Steel sheet and manufacturing method thereof

Abstract

The present invention relates to a material used for vehicle interior panels, reinforcing materials, and the like and, more specifically, to a high-strength steel sheet having excellent material uniformity and a manufacturing method thereof.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a material used for automobile interior panels, reinforcing materials, etc., and more specifically, to a high-strength steel plate having excellent material uniformity and a method for manufacturing the same.

[0002] Recently, demand for lightweight vehicle bodies is increasing worldwide to meet stricter fuel efficiency regulations. This demand is driving demand for higher-strength steel plates, particularly HSLA (High Strength Low Alloy) steels, widely used for automotive interior panels. HSLA steels possess a high yield ratio, which is advantageous for crashworthiness, and a low Ceq, which is advantageous for welding. Typically, a maximum yield strength of 450 MPa is widely used.

[0003] HSLA steels with a yield strength of 450 MPa are mainly designed as precipitation-strengthened types, and in precipitation-strengthened steels, strength is secured by forming carbonitrides using precipitated elements such as Ti, Nb, and V in a matrix composed of ferrite and pearlite. These carbonitrides not only restrict the movement of dislocations to produce a precipitation-strengthening effect, but also delay recrystallization during annealing after cold rolling, leaving unrecrystallized ferrite in the final microstructure. If unrecrystallized ferrite is excessive, the yield strength increases, but it becomes a major cause of material deviation in the longitudinal or transverse direction of the coil.

[0004] Patent Document 1 discloses a steel sheet that utilizes precipitation-strengthening elements to delay the recrystallization of ferrite, thereby utilizing both recrystallized and transformed ferrite, along with non-recrystallized ferrite. While the presence of a large amount of non-recrystallized ferrite within the steel sheet is advantageous in terms of securing yield strength and yield ratio, it also suffers from the disadvantages of low elongation and large material variation. In the case of Patent Document 1, because non-recrystallized ferrite is utilized, the steel sheet is sensitive to processing conditions, resulting in significant material variation.

[0005] Patent Document 2 also utilizes unrecrystallized ferrite, and discloses a steel sheet with improved stretch flangeability through the use of soft ferrite and unrecrystallized ferrite. However, due to the inclusion of unrecrystallized ferrite, the aforementioned steel sheet suffers from material variation in different directions, and, most importantly, material non-uniformity along the coil length during coil manufacturing. In practice, material differences are evident at the top, middle, and bottom of the coil.

[0006] Meanwhile, in the case of steel containing unrecrystallized structures, the fraction of unrecrystallized structures formed during cooling varies depending on the cooling conditions, which causes a large material deviation depending on the location of the steel.

[0007] (Patent Document 1) Japanese Patent Publication No. 2008-190032

[0008] (Patent Document 2) Japanese Patent Publication No. 2008-106351

[0009] One aspect of the present invention relates to a steel sheet suitable for automobile interior panels, reinforcing materials, etc., and aims to provide a high-strength steel sheet having an excellent yield ratio and a small material deviation depending on the location of the steel sheet, and a method for manufacturing the same.

[0010] The objectives of the present invention are not limited to the above-described content. Anyone with ordinary skill in the art to which the present invention pertains will have no difficulty understanding the additional objectives of the present invention from the entire disclosure of the present invention.

[0011] According to one aspect of the present invention, a steel sheet is provided that includes, in wt%, carbon (C): 0.03 to 0.12%, manganese (Mn): 1.0 to 1.6%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.06%, niobium (Nb): 0.005 to 0.060%, titanium (Ti): 0.003 to 0.060%, the remainder being Fe and other unavoidable impurities.

[0012] In one embodiment of the present invention, the steel plate includes ferrite with an area fraction of 85% or more and a remainder of pearlite as a microstructure, and the ferrite may include unrecrystallized ferrite with an area fraction of 5% or less (including 0%) of the entire microstructure.

[0013] In this way, by appropriately controlling the alloy composition and microstructure of the steel plate, it is possible to provide a steel plate with an excellent yield ratio and excellent material deviation by steel location.

[0014] In one embodiment of the present invention, the ratio (b / a) of the average grain size (a) of the recrystallized ferrite and the average grain size (b) of the non-recrystallized ferrite may be 0.50 or less.

[0015] In one embodiment of the present invention, the unrecrystallized ferrite may have an aspect ratio (major axis / minor axis) of 3.0 or less.

[0016] In one embodiment of the present invention, the steel plate may further include boron (B): 0.003% or less.

