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, in weight percent, C: 0.03 to 0.18%, Si: 0 to 0.70%, Mn: 0.25 to 1.80%, Al: 0 to 0.70%, P: 0.050% or less, S: 0.0300% or less, N: 0.0300% or less, Ti: 0 to 0.08%, Nb: 0 to 0.06%, V: 0 to 0.07%, Ti + Nb + V: 0.03 to 0.10%, Cr: 0 to 0.80%, Mo: 0 to 0.80%, Cu: 0 to 0.80%, Ni: 0 to 0.80%. %, B: 0-0.0050%, Ca: 0-0.050%, REM excluding Y: 0-0.050%, Mg: 0-0.050%, W: 0-0.50%, Zr: 0-0.50%, Sb: 0-0.50%, Sn: 0-0.50%, Y: 0-0.20%, Hf: 0-0.20%, the balance being Fe and other unavoidable impurities, and has a structure in area percentages of 1-15% pearlite including cementite, and the balance being ferrite and unavoidably flowing in ferrite.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet that can be used for various applications including parts for automobiles, home appliances, steel structures, etc., and in particular to a steel sheet that has an excellent balance of ductility, formability, and yield ratio, and a method for manufacturing the same. [Background technology]

[0002] Cold-rolled or plated steel sheets used in automobiles, home appliances, steel structures, etc. are required to have excellent ductility and formability as well as a sufficient yield ratio.

[0003] Methods for increasing yield strength include solid solution hardening and precipitation hardening.

[0004] Solid-solution strengthened steel is a steel sheet in which solid-solution strengthening elements (e.g., Mn, Si, Cr) are dissolved in ferrite, which has excellent formability, to increase the yield strength. However, Si and Cr are elements that tend to form oxides on the steel sheet surface during continuous annealing or continuous hot-dip galvanizing. Furthermore, Mn promotes the formation of low-temperature transformation phases (bainite or martensite), which tend to reduce yield strength. Therefore, solid-solution strengthened steel with large amounts of Mn, Si, and Cr added is not suitable as a method for increasing the yield ratio of steel sheets with a tensile strength of 410 MPa or more.

[0005] Precipitation-hardened steel, which utilizes elements such as Nb, Ti, and V, is a steel sheet that improves yield strength by precipitating fine carbides within the ferrite. Precipitation-hardened steel increases the yield ratio without degrading workability, making it a suitable strengthening mechanism for steel sheets with excellent workability and a tensile strength of 410 MPa or more.

[0006] As a technique for improving the formability and yield ratio of steel sheets, methods utilizing the introduction of non-recrystallized ferrite and the addition of Ti or Nb are disclosed in Patent Documents 1 and 2. Precipitation strengthening using Ti or Nb and non-recrystallized ferrite are effective in increasing yield strength without significantly increasing tensile strength by directly strengthening ferrite.

[0007] The manufacturing method described in Patent Document 1 satisfies a certain yield ratio (YR) of 0.7 to 0.85 or a certain product of the square of the tensile strength and the square root of the hole expandability (TS 2 ×√HER) is 1.4 to 3.0 × 10 6 (MPa) 2 % 0.5 However, the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) is 0.7 to 1.7 × 10 6 (MPa) 2 % 0.5 does not meet the above criteria.

[0008] In addition, the manufacturing method described in Patent Document 2 uses the product of the square of the tensile strength and the square root of the hole expandability (TS 2 ×√HER) is 1.4 to 3.0 × 10 6 (MPa) 2 % 0.5 or the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) is 0.7 to 1.7 × 10 6 (MPa) 2 % 0.5 However, the yield ratio (YR) does not satisfy the requirement of 0.7 to 0.85.

[0009] This means that the introduction of unrecrystallized ferrite into a steel sheet is not suitable for producing a steel sheet having an excellent balance of ductility, formability, and yield ratio, and therefore there is a continuing demand for technological development in this regard. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2009-114523 [Patent Document 2] Japanese Patent Publication No. 2017-002333 Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, an object of the present invention is to provide a cold-rolled steel sheet or an alloy-plated steel sheet which does not contain unrecrystallized ferrite and has an excellent balance of ductility, formability and yield ratio, and a method for producing the same. 2 ×√EL and TS 2 The object of the present invention is to provide a steel sheet with an excellent balance of ×√HER and a manufacturing method thereof.

[0012] Furthermore, the technical problems that the present invention aims to solve are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by a person having ordinary knowledge in the technical field to which the present invention pertains from the following description. [Means for solving the problem]

[0013] One embodiment of the present invention comprises: By weight, C: 0.03-0.18%, Si: 0-0.70%, Mn: 0.25-1.80%, Al: 0-0.70%, P: 0.050% or less, S: 0.0300% or less, N: 0.0300% or less, Ti: 0-0.08%, Nb: 0-0.06%, V: 0-0.07%, Ti+Nb+V: 0.03-0.10%, Cr: 0-0.80%, Mo: 0-0.80% , Cu: 0-0.80%, Ni: 0-0.80%, B: 0-0.0050%, Ca: 0-0.050%, REM excluding Y: 0-0.050%, Mg: 0-0.050%, W: 0-0.50%, Zr: 0-0.50%, Sb: 0-0.50%, Sn: 0-0.50%, Y: 0-0.20%, Hf: 0-0.20%, balance Fe and other unavoidable impurities, The steel sheet microstructure includes, by area%, 1 to 15% pearlite including cementite, residual ferrite, and an unavoidably inflowing structure; and The yield ratio (YR) of the steel plate is 0.70 to 0.85, and the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) is 0.7×10 6 ~1.7×10 6 (MPa) 2 %0.5 and the square root of the hole expandability (TS 2 ×√HER) is 1.4×10 6 ~3.0×10 6 (MPa) 2 % 0.5 The present invention relates to a steel plate that satisfies the above.

