WO2016059701A1 - 高炭素鋼板及びその製造方法 - Google Patents

高炭素鋼板及びその製造方法 Download PDF

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WO2016059701A1
WO2016059701A1 PCT/JP2014/077544 JP2014077544W WO2016059701A1 WO 2016059701 A1 WO2016059701 A1 WO 2016059701A1 JP 2014077544 W JP2014077544 W JP 2014077544W WO 2016059701 A1 WO2016059701 A1 WO 2016059701A1
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cementite
ferrite
steel sheet
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PCT/JP2014/077544
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English (en)
French (fr)
Japanese (ja)
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健悟 竹田
友清　寿雅
保嗣 塚野
荒牧　高志
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新日鐵住金株式会社
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Priority to CN201480082444.2A priority Critical patent/CN107075625B/zh
Priority to EP14903999.2A priority patent/EP3208357B1/en
Priority to US15/513,130 priority patent/US20170306434A1/en
Priority to ES14903999T priority patent/ES2807553T3/es
Priority to PL14903999T priority patent/PL3208357T3/pl
Priority to JP2016553926A priority patent/JP6388034B2/ja
Priority to KR1020177009862A priority patent/KR101919262B1/ko
Priority to MX2017004601A priority patent/MX2017004601A/es
Priority to PCT/JP2014/077544 priority patent/WO2016059701A1/ja
Priority to BR112017007275A priority patent/BR112017007275A2/pt
Publication of WO2016059701A1 publication Critical patent/WO2016059701A1/ja

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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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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
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    • C21D2211/00—Microstructure comprising significant phases
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Definitions

  • the present invention relates to a high carbon steel sheet with improved formability and a method for manufacturing the same.
  • High carbon steel plates are used in various steel products such as drive chain parts such as automobile chains, gears and clutches, saws and blades.
  • a high carbon steel sheet is formed and heat-treated.
  • punching, tension, compression, shearing and the like are performed, and as the heat treatment, quenching, tempering, carburizing, nitriding, soft nitriding and the like are performed. Since the strength level of the high carbon steel plate is higher than that of the mild steel plate, the mold used for forming the high carbon steel plate is more easily worn than the mold used for forming the mild steel plate. Moreover, a high carbon steel plate is easier to crack during forming than a mild steel plate.
  • Patent Documents 1 to 5 Improvement of the lubricity of the surface of the high carbon steel plate is effective for suppressing the wear of the mold, and softening of the high carbon steel plate is effective for suppressing cracking during forming. Therefore, techniques aimed at improving lubricity and softening have been proposed (Patent Documents 1 to 5).
  • Patent Document 6 A carbon steel sheet for the purpose of improving punchability is described in Patent Document 6, and a high carbon steel sheet for the purpose of improving formability is described in Patent Document 7, but sufficient formability can also be obtained by these. I can't.
  • An object of the present invention is to provide a high carbon steel sheet that can obtain good formability while avoiding a significant increase in cost, and a method for producing the same.
  • a high carbon steel sheet contain a predetermined amount of B, have a predetermined micro friction coefficient of ferrite on the surface, and have a predetermined cementite mode. I found out that it was important. Furthermore, in order to manufacture such a high-carbon steel sheet, it has also been found that it is important to set the conditions for hot rolling and annealing as predetermined after considering these as so-called integrated processes. The inventors of the present application have arrived at the following aspects of the invention based on these findings.
  • Nb 0.001% to 0.500%
  • Mo 0.001% to 0.500%
  • V 0.001% to 0.500%
  • W 0.001% to 0.500%
  • Ta 0.001% to 0.500%
  • Ni 0.001% to 0.500%
  • Mg 0.001% to 0.500%
  • Ca 0.001% to 0.500%
  • Y 0.001% to 0.500%
  • Zr 0.001% to 0.500%
  • La 0.001% to 0.500%
  • Ce Ce: 0.001% to 0.500%
  • the slab is % By mass C: 0.30% to 0.70%, Si: 0.07% to 1.00%, Mn: 0.20% to 3.00%, Ti: 0.010% to 0.500%, Cr: 0.01% to 1.50%, B: 0.0004% to 0.0035%, P: 0.025% or less, Al: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, Cu: 0.500% or less, Nb: 0.000% to 0.500%, Mo: 0.000% to 0.500%, V: 0.000% to 0.500%, W: 0.000% to 0.500%, Ta: 0.000% to 0.500%, Ni: 0.000% to 0.500%, Mg: 0.000% to 0.500%, Ca: 0.000% to 0.500%, Y: 0.000% to 0.500%, Zr: 0.000% to 0.