[0017] In one embodiment of the present invention, the steel plate may have a yield strength of 380 MPa or more and a tensile strength of 440 MPa or more.

[0018] In one embodiment of the present invention, a plating layer may be included on at least one surface of the steel plate.

[0019] According to another aspect of the present invention, a method for manufacturing a steel sheet is provided, comprising the steps of: preparing a steel slab having the above-described alloy composition; heating the steel slab in a temperature range of 1100 to 1300°C; finishing hot-rolling the heated steel slab at 880°C or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet in a temperature range of 400 to 650°C; cold-rolling the coiled hot-rolled steel sheet at a cold reduction ratio of 40 to 80% after pickling to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet in a temperature range of 760 to 850°C; and cooling the annealed cold-rolled steel sheet to room temperature at a cooling rate of 3°C / s or higher.

[0020] In one embodiment of the present invention, the cold rolling and annealing heat treatment step satisfies the following relational expression 1, and when the cold rolling reduction ratio is less than 70%, the following relational expression 1 may be 864 or more.

[0021] [Relationship 1]

[0022] Cold rolling reduction + annealing temperature > 840

[0023] (In equation 1, units are not considered.)

[0024] In one embodiment of the present invention, a step of obtaining a hot-dip galvanized steel sheet by hot-dip galvanizing the cooled cold-rolled steel sheet at a temperature range of 440 to 480°C may be further included.

[0025] In one embodiment of the present invention, a step of optionally performing alloying heat treatment after the molten zinc plating may be further included.

[0026] According to the present invention, there is an effect of providing a steel sheet suitable as an automobile material, particularly as an inner plate material, by having excellent material deviation of the steel and high yield ratio characteristics.

[0027] Figure 1 shows the EBSD measurement results of an invention example according to one embodiment.

[0028] Figure 2 shows the EBSD measurement results of a comparative example according to one embodiment.

[0029] Figure 3 shows SEM images of an inventive example and a comparative example according to one embodiment.

[0030] The inventors of the present invention have conducted in-depth research to obtain steel having excellent material uniformity, that is, minimizing material deviation by location of the steel, in addition to generally required physical properties such as strength, in order to provide a steel suitable as an interior panel material for automobiles.

[0031] As a result, it was confirmed that the material uniformity of the steel plate can be improved by suppressing the formation of unrecrystallized ferrite when including ferrite in the microstructure of the steel plate, and the present invention was completed.

[0032] In particular, in suppressing the formation of unrecrystallized ferrite among the microstructures of steel plates, there is technical significance in optimizing the manufacturing conditions, especially the cold rolling and annealing heat treatment processes, along with the alloy composition.

[0033] Hereinafter, the present invention will be described in detail.

[0034] A steel sheet according to one aspect of the present invention may contain, in wt%, carbon (C): 0.03 to 0.12%, manganese (Mn): 1.0 to 1.6%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.06%, niobium (Nb): 0.005 to 0.060%, and titanium (Ti): 0.003 to 0.060%.

[0035] Hereinafter, the reason for limiting the alloy composition of the steel plate according to one aspect of the present invention as described above will be described in detail.

[0036] Meanwhile, unless otherwise specified, the content of each element is based on weight, and the ratio of the tissue is based on area.

[0037] Carbon (C): 0.03~0.12%

[0038] Carbon (C) is an interstitial solid-solution element that effectively contributes to securing the strength of steel. In addition, it is an important element for the formation of pearlite and Ti-based and / or Nb-based precipitates in steel, and it is necessary to add C above a certain level to realize the microstructure targeted in the present invention. In particular, in terms of securing the strength targeted in the present invention, C may be included in an amount of 0.03% or more. However, if the content is excessive, it may cause a decrease in elongation, which increases the possibility of causing bending defects, etc., on the surface of the product during subsequent processing of the part. Therefore, C may be included in an amount of 0.12% or less.

[0039] In this way, the C may be included in an amount of 0.03% or more, more preferably 0.04% or more, and even more preferably 0.05% or more. In addition, the C may be included in an amount of 0.12% or less, more preferably 0.11% or less, and even more preferably 0.10% or less. Manganese (Mn): 1.0 to 1.6%

[0040] Manganese (Mn) not only contributes to increasing strength as a solid solution strengthening element, but also has the effect of suppressing brittleness caused by S by precipitating S in the steel as MnS. To achieve this effect, the Mn may be included in an amount of 1.0% or more. However, if the content of the Mn is excessive, problems such as surface defects may occur due to the formation of annealing oxides, and there is a concern that the elongation may decrease, resulting in poor workability. Therefore, the Mn may be included in an amount of 1.6% or less.

