Bake-hardened hot-dip galvanized steel sheet with excellent powdering resistance, and manufacturing method therefor

Abstract

To provide a bake-hardened hot-dip galvanized steel sheet with excellent powdering resistance, and a manufacturing method therefor.SOLUTION: The bake-hardened hot-dip galvanized steel sheet comprises, by wt.%, carbon (C): 0.0001-0.005%, manganese (Mn): 0.1-1.2%, silicon (Si): 0.02% or less (excluding 0%), phosphorus (P): 0.01-0.04%, sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (sol.Al): 0.01-0.06%, niobium (Nb): 0.003-0.015%, boron (B): 0.0005-0.0035%, chromium (Cr): 0.01-0.1%, molybdenum (Mo): 0.005-0.05%, titanium (Ti): 0.003% or less (excluding 0%), and the balance of Fe and inevitable impurities, wherein the microstructure is ferrite single phase.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the manufacture of bake-hardenable hot-dip galvanized steel sheets having excellent powdering resistance, More specifically, the C content is at a level of 0.005% by weight or less, and the P content is at a level of 400 ppm or less. The main process of the hot-dip galvanizing line for bake-hardenable hot-dip galvanized steel sheets is controlled by The factors are the annealing temperature, the alloying temperature of hot dip galvanizing, SPM (skin pass mill) (2) Temper rolling) By controlling the elongation rate, etc., it has excellent powdering resistance. The present invention relates to a bake hardening type hot-dip galvanized steel sheet capable of being subjected to the above-mentioned processes and a method for manufacturing the same. [Background technology]

[0002] Recently, in the automotive field, fuel efficiency regulations and performance standards have been introduced in developed countries, including Europe. Research into reducing the weight of car bodies is being actively conducted due to improvements in the steel industry. In order to meet the demands of automakers for weight reduction, The thickness of the steel plate has been further reduced compared to competing materials (Mg, Al, CFRP, etc.) In order to use it as a material for the outer panels of automobiles, The bake hardening phenomenon occurs during the processing of pressed parts. The dislocations generated during the process are fixed by the activated solute carbon and nitrogen during the paint baking process, resulting in the yield strength. Steel with excellent bake hardening properties is easy to form before painting and the final It has the property of improving dent resistance in products, and is used as a material for outer panels of automobiles. Furthermore, in order to apply it as a material for the outer panels of automobiles, A certain level of coverage is required to provide coverage against aging for a certain period of time. It is also required to have anti-aging properties.

[0003] Generally, the manufacturing method of cold-rolled steel sheet with bake hardenability is low-carbon P-added Al-kil The hot rolling temperature is in the range of 400 to 500°C. A steel sheet with a bake hardening level of approximately 40 to 50 MPa is produced by box annealing using low-temperature coiling. This is because box annealing makes it easier to achieve both formability and bake hardenability. In the case of P-added Al-killed steels produced by the continuous annealing method, the cooling rate was relatively fast. Therefore, it is easy to ensure bake hardenability, while rapid heating and short-time annealing However, there is a problem that formability deteriorates, and it is limited to only the outer panels of automobiles, which do not require formability. Recently, with the rapid development of steelmaking technology, it has become possible to control the amount of solid solution elements in steel. It is possible to use Al-Kille with the addition of strong carbonitride forming elements such as Ti or Nb. By using d steel sheet, bake hardening type cold rolled steel sheet with excellent formability can be manufactured, and it is suitable for applications where dent resistance is required. There is a growing trend for its use as exterior panel material for automobiles.

[0004] On the other hand, nano-sized CuS / MnS precipitates are utilized to form fine grain MAFE (Mic Bake-hardenable molten zinc alloy based on the "Ro Alloy Free for Exposure" concept Although a technology for producing lead-plated steel sheets has been proposed, the high P content of 500 ppm In addition, the addition of P retards alloying and reduces the amount of molten zinc. It is necessary to raise the alloying temperature of lead plating, which gives the plating layer high hardness. As the brittle Γ phase increases, the coating layer turns into powder due to compressive stress during press forming of parts. This can cause problems with powder falling off in small pieces. The defects are caused by the deterioration of corrosion resistance due to peeling of the coating layer, and the peeled powder is burned onto the mold, causing the steel sheet This may cause defects such as dents in the surface. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Korean Patent Publication KR2014-0048668 (Published on April 24, 2014) [Patent Document 2] Korean Patent Publication KR2011-0005414 (Published on January 18, 2011) Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has a carbon content of 0.005 wt. or less and a phosphorus content of 400 ppm or less. In controlled bake hardening hot-dip galvanized steel sheets, the steel composition is strictly controlled and hot-dip galvanization is performed. By controlling the main process conditions of the plating line, bake hardening with excellent powdering resistance is achieved. The object of the present invention is to provide a hot-dip galvanized steel sheet and a method for manufacturing the same.

