WO2015125463A1 - 高強度鋼板およびその製造方法 - Google Patents
高強度鋼板およびその製造方法 Download PDFInfo
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- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C21D1/26—Methods of annealing
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- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
- C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
- C25F1/02—Pickling; Descaling
- C25F1/04—Pickling; Descaling in solution
- C25F1/06—Iron or steel
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- C—CHEMISTRY; METALLURGY
- 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
- C21D1/34—Methods of heating
- C21D1/42—Induction heating
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- C—CHEMISTRY; METALLURGY
- 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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C—CHEMISTRY; METALLURGY
- 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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/25—Process efficiency
Definitions
- the present invention relates to a high-strength steel sheet having excellent chemical conversion property and corrosion resistance after electrodeposition coating even when the content of Si or Mn is large, and a method for producing the same.
- automotive steel plates are used after painting.
- a chemical conversion treatment called a phosphate treatment is performed on the steel plate for automobiles.
- Chemical conversion treatment of steel sheets for automobiles is one of the important treatments for ensuring corrosion resistance after painting.
- Si or Mn are oxidized even when annealing is performed in a reducing N 2 + H 2 gas atmosphere in which no oxidation of Fe occurs (which reduces Fe oxide).
- Oxidation of Si or Mn selectively forms an oxide containing Sn or Mn (SiO 2 , MnO, etc .; hereinafter referred to as a selective surface oxide) on the steel sheet surface. Since this selective surface oxide inhibits the formation reaction of the chemical conversion film during the chemical conversion treatment, a fine region where the chemical conversion film is not formed (hereinafter referred to as scale) is formed on the steel sheet surface, and the chemical conversion treatment performance is lowered.
- Patent Document 1 discloses a method of forming an iron coating layer of 20 to 1500 mg / m 2 on a steel sheet using an electroplating method. ing.
- this method there is a problem that the cost is increased due to the additional steps required for the electroplating equipment.
- Patent Document 2 the Mn / Si ratio is defined, and in Patent Document 3, Ni is added to improve the phosphate processability.
- the effect depends on the contents of Si and Mn in the steel sheet, and further improvement is necessary for the steel sheet having a high Si and Mn content.
- Patent Document 4 by setting the dew point during annealing to ⁇ 25 to 0 ° C., an internal oxide layer made of an oxide containing Si is formed within a depth of 1 ⁇ m from the steel sheet surface, and the steel sheet surface length is 10 ⁇ m. A method is disclosed in which the ratio of the Si-containing oxide to 80% is 80% or less.
- the area for controlling the dew point since the area for controlling the dew point is premised on the entire inside of the furnace, the controllability of the dew point is difficult and stable operation is difficult.
- Patent Document 5 describes a method in which a steel sheet temperature reaches 350 to 650 ° C. in an oxidizing atmosphere to form an oxide film on the steel sheet surface, and then heated to a recrystallization temperature in a reducing atmosphere and cooled. Yes.
- this method there is a difference in the thickness of the oxide film formed on the surface of the steel sheet depending on the method of oxidation, and oxidation may not occur sufficiently.
- the oxide film becomes too thick, and the oxide film may remain or peel off during annealing in a reducing atmosphere, which may deteriorate the surface properties.
- atmosphere is described. Oxidation in the air has a problem that a thick oxide is formed and subsequent reduction is difficult, or a reduction atmosphere with a high hydrogen concentration is necessary.
- Patent Document 6 a cold rolled steel sheet containing, by mass%, Si of 0.1% or more and / or Mn of 1.0% or more, a condition that the steel sheet temperature is 400 ° C. or more and under an iron oxidizing atmosphere. Describes a method of forming an oxide film on the steel sheet surface and then reducing the oxide film on the steel sheet surface in an iron reducing atmosphere. Specifically, N 2 + H 2 gas that reduces Fe oxide after oxidizing Fe on the steel sheet surface under the condition of using a direct fire burner with a steel sheet temperature of 400 ° C. or more and an air ratio of 0.93 or more and 1.10 or less. Annealing the steel sheet in an atmosphere.
- Patent Document 6 does not specifically describe the heating temperature of an open flame burner.
- a large amount of Si is contained (approximately 0.6% or more)
- the amount of oxidation of Si which is easier to oxidize than Fe, is increased, and the oxidation of Fe is suppressed. Too little.
- formation of the surface Fe reduced layer after reduction may be insufficient, or SiO 2 may be present on the surface of the steel plate after reduction, resulting in the occurrence of scaling of the conversion coating.
- JP-A-5-320952 Japanese Patent No. 4319559 Japanese Patent No. 2951480 Japanese Patent No. 3840392 JP 55-145122 A JP 2006-45615 A
- the present invention has been made in view of such circumstances, and even when the content of Si and Mn is large, a high-strength steel sheet having excellent workability, chemical conversion treatment, and corrosion resistance after electrodeposition coating, and its production It aims to provide a method.
- the dew point or oxygen concentration is increased by simply increasing the partial pressure of water vapor or oxygen in the entire annealing furnace, and the inside of the steel sheet is excessively oxidized. For this reason, as described above, in the conventional technique, there is a problem in dew point control or oxidation controllability in the entire furnace, unevenness in chemical conversion treatment occurs, or corrosion resistance after electrodeposition coating deteriorates.
- the steel sheet is heated at a temperature rising rate of 7 ° C./s or higher. .
- Condition 2 In continuous annealing, the maximum temperature reached by the steel sheet is 600 ° C. or higher and 750 ° C. or lower.
