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WO2018186335A1 - Tôle d'acier haute résistance laminée à froid et son procédé de fabrication - Google Patents

Tôle d'acier haute résistance laminée à froid et son procédé de fabrication Download PDF

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Publication number
WO2018186335A1
WO2018186335A1 PCT/JP2018/014075 JP2018014075W WO2018186335A1 WO 2018186335 A1 WO2018186335 A1 WO 2018186335A1 JP 2018014075 W JP2018014075 W JP 2018014075W WO 2018186335 A1 WO2018186335 A1 WO 2018186335A1
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WIPO (PCT)
Prior art keywords
less
steel sheet
cold
rolled
hot
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/JP2018/014075
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English (en)
Japanese (ja)
Inventor
田中　孝明
勇樹 田路
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JFE Steel Corp
Original Assignee
JFE Steel Corp
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Publication date
Priority claimed from JP2018017145A external-priority patent/JP6409991B1/ja
Application filed by JFE Steel Corp filed Critical JFE Steel Corp
Priority to US16/499,592 priority Critical patent/US11365459B2/en
Priority to CN201880020730.4A priority patent/CN110475892B/zh
Priority to KR1020197028975A priority patent/KR102274284B1/ko
Priority to EP18780802.7A priority patent/EP3591087B1/fr
Publication of WO2018186335A1 publication Critical patent/WO2018186335A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C21D8/0226—Hot rolling

Definitions

  • the present invention relates to a high-strength cold-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a high-strength cold-rolled steel sheet having a high strength of tensile strength (TS): 980 MPa or more suitable for parts of transportation machinery including automobiles, and a method for producing the same.
  • TS tensile strength
  • the steel sheet may be required to have excellent stretch flangeability.
  • the stretch flangeability is evaluated as good when, for example, the average value of the hole expansion ratio obtained by a predetermined hole expansion test is large.
  • the defect rate of the hole expansion test A steel sheet with a high defect rate in the hole expansion test is more likely to be defective even during actual pressing. Such defects are difficult to ignore in mass production and mass production of parts. In order to reduce the defective rate of press molding, a steel sheet having a low defective rate in the hole expansion test is required.
  • the present invention provides a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, excellent in ductility and stretch flangeability, and having a low defect rate in a hole expansion test, and a method for producing the same. For the purpose.
  • the present inventors have intensively studied to achieve the above object. As a result, when a large number of massive retained austenite contained in the steel sheet is exposed on the punched end surface during the punching prior to the hole expansion test, end face cracks are induced and the hole expansion rate is greatly reduced. I found out. Furthermore, the present inventors have found that when acicular retained austenite having a small aspect ratio is present at the Bain group boundary, there is an effect of suppressing the occurrence of the end face cracks. Further, the inventors of the present invention have a high ratio of acicular residual austenite with a small aspect ratio and a steel sheet having a microstructure in which most of the acicular residual austenite with a small aspect ratio exists at the boundary of the Bain group.
  • the failure rate of the spreading test was remarkably small.
  • the inventors have further studied.
  • the heat treatment (annealing process) of the steel sheet was performed twice, and in particular, by optimizing the thermal history in the first annealing process, it was found that the microstructure of the steel sheet can be stably made the above microstructure. .
  • the present inventors have further studied and completed the present invention.
  • the present invention provides the following [1] to [6].
  • the total area ratio of ferrite and bainitic ferrite is 20% or more and 80% or less
  • the area ratio of retained austenite is more than 10% and 40% or less
  • the area ratio of martensite is 0%.
  • the ratio of the remaining austenite that is 50% or less and the aspect ratio is 0.5 or less in the retained austenite is 75% or more in area ratio and the remaining austenite that has the aspect ratio is 0.5 or less.
  • the percentage of those present in-loop boundary is an area ratio of 50% or more, high strength cold rolled steel sheet.
  • the first Danhiyanobe annealed sheets a first stage annealing step of obtaining, the first Danhiyanobe annealed sheets, heated at an annealing temperature T 3 of 700 ° C. or higher 850 ° C. or less, From serial annealing temperature T 3, by cooling to 300 ° C. or higher 500 ° C. or less of the cooling stop temperature T 4, the production of high strength cold rolled steel sheet comprising: a second-stage annealing process of obtaining a second Danhiyanobe annealed sheets, the Method.
  • the present invention it is possible to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, excellent in ductility and stretch flangeability, and having a low defect rate in a hole expansion test, and a method for producing the same.
  • the high-strength cold-rolled steel sheet of the present invention is suitable for structural steel materials such as parts for transportation machinery including automobiles and steel materials for construction.
  • ADVANTAGE OF THE INVENTION According to this invention, the further use expansion
  • the proportion of residual austenite having an aspect ratio of 0.5 or less and the proportion of residual austenite having an aspect ratio of 0.5 or less at the Bain group boundary are the defect rate of the hole expansion test. It is a graph which shows the influence which acts.
  • the high-strength cold-rolled steel sheet of the present invention is, in mass%, C: more than 0.15% and 0.45% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 3.00.
