Steel plate and its manufacturing method

Abstract

A steel sheet and a method for manufacturing the same are provided. The steel sheet of the present invention has excellent ductility, formability and weldability, and can therefore be used for automotive structural members.

Description

[Technical Field]

[0001] The present invention relates to the manufacture of steel sheets used for automobile structural members, and more particularly to steel sheets having excellent ductility, formability and weldability, and a method for manufacturing the same. [Background technology]

[0002] Recently, the automotive industry has been facing increasingly stringent regulations for global environmental conservation. Accordingly, carbon reduction and fuel efficiency regulations are becoming increasingly stringent. High-strength steels are increasingly being adopted to ensure passenger stability in the event of a collision or other accident. To improve the crashworthiness of vehicle bodies, high-strength steels with excellent yield strength are being used in structural components such as members, seat rails, and pillars. However, increasing the strength of steel sheets can lead to reduced ductility and formability. To address this issue, the development of materials that simultaneously achieve high strength and high formability is needed. Generally, increasing the strength of steel sheets leads to a decrease in elongation, which reduces workability. Therefore, the development of materials that can address this issue is needed. Conventional steel strengthening methods, such as solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening, have been studied. However, among the above methods, solid solution strengthening and grain refinement have the drawback of making high-strength steels with tensile strengths of 490 MPa or higher.

[0003] Precipitation-hardened high-strength steel is a technology that ensures strength by precipitating carbonitrides through the addition of carbonitride-forming elements such as Nb, Ti, and V. The fine precipitates inhibit grain growth, resulting in finer grains. This technology has the advantage of easily achieving high strength at low manufacturing costs, but the drawback is that the fine precipitates cause a rapid rise in recrystallization temperature, necessitating high-temperature annealing to induce sufficient recrystallization and ensure ductility. Another problem with precipitation-hardened steel, which is strengthened by the precipitation of carbonitrides in a ferrite matrix, is that it is difficult to obtain high-strength steel of 600 MPa or higher.

[0004] Meanwhile, various transformation-strengthened high-strength steels have been developed, including dual-phase (DP) steel, which is composed of a soft ferrite matrix and a hard martensite matrix; transformation-induced plasticity (TRIP) steel, which achieves high ductility by utilizing the transformation-induced plasticity of retained austenite; and complex-phase (CP) steel, which is composed of a composite structure of ferrite and hard bainite or martensite. Recently, automotive steel sheets have been required to achieve higher strength to improve fuel efficiency and durability. Demand for high-strength steel sheets with tensile strengths of 780 MPa or higher is increasing for body structures and reinforcements to enhance crash safety and passenger protection. Among these, DP steel is the most commonly used automotive steel sheet due to its excellent ductility. However, it suffers from a low yield ratio (YR) and poor formability and workability. Furthermore, as steel sheets continue to become increasingly stronger, cracks and wrinkles occur during press forming of automotive parts, making it difficult to manufacture complex parts. Among high-strength steels, TRIP and XF steels have better ductility and yield ratios than existing DP steels, making them easier to work with. However, they suffer from poor weldability due to the large amounts of Si and Al added to ensure high elongation. To overcome these drawbacks of existing high-strength steels, reducing the Si and Al content can create a composition with good weldability while still maintaining a certain level of formability and ductility. This can expand the application of high-strength steels to more complex parts. This can be achieved by utilizing Quenching and Partitioning (Q&P) heat treatment, a cutting-edge heat treatment technology that preserves retained austenite.

[0005] An example of a conventional technique for simultaneously ensuring the ductility and workability of the high-tensile steel sheet is the invention disclosed in Patent Document 1. In the invention disclosed in Patent Document 1, it is necessary to precisely control the slow cooling and rapid cooling temperatures to form retained austenite, and to control the heat treatment temperature so as to ensure a high elongation percentage.

[0006] Another prior art is the invention disclosed in Patent Document 2. The invention disclosed in Patent Document 2 is characterized by producing a steel sheet having a high yield ratio by a Q&P process and a painting treatment.

[0007] Still another prior art is the invention disclosed in Patent Document 3. The invention disclosed in Patent Document 3 provides a method for producing a high-strength cold-rolled steel sheet having a high bainite fraction by cooling to the bainite region, but has the problem of a deterioration in elongation due to a reduced carbon partitioning effect compared to the Q&P process. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 2018-0165176 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-090432 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-106696 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a steel sheet that can be used mainly for automobile structural members, and a method for manufacturing the same.