[0041] In this way, the Mn may be included at 1.0% or more, more preferably 1.1% or more, and even more preferably 1.15% or more. In addition, the Mn may be included at 1.6% or less, more preferably 1.5% or less, and even more preferably 1.45% or less.

[0042] Silicon (Si): 0.6% or less (excluding 0%)

[0043] Silicon (Si) is an element that contributes to increasing the strength of steel through solid solution strengthening. However, in the present invention, since the intended properties can be secured even without adding Si, Si is not intentionally added.

[0044] In the present invention, if the content of Si exceeds 0.6%, there is a problem of deteriorating the surface properties of the plating, so the upper limit may be limited to 0.6%. More advantageously, it may be 0.5% or less, and even more advantageously, 0.4% or less. Meanwhile, considering the level of Si that is inevitably introduced during the steel manufacturing process, 0% may be excluded.

[0045] Phosphorus (P): 0.03% or less (excluding 0%)

[0046] Phosphorus (P) is the most effective element for securing the strength of steel through solid solution strengthening without significantly impairing the steel's drawability. However, if P is added excessively, the possibility of brittle fracture increases, which not only causes slab fracture during hot rolling, but also significantly deteriorates the surface properties of the coated steel sheet. Therefore, the P content may be limited to 0.03% or less. However, considering the level that is inevitably introduced during the steel manufacturing process, 0% may be excluded.

[0047] Sulfur (S): 0.01% or less (excluding 0%)

[0048] Sulfur (S) is an impurity that is inevitably added to steel. To ensure excellent weldability, it is advantageous to keep the S content as low as possible. In particular, S present in steel can cause red-hot embrittlement, so its content may be limited to 0.01% or less. However, considering the level that is inevitably introduced during the steel manufacturing process, 0% may be excluded.

[0049] Nitrogen (N): 0.01% or less (excluding 0%)

[0050] Nitrogen (N) is also an unavoidable impurity added to steel, so it is advantageous to keep its content as low as possible. However, considering factors such as steelmaking load and operating conditions, its content can be limited to 0.01% or less. Meanwhile, considering the level of N that is inevitably introduced during the steelmaking process, 0% can be excluded.

[0051] Aluminum (Al): 0.01~0.06%

[0052] Aluminum (Al) is an element added to achieve grain refinement and deoxidation effects. In particular, in the present invention, it is preferable to include Al in an amount of 0.01% or more to obtain a stable Al-killed steel. While excessive addition of Al is beneficial for increasing strength through grain refinement, it may increase the possibility of excessive inclusion formation during steelmaking and continuous casting, which may deteriorate the surface quality of the steel sheet and lead to an increase in manufacturing costs. Therefore, Al may be included in an amount of 0.06% or less.

[0053] In this way, the Al may be included in an amount of 0.01% or more, more preferably 0.015% or more. In addition, the Al may be included in an amount of 0.06% or less, more preferably 0.055% or less.

[0054] Niobium (Nb): 0.005–0.060%

[0055] Niobium (Nb) is an element that forms carbonitrides in steel. Carbonitrides formed in steel not only restrict dislocation movement and produce a precipitation strengthening effect, but also delay recrystallization during annealing after cold rolling, leaving unrecrystallized ferrite in the final microstructure. Therefore, in the present invention, it is desirable to limit the Nb content to secure the intended strength while preventing the recrystallization delay effect from becoming excessive.

[0056] If the content of Nb is less than 0.005%, the precipitation strengthening effect is insufficient, making it impossible to secure the target strength. On the other hand, if the content exceeds 0.060%, not only will precipitation hardening become excessive, but the recrystallization delay effect will also increase, raising concerns that a large amount of unrecrystallized ferrite may exist.

[0057] Titanium (Ti): 0.003~0.060%

[0058] Titanium (Ti) is also an element that exhibits the same or similar effects as the above-mentioned Nb. If the content of Ti exceeds 0.060%, not only will precipitation hardening become excessive, but the recrystallization delay effect will also increase, raising concerns about the presence of a large amount of unrecrystallized ferrite. On the other hand, if the content is less than 0.003%, it becomes difficult to obtain the precipitation strengthening effect.

[0059] Meanwhile, the steel plate of the present invention may further include boron in addition to the above-described alloy composition.