  • the total area ratio of ferrite and bainitic ferrite is 20% or more and 80% or less in the microstructure, the area ratio of retained austenite is more than 10% and 40% or less, and martensite.
  • the area ratio of the site is more than 0% and 50% or less, and the proportion of the retained austenite whose aspect ratio is 0.5 or less is 75% or more in area ratio and the aspect ratio is 0.5 or less. Residual austenite Of the percentage of those present in Bain group boundary is 50% or more in area ratio, a high strength cold rolled steel sheet.
  • strength cold-rolled steel plate of this invention is 5 mm or less, for example.
  • composition which the high-strength cold-rolled steel sheet of this invention has is demonstrated first.
  • the unit of element content in the component composition is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
  • C stabilizes austenite, ensures retained austenite with a desired area ratio, contributes effectively to improving ductility, increases the hardness of martensite, and contributes to an increase in strength.
  • C needs to contain more than 0.15%.
  • a large content exceeding 0.45% leads to deterioration of toughness, weldability and delayed fracture resistance, and makes the amount of martensite generated excessively and reduces ductility and stretch flangeability. Therefore, the C content is more than 0.15% and 0.45% or less, preferably 0.18% or more and 0.42% or less, and more preferably 0.20% or more and 0.40% or less.
  • Si suppresses the formation of carbide (cementite) and promotes the concentration of C to austenite, thereby stabilizing austenite and contributing to the improvement of the ductility of the steel sheet.
  • Si dissolved in ferrite improves work hardening ability and contributes to improvement of ductility of ferrite itself. In order to sufficiently obtain such an effect, Si needs to be contained in an amount of 0.50% or more.
  • Si exceeds 2.50% not only the effect of suppressing the formation of carbide (cementite) and stabilizing the retained austenite is saturated, but the amount of Si dissolved in ferrite becomes excessive. Therefore, ductility is reduced.
  • the Si content is 0.50% or more and 2.50% or less, preferably 0.80% or more and 2.30% or less, and more preferably 1.00% or more and 2.10% or less.
  • Mn is an austenite stabilizing element, and contributes to the improvement of ductility by stabilizing austenite, and also promotes the formation of martensite by increasing the hardenability and contributes to increasing the strength of the steel sheet.
  • Mn needs to be contained in an amount of 1.50% or more.
  • content of Mn is 1.50% or more and 3.00% or less, and 1.80% or more and 2.70% or less are preferable.
  • P is a harmful element that segregates at grain boundaries to reduce elongation, induces cracking during processing, and further degrades impact resistance. Therefore, the P content is 0.050% or less. Preferably it is 0.010% or less. However, excessive P removal causes an increase in refining time and cost, and therefore the P content is preferably 0.002% or more.
  • S exists as MnS in the steel, promotes the generation of voids during the punching process, and further decreases the stretch flangeability because it becomes a starting point for the generation of voids during the processing. Therefore, the amount of S is preferably reduced as much as possible, and is 0.0100% or less. Preferably it is 0.0050% or less. However, excessive desulfurization causes an increase in refining time and cost, and therefore the S content is preferably 0.0002% or more.
  • Al is an element that acts as a deoxidizer. In order to obtain such an effect, 0.010% or more of Al is contained. However, when the Al content is excessive, it remains as an Al oxide in the steel sheet, and the Al oxide is agglomerated and easily coarsened, causing the stretch flangeability to deteriorate. Therefore, the Al content is set to 0.100% or less.
  • N exists as AlN in the steel, promotes the generation of coarse voids during the punching process, and further reduces the stretch flangeability because it becomes the starting point for the generation of coarse voids during the machining. For this reason, it is preferable to reduce N amount as much as possible, and N content shall be 0.0100% or less. Preferably it is 0.0060% or less. However, excessive de-N causes an increase in refining time and an increase in cost, so the N content is preferably 0.0005% or more.
  • C and Mn are both elements that contribute to the formation of hard martensite. Even when the content of each element is individually within the above range, 7.5 ⁇ C + Mn is less than 5.0. In some cases, the stretch flangeability tends to be more excellent. This is because C and Mn do not independently determine the properties of martensite but influence each other. When 7.5 ⁇ C + Mn is less than 5.0, the martensite is excessive. It is considered that it is suppressed from becoming hard and stretch flangeability is more excellent. For this reason, it is preferable that C and Mn are mass% and satisfy the following formula (z).
  • C and Mn indicate the content of each element. Although a minimum is not specifically limited, For example, 7.5 * C + Mn has preferable 3.0 or more, and 3.5 or more is more preferable.
  • the above composition is further, if necessary, in mass%, Ti: 0.005% or more and 0.035% or less, Nb: 0.005% or more and 0.035% or less.