  • the finish rolling temperature is 830 ° C. or more and 950 ° C. or less
  • the winding temperature is set to 450 ° C. or more and 700 ° C. or less
  • the annealing step includes Holding the hot-rolled steel sheet at a temperature of 730 ° C. or more and 770 ° C. or less for 3 hours or more and 60 hours or less; Next, the step of cooling the hot-rolled steel sheet to 650 ° C. at a cooling rate of 1 ° C./hr to 60 ° C./hr, The manufacturing method of the high carbon steel plate characterized by having.
  • Nb 0.001% to 0.500%
  • Mo 0.001% to 0.500%
  • V 0.001% to 0.500%
  • W 0.001% to 0.500%
  • Ta 0.001% to 0.500%
  • Ni 0.001% to 0.500%
  • Mg 0.001% to 0.500%
  • Ca 0.001% to 0.500%
  • Y 0.001% to 0.500%
  • Zr 0.001% to 0.500%
  • La 0.001% to 0.500%
  • the B content, the micro friction coefficient of ferrite on the surface, and the like are appropriate, good formability can be obtained while avoiding a significant increase in cost.
  • FIG. 1 is a graph showing the relationship between the micro friction coefficient of ferrite and the B content.
  • FIG. 2 is a diagram showing the relationship between the micro friction coefficient of ferrite and the number of presses until flaws occur in the mold or high carbon steel sheet.
  • FIG. 3A is a photomicrograph showing the surface of the high carbon steel sheet before measuring the micro friction coefficient.
  • FIG. 3B is a photomicrograph showing the surface of the high carbon steel sheet after measuring the micro friction coefficient.
  • FIG. 4 is a schematic diagram showing a change in temperature from hot rolling to cooling.
  • Figure 5A is a schematic diagram showing a structure at the time t A.
  • FIG. 5B is a schematic diagram showing the structure at time t B.
  • FIG. 5C is a schematic diagram showing the structure at time point t C.
  • FIG. 5D is a schematic diagram showing a structure at the time t D.
  • Figure 5E is a schematic diagram showing a structure at the time t E.
  • FIG. 6A is a diagram showing a structure when the temperature of slab heating exceeds 1150 ° C.
  • FIG. 6B is a diagram showing a structure when the temperature of slab heating is less than 1000 ° C.
  • FIG. 6C is a diagram showing the structure when the annealing holding temperature is lower than 730 ° C.
  • FIG. 6D is a diagram showing a structure when the holding temperature of annealing exceeds 770 ° C. or when the holding time of annealing exceeds 60 hours.
  • FIG. 6E is a diagram showing the structure when the annealing holding temperature is less than 3 hours.
  • FIG. 6F is a diagram showing a structure when the cooling rate is less than 1 ° C./hr.
  • FIG. 6G is a diagram showing the structure when the cooling rate exceeds 60 ° C./hr.
  • FIG. 7 is a diagram showing the relationship between the microscopic friction coefficient of ferrite and the B content for a part of examples in the first experiment or the third experiment.
  • the chemical composition of the high carbon steel plate according to the embodiment of the present invention and the slab (steel ingot) used for the production will be described. Although details will be described later, the high carbon steel sheet according to the embodiment of the present invention is manufactured through hot rolling and annealing of a slab. Therefore, the chemical composition of the high carbon steel sheet and the slab considers not only the characteristics of the high carbon steel sheet but also these treatments.
  • the high carbon steel sheet according to the present embodiment and the slab used for the production thereof are: C: 0.30% to 0.70%, Si: 0.07% to 1.00%, Mn: 0.20% to 3. 00%, Ti: 0.010% to 0.500%, Cr: 0.01% to 1.50%, B: 0.0004% to 0.0035%, P: 0.025% or less, Al: 0 100% or less, S: 0.0100% or less, N: 0.010% or less, Cu: 0.500% or less, Nb: 0.000% to 0.500%, Mo: 0.000% to 0.
  • V 0.000% to 0.500%
  • W 0.000% to 0.500%
  • Ta 0.000% to 0.500%
  • Ni 0.000% to 0.500%
  • Mg 0.000% to 0.500%
  • Ca 0.000% to 0.500%
  • Y 0.000% to 0.500%
  • Z 0.000% to 0.500%
  • La 0.000% to 0.500%
  • Ce Ce: 0.000% to 0.500%
  • chemical composition represented by Fe and impurities ing include those contained in raw materials such as ore and scrap and those contained in the manufacturing process. For example, when scrap is used as a raw material, Sn, Sb, As, or any combination thereof may be mixed by 0.003% or more.
  • the effect of the present embodiment is not hindered, so that it is acceptable as an impurity.
  • O is acceptable as an impurity with a limit of 0.0025%.
