KR20090119264A - High strength hot dip galvanized steel plate with excellent plating property and manufacturing method - Google Patents

Abstract

The present invention includes, by weight, C: 0.05-0.25%, Si: 0.5-1.8%, Mn: 0.5-2.5%, N: 0.006-0.012%, Sb 0.01-0.04%, balance Fe and other unavoidable impurities. It includes, and provides a high-strength hot dip galvanized steel sheet characterized by having a stable Si and Mn oxide by controlling the dew point temperature and hydrogen concentration in the heat treatment step and the cooling step.

By using the steel sheet of the present invention, it is possible to provide an automotive steel sheet having high strength properties, excellent plating properties and improved surface appearance without adding expensive equipment and a large amount of alloying elements.

Description

High Strength Steel Sheet for Hot-Dip Galvanization with Excellent Galvanizing Properties and Manufacturing Method Thereof}

The present invention relates to a high-strength hot dip galvanized steel sheet and a method for manufacturing the same, which are used as a rust preventive steel sheet for automobiles, and more particularly, Si and Mn, which have tensile strengths of about 590 to 780 MPa, and which adversely affect plating properties. It relates to a high-strength hot-dip galvanized steel sheet including a plated surface excellent in appearance and unplated, and a method of manufacturing the same.

Parts such as cross members and side members, which are used in automobiles, have been examined for high strength in order to reduce fuel consumption, and high strength has been advanced to ensure strength even if the thickness of the material is thin. In order to achieve this purpose, techniques for developing a hot rolled steel sheet and a cold rolled steel sheet using transformation organic plasticity of the retained austenite contained in steel have been shown in the related art. It does not add various expensive alloying elements, and it is based on 0.07 ~ 0.4% by weight of C, 0.3 ~ 2.0% by weight of Si, and 0.2 ~ 2.5% of Mn as basic element. Bainite transformation at a temperature before and after the ℃ is a technique that includes the retained austenite in the metal structure. These steel grades can be manufactured from hot rolled steel as well as cold rolled steel by controlling the cooling and winding temperature in the hot rolled run-out table. Furthermore, since the ideal structure steel is quenched to the martensite transformation temperature by quenching to secure the martensite structure, there is no need to use much Si, so the degree of freedom in component design is greater than that of the transformation organic plastic steel.

However, in recent years, the improvement of corrosion resistance and appearance has become very important due to the trend of high quality of automobile parts, and the plating of automobile parts is progressing for the purpose of improving this. Particularly, galvanized steel sheet is used in many parts except for specific parts mounted inside the car body, and the steel sheet used for the galvanized steel sheet contains a large amount of Si, Mn, Al, etc. in the steel for high strength and high tensile properties. Since the oxide film is easily formed on the surface of the steel sheet, fine unplated portions may be generated during hot dip galvanizing, and there is a problem that the plating property of the processed portion is degraded after alloying.

In addition, the steel used as a conventional galvanized steel sheet still has the following problems. First, since the mechanism for increasing the strength of the steel sheet is to transform the austenite phase into the martensite phase or bainite phase to obtain high strength, the strength and material variation such as tensile strength are large. Second, when joining by spot welding, the crystal grains of the heat affected zone become large, and thus the breaking strength of the weld zone decreases. Third, due to the high content of Si and Mn in the steel, the plating wettability is poor and the unplated phenomenon is likely to appear. Therefore, it is necessary to conduct the lead of Fe or Ni-based before the hot dip galvanizing process, the process is added, the plating composition, adhesion amount and plating distribution becomes important. Fourth, it is easy to generate a pressing phenomenon toward the coil inner diameter after hot rolling and winding in the form of a coil. Therefore, a process for restoring the original state of the coil inner diameter needs to be added, thereby increasing the manufacturing cost.

In order to prevent the unplating phenomenon caused by the addition of non-plating elements such as Si and Mn, various techniques have been shown in the past. In Japan, for example, 2004-292869, a 590 ~ 1080 MPa class high strength GA manufacturing method with excellent press formability, C 0.05 ~ 0.25%, Si 0.1 ~ 1.8%, Mn 0.5 ~ 2.5%, Al 0.01 ~ 1.5%, Ni 0.4 (%) ≤Si (%) + Al (%) ≤2.0 (%), Ni (%) ≥2 / 5 X Si (%) + 1 /, containing 0.02-1.0%, Nb 0.002-0.05% 5 x Al (%), 1/200 X Ni (%) <Nb (%) <1/10 X Ni (%) conditions were presented. However, this was intended to improve the plating property by changing the form of Si, Mn oxide by concentrating Ni on the surface, which is harder to oxidize than Fe in the steel, but may have a disadvantage in that the manufacturing cost increases with the increase of expensive additive elements in the steel.