[0010] Furthermore, the technical problems to be achieved by the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention pertains from the following description. [Means for solving the problem]

[0011] Thus, one aspect of the present invention is In weight percent, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 1.6 to 2.6%, molybdenum (Mo): 0.40% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.10% or less (excluding 0%), sulfur (S): 0.020% or less (excluding 0%), aluminum (sol.Al): 0.60% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, nitrogen (N): 0.01% or less (excluding 0%), boron (B): 0.010% or less (including 0%), the balance being Fe and other unavoidable impurities, wherein the C, Si, Mn, Cr and Mo satisfy the following relational expression 1: The microstructure contains, in area percentage, ferrite of 20% to 40%, bainite and tempered martensite in total of 30% to 60%, and the remainder fresh martensite and retained austenite, and the area fraction of the retained austenite is less than 5% (excluding 0%), and The present invention relates to a steel sheet having a hole expandability (HER) of 30% or more, a value of the hole expandability (HER), yield strength (YS), tensile strength (TS) and elongation (EL) according to the following relational expression 2 of 500 or more, and a yield ratio (YS / TS) of 0.65 or more.

[0012] [Equation 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.300 Here, C, Si, Mn, Cr and Mo refer to the content % of the steel components in the base structure at 1 / 4t point of the thickness of the base steel sheet.

[0013] [Equation 2] HER*EL*YS / TS≧500

[0014] Another aspect of the present invention is a step of preparing a steel slab that satisfies the compositional composition of the steel and Relational Formula 1, and then reheating the steel slab; a step of hot rolling the reheated steel slab so that the temperature at the exit of the finish rolling mill is Ar3 to Ar3+50°C, and then coiling the slab at 400 to 650°C, followed by cooling the slab to room temperature at an average cooling rate of 0.1°C / s or less; cold rolling the cooled hot-rolled steel sheet at a reduction rate of 40 to 70% to produce a cold-rolled steel sheet; a step of continuously annealing the cold-rolled steel sheet at a temperature of Ac3-50°C to Ac3-20°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 650 to 680°C at an average cooling rate of 10°C / s or less, and then secondarily cooling to a temperature of 300 to 340°C at an average cooling rate of 5°C / s or more; Reheating the secondarily cooled cold-rolled steel sheet to a temperature of Ms or higher and then holding the temperature for 60 seconds or more; and cooling the cold-rolled steel sheet thus held to a temperature of 150°C or lower at an average cooling rate of 5°C / s or higher. [Effects of the Invention]

[0015] The present invention, configured as described above, optimizes alloy elements and manufacturing processes to produce high-strength steel sheets that have excellent yield strength (YS / TS) and hole expandability compared to conventional DP steels while still maintaining the high ductility characteristic of DP steels. This prevents processing defects, such as cracks, that occur during press forming, enabling the steel to be used in a variety of automotive structural components with complex shapes that require high formability. Another advantage is that the reduced Si and Al content results in better weldability compared to existing TRIP steels. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing changes in the relational expression HER*EL*YS / TS between hole expandability (HER), yield strength (YS), tensile strength (TS), and elongation (EL) according to Relational Expression 1 for an inventive steel and a comparative steel in an example of the present invention. [Figure 2] FIG. 1 is a diagram showing the change in hole expandability (HER) according to Relational Formula 1 between an inventive steel and a comparative steel in an example of the present invention. [Figure 3] FIG. 1 is a diagram showing the change in yield ratio according to Relational Formula 1 between an inventive steel and a comparative steel in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the phrase clearly dictates otherwise. As used in the specification, the term "comprises" embodies certain properties, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other specific properties, regions, integers, steps, operations, elements, components, and / or groups.

[0018] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an idealized or very formal sense unless defined.

[0019] The inventors have experimentally confirmed that by optimizing the alloy components and manufacturing process to introduce a final microstructure containing 20% ​​to 40% ferrite, 30% to 60% bainite and tempered martensite in total, and the remainder being fresh martensite and retained austenite, it is possible to increase the yield ratio and improve workability compared to conventional DP steels, and further improve ductility by forming less than 5% retained austenite during final cooling.Based on these results, the present invention was completed.

[0020] First, the inventors produced a hot-rolled steel sheet in which carbides that serve as austenite nucleation sites during annealing are finely dispersed, by hot-rolling a slab in which the C, Si, Mn, Cr, and Mo contents in the base structure at a point 1 / 4t of the thickness of the base steel sheet were controlled to satisfy Relation 1 so that the finish rolling outlet temperature was Ar3 to Ar3 + 50°C, coiling was performed at 400 to 650°C, and then cooling to room temperature at an average cooling rate of 0.1°C / s or less. The hot-rolled steel sheet was then cold-rolled at a reduction of 40 to 70% to produce a cold-rolled steel sheet, which was then continuously annealed at a temperature of Ac3 - 50°C to Ac3 - 20°C. The continuously annealed steel sheet was then primarily cooled to a temperature range of 650 to 680°C at an average cooling rate of 10°C / s or less, and then secondarily cooled to a temperature range of 300 to 340°C at an average cooling rate of 5°C / s or more to introduce fresh martensite. The steel plate was then reheated to a temperature above Ms, held for 60 seconds or more to form tempered martensite and bainite, concentrating carbon in the surrounding untransformed austenite, and then cooled to a temperature of 150°C or less at an average cooling rate of 5°C / s or more to introduce fine fresh martensite into the remaining portion.