- Condition 3 In continuous annealing, the steel plate passage time in the temperature range of 600 ° C. to 750 ° C. is 30 seconds to 10 minutes, and the dew point of the atmosphere is ⁇ 10 ° C. or higher.
- the structure and structure of the steel sheet surface layer of the high-strength steel sheet obtained by the above method has the following features 1 and 2.
- the present invention is based on the above findings, and features are as follows.
- the annealing furnace temperature in the heating process of the continuous annealing In the temperature range of 450 ° C. or higher and A ° C. or lower (A: 500 ⁇ A), the steel sheet is heated under the condition of a temperature increase rate of 7 ° C./s or higher, and the maximum temperature reached in the continuous annealing is 600 ° C. or higher and 750 ° C.
- a high-strength steel sheet characterized in that, in the continuous annealing, a steel sheet passage time in a temperature range of 600 ° C. or more and 750 ° C. or less is 30 seconds or more and 10 minutes or less, and the dew point of the atmosphere is ⁇ 10 ° C. or more. Production method.
- the steel sheet has a component composition in mass%, further B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Sn: 0.001 to Among 0.20%, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W: 0.001 to 0.10% and V: 0.001 to 0.10% (1)
- the manufacturing method of the high-strength steel plate as described in (1) characterized by including 1 or more types of elements chosen from.
- a high-strength steel sheet having excellent workability, chemical conversion treatment, and corrosion resistance after electrodeposition coating can be obtained even when the content of Si or Mn is large.
- being excellent in chemical conversion treatment means having a non-scaling and uneven appearance after chemical conversion treatment.
- An excellent appearance is achieved by suppressing the occurrence of scale and unevenness.
- the method for producing a high-strength steel sheet according to the present invention employs the following (Condition 1) to (Condition 3) when the steel sheet is continuously annealed.
- (Condition 1) In the heating process of continuous annealing, in the temperature range of annealing furnace temperature: 450 ° C. or higher and A ° C. or lower (500 ⁇ A), the steel sheet is heated under the condition of the temperature rising rate: 7 ° C./s or higher.
- Consdition 2 In continuous annealing, the maximum temperature reached by the steel sheet is 600 ° C. or higher and 750 ° C. or lower.
- Condition 3) In continuous annealing, the steel plate passage time in the temperature range of 600 ° C. to 750 ° C. is 30 seconds to 10 minutes, and the dew point of the atmosphere is ⁇ 10 ° C. or higher.
- the manufacturing method of a steel plate used as the object of continuous annealing is demonstrated.
- the manufacturing method of a steel plate is not specifically limited.
- a method of producing a hot-rolled sheet by hot rolling steel a method of producing a cold-rolled sheet by cold rolling after hot rolling the steel, hot rolling the steel, pickling, A method of manufacturing a cold-rolled sheet by cold rolling can be employed.
- the hot-rolled sheet and the cold-rolled sheet thus obtained can be used as an object for continuous annealing.
- the conditions for hot rolling and pickling at the time of manufacturing the steel sheet are not particularly limited, and may be set as appropriate.
- the cold rolling is preferably performed at a rolling reduction of 40% or more and 80% or less. If the rolling reduction is less than 40%, the recrystallization temperature is lowered, and the mechanical characteristics are likely to deteriorate. On the other hand, when the rolling reduction exceeds 80%, the steel sheet is a high-strength steel sheet, so that not only the rolling cost is increased, but surface concentration during annealing is increased, and chemical conversion treatment properties may be deteriorated.
- Continuous annealing can be performed using a general continuous annealing facility.
- a general continuous annealing facility has an annealing furnace having a heating zone in the front stage and a soaking zone in the rear stage.
- the steel sheet is heated to a predetermined temperature in the preceding heating zone, and the steel sheet is held under conditions of a predetermined temperature and a predetermined time in the latter-stage soaking zone.
- the present invention is characterized in that the above (Condition 1) to (Condition 3) are employed during continuous annealing.
- the rate of temperature increase is 7 ° C./s or higher. It is controlled to be heated. Due to this heating, oxidation of easily oxidizable elements (Si, Mn, etc.) (hereinafter referred to as internal oxidation) does not occur inside the steel sheet, but surface concentration occurs, but not less than 450 ° C. and not more than A ° C. (A: 500 ⁇ ).
- the temperature range of A) can be passed as soon as possible, and surface concentration can be suppressed. That is, it becomes possible to suppress the scale and unevenness after the chemical conversion treatment.
- the effect of improving the chemical conversion treatment as described above is caused by performing all of (Condition 1) to (Condition 3).
- (Condition 1) and (Condition 3) are considered important.
- the generation of the surface concentrate is suppressed as much as possible under (Condition 1).
- surface enrichment on the steel sheet surface such as Si and Mn in the steel deteriorates the chemical conversion property after annealing by allowing an appropriate amount of internal oxide to be present inside the steel sheet within 10 ⁇ m from the steel sheet surface. Can be suppressed.
- excellent chemical conversion processability without any scaling and unevenness and higher corrosion resistance after electrodeposition coating can be obtained.
- thermometer a multiple reflection thermometer, a radiation thermometer, etc. can be illustrated, and the system of a thermometer is not specifically limited.
- the temperature range for controlling the heating rate is set to 450 ° C. or higher is as follows. In the temperature range below 450 ° C., surface enrichment and internal oxidation that cause problems such as scale, unevenness, and deterioration of corrosion resistance do not occur. Therefore, the temperature range for controlling the rate of temperature rise is set to 450 ° C. or more, which is the temperature range where the effects of the present invention are manifested.