  • V 0.005% to 0.035%
  • Mo 0.005% to 0.035%
  • B 0.0003% to 0.0100%
  • Cr 0.05% to 1.00 %
  • Ni 0.05% to 1.00%
  • Cu 0.05% to 1.00%
  • Sb 0.002% to 0.050%
  • Sn 0.002% to 0 .050% or less
  • Ca 0.0005% or more and 0.0050% or less
  • Mg 0.0005% or more and 0.0050% or less
  • REM 0.0005% or more and 0.0050% or less At least one element.
  • Ti forms carbonitrides and increases the strength of the steel by precipitation strengthening action.
  • the content of Ti is preferably 0.005% or more and 0.035% or less, and more preferably 0.005% or more and 0.020% or less.
  • Nb forms carbonitride and increases the strength of the steel by precipitation strengthening action.
  • the Nb content is preferably 0.005% or more in order to effectively exhibit the above-described action.
  • the Nb content is preferably 0.005% or more and 0.035% or less, and more preferably 0.005% or more and 0.030% or less.
  • V (V: 0.005% to 0.035%) V forms carbonitride and increases the strength of the steel by precipitation strengthening action.
  • V content 0.005% or more.
  • the content of V is preferably 0.005% or more and 0.035% or less, and more preferably 0.005% or more and 0.030% or less.
  • Mo 0.005% to 0.035%
  • Mo forms carbonitrides and increases the strength of the steel by precipitation strengthening action.
  • Mo content 0.005% or more.
  • Mo is preferably 0.005% or more and 0.035% or less, and more preferably 0.005% or more and 0.030% or less.
  • B has an effect of enhancing hardenability and promoting the formation of martensite, and thus is useful as a steel strengthening element.
  • the B content is preferably 0.0003% or more.
  • the content of B is preferably 0.0003% or more and 0.0100% or less.
  • Cr 0.05% to 1.00%
  • Cr has a function of enhancing hardenability and promoting martensite formation, and thus is useful as a steel strengthening element.
  • the Cr content is preferably 0.05% or more.
  • the Cr content is preferably 0.05% or more and 1.00% or less.
  • Ni is useful as a steel strengthening element because it has the effect of enhancing hardenability and promoting the formation of martensite.
  • the Ni content is preferably 0.05% or more.
  • the Ni content is preferably 0.05% or more and 1.00% or less.
  • Cu has a function of enhancing hardenability and promoting the formation of martensite, and thus is useful as a steel strengthening element.
  • the Cu content is preferably 0.05% or more.
  • the content of Cu is preferably 0.05% or more and 1.00% or less.
  • Sb has the effect
  • the Sb content is preferably 0.002% or more.
  • the toughness may be reduced. For this reason, the content of Sb is preferably 0.002% or more and 0.050% or less.
  • Sn has an effect of suppressing decarburization of the steel sheet surface layer (region of about several tens of ⁇ m) caused by nitriding and oxidation of the steel sheet surface. Thereby, it can prevent that the production amount of austenite reduces on the steel plate surface, and it is effective for ensuring desired ductility.
  • the Sn content is preferably 0.002% or more.
  • the content of Sn is preferably 0.002% or more and 0.050% or less.
  • Ca (Ca: 0.0005% or more and 0.0050% or less)
  • Ca has the effect
  • the content of Ca is preferably 0.0005% or more and 0.0050% or less.
  • Mg has the effect
  • Mg content 0.0005% or more.
  • the content of Mg is preferably 0.0005% or more and 0.0050% or less.
  • REM 0.0005% or more and 0.0050% or less
  • REM rare earth element
  • the content of REM is preferably 0.0005% or more and 0.0050% or less.
  • Remainder Fe and inevitable impurities In the above composition, the balance other than the above components consists of Fe (remainder Fe) and inevitable impurities.
  • ferrite and bainitic ferrite are soft steel structures and contribute to improving the ductility of the steel sheet. Since carbon does not dissolve so much in these structures, discharging C into the austenite increases the stability of the austenite and contributes to the improvement of ductility. In order to impart the required ductility to the steel sheet, the total area ratio of ferrite and bainitic ferrite is required to be 20% or more. On the other hand, when the sum of the area ratios of ferrite and bainitic ferrite exceeds 80%, it becomes difficult to ensure a tensile strength of 980 MPa or more. For this reason, the sum total of the area ratios of ferrite and bainitic ferrite is 20% or more and 80% or less.
  • the retained austenite is a structure that is rich in ductility per se, but is a structure that contributes to further improving ductility by strain-induced transformation. In order to obtain such an effect, the retained austenite needs to be more than 10% in terms of area ratio. On the other hand, if the retained austenite increases in area ratio exceeding 40%, the stability of the retained austenite is lowered, so that strain-induced transformation occurs early and ductility is lowered. For this reason, the area ratio of retained austenite is more than 10% and 40% or less. In this specification, the volume ratio of retained austenite is calculated by the method described later, and this is treated as the area ratio.
  • the “martensite” here includes fresh martensite and tempered martensite. Martensite is a very hard structure and contributes to increasing the strength of the steel sheet. For the purpose of increasing the strength of the steel sheet, martensite has an area ratio of more than 0% (not including 0%), preferably 3% or more. On the other hand, if the area ratio exceeds 50%, desired ductility and stretch flangeability cannot be ensured. For this reason, the sum total of the area ratio of martensite is more than 0% and 50% or less, and preferably 3% or more and 50% or less.