[0007] Furthermore, the technical problems to be solved by the present invention are not limited to the above-mentioned technical problems. Further technical problems that have not been specified or mentioned will become apparent from the following description of the technology to which the present invention pertains. It can be clearly understood by a person having ordinary skill in the art. [Means for solving the problem]

[0008] Thus, one aspect of the present invention is By weight, carbon (C): 0.0001 to 0.005%, manganese (Mn): 0.1 to 1 0.2%, Silicon (Si): 0.02% or less (excluding 0%), Phosphorus (P): 0.01 to 0 0.04%, Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less ( 0%, aluminum (sol.Al): 0.01-0.06%, niobium (Nb) : 0.003~0.015%, Boron (B): 0.0005~0.0035%, Chromium ( Cr): 0.01-0.1%, Molybdenum (Mo): 0.005-0.05%, balance Fe and unavoidable impurities, satisfying the following relational expression 1-2, and the microstructure is a ferrite single phase. and bake-hardenable galvannealed coating with excellent powdering resistance that satisfies the following relational expression 3. This relates to steel plates.

[0009] [Equation 1] [Nb] / ((93 / 12)×[C])<0.55

[0010] [Equation 2] [Mn] / ((55 / 28) × [Si])>10

[0011] [Equation 3] [TS]×[El.]×[Upper-BH]>500,000(MPa×%) However, [Upper-BH] is: after 2% pre-strain, baked at 170°C for 20 minutes. Upper Yield Stress when baking.

[0012] Another aspect of the present invention is By weight, carbon (C): 0.0001 to 0.005%, manganese (Mn): 0.1 to 1 0.2%, Silicon (Si): 0.02% or less (excluding 0%), Phosphorus (P): 0.01 to 0 0.04%, Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less ( 0%, aluminum (sol.Al): 0.01-0.06%, niobium (Nb) : 0.003~0.015%, Boron (B): 0.0005~0.0035%, Chromium ( Cr): 0.01-0.1%, Molybdenum (Mo): 0.005-0.05%, balance Fe and unavoidable impurities, and the steel slab satisfies the following relational expression 1-2. reheating at a temperature hot rolling the reheated steel slab to a temperature in the range of 900 to 1100°C; The above hot-rolled steel sheet is cooled to a temperature in the range of 500 to 700°C at an average cooling rate of 10 to 70°C. Cooling at ° C. / sec and then winding; The coiled hot-rolled steel sheet is cold-rolled at a reduction ratio of 70 to 90%, and then rolled to a rolling speed of 740 to 850°C. continuous annealing in a temperature range of ° C.; The continuously annealed cold rolled steel sheet is immersed in a hot dip galvanized bath at 450 to 540°C. producing a hot-dip galvanized steel sheet by alloying in a temperature range; The hot-dip galvanized steel sheet manufactured as above is subjected to temper rolling at a reduction ratio of 0.5 to 2.0%. and a bake-hardenable alloying melt having excellent powdering resistance that satisfies the following relational expression 4. The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet.

[0013] [Equation 1] [Nb] / ((93 / 12)×[C])<0.55

[0014] [Equation 2] [Mn] / ((55 / 28) × [Si])>10

[0015] [Equation 4] Powdering=[-14.2+0.0362×(GA)-0.970×(CGL SPM El.)]<3 However, (GA) is the alloying temperature of hot dip galvanizing, and (CGL SPM El.) is the alloying temperature of continuous galvanizing. This is the temper rolling elongation rate of the hot dip galvanizing line.

[0016] The above-mentioned temper-rolled hot-dip galvanized steel sheet can satisfy the following relational expression 3.

[0017] [Equation 3] [TS]×[El.]×[Upper-BH]>500,000(MPa×%) However, [Upper-BH] is: after 2% pre-strain, baked at 170°C for 20 minutes. Upper Yield Stress when baking. [Effects of the Invention]

[0018] According to the present invention having the above-mentioned configuration, the composition of the alloying elements to be added and the manufacturing method can be appropriately controlled. By controlling the temperature, Upper BH (Baking Hardness) can be evaluated. ning) value and the basic tensile test strength, TS × El. × Upp, which is a comprehensive index of elongation. The er-BH value is 500,000 (MPa × %) or more, and the powdering resistance is 3 or less. Therefore, it is possible to effectively provide a hot-dip galvanized steel sheet having excellent bake hardening properties. . [Brief explanation of the drawings]

[0019] [Figure 1] 1A and 1B are diagrams showing BSE images (×7000) and the results of EDS element analysis of the alloyed coating layer after hot-dip galvanizing of the hot-dip galvanized steel sheets of Inventive Example 1 and Comparative Example 4 in the present Examples, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described below.

