Steel sheet and method for manufacturing the same

Abstract

This invention relates to a material used in automotive interior panels, reinforcing materials, etc., and more specifically, to a high-strength steel plate with excellent material uniformity and a method for manufacturing the same.

Description

Technical Field

[0001] This invention relates to a material used in automotive interior panels, reinforcing materials, etc., and more specifically, to a high-strength steel plate with excellent material uniformity and a method for manufacturing the same. Background Technology

[0002] In recent years, countries worldwide have increasingly demanded lightweight vehicle bodies to meet stricter fuel consumption regulations. To achieve this, higher strength steel sheets are required, and high-strength low-alloy (HSLA) steels, widely used as materials for automotive inner panels, are also required to possess high strength properties. HSLA steels have a high yield ratio that is beneficial for impact resistance and a low carbon equivalent (Ceq) that is beneficial for welding; they are typically widely used with a maximum yield strength standard of 450 MPa.

[0003] HSLA steel grades with a yield strength of 450 MPa are primarily designed for precipitation strengthening. In precipitation-strengthened steel, strength is ensured by generating carbonitrides using precipitating elements such as Ti, Nb, and V within a matrix composed of ferrite and pearlite. These carbonitrides not only exert precipitation strengthening effects by restricting dislocation movement but also delay recrystallization during cold rolling and subsequent annealing, resulting in residual unrecrystallized ferrite within the final fine microstructure. While excessive unrecrystallized ferrite increases the yield strength, it also becomes a major cause of material deviations in the length or width directions of the coil.

[0004] Patent Document 1 discloses a steel sheet that utilizes precipitation strengthening elements to delay the recrystallization of ferrite, and simultaneously utilizes recrystallized, phase-transformed ferrite, and unrecrystallized ferrite. In this case, the presence of a large amount of unrecrystallized ferrite within the steel sheet is advantageous in terms of ensuring yield strength and yield ratio, but it suffers from disadvantages such as low elongation and large material deviation. In the case of Patent Document 1, because of the use of unrecrystallized ferrite, the steel sheet is sensitive to process conditions, resulting in significant material deviation.

[0005] Patent document 2 also utilizes non-recrystallized ferrite, disclosing a steel sheet with improved elongation flange properties through soft ferrite and non-recrystallized ferrite. However, due to the presence of non-recrystallized ferrite, the aforementioned steel sheet suffers from material deviations in different directions, and most importantly, uneven material distribution along the length of the coil during manufacturing. In fact, significant material differences are observed in the top, middle, and bottom portions of the coil.

[0006] In addition, in the case of steel containing non-recrystallized structures, the fraction of non-recrystallized material formed during cooling varies depending on the cooling conditions, resulting in significant material deviations at different locations of the steel.

[0007] (Patent Document 1) Japanese Patent Publication No. 2008-190032 (Patent Document 2) Japanese Patent Publication No. 2008-106351 Summary of the Invention

[0008] (a) Technical problems to be solved One aspect of the present invention relates to a steel sheet suitable for automotive interior panels, reinforcing materials, etc., with the aim of providing a high-strength steel sheet with not only an excellent yield ratio but also small material deviations at different locations of the steel, and a method for manufacturing the same.

[0009] The technical problems of this invention are not limited to those described above. Anyone skilled in the art can readily understand the additional technical problems of this invention from the entirety of this specification.

[0010] (II) Technical Solution According to one aspect of the invention, a steel plate is provided, comprising, by weight percent: carbon (C): 0.03-0.12%, manganese (Mn): 1.0-1.6%, silicon (Si): less than 0.6% (excluding 0%), phosphorus (P): less than 0.03% (excluding 0%), sulfur (S): less than 0.01% (excluding 0%), nitrogen (N): less than 0.01% (excluding 0%), aluminum (Al): 0.01-0.06%, niobium (Nb): 0.005-0.060%, titanium (Ti): 0.003-0.060%, with the balance being Fe and other unavoidable impurities.