  • O forms an oxide, and if the oxide aggregates and becomes coarse, sufficient moldability cannot be obtained. For this reason, the lower the O content, the better.
  • C 0.30% to 0.70% C combines with Fe to form cementite with a small coefficient of friction, and is therefore an important element for ensuring macro-lubricating properties of high carbon steel sheets. If the C content is less than 0.30%, the amount of cementite is insufficient, sufficient lubricity cannot be obtained, and adhesion with the mold occurs during molding. Therefore, the C content is 0.30% or more, preferably 0.35% or more. If the C content exceeds 0.70%, the amount of cementite becomes excessive, and cracks starting from cementite are likely to occur during molding. Therefore, the C content is 0.70% or less, preferably 0.65% or less.
  • Si acts as a deoxidizing agent and is an element effective for suppressing excessive coarsening of cementite during annealing.
  • the Si content is 0.07% or more, preferably 0.10% or more. If the Si content exceeds 1.00%, the ductility of the ferrite is low, and cracks originating from the intragranular cracks of the ferrite tend to occur during molding. Therefore, the Si content is 1.00% or less, preferably 0.80% or less.
  • Mn is an important element for controlling pearlite transformation.
  • the Mn content is less than 0.20%, the effect by the above action cannot be sufficiently obtained. That is, if the Mn content is less than 0.20%, pearlite transformation occurs in the cooling process after annealing in the two-phase region, and the cementite spheroidization rate is insufficient. Therefore, the Mn content is 0.20% or more, preferably 0.25% or more. If the Mn content exceeds 3.00%, the ductility of the ferrite is low, and cracking starting from the intragranular cracking of the ferrite is likely to occur during molding. Therefore, the Mn content is 3.00% or less, preferably 2.00% or less.
  • Ti 0.010% to 0.500%
  • Ti is an element that forms nitrides at the molten steel stage and is effective in preventing the formation of BN.
  • the Ti content is 0.010% or more, preferably 0.040% or more. If the Ti content is more than 0.500%, cracks starting from coarse Ti oxides are likely to occur during molding. This is because coarse Ti oxide is formed during continuous casting and is wound inside the slab. Therefore, the Ti content is 0.500% or less, preferably 0.450% or less.
  • Cr 0.01% to 1.50%
  • Cr is an element that has a high affinity with N and is effective in suppressing the formation of BN, and is also effective in controlling pearlite transformation. If the Cr content is less than 0.01%, the effect by the above action cannot be sufficiently obtained. Therefore, the Cr content is 0.01% or more, preferably 0.05% or more. If the Cr content exceeds 1.50%, spheroidization of cementite during annealing is inhibited, and coarsening of cementite is significantly suppressed. Therefore, the Cr content is 1.50% or less, preferably 0.90% or less.
  • B is an element that reduces the micro friction coefficient of ferrite on the surface of the high carbon steel sheet.
  • B is an element that segregates and concentrates at the interface between ferrite and cementite during annealing described later, suppresses peeling at the interface during molding, and is effective in preventing cracking.
  • the B content is 0.0004% or more, preferably 0.0008% or more. If the B content exceeds 0.0035%, cracks starting from borides such as Fe and B carbides tend to occur during molding. Therefore, the B content is 0.0035% or less, preferably 0.0030% or less.
  • FIG. 1 is a graph showing the relationship between the micro friction coefficient of ferrite and the B content.
  • the micro friction coefficient of ferrite is remarkably low as compared with the case where the B content is less than 0.0004%.
  • the reason why the wear of the mold can be suppressed as the micro friction coefficient of ferrite is lower is that a hard B film is formed on the surface of the high carbon steel sheet as described later.
  • Another factor is that B segregated and concentrated at the interface between ferrite and cementite improves the strength of this interface, suppresses cracking of the high carbon steel sheet, and suppresses wear of the mold accompanying cracking. Can be guessed.
  • P 0.025% or less
  • P is not an essential element but is contained as an impurity in, for example, a steel plate.
  • P is strongly segregated at the interface between ferrite and cementite, the segregation of B on the interface is inhibited, and peeling at the interface is caused. For this reason, the lower the P content, the better.
  • the P content exceeds 0.025%, the adverse effect becomes remarkable. Therefore, the P content is 0.025% or less.
  • the reduction of the P content requires a refining cost, and if it is attempted to reduce it to less than 0.0001%, the refining cost increases remarkably. For this reason, the P content may be 0.0001% or more.
  • Al is an element that acts as a deoxidizer and is effective for fixing N in the steelmaking stage, but is not an essential element of the high-carbon steel sheet, and is contained as an impurity in the steel sheet, for example. If the Al content exceeds 0.100%, the ductility of the ferrite is low, cracking starting from the intragranular cracking of the ferrite tends to occur during molding, and the strength becomes excessive, resulting in an increase in molding load. Therefore, the Al content is 0.100% or less. If the Al content of the high-carbon steel sheet is less than 0.001%, the fixation of N may not be sufficient. Therefore, the Al content may be 0.001% or more.