[0021] By controlling the chemical composition and manufacturing process described above, it is possible to reduce the fraction of ferrite and fresh martensite compared to conventional DP steels and introduce tempered martensite, bainite, and retained austenite, thereby increasing the yield ratio and ensuring workability compared to conventional DP steels. Furthermore, a large number of mobile dislocations are formed around the retained austenite during plastic deformation, contributing to improved ductility. This precisely controlled dual-phase steel ensures ductility while maintaining a high yield ratio compared to conventional DP steels. This allows for the production of high-strength steel sheets with excellent ductility, formability, and weldability.

[0022] DETAILED DESCRIPTION OF THE INVENTION The technical configuration of the present invention will be described in more detail below with reference to various embodiments and accompanying drawings.

[0023] First, the steel sheet of the present invention contains, by weight %, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 1.6 to 2.6%, molybdenum (Mo): 0.40% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.10% or less (excluding 0%), sulfur (S): 0.020% or less (excluding 0%), aluminum (sol.Al): 0.60% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, nitrogen (N): 0.01% or less (excluding 0%), boron (B): 0.010% or less (including 0%), the balance being Fe and other unavoidable impurities, and satisfies Relational Formula 1.

[0024] The properties of each alloying element and the critical meaning of the composition range will be briefly explained below, and the content of each element is expressed in weight percent unless otherwise specified.

[0025] C: 0.06 to 0.16% Carbon (C) is a very important element added to strengthen the transformed structure. In dual-phase steels, carbon promotes the formation of hard martensite, improving strength. As the carbon content increases, the amount of martensite also increases. However, if the carbon content exceeds 0.16%, the strength of martensite increases, but the strength difference between martensite and ferrite, which has a low carbon concentration, becomes significant. This strength difference facilitates fracture at the interface between phases during plastic deformation, reducing ductility and work hardening rates. It also deteriorates weldability, resulting in welding defects during customer part processing. However, if the carbon content is less than 0.06%, it becomes difficult to achieve the desired strength. Therefore, it is preferable to limit the carbon content to 0.06 to 0.16%. More preferably, the carbon content can be limited to the range of 0.07 to 0.15%.

[0026] Si: 0.8% or less (excluding 0%) Silicon (Si) is a ferrite-stabilizing element that promotes ferrite transformation and promotes carbon enrichment in untransformed austenite during the Q&P process, contributing to the formation of retained austenite. It also effectively strengthens ferrite through solid solution strengthening, reducing the hardness difference between phases, making it a useful element that ensures strength without reducing the ductility of steel sheets. However, if its content exceeds 0.8%, it can induce surface scale defects, adversely affecting surface quality and reducing weldability and phosphatability. Therefore, its addition amount is preferably limited to 0.8% or less, and more preferably to 0.7% or less.

[0027] Mn: 1.6 to 2.6% Manganese (Mn) refines grain size without reducing ductility, completely precipitates sulfur (S) in steel as MnS, prevents hot embrittlement due to the formation of FeS, and strengthens the steel. At the same time, in dual-phase steels, it lowers the critical cooling rate at which martensite is obtained, facilitating martensite formation. If the manganese content is less than 1.6%, it is difficult to achieve the strength targeted in the present invention. On the other hand, if the manganese content exceeds 2.6%, problems such as weldability and hot rolling are likely to occur. Excessive martensite formation leads to instability in the material, and Mn-bands (bands of Mn oxides) form within the structure, increasing the risk of processing cracks and sheet fracture. Furthermore, Mn oxides dissolve on the surface during annealing, significantly impairing surface quality. Therefore, in the present invention, the Mn content is preferably limited to the range of 1.6 to 2.6%, more preferably 1.8 to 2.4%.

[0028] Mo: 0.40% or less (excluding 0%) Molybdenum (Mo) is an element that delays the transformation of austenite to pearlite and refines ferrite, improving strength. Mo has the advantages of improving the hardenability of steel and finely forming martensite at grain boundaries, enabling control of the yield ratio. However, Mo is an expensive element, and a higher Mo content increases manufacturing costs, resulting in cost disadvantages. Therefore, it is preferable to appropriately control Mo content. To achieve the above effects, a maximum of 0.40% is preferably added. However, if the Mo content exceeds 0.40%, the alloy cost increases sharply, reducing economic viability, and the excessive grain refinement and solid solution strengthening effects can actually reduce the ductility of the steel. Therefore, in the present invention, the Mo content is preferably limited to 0.40% or less, more preferably 0.20% or less. Meanwhile, in the present invention, 0% Mo is excluded in consideration of the amount of Mo that is inevitably added during manufacturing.