[0060] Boron (B): 0.003% or less

[0061] Boron (B) is a precursor that facilitates grain boundary segregation, and can be added to prevent secondary processing embrittlement that may occur when a certain amount of P is added. In addition, in composite phase steel, the hardenability is greatly improved even with a small amount of B, which is advantageous for securing strength. To achieve this effect, B can be added. However, if the content exceeds 0.003%, there is a problem that it causes a decrease in elongation. Therefore, when adding B, the content can be limited to 0.003% or less.

[0062] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0063] According to one embodiment of the present invention, the steel sheet may include a ferrite phase as a main phase in its microstructure and a pearlite phase as a secondary phase. However, the steel sheet does not exclude other phases that are inevitably formed during the steel manufacturing process in addition to the aforementioned phases as its microstructure. As one example, the steel sheet may include a hard phase such as a bainite phase. Here, the microstructure fraction of the steel sheet may be a result measured based on the entire thickness of the steel sheet.

[0064] More specifically, the steel plate may include ferrite with an area fraction of 85% or more and a remainder of pearlite as a microstructure. If the ferrite fraction is less than 85% or the pearlite fraction exceeds 15%, the strength of the steel plate may increase excessively and the elongation may decrease significantly, which may result in poor processing characteristics.

[0065] As the fraction of ferrite decreases, the fraction of hard phases such as pearlite increases relatively, which can be advantageous in implementing a composite structure. However, as the fraction of hard phases such as pearlite increases, the yield strength and yield ratio of the steel sheet inevitably increase, which increases the possibility of surface waviness defects occurring during subsequent processing into parts. Therefore, in the present invention, the fraction of ferrite is limited to 85% or more based on the total thickness of the steel sheet.

[0066] A steel sheet according to one embodiment of the present invention includes a ferrite phase, mainly including a recrystallized ferrite phase, and preferably, the fraction of unrecrystallized ferrite among the ferrite may be 5 area% or less of the entire microstructure.

[0067] That is, in one embodiment of the present invention, by suppressing the formation of unrecrystallized ferrite during the process of manufacturing a steel sheet, the material uniformity of the manufactured steel sheet can be secured. In particular, the effect of minimizing material deviation in the longitudinal direction (rolling direction) can be obtained.

[0068] More preferably, the fraction of the unrecrystallized ferrite may be less than 5 area%, more advantageously less than 4 area%, even more advantageously less than 3 area%, and also includes 0 area%.

[0069] In one embodiment of the present invention, when unrecrystallized ferrite is included in the ferrite, it has the characteristic of being fine in size. In particular, when compared to the size of the recrystallized ferrite, the ratio (b / a) of the average grain size (a) of the recrystallized ferrite and the average grain size (b) of the unrecrystallized ferrite may be 0.50 or less. In this way, unrecrystallized ferrite having a fine size can be more advantageous in securing the intended physical properties.

[0070] In addition, the non-recrystallized ferrite may have an aspect ratio of 3.0 or less. If the aspect ratio of the non-recrystallized ferrite exceeds 3.0, the formation of elongated ferrite may have a negative impact on securing physical properties. Here, the aspect ratio represents the value obtained by dividing the major axis by the minor axis when the crystal grain size is converted to an equivalent diameter (major axis / minor axis). At this time, the closer the value is to 1.0, the more it indicates a spherical shape.

[0071] In one embodiment of the present invention, the steel plate may have high strength characteristics, and more preferably, may have characteristics such as a yield strength of 380 MPa or more and a tensile strength of 440 MPa or more. In particular, the tensile strength may be 500 MPa or more.

[0072] In addition, the steel plate has the characteristic of excellent material uniformity, and specifically, has the effect of minimizing material deviation in the longitudinal direction (rolling direction). In particular, it can have the characteristic of having a strength difference in the longitudinal direction, and a strength difference between the top, middle, and bottom parts of the coil of 30 MPa or less.

[0073] As will be described in detail later, a steel sheet according to one embodiment of the present invention is a cold-rolled steel sheet manufactured through a series of rolling processes, and may also include a plated steel sheet including a plated layer on at least one surface of the cold-rolled steel sheet.

[0074] Hereinafter, a method for manufacturing a steel plate according to another aspect of the present invention will be described.

[0075] According to one embodiment of the present invention, a desired steel plate can be manufactured through the process of [steel slab heating - hot rolling - coiling - cold rolling - annealing heat treatment], and the conditions for each step are described in detail below. However, it should be noted that the manufacturing process described below is only one example for manufacturing the steel plate of the present invention.