  • S is not an essential element but is contained as an impurity in, for example, a steel plate.
  • S forms coarse non-metallic inclusions such as MnS and deteriorates moldability. For this reason, the lower the S content, the better. In particular, when the S content exceeds 0.0100%, the adverse effect becomes remarkable. Therefore, the S content is 0.0100% or less.
  • refining costs are required to reduce the S content, and refining costs significantly increase if it is attempted to reduce the content to less than 0.0001%. For this reason, S content is good also as 0.0001% or more.
  • N is not an essential element but is contained as an impurity in, for example, a steel plate. N decreases the solid solution amount of B due to the formation of BN, and causes adhesion with a mold and cracking during molding. For this reason, the lower the N content, the better. In particular, when the N content exceeds 0.010%, the adverse effects become significant. Therefore, the N content is 0.010% or less. In addition, the refining cost is required to reduce the N content, and the refining cost significantly increases when trying to reduce the N content to less than 0.001%. For this reason, N content is good also as 0.001% or more.
  • Cu is not an essential element, but is mixed from, for example, scrap and contained as an impurity in the steel sheet.
  • Cu causes an increase in strength and brittleness in the heat. For this reason, the lower the Cu content, the better. In particular, when the Cu content exceeds 0.500%, the adverse effect becomes significant. Therefore, the Cu content is 0.500% or less.
  • refining costs are required to reduce the Cu content, and refining costs significantly increase when trying to reduce the Cu content to less than 0.001%. For this reason, Cu content is good also as 0.001% or more.
  • Nb, Mo, V, W, Ta, Ni, Mg, Ca, Y, Zr, La, and Ce are not essential elements, but are arbitrary elements that may be appropriately contained in high carbon steel sheets and slabs up to a predetermined amount. It is.
  • Nb is an element that forms nitrides and is effective in suppressing the formation of BN. Therefore, Nb may be contained. However, if the Nb content exceeds 0.500%, the ductility of ferrite is low and sufficient formability cannot be obtained. Therefore, the Nb content is 0.500% or less. In order to surely obtain the effect by the above action, the Nb content is preferably 0.001% or more.
  • Mo 0.000% to 0.500%
  • Mo is an element effective for improving hardenability. Therefore, Mo may be contained. However, if the Mo content exceeds 0.500%, the ductility of ferrite is low and sufficient formability cannot be obtained. Therefore, the Mo content is 0.500% or less. In order to surely obtain the effect by the above action, the Mo content is preferably 0.001% or more.
  • V 0.000% to 0.500%
  • Nb is an element that forms nitrides and is effective in suppressing the formation of BN. Therefore, V may be contained. However, if the V content exceeds 0.500%, the ductility of ferrite is low and sufficient formability cannot be obtained. Therefore, the V content is 0.500% or less. In order to surely obtain the effect by the above action, the V content is preferably 0.001% or more.
  • W is an element effective for improving hardenability. Therefore, W may be contained. However, if the W content exceeds 0.500%, the ductility of ferrite is low and sufficient formability cannot be obtained. Therefore, the W content is 0.500% or less. In order to surely obtain the effect by the above action, the W content is preferably 0.001% or more.
  • Ta 0.000% to 0.500%
  • Nb and V is an element that forms nitrides and is effective in suppressing the formation of BN. Therefore, Ta may be contained. However, if the Ta content exceeds 0.500%, the ductility of ferrite is low and sufficient formability cannot be obtained. Therefore, the Ta content is 0.500% or less. In order to surely obtain the effect by the above action, the Ta content is preferably 0.001% or more.
  • Ni is an element effective for improving toughness and hardenability. Therefore, Ni may be contained. However, if the Ni content exceeds 0.500%, the micro friction coefficient of ferrite becomes high and adhesion with the mold tends to occur. Therefore, the Ni content is 0.500% or less. In order to surely obtain the effect by the above action, the Ni content is preferably 0.001% or more.
  • Mg is an effective element for controlling the form of sulfide. Therefore, Mg may be contained. However, Mg easily forms an oxide, and if the Mg content exceeds 0.500%, sufficient formability cannot be obtained due to cracks originating from the oxide. Therefore, the Mg content is 0.500% or less. In order to reliably obtain the effect by the above action, the Mg content is preferably 0.001% or more.
  • Ca 0.000% to 0.500%
  • Ca is an element effective for controlling the form of sulfide. Therefore, Ca may be contained.