[0029] Cr: 1.0% or less (excluding 0%) Chromium (Cr) is an element added to improve the hardenability of steel and ensure high strength. It also plays a very important role in the formation of martensite, minimizing the decrease in elongation rate while increasing strength, and is advantageous for producing dual-phase steel with high ductility. In particular, Cr is used in the hot rolling process. 23 The Cr content of Cr forms Cr-based carbides such as C6, some of which dissolve during annealing and some of which remain undissolved. This allows the amount of solute C in martensite to be controlled below an appropriate level after cooling. This makes Cr an advantageous element for producing dual-phase steels with low yield ratios by suppressing the occurrence of elongation at the yield point. However, if the Cr content exceeds 1.0%, not only does this effect saturate, but there are also problems with the deterioration of cold rolling due to an excessive increase in hot rolling strength. Furthermore, the increased fraction of Cr-based carbides causes coarsening, resulting in coarsening of the martensite size after annealing, leading to a decrease in elongation. Therefore, in the present invention, the Cr content is preferably limited to 1.0% or less, and more preferably to 0.8% or less. Meanwhile, in the present invention, the Cr content is excluded from 0% in consideration of the amount of Cr that is inevitably added during manufacturing.

[0030] P: 0.10% or less (excluding 0%) Phosphorus (P) is a substitutional element with the greatest solid solution strengthening effect, and is the most advantageous element for improving in-plane anisotropy and ensuring strength without significantly impairing formability. However, if added in excess of a certain level, it can significantly increase the likelihood of brittle fracture, potentially causing slab breakage during hot rolling, and can also act as an element that impairs the properties of the coating surface. Therefore, in the present invention, its content is limited to a maximum of 0.10%. However, 0% is excluded to take into account the level of unavoidable addition.

[0031] S: 0.020% or less (excluding 0%) Sulfur (S) is an unavoidable impurity element in steel, and since it reduces ductility and weldability, it is important to keep its content as low as possible. In particular, since it increases the likelihood of red shortness, it is preferable to control its content to 0.020% or less. However, 0% is excluded in consideration of the level that is inevitably added during the manufacturing process.

[0032] sol.Al: 0.60% or less (excluding 0%) Acid-soluble aluminum (sol. Al) is an element added to steel for grain refinement and deoxidation, and, like Si, is a ferrite stabilizing element. It also distributes carbon from ferrite to austenite, improving martensite hardening ability and forming retained austenite. By maintaining the bainite region during annealing, it effectively suppresses carbide precipitation in bainite, thereby improving the ductility of steel sheets. However, if its content exceeds 0.60%, although it is advantageous for increasing strength due to its grain refinement effect, it increases the likelihood of surface defects in plated steel sheets due to excessive formation of inclusions during continuous casting operations, and also increases production costs. Therefore, in the present invention, the sol. Al content is preferably controlled to 0.60% or less, more preferably 0.40% or less.

[0033] Ti, Nb: 0.001~0.04% each Titanium (Ti) and niobium (Nb) are elements effective in increasing the strength of steel sheets and refining grain size through the formation of nanoprecipitates. When these elements are added, they combine with carbon to form very fine nanoprecipitates, which strengthen the matrix and reduce the hardness difference between phases. If the Ti and Nb contents are less than 0.001%, it is difficult to ensure these effects. However, if the Ti and Nb contents exceed 0.04%, manufacturing costs may increase and ductility may be significantly reduced due to excessive precipitates. Therefore, in the present invention, the Ti and Nb contents are limited to 0.001-0.04%.

[0034] N: 0.01% or less (excluding 0%) Nitrogen (N) is an element that has an effective effect on stabilizing austenite, but if its content exceeds 0.01%, there are problems such as a sharp increase in the cost of refining steel and a significant increase in the risk of cracks occurring during continuous casting due to the formation of AlN, etc. Therefore, it is preferable to limit its upper limit to 0.01%. However, 0% is excluded in consideration of the level at which unavoidable addition occurs.

[0035] B: 0.010% or less (including 0%) Boron (B) is a component that delays the transformation of austenite to pearlite during the cooling process during annealing, and is a hardening element that suppresses the formation of ferrite and promotes the formation of martensite. However, if the B content exceeds 0.01%, excessive B may concentrate on the surface, causing a deterioration in plating adhesion. Therefore, the B content is controlled to 0.010% or less, and more preferably, 0.005% or less.

[0036] [Equation 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.300 Here, C, Si, Mn, Cr and Mo refer to the content % of the steel components in the base structure at 1 / 4t point of the thickness of the base steel plate.