[0021] The inventors have discovered that the target steel can be obtained when the steel composition, annealing and operating conditions satisfy a specific relationship. The present invention was completed by confirming through experiments that the desired physical properties can be secured. The present invention relates to a bake-hardenable hot-dip galvanized steel sheet in which a hot-dip galvanized layer is formed on a base steel sheet. The base steel sheet has, in weight percent, carbon (C): 0.0001 to 0.0 05%, Manganese (Mn): 0.1 to 1.2%, Silicon (Si): 0.02% or less ( 0%, Phosphorus (P): 0.01-0.04%, Sulfur (S): 0.01% or less (0% Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al ): 0.01-0.06%, Niobium (Nb): 0.003-0.015%, Boron (B) : 0.0005~0.0035%, Chromium (Cr): 0.01~0.1%, Molybdenum ( Mo): 0.005 to 0.05%, the balance including Fe and unavoidable impurities, and the following relational expression 1-2 The microstructure is a single ferrite phase, and the following relational expression 3 is satisfied.

[0022] First, the composition of the steel of the base steel sheet constituting the hot-dip galvanized steel sheet of the present invention and its The reason for limiting the content will be explained. Meanwhile, "%" here means "by weight" unless otherwise specified. %".

[0023] ·Carbon (C): 0.0001~0.005% Carbon is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. In ultra-low carbon steel, it dissolves inside the steel sheet during cold rolling and annealing, and is formed by temper rolling. Basically, the carbon content is However, if there is too much solute carbon in the material, the part may break down. When molding, a defect called orange peel occurs on the surface. According to the present invention, if the content exceeds 0.005%, it is difficult to form the alloy. It is also disadvantageous in that it has poor room temperature aging resistance and is therefore limited in its application to parts. The C content is limited to 0.0001 to 0.005%. Preferably, the C content is limited to 0.0005 to 0.0 Manage within the range of 0.4%.

[0024] Manganese (Mn): 0.1-1.2% Manganese is a solid solution strengthening element that not only contributes to increasing strength but also converts S in steel into MnS When the Mn content is less than 0.1%, MnS is effectively precipitated. On the other hand, if it exceeds 1.2%, the strength increases. However, even if the Mn content is excessive, it will be dissolved and the drawability will be reduced. Therefore, it is preferable to limit the Mn content to 0.1 to 1.2%. The content is controlled within the range of 0.2 to 1.0%.

[0025] Silicon (Si): 0.02% or less (excluding 0%) Silicon contributes to increasing the strength of steel sheets through solid solution strengthening, but when added in an amount of 0.02% or more, In particular, there is a problem that the [Mn] / [S i] exceeds 10, hot-dip galvanized steel sheets with excellent surface quality can be produced. can be done.

[0026] Phosphorus (P): 0.01-0.04% Phosphorus has the greatest effect on solid solution strengthening in ultra-low carbon steel, and does not significantly impair drawability. It is an element that is effective in ensuring the strength of steel. In particular, the above-mentioned P easily segregates at the grain boundaries. This inhibits the growth of crystal grains during annealing, resulting in finer crystal grains and improving room temperature aging resistance. However, if the P content is less than 0.01%, it is impossible to ensure the desired strength. On the other hand, if it exceeds 0.04%, the surface after hot dip galvanizing will be affected by P segregation in the surface layer. High P content also causes linear defects such as knife marks in the hot dip galvanizing alloy. To slow the alloying process, the alloying temperature must be increased, which makes the plating layer brittle. However, there is a problem in that the amount of Fe-Zn intermetallic compounds (Γ) increases, resulting in poor powdering properties. Preferably, the P content is controlled to a range of 0.015 to 0.04%.

[0027] In the present invention, the content of the key element P is controlled to ensure bake hardenability and powdering resistance. To achieve this, other alloying elements and manufacturing conditions were optimized.

[0028] Sulfur (S): 0.01% or less (excluding 0%) Sulfur is an unavoidable impurity contained in steel, and its content should be kept as low as possible. In particular, sulfur in steel increases the possibility of generating red shortness, so its content is Control to 0.01% or less.