In addition, in Japan, 2000-239788, 45kg / mm class ² high tensile strength steel sheet with excellent plating and deep workability is C 0.0005 ~ 0.005%, Si 0.1 ~ 1.2%, Mn 0.5 ~ 3.0%, Al 0.10% or less, P 0.04 ~ Steel components containing 0.13%, Nb 0.003-0.10%, Ti 0.015-0.10%, and Zr 0.02-0.2% were presented. The Japanese Laid-Open Patent attempts to suppress the formation of Si oxide films (SiO ₂ , Mn ₂ SiO ₄ , MnSiO ₃ ) by the addition of Zr components, but the maximum amount of Si added in steel is 1.2%, which is the amount of general metamorphic organic steel or abnormal tissue steel. It is less and shows the disadvantage that the manufacturing cost rises with the increase of additive elements.

In addition, Japanese Patent 7-118796 and Japanese Patent 8-291367 disclose a method for manufacturing 40kg / mm class ² high tensile strength steel with excellent plating and deep workability, C 0.0005 ~ 0.005%, Si 0.1 ~ 1.2%, Mn It relates to steel materials containing 1.0 to 3.0%, Al 0.10% or less, B 0.005% or less, Nb 0.003 to 0.10%, Ti 0.015 to 0.10%, La, Ce, Pr, Nd, and Gd 0.005 to 0.10%. The Japanese Laid-Open Patents seek to suppress the amount of oxidation of the surface oxide layer and the formation of Mn ₂ SiO ₄ and MnSiO ₃ oxides, in particular by the addition of Rare Earth Metal (REM). However, the addition of REM made it difficult to manage inclusions in steelmaking and to control oxide in the hot rolling stage.

In addition, in Japan 2001-131698, a high tensile strength hot-dip steel sheet manufacturing method is C 0.0005 ~ 0.005%, Si 0.1 ~ 1.2%, Mn 0.5 ~ 2.5%, Al, S 0.10% or less, B, N 0.005% or less, Nb 0.02 ~ It contains a 0.20% component, heat treatment at dew point temperature (DP) below 0 ℃, annealing temperature above 780 ℃, removes oxide, and after heat treatment at DP-20 ℃ below annealing temperature above 650 ℃, the method is characterized by plating. However, this oxidation-reduction technique uses two heat treatment processes to increase manufacturing costs and complicated production management.

Furthermore, in Japan 2001-279409, a high-strength GA steel sheet manufacturing method with excellent workability was proposed to improve the plating property by producing 50 ~ 1000nm oxide film by Fe-based electroplating in the pretreatment stage. There is a disadvantage that additional cost and time are added. Similar to this lead method, the Japanese JP00-212711 patent discloses that when a metal compound is attached to the surface of a steel sheet and then heat-annealed, metal emulsions are generated as the metal is dissolved on the surface of the steel sheet, and these metal emulsions inhibit the growth of grains. Was formed to produce fine Fe-Zn crystals during hot dip galvanizing, which is said to be able to manufacture hot dip galvanized steel sheets with good adhesion and uniformity, but the production cost increases due to the addition of pretreatment facilities.

Even with reference to the above-mentioned prior arts, there are various problems. Therefore, the practical use of Si, Mn, and Al-based high-strength hot-dip galvanized steel sheets which are excellent in corrosion resistance while preventing unplated parts due to excellent processing part plating property is delayed. It is a reality.

The present invention can solve the problems presented by the prior art in the production of 590 ~ 780MPa grade ultra-high strength alloyed hot-dip galvanizing and prevent the unplated phenomenon, and also improve the plating properties and the unplated without the addition of additional equipment or additional elements. It is to provide a high strength hot dip galvanized steel sheet that can be prevented.

MEANS TO SOLVE THE PROBLEM In order to solve the above-mentioned subject, this invention makes C: 0.05-0.25%, Si: 0.5-1.8%, Mn: 0.5-2.5%, N: 0.006-0.012%, Sb 0.01-0.04% by weight%. It provides a high strength hot dip galvanized steel sheet comprising a balance Fe and other unavoidable impurities.