  • the microstructure of the high-strength cold-rolled steel sheet according to the present invention is not limited to the case where the total area ratio of each of the ferrite and bainitic ferrite, retained austenite, and martensite is 100%. In some cases, the area ratio is 100%.
  • Ratio of retained austenite with an aspect ratio of 0.5 or less: 75% or more in area ratio Residual austenite improves the ductility of the steel sheet, but its contribution to the improvement of ductility differs depending on its shape. Residual austenite having an aspect ratio of 0.5 or less is more stable to processing than the retained austenite having an aspect ratio of more than 0.5, and the effect of improving ductility is great. Residual austenite having a low processing stability and an aspect ratio of more than 0.5 becomes hard martensite at an early stage in the punching prior to the hole expansion test, so that it is easy to form coarse voids around it.
  • the retained austenite having an aspect ratio of 0.5 or less is deformed along the flow of the microstructure, and it is difficult to form voids around the austenite.
  • the ratio of the retained austenite having an aspect ratio of 0.5 or less in the retained austenite is 75% or more in terms of area ratio. If it is. Preferably it is 80% or more. The upper limit of this ratio is not particularly limited, and may be 100%.
  • FIG. 1 is a schematic diagram showing a part of the microstructure of a steel plate (region considered to be generated from one prior austenite grain).
  • the microstructure of the steel sheet shown in FIG. 1 is composed of three Bain groups (B1 to B3). The same Bain group is given the same hatching. Residual austenite is also present in the microstructure of the steel sheet shown in FIG.
  • the retained austenite indicated by the symbol “RA 2 ” exists inside one Bain group B2.
  • the retained austenite indicated by the symbol “RA 1 ” is present at the boundary between the Bain group B1 and another Bain group B3.
  • the retained austenite indicated by the symbol “RA 1 ” corresponds to the retained austenite existing at the Bain group boundary.
  • the proportion of the remaining austenite having an aspect ratio of 0.5 or less at the boundary of the Bain group may be 50% or more in terms of area ratio. Preferably it is 65% or more.
  • the upper limit of this ratio is not particularly limited, and may be 100%. Preferably, it is 95% or less.
  • the high-strength cold-rolled steel sheet of the present invention may further have a plating layer on the surface from the viewpoint of improving corrosion resistance and the like.
  • a hot dip galvanized layer, an alloyed hot dip galvanized layer, or an electrogalvanized layer is preferable.
  • the hot-dip galvanized layer, the alloyed hot-dip galvanized layer, and the electrogalvanized layer are not particularly limited, and are conventionally known hot-dip galvanized layer, conventionally known alloyed hot-dip galvanized layer, and conventionally known, respectively.
  • the electrogalvanized layer is preferably used.
  • the electrogalvanized layer may be a zinc alloy plated layer obtained by adding an appropriate amount of elements such as Fe, Cr, Ni, Mn, Co, Sn, Pb, or Mo to Zn according to the purpose. .
  • the production method of the present invention generally includes the above-described high-strength cold-rolled steel sheet according to the present invention by sequentially subjecting a steel material having the above composition to hot rolling, pickling, cold rolling, and annealing. Is the way to get. And in the manufacturing method of this invention, the process of annealing is divided into two processes.
  • the steel material is not particularly limited as long as it is a steel material having the above composition.
  • the melting method of the steel material is not particularly limited, and a known melting method using a converter or an electric furnace can be employed. From the viewpoint of productivity and the like, it is preferable to form a slab (steel material) by a continuous casting method after melting, but the slab may be formed by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. Good.
  • a hot rolling process is a process of obtaining a hot-rolled sheet by hot-rolling the steel raw material which has the said composition.
  • the hot rolling process is not particularly limited as long as it is a process in which a steel material having the above composition is heated and subjected to hot rolling to obtain a hot rolled sheet having a predetermined size, and a normal hot rolling process is applied. it can.
  • a normal hot rolling process for example, a steel material is heated to a heating temperature of 1100 ° C. or more and 1300 ° C. or less, and hot rolling is performed on the heated steel material at a finish rolling outlet temperature of 850 ° C. or more and 950 ° C. or less.
  • cooling after appropriate rolling (specifically, for example, a temperature range of 450 ° C. or more and 950 ° C. or less is performed at an average cooling rate of 20 ° C./s or more and 100 ° C./s or less).
  • An example is a hot rolling process in which cooling is performed after cooling and winding is performed at a coiling temperature of 400 ° C. or more and 700 ° C. or less to obtain a hot-rolled sheet having a predetermined size and shape.
  • the pickling step is a step of pickling the hot-rolled sheet obtained through the hot rolling step.
  • the pickling step is not particularly limited as long as it can be pickled to such an extent that cold rolling can be performed on the hot-rolled sheet.