  • Ca tends to form an oxide, and if the Ca content exceeds 0.500%, sufficient formability cannot be obtained due to cracks originating from the oxide. Therefore, the Ca content is 0.500% or less.
  • the Ca content is preferably 0.001% or more.
  • Y like Mg and Ca, is an element effective for controlling the form of sulfide. Therefore, Y may be contained. However, Y tends to form an oxide, and if the Y content exceeds 0.500%, sufficient formability cannot be obtained due to cracks originating from the oxide. Therefore, the Y content is 0.500% or less. In order to surely obtain the effect by the above action, the Y content is preferably 0.001% or more.
  • Zr 0.000% to 0.500%
  • Zr is an element effective for controlling the form of sulfide. Therefore, Zr may be contained. However, Zr tends to form an oxide, and if the Zr content exceeds 0.500%, sufficient formability cannot be obtained due to cracks originating from the oxide. Therefore, the Zr content is 0.500% or less.
  • the Zr content is preferably 0.001% or more in order to surely obtain the effect by the above action.
  • La like Mg, Ca, Y and Zr, is an element effective for controlling the form of sulfide. Therefore, La may be contained. However, La tends to form an oxide, and if the La content exceeds 0.500%, sufficient formability cannot be obtained due to cracks originating from the oxide. Therefore, the La content is 0.500% or less. In order to surely obtain the effect by the above action, the La content is preferably 0.001% or more.
  • Ce 0.000% to 0.500%
  • Ce is an element effective for controlling the form of sulfide. Therefore, Ce may be contained.
  • Ce tends to form an oxide, and if the Ce content exceeds 0.500%, sufficient formability cannot be obtained due to cracks originating from the oxide. Therefore, the Ce content is 0.500% or less.
  • the Ce content is preferably 0.001% or more in order to surely obtain the effect of the above action.
  • Nb, Mo, V, W, Ta, Ni, Mg, Ca, Y, Zr, La, and Ce are optional elements, such as “Nb: 0.001% to 0.500%”, “Mo: “0.001% to 0.500%”, “V: 0.001% to 0.500%”, “W: 0.001% to 0.500%”, “Ta: 0.001% to 0.500%” % "," Ni: 0.001% to 0.500% ",” Mg: 0.001% to 0.500% ",” Ca: 0.001% to 0.500% ",” Y: 0. "001% to 0.500%”, “Zr: 0.001% to 0.500%", "La: 0.001% to 0.500%", or "Ce: 0.001% to 0.500%” Or any combination thereof is preferably satisfied.
  • micro friction coefficient of ferrite on the surface of the high carbon steel sheet according to this embodiment will be described.
  • the micro friction coefficient of ferrite on the surface of the high carbon steel sheet according to this embodiment is less than 0.5.
  • the micro friction coefficient of ferrite on the surface is closely related to the adhesion between the mold during forming and the high carbon steel sheet.
  • the micro friction coefficient of ferrite is 0.5 or more, micro adhesion occurs between the high carbon steel sheet and the mold during the molding using the mold.
  • the micro friction coefficient of ferrite is less than 0.5.
  • the micro friction coefficient is preferably as low as possible.
  • the micro friction coefficient is often 0.35 or more.
  • FIG. 2 is a diagram showing the relationship between the micro friction coefficient of ferrite and the number of presses (shots) until wrinkles occur in the die or the high carbon steel plate in the punching forming of the high carbon steel plate. As shown in FIG. 2, if the micro friction coefficient is less than 0.5, the number of presses until wrinkles are generated is significantly higher than when 0.5 or more.
  • the micro friction coefficient can be measured using a nanoindenter. That is, a dynamic friction force F generated when a vertical load P of 10 ⁇ N is applied to the surface of the high carbon steel plate with a diamond indenter and moved in the horizontal direction is acquired. The moving speed at this time is, for example, 1 ⁇ m / second. Then, a micro friction coefficient ⁇ (dynamic friction coefficient) is calculated from the following equation (1).
  • FIG. 3A is a micrograph showing the surface of the high carbon steel plate before measuring the micro friction coefficient
  • FIG. 3B is a micro photo showing the surface of the high carbon steel plate after measuring the micro friction coefficient.
  • 3A and 3B show an example of a visual field of 10 ⁇ m ⁇ 10 ⁇ m.
  • ferrite 31 and cementite 32 are present in this visual field example.
  • FIG. 3B after the measurement, there is a measuring rod 33 generated as the diamond indenter moves in the horizontal direction.
  • the micro friction coefficient of cementite was 0.4 or less.
  • the high carbon steel sheet according to the present embodiment has a structure represented by a cementite spheroidization ratio of 80% or more and an average particle diameter of cementite of 0.3 ⁇ m to 2.2 ⁇ m.