[0014] The content of C is preferably in the range of 0.04 to 0.17% by weight.

[0015] The Mn content is preferably in the range of 0.27 to 1.78% by weight.

[0016] It is preferable that the steel sheet has a microstructure that contains, by area %, 3 to 13% pearlite containing cementite, residual ferrite, and structures that inevitably flow in.

[0017] The yield ratio (YR=YS / TS), which is the value obtained by dividing the yield strength (YS) of the steel plate by the tensile strength (TS), preferably satisfies 0.71 to 0.84.

[0018] The product of the square of the tensile strength and the square root of the elongation of the above steel plate (TS 2 ×√EL) is 0.71×10 6 ~1.69×10 6 (MPa) 2 % 0.5 It is preferable that the following is satisfied.

[0019] The product of the square of the tensile strength and the square root of the hole expandability (TS 2 ×√HER) is 1.43×10 6 ~2.93×10 6 (MPa) 2 % 0.5 It is preferable that the following is satisfied.

[0020] Another embodiment of the present invention is heating a steel ingot or slab having the above-mentioned composition to a temperature range of 1000 to 1350°C; a step of producing a hot-rolled steel sheet by hot-rolling the heated steel ingot or slab at a finish rolling temperature of 800 to 1000°C; cooling the hot-rolled steel sheet and then coiling it at a temperature in the range of 300 to 600°C; heat-treating the coiled hot-rolled steel sheet at a temperature range of 650 to 800°C for 600 to 1700 seconds, and then cold-rolling it at a reduction rate of 30 to 90%; heating the rolled cold-rolled steel sheet in a temperature range of 740 to 860°C and holding it for 50 seconds or more; performing primary cooling of the primarily held cold-rolled steel sheet to a temperature range of 600 to 760°C at an average cooling rate of 5°C / s or less; and The method for manufacturing a steel sheet includes the steps of: secondarily cooling the primarily cooled cold-rolled steel sheet to a temperature range of 450 to 550°C at an average cooling rate of more than 5°C / s and not more than 20°C / s; secondarily holding the steel sheet in this temperature range for 50 seconds or more; and then cooling the steel sheet to room temperature. [Effects of the Invention]

[0021] The present invention as described above can effectively provide a cold-rolled steel sheet or an alloy-plated steel sheet which does not contain unrecrystallized ferrite and has an excellent balance of ductility, formability, and yield ratio, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described below.

[0023] The present inventors have confirmed that an optimal microstructure fraction, the addition of elements to realize this, and an optimal manufacturing process are necessary to produce a steel sheet having excellent formability and a high yield ratio.

[0024] In particular, YR and TS of steel plates 2 ×√EL, TS 2 To improve ×√HER, it is important to optimize the ferrite fraction. Specifically, if the recrystallized ferrite fraction is not 85-99%, the YR will be 0.7-0.85 or TS 2 ×√EL is 0.7×106 ~1.7×10 6 (MPa) 2 % 0.5 or T.S. 2 ×√HER is 1.4×10 6 ~3×10 6 (MPa) 2 % 0.5 This has confirmed that the above-mentioned problem cannot be solved, and the present invention is presented.

[0025] The steel sheet of the present invention having an excellent balance of ductility, formability and yield ratio contains, in weight percent, C: 0.03 to 0.18%, Si: 0 to 0.70%, Mn: 0.25 to 1.80%, Al: 0 to 0.70%, P: 0.050% or less, S: 0.0300% or less, N: 0.0300% or less, Ti: 0 to 0.08%, Nb: 0 to 0.06%, V: 0 to 0.07%, Ti+Nb+V: 0.03 to 0.10%, Cr: 0 to 0.80%, Mo: 0 to 0.80%, Cu: 0 to 0.80%, Ni: 0 to 0.80%, B: 0 to 0.0050%, Ca: 0 to 0.80%, and Si: 0 to 0.70%. : 0-0.050%, REM excluding Y: 0-0.050%, Mg: 0-0.050%, W: 0-0.50%, Zr: 0-0.50%, Sb: 0-0.50%, Sn: 0-0.50%, Y: 0-0.20%, Hf: 0-0.20%, the balance being Fe and other unavoidable impurities, and the steel sheet microstructure includes, in area %, 1-15% pearlite including cementite, and the remaining ferrite and a structure that inevitably flows in, and the yield ratio (YR) of the steel sheet is 0.7-0.85, and the product of the square of the tensile strength and the square root of the elongation (TS) of the steel sheet is 0.7-0.85. 2 ×√EL) is 0.7×10 6 ~1.7×10 6 (MPa) 2 % 0.5 and the square root of the hole expandability (TS 2 ×√HER) is 1.4×10 6 ~3×10 6 (MPa) 2 % 0.5 Meet the following.