[0029] Nitrogen (N): 0.01% or less (excluding 0%) Nitrogen is an unavoidable impurity contained in steel, and its content should be kept as low as possible. However, this would pose a problem of a sharp rise in steel refining costs. Therefore, the content is controlled to 0.01% or less, which is the range that is possible under the operating conditions.

[0030] Aluminum (sol.Al): 0.01-0.06% Acid-soluble aluminum is an element added for grain refinement and deoxidation, and its content If the content is less than 0.01%, it is considered to be aluminum killed (Al-kill) under normal stable conditions. On the other hand, if its content exceeds 0.06%, the grain size becomes smaller. Although the refinement effect is advantageous for increasing strength, excessive inclusions are formed during continuous casting of steel. This not only increases the possibility of surface defects on plated steel sheets, but also increases the manufacturing cost. Therefore, in the present invention, the content of acid-soluble aluminum is is controlled to 0.01 to 0.06%.

[0031] Niobium (Nb): 0.003-0.015% Niobium combines with carbon in the steel during hot rolling and precipitates as NbC, forming solid solution carbon. This reduces the carbon content in the steel, which precipitates as NbC, and affects the bake hardenability and aging resistance. As the amount of carbon increases, the amount of carbon dissolved in the solid solution decreases. This is advantageous from the viewpoint of aging resistance, but it may cause problems when sintering. The control of solute carbon at an appropriate level ensures room temperature aging resistance. This is the premise for obtaining excellent bake hardenability, and it is an important element for controlling such solute carbon. The element is niobium.

[0032] When the niobium content is less than 0.003%, very little carbon precipitates as NbC. However, most of the carbon in the steel remains as solid solution, which is advantageous for bake hardenability. However, there is a problem that the room temperature aging resistance is poor, and there are limitations to its application to parts. If the content exceeds 0.015%, on the other hand, most of the C in the steel precipitates as NbC, Although the solute C content is advantageous for room temperature aging resistance, it is not sufficient for GA steel sheets. Therefore, it is not possible to ensure an Upper BH value of 30 MPa or more. In the present invention, it is preferable to control the Nb content to 0.003 to 0.015%. In general, the Nb content is controlled to be in the range of 0.0035 to 0.010%.

[0033] In addition, as described in relational expression 1 below, the atomic ratio of [Nb] / [C] is less than 0.55. In this case, excellent bake hardenability can be achieved without impairing aging resistance.

[0034] Boron (B): 0.0005 to 0.0035% Boron is a compound that prevents secondary work embrittlement due to grain boundary embrittlement in ultra-low carbon steels containing a large amount of phosphorus. B is an element added to prevent grain boundary segregation. The addition of boron suppresses P segregation at grain boundaries, preventing secondary work embrittlement. In addition, when boron is added within the range of the present invention, it plays a role in preventing baking. ) It may be possible to ensure bake hardenability by increasing the interaction with dislocations. If it exceeds 0.35%, peeling of the coating layer of the hot-dip galvanized steel sheet may occur. Therefore, it is preferable to limit its content to 0.0005 to 0.0035%. Preferably, the B content is controlled within the range of 0.0005 to 0.003%.

[0035] Chromium (Cr): 0.01-0.1% Chromium is an element added to improve the hardenability of steel and ensure high strength. This element plays a very important role in the formation of rutensite. In the case of ferrite, the P content is controlled to increase strength and ensure strength. If the amount exceeds the range of the present invention, problems with the corrosion resistance of bare plates may occur. This may result in excessive increases in the cost of ferroalloys, and therefore, it is considered that the scope of the present invention is More preferably, the Cr content is controlled to the range of 0.02 to 0.08%. .

[0036] Molybdenum (Mo): 0.005-0.05% Molybdenum, like chromium, is an element that improves the hardenability of steel, and is present in concentrations of 0.005 to 0. It is an element that can provide a high hardening effect when added in an amount of about 0.5%. In the case of , it is a ferrite stabilizing element, and when molybdenum is added within the range of the present invention, The formation of Mo-P compound during rolling reduces the P segregation zone, and the molten zinc It is possible to improve the surface quality of the plated steel sheet. When added in an amount exceeding the above range, the alloy It is added at the range level of the present invention because it excessively increases the iron cost. The o content should be controlled within the range of 0.01 to 0.04%.

[0037] Relational Formula 1 and Relational Formula 2 In the present invention, the atomic ratio defined by the following relational expression 1 is controlled to be less than 0.55. If the atomic ratio of [Nb] to [C] is 0.55 or more, it is called NbC. The amount of precipitates increases, and the solute C content is absolutely insufficient, so the required It may become difficult to maintain an Upper BH value of 30 MPa or more.

