EP0620288A1 - Feuille laminee a froid, eventuellement galvanisee a chaud, aux qualites de sechage de peinture a chaud, de non-vieillissement a froid et de formage excellentes, et procede de production - Google Patents

Feuille laminee a froid, eventuellement galvanisee a chaud, aux qualites de sechage de peinture a chaud, de non-vieillissement a froid et de formage excellentes, et procede de production Download PDF

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EP0620288A1
EP0620288A1 EP93919599A EP93919599A EP0620288A1 EP 0620288 A1 EP0620288 A1 EP 0620288A1 EP 93919599 A EP93919599 A EP 93919599A EP 93919599 A EP93919599 A EP 93919599A EP 0620288 A1 EP0620288 A1 EP 0620288A1
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Prior art keywords
cold
rolled steel
hot
temperature
weight
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EP93919599A
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German (de)
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EP0620288A4 (fr
EP0620288B1 (fr
Inventor
Naoki; Yoshinaga
Kohsaku; Ushioda
Osamu; Akisue
Yoshikazu; Matsumura
Kunio Nishimura
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Nippon Steel Corp
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Nippon Steel Corp
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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
    • C21D8/0421—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 working steps
    • C21D8/0426—Hot rolling
    • 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
    • C21D8/0421—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 working steps
    • C21D8/0436—Cold rolling
    • 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
    • C21D8/0473—Final recrystallisation annealing

Definitions

  • the present invention relates to a cold-rolled steel sheet and a hot-dip galvanized cold-rolled steel sheet having excellent bake hardenability, non-aging properties at room temperature and good formability and a process for producing the same.
  • the cold-rolled steel sheet according to the present invention is subjected to press molding before use in automobiles, domestic electric appliances, buildings, etc. It includes both a cold-rolled steel sheet in a narrow sense, which has an untreated surface, and a cold-rolled steel sheet subjected to surface treatments, such as galvanizing or alloyed galvanizing, for rust preventive purposes. Since the steel sheet according to the present invention has a combination of strength with formability, use thereof enables the sheet thickness to be reduced to a greater extent than with conventional steel sheets. In other words, a reduction in weight is possible. Therefore, the steel sheet of the present invention can be expected to contribute to the protection of the environment.
  • extra low carbon steel sheets having a good formability.
  • Kanai Japanese Unexamined Patent Publication
  • 59-31827 and 59-38337 wherein Ti and Nb are added in combination
  • BH paint-bake hardenability
  • the BH level of these sheets does not exceed the level of the conventional BH steel sheets, and an attempt to further enhance the BH level unfavorably makes it impossible to ensure that non-aging properties at room temperature.
  • numerous extra low carbon steel sheets containing neither Ti or Nb and having an excellent formability have been disclosed, and examples thereof include those disclosed in Japanese Examined Patent Publication (Kokoku) No. 53-22052 and Japanese Unexamined Patent Publication (Kokai) Nos. 58-136721 and 58-141335.
  • Japanese Examined Patent Publication (Kokoku) No. 57-57945 discloses a representative prior art technique in which P is added to an extra low carbon steel containing Ti to produce a high-strength cold-rolled steel sheet. Further, with respect to extra low carbon steels containing neither Ti nor Nb, Japanese Examined Patent Publication (Kokoku) No. 58-57492 and Japanese Unexamined Patent Publication (Kokai) No. 58-48636 disclose a technique in which P is added to enhance the strength, and Japanese Unexamined Patent Publication (Kokai) No. 57-43932 discloses a technique in which Si is utilized.
  • P has hitherto been most extensively used as a reinforcing element with Si being the second most extensively used reinforcing element.
  • Si have been considered to have a very high solid solution strength capability, enable the strength to be increased by addition thereof in a minor amount, cause no significant lowering in ductility and deep drawability and further incur no significant increase in cost derived from the addition of these elements.
  • an attempt to attain the increase in strength by addition of these elements alone causes not only strength but also yield strength to be remarkably increased, which renders the face shape unsatisfactory, so that use thereof in panels for automobiles is often limited.
  • Si induces a failure in plating or P and Si remarkably lower the alloying rate, so that the productivity is lowered.
  • Japanese Unexamined Patent Publication (Kokai) Nos. 63-190141 and 64-62440 disclose a technique in which Mn is added to an extra low carbon steel sheet containing Ti
  • Japanese Examined Patent Publication (Kokoku) No. 59-42742 and the above-described Japanese Examined Patent Publication (Kokoku) No. 57-57945 disclose a technique in which Mn and Cr are added to an extra low carbon steel sheet containing Ti.