[0037] In the present invention, it is important to control the contents of C, Si, Al, Cr, and Mo among the alloy components in the base structure at the 1 / 4t point of the thickness of the base steel sheet so that the above relational expression 1 is satisfied.

[0038] Si and Al are ferrite stabilizing elements that promote ferrite transformation and contribute to the formation of retained austenite and martensite by promoting the enrichment of carbon in untransformed austenite. Carbon also contributes to the formation and fraction control of martensite by promoting the enrichment of carbon in untransformed austenite. However, C, Si, and Al have a negative effect on weldability, causing cracks on the surface and inside of the weld.

[0039] On the other hand, Mn, Cr, and Mo are elements that contribute to improving hardening ability, but their effect on concentrating C in austenite is relatively low compared to C, Si, and Al. Therefore, it is very important to carefully adjust the ratios of C, Si, and Al to the other hardening elements Mn, Cr, and Mo.

[0040] If the value defined by the above relational expression 1 is 0.300 or more, the relational expression HER*EL*YS / TS between hole expandability (HER), yield strength (YS), tensile strength (TS), and elongation (EL) can be ensured to be 500 or more. Furthermore, by ensuring that the total of bainite and tempered martensite is 30% or more and 60% or less, the hardness difference between the phases is reduced, and a hole expandability value of 30% or more can be ensured while simultaneously ensuring the required ductility. Furthermore, a yield ratio (YS / TS) of 0.65 or more can be ensured. However, if the value defined by the above relational expression 1 is less than 0.300, the above-mentioned effects cannot be expected.

[0041] In addition to the above components, the present invention may also comprise the balance Fe and other unavoidable impurities.

[0042] On the other hand, the steel sheet of the present invention is a dual-phase steel sheet, which can improve workability by increasing the yield ratio compared to conventional DP steels while maintaining a certain degree of ductility. To achieve this, in addition to the above-mentioned alloy composition, it is necessary to satisfy the following control conditions for the microstructure and phase fraction.

[0043] Specifically, the steel sheet of the present invention has a microstructure that contains, in area percentages, ferrite of 20% to 40%, bainite and tempered martensite in total of 30% to 60%, and the remainder being fresh martensite and retained austenite, with the area fraction of the retained austenite being less than 5% (excluding 0%).

[0044] To achieve a lean chemical composition and high ductility, while improving the yield strength ratio (YS / TS) compared to conventional DP steels, control of the structure and composition and careful heat treatment are essential. First, it is important to introduce a small amount of retained austenite. Retained austenite induces transformation-induced plasticity and contributes to improving the ductility of steel sheets. To introduce this retained austenite, the steel is rapidly cooled to a temperature below Ms to form fresh martensite, and then reheated to a temperature above Ms (Quenching & Partitioning (Q&P)). This process results in the formation of large amounts of tempered martensite and bainite, which stably distributes carbon and contributes to the formation of a small amount of retained austenite in the final structure. Careful control of the tempered martensite and bainite fractions is also important to ensure high hole expandability and ductility. Two-phase annealing, close to the single-phase region, controls the ferrite fraction during annealing, and the Q&P process, which involves rapid cooling below the Ms temperature and reheating above the Ms temperature, minimizes the formation of fresh martensite. Furthermore, the precipitation of fine nano-precipitates within the ferrite further reduces the hardness difference between the phases, improving workability. Finally, the introduction of a small amount of fresh martensite during final cooling ensures the desired strength.

[0045] Therefore, in the microstructure of the steel sheet of the present invention, the total of bainite and tempered martensite is controlled to be 30% or more and 60% or less. If the total of bainite and tempered martensite is less than 30%, there is a problem that the desired yield ratio and hole expandability cannot be ensured. More preferably, the total of bainite and tempered martensite is limited to 40% or more.

[0046] In the present invention, it is preferable to limit the fraction of ferrite in the microstructure of the steel sheet to 20% or more and 40% or less. If the fraction exceeds 40%, there is a problem that the desired yield ratio and hole expandability cannot be ensured. More preferably, the fraction of the ferrite structure is limited to 30% or less.

[0047] The steel sheet of the present invention has a microstructure containing the remainder fresh martensite and retained austenite, with the area fraction of the retained austenite being less than 5% (excluding 0%). For example, in order to increase the area fraction of the retained austenite to 5% or more, it is necessary to increase the Si content, which may cause the problem of LME. More preferably, the area fraction of the retained austenite is limited to the range of 1 to 4%.

[0048] The steel sheet of the present invention having the above-described microstructure has a superior yield ratio and improved workability and formability compared to conventional DP steels due to the effect of the homogenized structure. Specifically, this makes it possible to provide a steel sheet with excellent ductility, formability, and weldability, in which the value of hole expandability (HER), yield strength (YS), tensile strength (TS), and elongation (EL) calculated by the following relational expression 2 is 500 or more, and the yield ratio (YS / TS) is 0.65 or more, and LME cracking does not occur on the surface or inside of the weld.

