wt%, and if it exceeds 0.2wt%, the cost rises, workability is lowered due to excessive strength increase, and it is difficult to secure excellent plating property.

Therefore, the content of Mo is set to 0.01 ~ 0.2wt%.

Cu: 0.05 ~ 0.25wt%

Cu exists as an impurity, a tramp element, which cannot be removed in a steelmaking process using scrap as a raw material. Cu has an aspect of increasing the strength, but the elongation, r value is lowered and the surface quality is reduced, so the content is controlled to 0.25wt% or less.

If the Cu content is more than 0.25wt%, Cu is concentrated on the surface during the annealing and annealing heat treatment, so that red brittleness is generated. However, it should be contained more than 0.05wt% to enhance the solid solution.

Therefore, the content of Cu is regulated in the range 0.05 ~ 0.25 wt%.

Sn: 0.008 ~ 0.025wt%

Sn, like Cu, is present as an impurity, a tramp element, which cannot be removed in a steelmaking process using scrap as a raw material. Sn plays a decisive role in lowering the mechanical properties of steel, unlike other tramp elements.

If the content of Sn is less than 0.008wt%, the solid solution strengthening effect is lowered, so that it is difficult to secure the target strength value. Adversely affect the moldability.

Therefore, Sn is regulated in the range of 0.008 to 0.025wt%.

In addition to Cu and Sn, in the case of mini-mill production method, Ni and Cr may be contained in 0.07 wt% or less.

Since the impurity element is a cause of deteriorating the mechanical properties, it is regulated to the minimum possible. In particular, since the present invention steel is intended to improve the surface quality rather than the purpose of increasing the strength, it is regulated to 0.07wt% or less in accordance with the purpose of the present invention.

N: 0.002-0.01 wt%

N, like C, exists as a solid solution in the steel, lowers the elongation and lowers the workability and formability of the steel sheet. The smaller the content of N, the better the moldability. However, considering the steelmaking level and cost, the lower limit is set to 0.002wt%.

In addition, the higher the content of N, the lower the upper limit of 0.01wt% in order to reduce workability. Therefore, N is regulated in the range of 0.002 to 0.01 wt%.

In addition to the alloying elements described above, B may be added to prevent secondary work brittleness due to P. In this case, B may be contained in an amount of 0.001 wt% or less.

In order to secure the plating properties, the content of Si, Mn, Sb, and Mo satisfies Equation 2 ≦ (Si + Mn) / (Sb + Mo) ≦ 22.

If the (Si + Mn) / (Sb + Mo) value is less than 2, the Sb, Mo content is excessive compared to the Si and Mn content, the material properties may deteriorate, and the (Si + Mn) / (Sb + Mo) value When it exceeds 22, Si and Mn may diffuse-distribute at grain boundaries.

The steel sheet of the present invention contains the above components, the balance is Fe, and fine incorporation of other unavoidable impurities such as 0.01 wt% or less oxygen as an element contained according to the situation of raw materials, materials, manufacturing facilities and the like is also allowed.

The slabs having the composition as described above are obtained by molten steel of a desired composition through a steelmaking process, and then manufactured by a continuous casting process, and then made of a hot rolled steel sheet through reheating and hot rolling, and then pickling, cold rolling, annealing and melting. It is manufactured from high strength steel sheet by galvanizing process.

The high strength high strength steel sheet thus manufactured has a tensile strength of 440 MPa or more, an elongation of 40% or more, and an r value of 1.5 or more.

The grain size of the microstructure is 15 µm or less, and carbon nitrides of 0.2 µm or less are uniformly distributed in the grains.

The specific manufacturing process is as follows.

Reheating Process

The reheating process of the slab is to reclaim segregated components and precipitates during casting. Reheating is carried out for 1 to 3 hours above Ac3, the austenitic single phase. The Ac3 temperature or more is 1100-1200 degreeC in this Example.

If the reheating temperature is low, it promotes Ti precipitation. In the case of TiN and TiC, the precipitation tendency does not change significantly depending on the actual reheating temperature, but Ti4C2S2 and TiS precipitates change depending on the reheating temperature.

Therefore, by lowering the reheating temperature, Ti4C2S2 and TiS are easily precipitated, thereby removing the solid solution elements C and N. Therefore, the reheating temperature is in the range of 1100 to 1200 ° C., which is low temperature heating.

Toughness is improved and material variation is reduced at low temperature, but scale peelability is disadvantageous. However, in the present invention, the scale suppresses the formation by increasing the Si content, and thus does not adversely affect the surface quality even when the reheat temperature is a low temperature heating.

