High-strength hot-dip galvanized steel sheet with excellent formability and process for producing same

Abstract

Provided are a high-strength hot-dip galvanized steel sheet with excellent formability and a process for producing same. The high-strength hot-dip galvanized steel sheet comprises, in mass%: 0.005-0.009% of C; 0.05% or less of Si; 0.3-0.8% of Mn; 0.06-0.09% of P; 0.01% or less of S; 0.005% or less of N; 0.1% or less of Al; 0.05-0.08% of Mo; 0.01-0.03% of Ti; 0.03-0.045% of Nb; 0.06-0.1% of Cu; 0.0015% or less of B; and a balance of Fe and inevitable impurities, wherein C, Ti, and Nb satisfy relation 1, and alloy microstructures include 95% or more of ferrite in terms of area fraction, wherein the ferrite has an average grain size of 15 µm or less, with ultrafine grains of 6 µm or less occupying 5-10% in a 1 mm x 1 mm area, and has a surface nanohardness value of 1-1.5 GPa.

Description

High-strength zinc-plated steel sheet with excellent formability and manufacturing method thereof

[0001] The present invention relates to the production of ultra-high-strength, ultra-low-carbon steel plated steel sheets having excellent formability and for reducing the weight of automobiles, and more specifically, to a high-strength zinc-based plated steel sheet that can be preferably applied as an automobile exterior panel material and a method for producing the same.

[0002] Cold-rolled steel sheets processed through press forming and other methods are used as exterior panels for automobiles, and generally require high formability. Furthermore, with the aim of preventing global warming, new fuel economy targets have been established as carbon dioxide emission regulations, and preferential tax rates for low-emission vehicles have been introduced, increasing the demand for improved fuel efficiency. Reducing the weight of automobile bodies is an effective means of improving fuel efficiency, and from this perspective, slimmer steel sheets for automobile bodies are required. Meanwhile, to ensure safety, higher strength steel sheets for automobile bodies are required. To meet these requirements for slimness and strength, and to be pressed into complex shapes, zinc-plated high-strength steel sheets with excellent surface appearance and good press formability are in demand.

[0003] In order to improve the formability of automotive steel sheets, there is so-called IF steel (Interstitial Free Steel), which improves formability by adding Ti or Nb alone or in combination to ultra-low carbon cold-rolled steel sheets to precipitate solid-solution elements such as C, N, and S in the form of carbides and nitrides. Therefore, in the past, high purity was achieved during the steelmaking stage, and carbon-nitride-forming elements such as titanium were added to precipitate solid-solution elements, thereby limiting the aging phenomenon caused by solid-solution elements. In addition, in the case of high-strength steel sheets, solid-solution strengthening elements such as Si, Mn, and P are included in the steel to improve the strength of the steel sheet.

[0004] In particular, P is added to steel to increase the strength of the steel sheet, but P is an element that is very prone to segregation, and the P segregated on the surface of the slab is stretched in the longitudinal direction of the steel sheet by hot rolling and cold rolling, forming a P-concentrated layer on the coil surface. Since alloying is delayed in this P-concentrated layer during plating, this causes linear defects in the alloyed hot-dip galvanized steel sheet. To address this problem, a method for manufacturing an alloyed hot-dip galvanized steel sheet using a steel sheet having a P content of 0.03% or more as a base material has been proposed, in which the surface of the steel sheet is ground at a grinding amount according to the amount of P in the steel sheet in order to resolve the unevenness of the surface of the steel sheet, and the alloying treatment is performed in an alloying furnace using an induction heating method (Patent Document 1).

[0005] In these prior arts, in order to prevent linear defects in galvannealed steel sheets, for example, when using ultra-low carbon Ti-added steel sheets with a P content of 0.03% or more, the surface is scarfed (melted) by 3 mm or more during the continuous casting stage, and the surface is ground by 5 μm or more during the steel sheet stage before plating. This prevents the occurrence of shape defects after plating and secures surface quality, but this is the cause of a decrease in yield. Therefore, there is a growing need for the development of a method that can secure yield while simultaneously manufacturing high-formability, high-strength steel sheets with excellent surface appearance.