[0049] [Equation 2] HER*EL*YS / TS≧500

[0050] Next, the method for producing a steel sheet according to the present invention will be described in detail.

[0051] The method for producing a steel sheet of the present invention includes the steps of: providing a steel slab that satisfies the above-mentioned steel composition and Relational Formula 1, and then reheating the steel slab; hot rolling the reheated steel slab so that the temperature at the exit side of a finish rolling mill is Ar3 to Ar3 + 50°C, and then coiling it at 400 to 650°C, and cooling it to room temperature at an average cooling rate of 0.1°C or less; primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 650 to 680°C at an average cooling rate of 10°C / s or less, and then secondarily cooling it to a temperature of 300 to 340°C at an average cooling rate of 5°C / s or more; reheating the secondarily cooled cold-rolled steel sheet to a temperature of Ms or higher and holding the temperature for 60 seconds or more; and cooling the held cold-rolled steel sheet to a temperature of 150°C or lower at an average cooling rate of 5°C / s or more.

[0052] First, in the present invention, the steel slab having the above-described composition is reheated under normal conditions. The slab reheating process is a process of heating the steel slab to smoothly carry out the subsequent rolling process and to sufficiently obtain the target physical properties of the steel sheet. The present invention is not particularly limited to such reheating conditions, and normal reheating conditions may be used. One example is reheating in a temperature range of 1100 to 1300°C.

[0053] Next, in the present invention, the reheated steel slab is hot rolled so that the temperature at the exit side of the finish rolling is Ar3 to Ar3 + 50°C, and then coiled at 400 to 650°C, and then cooled to room temperature at an average cooling rate of 0.1°C / s or less.

[0054] The reheated steel slab is finish hot rolled under normal conditions at a temperature equal to or higher than the Ar3 transformation point. The present invention is not limited to specific hot rolling conditions, and normal hot rolling temperatures can be used. For example, finish hot rolling can be performed in the temperature range of 800 to 1000°C.

[0055] In the present invention, the finish hot-rolled steel sheet is then coiled in a temperature range of 400 to 650°C, and then cooled to room temperature at an average cooling rate of 0.1°C / s or less to produce a hot-rolled steel sheet in which carbides, which serve as nucleation sites for austenite, are finely dispersed. By uniformly dispersing fine carbides in this hot-rolling process, austenite is formed in a finely dispersed state as the carbides dissolve in the subsequent annealing process, resulting in uniformly dispersed fine martensite after annealing.

[0056] In the present invention, the cooled hot-rolled steel sheet is cold-rolled at a reduction rate of 40 to 70% to produce a cold-rolled steel sheet. If the cold reduction rate is less than 40%, it is difficult to obtain the target thickness and also difficult to correct the shape of the steel sheet. On the other hand, if it exceeds 70%, there is a high possibility that cracks will occur at the edge of the steel sheet, which will cause a problem of load during cold rolling. Therefore, in the present invention, it is preferable to limit the cold reduction rate to 40 to 70%.

[0057] Next, in the present invention, the cold-rolled steel sheet is subjected to continuous annealing in a temperature range of Ac3-50°C to Ac3-20°C. The purpose of this continuous annealing step is to adjust the fractions of ferrite and austenite to achieve an optimal microstructure. If the continuous annealing temperature is below Ac3-50°C, it is difficult to ensure a sufficient austenite fraction, and the ferrite fraction increases after annealing, making it difficult to ensure the desired formability and strength. On the other hand, if the temperature exceeds Ac3-20°C, excessive austenite is formed, resulting in excessively coarse grain size, making it difficult to ensure the desired ductility. In addition, surface segregation of elements that deteriorate surface quality, such as Si, Mn, and B, may become severe, potentially degrading surface quality. In consideration of this, in the present invention, it is preferable to limit the continuous annealing temperature to Ac3-50°C to Ac3-20°C. More preferably, continuous annealing is performed in a temperature range of 810 to 850°C.

[0058] In the present invention, fresh martensite is introduced by primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 650 to 680°C at an average cooling rate of 10°C / s or less, and then secondary cooling to a temperature of 300 to 340°C at an average cooling rate of 5°C / s or more. In this case, in the present invention, the steel sheet can also be cooled using hydrogen gas via hydrogen-grade cooling equipment during the secondary cooling.

[0059] In the present invention, it is very important to control the quenching temperature during secondary cooling to 300 to 340°C, which is below the martensite formation temperature Ms. However, if the temperature exceeds 340°C, the fraction of martensite formed initially will be very small, or it will be difficult to form martensite, making it difficult to form the desired fraction of tempered martensite and bainite during final cooling, and the desired hole expandability will not be obtained. Also, if the temperature is reduced to less than 300°C, the fraction of tempered martensite and bainite will be excessive, making it difficult to form the desired fraction of fresh martensite during final cooling, and the desired strength will not be obtained.