  • cementite may be a starting point for stress concentration during molding, and stress is particularly likely to concentrate locally in acicular cementite.
  • the spheroidization rate of cementite is less than 80%, there is a lot of acicular cementite that tends to concentrate stress, so stress concentration tends to occur, and peeling occurs at the interface between ferrite and cementite, resulting in sufficient formability. I can't. Therefore, the spheroidization rate of cementite is 80% or more, preferably 85% or more. From the viewpoint of moldability, the spheroidization rate of cementite is preferably as high as possible, and may be 100%. However, if the spheroidization rate of cementite is set to 100%, the productivity may decrease, and from the viewpoint of productivity, the spheroidization rate of cementite is preferably 80% or more and less than 100%.
  • the average particle size of cementite is closely related to the degree of stress concentration on cementite.
  • the average particle diameter of cementite is less than 0.3 ⁇ m, dislocations generated during molding form an Oro-one loop with respect to cementite, increasing the dislocation density around the cementite and generating cracks (voids). Therefore, the average particle diameter of cementite is 0.3 ⁇ m or more, preferably 0.5 ⁇ m or more.
  • the average particle size of cementite exceeds 2.2 ⁇ m, a large amount of dislocations generated during molding accumulates, local stress concentration occurs, and cracks occur. Therefore, the average particle diameter of cementite is 2.2 ⁇ m or less, preferably 2.0 ⁇ m or less.
  • the spheroidization rate and average particle diameter of cementite can be determined by structural observation using a scanning electron microscope.
  • the observation surface was mirror-finished by wet polishing with emery paper and diamond abrasive grains having a particle size of 1 ⁇ m, and then with an etching solution of 3% by volume nitric acid and 97% by volume alcohol. Etching is performed.
  • the observation magnification is 3000 to 10,000 times, for example, 10,000 times, and 16 fields of view containing 500 or more cementites on the observation surface are selected, and these tissue images are acquired. Then, the area of each cementite in the tissue image is measured using image processing software.
  • the image processing software for example, “Win ROOF” manufactured by Mitani Corporation can be used.
  • cementite having an area of 0.01 ⁇ m 2 or less is excluded from the evaluation target.
  • the average area of the cementite to be evaluated is obtained, the diameter of the circle from which this average area is obtained is obtained, and this diameter is taken as the average particle diameter of the cementite.
  • the average area of cementite is a value obtained by dividing the total area of cementite to be evaluated by the number of cementite.
  • cementite with a ratio of major axis length to minor axis length of 3 or more is acicular cementite, cementite less than 3 is spherical cementite, and the value obtained by dividing the number of spherical cementites by the total number of cementite is spheroidized. Rate.
  • the slab having the above chemical composition is hot-rolled to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is pickled, and then the hot-rolled steel sheet is annealed.
  • the temperature of slab heating is set to 1000 ° C. or higher and lower than 1150 ° C.
  • the temperature of finish rolling is set to 830 ° C. or higher and 950 ° C. or lower
  • the winding temperature is set to 450 ° C. or higher and 700 ° C. or lower.
  • the hot rolled steel sheet is held at a temperature of 730 ° C. or higher and 770 ° C.
  • the annealing atmosphere is, for example, an atmosphere containing 75% by volume or more of hydrogen in a temperature range where the atmospheric temperature exceeds 400 ° C., but is not limited thereto.
  • FIG. 4 is a schematic diagram showing changes in temperature.
  • FIG. 5A to FIG. 5E are schematic diagrams showing changes in tissue.
  • the hot rolling S1 includes slab heating S11, finish rolling S12 and winding S13, and the annealing S3 includes high temperature holding S31 and cooling S32.
  • pickling S2 between hot rolling S1 and annealing S3, and cooling S4 after annealing S3.
  • B atoms 13 are segregated at the interface between the austenite 12 and the austenite 12, as shown in FIG. 5A.
  • the structure of the steel sheet includes ferrite 11 and austenite 12. Further, B atoms 13 are segregated at the interface between the ferrite 11 and the austenite 12. Some of the B atoms 13 are also present on the surface 15 of the steel sheet, and the B atoms 13 existing on the surface of the steel sheet are bonded to each other by a covalent bond 14. Although not shown in FIG.
  • cementite is also included in the structure of the steel sheet, and some of the B atoms 13 are segregated also at the interface between the ferrite 11 and cementite.
  • the ratio of ferrite 11 increases and the ratio of austenite 12 decreases compared to the structure shown in FIG. 5B. Is moving.
  • the B atom 13 which exists in the surface of a steel plate increases with the movement of an interface.
  • cooling S32 is proceeded, as shown in FIG. 5D, than the tissue shown in FIG.