[0011] In one embodiment of the present invention, the steel plate, as a fine structure, comprises ferrite with an area fraction of 85% or more and the balance of pearlite, wherein the ferrite may comprise unrecrystallized ferrite with an area fraction of less than 5% (including 0%) in the entire fine structure.

[0012] In this way, by properly controlling the alloy composition and microstructure of the steel plate, a steel plate with excellent yield ratio and excellent material deviation at different locations of the steel can be provided.

[0013] In one embodiment of the present invention, the ratio (b / a) of the average grain size (b) of the unrecrystallized ferrite to the average grain size (a) of the recrystallized ferrite can be 0.50 or less.

[0014] In one embodiment of the present invention, the aspect ratio (major diameter / minor diameter) of the unrecrystallized ferrite can be 3.0 or less.

[0015] In one embodiment of the invention, the steel plate may further contain less than 0.003% boron (B).

[0016] In one embodiment of the present invention, the yield strength of the steel plate can be above 380 MPa, and the tensile strength can be above 440 MPa.

[0017] In one embodiment of the invention, at least one side of the steel plate may include a coating.

[0018] According to another aspect of the present invention, a method for manufacturing a steel plate is provided, comprising the following steps: preparing a steel billet having the above-mentioned alloy composition; heating the steel billet at a temperature range of 1100-1300°C; hot-rolling the heated steel billet at a temperature of 880°C or higher to obtain a hot-rolled steel plate; coiling the hot-rolled steel plate at a temperature range of 400-650°C; pickling the coiled hot-rolled steel plate and then cold-rolling it at a cold reduction rate of 40-80% to obtain a cold-rolled steel plate; annealing the cold-rolled steel plate at a temperature range of 760-850°C; and cooling the annealed cold-rolled steel plate to room temperature at a cooling rate of 3°C / second or higher.

[0019] In one embodiment of the present invention, the cold rolling and annealing heat treatment steps satisfy the following relationship 1, and when the cold reduction rate is less than 70%, the following relationship 1 can be 864 or higher.

[0020] [Relation 1] Cold pressing reduction + annealing temperature > 840 (Units are not considered in relation 1.) In one embodiment of the invention, the step of hot-dip galvanizing the cooled cold-rolled steel sheet at a temperature range of 440-480°C may be further included to obtain a hot-dip galvanized steel sheet.

[0021] In one embodiment of the invention, the step of selectively performing alloying heat treatment after hot-dip galvanizing may be further included.

[0022] (III) Beneficial Effects According to the present invention, the following effects are achieved: due to the excellent material properties of the steel and its high yield ratio, steel sheets suitable for automotive materials, especially for inner panel materials, are provided. Attached Figure Description

[0023] Figure 1 The invention example shown is an EBSD measurement result according to one embodiment.

[0024] Figure 2 The results of EBSD measurements according to a comparative example based on one implementation are shown.

[0025] Figure 3SEM images of an inventive example and a comparative example according to one embodiment are shown. Best practice

[0026] The inventors of this invention conducted in-depth research in order to obtain steel with minimal material deviation at different locations, i.e., excellent material uniformity, in addition to the generally required physical properties such as strength, when providing steel suitable for automotive inner panel materials.

[0027] As a result, it was confirmed that when the microstructure of the steel plate contains ferrite, the material uniformity of the steel plate can be improved by suppressing the formation of non-recrystallized ferrite, thus completing the present invention.

[0028] In particular, in suppressing the formation of non-recrystallized ferrite in the microstructure of steel sheets, optimizing manufacturing conditions, especially cold rolling and annealing heat treatment processes, is of technical significance in addition to alloy composition.

[0029] The present invention will now be described in detail.

[0030] According to one aspect of the present invention, the steel plate may contain, by weight percent: carbon (C): 0.03-0.12%, manganese (Mn): 1.0-1.6%, silicon (Si): less than 0.6% (except 0%), phosphorus (P): less than 0.03% (except 0%), sulfur (S): less than 0.01% (except 0%), nitrogen (N): less than 0.01% (except 0%), aluminum (Al): 0.01-0.06%, niobium (Nb): 0.005-0.060%, and titanium (Ti): 0.003-0.060%.