  • a conventional pickling step using hydrochloric acid or sulfuric acid can be applied.
  • the cold rolling process is a process of performing cold rolling on the hot-rolled sheet that has undergone the pickling process. More specifically, the cold rolling step is a step of obtaining a cold rolled plate having a predetermined thickness by subjecting the hot rolled plate subjected to pickling to cold rolling with a rolling reduction of 30% or more.
  • ⁇ Cold rolling reduction 30% or more>
  • the rolling reduction of cold rolling is 30% or more.
  • the processing amount is insufficient, and the number of austenite nucleation sites decreases.
  • austenite becomes coarse and non-uniform in the first-stage annealing process of the next process, and the lower bainite transformation in the holding process of the subsequent first-stage annealing process is suppressed, and martensite is generated excessively.
  • the microstructure of the steel sheet after the first stage annealing process cannot be made a microstructure mainly composed of lower bainite.
  • the portion that is martensite after the first stage annealing step tends to generate retained austenite having an aspect ratio of more than 0.5 in the subsequent second stage annealing step.
  • the upper limit of the rolling reduction is determined by the capability of the cold rolling mill, but if the rolling reduction is too high, the rolling load increases and the productivity may decrease. For this reason, the rolling reduction is preferably 70% or less.
  • the number of rolling passes and the rolling reduction per pass are not particularly limited.
  • An annealing process is a process which anneals the cold-rolled sheet obtained through the cold rolling process, and is a process including the 1st stage annealing process and 2nd stage annealing process mentioned later in detail.
  • First stage annealing process the cold-rolled sheet obtained through the cold rolling step is heated at an annealing temperature T 1 of Ac 3 points or more and 950 ° C. or less, and from the annealing temperature T 1 , an average of more than 10 ° C./s at a cooling rate, cooling to cooling stop temperature T 2 less than 250 ° C. or higher 350 ° C., by holding at the cooling stop temperature T 2 10s or more, a step of obtaining a first Danhiyanobe annealed sheets.
  • the purpose of this process is to make the microstructure of the steel sheet at the completion of the first stage annealing process into lower bainite.
  • the portion that is martensite after the first stage annealing step tends to generate retained austenite having an aspect ratio of more than 0.5 in the subsequent second stage annealing step, so that the martensite is excessive in the first stage annealing step.
  • generates it will become difficult to obtain the microstructure of a desired steel plate.
  • the parentheses in the above formula represent the content (unit: mass%) of the element in the parentheses in the steel sheet. When no element is contained, it is calculated as 0.
  • the annealing temperature T 1 is excessively coarsened austenite grains exceeds 950 ° C., since the formation of lower bainite is suppressed in the course retained after cooling, because the martensite excessively generated, the first stage annealing step
  • the microstructure of the later steel sheet cannot be made into a microstructure mainly composed of lower bainite.
  • the portion that is martensite after the first stage annealing step tends to generate retained austenite having an aspect ratio of more than 0.5 in the subsequent second stage annealing step.
  • annealing temperatures T 1 is Ac 3 point or more 950 ° C. or less.
  • Holding time at the annealing temperatures T 1 is not particularly limited, for example, is 10s or 1000s or less.
  • the average cooling rate from the annealing temperature T 1 of to the cooling stop temperature T 2 is 10 ° C. / s greater, preferably 15 ° C. / s or higher.
  • the upper limit of the average cooling rate is not particularly limited, but an excessively large cooling device is required to ensure an excessively high cooling rate. From the viewpoint of production technology and capital investment, the average cooling rate is 50 It is preferably at most ° C / s.
  • the cooling is preferably gas cooling, but can be performed by combining furnace cooling and mist cooling.
  • the cooling stop temperature T 2 is less than 250 ° C. or higher 350 ° C.. More preferably, it is 270 degreeC or more and 340 degrees C or less.
  • the holding time at the cooling stop temperature T 2 is 10s or more.
  • the holding time at the cooling stop temperature T 2 is 10s or more.
  • it is 30 s or more.
  • the upper limit of the holding time at the cooling stop temperature T 2 is not particularly limited, if it is excessively held for a long time, as well as it requires a long production facilities, because the productivity of the steel sheet is remarkably reduced, 1800 s or less Is preferred.
  • the second stage annealing step following step for example it may be cooled to room temperature, subsequently it is subjected to heating and second stage annealing step without cooling.
  • two heating furnaces of a normal continuous annealing facility (CAL) are required in one line. After performing the first stage annealing process by CAL, the second stage annealing process is performed by passing the CAL once again.
  • Second stage annealing process a first Danhiyanobe annealed sheets obtained through the first-stage annealing process, was heated at 700 ° C. or higher 850 ° C. below the annealing temperature T 3 (reheat), from annealing temperature T 3 , by cooling to 300 of the cooling stop ° C. or higher 500 ° C. or less temperature T 4, which is a step of obtaining a second Danhiyanobe annealed sheets.