- the reason why the temperature range for controlling the rate of temperature rise is A ° C. or lower (A: any value selected from 500 ⁇ A) is as follows. First, when the upper limit of the temperature range for controlling the temperature increase rate is below 500 ° C., the time for controlling the temperature increase rate to 7 ° C./s or more is short, and the effect of the present invention cannot be sufficiently obtained. For this reason, A shall be 500 degreeC or more. Further, when the upper limit of the temperature range for controlling the rate of temperature rise exceeds 600 ° C., there is no problem with the effect of the present invention.
- the upper limit exceeds 600 ° C., it is disadvantageous from the viewpoint of an increase in cost (equipment of an induction heater, etc.) required for the equipment in the annealing furnace. Therefore, 600 ° C. or lower is preferable.
- the heating rate in the above temperature range is 7 ° C./s or more is as follows. It is at a temperature increase rate of 7 ° C./s or more that the effect of suppressing surface concentration begins to be recognized. Although there is no particular upper limit for the rate of temperature increase, the effect is saturated at 500 ° C./s or more, which is disadvantageous in terms of cost. For this reason, the heating rate is desirably 500 ° C./s or less. Note that it is possible to set the rate of temperature rise to 7 ° C./s or more, for example, by placing an induction heater in an annealing furnace in which the steel plate temperature is 450 ° C. or more and A ° C. or less.
- the highest temperature reached in the steel sheet is 600 ° C. or higher and 750 ° C. or lower in annealing.
- the steel plate maximum temperature is a temperature that is further increased by heating from the maximum temperature A ° C. in the heating in the heating process, except when A ° C. is the same as the steel plate maximum temperature.
- the maximum steel plate temperature refers to the maximum value during annealing when measured by the same method as the method for measuring the steel plate temperature.
- the maximum steel sheet temperature in the annealing furnace is 600 ° C. or higher and 750 ° C. or lower is as follows. If the maximum steel plate temperature is below 600 ° C., a good material cannot be obtained. Therefore, in this invention, the said steel plate highest reached temperature shall be 600 degreeC or more. On the other hand, when the maximum temperature reached by the steel sheet exceeds 750 ° C., the surface concentration becomes remarkable, and deterioration of the chemical conversion property starts to be recognized. Furthermore, from the viewpoint of the material, when the maximum steel sheet temperature exceeds 750 ° C., the effect of balance between strength and ductility is saturated. From the above, the maximum temperature reached by the steel sheet is set to 600 ° C. or higher and 750 ° C. or lower.
- the steel plate temperature in the temperature range of 600 ° C. to 750 ° C. is set to 30 seconds or more and 10 minutes or less, and the dew point of the atmosphere is ⁇ 10 ° C. or more.
- the target material (TS, El) cannot be obtained.
- the steel plate passage time exceeds 10 minutes, the effect of balance between strength and ductility is saturated.
- the dew point of the atmosphere in which the steel sheet temperature is 600 ° C. or higher and 750 ° C. or lower is set to ⁇ 10 ° C. or higher in annealing, the dew point increases to increase the O 2 potential resulting from the decomposition of H 2 O and promote internal oxidation. It is possible. In the temperature range below -10 ° C, the amount of internal oxidation formed is small. There is no particular upper limit on the dew point. However, if the dew point exceeds 90 ° C., the amount of Fe oxidation increases, and there is a concern about deterioration of the annealing furnace wall and roll. For this reason, the dew point is desirably 90 ° C. or lower.
- the dew point in other temperature ranges is not particularly limited, and may be in the range of ⁇ 10 to ⁇ 40 ° C., for example.
- the conditions of the soaking temperature and soaking time in the soaking zone are not particularly limited, and may be set as appropriate.
- the atmosphere gas is not particularly limited as long as it does not impair the effects of the present invention.
- the atmosphere gas is composed of hydrogen gas, nitrogen gas, and inevitable impurity gas. Further, other gases (H 2 O, CO 2 , CO, etc.) may be included as long as the effects of the present invention are not impaired.
- the hydrogen concentration of the atmosphere in continuous annealing is 1 vol% or more and 50 vol% or less.
- the hydrogen concentration is less than 1 vol%, the activation effect due to the reduction cannot be obtained, and the chemical conversion treatment property may be deteriorated.
- the upper limit is not particularly defined, but if it exceeds 50 vol%, the production cost increases to increase the hydrogen concentration, and the effect of adjusting the hydrogen concentration is saturated. Therefore, the hydrogen concentration is preferably 1 vol% or more and 50 vol% or less. Furthermore, 5 vol% or more and 30 vol% or less are desirable.
- the following treatment may be performed after the continuous annealing.
- quenching and tempering may be performed as necessary.
- the conditions for quenching and tempering are not particularly limited. Tempering is preferably performed at a temperature of 150 to 400 ° C. If the temperature is lower than 150 ° C., the elongation of the steel sheet tends to deteriorate, and if it exceeds 400 ° C., the hardness of the steel sheet tends to decrease.
- electrolytic pickling in an aqueous solution containing sulfuric acid is performed on a high-strength steel sheet after continuous annealing. It is preferable to carry out.