[0031] The reasons for limiting the alloy composition of the steel plate according to one aspect of the present invention to the above-mentioned conditions will be explained in detail below.

[0032] In addition, unless otherwise specified, the content of each element is based on weight, and the proportion of tissue is based on area.

[0033] Carbon (C): 0.03-0.12% Carbon (C) is an interstitial solid solution element that effectively contributes to ensuring the strength of steel. Furthermore, as an important element for the formation of pearlite and Ti-based and / or Nb-based precipitates within the steel, C needs to be added to a certain level or higher to achieve the fine microstructure desired in this invention. Specifically, regarding ensuring the strength desired in this invention, the C content can be 0.03% or more. However, excessive C content leads to a decrease in elongation and increases the likelihood of surface bending defects during subsequent component processing. Therefore, the C content can be 0.12% or less.

[0034] In this way, when the content of C is more than 0.03%, it is more advantageous to be more than 0.04%, and more advantageous to be more than 0.05%. In addition, when the content of C is less than 0.12%, it is more advantageous to be less than 0.11%, and more advantageous to be less than 0.10%.

[0035] Manganese (Mn): 1.0-1.6% Manganese (Mn), as a solid solution strengthening element, not only contributes to increased strength but also has the effect of precipitating sulfur (S) in steel as MnS, thus suppressing S-induced brittleness. To achieve this effect, the Mn content can be 1.0% or higher. However, when the Mn content is too high, annealing oxides can form, leading to surface defects, reduced elongation, and potentially worsened processability. Therefore, the Mn content can be 1.6% or lower.

[0036] When the Mn content is 1.0% or more, it is more advantageous to have 1.1% or more, and more advantageous to have 1.15% or more. Furthermore, when the Mn content is 1.6% or less, it is more advantageous to have 1.5% or less, and more advantageous to have 1.45% or less.

[0037] Silicon (Si): Less than 0.6% (except 0%) Silicon (Si) is an element that contributes to the increase of steel strength through solid solution strengthening. However, the target properties can be ensured even without the addition of Si in this invention, so the addition of Si is not intentional.

[0038] In this invention, if the Si content exceeds 0.6%, it can degrade the surface properties of the coating; therefore, its upper limit can be limited to 0.6%. More advantageously, it can be 0.5% or less, and even more advantageously, it can be 0.4% or less. Furthermore, considering the level of Si inevitably introduced during the steel manufacturing process, the 0% limit is excluded.

[0039] Phosphorus (P): less than 0.03% (except 0%) Phosphorus (P) is the most effective element for ensuring the strength of steel through solid solution strengthening without significantly impairing its deep-drawing properties. However, excessive addition of P increases the likelihood of brittle fracture, not only causing sheet breakage during hot rolling but also potentially significantly damaging the surface properties of coated steel sheets. Therefore, the P content can be limited to below 0.03%. However, 0% is an exception, considering the unavoidable level of contamination during steel manufacturing.

[0040] Sulfur (S): less than 0.01% (except 0%) Sulfur (S) is an unavoidable impurity added to steel, and to ensure excellent weldability, it is advantageous to manage the S content as low as possible. In particular, S present in steel can cause red brittleness, so taking this into consideration, the S content can be limited to below 0.01%. However, 0% is an exception, considering the level that is unavoidably introduced during the steel manufacturing process.

[0041] Nitrogen (N): less than 0.01% (except 0%) Nitrogen (N) is also an unavoidable impurity added to steel, so it is advantageous to manage its content as low as possible. However, considering factors such as steelmaking load and operating conditions, the N content can be limited to below 0.01%. Furthermore, considering the unavoidable level of N contamination during steelmaking, this excludes 0%.

[0042] Aluminum (Al): 0.01-0.06% Aluminum (Al) is an element added to achieve grain refinement and deoxidation effects in steel. Specifically, in this invention, to obtain stable aluminum-killed steel, the Al content is preferably 0.01% or more. Excessive Al addition is beneficial for strength increase through grain refinement; conversely, excessive inclusion formation during continuous casting increases the likelihood of deteriorated surface quality of the steel plate, leading to increased manufacturing costs. Therefore, the Al content can be 0.06% or less.