On the other hand, if the reheating temperature is lower than 1100 ° C, segregated components during casting cannot be re-used. And, if the reheating temperature exceeds 1200 ℃ scale peelability and rollability is good but toughness is reduced.

[Hot Rolling and Winding Process]

The slab reheated in the furnace process is finished hot-rolled at a temperature of Ar3-Ar3 + 50 ° C, which is an austenitic single phase, and wound in the form of a steel sheet coil at 500-700 ° C. After hot rolling, finish air cooling.

If the finish hot rolling temperature is above Ar3, austenite is transformed into ferrite during the cooling process. In this metamorphosis, after the cold rolling annealing, a collective tissue is formed which is the basis of the (111) aggregate development. However, when the finish hot rolling temperature exceeds Ar3 + 50 ° C., it is easy to be exposed to defects occurring at high temperatures such as scale.

In this embodiment, Ar3 ~ Ar3 + 50 ℃ is a temperature range of 880 ~ 930 ℃.

Winding is performed at 500-700 ° C. to prevent grain coarsening. If the winding temperature is less than 500 ° C., the shape may be distorted due to non-uniform friction. If the winding temperature is higher than 700 ° C., the grains may grow coarse, and thus, it may be difficult to secure the target strength.

Preferably the winding range is 600 ~ 630 ℃ for excellent surface properties. This is because the range is most stable in terms of preventing grain coarsening and minimizing non-uniform friction.

In the hot rolled steel sheet manufactured as described above, a silicon-based oxide (2FeSiO ₂ ) is thinly formed on the surface due to the addition of Si, and the 2FeSiO ₂ prevents contact between the steel sheet and oxygen to suppress scale formation. In addition, since 2FeSiO ₂ is thin, it is easy to peel off during pickling.

[Cold rolling process]

After hot rolling, the hot rolled steel sheet is subjected to a pickling process to remove the scale and cold rolled.

The cold rolling process is a step of cold rolling to obtain the final desired thickness of the steel sheet and to obtain the desired material. Cold rolling is carried out at a reduction ratio of 30 to 80% and more preferably at 60 to 75%.

The cold rolling rate plays an important role in the development of (111) aggregates. The higher the reduction ratio of cold rolling, the better the (111) aggregate structure is developed. However, if the cold reduction rate is excessively high, the annealing grains become too fine and the strength increases, which adversely affects the workability.

Therefore, cold rolling is performed at a reduction ratio of 30 to 80%, preferably 60 to 75%, in consideration of the development and processability of the texture.

[Annealing process]

The cold rolled steel sheet is maintained at 800 to 860 ° C. for 30 to 100 seconds, and then cooled to 400 to 500 ° C. at a cooling rate of 10 to 20 ° C./sec for 180 to 300 seconds. After air cooling.

At this time, when the annealing temperature is less than 800 ℃, the development of the recrystallized texture is insufficient, the moldability is lowered, if the temperature exceeds 860 ℃ abnormal grain growth may occur.

If the cooling rate after annealing is less than 10 ℃ / sec ductility is lowered, if it exceeds 20 ℃ / sec may cause a problem of material unevenness.

After the annealing is completed, the cold rolled steel sheet is immersed in a hot dip galvanizing pot of 460 to 480 ° C. to perform hot dip galvanizing for 3 to 5 seconds, and an alloy heat treatment is maintained for 3 to 5 seconds at 480 to 520 ° C. Then cool.

In the alloying heat treatment, when the temperature is lower than 480 ° C., it is difficult to stably grow the plating layer, and when the temperature is higher than 520 ° C., material degradation occurs.

The steel sheet thus manufactured has a tensile strength of 440 MPa or more, an elongation of 40% or more, an r value of 1.5 or more, and a plating surface grade of 2 or less.

Hereinafter, the high strength high strength steel sheet and its manufacturing method excellent in the surface characteristics described above will be described in comparison with the invention example and other comparative examples.

Table 1 shows the component ratio of the invention example of this invention and another comparative example.