  • Japanese Unexamined Patent Publication (Kokai) No. 62-40352 discloses a technique in which Mn is added to an extra low carbon steel containing neither Ti nor Nb.
  • Japanese Unexamined Patent Publication (Kokai) No. 2-111841 discloses a cold-rolled steel sheet and a hot-dip galvanized steel sheet having a bake hardenability and a good formability, comprising a Ti-containing extra low carbon steel and, added thereto, from 1.5 to less than 3.5% of Mn.
  • an improvement in operating stability of hot rolling and in the homogeneity of the metallic structure through a lowering in Ar3, transformation point is intended by the addition of a large amount of Mn.
  • the addition of Cr and V in amounts in the range of from 0.2 to 1.0% is also disclosed with a view to further improving the ductility.
  • DP steel dual phase steel
  • YR very low yield ratio
  • Japanese Examined Patent Publication (Kokoku) Nos. 3-2224 and 3-21611 and Japanese Unexamined Patent Publication (Kokai) No. 3-277741 disclose steel sheets having a composite structure comprising an extra low carbon steel as a raw material.
  • the present inventors have found that the above technique has the following problems. Specifically, in a steel comprising a composite structure having a percentage volume of not more than 5% in the second phase, it is difficult to impart a BH property on a level comparable or superior to the conventional level, that is, on the level of not less than 5 kgf/mm2. Further, even though the BH level could exceed 5 kgf/mm2, the YP-E1 after artificial aging unfavorably exceeds 0.2%, so that it is very difficult to ensure the non-aging properties at room temperature. This problem is considered attributable to a low percentage volume of the second phase which results in a unsatisfactory movable dislocation density introduced into ferrite.
  • the steel sheet after press molding is required to have denting resistance.
  • denting resistance is intended to mean resistance of the steel sheet to permanent depression deformation when stones or the like hit against assembled automobiles or the like. Assuming the sheet thickness is constant, the denting resistance becomes better with increasing the deformation stress after press molding and painting/baking. Therefore, when steels sheets have the same yield strength, the denting resistance improves with increasing the paint-bake hardenability and increasing the work hardenability.
  • steel sheets desirable for use in panels for automobiles are those having a combination of such properties that the yield strength is not very high, work hardening is significant and paint-bake hardenability is high. It is a matter of course that they should be excellent also in formability in respect of the average r value (deep drawability) and elongation (punch stretchability). Further, they should have substantially non-aging at room temperature.
  • An object of the present invention is to provide a cold-rolled steel sheet and a hot-dip galvanized cold-rolled steel sheet unattainable by the prior art, which can satisfy the above-described demands, particularly with respect to the paint-bake hardenability, can impart a BH property on a high level of not less than 5 kgf/mm2 depending upon purposes and also have a non-aging properties at room temperature, and a process for producing the same.
  • the present inventors have made extensive and intensive studies with a view to attaining the above-described object and, as a result, have obtained the following novel finding.
  • Mn, B and Cr were added to a base material comprising an extra low carbon steel containing neither Nb nor Ti or a base material comprising an extra low carbon steel and, added thereto, one or a combination of Nb and Ti (for example, a steel having a composition of 0.003%C-0.01%Si-0.15%Mn-0.008%P-0.003%S-0.05%Al-0.012%Ti-0.02%Nb-0.0015%B), and studies have been made on the structure and tensile properties after cold rolling, annealing and temper rolling, particularly a difference in the structure and tensile properties between annealing in a two-phase region of ⁇ + ⁇ and annealing in a ⁇ single-phase region.
  • annealing in a ⁇ single-phase region has the following features as compared with annealing in an ( ⁇ + ⁇ ) two-phase region.
  • the structure after annealing can be brought to a single-phase structure of a low-temperature transformation product, so that the variation in quality during production is very small.
  • low-temperature transformation product used herein is intended to mean all structures provided in annealing in a ferritic single-phase temperature region except for the so-called "polygonal ferrite.”
  • the structure comprises at least one member selected from massive ferrite, bainite, Widmanstatten ferrite, martensite and acicular ferrite.
  • the present inventors have conducted further studies on the relationship between these elements and the amount of Mn added and, as a result, have found that, when the Mn content is less than 0.3%, in order to improve the non-aging properties, addition of Ti or Nb is necessary and it is preferred to satisfy the requirement represented by the above formula.
  • annealing in a ⁇ region enabled a BH property on a level of not less than 5 kgf/mm2 to be stably imparted.