[0076] [Heating of steel slabs]

[0077] After preparing a steel slab having the above-described alloy composition, the steel slab can be heated in a temperature range of 1100 to 1300°C.

[0078] The heating process of the above steel slab is a process for smoothly performing the hot rolling process described later. If the heating temperature is less than 1100°C, there is a concern that a rolling load may occur in the subsequent hot rolling process, and on the other hand, if the temperature exceeds 1300°C, there is a concern that surface scale defects may occur.

[0079] [Hot rolling]

[0080] A hot-rolled steel sheet can be obtained by hot-rolling the steel slab heated as described above, and at this time, finishing hot rolling can be performed at 880°C or higher. If the temperature during the finishing hot rolling is lower than 880°C, abnormal rolling may occur, which may cause uneven texture.

[0081] [Winding]

[0082] The above hot-rolled steel sheet can be coiled. The coiling can be performed at a temperature range of 400 to 650°C.

[0083] While minimizing the grain size during the above-described coiling process and achieving precipitation strengthening during the subsequent annealing process requires a low temperature, a temperature below 400°C can lead to a water film forming on the coil surface, making it difficult to maintain temperature uniformity. Conversely, if the temperature exceeds 650°C, secondary scale formation can occur, potentially degrading the steel plate surface.

[0084] [Cold rolling]

[0085] The above-mentioned hot-rolled steel sheet can be uncoiled and cold-rolled to obtain a cold-rolled steel sheet. At this time, the reduction ratio can be controlled so as to obtain the desired thickness, and in the present invention, it can be controlled to 40 to 80%.

[0086] If the reduction ratio during the above cold rolling is less than 40%, the recrystallization driving force by cold rolling is insufficient, so that the ferrite recrystallization is not completed, and there is a problem that an excessive amount of unrecrystallized ferrite structure remains. On the other hand, if it exceeds 80%, the load on the rolling roll becomes very severe, resulting in a poor shape.

[0087] More advantageously, the cold rolling can be performed at a cold rolling reduction ratio of 50% or more and 70% or less.

[0088] In one embodiment of the present invention, prior to performing the cold rolling, a pickling process may be additionally performed for the purpose of removing surface scale from the coiled hot-rolled steel sheet. The pickling process may be performed under conventional conditions, and the present invention does not specifically limit the conditions.

[0089] [Annealing heat treatment]

[0090] The above cold rolled steel sheet can be annealed and heat treated.

[0091] The above annealing heat treatment can be performed at a temperature range of 760 to 850°C to ensure sufficient recrystallization of the steel sheet. If the temperature is lower than 760°C during the above annealing heat treatment, there is a risk that excessive unrecrystallized ferrite will remain after the annealing is completed. On the other hand, if the temperature exceeds 850°C, the grains will become coarser, making it impossible to secure the target level of strength.

[0092] Meanwhile, when the reduction ratio is low in the cold rolling process performed previously, especially when a reduction ratio of 40% or more but less than 70% is applied, the recrystallization driving force is relatively low, so the temperature can be set to 800°C or higher, more preferably 830°C or higher, so that recrystallization can sufficiently occur in the subsequent annealing heat treatment process.

[0093] In particular, according to one embodiment of the present invention, when performing the cold rolling and annealing heat treatment process, the following [Relationship 1] is satisfied, but when the reduction ratio during the cold rolling is less than 70%, it is preferable that the following Relationship 1 is 864 or more.

[0094] [Relationship 1]

[0095] Cold rolling reduction + annealing temperature > 840

[0096] (In equation 1, units are not considered.)

[0097] [cooling]

[0098] The cold-rolled steel sheet, which has undergone annealing heat treatment as described above, can be cooled to room temperature. This can be accomplished at a cooling rate of 3°C / s or higher. If the cooling rate is less than 3°C / s, there is a risk that a hard phase (low-temperature transformation phase) will form during cooling, resulting in an excessively high fraction.

[0099] There is no specific limitation on the upper limit of the above cooling speed, and it is noted that the upper limit can be appropriately set according to the equipment specifications.

[0100] The intended steel sheet can be obtained through the aforementioned series of processes. In particular, sufficient recrystallization can be achieved during the cold rolling and annealing heat treatment processes, resulting in a steel sheet with a minimized fraction of unrecrystallized ferrite. As a result, the steel sheet of the present invention can possess both high strength and excellent material uniformity.