[0043] When the Al content is 0.01% or more, it is more advantageous to have 0.015% or more. Furthermore, when the Al content is 0.06% or less, it is more advantageous to have 0.055% or less.

[0044] Niobium (Nb): 0.005-0.060% Niobium (Nb) is an element that forms carbonitrides in steel. These carbonitrides exert a precipitation strengthening effect by restricting dislocation movement and also contribute to delayed recrystallization during cold rolling followed by annealing, resulting in the presence of unrecrystallized ferrite within the final fine microstructure. Therefore, in this invention, by limiting the Nb content, it is preferable to ensure the target strength while preventing excessive delay in recrystallization.

[0045] When the Nb content is less than 0.005%, the precipitation hardening effect becomes insufficient and the target strength cannot be guaranteed. However, when the Nb content exceeds 0.060%, not only does precipitation hardening become excessive, but the recrystallization delay effect also increases, and a large amount of unrecrystallized ferrite may exist.

[0046] Titanium (Ti): 0.003-0.060% Titanium (Ti) is also an element that exerts the same / similar effect as Nb. When the Ti content exceeds 0.060%, not only does precipitation hardening become excessive, but the recrystallization delay effect also increases, and a large amount of unrecrystallized ferrite may exist. However, when the Ti content is less than 0.003%, it is difficult to obtain precipitation strengthening effect.

[0047] In addition to the alloy composition described above, the steel plate of the present invention may further contain boron.

[0048] Boron (B): less than 0.003% Boron (B) is an element that easily segregates at grain boundaries. Adding B can prevent secondary processing brittleness that may occur when a certain amount of phosphorus (P) is added. Furthermore, in composite steels, even a small amount of B significantly improves hardenability, thus benefiting strength. To achieve this effect, B can be added. However, when the B content exceeds 0.003%, there is a problem of reduced elongation. Therefore, when adding B, the B content can be limited to below 0.003%.

[0049] The remaining component of this invention is iron (Fe). However, unintentional impurities inevitably enter the material from raw materials or the surrounding environment during normal manufacturing processes, and therefore these impurities cannot be excluded. These impurities are known to anyone skilled in the art during normal manufacturing processes, and therefore their contents are not specifically mentioned in this specification.

[0050] According to one embodiment of the present invention, the fine microstructure of the steel plate may include a ferrite phase as the main phase and a pearlite phase as the second phase. However, the fine microstructure of the steel plate, in addition to the aforementioned phases, does not exclude other microstructures inevitably generated during the steel manufacturing process. As an example, the steel plate may include hard phases such as bainite. Here, the fraction of the fine microstructure of the steel plate can be the result of measurement based on the full thickness of the steel plate.

[0051] More specifically, the steel plate, as a fine-grained structure, may contain ferrite with an area fraction of 85% or more and the remainder pearlite. When the ferrite fraction is less than 85% or the pearlite fraction exceeds 15%, the strength of the steel plate increases excessively, the elongation decreases significantly, and the processing characteristics may deteriorate.

[0052] A lower ferrite content corresponds to a relatively higher content of hard phases such as pearlite, which may be advantageous for achieving a composite microstructure. However, a higher content of hard phases such as pearlite inevitably leads to an increase in the yield strength and yield ratio of the steel sheet, thus increasing the likelihood of surface bending defects during subsequent processing into components. Therefore, in this invention, the ferrite content is limited to 85% or more based on the full thickness of the steel sheet.

[0053] According to one embodiment of the present invention, when the steel plate contains a ferrite phase, it mainly contains a recrystallized ferrite phase, preferably the fraction of non-recrystallized ferrite in the ferrite is less than 5% of the total area of ​​the fine structure.

[0054] That is, in one embodiment of the present invention, by suppressing the formation of non-recrystallized ferrite during the manufacturing process of the steel sheet, the material uniformity of the manufactured steel sheet can be ensured. In particular, the effect of minimizing material deviation in the length direction (rolling direction) can be obtained.