  • reheat annealing temperature
  • the ratio of the remaining austenite having an aspect ratio of 0.5 or less and the ratio of the remaining austenite having an aspect ratio of 0.5 or less at the Bain group boundary may be set to desired values. It becomes difficult. Therefore, the annealing temperature T 3 is no more than 850 ° C. 700 ° C. or higher, preferably 710 ° C. or higher 830 ° C. or less.
  • Holding time at the annealing temperature T 3 is not particularly limited, for example, is 10s or 1000s or less.
  • the average cooling rate from the annealing temperature T 3 to a cooling stop temperature T 4 is not particularly limited, for example, is 50 ° C. / s or less 5 ° C. / s or higher.
  • cooling stop temperature T 4 300 ° C. or more and 500 ° C. or less
  • the cooling stop temperature T 4 is lower than 300 ° C., enrichment of C into austenite becomes insufficient, a large amount of martensite with retained austenite amount decreases is produced, it can not be obtained microstructure of the desired steel sheet .
  • the cooling stop temperature T 4 is greater than 500 ° C., obtained with ferrite and bainitic ferrite are produced in large quantities, since the pearlite from austenite is generated, the amount of retained austenite is reduced, the microstructure of the desired steel sheet I can't.
  • Holding time at the cooling stop temperature T 4 is not particularly limited, for example, is 10s or 1800s or less.
  • Second Danhiyanobe annealed sheet after holding in the cooling stop temperature T 4 is preferably cooled.
  • This cooling is not particularly limited, and the cooling can be performed to a desired temperature such as room temperature by an arbitrary method such as cooling.
  • the second-stage cold-rolled annealed sheet obtained through the second-stage annealing process becomes the high-strength cold-rolled steel sheet of the present invention.
  • the second-stage cold-rolled annealed plate obtained through the second-stage annealing step may be further subjected to a plating treatment to form a plating layer on the surface thereof.
  • the second-stage cold-rolled annealed plate having a plating layer formed on the surface is the high-strength cold-rolled steel plate of the present invention.
  • hot dip galvanizing treatment hot dip galvanizing treatment and alloying treatment, or electrogalvanizing treatment is preferable.
  • the hot dip galvanizing treatment, the hot dip galvanizing treatment and the alloying treatment, and the electrogalvanizing treatment are not particularly limited, and are conventionally known hot dip galvanizing treatment, conventionally known hot dip galvanizing treatment and alloying treatment, respectively.
  • a conventionally known electrogalvanizing treatment is preferably used.
  • pretreatment such as degreasing and phosphate treatment may be performed prior to the plating treatment.
  • the hot dip galvanizing treatment for example, a conventional continuous hot dip galvanizing line is used to immerse the second stage cold-rolled annealing plate in a hot dip galvanizing bath and form a predetermined amount of hot dip galvanized layer on the surface. It is preferable that When immersed in a hot dip galvanizing bath, the temperature of the second-stage cold-rolled annealed plate is not less than the temperature of the hot dip galvanizing bath temperature ⁇ 50 ° C. and not more than the temperature of the hot dip galvanizing bath temperature + 80 ° C. by reheating or cooling. It is preferable to adjust within the range.
  • the temperature of the hot dip galvanizing bath is preferably 440 ° C. or higher and 500 ° C. or lower.
  • the hot dip galvanizing bath may contain Al, Fe, Mg, Si or the like in addition to pure zinc.
  • the adhesion amount of the hot-dip galvanized layer can be adjusted to a desired adhesion amount by adjusting gas wiping or the like, and is preferably about 45 g / m 2 per side.
  • the plated layer (hot galvanized layer) formed by the hot dip galvanizing process may be an alloyed hot dip galvanized layer by performing a usual alloying process as necessary.
  • the temperature for the alloying treatment is preferably 460 ° C. or more and 600 ° C. or less.
  • adjusting the effective Al concentration in the hot dip galvanizing bath to a range of 0.10% by mass or more and 0.22% by mass or less from the viewpoint of securing a desired plating appearance. preferable.
  • the electrogalvanizing treatment is preferably, for example, a treatment of forming a predetermined amount of electrogalvanized layer on the surface of the second stage cold-rolled annealed plate using a conventional electrogalvanizing line.
  • the adhesion amount of the electrogalvanized layer can be adjusted to a predetermined adhesion amount by adjusting the sheet passing speed or the current value, and is preferably about 30 g / m 2 per side.
  • the obtained cold-rolled sheet was annealed under the conditions shown in Tables 2 to 3 below to obtain a second-stage cold-rolled annealed sheet.
  • the annealing process was a two-stage process consisting of a first stage annealing process and a second stage annealing process. It cooled to room temperature between the 1st stage annealing process and the 2nd stage annealing process. Holding time at the annealing temperature T 1 of the first stage annealing process was 100s.
  • the holding time at the annealing temperature T 3 is 100 s
  • the average cooling rate from the annealing temperature T 3 to the cooling stop temperature T 4 is 20 ° C./s
  • the holding time at the cooling stop temperature T 4 was 250 s.