[0101] Meanwhile, a plated steel sheet can be manufactured by performing a plating process on the cold-rolled steel sheet manufactured as described above. The plating process in this case is a hot-dip galvanizing process, and is described in detail below.

[0102] [Hot-dip galvanizing]

[0103] By performing hot-dip galvanization on the cold-rolled steel sheet subjected to the above annealing treatment in a continuous hot-dip galvanization line, a galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the cold-rolled steel sheet can be obtained.

[0104] The above hot-dip galvanizing can be performed by immersing the annealed cold-rolled steel sheet in a plating bath containing zinc as the main component. The components in the plating bath are not particularly limited and can be performed under normal conditions. As an example, the above hot-dip galvanizing can be performed at a temperature range of 440 to 480°C.

[0105] After performing the above-mentioned hot-dip galvanizing, an alloying heat treatment process can be additionally performed, and this can also be performed under normal conditions.

[0106] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended solely to illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0107] (Example)

[0108] Traditional example

[0109] After manufacturing cold-rolled steel sheets under conventional process conditions, the mechanical properties of each cold-rolled steel sheet were evaluated. At this time, steel slabs of the C grade shown in Table 2 below were prepared, and cold-rolled steel sheets were manufactured by applying the reheating temperature, coiling temperature, cold reduction ratio, annealing temperature, and cooling rate shown in Table 1 below to these steel slabs. Meanwhile, after reheating the steel slabs, the finishing hot rolling was performed at 900°C during hot rolling.

[0110] The yield strength and tensile strength of each manufactured cold-rolled steel sheet were measured, and the measurement method was applied in the same manner as shown below.

[0111] Classification Process Conditions Mechanical Properties Reheating Temperature (℃) Coiling Temperature (℃) Cold Reduction Ratio (%) Annealing Temperature (℃) Cooling Speed ​​(℃) Yield Strength (MPa) Tensile Strength (MPa) Conventional Example 11 200 450 60 770 35 34 598 Conventional Example 21 200 60 0 770 34 9 0 567 Conventional Example 31 250 550 60 770 35 34 663 Conventional Example 41 200 450 50 770 35 76 643 Conventional Example 51 250 560 50 770 35 6 5 42 Conventional Example 61 150 500 50 770 35 6 2 635

[0112]

[0113] As shown in Table 1 above, the yield strength tends to increase as the reheating temperature increases and the coiling temperature decreases. This is because precipitates are minimized during the hot rolling process and finely distributed during the cold rolling and annealing processes. Furthermore, since annealing is performed at a relatively low temperature, the remaining unrecrystallized structure generally results in high strength. Therefore, it is expected that the conventional examples 1 to 6 will cause longitudinal material deviation in the coil state.

[0114]

[0115] Invention examples and comparative examples

[0116] After preparing slabs having the alloy compositions shown in Table 2 below, each slab was reheated to 1200°C, hot-rolled at a finishing rolling temperature of 900 to 950°C, and coiled to obtain hot-rolled steel sheets. Subsequently, cold-rolled at a reduction ratio of 54%, and then continuously annealed to produce cold-rolled steel sheets. At this time, some cold-rolled steel sheets were subjected to hot-dip galvanizing to produce coated steel sheets. The process conditions are shown in Table 3 below.

[0117] For the cold-rolled steel sheets and coated steel sheets manufactured as described above, a tensile test was performed in the rolling direction according to JIS-5 standards, and the properties of yield strength (YS), tensile strength (TS), and elongation (El) were measured.

[0118] In addition, to measure the microstructure of each steel plate, specimens obtained by JIS-5 were observed using an optical microscope and a scanning electron microscope (SEM) to measure the fraction of each phase, and among these, the fraction of unrecrystallized ferrite was measured using EBSD (Electron Back Scattering Diffraction).

[0119] Then, the average size of ferrite was calculated by calculating the average grain size (a) of recrystallized ferrite and the average grain size (b) of non-recrystallized ferrite, and then calculating the value of b / a. At this time, the grain size distribution was measured using an application program based on the results measured using SEM, and then the average value was calculated. Each result is shown in Table 4 below.