- the pickling solution used for electrolytic pickling is not particularly limited. However, nitric acid and hydrofluoric acid are not preferred because they are highly corrosive to equipment and require careful handling. Hydrochloric acid is not preferred because it may generate chlorine gas from the cathode. For this reason, use of sulfuric acid is preferable in consideration of corrosivity and environment.
- the sulfuric acid concentration is preferably 5% by mass or more and 20% by mass or less. If the sulfuric acid concentration is less than 5% by mass, the electrical conductivity will be low, so that the bath voltage during electrolysis will rise and the power load may become large. On the other hand, when the sulfuric acid concentration exceeds 20% by mass, loss due to drag-out is large, which causes a problem in cost.
- the conditions for electrolytic pickling are not particularly limited. In order to efficiently remove oxides of Si and Mn that are inevitably surface-enriched formed after annealing, it is desirable to perform alternating electrolysis with a current density of 1 A / dm 2 or more.
- the reason for the alternating electrolysis is as follows. The pickling effect is small when the steel plate is held on the cathode. In addition, if the steel plate is held on the anode, Fe eluted during electrolysis accumulates in the pickling solution, and the Fe concentration in the pickling solution increases. Will occur.
- the temperature of the electrolytic solution is preferably 40 ° C. or higher and 70 ° C. or lower. Since the bath temperature rises due to heat generated by continuous electrolysis, it may be difficult to maintain the temperature below 40 ° C. Moreover, it is not preferable that temperature exceeds 70 degreeC from a durable viewpoint of the lining of an electrolytic cell. In addition, when it is less than 40 degreeC, since the pickling effect becomes small, 40 degreeC or more is preferable.
- the present invention is a manufacturing method characterized by continuous annealing conditions for steel sheets.
- the steel plate that is subject to this continuous annealing will be described.
- “%” means “mass%”.
- C 0.03-0.35%
- C improves workability by forming martensite or the like in the steel structure.
- the C content needs to be 0.03% or more.
- the C content is 0.03% or more and 0.35% or less.
- Si 0.01 to 0.50% Si is an element effective for strengthening steel and obtaining a good material.
- Si is an easily oxidizable element, it is disadvantageous for chemical conversion treatment. From this viewpoint, Si is an element that should be avoided as much as possible. Moreover, about 0.01% Si is inevitably contained in the steel, and the cost increases in order to reduce the Si content below this. From the above, the lower limit of the Si content is 0.01%.
- the Si content exceeds 0.50%, the steel strengthening ability and elongation improvement effect are saturated. Moreover, the chemical conversion property of a high strength steel plate deteriorates. Therefore, the Si amount is set to 0.01% or more and 0.50% or less.
- One feature of the present invention is that the chemical conversion treatment can be improved even when the Si content is high.
- Mn 3.6 to 8.0%
- Mn is an element effective for increasing the strength of steel.
- the Mn content needs to be 3.6% or more.
- the content of Mn exceeds 8.0%, it becomes difficult to secure chemical conversion property and secure a balance between strength and ductility. Further, it is disadvantageous in terms of cost. Therefore, the Mn content is 3.6% or more and 8.0% or less.
- Al 0.01 to 1.0% Al is added for the purpose of deoxidizing molten steel. If the Al content is less than 0.01%, the object is not achieved. The effect of deoxidation of molten steel can be obtained by making the Al content 0.01% or more. On the other hand, if the Al content exceeds 1.0%, the cost increases. Furthermore, if the Al content exceeds 1.0%, the surface concentration of Al increases, and it becomes difficult to improve the chemical conversion property. Therefore, the Al content is set to 0.01% to 1.0%.
- P 0.10% or less
- P is one of elements inevitably contained, and may not be contained. In order to reduce the P content to less than 0.005%, there is a concern about an increase in cost. Therefore, the P content is preferably 0.005% or more. On the other hand, if the P content exceeds 0.10%, the weldability deteriorates. Furthermore, when the P content exceeds 0.10%, the chemical conversion treatment property deteriorates so severely that it is difficult to improve the chemical conversion treatment property even with the present invention. Accordingly, the P content is preferably 0.10% or less, and the lower limit is preferably 0.005%.
- S 0.010% or less S is one of elements inevitably contained, and S may not be contained.
- the lower limit of the S content is not specified.
- content of S shall be 0.010% or less.
- the steel sheet subjected to continuous annealing is B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Sn: 0.001 to 0.20%, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W : One or more elements selected from 0.001 to 0.10% and V: 0.001 to 0.10% may be included as necessary. The reason for limiting the proper content in the case of containing these elements is as follows.
- B 0.001 to 0.005%
- the content of B is less than 0.001%, it is difficult to obtain the quenching promoting effect.
- the content of B exceeds 0.005%, chemical conversion property may be deteriorated. Therefore, when it contains B, it is preferable to make B amount into 0.001% or more and 0.005% or less.
- the steel plate does not need to contain B. It is the same for other optional elements to be contained as necessary.
- Nb 0.005 to 0.05% If the Nb content is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, if the Nb content exceeds 0.05%, the cost increases. Therefore, when Nb is contained, the Nb content is 0.005% or more and 0.05% or less.
- Ti 0.005 to 0.05% If the Ti content is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, when the Ti content exceeds 0.05%, chemical conversion treatment may be deteriorated. Therefore, when Ti is contained, the Ti content is preferably 0.005% or more and 0.05% or less.
- Cr 0.001 to 1.0%
- Cr content is less than 0.001%, it is difficult to obtain a hardenability effect.