[0060] Next, in the present invention, the above-mentioned secondarily cooled cold-rolled steel sheet is reheated to a temperature of Ms or higher, and then held at that temperature for 60 seconds or longer.

[0061] In the present invention, it is important to control the quenching temperature and reheating temperature in the Q&P process, in which the steel sheet is reheated to above Ms after the aforementioned first slow cooling process and second rapid cooling process in which the steel sheet is cooled to below Ms, to form a desired microstructure. By holding the reheated cold-rolled steel sheet for 60 seconds or more, tempered martensite and bainite are formed, and carbon is concentrated in the surrounding untransformed austenite.

[0062] Thereafter, in the present invention, the cold-rolled steel sheet thus held is cooled to a temperature of 150°C or less at an average cooling rate of 5°C / s or more to introduce fine fresh martensite.

[0063] If necessary, the cooled steel sheet may be subjected to temper rolling of less than 1%.

[0064] Furthermore, if necessary, the cooled steel sheet may be subjected to one of hot-dip galvanizing and galvannealed hot-dip galvanizing to produce a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet. [Example]

[0065] The present invention will be described in detail below with reference to preferred examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0066] (Example) A steel slab having the composition shown in Table 1 below was prepared, reheated to a temperature range of 1050 to 1250°C, and finish hot-rolled at a temperature range of Ar3 + 50°C, above the Ar3 transformation point. The hot-rolled steel sheet was then coiled at 400 to 650°C and cooled at a cooling rate of 0.1°C per second or less to produce a hot-rolled steel sheet. The hot-rolled steel sheet was pickled, cold-rolled at a reduction of 40 to 70%, and then continuously annealed under the conditions shown in Table 2 below. Q&P heat treatment was then performed. The steel sheet was then cooled to a temperature of 150°C or less at an average cooling rate of 5°C / s or more, and then temper-rolled to a temperature of less than 1°C to produce a final steel sheet. Meanwhile, the Q&P heat treatment in this experiment was performed with a primary cooling rate of 6°C / s, a secondary cooling rate of 15°C / s, and a holding time of 200 seconds after reheating.

[0067] The mechanical properties and microstructural properties of each steel sheet manufactured as described above were evaluated, and the results are shown in Table 3 below. Tensile tests were performed on each test piece in the C direction according to JIS standards, and tensile properties were evaluated. The microstructural fraction was determined by analyzing the matrix structure at 1 / 4 of the thickness of the annealed steel sheet. Specifically, after Nital corrosion, the fractions of ferrite, bainite, martensite, and austenite were measured using an FE-SEM and an image analyzer. Hole expandability was also measured using a hole expandability tester. Furthermore, the occurrence of LME cracking in the spot welds was evaluated.

[0068] [Table 1]

[0069] [Table 2]

[0070] [Table 3]

[0071] As shown in Tables 1 to 3 above, in the case of Invention Examples 1 to 4, in which the ranges of steel composition, manufacturing conditions, and steel microstructure satisfy the ranges of the present invention, the hole expandability (HER) is 45% or more, and the value of the relational expression 2 between the hole expandability (HER), yield strength (YS), tensile strength (TS), and elongation (EL) is 500 or more. Furthermore, it is clear that no LME cracks occur on the surface or inside of the weld, the yield ratio (YS / TS) is 0.65 or more, and the material properties, hole expandability, and weldability of the steel sheet targeted in the present invention can be ensured.

[0072] In contrast, in Comparative Examples 1 to 8, where the steel composition (Relational Formula 1) and manufacturing process are outside the scope of the present invention, or the fraction of the steel microstructure is outside the scope of the present invention, the values ​​according to Relational Formula 2 for hole expandability (HER), yield strength (YS), tensile strength (TS), and elongation (EL) are all less than 500, or the yield ratio (YS / TS) is less than 0.65, and the hole expandability is less than 30%. Therefore, it is clear that the strength, ductility, hole expandability, and weldability of the steel sheet targeted by the present invention cannot be simultaneously ensured.

[0073] Specifically, in Comparative Example 1, the annealing temperature was too high, resulting in low elongation and failure to ensure hole expandability, while in Comparative Example 2, the annealing temperature was too low, resulting in excessive formation of two-phase ferrite and failure to achieve the target strength.

[0074] In Comparative Example 3, the secondary cooling end temperature was too high, so that the tempered martensite could not be sufficiently obtained, and the hole expandability could not be ensured.

[0075] In Comparative Example 4, the secondary cooling end temperature was too low, resulting in the formation of excessive tempered martensite and bainite, and the strength was deteriorated.