[0120] Steel alloy composition (weight %) CSiMnAlPSNNbTiA0.0850.211.100.0480.0190.0020.0030.0360.005 B0.0800.201.090.0470.0190.0020.0030.0340.005 C0.0820.301.350.0430.0190.0020.004 0.0400.005D0.0820.301.340.0390.0190.0020.0040.0410.005E0.1250.021.350.0 380.0200.0020.0030.0450.045F0.1220.011.270.0370.0200.0020.0050.0440.048

[0121] Steel grade heating temperature (℃) Coiling temperature (℃) Cold reduction ratio (%) Annealing temperature (℃) Relationship equation 1 Cooling speed (℃ / s) Classification A1 200 56 0 54 8 10 8 6 4 3 Invention example 1 B1 2 2 0 4 5 0 5 4 8 10 8 6 4 3 Invention example 2 C1 2 0 0 5 6 0 5 4 8 30 8 8 4 3 Invention example 3 D1 2 2 0 4 5 0 5 4 8 20 8 7 4 3 Invention example 4 A1 1 8 0 5 6 0 5 4 8 30 8 8 4 3 Invention example 5 E1 2 0 0 5 6 0 5 5 8 10 8 6 5 3 Comparative example 1 F1 2 0 0 4 5 0 5 5 8 10 8 6 5 3 Comparative example 2 A1 2 0 0 5 6 0 5 4 7 8 0 8 3 4 3 Comparative example 3 C1 2 0 0 5 6 0 5 4 7 7 0 8 2 4 3 Comparative example 4

[0122] Classification Microstructure Mechanical composition (area %) F Average size Tensile strength (MPa) Yield strength (MPa) F P Non-recrystallized F Recrystallized F (a) Non-recrystallized F (b) b / a Top part Middle part Bottom part Top part Middle part Bottom part Invention example 19550~0.96.52.00.31534542559431445421 Invention example 29460~1.36.42.10.33531538554434441428 Invention example 39460~2.16.22.40.39556550565452450443 Invention example 49370~2.45.82.60.45556555557460458462 Invention example 59550~0.76.51.90.29536528532404398386Comparative Example 1831740~605.34.74.36.30.811.34658615681572530613Comparative Example 2821840~605.44.84.36.10.801.27667632672580545592Comparative Example 394610~405.85.43.64.30.620.80552528559453415469Comparative Example 493730~505.45.34.16.00.761.13592558560487454456F: Ferrite, P: Pearlite In Table 4, ferrite refers to the total ferrite fraction including unrecrystallized ferrite. In addition, since the fraction of unrecrystallized ferrite varies depending on the measurement location (Top, Middle, Bottom), it is written as a range based on the minimum fraction and the maximum fraction. The average size of ferrite (F) refers to the average grain size of recrystallized ferrite (a) and the average grain size of unrecrystallized ferrite (b), and their ratio (b / a), and the unit of each size is ㎛. At this time, since the comparative examples showed a large difference in the unrecrystallized fraction of the Top and Middle parts, the average size of ferrite of each part (top: Top, bottom: Middle) was calculated and shown.

[0123]

[0124] As shown in Tables 2 to 4 above, Inventive Examples 1 to 5, which satisfy both the alloy composition and manufacturing conditions proposed in the present invention, contain a ferrite phase as the main phase in the microstructure, while minimizing the fraction of unrecrystallized ferrite to 5% or less. In particular, it can be confirmed that the unrecrystallized ferrite is formed in a fine size. As a result, the longitudinal material uniformity of the steel sheet was excellent, and the target level of strength was also secured.

[0125] However, Comparative Examples 1 and 2, whose alloy compositions deviate from the present invention, and Comparative Examples 3 and 4, whose alloy compositions satisfy the present invention but whose manufacturing conditions deviate from Equation 1, had an excessive fraction of unrecrystallized ferrite phase and a large size, making it impossible to secure longitudinal material uniformity of the steel sheet. In particular, based on the strength value of the middle portion, there was a part where the strength difference exceeded 30 MPa and reached a maximum of 54 MPa. That is, as expected in the prior art examples, it can be confirmed that longitudinal material deviation occurs in Comparative Examples 3 and 4, which substantially deviate from Equation 1.

[0126] Meanwhile, although not described, the aspect ratio of the unrecrystallized ferrite of invention examples 1 to 5 was 3.0 or less, which can be confirmed through the microstructure photograph of invention example 4 shown in Fig. 1.

[0127] On the other hand, Comparative Examples 1 to 4 contain a large amount of unrecrystallized ferrite having an aspect ratio exceeding 3.0, as can be confirmed through the microstructure photograph of Comparative Example 3 shown in Fig. 2.

[0128]

[0129] Figure 1 is an EBSD result observing the microstructure of Invention Example 4, and it can be confirmed that recrystallization is almost complete in both the middle part (a) and the top part (b).