- Cr content exceeds 1.0%, Cr is concentrated on the surface, so that weldability is deteriorated. Therefore, when it contains Cr, it is preferable that Cr amount shall be 0.001% or more and 1.0% or less.
- Mo 0.05 to 1.0% If the Mo content is less than 0.05%, the effect of adjusting the strength is difficult to obtain. On the other hand, if the Mo content exceeds 1.0%, the cost increases. Therefore, when it contains Mo, it is preferable to make Mo amount into 0.05% or more and 1.0% or less.
- the amount of Cu is preferably 0.05% or more and 1.0% or less.
- Ni 0.05 to 1.0%
- the amount of Ni is preferably 0.05% or more and 1.0% or less.
- Sn and Sb can be contained from the viewpoint of suppressing decarburization in the region of several tens of microns from the steel sheet surface caused by nitriding, oxidation, or oxidation of the steel sheet surface.
- the content is 0.001% or more.
- these content shall be 0.20% or less, respectively.
- Ta 0.001 to 0.10%
- Ta contributes to higher strength by forming carbides and carbonitrides with C and N, and further contributes to higher yield ratio (high YR).
- Ta has the effect of refining the hot-rolled sheet structure, and this effect refines the ferrite grain size after cold rolling and annealing.
- the amount of C segregation to the grain boundary accompanying the increase in grain boundary area increases, and a high seizure hardening amount (BH amount) can be obtained.
- BH amount high seizure hardening amount
- 0.001% or more of Ta can be contained.
- the content of Ta exceeds 0.10%, not only the raw material cost is increased, but also the formation of martensite in the cooling process after annealing may be hindered.
- TaC precipitated in the hot-rolled sheet increases the deformation resistance during cold rolling, and may make it difficult to manufacture a stable actual machine. Therefore, when Ta is contained, the content is preferably 0.10% or less.
- W and V are elements that form carbonitrides and have the effect of increasing the strength of steel by precipitation effects, and can be added as necessary. Such an effect is observed when both W and / or V are added, containing 0.001% or more. On the other hand, when the content of these elements exceeds 0.10%, the steel sheet becomes excessively strong and the ductility may be deteriorated. From the above, when W and / or V are contained, the content is preferably 0.001% or more and 0.10% or less.
- the remainder other than the above is Fe and inevitable impurities. Even if elements other than the elements described above are contained, the present invention is not adversely affected, and the upper limit is made 0.10%.
- the structure and structure of the steel sheet surface layer which may be the starting point of corrosion cracking, are controlled to a higher degree.
- the dew point was controlled in continuous annealing to increase the oxygen potential.
- Si, Mn, and the like which are easily oxidizable elements, are internally oxidized immediately before the chemical conversion treatment, and the activities of Si and Mn on the steel sheet surface are reduced. And external oxidation of these elements is suppressed, and as a result, chemical conversion property and corrosion resistance after electrodeposition coating are improved.
- the structure and structure of the steel sheet surface layer of the steel strength steel sheet manufactured by the manufacturing method of the present invention has the following characteristics.
- (Characteristic 1) Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V are in a region within 100 ⁇ m from the steel plate surface of the high-strength steel plate. In total, at least one oxide selected from the group consisting of 0.010 to 0.050 g / m 2 per side.
- (Feature 2) It has an oxide containing Mn in grains within 1 ⁇ m from the grain boundary of the steel sheet in a region within 10 ⁇ m from the steel sheet surface.
- Fe, Si, Mn, Al, P, and B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta are formed in a region within 100 ⁇ m from the steel sheet surface.
- At least one oxide selected from W and V is present in a total amount of 0.010 g / m 2 or more per side.
- the amount of oxide formation exceeds 0.050 g / m 2 , there is a concern that it becomes a starting point of corrosion cracking. Further, even if the formation amount exceeds 0.050 g / m 2 , the effect of improving the chemical conversion treatment property is saturated, so the formation amount does not need to exceed 0.050 g / m 2 .
- an oxide containing Mn is present in the grains within 1 ⁇ m from the steel grain boundaries.
- the grain boundary diffusion of easily oxidizable elements in the steel can be suppressed, but the intragranular diffusion may not be sufficiently suppressed. Therefore, in the present invention, as described above, in the temperature range of the annealing furnace temperature: 450 ° C. or higher and A ° C. or lower (A: 500 ⁇ A ⁇ 600), the temperature rising rate is controlled to be 7 ° C./s or higher. By doing so, internal oxidation is performed not only at the grain boundaries but also within the grains.
- crystalline Si and Mn-based oxides are present in the ground iron grains within 1 ⁇ m from the grain boundary.
- the presence of oxide in the grains reduces the amount of solute Si and Mn in grains near the oxide. As a result, concentration on the surface due to intragranular diffusion of Si and Mn can be suppressed.
- the structure of the steel sheet surface of the high-strength steel sheet obtained by the production method of the present invention is as described above. For example, there is no problem even if the oxide grows in a region exceeding 100 ⁇ m from the steel sheet surface. . In addition, there is no problem even if crystalline Si or Mn-based oxides are present in the grain of 1 ⁇ m or more from the grain boundary in a region exceeding 10 ⁇ m from the steel plate surface.
- the high-strength steel plate of the present invention may be formed by subjecting the high-strength steel plate to a chemical conversion treatment.
- the type of chemical conversion treatment liquid is not particularly limited, and general chemicals such as a chromate treatment liquid and a non-chromate treatment liquid can be used.
- the chemical conversion treatment method is not limited, and various methods such as immersion (dip) treatment, spray treatment, and electrolytic treatment can be applied.