Further, the present invention, by weight percent, C: 0.05-0.25%, Si: 0.5-1.8%, Mn: 0.5-2.5%, N: 0.006-0.012%, Sb 0.01-0.04%, balance Fe and other For steel slabs containing inevitable impurities,

Heating the slab to 1150-1300 ° C .;

A filamentous rolling step hot-rolled at 850 to 950 ° C;

Winding the cooling rate to 30 ° C./sec or more and the temperature to 500 ° C. to 600 ° C .;

Pickling and cold rolling at a rolling reduction of at least 60%;

A heat treatment step of heat treatment at 800 ° C. or lower; And

A cooling step of annealing at a temperature of 20 ° C./sec or more to 400 ° C. with a crack time of 20 to 200 seconds;

It provides a method for producing a high strength hot dip galvanized steel sheet comprising a. In particular, the heat treatment step may be made at a dew point temperature of -40 ~ -60 ℃, hydrogen concentration 10 ~ 20%. In addition, the cooling step may be made at a dew point temperature of -20 ~ 40 ℃, hydrogen concentration 50 ~ 80%.

By using the steel sheet of the present invention, it is possible to provide an automotive steel sheet having high strength properties, excellent plating properties and improved surface appearance without adding expensive equipment and a large amount of alloying elements.

The present invention provides an ultra-high strength alloyed hot dip galvanized steel sheet and a method of manufacturing the same by controlling the composition and dew point of a reducing atmosphere gas in a heat treatment process before plating. In particular, the present invention does not require additional processes of ancillary processes to improve the plating property and does not require the addition of expensive alloying elements such as Ni and Cu.

As a result of observing the conventional unplated steels, it was found that a hemispherical oxide (selective oxide produced during cold rolling annealing) having a Si-Mn-O component was present directly on the plating layer / ferrous iron interface. . That is, Si-based oxides that inhibit the plating properties, for example, SiO ₂ , Fe ₂ SiO ₄ , Mn ₂ SiO ₄ , MnSiO _3, etc. are concentrated on the surface of the steel sheet to reduce wettability with molten zinc during plating. Due to the presence of such Si-based oxide, when the ultra-high strength alloyed hot dip galvanized steel sheet was manufactured by a conventional method, a large amount of unplated steel having a diameter of 2 to 4 mm or less was generated.

Accordingly, the present inventors have led to the present invention that can suppress such unplated as a result of repeated studies, it will be described in detail.

First, degreasing cold rolled steel sheet was heat-treated at a dew point temperature of -40 to -60 ° C and a hydrogen concentration of 10 to 20% at a temperature of 800 ° C or lower, and then slowly cooled to 650 ° C to inhibit oxidation of Mn to the maximum. Mn oxide formation can be suppressed by controlling dew point temperature at -20 ~ -40 ℃, but Si is produced as Si oxide by the following reaction.

Si + 2H ₂ O → SiO ₂ + 2H ₂

The hydrogen concentration is required to appropriate level for the inhibition of the oxide, in the present invention, the amount of hydrogen that can inhibit the oxidation of Mn experimentally was limited to 10 to 20%.

When the dew point temperature rises, the appearance frequency decreases rapidly on the surface of the steel sheet due to internal oxidation, and the surface appearance is improved by plating in this state. Accordingly, the steel sheet is then quenched to 400 ° C. while maintaining a hydrogen concentration of 50 to 80% and a dew point temperature of −20 ° to −40 ° C. By the quenching treatment carried out in such a high hydrogen reducing atmosphere, the Si oxide is changed into Si with water to suppress the generation of Si oxide. After reheating to the plating bath bathing temperature and introduced into the plating bath 460 ℃. After plating, the alloying treatment is performed at the temperature of 540 ~ 560 ℃, and after this process, it has an Fe content of about 10%, ultra-high strength alloyed hot-dip galvanized steel sheet which has an unplated after plating and a sharp surface appearance. Can be prepared.

Hereinafter, the reason for limiting the composition of the molten steel sheet of the present invention will be described in more detail (hereinafter, referred to as weight%).

C: 0.05 ~ 0.25%

C is an austenite stabilizing element and moves from ferrite in the two-phase coexistence temperature region and bainite transformation temperature region, and is concentrated in austenite. As a result, even when cooled to room temperature, stabilized austenite remains at a level of 2 to 20%, thereby securing moldability due to transformation of organic plastics. If C is less than 0.05%, it is difficult to secure more than 2% of the retained austenite tissue, whereas if C is added more than 0.25%, the weldability may deteriorate.