[0026] First, the chemical composition of the steel sheet provided by the present invention will be described in detail below. Here, unless otherwise specified, the content of each component means % by weight.

[0027] C: 0.03 to 0.18% Carbon (C) is an essential element for forming precipitates together with Ti, Nb, or V in the ferrite phase to impart strength to steel sheets. If the amount of carbon added is less than 0.03%, it is difficult to ensure a tensile strength of 410 MPa or more. On the other hand, if the amount of carbon added exceeds 0.18%, it is difficult to ensure the weld strength of welded parts. Therefore, in the present invention, the C content is preferably controlled to a range of 0.03 to 0.18%. More preferably, the C content is controlled to a range of 0.04 to 0.17%.

[0028] Si: 0 to 0.70% Silicon (Si) is an element that has the effect of improving strength through solid solution strengthening, strengthening ferrite, homogenizing the structure, and improving workability. It is also an element necessary for deoxidation during steelmaking. If the amount of Si added exceeds 0.70%, it can cause plating defects such as unplated areas during the plating process and reduce the weldability of the steel sheet. Therefore, in the present invention, the Si content is preferably controlled to a range of 0 to 0.70%, and more preferably to a range of 0 to 0.50%.

[0029] Mn: 0.25 to 1.80% Manganese (Mn) is a useful element for increasing both strength and ductility. Its effect is apparent when its content is 0.25% or more. However, if its content exceeds 1.80%, the low-temperature transformation of austenite to martensite or bainite is promoted, resulting in a decrease in the yield ratio of the steel sheet. Therefore, in the present invention, the Mn content is preferably controlled to 0.25 to 1.80%. More preferably, the Mn content is controlled to the range of 0.27 to 1.78%.

[0030] Al: 0 to 0.70% Aluminum (Al) is an element that combines with oxygen in steel to deoxidize it. Similarly to Si, it strengthens ferrite, homogenizes the structure, and improves workability. Addition of more than 0.70% of Al can lead to plating defects, such as unplated areas, during the plating process and can also reduce the weldability of the steel sheet. Therefore, in the present invention, the Al content is preferably controlled to a range of 0 to 0.70%, more preferably 0 to 0.50%.

[0031] P:0.050% or less P is an element that is contained as an impurity and deteriorates impact toughness. Therefore, in the present invention, the P content is preferably controlled to 0.050% or less. The P content is more preferably limited to 0.020% or less, and further preferably to 0.015% or less.

[0032] S: 0.0300% or less S is an element that is contained as an impurity and forms MnS in the steel sheet, which deteriorates ductility. Therefore, in the present invention, the S content is preferably controlled to 0.0300 wt% or less. The S content is more preferably limited to 0.0100% or less, and further preferably to 0.0050% or less.

[0033] N: 0.0300% or less N is an element that is contained as an impurity and forms nitrides during continuous casting, causing cracks in the slab. Therefore, in the present invention, the N content is preferably controlled to 0.0300 wt% or less. The N content is more preferably limited to 0.0100% or less, and even more preferably to 0.0050% or less.

[0034] Ti: 0-0.08%, Nb: 0-0.06%, and V: 0-0.07% Ti+Nb+V: 0.03 to 0.10% Ti, Nb, and V are important elements that form precipitates in steel sheets and may be added to improve the strength and impact toughness of the steel sheets. In the present invention, the contents of these elements are preferably controlled to the ranges of Ti: 0 to 0.08%, Nb: 0 to 0.06%, and V: 0 to 0.07%, respectively. If each of the Ti, Nb, and V contents exceeds its upper limit, excessive precipitate formation occurs, resulting in the generation of unrecrystallized ferrite, which may result in excessive property effects and increased manufacturing costs.

[0035] Preferably, in the present invention, the total content (Ti + Nb + V) of one or more of Ti, Nb, and V is controlled to a range of 0.03 to 0.10%. If the content of Ti + Nb + V is less than 0.03%, the effect of adding the component elements cannot be expected, and if it exceeds 0.10%, excessive precipitates are formed, resulting in the generation of unrecrystallized ferrite, which may result in excessive property effects as well as an increase in manufacturing costs.

[0036] Cr: 0-0.80% and Mo: 0-0.80% Cr and Mo are optional elements used in the present invention, which inhibit austenite decomposition during alloying and stabilize austenite similarly to Mn. If the Cr and Mo contents exceed 0.80%, the formation of low-temperature transformation phases such as martensite or bainite is promoted, resulting in a decrease in the yield ratio of the steel sheet. Therefore, it is preferable to control the Cr and Mo contents to 0.80% or less, and more preferably to 0.50% or less.

[0037] Cu: 0-0.80%, Ni: 0-0.80% Cu and Ni are optional elements used in the present invention to stabilize austenite and inhibit corrosion. Furthermore, Cu and Ni concentrate on the steel sheet surface to prevent hydrogen penetration and migration into the steel sheet, thereby inhibiting delayed hydrogen fracture. In the present invention, if the Cu and Ni contents exceed 0.80%, not only will excessive property effects occur but also manufacturing costs will increase. Therefore, in the present invention, the Cu and Ni contents are preferably controlled to 0.80% or less, and more preferably 0.50% or less.