  • samples were prepared in the same manner as that of Example 1 using sample No. 4-1 (steel of the present invention) and sample No. 4-4 (comparative steel) specified in Table 1 and used to examine the relationship between the deep drawability (r value) and the annealing temperature of the samples.
  • the results are given in Fig. 3.
  • the present inventors have found that the steel of the present invention is advantageous also as a hot-dip galvanized cold-rolled steel sheet. Specifically, addition of large amounts of Si or P to steels is known to deteriorate the platability of the steels in the hot-dip galvanizing and, further, causes delay of the subsequent alloying reaction. By contrast, steels containing Mn or Cr cause no deterioration in platability in the hot-dip galvanizing even when they contain large amounts of Si and P. The present inventors have also carried out studies on the influence of B and, as a result, have found that a large amount of B has an adverse effect on platability in the hot-dip galvanizing and the alloying reaction.
  • a lowering in the P and Si contents is advantageous also from the viewpoint of lowering the Ac3 point.
  • the present invention provides a novel steel sheet based on the above-described idea and novel finding, and the subject matter of the present invention resides in a cold-rolled steel sheet or a hot-dip galvanized cold-rolled steel sheet, comprising, in terms of % by weight, 0.0005 to 0.0070% of C, 0.001 to 0.8% of Si, 0.3 to 4.0% of Mn, 0.002 to 0.15% of P, 0.0005 to 0.015% of S, 0.005 to 0.1% of Al and 0.0003 to 0.0060% of N and optionally further comprising at least one element selected from B in an amount capable of satisfying a requirement, of less than 0.0030% and B/N ⁇ 1.5 and 0.01 to 3.0% of Cr with the balance consisting of Fe and unavoidable impurities and having a single-phase structure of a low-temperature transformation product, and a cold-rolled steel sheet or a hot-dip galvanized cold-rolled steel sheet, comprising, in terms of % by weight, 0.0005 to 0.0070%
  • C is a very important element for determining the quality of products.
  • the present invention is on the premise that the steel is an extra low carbon steel which has been subjected to vacuum degassing.
  • the C content is less than 0.0005%, there occur a lowering in grain boundary strength, a deterioration in fabricability and a remarkable increase in production cost. For this reason, the lower limit of the C content is 0.0005%.
  • the C content exceeds 0.0070%, the moldability is deteriorated and non-aging properties at room temperature is not ensured. For this reason, the upper limit of the C content is 0.0070%.
  • Si is known to be an element that can economically increase the strength.
  • the amount of Si to be added varies depending upon the target strength level. However, when it exceeds 0.8%, an increase in yield strength becomes so large that face strain occurs during press molding. Further, in this case, the Ac3 transformation point increases, so that the annealing temperature for providing the single-phase structure of a low-temperature transformation product becomes remarkably high. Further, there occur problems such as a lowering in conversion treatability, a lowering in adhesion of hot-dip galvanized coating and a lowering in productivity due to delay of the alloying reaction.
  • the lower limit of Si is 0.001% from the viewpoint of steelmaking techniques and cost.
  • Mn is the most important element for the present invention. Specifically, since Mn lowers the Ac3 transformation point, the temperature necessary for the formation of a single-phase structure of a low-temperature transformation product is not very high. Further, in a steel sheet having a single-phase structure of a low-temperature transformation product provided by utilizing Mn, it is possible to easily impart a BH level of not less than 5 kgf/mm2 unattainable by the conventional techniques. Further, a excellent non-aging properties at room temperature can be attained also when the BH level is 5 kgf/mm2 or more.
  • This property is characteristic of steel sheets having a single-phase structure of a low-temperature transformation product and cannot be provided in steel sheets having a ferritic single-phase structure and steel sheets having a composite structure provided by adding a large amount of B. What is more important is as follows.
  • the conventional steels are commonly known to cause a remarkable deterioration in the r value when annealed in a ⁇ single-phase region. For this reason, the annealing temperature has been limited to the Ae3 or Ac3 point or lower temperature.
  • the r value is hardly deteriorated even when annealing is effected in a ⁇ single-phase region.
  • Mn is a solid solution strengthening element useful for increasing the strength without causing a significant increase in yield strength and also has the effect of improving the conversion treatability or improving the platability in hot-dip galvanizing.
  • the lower limit is 0.01% from the viewpoint of steelmaking techniques.
  • Mn is added in an amount of 0.6% or more, it becomes possible to most significantly attain the effect of lowering the annealing temperature necessary for the formation of a single-phase structure of a low-temperature transformation product, the effect of improving the non-aging properties and other effects.
  • the amount of Mn added exceeds 4.0%, the cost becomes high and the formability is deteriorated.