Si: 0.5 ~ 1.8%

Si does not dissolve in cementite and inhibits the precipitation of cementite, thus delaying the transformation from austenite at a temperature of 350 to 600 ° C. Therefore, it is possible to promote the concentration of austenite C, thereby improving the chemical stability of the austenite, and it is possible to secure residual moldable austenite, which causes transformation organic plasticity and improves moldability. In the case where the Si content is less than 0.5%, such an effect is insignificant, whereas the addition of more than 1.8% may deteriorate the plating property, so the amount of Si added is limited to the above range.

Mn: 0.5 ~ 2.5%

Mn is an element that promotes austenite formation and prevents austenite from being decomposed into pearlite during annealing in an abnormal temperature range and then cooled to 350 ~ 600 ℃ so that austenite remains in the metal structure during cooling to room temperature. . Therefore, in the present invention, 0.5% or more is added to suppress the retaining of austenite and decomposition into pearlite, but when the content exceeds 2.5%, the band structure becomes remarkable and the physical properties are lowered, and the spot welding part breaks in the nugget. So its content is controlled.

N: 0.006-0.012%

N is an austenite stabilizing element, similar to C, serves to lower martensite transformation initiation temperature. At low concentrations, N exhibits a similar behavior to C during intercritical annealing. It is also an invasive element that interferes with vane deformation and lowers the martensite starting temperature. Particularly, in high Si-containing steel, a considerable amount of Si oxide film is formed to about 100 nm in the surface layer during annealing, and stable SiO ₂ oxide film adversely affects Zn wettability. By inducing the formation of a nitride film, it is possible to produce a hot-dip galvanized steel sheet having improved wettability between Zn and the steel sheet and having good surface properties. If N is less than 0.006%, this effect does not appear or insignificant, whereas if it exceeds 0.12%, the material of the steel sheet may be too hardened, so the content is controlled.

Sb: 0.01 ~ 0.04%

Sb is an element that inhibits high-temperature oxidation by grain boundary segregation and thickening effects, and has an effect of miniaturizing and homogenizing annealing oxide even with a small amount of addition. The addition of 0.01% or more of Sb inhibits the development of a film-type oxide film that is detrimental to plating properties, promotes fine and uniform granular oxides to make the plating surface uniform, and prevents defects caused by oxides, but the content is 0.04%. If exceeded, there is no difference in plating property and the mechanical properties are lowered so that the content is controlled.

In addition, the present invention may include inevitable impurities such as P and S. The smaller the amount of P and S is preferable, but it does not particularly limit the content because it does not affect the steel of the present invention.

The steel sheet of the present invention can be manufactured by a general method for producing steel using the steel of the above-mentioned composition. That is, after finishing the hot rolling process in which the slab heating temperature is 1150 to 1300 ° C. and the finishing rolling temperature is 850 to 950 ° C., the coil is wound at a cooling rate of 30 ° C./sec or more and a temperature of 500 ° C. to 600 ° C. After pickling and cold rolling, the appropriate reduction ratio is more than 60%. Heat treatment temperature is 800 degrees C or less, a crack time 20-200 second, and cooling is performed by annealing 20 degrees C / sec or more to 400 degreeC.

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

In this embodiment, a steel sheet for hot dip galvanizing was prepared using a steel slab composed of a component system disclosed in Table 1 below. The hot dip galvanized steel sheet was coated with a bath temperature of 460 ° C., an Al content of a plating solution of 0.13% by weight, and a plating adhesion amount. : Plated to 60g / m ² to prepare a hot-dip galvanized steel sheet.