[0076] In addition, in Comparative Examples 5 to 8, the steel composition (relationship 1) was outside the range of the present invention, and none of them satisfied the target mechanical properties, and furthermore, LME cracks occurred on the surface and inside of the weld.

[0077] On the other hand, Fig. 1 is a diagram showing the changes in values ​​of hole expandability (HER) according to Relational Formula 1 and yield strength (YS), tensile strength (TS), and elongation (EL) according to Relational Formula 2 for the invention steel and the comparative steel in the examples of the present invention, Fig. 2 is a diagram showing the changes in hole expandability (HER) according to Relational Formula 1 for the invention steel and the comparative steel in the examples of the present invention, and Fig. 3 is a diagram showing the changes in yield ratio according to Relational Formula 1 for the invention steel and the comparative steel in the examples of the present invention. Note that in Figs. 1 to 3 above, the invention steel refers to the steels corresponding to invention examples 1 to 4, and the comparative steel refers to the steels corresponding to comparative examples 5 to 8.

[0078] As described above, the detailed description of the present invention has been given with reference to the preferred embodiment of the present invention, but it goes without saying that a person skilled in the art to which the present invention pertains can make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be determined by the claims below as well as their equivalents.

Claims

1. In weight percent, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 1.6 to 2.6%, molybdenum (Mo): 0.40% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.10% or less (excluding 0%), sulfur (S): 0.020% or less (excluding 0%), aluminum (sol. Al): 0.60% or less (excluding 0%), titanium (Ti): 0.001 to 0.04%, niobium (Nb): 0.001 to 0.04%, nitrogen (N): 0.01% or less (excluding 0%), boron (B): 0.010% or less (including 0%), the balance being Fe and other unavoidable impurities, wherein the C, Si, Mn, Cr, and Mo satisfy the following relational expression 1, The steel has a microstructure containing, in area percentages, ferrite in an amount of 20% to 40%, bainite and tempered martensite in total in an amount of 30% to 60%, and the remainder being fresh martensite and retained austenite, with the area fraction of the retained austenite being less than 5% (excluding 0%); and A steel sheet having a hole expandability (HER) of 30% or more, a value of 500 or more according to the following relational expression 2 for the hole expandability (HER), yield strength (YS), tensile strength (TS), and elongation (EL), and a yield ratio (YS / TS) of 0.65 or more. [Relationship 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.300 Here, C, Si, Mn, Cr and Mo mean the content % of the steel components in the base structure at 1 / 4t point of the thickness of the base steel sheet. [Relationship 2] HER*EL*YS / TS≧500

2. The steel plate according to claim 1, wherein the fraction of the retained austenite satisfies 1 to 4% in terms of area %.

3. The steel sheet according to claim 1, wherein at least one surface of the steel sheet is formed with one of a hot-dip galvanized layer and a hot-dip galvannealed layer.

4. In weight percent, carbon (C): 0.06 to 0.16%, silicon (Si): 0.8% or less (excluding 0%), manganese (Mn): 1.6 to 2.6%, molybdenum (Mo): 0.40% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.10% or less (excluding 0%), sulfur (S): 0.020% or less (excluding 0%), aluminum (sol. Al) a step of preparing a steel slab containing C, Si, Mn, Cr, and Mo satisfying the following relational expression 1, and then reheating the steel slab; a step of hot rolling the reheated steel slab so that the temperature at the exit side of the finish rolling is Ar3 to Ar3 + 50°C, and then coiling the slab at 400 to 650°C, followed by cooling the slab to room temperature at an average cooling rate of 0.1°C / s or less; cold rolling the cooled hot-rolled steel sheet at a reduction ratio of 40 to 70% to produce a cold-rolled steel sheet; A step of continuously annealing the cold-rolled steel sheet at a temperature of Ac3-50°C to Ac3-20°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to a temperature range of 650 to 680°C at an average cooling rate of 10°C / s or less, and then secondarily cooling to a temperature of 300 to 340°C at an average cooling rate of 5°C / s or more; Reheating the secondarily cooled cold-rolled steel sheet to a temperature of Ms or higher and then holding the temperature for 60 seconds or more; and cooling the held cold-rolled steel sheet to a temperature of 150°C or less at an average cooling rate of 5°C / s or more. [Relationship 1] C+Si / 30+Mn / 20+(Cr+Mo) / 5≧0.300 Here, C, Si, Mn, Cr and Mo mean the content % of the steel components in the base structure at 1 / 4t point of the thickness of the base steel sheet.

5. The method for producing a steel sheet according to claim 4, wherein continuous annealing is performed in a temperature range of 810 to 850°C.

6. The method for producing a steel plate according to claim 4, wherein the cooled steel plate is further subjected to temper rolling of less than 1%.

7. The method for manufacturing a steel sheet according to claim 4, wherein the cooled steel sheet is further subjected to one of hot-dip galvanizing and galvannealing.

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