[0055] More preferably, the fraction of the unrecrystallized ferrite can be less than 5% by area, more advantageously less than 4% by area, more advantageously less than 3% by area, and also includes 0% by area.

[0056] In one embodiment of the invention, when the ferrite contains unrecrystallized ferrite, it is characterized by its fine size. Specifically, when compared to the size of the recrystallized ferrite, the ratio (b / a) of the average grain size (b) of the unrecrystallized ferrite to the average grain size (a) of the recrystallized ferrite can be 0.50 or less. In this way, the fine-sized unrecrystallized ferrite can play a more advantageous role in ensuring the target physical properties.

[0057] In addition, the aspect ratio (or major-to-minor axis ratio) of the unrecrystallized ferrite can be 3.0 or less. When the aspect ratio of the unrecrystallized ferrite exceeds 3.0, the formation of extended ferrite may adversely affect the assurance of physical properties. Here, the aspect ratio represents the value of the major axis divided by the minor axis when the grain size is converted to the equivalent diameter of a circle (major axis / minor axis). In this case, the closer this value is to 1.0, the closer it is to a sphere.

[0058] In one embodiment of the invention, the steel plate may have high strength characteristics, more preferably 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.

[0059] Furthermore, the steel plate exhibits excellent material uniformity, specifically minimizing material deviation along the length direction (rolling direction). In particular, it can have a strength difference along the length direction, and a strength difference between the top, middle, and bottom of the coil of less than 30 MPa.

[0060] The following will describe in detail, but according to one embodiment of the invention, the steel plate is a cold-rolled steel plate manufactured by a series of rolling processes, and may also include a coated steel plate having a coating on at least one side of the cold-rolled steel plate.

[0061] Hereinafter, a method for manufacturing a steel plate according to another aspect of the present invention will be described.

[0062] According to one embodiment of the present invention, the target steel sheet can be manufactured through a process of [billet heating - hot rolling - coiling - cold rolling - annealing heat treatment], the conditions of each step of which will be described in detail below. However, it is hereby noted that the following manufacturing process corresponds to an example for manufacturing the steel sheet of the present invention.

[0063] [Bill Heating] Prepare a steel billet with the above alloy composition system, and then heat the steel billet in a temperature range of 1100-1300℃.

[0064] The billet heating process is for the smooth execution of the hot rolling process described later. When the heating temperature is below 1100°C, rolling load may occur in the subsequent hot rolling process, but when the heating temperature exceeds 1300°C, surface oxide scale defects may occur.

[0065] [Hot Rolled] The steel billet heated according to the above description can be hot-rolled to obtain hot-rolled steel plate. At this time, hot finishing rolling can be performed at a temperature above 880°C. When the temperature during hot finishing rolling is below 880°C, rolling in the two-phase region will occur, which may lead to uneven microstructure.

[0066] [Collection] The hot-rolled steel sheet can be coiled. The coiling can be carried out in a temperature range of 400-650°C.

[0067] To achieve grain refinement during coiling and precipitation strengthening during subsequent annealing, a low temperature is advantageous. However, below 400°C, a water film forms on the coil surface, making it difficult to ensure temperature uniformity. On the other hand, above 650°C, secondary oxide scale forms, potentially deteriorating the steel plate surface.

[0068] [Cold Rolled] The hot-rolled steel sheet can be uncoiled and cold-rolled simultaneously to obtain a cold-rolled steel sheet. At this time, the reduction rate can be controlled to obtain the target thickness; in this invention, it can be controlled to be 40-80%.

[0069] When the reduction rate during cold rolling is less than 40%, the recrystallization driving force through cold rolling is insufficient, and the recrystallization of ferrite cannot be completed, resulting in excessive residue of unrecrystallized ferrite. On the other hand, when the reduction rate during cold rolling exceeds 80%, the load on the rolling rolls becomes very large, and the shape deteriorates.

[0070] More advantageously, the cold rolling can be carried out at a cold reduction rate of 50% or more and 70% or less.

[0071] In one embodiment of the invention, prior to the cold rolling, a pickling process may be performed to remove the surface oxide scale from the coiled hot-rolled steel sheet. The pickling process can be carried out under normal conditions, and these conditions are not particularly limited in this invention.