[0038] B: 0 to 0.0050% B is an optional element used in the present invention, which improves hardenability, increases strength, and suppresses nucleation at grain boundaries. If the B content exceeds 0.0050%, it not only has excessively adverse effects on properties but also increases manufacturing costs. Therefore, in the present invention, it is preferable to control the B content to 0.0050% or less.

[0039] Ca: 0-0.050%, Mg: 0-0.050%, and REM excluding Y: 0-0.050% REM refers to a total of 17 elements, including Sc, Y, and lanthanides. REM, excluding Ca, Mg, and Y, are optional elements used in the present invention to improve the ductility of steel sheets by spheroidizing sulfides. If the content of each of the REM elements excluding Ca, Mg, and Y exceeds 0.050%, this not only results in excessively high performance but also increases manufacturing costs. Therefore, in the present invention, the content of each of the REM elements excluding Ca, Mg, and Y is preferably controlled to 0.050% or less, and more preferably to 0.010% or less.

[0040] W: 0-0.50%, Zr: 0-0.50% W and Zr are optional elements used in the present invention to improve hardenability and increase the strength of the steel sheet. If the W and Zr contents exceed 0.50%, not only will excessive property effects occur but also manufacturing costs will increase. Therefore, in the present invention, the W and Zr contents are preferably controlled to 0.50% or less, and more preferably 0.20% or less.

[0041] Sb: 0-0.50%, Sn: 0-0.50% Sb and Sn are optional elements used in the present invention to improve the plating wettability and plating adhesion of steel sheets. If the Sb and Sn contents exceed 0.50%, the brittleness of the steel sheet increases, and cracks may occur during hot or cold working. Therefore, in the present invention, the content of at least one of Sb and Sn is preferably controlled to 0.50 wt% or less, and more preferably 0.20% or less.

[0042] Y: 0-0.20%, Hf: 0-0.20% Y and Hf are optional elements used in the present invention to improve the corrosion resistance of the steel sheet. If the Y and Hf contents exceed 0.20%, the ductility of the steel sheet may deteriorate. Therefore, in the present invention, the Y and Hf contents are preferably controlled to 0.20% or less, and more preferably 0.10% or less.

[0043] In addition to the above composition, the remainder preferably contains Fe and inevitable impurities, and the steel material of the present invention does not exclude the addition of other components. The inevitable impurities mentioned above cannot be excluded because they may be unintentionally mixed in from raw materials or the surrounding environment during the normal steel manufacturing process. The meaning of the inevitable impurities mentioned above is understood by engineers in the field of normal steel manufacturing.

[0044] Furthermore, the present invention may have a steel sheet microstructure that includes, by area %, 1 to 15% pearlite containing cementite, residual ferrite, and structures that inevitably flow in. If the pearlite fraction is less than 1% or exceeds 15%, the yield ratio, elongation, or hole expandability of the steel sheet may decrease. This ultimately leads to the steel sheet having a YR of 0.70 to 0.85 and a TS of 0.70 to 0.85, which will be described later. 2 ×√EL 0.7×10 6 ~1.7×10 6 (MPa) 2 % 0.5 , or TS 2 ×√HER 1.4×10 6 ~3.0×10 6 (MPa) 2 % 0.5 This does not satisfy the above requirements and is therefore undesirable.

[0045] More preferably, the steel sheet has a microstructure containing, in area %, 3 to 13% pearlite containing cementite, residual ferrite, and a structure that inevitably flows in.

[0046] On the other hand, the steel sheet of the present invention having the above-mentioned fraction of fine structure has a tensile strength of 410 MPa or more (preferably 410 to 580 MPa) and a yield ratio (YR) of 0.70 to 0.85, and the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) is 0.7×10 6 ~1.7×10 6 (MPa) 2 % 0.5 and the product of the square of the tensile strength and the square root of the hole expandability (TS 2 ×√HER) is 1.4×10 6 ~3×10 6 (MPa) 2 % 0.5 can be satisfied.

[0047] More preferably, the yield ratio (YR=YS / TS) can be controlled to 0.71 to 0.84.

[0048] In addition, the product of the square of the tensile strength of the steel plate and the square root of the elongation (TS 2 ×√EL) is 0.71×10 6~1.69×10 6 (MPa) 2 % 0.5 It is more preferable that the following be satisfied.

[0049] In addition, the product of the square of the tensile strength of the steel sheet and the square root of the hole expandability (TS 2 ×√HER) is 1.43×10 6 ~2.93×10 6 (MPa) 2 % 0.5 It is more preferable that the following be satisfied.