[0038] In the present invention, the atomic ratio of [Mn] to [Si] defined by the following relational expression 2 is 10 If the atomic ratio is less than 10, the surface Mn-S i The formation of complex oxides may deteriorate the surface quality of the steel sheet.

[0039] [Equation 1] [Nb] / ((93 / 12)×[C])<0.55

[0040] [Equation 2] [Mn] / ((55 / 28) × [Si])>10

[0041] The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, Since unintended impurities from the surrounding environment can be unavoidably mixed in, it is important to eliminate them. These impurities are obvious to any engineer in the normal manufacturing process. Therefore, the entire contents of the present specification will not be specifically mentioned.

[0042] On the other hand, the base steel sheet constituting the hot-dip galvanized steel sheet of the present invention has a microstructure consisting of a single ferrite phase. and satisfies the following relational expression 3.

[0043] [Equation 3] [TS]×[El.]×[Upper-BH]>500,000(MPa×%) However, [Upper-BH] is: after 2% pre-strain, baked at 170°C for 20 minutes. Upper Yield Stress when baking.

[0044] The above relational expression 3 is the relationship between the [Upper-BH] value, which is an index of bake hardenability, and basic tensile properties. This is a formula related to the overall index of tensile strength and elongation. If the relational expression 3 is 500,00 If it is below 0 (MPa × %), the normally required tensile strength, elongation, and U-BH value will be at the lower limit. exceeding the specified temperature, the physical properties may be inferior.

[0045] Next, the method for producing a hot-dip galvanized steel sheet having excellent powdering resistance and bake hardenability according to the present invention will be described. The law is explained in detail.

[0046] The method for producing a hot-dip galvanized steel sheet of the present invention comprises the steps of: Reheating the steel slab to be filled at a temperature of 1000 to 1250°C; hot rolling the slab to a temperature in the range of 900 to 1100 ° C; After cooling the plate to a temperature in the range of 500 to 700°C at an average cooling rate of 10 to 70°C / sec. , and the coiled hot-rolled steel sheet is cold-rolled at a reduction rate of 70 to 90%. and continuously annealing the cold-rolled steel sheet at a temperature in the range of 740 to 850°C. After immersion in a hot dip galvanizing bath, the material is alloyed at a temperature range of 450 to 540°C. a step of manufacturing a hot-dip galvanized steel sheet, and and performing temper rolling within a rolling reduction range of 0.0%, and the following relational expression 4 is satisfied.

[0047] First, a steel slab having the above-mentioned component system is reheated at a temperature of 1000 to 1250°C. This process is carried out in order to smoothly carry out the subsequent hot rolling process and to fully obtain the target physical properties of the steel sheet. If the reheating temperature is less than 1000°C, the slab inclusions and the like will not be sufficiently removed. This can cause variations in material quality and surface defects after hot rolling. At temperatures above 1250°C, the strength decreases due to abnormal grain growth of austenite grains. Therefore, it is preferable to limit the reheating temperature to 1000 to 1250°C.

[0048] Next, in the present invention, the reheated steel slab is heated to a temperature in the range of 900 to 1100°C. If hot rolling is started at a temperature higher than 1100°C, the hot rolled steel The temperature of the plate increases, the grain size becomes coarse, and the surface quality of the hot-rolled steel plate deteriorates. In addition, if hot rolling is completed at a temperature lower than 900°C, the recrystallization will be delayed excessively, resulting in poor elongation. As a result of the development of grains and a high yield ratio, the cold rolling property is poor and the shear workability is also poor. It becomes.

[0049] In the present invention, the hot-rolled steel sheet is subjected to a flattening treatment at a temperature in the range of 500 to 700°C. After cooling at an average cooling rate of 10 to 70°C / sec, the sheet is wound up.

[0050] If the steel sheet is cooled to less than 500°C and then coiled, the coiling temperature will be too low, resulting in the steel sheet shape becoming distorted. On the other hand, cooling above 700°C is expected to result in a deterioration in ductility due to the formation of fine crystal grains. When the wire is wound after being heated, coarse ferrite grains are formed, and coarse carbides and nitrides are formed. This makes it easier for cracks to form, which may result in inferior steel quality.

[0051] Furthermore, if the average cooling rate during the cooling is less than 10°C / sec, coarse ferrite crystals will form. When grains are formed, the microstructure becomes non-uniform, and the average cooling rate exceeds 70°C / sec, This not only causes distortion of the plate shape, but also makes the microstructure uneven in the thickness direction of the plate. This may result in poor shear workability of the steel.