- the high-strength steel sheet of the present invention may be formed by forming a coating film by electrodeposition coating on the chemical conversion film of the steel sheet subjected to the chemical conversion treatment.
- the conditions for electrodeposition coating are not particularly limited, and may be set as appropriate.
- the hot-rolled steel sheet having the steel composition shown in Table 1 was pickled, removed the black scale, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. A part of the hot-rolled steel sheet (thickness: 2.0 mm) after removing the black scale was used without performing cold rolling.
- the steel sheet temperature of 450 ° C. to A ° C. (A: 500 ⁇ A ⁇ 600) is controlled by controlling the heating rate in the temperature range, the dew point and the steel plate passage time in the temperature range of 600 ° C. to 750 ° C., and the maximum temperature reached by the steel plate. Threaded and annealed.
- the dew point in the annealing furnace other than the region where the dew point was controlled was ⁇ 35 ° C.
- the dew point was controlled by absorbing and removing moisture in the atmosphere.
- the atmospheric gas component is composed of nitrogen gas, hydrogen gas, and unavoidable impurity gas.
- the hydrogen concentration in the atmosphere was 10 vol%.
- TS and El were measured for the specimens obtained as described above.
- chemical conversion treatment, corrosion resistance after electrodeposition coating, and workability were investigated. Further, the amount of oxide (internal oxidation amount) present in the region of 100 ⁇ m from the steel plate surface was measured. The results are shown in Tables 2 and 3. Moreover, a measuring method and evaluation criteria are shown below.
- TS Tensile strength
- El elongation
- the evaluation method of chemical conversion property is as follows.
- a chemical conversion treatment liquid (Palbond L3080 (registered trademark)) manufactured by Nippon Parkerizing Co., Ltd. was used as the chemical conversion treatment liquid, and chemical conversion treatment was performed by the following method.
- the test material is degreased with a degreasing liquid fine cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., washed with water, and then subjected to a surface adjustment for 30 seconds with a surface conditioning liquid preparen Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. It was. After the surface adjustment, the substrate was immersed in a chemical conversion solution (Palbond L3080) at 43 ° C. for 120 seconds, washed with water, and dried with warm air.
- a chemical conversion solution Palbond L3080
- test sample was immersed in a 5% NaCl aqueous solution (55 ° C.) for 240 hours, then taken out, washed with water and dried, and then the cross-cut portion was peeled off to measure the peeling width.
- the measurement results were evaluated according to the following evaluation criteria. " ⁇ " is an acceptable level ⁇ : Peeling width is less than 2.5 mm on one side ⁇ : Peeling width is 2.5 mm or more on one side
- a JIS No. 5 tensile test piece was sampled from the test material in the direction of 90 ° with respect to the rolling direction, and a tensile test was conducted under the condition of a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241.
- the strength TS (MPa) and the elongation El (%) were measured, and those with TS ⁇ E1 ⁇ 24000 were good and those with TS ⁇ El ⁇ 24000 were bad.
- the amount of internal oxidation is measured by “impulse furnace melting-infrared absorption method”. However, it is necessary to subtract the amount of oxygen contained in the material (that is, the steel plate before annealing).
- the oxygen concentration in the steel was measured at positions polished by 100 ⁇ m or more from both surfaces of the high-strength steel sheet after continuous annealing, and the measured value was defined as the amount of oxygen OH contained in the material.
- board thickness direction of the high strength steel plate surface after continuous annealing was measured, and the measured value was made into the oxygen amount OI after internal oxidation.
- the high-strength steel plate produced by the method of the present invention is a high-strength steel plate containing a large amount of oxidizable elements such as Si and Mn. It turns out that it is excellent in workability. On the other hand, in a comparative example, any one or more of chemical conversion treatment property, corrosion resistance, and workability is inferior.
- the high-strength steel sheet of the present invention is excellent in chemical conversion property, corrosion resistance, and workability, and can be used as a surface-treated steel sheet for reducing the weight and strength of the automobile body itself.
- the steel sheet can be applied in a wide range of fields such as home appliances and building materials as a surface-treated steel sheet provided with rust prevention properties.