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

[0051] The method for producing a steel sheet of the present invention includes the steps of: heating a steel ingot or slab having the above-described composition to a temperature of 1000 to 1350°C; hot-rolling the heated steel ingot or slab at a finish rolling temperature of 800 to 1000°C to produce a hot-rolled steel sheet; cooling the produced hot-rolled steel sheet and then coiling it at a temperature range of 300 to 600°C; heat-treating the coiled hot-rolled steel sheet at a temperature range of 650 to 800°C for 600 to 1700 seconds, and then reducing it by 30 to 90%. the step of heating the rolled cold-rolled steel sheet to a temperature range of 740 to 860°C and holding it for 50 seconds or more; the step of primarily cooling the primarily-held cold-rolled steel sheet to a temperature range of 600 to 760°C at an average cooling rate of 5°C / s or less; and the step of secondarily cooling the primarily-cooled cold-rolled steel sheet to a temperature range of 450 to 550°C at an average cooling rate of more than 5°C / s and 20°C / s or less, and then holding it in this temperature range for 50 seconds or more, followed by cooling to room temperature.

[0052] heating In the present invention, first, a steel ingot or slab having the above-mentioned composition is heated to a temperature range of 1000 to 1350°C.

[0053] Although the present invention is not particularly limited to the flux process for producing the steel ingot or slab, it is preferable to limit the heating temperature to a range of 1000 to 1350°C. This is because if the heating temperature is less than 1000°C, the base material will be hot rolled below the finish rolling temperature range, and if the heating temperature exceeds 1350°C, the base material may reach the melting point of the steel and melt.

[0054] Finishing hot rolling Next, in the present invention, the heated steel ingot or slab is hot rolled at a finish rolling temperature of 800 to 1000°C to produce a hot rolled steel sheet.

[0055] If the finish rolling temperature is less than 800°C, the high strength of the steel may place a heavy burden on the hot rolling mill, while if the finish temperature exceeds 1000°C, the grains of the steel sheet after hot rolling may become coarse, which may deteriorate the physical properties of the steel sheet.

[0056] Preferably, the finish hot rolling is carried out in the temperature range of 830 to 950°C.

[0057] Winding after cooling In the present invention, the hot-rolled steel sheet produced as described above is cooled and then coiled at a temperature in the range of 300 to 600°C.

[0058] After the finish hot rolling, the finish hot rolled steel sheet is cooled in order to refine the crystal grains of the hot rolled steel sheet, and at this time, it is preferable to set the average cooling rate to 10°C / s or more.

[0059] Furthermore, hot-rolled steel sheets whose main phase is a low-temperature transformation phase of martensite or bainite, or in which scale has formed deep inside the steel sheet, are not easy to coil or pickle, and therefore the coiling temperature of the hot-rolled steel sheet is preferably 300 to 600°C.

[0060] If the coiling temperature is less than 300°C, the main phase of the hot-rolled steel sheet will be composed of a high-strength low-temperature transformation phase, which may make it difficult to coil the hot-rolled steel sheet. On the other hand, if the coiling temperature exceeds 600°C, scale formed on the surface of the hot-rolled steel sheet may extend deep into the hot-rolled steel sheet, which may make pickling difficult.

[0061] Preferably, the coiling is carried out at a temperature in the range of 350 to 550°C.

[0062] Cold rolling after precipitate forming annealing heat treatment Next, in the present invention, the coiled hot-rolled steel sheet is heat-treated in a temperature range of 650 to 800°C for 600 to 1700 seconds, and then cold-rolled at a reduction ratio of 30 to 90%.

[0063] The present inventors have confirmed that annealing a hot-rolled steel sheet at an appropriate high temperature for a short time facilitates optimization of the precipitates in the hot-rolled steel sheet. This is because Ti, Nb, or V, which was not precipitated during coiling in the hot-rolling process, is re-precipitated in the hot-rolled steel sheet during annealing at an appropriate temperature and time, contributing to optimization of the precipitates in the steel sheet. That is, the hot-rolled steel sheet of the present invention is required to be annealed at an appropriate high temperature for a short time. This is because annealing at an appropriate temperature and time promotes the formation of precipitates in the hot-rolled steel sheet, thereby improving the yield ratio of the final product.

[0064] Therefore, in the present invention, the annealing heat treatment of the hot-rolled steel sheet is preferably performed in a temperature range of 650 to 800°C for 600 to 1700 seconds. If the heat treatment conditions are lower than 650°C or shorter than 600 seconds, it may not be easy to optimize the precipitates in the annealed steel sheet. On the other hand, if the heat treatment conditions are higher than 800°C or longer than 1700 seconds, it may not be easy to obtain precipitation in the annealed steel sheet. As a result, there is a problem that the desired yield ratio of the final steel sheet cannot be obtained.

[0065] Preferably, the annealing heat treatment is carried out in the temperature range of 680 to 750°C.

[0066] Subsequently, in the present invention, the hot-rolled steel sheet that has been subjected to the annealing heat treatment may be subjected to a conventional pickling treatment. The pickled hot-rolled steel sheet is then preferably cold-rolled at a cumulative reduction of 30 to 90%. If the cumulative reduction of cold rolling exceeds 90%, it may be difficult to perform cold rolling in a short time due to the high strength of the steel sheet.

[0067] The cold-rolled steel sheet according to the present invention may be manufactured as an uncoated cold-rolled steel sheet after an annealing process, or may be manufactured as a coated steel sheet after a coating process to impart corrosion resistance. Coating may be performed by a coating method such as hot-dip galvanizing, electrogalvanizing, or hot-dip aluminum coating.

[0068] Primary retention In the present invention, the rolled cold-rolled steel sheet is heated in a temperature range of 740 to 860°C and held for 50 seconds or more.