[0072] [Annealing heat treatment] The cold-rolled steel sheet can be subjected to annealing heat treatment.

[0073] During the annealing heat treatment, to ensure sufficient recrystallization of the steel sheet, the temperature range of 760-850℃ can be maintained. When the annealing heat treatment temperature is below 760℃, excessive unrecrystallized ferrite may remain after annealing. However, when the temperature exceeds 850℃, the grains coarsen, making it impossible to ensure the target strength level.

[0074] In addition, when the reduction rate is low in the cold rolling process described above, especially when the reduction rate is 40% or more but less than 70%, the recrystallization driving force is relatively low. Therefore, in order to ensure that recrystallization can be fully carried out in the subsequent annealing heat treatment, the temperature can be set to 800°C or more, and more preferably 830°C or more.

[0075] 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, and when the reduction rate during cold rolling is less than 70%, the following relationship 1 is preferably 864 or more.

[0076] [Relation 1] Cold pressing reduction + annealing temperature > 840 (Units are not considered in relation 1.) [cool down] The cold-rolled steel sheet that has undergone annealing heat treatment according to the above description can be cooled to room temperature. At this time, a cooling rate of 3°C / second or higher can be used. When the cooling rate is less than 3°C / second, a hard phase (low-temperature phase transformation) will form during cooling, and its fraction may become too high.

[0077] There is no particular upper limit to the cooling rate; it can be appropriately determined according to the equipment specifications.

[0078] Through the aforementioned series of processes, the target steel sheet can be obtained. In particular, by carrying out sufficient recrystallization during cold rolling and annealing heat treatment, a steel sheet with a minimized non-recrystallized ferrite fraction can be obtained. Therefore, in addition to high strength, the steel sheet of the present invention also possesses excellent material uniformity.

[0079] Furthermore, by performing a coating process on the cold-rolled steel sheet manufactured according to the above description, coated steel sheet can be produced. The coating process in this case is hot-dip galvanizing, which will be explained in detail below.

[0080] Hot-dip galvanizing By hot-dip galvanizing the annealed cold-rolled steel sheet in a continuous hot-dip galvanizing production line, a galvanized steel sheet with a zinc-based coating formed on at least one side of the cold-rolled steel sheet can be obtained.

[0081] The hot-dip galvanizing is carried out by immersing the annealed cold-rolled steel sheet in a zinc-based galvanizing bath. It should be noted that the composition of the galvanizing bath is not particularly limited and can be carried out under normal conditions. As an example, the hot-dip galvanizing can be performed in a temperature range of 440-480°C.

[0082] After hot-dip galvanizing, an additional alloying heat treatment process can be performed, which can also be carried out under normal conditions. Detailed Implementation

[0083] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative and not intended to limit the scope of the invention. The scope of the invention is determined by the matters set forth in the claims and by reasonable analogy therefrom.

[0084] (Example) Existing examples After manufacturing cold-rolled steel sheets using existing process conditions, the mechanical properties of each cold-rolled steel sheet are evaluated. At this time, steel billets of grade C shown in Table 2 are prepared, and then cold-rolled steel sheets are manufactured from these billets by applying the reheating temperature, coiling temperature, cold reduction rate, annealing temperature, and cooling rate as shown in Table 1. Furthermore, the hot finishing rolling of the billet after reheating is performed at 900°C.

[0085] The yield strength and tensile strength of each manufactured cold-rolled steel sheet are measured using the same methods as those shown below.

[0086] [Table 1] As shown in Table 1 above, it can be confirmed that the higher the reheating temperature and the lower the winding temperature, the higher the yield strength tends to be. This is because the precipitates are suppressed to the greatest extent during hot rolling, while the precipitates are finely distributed during cold rolling and annealing. In addition, due to the annealing at a relatively low temperature, the absence of recrystallized structures results in high overall strength. Therefore, it is expected that existing Examples 1 to 6 will cause material deviations in the length direction when in the coil state.