  • the micro friction coefficient of ferrite on the surface 15 can be made less than 0.5.
  • the slab heating temperature is 1150 ° C. or lower, preferably 1140 ° C. or lower.
  • the slab heating temperature is set to 1000 ° C. or higher, preferably 1030 ° C. or higher.
  • the finish rolling temperature is 950 ° C. or lower, preferably 940 ° C. or lower.
  • the temperature of finish rolling is less than 830 degreeC, the adhesiveness with the steel plate of the scale produced until completion of winding is very high, and it is difficult to remove by pickling. Although it can be removed by performing strong pickling, the surface of the steel sheet is roughened by strong pickling, so that adhesion to the mold may occur during forming. Moreover, if the temperature of finish rolling is less than 830 degreeC, since the recrystallization of austenite is not completed before winding, the anisotropy of a hot-rolled steel plate increases. Since the anisotropy of the hot-rolled steel sheet is inherited even after annealing, sufficient formability cannot be obtained. Accordingly, the finish rolling temperature is 830 ° C. or higher, preferably 840 ° C. or higher.
  • the winding temperature is 700 ° C. or less.
  • the winding temperature exceeds 570 ° C., a coarse scale is generated until the winding is completed. For this reason, for the same reason as when the finish rolling temperature exceeds 950 ° C., adhesion to the mold may occur during molding. Therefore, the winding temperature is preferably 570 ° C.
  • the winding temperature is 450 ° C. or higher, preferably 460 ° C. or higher.
  • the temperature of the coarse bar may be raised before entering the finishing mill.
  • the apparatus used for the temperature increase and the method for the temperature increase are not particularly limited, but it is desirable to perform the temperature increase by high frequency induction heating.
  • the preferred temperature range of the heated coarse bar is 850 ° C. to 1100 ° C. Since the temperature below 850 ° C. is close to the transformation temperature from austenite to ferrite, if the temperature of the heated coarse bar is less than 850 ° C., heat generation and endotherm in transformation and reverse transformation may occur, and temperature control becomes unstable.
  • the temperature raising temperature is preferably 850 ° C. or higher. Setting the temperature of the coarse bar to over 1100 ° C. takes excessive time and decreases the productivity. For this reason, when raising the temperature of the coarse bar, the temperature raising temperature is preferably 1100 ° C. or less.
  • the holding temperature for annealing is set to 730 ° C. or higher, preferably 735 ° C. or higher.
  • the annealing holding temperature exceeds 770 ° C., as shown in FIG. 6D, the B atoms 13 are concentrated in the vicinity of the triple point of the ferrite 11, the austenite 12, and the surface of the steel sheet, and coarse B crystals are generated.
  • the annealing holding temperature exceeds 770 ° C., the hot-rolled steel sheet wound in a coil shape has a large thermal expansion, and the hot-rolled steel sheets may rub against each other during the annealing, resulting in scuffing on the surface.
  • the surface aesthetics may be damaged or the yield may be reduced by rubbing.
  • the annealing holding temperature is set to 770 ° C. or lower, preferably 765 ° C. or lower.
  • the annealing holding time 3 hours or more and 60 hours or less
  • the B atoms 13 do not sufficiently segregate at the interface between the ferrite 11 and the austenite 12, so that even if the subsequent treatment is appropriately performed, A good covered surface cannot be obtained, and the micro friction coefficient of ferrite on the surface cannot be less than 0.5.
  • the annealing holding time is less than 3 hours, the cementite is not sufficiently coarsened, and the average particle size of the cementite cannot be 0.3 ⁇ m or more. Therefore, the annealing holding time is 3 hours or more, preferably 5 hours or more.
  • the annealing holding time exceeds 60 hours, the micro friction coefficient of ferrite on the surface cannot be made less than 0.5 for the same reason as when the annealing holding temperature exceeds 770 ° C. Further, when the annealing holding time exceeds 60 hours, the cementite becomes excessively coarse, and the average particle size of the cementite cannot be made 2.2 ⁇ m or less. Accordingly, the annealing holding time is 60 hours or less, preferably 40 hours or less.
  • the cooling rate to 650 ° C. is 60 ° C./hr or less, and 50 ° C./hr or less.
  • an evaluation of adhesion suppression and an evaluation of crack sensitivity were performed.
  • a draw bead test was performed for evaluation of adhesion suppression. That is, an indentation bead with a tip radius R of 20 mm was pressed against a high carbon steel plate with a load of 10 kN and pulled out. Then, the presence or absence of an adherent at the tip of the indentation bead was observed. The presence of an adhesive in this test indicates that an adhesive is generated on the mold at an early stage in press molding of several thousand to several tens of thousands of shots, thereby reducing the formability.