[0069] If the primary holding temperature is less than 740°C, unrecrystallized ferrite is generated, resulting in the YR and TS of the steel sheet. 2 ×√EL and TS 2 In addition, if the primary holding temperature exceeds 860°C, the fraction of pearlite containing cementite exceeds 15%, and the YR and TS of the above steel plate decrease. 2 ×√EL and TS 2 ×√Decrease HER.

[0070] Furthermore, if the first holding time is less than 50 seconds, the heat treatment time will be insufficient, resulting in the YR and TS of the steel sheet being reduced. 2 ×√EL and TS 2 ×√HER may be reduced.

[0071] Primary cooling The cold-rolled steel sheet that has been primarily held is primarily cooled to a temperature range of 600 to 760°C at an average cooling rate of 5°C / s or less.

[0072] If the cooling stop temperature is less than 600°C, the cooling stop temperature is too low and TS 2×√EL and TS 2 In addition, if the cooling end temperature exceeds 760°C, the cooling end temperature is too high and TS 2 ×√EL and TS 2 ×√HER may be reduced.

[0073] Furthermore, if the primary cooling rate exceeds 5°C / s, problems with equipment specifications may arise.

[0074] Secondary holding after secondary cooling Subsequently, in the present invention, the primarily cooled cold-rolled steel sheet is secondarily cooled to a temperature range of 450 to 550°C at an average cooling rate of more than 5°C / s and not more than 20°C / s, and then held in this temperature range for 50 seconds or more, and then cooled to room temperature.

[0075] When the secondary average cooling rate is 5℃ / s or less, the pearlite fraction including cementite exceeds 15% and TS 2 ×√EL and TS 2 ×√HER may decrease, and if the secondary average cooling rate exceeds 20°C / s, problems may arise with equipment specifications.

[0076] In addition, if the secondary holding temperature is less than 450°C, the TS of the above steel sheet can be obtained at a low heat treatment temperature. 2 ×√EL and TS 2 In addition, if the secondary holding temperature exceeds 550°C, the YR and TS of the above steel sheets will decrease. 2 ×√EL and TS 2 ×√HER may be reduced.

[0077] Furthermore, if the second holding time is less than 50 seconds, the heat treatment time will be insufficient, and the TS of the steel sheet will be 2 ×√EL and TS 2 ×√HER may be reduced.

[0078] The steel plate is then cooled to room temperature after the second holding as described above, and the YR and TS are then measured. 2 ×√EL, TS 2This makes it possible to effectively manufacture steel plates with an excellent balance of ×√HER. [Example]

[0079] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0080] (Example) A 100 mm thick slab having the chemical composition shown in Table 1 below was manufactured. The slab was heated to 1200°C and then hot rolled at a finish rolling temperature of 900°C to manufacture a 3 mm thick hot-rolled steel sheet. The hot-rolled steel sheet was then cooled at an average cooling rate of 30°C / s and coiled at the hot-rolled steel sheet coiling temperature shown in Table 2 below. The coiled hot-rolled steel sheet was then heated to the annealing temperature shown in Table 2 below, held for a certain annealing time, and cooled to room temperature. The annealed steel sheet was pickled to remove surface scale, and then cold-rolled to a thickness of 1.5 mm. The cold-rolled steel sheet was heated to the first holding temperature shown in Table 2 below and held at the first holding temperature for the first holding time. The cold-rolled steel sheet after the first holding was then cooled at an average primary cooling rate to the first cooling stop temperature shown in Table 3 below. Next, the primarily cooled cold-rolled steel sheet was cooled to a secondary holding temperature at a secondary average cooling rate shown in Table 3 below, held for a secondary holding time, and then cooled to room temperature at a tertiary average cooling rate shown in Table 3 below.

[0081] The microstructure of each steel sheet manufactured as described above was measured and is shown in Table 4 below. The presence of recrystallized ferrite, unrecrystallized ferrite, cementite, and low-temperature transformed structures (bainite and martensite) in the microstructure of the steel sheets was examined using a SEM after nital etching of the polished cross section of the test specimen. After nital etching, the smooth structure on the test specimen surface was identified as ferrite, while the structure with a globular or lamellar structure was identified as cementite. Unrecrystallized ferrite, which contains a large amount of potential, has crystal misorientation within the grains. Therefore, the crystal orientation of ferrite was measured using FESEM-EBSD, and the unrecrystallized ferrite within the ferrite was distinguished using the KAM (Kernel Average Misorientation) method.

[0082] The mechanical properties of each of the steel sheets produced above were measured and are shown in Table 5 below. The properties of the steel sheets were evaluated by tensile tests and hole expansion tests. The tensile tests were performed using test pieces taken in the 0° direction relative to the rolling direction of the rolled sheet material in accordance with JIS No. 5 standard, and the YR, TS 2 The hole expansion test was carried out by compressing and expanding a 10 mmφ punched hole (die inner diameter 10.3 mm, clearance 12.5%) with a conical punch having an apex angle of 60° at a rate of 20 mm / min in the direction in which the burr of the punched hole faces outward. Hole expansion ratio: HER(%)={(D-D0) / D0}×100 D: Hole diameter when the crack penetrates the plate thickness (mm) D0: Initial hole diameter (mm)

[0083] [Table 1]

[0084] *The remaining components in Table 1 are Fe and unavoidable impurities.