[0052] Next, in the present invention, the coiled hot-rolled steel sheet is cold-rolled at a reduction ratio of 70 to 90%. After that, it is subjected to continuous annealing in the temperature range of 740 to 850°C.

[0053] In the present invention, when the coiled hot-rolled steel sheet is cold-rolled, the cold reduction ratio is 70 to 90 If the cold rolling reduction is less than 70%, the target thickness is On the other hand, it is difficult to ensure the required cold reduction ratio, which may make it difficult to correct the shape of the steel sheet. If the temperature exceeds this limit, cracks may occur at the edge of the steel plate, and the cooling Cold rolling loads may occur.

[0054] On the other hand, when rolling is performed using a rolling mill that is mainly composed of 5 to 6 stands, The first stand reduction is set to 20-40% to produce cold-rolled steel sheets. When the stand reduction rate is less than 20%, the low reduction rate limits the shape control of the hot rolled steel sheet. If it exceeds 40%, the initial stand reduction rate increases, which places a load on the equipment. It is more preferable that the initial stand reduction is 25 to 35%.

[0055] The cold-rolled steel sheet is then continuously annealed in a continuous annealing line including a plating line. The annealing temperature is preferably controlled within the range of 740 to 850°C. If the annealing temperature is less than 740°C, the structure during cold rolling will not undergo sufficient ferrite reforming. If crystallization is not completed and a duplex structure occurs, and if the temperature exceeds 850°C, the on-site setting due to excessively high temperature annealing will be This increases the possibility of equipment troubles and the crystal grains become too coarse. After continuous annealing, the steel is cooled under normal operating conditions.

[0056] Next, in the present invention, the continuously annealed cold rolled steel sheet is immersed in a hot dip galvanized bath, and hot-dip galvanized steel sheets are manufactured by alloying them in the temperature range of 450 to 540°C. .

[0057] In the case of hot-dip galvanized materials, the temperature is usually in the range of 440 to 480°C, which is the temperature of the hot-dip galvanized bath. It can be carried out under normal conditions.

[0058] After the hot dip galvanizing, it is preferable to perform alloying in the temperature range of 450 to 540°C. If the alloying temperature is lower than 450°C, the unplated areas of the entire width of the annealed steel sheet If the alloying temperature exceeds 540°C, embrittlement may occur due to excessive alloying. The powdering properties become poor due to the influence of the Fe-Zn intermetallic compound (Γ).

[0059] In the present invention, the temper rolling after the hot dip galvanizing is carried out in the range of 0.5 to 2.0%. If the temper rolling elongation is less than 0.5%, sufficient dislocations are not formed, and the sheet shape is not visible. On the other hand, if the content exceeds 2%, it is disadvantageous from the viewpoint of the surface defects of the plating. In this case, high loads may cause equipment loads, which may result in side effects such as plate breakage. There is a saying.

[0060] On the other hand, in the present invention, a bake hardenable hot-dip galvanized steel sheet having excellent powdering resistance is produced. In order to achieve this, the alloying temperature (GA) of hot dip galvanizing and the continuous hot dip It is preferable to control the temper rolling elongation (CGL SPM El.) of the galvanizing line. The following relational expression 3 is a regression relational expression between each variable and powdering resistance, and the When the defined value is less than 3, the plating quality can be excellent.

[0061] [Equation 4] Powdering=[-14.2+0.0362×(GA)-0.970×(CGL SPM El.)]<3 However, (GA) is the alloying temperature of hot dip galvanizing, and (CGL SPM El.) is the alloying temperature of continuous galvanizing. This is the temper rolling elongation rate of the hot dip galvanizing line. [Example]

[0062] The present invention will be described in more detail below with reference to examples.

[0063] (Example) A steel slab having the chemical composition of the alloying elements shown in Table 1 below was prepared. The lab was manufactured using a process for manufacturing ordinary bake-hardening cold-rolled steel sheets. Specifically, the reheating temperature of the steel slab is about 1100°C, and the hot rolling finishing temperature is Ar3 or higher. The hot-rolled steel sheet was cooled at a temperature of 900 to 940°C, and then at a cooling rate of 30 to 50°C / sec. After that, the hot-rolled sheet was coiled at 580 to 620°C. Cold-rolled steel sheets were produced by cold rolling at a cold rolling reduction of 0%.