 (Unit: wt%, balance Fe) division C Si Mn P S Al Ti Nb Sb Mo Cu Sn N Comparative Example 1 0.004 0.03 1.5 0.07 0.008 0.04 0.04 0.02 - - - - 0.004 Comparative Example 2 0.003 0.03 1.5 0.07 0.008 0.04 0.04 0.02 - - - - 0.004 Comparative Example 3 0.003 0.03 1.5 0.07 0.008 0.04 0.04 0.02 - - 0.1 0.01 0.004 Comparative Example 4 0.003 0.03 1.5 0.07 0.008 0.04 0.04 0.02 - - 0.1 0.01 0.004 Comparative Example 5 0.004 0.03 1.5 0.07 0.008 0.04 0.04 0.02 - - 0.1 0.01 0.004 Inventory 1 0.003 0.15 0.8 0.045 0.008 0.04 0.04 0.015 0.02 0.02 0.1 0.01 0.004 Inventive Example 2 0.003 0.2 0.8 0.045 0.008 0.04 0.04 0.015 0.05 0.02 0.1 0.015 0.004 Inventory 3 0.004 0.2 0.8 0.045 0.008 0.04 0.04 0.015 0.06 0.02 0.1 0.015 0.004 Honorable 4 0.004 0.2 0.8 0.045 0.008 0.04 0.04 0.015 0.07 0.02 0.1 0.015 0.004 Inventory 5 0.004 0.25 0.8 0.045 0.008 0.04 0.04 0.015 0.03 0.01 0.1 0.02 0.004

division
(Si + Mn) / (Sb + Mo) Mechanical properties Grain size
Carbonitride
(Precipitate) size Plating surface
Rating TS YP El r Comparative Example 1 - 444 310 32 1.4 8 ~ 17㎛ 0.2㎛ or less 4 Comparative Example 2 - 441 287 34 1.4 5 ~ 17㎛ 0.2㎛ or less 3 Comparative Example 3 - 450 300 32 1.5 7 ~ 18㎛ Less than 0.4㎛ 4 Comparative Example 4 - 451 288 31 1.4 8 ~ 16㎛ Less than 0.3㎛ 4 Comparative Example 5 - 456 279 31 1.3 7 ~ 17㎛ Less than 0.3㎛ 5 Inventory 1 23.8 450 310 42 1.7 8 ~ 13㎛ 0.1㎛ less than 2 Inventive Example 2 14.3 458 304 41 1.6 8 ~ 13㎛ 0.2㎛ or less One Inventory 3 12.5 457 325 40 1.8 8 ~ 13㎛ 0.15㎛ or less One Honorable 4 11.1 468 315 42 1.7 8 ~ 13㎛ 0.1㎛ less than One Inventory 5 26.3 465 323 41 1.8 8 ~ 13㎛ 0.1㎛ less than 2

[TS (MPa): Tensile Strength, YP (MPa): Yield Strength, El (%): Elongation, r: Plastic Strain Ratio]

Here, the r value means a value obtained by dividing the widthwise deformation by the thicknesswise deformation during drawing.

The results of measuring the mechanical properties of the specimens prepared by the following hot rolling, cold rolling and annealing conditions using the slab composition as shown in Table 1 are shown in Table 2.

In the manufacturing method, the slab having the alloy composition of Table 1 was reheated at 1150 ° C. for 2 hours, followed by finishing hot rolling at 900 ° C., and wound at 500 ° C.

The hot rolled steel sheet is pickled and cold rolled at a reduction rate of 65%. The cold rolled steel sheet was held at 850 ° C. for 50 seconds, and then subjected to annealing heat treatment for cooling to 500 ° C. at 200 ° C. for 200 seconds.

Subsequently, the annealing cold rolled steel sheet was immersed in a zinc plating bath at 460 ° C. to perform hot dip galvanizing. After hot dip galvanizing, the alloy was subjected to alloy heat treatment.

Here, grade 1 means less than 3% of scale generation area, total grade 2 is less than 5%, grade 3 is less than 10%, grade 4 is less than 20%, grade 5 is less than 30%, and grade 5 is less than 30%. do.

Looking at Table 1 and Table 2, the content of Si was increased, the content of Mn was lowered, and when the Mo and Sb were added, the plating surface grade was excellent.

In particular, even in Inventive Example 2 and Inventive Example 3 in which the contents of Si, Mn, Sb, and Mo satisfy the expression 2? (Si + Mn) / (Sb + Mo)? It can be seen that the most effective in forming an even plating layer when the content of Si, Mn and Sb, Mo satisfy the above range.

1 is a SEM photograph showing a step between a region where scale defects exist and a region where scale defects of Comparative Example 1 and Example 3 exist.

As shown in FIG. 1, in the case of Comparative Example 1, the step difference in the region where the scale defect exists is wider than 40 μm. On the other hand, in the case of Inventive Example 3, even if there is a scale defect, it can be seen that it is 10 µm or less and is similar to the base material.