[0085] [Table 2]

[0086] [Table 3]

[0087] [Table 4]

[0088] [Table 5]

[0089] As shown in Tables 1 to 4 above, in Examples 1 and 15 to 31, which satisfy the alloy composition, manufacturing process, and microstructure conditions of the present invention, the yield ratio (YR) of the steel sheet is 0.7 to 0.85, and the product of the square of the tensile strength and the square root of the elongation (TS 2 ×√EL) is 0.7×10 6 ~1.7×10 6 (MPa) 2 % 0.5 , and the product of the square of the tensile strength and the square root of the hole expandability (TS 2 ×√HER) is 1.4×10 6 ~3×10 6 (MPa) 2 % 0.5 It can be seen that the above condition is satisfied, and it can be confirmed that a steel sheet having an excellent balance of ductility, formability, and yield ratio can be manufactured.

[0090] On the other hand, in Comparative Example 2, the annealing temperature of the hot-rolled steel sheet was high, making it difficult for the steel sheet to precipitate. As a result, TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0091] In Comparative Example 3, the annealing temperature of the hot-rolled steel sheet was low, making it difficult to optimize the precipitation of the steel sheet. 2 ×√EL is 0.7×10 6 (MPa)2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0092] In Comparative Example 4, the annealing time of the hot-rolled steel sheet was long, and the precipitation of the steel sheet was not easy. As a result, TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0093] In Comparative Example 5, the annealing time of the hot-rolled steel sheet was short, making it difficult to optimize the precipitation of the steel sheet. 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0094] In Comparative Example 6, the primary holding temperature was high, and the fraction of pearlite containing cementite in the steel sheet exceeded 15%, resulting in a YR of less than 0.7 and a TS of 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0095] In Comparative Example 7, the first holding temperature was low, and unrecrystallized ferrite was generated in the steel sheet. As a result, the YR exceeded 0.85 and the TS 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5 Exceeded, TS2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0096] In Comparative Example 8, the first holding time was short, and unrecrystallized ferrite was generated in the steel sheet. As a result, the YR exceeded 0.85 and the TS 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5 Exceeded, TS 2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0097] In Comparative Example 9, the primary cooling stop temperature was high and TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0098] In Comparative Example 10, the primary cooling stop temperature was low and TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0099] In Comparative Example 11, the secondary average cooling rate was low, and the fraction of pearlite containing cementite exceeded 15%, resulting in TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0100] In Comparative Example 12, the secondary holding temperature was high, and YR was less than 0.7. 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0101] In Comparative Example 13, the secondary holding temperature was low and TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0102] In Comparative Example 14, the secondary holding time was short and TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0103] In Comparative Example 32, the C content was low, and the YR was less than 0.7. 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0104] In Comparative Example 33, the C content was high, and unrecrystallized ferrite was generated, resulting in TS. 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5Exceeded, TS 2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0105] In Comparative Example 34, the Si content was high, which increased embrittlement and TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0106] Comparative Example 35 has a low Mn content and TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0107] In Comparative Example 36, the Mn content was high, and unrecrystallized ferrite was generated, resulting in TS. 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5 Exceeded, TS 2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0108] Comparative Example 37 has a high Al content and TS 2 ×√EL is 0.7×10 6 (MPa) 2 % 0.5 Less than TS 2 ×√HER is 1.4×10 6 (MPa) 2 % 0.5 It was less than.

[0109] In Comparative Example 38, the Ti content was high, and unrecrystallized ferrite was generated, resulting in a YR exceeding 0.85 and a TS 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5 Exceeded, TS 2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0110] In Comparative Example 39, the Nb content was high, and unrecrystallized ferrite was generated, resulting in a YR exceeding 0.85 and a TS 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5 Exceeded, TS 2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0111] In Comparative Example 40, the V content was high, and unrecrystallized ferrite was generated, resulting in a YR exceeding 0.85 and a TS 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5 Exceeded, TS 2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0112] In Comparative Example 41, the content of Ti+Nb+V was high, so that unrecrystallized ferrite was generated, YR exceeded 0.85, and TS 2 ×√EL is 1.7×10 6 (MPa) 2 % 0.5 Exceeded, TS 2 ×√HER is 3.0×10 6 (MPa) 2 % 0.5 Exceeded.

[0113] Although the present invention has been described above with reference to the embodiments, it is understood that those skilled in the art can make various modifications and changes to the present invention within the scope of the basic idea of ​​the present invention, and that the scope of the rights of the present invention can be interpreted based on the scope of the claims.