[0006] [Prior Art Literature]

[0007] (Patent Document 1) Japanese Patent Publication No. 2004-169160

[0008] The purpose of the present invention is to provide a high-strength hot-dip galvanized steel sheet having excellent formability and sharpness by controlling grain size distribution by adding P, Nb and Ti to ultra-low carbon steel applied to automobile exterior panels requiring formability, and a method for manufacturing the same.

[0009] Meanwhile, the objectives of the present invention are not limited to the above-described content. The objectives of the present invention can be understood from the overall content of this specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention.

[0010] One aspect of the present invention is

[0011] A steel sheet containing, in mass%, C: 0.005 to 0.009%, Si: 0.05% or less, Mn: 0.3 to 0.8%, P: 0.06 to 0.09%, S: 0.01% or less, N: 0.005% or less, S. Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.01 to 0.03%, Nb: 0.03 to 0.045%, Cu: 0.06 to 0.1%, B: 0.0015% or less, the remainder being Fe and unavoidable impurities, and wherein C, Ti and Nb satisfy the following relationship 1:

[0012] The alloy microstructure is a high-strength galvanized steel sheet having excellent formability, in which the area fraction of ferrite is 95% or more, the average crystal grain size of the ferrite is 15 ㎛ or less, and the ultrafine grains of 6 ㎛ or less have a ratio of 5 to 10% within an area of ​​1 mm × 1 mm, and the surface roughness value is 1 to 1.5 GPa.

[0013] [Relationship 1]

[0014] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.065

[0015]

[0016] The above hot-dip galvanized steel sheet may have a tensile strength of 440 MPa or more and an r value of 1.4 or more.

[0017]

[0018] Another aspect of the present invention is

[0019] A process of heating a steel slab satisfying the above composition to 1100 to 1300°C;

[0020] A process of manufacturing a hot-rolled steel plate by hot-rolling the above-mentioned heated steel slab to a finishing rolling temperature of 920 to 970°C and then coiling it at a temperature of 600 to 650°C;

[0021] A process for obtaining a cold rolled steel sheet by cold rolling the above-mentioned hot rolled steel sheet at a reduction ratio of 70 to 83% after pickling;

[0022] A process of annealing the above cold rolled steel sheet within a temperature range of 760 to 830°C and then performing hot-dip galvanizing; and

[0023] The present invention relates to a method for manufacturing a high-strength hot-dip galvanized steel sheet having excellent formability, including a process of alloying the hot-dip galvanized steel sheet at a temperature range of 500 to 560°C.

[0024]

[0025] For the above-mentioned alloyed heat-treated hot-dip galvanized steel sheet, a skin pass roll having a roughness (Ra) of 1.0 to 1.6 ㎛ can be used to perform temper rolling at a rate of 0.6 to 1.2%.

[0026] The hot-dip galvanized steel sheet of the present invention, having the above-described composition, exhibits excellent formability and high strength, and thus can be reliably utilized as a steel sheet for automobile exterior panels. Therefore, the scope of application of high-strength cold-rolled steel sheets containing P to automobile bodies can be expanded to previously unavailable applications, such as side outer panels, thereby further contributing to weight reduction of automobile bodies.

[0027] Figure 1 is a graph showing the correlation between the ratio of ultra-fine grains having an average grain size of 6 ㎛ or less and the surface roughness in an embodiment of the present invention.

[0028] Hereinafter, the present invention will be described.

[0029] The present inventors have conducted in-depth research to solve the problems of the above-mentioned prior art, and as a result, they have confirmed that by adding titanium (Ti) and / or niobium (Nb), which are strong carbonitride-forming elements in steel, to minimize the solid solution elements such as carbon (C), nitrogen (N), and sulfur (S), and thereby secure formability, and at the same time, by adding phosphorus (P), molybdenum (Mo), etc., it is possible to manufacture a high-strength, high-formability steel sheet for automobile exterior panels with excellent surface quality and a tensile strength of 440 MPa or higher, and have thus completed the present invention. In general, steel sheets for automobile exterior panels must satisfy not only high strength but also press formability such as deep drawability. Therefore, as the base material of the alloyed hot-dip galvanized steel sheet of the present invention, a high-strength steel sheet containing ultra-low carbon steel as a basic component and adding reinforcing elements such as manganese (Mn), phosphorus (P), etc., to improve workability was used.