[0130] From this, as shown in Table 3 above, the strength measurements of specimens taken from the top and middle sections showed that the yield strength and tensile strength were almost uniform. In addition, it can be confirmed that the aspect ratio of the unrecrystallized ferrite was 3.0 or less.

[0131] Figure 2 shows the EBSD results observing the microstructure of Comparative Example 3. It can be confirmed that both the middle part (a) and the top part (b) were not completely recrystallized and a large amount of unrecrystallized structure remained. In particular, the unrecrystallized fraction was measured to be 10.9% in the middle part, and 32.9% in the top part. In addition, it can be confirmed that the aspect ratio of the unrecrystallized ferrite exceeds 3.0.

[0132] Thus, due to differences in the unrecrystallized fraction at different locations in the coil, yield strength differences occurred, amounting to approximately 38 MPa. At this time, localized inhomogeneity occurred as yield strength increased in areas with a greater unrecrystallized fraction, resulting in greater material deviation.

[0133] Figure 3 shows the SEM results of observing the microstructures of Comparative Example 4 (a) and Inventive Example 3 (b). It can be confirmed that Comparative Example 4, in which the annealing process was performed at 770°C, has an unrecrystallized structure of approximately 50 area%. However, in the case of Inventive Example 3, in which the annealing process was performed at 830°C, an unrecrystallized structure was observed to be less than 5 area%.

[0134] At this time, as can be confirmed in (a) of Fig. 3, the crystal grains of the unrecrystallized structure exist in an elongated form in the rolling direction, and it is observed that a deformed structure remains within the grains. In contrast, as shown in (b) of Fig. 3, it can be seen that the structure in which recrystallization is almost complete exists in a shape close to a spherical shape.

Claims

1. Contains, in wt%, carbon (C): 0.03 to 0.12%, manganese (Mn): 1.0 to 1.6%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.06%, niobium (Nb): 0.005 to 0.060%, titanium (Ti): 0.003 to 0.060%, the remainder being Fe and other unavoidable impurities. It contains ferrite with an area fraction of 85% or more and residual pearlite in the microstructure. The above ferrite is composed of recrystallized ferrite and non-recrystallized ferrite, and the steel plate has an area fraction of non-recrystallized ferrite of 5% or less (including 0%) in the entire microstructure.

2. In paragraph 1, A steel plate having a ratio (b / a) of the average grain size (a) of the recrystallized ferrite and the average grain size (b) of the non-recrystallized ferrite of 0.50 or less.

3. In paragraph 1, The above unrecrystallized ferrite is a steel plate having an aspect ratio (major axis / minor axis) of 3.0 or less.

4. In paragraph 1, The above steel plate is a steel plate further comprising, in weight %, boron (B): 0.003% or less.

5. In paragraph 1, The above steel plate is a steel plate having a yield strength of 380 MPa or more and a tensile strength of 440 MPa or more.

6. In paragraph 1, A steel plate comprising a plating layer on at least one surface of the steel plate.

7. A step for preparing a steel slab containing, by weight%, carbon (C): 0.03 to 0.12%, manganese (Mn): 1.0 to 1.6%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.01 to 0.06%, niobium (Nb): 0.005 to 0.060%, titanium (Ti): 0.003 to 0.060%, the remainder being Fe and other unavoidable impurities; A step of heating the above steel slab in a temperature range of 1100 to 1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab at 880℃ or higher; A step of coiling the above hot-rolled steel plate at a temperature range of 400 to 650°C; A step of obtaining a cold rolled steel sheet by cold rolling the above-mentioned hot rolled steel sheet at a cold reduction ratio of 40 to 80% after pickling; A step of annealing the above cold rolled steel sheet at a temperature range of 760 to 850°C; and It includes a step of cooling the above annealed cold rolled steel sheet to room temperature at a cooling rate of 3℃ / s or more, A method for manufacturing a steel plate wherein the cold rolling and annealing heat treatment steps satisfy the following relational expression 1, and when the cold rolling reduction is less than 70%, the following relational expression 1 is 864 or higher. [Relationship 1] Cold compression ratio + annealing temperature > 840 (Units are not considered in equation 1.) 8. In paragraph 7, A method for manufacturing a steel sheet further comprising the step of hot-dip galvanizing the cooled cold-rolled steel sheet at a temperature range of 440 to 480°C to obtain a hot-dip galvanized steel sheet.

9. In paragraph 8, A method for manufacturing a steel sheet further comprising the step of optionally performing alloying heat treatment after the above-mentioned hot-dip galvanizing.

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