Claims

1. In weight percent, C: 0.03 to 0.18%, Si: 0 to 0.70%, Mn: 0.25 to 1.80%, Al: 0 to 0.70%, P: 0.050% or less, S: 0.0300% or less, N: 0.0300% or less, Ti: 0 to 0.08%, Nb: 0 to 0.06%, V: 0 to 0.07%, Ti + Nb + V: 0.03 to 0.10%, Cr: 0 to 0.80%, Mo: 0 to 0.80% , Cu: 0 to 0.80%, Ni: 0 to 0.80%, B: 0 to 0.0050%, Ca: 0 to 0.050%, REM excluding Y: 0 to 0.050%, Mg: 0 to 0.050%, W: 0 to 0.50%, Zr: 0 to 0.50%, Sb: 0 to 0.50%, Sn: 0 to 0.50%, Y: 0 to 0.20%, Hf: 0 to 0.20%, the balance being Fe and other unavoidable impurities, The steel sheet microstructure includes, in area percent, 1 to 15% pearlite including cementite, residual ferrite, and an unavoidably inflowing structure; and The yield ratio (YR) of the steel plate is 0.70 to 0.85, and the product of the square of the tensile strength of the steel plate and the square root of the elongation (TS ２ ×√EL) is 0.7 × 10 ６ ~1.7 × 10 ６ (MPa) ２ % ０．５ and the product of the square of the tensile strength of the steel plate and the square root of the hole expandability (TS ２ ×√HER) is 1.4 × 10 ６ ~3.0 x 10 ６ (MPa) ２ % ０．５ Meet the steel plate.

2. The steel plate according to claim 1, wherein the steel plate contains C in the range of 0.04 to 0.17%.

3. The steel plate according to claim 1, wherein the steel plate contains Mn in the range of 0.27 to 1.78%.

4. The steel plate according to claim 1, wherein the steel plate has a microstructure containing, by area %, 3 to 13% pearlite containing cementite, the remainder being ferrite and a structure that inevitably flows in.

5. The steel plate according to claim 1, wherein the yield ratio (YR = YS / TS), which is the value obtained by dividing the yield strength (YS) of the steel plate by the tensile strength (TS), satisfies 0.71 to 0.

84.

6. The product of the square of the tensile strength of the steel plate and the square root of the elongation (TS ２ ×√EL) is 0.71×10 ６ ~1.69 x 10 ６ (MPa) ２ % ０．５ The steel plate according to claim 1, which satisfies the following:

7. The product of the square of the tensile strength of the steel plate and the square root of the hole expandability (TS ２ ×√HER) is 1.43 × 10 ６ ~2.93 x 10 ６ (MPa) ２ % ０．５ The steel plate according to claim 1, which satisfies the following:

8. In weight percent, C: 0.03 to 0.18%, Si: 0 to 0.70%, Mn: 0.25 to 1.80%, Al: 0 to 0.70%, P: 0.050% or less, S: 0.0300% or less, N: 0.0300% or less, Ti: 0 to 0.08%, Nb: 0 to 0.06%, V: 0 to 0.07%, Ti + Nb + V: 0.03 to 0.10%, Cr: 0 to 0.80%, Mo: 0 to 0.80%, Cu: 0 to 0.80%, Ni: heating a steel ingot or slab containing 0 to 0.80%, B: 0 to 0.0050%, Ca: 0 to 0.050%, REM excluding Y: 0 to 0.050%, Mg: 0 to 0.050%, W: 0 to 0.50%, Zr: 0 to 0.50%, Sb: 0 to 0.50%, Sn: 0 to 0.50%, Y: 0 to 0.20%, Hf: 0 to 0.20%, the balance being Fe and other unavoidable impurities, in a temperature range of 1000 to 1350°C; hot rolling the heated steel ingot or slab at a finish rolling temperature of 800 to 1000°C to produce a hot-rolled steel sheet; cooling the manufactured hot-rolled steel sheet and then coiling it at a temperature in the range of 300 to 600°C; heat-treating the coiled hot-rolled steel sheet at a temperature range of 650 to 800°C for 600 to 1700 seconds, and then cold-rolling it at a reduction rate of 30 to 90%; heating the rolled cold-rolled steel sheet in a temperature range of 740 to 860°C and holding the same for 50 seconds or more; subjecting the primarily held cold-rolled steel sheet to a primary cooling at an average cooling rate of 5°C / s or less to a temperature range of 600 to 760°C; and secondarily cooling the primarily cooled cold-rolled steel sheet to a temperature range of 450 to 550°C at an average cooling rate of more than 5°C / s and not more than 20°C / s, and then holding the steel sheet in this temperature range for 50 seconds or more, followed by cooling the steel sheet to room temperature.

9. The method for producing a steel plate according to claim 8, wherein the steel ingot or slab contains C in the range of 0.04 to 0.17%.

10. The method for producing a steel plate according to claim 8, wherein the steel ingot or slab contains Mn in the range of 0.27 to 1.78%.

11. The steel plate cooled to room temperature after the second holding is The steel sheet microstructure includes, in area percent, 1 to 15% pearlite including cementite, residual ferrite, and an unavoidably inflowing structure; and The yield ratio (YR) of the steel plate is 0.70 to 0.85, and the product of the square of the tensile strength of the steel plate and the square root of the elongation (TS ２ ×√EL) is 0.7 × 10 ６ ~1.7 × 10 ６ (MPa) ２ % ０．５ and the product of the square of the tensile strength of the steel plate and the square root of the hole expandability (TS ２ ×√HER) is 1.4 × 10 ６ ~3.0 x 10 ６ (MPa) ２ % ０．５ The method for producing a steel sheet according to claim 8, which satisfies the above.

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