[0030]

[0031] Therefore, the high-strength hot-dip galvanized steel sheet of the present invention having excellent formability prepared from this point of view is a steel sheet containing, in mass%, C: 0.005 to 0.009%, Si: 0.05% or less, Mn: 0.3 to 0.8%, P: 0.06 to 0.09%, S: 0.01% or less, N: 0.005% or less, S. Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.01 to 0.03%, Nb: 0.03 to 0.045%, Cu: 0.06 to 0.1%, B: 0.0015% or less, the remainder being Fe and unavoidable impurities, and C, Ti and Nb satisfying the following relational expression 1, wherein the alloy microstructure is a steel sheet in which the area fraction of ferrite is 95% or more, the average grain size of the ferrite is 15 ㎛ or less, Ultrafine particles of 6㎛ or less occupy a proportion of 5 to 10% within an area of ​​1 mm × 1 mm, and the surface roughness value is 1 to 1.5 GPa.

[0032] First, the alloy components of the cold-rolled steel sheet forming the base of the hot-dip galvanized steel sheet of the present invention and the reasons for limiting the content thereof will be explained. In addition, "%" herein means "weight %" unless otherwise specified.

[0033]

[0034] Carbon (C): 0.005~0.009%

[0035] C is an interstitial solid solution element that has a great influence on the formation of the texture of steel sheets during cold rolling and annealing processes, and for this purpose, it is necessary to add at least 0.005% or more. However, when the amount of dissolved carbon in steel increases, the growth of grains having a {111} gamma (γ)-fiber texture, which is advantageous for drawing processing, is suppressed, and the growth of grains having a {110} and {100} texture is promoted, thereby reducing the drawability of the annealed sheet. Furthermore, when the content of C exceeds 0.009%, the contents of Ti and Nb required to precipitate it as carbides increase, which is not only disadvantageous in terms of economic efficiency, but also may lower formability by forming pearlite, etc. Therefore, in the present invention, it is preferable to limit the content of C to the range of 0.005 to 0.009%.

[0036]

[0037] ·Silicon (Si): 0.05% or less (excluding 0%)

[0038] Si is an element that contributes to increased strength through solidification. If the Si content exceeds 0.05%, surface scale defects may occur, which may deteriorate the plating surface properties. Therefore, in the present invention, it is preferable to control the Si content to 0.05% or less.

[0039]

[0040] ·Manganese (Mn): 0.3~0.8%

[0041] Mn is a reinforcing element that not only contributes to increasing strength, but also plays a role in precipitating S in steel as MnS. If the content of Mn is less than 0.3%, there is concern about a decrease in strength, while if it exceeds 0.8%, surface problems due to oxides may occur. Therefore, it is desirable to limit the content of Mn to 0.3 to 0.8%.

[0042]

[0043] ·P: 0.06~0.09%

[0044] P is the most effective element for securing the strength of steel without significantly impairing the drawability and having the best employment effect. If the content of P is less than 0.06%, it is impossible to secure the desired strength, whereas if it exceeds 0.09%, secondary embrittlement and surface stripe defects due to P segregation may occur. Therefore, it is preferable to limit the content of P to the range of 0.06 to 0.09%.

[0045]

[0046] ·Molybdenum (Mo): 0.05~0.08%

[0047] Mo is an element with high affinity for P (phosphorus) and plays a role in suppressing P segregation. In order to secure high strength in ultra-low carbon steel, P must be utilized inevitably, and adding an appropriate amount of Mo can contribute to improving surface defects caused by P segregation to some extent. If the content of Mo is less than 0.05%, it is not very effective in improving the desired surface, and if it exceeds 0.08%, the price becomes high and cost competitiveness decreases, so it is desirable to limit the content of Mo to the range of 0.05 to 0.08%.

[0048]

[0049] Sulfur (S): 0.01% or less, Nitrogen (N): 0.005% or less

[0050] S and N are impurities that exist in steel and are inevitably added. In order to secure excellent welding properties, it is desirable to control their contents as low as possible. In the present invention, the content of S is controlled to 0.01% or less, and the content of N is preferably managed to 0.005% or less.

[0051]

[0052] · Aluminum (Al): 0.1% or less (excluding 0%)

[0053] Al contributes to improving the drawability and ductility of steel by precipitating AlN. However, if the Al content exceeds 0.1%, there is a problem of internal defects in the steel sheet due to excessive formation of Al inclusions during steelmaking operations. Therefore, it is preferable to control the Al content to 0.1% or less.