[0087] Invention Examples and Comparative Examples After preparing billets with the alloy compositions shown in Table 2 below, each billet is reheated to 1200°C and hot-rolled at a finishing temperature of 900-950°C to obtain hot-rolled steel sheets. Subsequently, after cold rolling with a reduction rate of 54%, continuous annealing heat treatment is performed to manufacture cold-rolled steel sheets. At this time, some of the cold-rolled steel sheets are hot-dip galvanized to produce coated steel sheets. The process conditions are shown in Table 3 below.

[0088] For cold-rolled steel sheets and coated steel sheets manufactured according to the above content, tensile tests are carried out along the rolling direction according to JIS-5 specification to measure the physical properties of yield strength (YS), tensile strength (TS) and elongation (E1).

[0089] In addition, in order to measure the microstructure of each steel plate, the samples obtained by JIS-5 were observed using an optical microscope and a scanning electron microscope (SEM), and the fraction of each phase was measured. The fraction of unrecrystallized ferrite was measured using electron backscatter diffraction (EBSD).

[0090] Furthermore, after determining the average grain size of recrystallized ferrite (a) and the average grain size of unrecrystallized ferrite (b), the value of b / a was calculated. Then, in the results measured using SEM, the average value was calculated after measuring the grain size distribution using an application program. The results are shown in Table 4 below.

[0091] [Table 2] [Table 3] [Table 4] As shown in Tables 2 to 4 above, in Invention Examples 1 to 5, which fully meet the alloy composition and manufacturing conditions proposed in this invention, the fine microstructure contains a ferrite phase as the main phase, and the fraction of unrecrystallized ferrite is minimized to 5% or less. In particular, it can be confirmed that the unrecrystallized ferrite is formed at a fine size. Therefore, the steel plate exhibits excellent material uniformity along its length, and the target level of strength is also ensured.

[0092] However, in Comparative Examples 1 and 2, where the alloy composition deviates from the present invention, and in Comparative Examples 3 and 4, where the alloy composition satisfies the present invention but the manufacturing conditions deviate from Relationship 1, the fraction of the non-recrystallized ferrite phase is excessive, and its size is also coarse, making it impossible to ensure the material uniformity along the length of the steel plate. In particular, based on the strength value at the center, the strength difference exceeds 30 MPa, with some sections reaching a maximum of 54 MPa. That is, as expected in the aforementioned prior examples, it can be confirmed that material deviation along the length actually occurred in Comparative Examples 3 and 4, where Relationship 1 deviated.

[0093] Furthermore, although not explicitly stated, the aspect ratio of the unrecrystallized ferrite in Examples 1 to 5 is 3.0 or less, which is representative of the properties that can be achieved through... Figure 1 The microstructure photographs of Example 4 of the invention shown confirm this.

[0094] On the other hand, Comparative Examples 1 to 4 contain a large amount of unrecrystallized ferrite with an aspect ratio exceeding 3.0, which can be representatively observed through... Figure 2 The microstructure photographs of Comparative Example 3 shown confirm this.

[0095] Figure 1 The EBSD results of the microstructure observed in Example 4 of the invention confirm that the middle (a) and top (b) sections have almost completely undergone recrystallization.

[0096] Therefore, as shown in Table 3 above, the strength measurements of the specimens collected from the top and middle sections show almost uniform results in terms of yield strength and tensile strength. Furthermore, it can be confirmed that the aspect ratio of the unrecrystallized ferrite is below 3.0.

[0097] Figure 2 Based on the EBSD results of the microstructure observed in Comparative Example 3, it can be confirmed that recrystallization was incomplete in both the middle (a) and top (b), with a large amount of unrecrystallized microstructure remaining. In particular, the unrecrystallized fraction was measured to be 10.9% in the middle and 32.9% in the top. Furthermore, it can be confirmed that the aspect ratio of the unrecrystallized ferrite exceeds 3.0.

[0098] As such, due to the difference in the non-recrystallization fraction at different locations on the coil, a yield strength difference occurs, with a difference of approximately 38 MPa. At this point, the yield strength increases in areas with a larger non-recrystallization fraction, resulting in localized inhomogeneity and greater material deviation.