  • a compression processing test was performed.
  • sample No. 10 since the C content of steel type J was too low, the amount of cementite was insufficient, sufficient lubricity was not obtained, and adhesion with the mold occurred during molding.
  • Sample No. In No. 11 since the N content of steel type K was too high, BN was precipitated, the solid solution amount of B was insufficient, the micro friction coefficient of ferrite was low, and cracking occurred during adhesion and compression tests.
  • Sample No. In No. 12 since the Al content of steel type L was too high, the ductility of ferrite was low, and cracks originating from the intragranular cracking of ferrite occurred during the compression test. Sample No. In No.
  • sample no. 31-No. No. 43 was within the scope of the present invention, so that good adhesion suppression and cracking sensitivity could be obtained.
  • sample No. In No. 44 since the C content of steel grade AM was too low, the amount of cementite was insufficient, sufficient lubricity could not be obtained, and adhesion with the mold occurred during molding.
  • Sample No. In No. 45 since the Cu content of steel grade AN was too high, soot was generated during hot rolling, and adhesion starting from this soot occurred.
  • Sample No. In No. 46 since the Ca content of the steel type AO was too high, coarse Ca oxide was formed, and cracks originating from the coarse Ca oxide occurred during the compression test. Sample No. In No.
  • FIG. 1-No. 25 and no. 31-No. 67 sample no. 11, no. 51, no. 53 and no. Although excluding 62, the relationship between the micro friction coefficient of ferrite and the B content is shown. As shown in FIG. 1, when the B content is 0.0004% or more, the micro friction coefficient of ferrite is remarkably low as compared with the case where the B content is less than 0.0004%.
  • sample no. 72, no. 74, no. 77-No. 80, no. 82, no. 83, no. 85 and no. Nos. 88 to 92 were within the scope of the present invention, so that good adhesion suppression and cracking sensitivity could be obtained.
  • sample no. 103, no. 105, no. 106, no. 108-No. 111, no. 114-No. 117 and no. 120-No. Even 122 since it is within the scope of the present invention, good adhesion suppression and cracking sensitivity could be obtained.
  • sample No. In No. 71 since the holding temperature of annealing was too high, the volume expansion was large, the hot rolled coil was unwound and rubs were generated, and pressing wrinkles were also generated by the binding band. Further, the variation in the thickness of the B crystal film was large, and the micro friction coefficient of ferrite was large. For this reason, adhesion occurred. Furthermore, the cementite was excessively coarsened, and cracks originating from the coarse cementite occurred during the compression test. Sample No. In No.
  • Sample No. 101 since the annealing holding temperature was too low, segregation at the interface between B ferrite and austenite was suppressed, the micro friction coefficient of ferrite was large, and adhesion occurred. In addition, segregation at the interface between B ferrite and cementite was suppressed, and cracking occurred during the compression test.
  • Sample No. 102 since the temperature of the finish rolling was too high, large irregularities were formed along with the removal of the scale, and the micro friction coefficient of ferrite was large. For this reason, adhesion occurred.
  • Sample No. In 104 since the temperature of slab heating was too high, B atoms were oxidized during slab heating, and the micro friction coefficient of ferrite was large. For this reason, adhesion occurred. Sample No.
  • Sample No. In 132 since the cooling rate was too low, the variation in the thickness of the B crystal film was large, and the micro friction coefficient of ferrite was large. For this reason, adhesion occurred. Also, the cementite was excessively coarsened, and cracks originating from the coarse cementite occurred during the compression test. Sample No. In No. 135, since the temperature of finish rolling was too low, the anisotropy of the structure was strong, and cracks originating from the non-uniform structure occurred during the compression test. Further, as a result of removing the scale, the surface of the steel sheet was rough and adhesion occurred. Sample No. In No.
  • FIG. 7 shows the relationship between the micro friction coefficient of ferrite and the B content in the samples extracted from the examples in the first experiment or the third experiment. As shown in FIG. 7, if the B content is 0.0008% or more, the micro friction coefficient of ferrite is further lower than that in the case of less than 0.0008%.
  • the present invention can be used in, for example, manufacturing industries and utilization industries of high carbon steel sheets used in various steel products such as automobile drive system parts, saws and blades.

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PCT/JP2014/077544 2014-10-16 2014-10-16 高炭素鋼板及びその製造方法 WO2016059701A1 (ja)

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US15/513,130 US20170306434A1 (en) 2014-10-16 2014-10-16 High-carbon steel sheet and method of manufacturing the same
ES14903999T ES2807553T3 (es) 2014-10-16 2014-10-16 Lámina de acero con alto contenido de carbono y método de fabricación de la misma
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