[0054]

[0055] Titanium (Ti): 0.01~0.03%

[0056] Ti is an element that greatly contributes to improving the drawability of steel sheets by reacting with dissolved carbon and dissolved nitrogen during hot rolling to precipitate Ti-based carbonitrides. If the Ti content is less than 0.01%, carbonitrides are not sufficiently precipitated, resulting in poor drawability. On the other hand, if it exceeds 0.03%, inclusion management during steelmaking can be difficult, resulting in inclusion defects. Therefore, it is preferable to limit the Ti content to the range of 0.01 to 0.03%.

[0057]

[0058] ·Niobium (Nb): 0.03~0.045%

[0059] Nitrogen is the most effective element for creating very fine grains through the rolling and cooling processes, as the austenite non-recrystallization region expands at high temperatures due to the solute drag and precipitate pinning effects of hot rolling. When the Nb content is less than 0.03%, the austenite non-recrystallization temperature range in the steel narrows, and the grain size refinement effect is minimal. On the other hand, when it exceeds 0.045%, there is a problem that high-temperature strength increases, making hot rolling difficult. Therefore, it is preferable to limit the Nb content to the range of 0.03 to 0.045%.

[0060]

[0061] ·Boron (B): 0.003% or less (excluding 0%)

[0062] B is an element added to prevent secondary processing embrittlement due to P addition to steel, but if its content exceeds 0.003%, it causes a decrease in ductility of the steel plate, so it is desirable to limit the B content to 0.003% or less.

[0063]

[0064] Copper (Cu): 0.04~0.1%

[0065] Cu is an element that is difficult to remove when adjusting the steel composition through steelmaking, and although it is contained in trace amounts (for example, 0.04% or more), if it exceeds 0.1%, it is easy for a shape to occur in a hot-dip galvanized steel sheet, and it also leads to grain boundary embrittlement and increased cost, so it is desirable to limit it to the range of 0.04 to 0.1%.

[0066]

[0067] ·Relationship 1

[0068] In the present invention, it is required to control the contents of C, Ti, and Nb so that the values ​​defined by the following relational expression 1 satisfy 0.05 to 0.065. The reason why relational expression 1 is set in the present invention is that the most effective elements for grain size refinement are Ti and Nb, and these two elements affect the recrystallization behavior in a solid solution state and / or in a precipitate state by combining with C. Therefore, in order to achieve the purpose pursued in the present invention, it is important to control the contents of C, Ti, and Nb.

[0069] If the value defined in the following relational expression 1 is less than 0.05, grain size refinement is not sufficiently achieved, so the desired strength cannot be secured, or surface stripe defects may occur due to the yield point phenomenon as the dissolved C increases. On the other hand, if it exceeds 0.065, there is a problem of loss of competitiveness in terms of cost as the amount of Ti and Nb elements added relatively increases.

[0070] [Relationship 1]

[0071] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.065

[0072] In addition, it contains residual Fe and unavoidable impurities. The addition of effective ingredients other than the above composition is not excluded.

[0073]

[0074] The present invention is a hot-dip galvanized steel sheet made of ultra-low carbon steel having a C content of 0.009% or less, so the microstructure is composed of a single-phase ferrite structure. However, since the single-phase ferrite structure may inevitably include other structures that are created, the alloy microstructure of the present invention has an area fraction of 95% or more of ferrite, and a trace amount of pearlite or the like may remain as residual components.

[0075] In addition, it is preferable that the average grain size of the microstructure crystal grains of the cold-rolled steel sheet, which is the base material of the molten zinc steel sheet of the present invention, is 15 µm or less. If the average grain size exceeds 15 µm or less, the strength aimed at by the present invention cannot be sufficiently secured.

[0076] Furthermore, it is preferable that the cold-rolled steel sheet of the present invention have an ultra-fine grain size of 6㎛ or less in a ratio of 5 to 10% within an area of ​​1mm x 1mm. By having such a ratio, a hot-dip galvanized steel sheet with excellent formability can be obtained. If the ratio is less than 5%, the strength aimed for by the present invention cannot be sufficiently secured, and if it exceeds 10%, the strength becomes too high and the elongation decreases, which causes a problem of poor formability.

[0077] In addition, in consideration of securing surface clarity, it is preferable to control the surface roughness hardness value of the cold-rolled steel sheet of the present invention to a range of 1 to 1.5 GPa.