  • the second-stage cold-rolled annealed sheets were subjected to hot dip galvanizing treatment after the end of annealing, thereby forming a hot dip galvanized layer on the surface to obtain hot dip galvanized steel sheets.
  • the hot dip galvanizing treatment the second-stage cold-rolled annealed plate is reheated to a temperature in the range of 430 ° C. or higher and 480 ° C. or lower as necessary using a continuous hot dip galvanizing line, and a hot dip galvanizing bath (bath temperature) is used. : 470 ° C.), and the amount of adhesion of the plating layer was adjusted to 45 g / m 2 per side.
  • the bath composition was Zn-0.18 mass% Al.
  • the hot dip galvanized steel sheets had a bath composition of Zn-0.14 mass% Al, and after the plating process, an alloying process was performed at 520 ° C. to obtain an alloyed hot dip galvanized steel sheet.
  • the Fe concentration in the plating layer was 9% by mass or more and 12% by mass or less.
  • Another part of the second-stage cold-rolled annealed plate is subjected to an electrogalvanizing treatment after the annealing, and further using an electrogalvanizing line so that the amount of plating is 30 g / m 2 per side. To give an electrogalvanized steel sheet.
  • the second-stage cold-rolled annealed sheet that does not form a plating layer is “CR”
  • the hot-dip galvanized steel sheet is “GI”
  • the galvannealed steel sheet is “GA”
  • the electrogalvanized steel sheet was written as “EG”.
  • a second-stage cold-rolled annealed plate that does not form a plated layer, and a second-stage cold-rolled annealed plate that forms a plated layer hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and electrogalvanized steel sheet
  • cold-rolled steel sheet A cold-rolled steel sheet was produced as described above.
  • test piece was collected from the obtained cold-rolled steel sheet and subjected to microstructure observation, measurement of residual austenite area ratio, tensile test, and hole expansion test.
  • the test method was as follows.
  • ⁇ Microstructure observation a specimen for microstructural observation was collected from the cold rolled steel sheet. Next, the collected specimen was polished so that the position corresponding to 1/4 of the plate thickness in the rolling direction cross section (L cross section) was the observation surface. The observation surface is corroded (1% by volume nital liquid corrosion), and then observed using a scanning electron microscope (SEM, magnification: 3000 times) in a visual field range of 30 ⁇ m ⁇ 35 ⁇ m, and imaged. SEM images were obtained. The area ratio of each tissue was determined by image analysis using the obtained SEM image. The area ratio was an average value of 10 fields of view.
  • SEM scanning electron microscope
  • test piece was polished by colloidal silica vibration polishing so that a position corresponding to 1/4 of the plate thickness in the cross section in the rolling direction (L cross section) became the observation surface.
  • the observation surface was a mirror surface.
  • processing transformation phase of the observation surface due to polishing strain was removed by ultra-low acceleration ion milling, and then electron beam backscatter diffraction (EBSD) measurement was performed to obtain local crystal orientation data.
  • EBSD electron beam backscatter diffraction
  • the SEM magnification was 1500 times
  • the step size was 0.04 ⁇ m
  • the measurement area was 40 ⁇ m square
  • WD was 15 mm.
  • Analysis software OIM Analysis 7 was used to analyze the obtained local orientation data. The analysis was performed for three visual fields, and the average value was used.
  • Grain Dilution function (Grain Tolerance Angle: 5 °, Minimum Grain Size: 5, Single Iteration: ON) and Grain CI Standardization Function (Grain Tolerance Angle: 5 °, Grain Tolerance Angle: 5 °, Grain Tolerance Angle: 5 °
  • Grain Tolerance Angle 5 °
  • the clean-up process according to was performed once in order. Thereafter, only measurement points with CI values> 0.1 were used for analysis.
  • the fcc phase data was analyzed using the Area Fraction of the Grain Shape Aspect Ratio chart, and the ratio (area ratio) of retained austenite having an aspect ratio of 0.5 or less was determined from the retained austenite. In the above analysis, Method 2 was used as the grain shape calculation method.
  • the different austenite with the aspect ratio obtained earlier of 0.5 or less is used in a different color.
  • the ratio of those existing at the boundary of the colored region, that is, the Bain group boundary (including the former austenite grain boundary) was obtained as an area ratio.
  • a specimen for X-ray diffraction is taken from the cold-rolled steel sheet, ground and polished so that the position corresponding to 1/4 of the plate thickness becomes the measurement surface, and diffracted X-ray intensity is measured by the X-ray diffraction method. From the volume fraction of retained austenite. CoK ⁇ rays were used as incident X-rays.
  • the volume fraction of retained austenite When calculating the volume fraction of retained austenite, the ⁇ 111 ⁇ , ⁇ 200 ⁇ , ⁇ 220 ⁇ and ⁇ 311 ⁇ faces of the fcc phase (residual austenite) and the ⁇ 110 ⁇ , ⁇ 200 ⁇ and ⁇ 211 ⁇ of the bcc phase The intensity ratio was calculated for all combinations of the integrated intensity of the peak of the surface, the average value was obtained, and the volume fraction of retained austenite was calculated. The volume ratio of austenite thus obtained was defined as the area ratio.