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Abstract
Description
(条件1)連続焼鈍の加熱過程において、焼鈍炉内温度:450℃以上A℃以下(A:500≦A)の温度域では、昇温速度:7℃/s以上の条件で鋼板を加熱する。
(条件2)連続焼鈍において鋼板最高到達温度を600℃以上750℃以下とする。
(条件3)連続焼鈍において鋼板温度が600℃以上750℃以下の温度域の鋼板通過時間を30秒以上10分以内、雰囲気の露点を-10℃以上とする。
(条件1)連続焼鈍の加熱過程において、焼鈍炉内温度:450℃以上A℃以下(500≦A)の温度域では、昇温速度:7℃/s以上の条件で鋼板を加熱する。
(条件2)連続焼鈍において鋼板最高到達温度を600℃以上750℃以下とする。
(条件3)連続焼鈍において鋼板温度が600℃以上750℃以下の温度域の鋼板通過時間を30秒以上10分以内、雰囲気の露点を-10℃以上とする。
Cは、鋼組織中にマルテンサイトなどを形成させることで加工性を向上させる。そのためには、Cの含有量を0.03%以上にする必要がある。一方、Cの含有量が0.35%を超えると強度が上昇しすぎて、延びが低下し、結果として加工性が劣化する。したがって、C量は0.03%以上0.35%以下とする。
Siは鋼を強化して良好な材質を得るのに有効な元素である。しかし、Siは易酸化性元素であるため、化成処理性には不利である。この観点からは、Siは極力添加することを避けるべき元素である。また、0.01%程度のSiは不可避的に鋼中に含まれ、Siの含有量をこれ以下に低減するためにはコストが上昇してしまう。以上より、Siの含有量は0.01%を下限とする。一方、Siの含有量が0.50%を超えると鋼の強化能や伸び向上効果が飽和してくる。また、高強度鋼板の化成処理性が劣化する。したがって、Si量は0.01%以上0.50%以下とする。本発明は、Siの含有量が多い場合であっても化成処理性を改善できる点が特徴の1つである。
Mnは鋼の高強度化に有効な元素である。機械特性や強度を確保するためは、Mnの含有量を3.6%以上にする必要がある。一方、Mnの含有量が8.0%を超えると化成処理性の確保、強度と延性のバランスの確保が困難になる。さらに、コスト的に不利となる。したがって、Mn量は3.6%以上8.0%以下とする。
Alは溶鋼の脱酸を目的に添加される。Alの含有量が0.01%未満の場合、その目的が達成されない。溶鋼の脱酸の効果はAlの含有量を0.01%以上にすることで得られる。一方、Alの含有量が1.0%を超えるとコストアップになる。さらに、Alの含有量が1.0%を超えると、Alの表面濃化が多くなり、化成処理性の改善が困難になってくる。したがって、Al量は0.01%以上1.0%以下とする。
Pは不可避的に含有される元素のひとつであり、含有しなくてもよい。Pの含有量を0.005%未満にするためには、コストの増大が懸念されるため、Pの含有量は0.005%以上が望ましい。一方、Pの含有量が0.10%を超えると溶接性が劣化する。さらに、Pの含有量が0.10%を超えると、化成処理性の劣化が激しくなり、本発明をもってしても化成処理性を向上させることが困難になる。したがって、P量は0.10%以下とし、下限としては0.005%が望ましい。
Sは不可避的に含有される元素のひとつであり、Sを含有しなくてもよい。Sの含有量の下限は規定しない。Sの含有量が多量になると溶接性および耐食性が劣化する。このため、Sの含有量は0.010%以下とする。
Bの含有量が0.001%未満では焼き入れ促進効果が得られにくい。一方、Bの含有量が0.005%超えでは化成処理性が劣化する場合がある。よって、Bを含有する場合、B量は0.001%以上0.005%以下とすることが好ましい。ただし、機械的特性改善上、Bを含有する必要がないと判断される場合、鋼板はBを含有する必要はない。必要に応じて含有することは、他の任意元素についても同様である。
Nbの含有量が0.005%未満では強度調整の効果が得られにくい。一方、Nbの含有量が0.05%超えではコストアップを招く。よって、Nbを含有する場合、Nb量は0.005%以上0.05%以下とする。
Tiの含有量が0.005%未満では強度調整の効果が得られにくい。一方、Tiの含有量が0.05%超えでは化成処理性の劣化を招く場合がある。よって、Tiを含有する場合、Ti量は0.005%以上0.05%以下とすることが好ましい。
Crの含有量が0.001%未満では焼き入れ性効果が得られにくい。一方、Crの含有量が1.0%超えではCrが表面濃化するため、溶接性が劣化する。よって、Crを含有する場合、Cr量は0.001%以上1.0%以下とすることが好ましい。
Moの含有量が0.05%未満では強度調整の効果が得られにくい。一方、Moの含有量が1.0%超えではコストアップを招く。よって、Moを含有する場合、Mo量は0.05%以上1.0%以下とすることが好ましい。
Cuの含有量が0.05%未満では残留γ相形成促進効果が得られにくい。一方、Cuの含有量が1.0%超えではコストアップを招く。よって、Cuを含有する場合、Cu量は0.05%以上1.0%以下とすることが好ましい。
Niの含有量が0.05%未満では残留γ相形成促進効果が得られにくい。一方、Niの含有量が1.0%超えではコストアップを招く。よって、Niを含有する場合、Ni量は0.05%以上1.0%以下とすることが好ましい。
SnやSbは鋼板表面の窒化、酸化、あるいは酸化により生じる鋼板表面から数十ミクロン領域の脱炭を抑制する観点から含有することができる。窒化や酸化を抑制することで鋼板表面においてマルテンサイトの生成量が減少するのを防止し、得られる高強度鋼板の疲労特性や表面品質が改善する。以上の観点から、Snおよび/またはSbを含有する場合は、いずれも0.001%以上の含有量とすることが好ましい。また、いずれかの含有量が0.20%を超えると靭性の劣化を招くので、これらの含有量はそれぞれ0.20%以下とすることが好ましい。