[0078]

[0079] Next, the method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent formability according to the present invention is described.

[0080] The method for manufacturing a high-strength hot-dip galvanized steel sheet of the present invention comprises: a step of heating a steel slab satisfying the above composition to 1100 to 1300°C; a step of hot-rolling the heated steel slab so that the finishing rolling temperature becomes 920 to 970°C, and then coiling it at a temperature of 600 to 650°C to manufacture a hot-rolled steel sheet; a step of cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 70 to 83% after pickling, thereby obtaining a cold-rolled steel sheet; a step of annealing the cold-rolled steel sheet within a temperature range of 760 to 830°C, and then performing hot-dip galvanizing; and a step of alloying heat-treating the hot-dip galvanized steel sheet within a temperature range of 500 to 560°C.

[0081]

[0082] First, in the present invention, a steel slab having the above composition is heated at a temperature range of 1100 to 1300°C. If the heating temperature is lower than 1100°C, problems may arise in production due to the FM section rolling load, and if it exceeds 1300°C, problems such as surface scale defects may arise.

[0083]

[0084] Next, in the present invention, the heated steel slab is hot-rolled so that the finishing rolling temperature becomes 920 to 970°C, and then coiled at a temperature of 600 to 650°C to manufacture a hot-rolled steel sheet.

[0085] In the present invention, it is preferable to limit the finishing rolling temperature to 920 to 970°C. If the finishing rolling temperature is lower than 920°C, coarse grains may be formed on the surface, which may result in an uneven material quality. If the temperature exceeds 970°C, the grain size may not be sufficiently fine, which may ultimately result in a material deficiency problem.

[0086] In addition, in the present invention, it is preferable to manage the coiling temperature in the range of 600 to 650°C. If the coiling temperature is less than 600°C, precipitates such as Ti(Nb)C are not formed, and the dissolved Ti and Nb increase, which may cause fine precipitation as TiC and Ti(Nb)C during heating in the annealing process, or may exist in a solid solution state of Ti and Nb, which may have an effect of inhibiting recrystallization and grain growth, which may cause problems in securing the desired strength and elongation. On the other hand, if it exceeds 630°C, the problem of surface heat may occur due to the formation of secondary scale.

[0087]

[0088] And, in the present invention, after the hot-rolled steel sheet is subjected to a pickling process to remove surface scale, the cold-rolled steel sheet is manufactured by cold-rolling at a reduction ratio of 70 to 83%. If the cold reduction ratio is less than 70%, there is a problem that the {111} texture does not grow sufficiently, resulting in poor formability. On the other hand, if it exceeds 83%, there is a problem that the rolling roll load is very severe during on-site manufacturing, resulting in poor shape. Therefore, the reduction ratio is preferably limited to 70 to 83%, and more preferably limited to 74 to 80%.

[0089]

[0090] Subsequently, the cold-rolled steel sheet manufactured as described above is subjected to an annealing process and then subjected to hot-dip galvanizing or alloyed hot-dip galvanizing.

[0091] When annealing cold-rolled steel sheets, annealing should be performed at a temperature above the recrystallization temperature, within the temperature range of 760–830°C. Annealing above the recrystallization temperature removes deformation caused by rolling and softens the steel, improving workability.

[0092] The above annealed cold rolled steel sheet is directly hot-dip galvanized in a continuous hot-dip galvanizing line.

[0093] And in the present invention, alloying heat treatment can be performed on the manufactured hot-dip galvanized steel sheet. The alloying heat treatment is performed within the range of 500 to 560°C after hot-dip galvanizing. If the alloying heat treatment temperature is less than 500°C, alloying does not proceed sufficiently, whereas if it exceeds 560°C, alloying proceeds excessively and the plating layer becomes embrittled, which may cause problems such as plating peeling off during processing such as pressing.

[0094]

[0095] At this time, in the present invention, if necessary, the hot-dip galvanized steel sheet subjected to the alloying heat treatment can be subjected to temper rolling at a rate of 0.6 to 1.2% using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm.

[0096]

[0097] Hereinafter, the present invention will be described in detail through examples.

[0098] (Example)

[0099] A 250 mm thick steel slab having the alloy composition described in Table 1 below was reheated to 1250°C, and then hot-rolled, cold-rolled, continuously annealed, and alloyed hot-dip galvanized steel was manufactured under the conditions described in Table 2 below.