[0099] Figure 3 Based on the SEM results of the fine structures observed in Comparative Example 4(a) and Invention Example 3(b), it can be confirmed that in Comparative Example 4, which underwent annealing at 770°C, there was approximately 50% by area of ​​non-recrystallized structure. However, in Invention Example 3, which underwent annealing at 830°C, the non-recrystallized structure was observed to be less than 5% by area.

[0100] At this time, as Figure 3 As can be confirmed in (a), the unrecrystallized grains exist in a form extending along the rolling direction, and residual deformed structures are observed within the grains. In contrast, as... Figure 3 As in (b), the almost completely recrystallized tissue exists in a near-spherical shape.

Claims

1. A steel plate, by weight percent, comprising: carbon (C): 0.03-0.12%, manganese (Mn): 1.0-1.6%, silicon (Si): less than 0.6% and excluding 0%, phosphorus (P): less than 0.03% and excluding 0%, sulfur (S): less than 0.01% and excluding 0%, nitrogen (N): less than 0.01% and excluding 0%, aluminum (Al): 0.01-0.06%, niobium (Nb): 0.005-0.060%, titanium (Ti): 0.003-0.060%, with the balance being Fe and other unavoidable impurities. The microstructure consists of ferrite with an area fraction of over 85% and the remainder pearlite. The ferrite is composed of recrystallized ferrite and non-recrystallized ferrite, and the area fraction of non-recrystallized ferrite in the entire microstructure is less than 5% and includes 0%.

2. The steel plate according to claim 1, wherein, The ratio of the average grain size b of the unrecrystallized ferrite to the average grain size a of the recrystallized ferrite, b / a, is 0.50 or less.

3. The steel plate according to claim 1, wherein, The aspect ratio of the unrecrystallized ferrite, i.e., the major axis to minor axis, is 3.0 or less.

4. The steel plate according to claim 1, wherein, The steel plate further contains less than 0.003% boron (B) by weight.

5. The steel plate according to claim 1, wherein, The steel plate has a yield strength of 380 MPa or higher and a tensile strength of 440 MPa or higher.

6. The steel plate according to claim 1, wherein, At least one side of the steel plate includes a coating.

7. A method for manufacturing a steel plate, comprising the following steps: Prepare a steel billet, which, by weight percent, comprises: carbon (C): 0.03-0.12%, manganese (Mn): 1.0-1.6%, silicon (Si): less than 0.6% and excluding 0%, phosphorus (P): less than 0.03% and excluding 0%, sulfur (S): less than 0.01% and excluding 0%, nitrogen (N): less than 0.01% and excluding 0%, aluminum (Al): 0.01-0.06%, niobium (Nb): 0.005-0.060%, titanium (Ti): 0.003-0.060%, with the balance being Fe and other unavoidable impurities; The steel billet is heated in a temperature range of 1100-1300℃; The heated steel billet is hot-rolled at a temperature above 880°C to obtain hot-rolled steel plate; The hot-rolled steel sheet is coiled at a temperature range of 400-650℃; The hot-rolled steel sheet that has been coiled is pickled and then cold-rolled at a cold reduction rate of 40-80% to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is subjected to annealing heat treatment at a temperature range of 760-850℃; as well as The cold-rolled steel sheet that has undergone the annealing heat treatment is cooled to room temperature at a cooling rate of 3°C / second or higher. Wherein, the cold rolling and annealing heat treatment steps satisfy the following relationship 1, and when the cold reduction rate is less than 70%, the following relationship 1 is 864 or higher. [Relation 1] Cold pressing reduction + annealing temperature > 840 Units are not considered in relation 1.

8. The method for manufacturing a steel plate according to claim 7, wherein, The manufacturing method further includes the step of hot-dip galvanizing the cooled cold-rolled steel sheet at a temperature range of 440-480°C to obtain a hot-dip galvanized steel sheet.

9. The method for manufacturing a steel plate according to claim 8, wherein, The manufacturing method further includes the step of selectively performing alloying heat treatment after hot-dip galvanizing.

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