[0064] The cold-rolled steel sheets that have been cold-rolled are annealed at the annealing temperatures shown in Table 2 below, and then rolled in the usual way. The cold-rolled steel sheet was then cooled under normal conditions to produce a hot-dip galvanized steel sheet. The molten zinc is immersed in a hot dip galvanizing pot maintained at a temperature of around 460°C. The plating was performed, and then alloying treatment was performed at different alloying temperatures as shown in Table 2 below. After hot dip galvanizing, the coated steel sheets were inspected under the conditions shown in Table 2 below. A temper rolling reduction was applied to produce the final hot-dip galvanized steel sheet.

[0065] For each of the hot-dip galvanized steel sheets manufactured above, the rolling perpendicularity was measured using the JIS standard. A tensile test was carried out in the direction of the tensile strength, and the yield strength (YP), tensile strength (TS) and elongation ( El.) was measured, and the results are shown in Table 2 below. Upper Yield after baking at 170°C for 20 minutes after straining The ld Stress [U-BH] was measured, and the results are shown in Table 2 below.

[0066] In addition, it is confirmed whether or not the above-mentioned relational expression 3-4 of the present invention is satisfied, and if it is satisfied, it is marked with ○. If it was not met, mark it with an X.

[0067] [Table 1] *In Table 1, the remaining components are Fe and unavoidable impurities.

[0068] [Table 2] *In Table 2, SS is the continuous annealing temperature, GA is the alloying temperature, and SPM El. is the preparation temperature. The relationship 3 is [TS] × [El.] × [Upper-BH]> 500,000 (MPa × %), and relation 4 is Powdering = [-14.2 +0.0362×(GA)-0.970×(CGL SPM El.)]<3 .

[0069] As shown in Table 1-2 above, the range of element content, process conditions and relationship 1-4 of the present invention are satisfied. In addition, all of the invention examples 1 to 7 have excellent surface quality and excellent powdering resistance. It has been shown that it is possible to manufacture bake-hardenable hot-dip galvanized steel sheets that ensure satisfactory tensile properties. There are.

[0070] In contrast, although the composition of the steel is within the range of the present invention, the manufacturing process conditions of the plated steel sheet However, in Comparative Examples 1 to 7, which are outside the scope of the present invention, the powdering resistance and mechanical properties are poor. Specifically, Comparative Examples 1, 4, and 5 did not satisfy Relational Expression 4. In the case where the alloying temperature is relatively high, the brittle Γ phase is formed in the alloyed plating layer. In addition, Comparative Examples 2-3 and 6-7 had poor SPM elongation. When the tensile strength is high (2% or more), the yield strength increases due to hardening caused by the formation of excessive mobile dislocations. However, the elongation rate is reduced, the BH increase is insufficient, and the Upper-BH value is poor. there was.

[0071] In addition, in Comparative Examples 8, 9, 10, and 12, the alloy The formation of brittle Γ phase in the plating layer basically results in poorer powdering resistance than the invention example. was inferior.

[0072] In addition, Comparative Example 9-14 has a high Nb / C atomic ratio, does not satisfy Relation 1, and Due to the lack of solute C, the U-BH value is poor, and Comparative Examples 9 and 13 do not satisfy Relation 3. There was no one there.

[0073] In Comparative Examples 8 and 10, B and Mo were not within the range of the present invention. did not satisfy the [Mn] and [Si] formulas in relational expression 2, and therefore the surface quality was poor.

[0074] In Comparative Example 15, the manufacturing conditions of the plated steel sheet were within the range of the present invention, but the composition of the steel was In the case where a steel sheet is used that does not satisfy the relational expression 2 in the composition, Mn-Si complex is formed during annealing. The surface quality of the plating was poor due to the formation of oxide composites.

[0075] On the other hand, FIG. 1 shows the hot-dip galvanized steel sheets of the above-mentioned Example 1 and Comparative Example 4. The BSE image (×7000) of the alloyed plating layer after plating and the results of EDS component analysis are shown. Figure.

[0076] When comparing Example 1 and Comparative Example 4, the basic composition is the same, but the difference in GA temperature The Fe content in the gold plating layer is different. When the results of the analysis of pt2 and pt3 were checked, it was found that in the case of Example 1, the Fe content was 13-14%. In the case of Comparative Example 4, the Fe content is at a level of 19 to 20%. When the amount is 7-12%, the FeZn7 delta intermetallic compound is formed, and when the Fe content is 17-20 %, Fe5Zn 21 The hardness of Γ is higher than that of δ, and the alloy The thicker the Γ phase in the plating layer, the poorer the powdering resistance becomes, and the more peeling occurs. That is, in the case of Comparative Example 4, the Γ phase of the alloyed plating layer is formed thicker than in Inventive Example 1. Therefore, it is clear that the powdering resistance is poor.