[0100] For each manufactured hot-dip galvanized steel sheet, the tensile properties, the r value (Lankford value), which is an indicator of deep drawing processing, grain size and distribution ratio, and surface nanohardness were measured. The measurement method is described below.

[0101] YS, TS, and T-El were measured in the tensile test. Here, YS, TS, and T-El represent yield strength, tensile strength, and fracture elongation, respectively. The tensile test was conducted using test specimens collected according to the JIS No. 5 standard. As a result of these measurements, a tensile strength of 440 MPa or higher was considered passing.

[0102] Meanwhile, for the evaluation of the r value, which is an indicator of deep drawing processing, JIS 5 tensile test pieces were taken from the galvanized steel sheet in three directions: parallel to the rolling direction, 45° direction, and perpendicular direction, and the r value of each test piece was measured. For example, to measure the r value, the change in plate thickness and the change in plate width at the time of performing a tensile strain of about 15% in the tensile test described above are measured, and the ratio of the change in plate width to the plate thickness is obtained. Then, the r value parallel to the rolling direction is r0, and the r value in the 45° direction is r 45 , the r value in the orthogonal direction is r 90 When , the r value in each direction was calculated using mathematical formula A.

[0103] [Mathematical Formula A]

[0104] A = r0+2*r 45 +r 90 / 4

[0105] Grain size and its distribution were evaluated using TSL OIM analysis software via EBSD measurements. Surface nanohardness was measured using a 500 nm deep indentation after pretreatment via surface electropolishing. A total of five observations were performed, and the average value should be 1 to 1.5 GPa.

[0106]

[0107] Steel alloy composition (weight%) relationship formula 1CSiPMoMnCuSTiNbB Inventive steel 10.00560.0250.080.050.60.080.00230.020.0390.00080.060 Inventive steel 20.00620.030.0780.0530.510.070.00340.0180.0410.00050.058 Inventive steel 30 .0070.0310.0810.05760.80.080.00210.0190.040.00060.059Inventive steel40.00580.0340.0750.0620.720.0690.0030.0170.0420.00070.056Inventive steel50.00660.0280.0770.0540.590.0780.0 0280.0210.0420.00060.064Inventive steel60.00840.0240.0810.080.50.10.00150.0190.0380.00090.059Comparative steel10.0030.050.0590.060.80.130.00540.0320.0350.0020.081Comparative steel20.0020.10. 040.010.50.020.00220.050.0210.0010.108Comparative Strength30.00560.030.080.040.720.050.00280.010.0460.0080.042Comparative Strength40.0080.0450.0780.0560.70.090.00340.0050.0320.0060.042

[0108] *In Table 1, Al and N were contained in the range of 0.02% and 0.0005%, respectively, in all steel grades, with the remainder being Fe and unavoidable impurities.

[0109]

[0110] Steel gradeFDT(℃)CT(℃)Cold reduction ratio(%)Annealing temperature(℃)GAAlloying temperature(℃)RemarksInventive steel 192562277770540Inventive example 193068072845530Comparative example 189063177800545Comparative example 2Inventive steel 293262077779535Inventive example 292562576835535Comparative example 388065065780540Comparative example 4Inventive steel 3940618 77782535Invention Example 3Invention Class 483061581768540Invention Example 4Invention Class 592062081780535Invention Example 5Invention Class 693362271790535Invention Example 6Comparative Class 184271072791550Comparative Example 5Comparative Class 292369070720580Comparative Example 6Comparative Class 391161870820610Comparative Example 7Comparative Class 493562077780535Comparative Example 8

[0111]

[0112]

[0113]

[0114] As shown in Table 1-3 above, it can be confirmed that Invention Example 1-6, which satisfies the scope of the present invention not only in terms of steel composition but also in the conditions of the manufacturing process of the plated steel sheet, exhibits excellent tensile properties, r-value, ultrafine grain ratio, and surface nano-hardness.

[0115]

[0116] In contrast, Comparative Examples 1-4 satisfy the scope of the present invention in terms of the stress component, but the manufacturing process of the plated steel sheet is outside the scope of the present invention.