Steel grade Chemical content (wt%) C Si Mn P S Sol-Al N Sb Inventive Steel 1 0.072 0.65 2.5 0.014 0.012 0.044 0.0068 0.015 Comparative Steel 1 0.103 0.88 1.6 0.012 0.010 0.035 0.0090 0.009 Inventive Steel 2 0.124 0.58 1.5 0.015 0.010 0.041 0.0113 0.014 Comparative Steel 2 0.093 0.95 0.3 0.011 0.012 0.036 0.0080 0.033 Invention Steel 3 0.112 1.2 1.2 0.011 0.015 0.047 0.0072 0.024 Comparative Steel 3 0.183 0.41 2.0 0.014 0.013 0.050 0.0098 0.035 Inventive Steel 4 0.153 1.75 1.9 0.010 0.010 0.048 0.011 0.04 Comparative Steel 4 0.144 1.09 1.6 0.013 0.009 0.048 0.0049 0.02 Inventive Steel 5 0.192 0.88 1.8 0.015 0.008 0.057 0.0106 0.026 Comparative Steel 5 0.023 0.98 1.1 0.009 0.014 0.049 0.0085 0.010  * Underscore is outside the scope of the present invention steel

Evaluation of the plating property of the hot-dip galvanized steel sheet was determined by obtaining the area ratio of the unplated portion after hot-dip galvanized and the size was measured by SEM and the results are shown in Table 2 below.

River bell Heating condition Cooling condition Unplated rating Mechanical properties Annealing Temperature (℃) Dew point (℃) Hydrogen concentration (H ₂ ,%) Dew point (℃) TS (MPa) T-El (%) Inventive Steel 1 780 -50 75 -30 One 598 30 Comparative Steel 1 790 -55 50 -55 2 670 26 Inventive Steel 2 800 -60 80 -20 One 780 23 Comparative Steel 2 785 -35 50 -10 3 570 29 Comparative Steel 2 785 -50 30 -10 3 568 28 Invention Steel 3 795 -40 80 -25 One 650 28 Comparative Steel 3 800 -30 75 -60 3 600 25 Comparative Steel 3 800 -55 35 -20 3 595 26 Inventive Steel 4 785 -50 80 -40 One 810 23 Comparative Steel 4 780 -45 30 -50 4 785 24 Inventive Steel 5 795 -55 50 -30 One 755 25 Comparative Steel 5 790 -30 60 -50 3 510 29

Rating 1: 0% unplated area

Rating 2: Less than 0.1% of unplated area, 0.1 mm or less of unplated size

Rating 3: Less than 0.1 ~ 0.3% of unplated area, less than 0.5 mm of unplated size

Rating 4: Less than 0.3 ~ 0.5% of unplated area, less than 1 mm of unplated size

Rating 5: More than 0.5% of unplated area, more than 3 mm of unplated size

As can be seen in Table 2, the plating property of the plated steel sheet using the steel sheet of the present invention was found to be excellent in the appearance of the surface and unplated. Furthermore, after removing the galvanized layer after plating and comparing the Al content in the Al-rich layer, it can be seen that there is much more present in the inventive steel, which means that the plating property is significantly improved. On the other hand, it can be seen that in the comparative steel, the plating property is deteriorated and the unplated generation is remarkable, and it is not preferable to be used as a high-grade steel for automobiles.

Therefore, using the steel sheet of the present invention as described above can provide an automotive steel sheet having high strength properties, excellent plating properties and improved surface appearance.

Claims (5)

% By weight, containing C: 0.05-0.25%, Si: 0.5-1.8%, Mn: 0.5-2.5%, N: 0.006-0.012%, Sb: 0.01-0.04% and containing residual Fe and other unavoidable impurities High strength hot dip galvanized steel sheet. The steel sheet for high-strength hot dip galvanizing according to claim 1, wherein the structure of the steel sheet includes 2 to 20% of retained austenite structure in an area fraction. By weight, steel containing C: 0.05-0.25%, Si: 0.5-1.8%, Mn: 0.5-2.5%, N: 0.006-0.012%, Sb 0.01-0.04% and containing residual Fe and other unavoidable impurities About the slab, Heating the slab to 1150-1300 ° C .; A filamentous rolling step hot-rolled at 850 to 950 ° C; Winding the cooling rate to 30 ° C./sec or more and the temperature to 500 ° C. to 600 ° C .; Pickling and cold rolling at a rolling reduction of at least 60%; A heat treatment step of heat treatment at 800 ° C. or lower; And A cooling step of annealing at a temperature of at least 20 ° C./sec to 400 ° C. with a crack time of 20 to 200 seconds; Method for producing a high strength hot dip galvanized steel sheet comprising a. The method of claim 3, wherein the heat treatment is performed at a dew point temperature of -40 to -60 ℃, hydrogen concentration of 10 to 20%. The method of claim 3, wherein the cooling is performed at a dew point temperature of −20 to −40 ° C. and a hydrogen concentration of 50 to 80%.