TaはCやNと炭化物や炭窒化物を形成することで高強度化に寄与し、さらに高降伏比(高YR)化に寄与する。さらに、Taは熱延板組織を微細化する作用を有し、この作用により、冷延、焼鈍後のフェライト粒径が微細化される。そして、粒界面積の増大に伴う粒界へのC偏析量が増大し、高焼付き硬化量(BH量)を得ることができる。このような観点から、Taを0.001%以上含有することができる。一方、Taの含有量が0.10%を超える場合、原料コストの増加を招くだけでなく、焼鈍後の冷却過程におけるマルテンサイトの形成を妨げる可能性がある。さらには、熱延板中に析出したTaCは、冷間圧延時の変形抵抗を高くし、安定した実機製造を困難にする場合がある。よって、Taを含有する場合、その含有量は0.10%以下とすることが好ましい。
WおよびVは炭窒化物を形成し、鋼を析出効果により高強度化する作用を有する元素であり、必要に応じて添加できる。このような作用は、Wおよび/またはVを添加する場合、いずれも0.001%以上含有して認められる。一方、これらの元素の含有量が0.10%を超える場合、鋼板が過度に高強度化し、延性が劣化してしまう場合がある。以上より、Wおよび/またはVを含有する場合、いずれも含有量は0.001%以上0.10%以下が好ましい。
(特徴1)高強度鋼板の鋼板表面から100μm以内の領域に、Fe、Si、Mn、Al、P、B、Nb、Ti、Cr、Mo、Cu、Ni、Sn、Sb、Ta、WおよびVの中から選ばれる少なくとも1種以上の酸化物を、合計で片面あたり0.010~0.050g/m2有する。
(特徴2)鋼板表面から10μm以内の領域における、鋼板結晶粒界から1μm以内の粒内に、Mnを含む酸化物を有する。
JIS Z 2241に準拠する金属材料引張試験方法に従い、引張強度(TS)、伸び(El)を測定した。測定結果は後述する加工性の評価に用いた。
化成処理性の評価方法は次の通りである。化成処理液は日本パーカライジング社製の化成処理液(パルボンドL3080(登録商標))を用い、下記方法で化成処理を施した。
○:10%以下
×:10%超
<電着塗装後の耐食性>
上記の方法で得られた化成処理鋼板より寸法70mm×150mmの試験片を切り出し、日本ペイント社製のPN-150G(登録商標)でカチオン電着塗装(焼付け条件:170℃×20分、膜厚25μm)を行った。その後、端部と評価しない側の面をAlテープでシールし、カッターナイフにて鋼板表面に達するクロスカット(クロス角度60°)を入れ、試験サンプルとした。
○: 剥離幅が片側2.5mm未満
×: 剥離幅が片側2.5mm以上
<加工性>
加工性は、供試材から圧延方向に対して90°方向にJIS5号引張試験片を採取し、JIS Z 2241の規定に準拠してクロスヘッド速度10mm/minの条件で引張試験を行い、引張り強度TS(MPa)と伸びEl(%)を測定し、TS×El≧24000のものを良好、TS×El<24000のものを不良とした。
内部酸化量は、「インパルス炉溶融-赤外線吸収法」により測定する。ただし、素材(すなわち焼鈍を施す前の鋼板)に含まれる酸素量を差し引く必要がある。本発明では、連続焼鈍後の高強度鋼板の両面からそれぞれ100μm以上研磨した位置での鋼中酸素濃度を測定し、その測定値を素材に含まれる酸素量OHとした。また、連続焼鈍後の高強度鋼板表面の板厚方向全体での鋼中酸素濃度を測定して、その測定値を内部酸化後の酸素量OIとした。このようにして得られた高強度鋼板の内部酸化後の酸素量OIと、素材に含まれる酸素量OHとを用いて、OIとOHの差(=OI-OH)を算出し、さらに片面単位面積(すなわち1m2)当たりの量に換算した値(g/m2)を内部酸化量とした。
SEMまたはTEM観察により、倍率20000倍の条件で無作為に5視野を観察し、必要に応じてEDX分析することで行った。
Claims (4)
- 質量%で、C:0.03~0.35%、Si:0.01~0.50%、Mn:3.6~8.0%、Al:0.01~1.0%、P:0.10%以下、S:0.010%以下を含有し、残部がFeおよび不可避的不純物からなる鋼板を連続焼鈍する際に、
前記連続焼鈍の加熱過程において、焼鈍炉内温度:450℃以上A℃以下(A:500≦A)の温度域では、昇温速度:7℃/s以上の条件で前記鋼板を加熱し、
前記連続焼鈍において鋼板最高到達温度を600℃以上750℃以下とし、
前記連続焼鈍において鋼板温度が600℃以上750℃以下の温度域の鋼板通過時間を30秒以上10分以内、雰囲気の露点を-10℃以上とすることを特徴とする高強度鋼板の製造方法。 - 前記鋼板は、成分組成として、質量%で、さらに、B:0.001~0.005%、Nb:0.005~0.05%、Ti:0.005~0.05%、Cr:0.001~1.0%、Mo:0.05~1.0%、Cu:0.05~1.0%、Ni:0.05~1.0%、Sn:0.001~0.20%、Sb:0.001~0.20%、Ta:0.001~0.10%、W:0.001~0.10%およびV:0.001~0.10%の中から選ばれる1種以上の元素を含有することを特徴とする請求項1に記載の高強度鋼板の製造方法。
- 前記連続焼鈍を行った後、硫酸を含む水溶液で電解酸洗を行うことを特徴とする請求項1または2に記載の高強度鋼板の製造方法。
- 請求項1又は2に記載の成分組成を有し、
鋼板表面から100μm以内の領域に、Fe、Si、Mn、Al、P、B、Nb、Ti、Cr、Mo、Cu、Ni、Sn、Sb、Ta、WおよびVの中から選ばれる少なくとも1種以上の酸化物を、合計で片面あたり0.010~0.050g/m2有し、
さらに、鋼板表面から10μm以内の領域における、鋼板結晶粒界から1μm以内の粒内に、Mnを含む酸化物を有することを特徴とする高強度鋼板。
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