This is because silicon-based oxide (2FeSiO ₂ ) is thinly formed on the surface due to the addition of Si, and the 2FeSiO ₂ prevents the contact between the steel sheet and the oxygen, thereby suppressing scale formation, minimizing scale formation and easy peeling during pickling. to be.

Moreover, in the case of the comparative example 1, it is confirmed that there is an unplated part by not adding Sb. Such an unplated portion adversely affects the plating characteristics.

In addition, in the case of the invention examples 1 to 5, even if Cu and Sn, which are inevitable impurities of the mini-mill process, are contained, the r-value is 1.5 or more, which is excellent in workability. The grain size of the microstructure also satisfies 15 μm or less, and the size of the carbonitride formed in the crystal grains is uniformly distributed to 0.2 μm or less. This may be due to the control of the solid solution by Ti and Nb and the formation of aggregates.

On the contrary, in Comparative Examples 1 to 5, although the Si content is low, the r value is low and the plating surface grade is low due to the absence of Mo and Sb. In addition, the crystallite size of the microstructure and the size of the carbonitride were not uniformly distributed.

Through this, even in the case of increasing the content of Si in the ultra-low carbon steel having a content of C and N of 100ppm or less, it is possible to manufacture a steel plate having high plating strength and high elongation with low Mn content and adding Mo and Sb. It can be seen.

The scope of the present invention is not limited to the embodiments described above, but may be defined by the scope of the claims, and those skilled in the art may make various modifications and alterations within the scope of the claims It is self-evident.

1 is a SEM photograph showing the step difference between the region where the scale defects of Comparative Example 1 and Inventive Example 3 of Table 1 is present and the region is not present.

Claims (5)

C: 0.002 to 0.01 wt%, Si: 0.05 to 0.25 wt%, Mn: greater than 0 wt% and less than 1.0, P: greater than 0 wt% and 0.05 wt% or less, S: greater than 0 wt% and 0.01 wt% or less, Al: 0.01 to 0.06 wt% , Ti: 0.02 ~ 0.07wt%, Nb: 0.01 ~ 0.05wt%, Sb: 0.01 ~ 0.2wt%, Mo: 0.01 ~ 0.2wt%, Cu: 0.05 ~ 0.25wt%, Sn: 0.008 ~ 0.025wt%, N : 0.002 ~ 0.01wt% and the rest is composed of Fe and inevitable impurities, Tensile strength is 440MPa or more, elongation is 40% or more, r value is 1.5 or more, The high-strength high strength steel sheet having excellent surface properties, characterized in that the content of Si, Mn, Sb, Mo satisfies the formula 2≤ (Si + Mn) / (Sb + Mo) ≤22. delete C: 0.002 to 0.01 wt%, Si: 0.05 to 0.25 wt%, Mn: greater than 0 wt% and less than 1.0, P: greater than 0 wt% and 0.05 wt% or less, S: greater than 0 wt% and 0.01 wt% or less, Al: 0.01 to 0.06 wt% , Ti: 0.02 ~ 0.07wt%, Nb: 0.01 ~ 0.05wt%, Sb: 0.01 ~ 0.2wt%, Mo: 0.01 ~ 0.2wt%, Cu: 0.05 ~ 0.25wt%, Sn: 0.008 ~ 0.025wt%, N : Slab composed of 0.002 ~ 0.01wt% and the remainder composed of Fe and inevitable impurities Reheating to 1100 ~ 1200 ° C., then performing finish hot rolling at 880 ~ 930 ° C. and winding at 500 ° C. to 700 ° C. to produce hot rolled steel sheet; Pickling the hot rolled steel sheet and cold rolling at a reduction ratio of 30 to 80% to produce a cold rolled steel sheet; And Annealing heat treatment and hot dip galvanizing the cold rolled steel sheet to develop the (111) aggregate structure, The Si, Mn, Sb, Mo content of the formula 2≤ (Si + Mn) / (Sb + Mo) ≤ 22 method for producing a high strength high strength steel sheet having excellent surface properties, characterized in that. The method of claim 3, The annealing heat treatment The cold rolled steel sheet is heated to 800 to 860 ° C. for 30 to 100 seconds, and then cooled to 400 to 500 ° C. at a cooling rate of 10 to 20 ° C./sec for 180 to 300 seconds. Method for producing high strength high strength steel sheet with excellent characteristics. delete

Images