[0117] Specifically, Comparative Examples 1 and 3 failed to secure the desired tensile strength and surface nano-hardness values ​​because the grain size was not sufficiently fine when the annealing temperature was high, 830°C or higher. In addition, Comparative Examples 2 and 4 failed to secure the desired surface nano-hardness values ​​because the grain size of the surface layer became large when the FDT (Finish Mill Delivery Temperature) in the hot rolling process was lower than the Ar3 temperature, resulting in a low fine grain ratio in the final annealed structure.

[0118] In addition, Comparative Examples 5-7, in which not only the steel composition but also the conditions for the manufacturing process of the coated steel sheet were outside the scope of the present invention, did not satisfy the ultrafine grain fraction and also did not meet the surface nanohardness value, so the desired strength could not be secured. In particular, it can be seen that Comparative Example 6 satisfied the ultrafine grain fraction and strength but did not satisfy the elongation and formability r value because the annealing temperature was too low and sufficient recrystallization did not occur.

[0119] In addition, Comparative Example 8 is a case where the relational expression 1 in the steel composition component is outside the scope of the present invention, and it can be confirmed that even when a plated steel sheet is manufactured using the plated steel sheet manufacturing process of the present invention, a sufficient fine particle fraction is not secured, and thus the desired surface roughness value is not secured.

[0120]

[0121] Meanwhile, Fig. 1 is a graph showing the correlation between the ratio of ultra-fine grains having an average grain size of 6 ㎛ or less and the surface roughness in an embodiment of the present invention.

[0122]

[0123] As described above, the detailed description of the present invention has described preferred embodiments of the present invention. However, it will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described below but also by equivalents thereof.

Claims

1. A steel sheet containing, in mass%, C: 0.005 to 0.009%, Si: 0.05% or less, Mn: 0.3 to 0.8%, P: 0.06 to 0.09%, S: 0.01% or less, N: 0.005% or less, S. Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.01 to 0.03%, Nb: 0.03 to 0.045%, Cu: 0.06 to 0.1%, B: 0.0015% or less, the remainder being Fe and unavoidable impurities, and wherein C, Ti and Nb satisfy the following relationship 1: A high-strength hot-dip galvanized steel sheet having excellent formability, wherein the alloy microstructure comprises an area fraction of 95% or more of ferrite, the average grain size of the ferrite is 15 ㎛ or less, and ultrafine grains of 6 ㎛ or less occupy a proportion of 5 to 10% within an area of ​​1 mm × 1 mm, and the surface roughness value is 1 to 1.5 GPa. [Relationship 1] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.065 2. In the first paragraph, the hot-dip galvanized steel sheet is a high-strength hot-dip galvanized steel sheet with excellent formability, characterized in that the tensile strength is 440 MPa or more and the r value is 1.4 or more.

3. A process for heating a steel slab, which contains, in mass%, C: 0.005 to 0.009%, Si: 0.05% or less, Mn: 0.3 to 0.8%, P: 0.06 to 0.09%, S: 0.01% or less, N: 0.005% or less, S. Al: 0.1% or less, Mo: 0.05 to 0.08%, Ti: 0.01 to 0.03%, Nb: 0.03 to 0.045%, Cu: 0.06 to 0.1%, B: 0.0015% or less, the remainder being Fe and unavoidable impurities, and in which C, Ti and Nb satisfy the following relationship 1, at 1100 to 1300°C; A process for manufacturing a hot-rolled steel sheet by hot-rolling the above-mentioned heated steel slab to a finishing rolling temperature of 920 to 970°C and then coiling it at a temperature of 600 to 650°C; A process for obtaining a cold rolled steel sheet by cold rolling the above-mentioned hot rolled steel sheet at a reduction ratio of 70 to 83% after pickling; A process of annealing the above cold rolled steel sheet within a temperature range of 760 to 830°C and then performing hot-dip galvanizing; and A method for manufacturing a high-strength hot-dip galvanized steel sheet having excellent formability, comprising: a process of alloying the hot-dip galvanized steel sheet at a temperature range of 500 to 560°C. [Relationship 1] 0.05≤[(Nb(48 / 93))+(Ti(93 / 48))+(C(12 / 48))]≤0.065 4. A method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent formability, characterized in that in the third paragraph, the hot-dip galvanized steel sheet subjected to the alloying heat treatment is subjected to a 0.6 to 1.2% temper rolling process using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 ㎛.