  • ⁇ Tensile test> A JIS No. 5 tensile test piece (JIS Z 2001) having a tensile direction in the direction perpendicular to the rolling direction (C direction) is taken from the cold-rolled steel sheet and subjected to a tensile test in accordance with the provisions of JIS Z 2241. Strength (TS) and elongation (El) were measured.
  • ⁇ Hole expansion test ⁇ A specimen (size: 100 mm ⁇ 100 mm) is taken from the cold rolled steel sheet, and a hole having an initial diameter d 0 : 10 mm ⁇ is formed in the specimen by punching (clearance: 12.5% of the specimen thickness). did. Using these test pieces, a hole expansion test was performed. That is, a conical punch having an apex angle of 60 ° is inserted into a hole having an initial diameter d 0 : 10 mm ⁇ from the punching side at the time of punching, and the hole is expanded and the hole penetrates through the steel plate (test piece). The diameter d (unit: mm) was measured, and the hole expansion ratio ⁇ (unit:%) was calculated by the following formula.
  • Hole expansion ratio ⁇ ⁇ (d ⁇ d 0 ) / d 0 ⁇ ⁇ 100
  • the hole expansion test was performed 100 times for each steel plate, and the average value was defined as the average hole expansion rate ⁇ (unit:%).
  • the average hole expansion ratio ⁇ is also expressed as “average ⁇ ”.
  • a probability that the value of the hole expansion rate ⁇ is a value equal to or less than half of the average hole expansion rate ⁇ was determined, and this was defined as a defective rate (unit:%) of the hole expansion test.
  • FIG. 2 is a graph plotting a part of the results of Tables 4-5. More specifically, FIG. 2 shows the proportion of residual austenite with an aspect ratio of 0.5 or less, and the proportion of residual austenite with an aspect ratio of 0.5 or less that exists at the Bain group boundary. These are graphs which show the influence which it has on the defect rate of a hole expansion test. As can be seen from the graph of FIG. 2, the ratio of residual austenite having an aspect ratio of 0.5 or less is 75% or more, and the residual austenite having an aspect ratio of 0.5 or less is present at the Bain group boundary. Only when the ratio of the objects is 50% or more, a steel sheet having a low defect rate in the hole expansion test is obtained.
  • the cold-rolled steel sheets of the examples of the present invention all have high strength with a tensile strength (TS) of 980 MPa or more, and have good ductility and stretch flanges.
  • TS tensile strength
  • the defect rate of the hole expansion test is small.
  • any of the above characteristics was insufficient.

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Abstract

La présente invention concerne une tôle d'acier laminée à froid haute résistance qui présente une résistance à la traction de 980 MPa ou plus, d'excellentes caractéristiques de ductilité et de déformabilité de bordage par étirage et un faible taux de défauts dans un test d'agrandissement de trou. La tôle d'acier laminée à froid haute résistance selon l'invention a une composition qui contient, en termes de % de masse, plus de 0,15 % et pas plus de 0,45 % de C, de 0,50 à 2,50 % de Si, de 1,50 à 3,00 % de Mn, pas plus de 0,050 % de P, pas plus de 0,0100 % de S, de 0,010 à 0,100 % d'Al et pas plus de 0,0100 % de N, le reste comprenant du Fe et des impuretés inévitables, a une teneur totale en ferrite et en ferrite bainitique de 20-80 %, a une teneur en austénite résiduelle supérieure à 10 % et inférieure ou égale à 40 %, présente une teneur en martensite supérieure à 0 % et inférieure ou égale à 50 % et est telle que la proportion d'austénite résiduelle ayant un rapport d'aspect inférieur ou égal à 0,5 n'est pas inférieure à 75 % et que la proportion d'austénite résiduelle ayant un rapport d'aspect inférieur ou égal à 0,5 présente au niveau des limites de groupe de Bain n'est pas inférieure à 50 %.
PCT/JP2018/014075 2017-04-05 2018-04-02 Tôle d'acier haute résistance laminée à froid et son procédé de fabrication Ceased WO2018186335A1 (fr)

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CN201880020730.4A CN110475892B (zh) 2017-04-05 2018-04-02 高强度冷轧钢板及其制造方法
KR1020197028975A KR102274284B1 (ko) 2017-04-05 2018-04-02 고강도 냉연 강판 및 그의 제조 방법
EP18780802.7A EP3591087B1 (fr) 2017-04-05 2018-04-02 Tôle d'acier haute résistance laminée à froid et son procédé de fabrication

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KR102468051B1 (ko) * 2020-10-23 2022-11-18 주식회사 포스코 연성이 우수한 초고강도 강판 및 그 제조방법
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CN119640138B (zh) * 2025-02-18 2025-07-04 江苏沙钢集团有限公司 用于制备套筒的圆钢的生产方法、该圆钢制备的套筒以及套筒和钢筋的连接方法

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