[0077] As mentioned above, the detailed description of the present invention describes the preferred embodiment of the present invention. However, a person having ordinary skill in the art to which the present invention pertains would be able to easily understand the above-mentioned invention from the perspective of the present invention. It goes without saying that various modifications are possible within the scope of the present invention. The scope of the invention should not be limited to the described embodiments, but should be understood as set forth in the following claims. It should be determined not only by the amount of the tax but also by something equivalent to it.

Claims

1. In weight percent, carbon (C): 0.0001 to 0.005%, manganese (Mn): 0.1 to 1 .2%, Silicon (Si): 0.02% or less (excluding 0%), Phosphorus (P): 0.01 to 0 .04%, sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less ( 0% or less), aluminum (sol. Al): 0.01 to 0.06%, niobium (Nb) : 0.003 to 0.015%, Boron (B): 0.0005 to 0.0035%, Chromium ( Cr): 0.01 to 0.1%, Molybdenum (Mo): 0.005 to 0.05%, Titanium ( Ti): 0.003% or less (excluding 0%), the balance being Fe and unavoidable impurities, and satisfying the following relationship: Formula 1-2 is satisfied, the microstructure is a ferrite single phase, and the following relational formula 3 is satisfied. Bake-hardenable galvannealed steel sheet with excellent resistance to wetting. [Relationship 1] [Nb] / ((93 / 12)×[C])<0.55 [Relationship 2] [Mn] / ((55 / 28)×[Si])>10 [Relationship 3] [TS]×[El. ]×[Upper-BH]>500,000 (MPa×%) However, [Upper-BH] is: after 2% pre-strain, baked at 170°C for 20 minutes. Upper Yield Stress during baking.

2. In weight percent, carbon (C): 0.0001 to 0.005%, manganese (Mn): 0.1 to 1 .2%, Silicon (Si): 0.02% or less (excluding 0%), Phosphorus (P): 0.01 to 0 .04%, sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less ( 0% or less), aluminum (sol. Al): 0.01 to 0.06%, niobium (Nb) : 0.003 to 0.015%, Boron (B): 0.0005 to 0.0035%, Chromium ( Cr): 0.01 to 0.1%, Molybdenum (Mo): 0.005 to 0.05%, Titanium ( Ti): 0.003% or less (excluding 0%), the balance being Fe and unavoidable impurities, and satisfying the following relationship: Reheating the steel slab satisfying Equation 1-2 at a temperature of 1000 to 1250°C; hot rolling the reheated steel slab to a temperature in the range of 900 to 1100°C; The hot-rolled steel sheet is cooled to a temperature in the range of 500 to 700°C at an average cooling rate of 10 to 70°C. Cooling at ° C. / sec and then winding; The coiled hot-rolled steel sheet is cold-rolled at a reduction of 70 to 90%, and then subjected to a rolling reduction of 740 to 850. continuous annealing in a temperature range of ° C.; The continuously annealed cold rolled steel sheet is immersed in a hot dip galvanizing bath and then subjected to galvanization at a temperature of 450 to 540°C. producing a hot-dip galvanized steel sheet by alloying in a temperature range; The hot-dip galvanized steel sheet produced above is subjected to temper rolling at a rolling reduction rate of 0.5 to 2.0%. and a bake hardenable alloying step of forming a powdered alloy having excellent powdering resistance, the step of forming a powdered alloy having excellent powdering resistance and satisfying the following relational expression 4: Manufacturing method for hot-dip galvanized steel sheet. [Relationship 1] [Nb] / ((93 / 12)×[C])<0.55 [Relationship 2] [Mn] / ((55 / 28)×[Si])>10 [Relationship 4] Powdering=[-14.2+0.0362×(GA)-0.970×(CGL SPM El. )] <3 where (GA) is the alloying temperature of hot dip galvanizing, and (CGL SPM El.) is the alloying temperature of continuous galvanizing. This is the temper rolling elongation rate of the hot dip galvanizing line.

3. The temper-rolled hot-dip galvanized steel sheet is characterized in that it satisfies the following relational expression 3: Manufacture of bake hardenable galvannealed steel sheet having excellent powdering resistance according to claim 2 method. [Relationship 3] [TS]×[El. ]×[Upper-BH]>500,000 (MPa×%) However, [Upper-BH] is: after 2% pre-strain, baked at 170°C for 20 minutes. Upper Yield Stress during baking.

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