  • P As with Si, P is known to be an element which can economically increase the strength, and the amount of P to be added varies depending upon the target strength level.
  • the amount added exceeds 0.15%, the annealing temperature necessary for the formation of a single-phase structure of a low-temperature transformation product becomes remarkably high and the yield strength becomes so high that a failure of face shape occurs during pressing. Further, in the stage of continuous hot-dip galvanizing, the alloying reaction rate is so low that the productivity lowers. Further, the fabricability too is deteriorated. For this reason, the upper limit is 0.15%.
  • the lower limit is 0.002% from the viewpoint of steelmaking techniques and cost. However, in order to attain the above-described effect, it is preferred for the P content to be not less than 0.005%.
  • Al is used for deoxidation and fixation of nitrogen.
  • the Al content is less than 0.005%, the effect is unsatisfactory.
  • it exceeds 0.2% the cost is increased, so that the upper limit is 0.2%.
  • N The lower the N content, the better the results. However, when the N content is lower than 0.0003%, the cost is remarkably increased. On the other hand, when it is excessively high, a large amount of Al becomes necessary or the formability is deteriorated. For this reason, the upper limit is 0.0060%.
  • Ti, Nb, B and Cr serve to fix the whole or part of N, C and S, thereby enabling the formability and non-aging properties of the extra low carbon steel to be ensured. Further, they refine grains of the hot-rolled sheet to render the formability of the product sheet good. Further, B is useful for preventing fabrication embrittlement, and Cr has an excellent effect of enhancing the BH property and work hardenability. The above elements are added when an enhancement in the above properties is desired.
  • Ti, Nb, B and Cr are useful for attaining excellent BH properties and non-aging properties when annealing is effected in a ⁇ region. They are also useful for maintaining a high r value.
  • Mn content is less than 0.3% by weight, an addition of Ti or Nb is necessary.
  • Ti and Nb it is preferred for Ti and Nb to be added in respective amounts capable of satisfying the requirement: 10(Ti + Nb) + 100B + Cr ⁇ 0.1.
  • the upper limit of the content of the able elements is 0.1% by weight for Ti and Nb, 0.0030% by weight for B and 3.0% by weight for Cr.
  • the lower limit of the content of the above elements is a minimum value necessary for attaining the intended effect.
  • the strength all the steel sheets having a strength of not less than 25 kgf/mm2 fall within the scope of the present invention. However, in order to provide a good r value as the single-phase structure of a low-temperature transformation product, it is preferred for the strength to be 35 kgf/mm2.
  • a slab having the above-described composition is heated in the temperature range of from 900 to 1,400°C and then hot-rolled.
  • the finishing temperature of the hot rolling should be not less than (Ar3 - 100)°C from the viewpoint of ensuring the formability of the product sheet.
  • the coiling temperature for the hot-rolled steel strip is in the range of from room temperature to 750°C.
  • the present invention is characterized in that the quality of the product is not significantly influenced by the coiling temperature in the hot rolling.
  • the upper limit of the coiling temperature is determined from the viewpoint of preventing a lowering in yield attributable to a deterioration in the quality at both ends of the coil.
  • the hot-rolled steel strip is then cold-rolled.
  • the rolling is effected with a reduction ratio of not less than 60% from the viewpoint of ensuring the deep drawability after annealing.
  • the cold-rolled steel strip thus obtained is transferred to a continuous annealing furnace while uncoiling the steel strip and annealed at the Ac3 transformation point or above.
  • the annealing temperature is below the Ac3 transformation point, it is impossible to provide the single-phase structure of a low-temperature transformation product characteristic of the present invention.
  • cooling is preferably effected at an average rate of 30°C/sec or less until the temperature is lowered from the annealing temperature to 600 to 700°C.
  • cooling is preferably effected at an average rate of 30°C/sec or more until the temperature is lowered from the annealing temperature to 600 to 700°C.
  • one of these conditions is essential.
  • the steel strip When hot-dip galvanizing is effected after the annealing, the steel strip is cooled from the above-described annealing temperature and immersed in a galvanizing bath (temperature: 420 to 520°C, Al concentration of the bath: 0.05 to 0.3%) to galvanize the surface of the steel strip. Thereafter, the galvanized steel strip may be subjected to an alloying treatment commonly effected in the conventional galvanizing.
  • a cold-rolled steel strip and a hot-dip galvanized steel strip are produced. Thereafter, if necessary, the steel strip is subjected to temper rolling with a reduction ratio of 0.2 to 3% for the purpose of correcting the shape. In the present invention, the temper rolling for improving the aging property is not necessary.
  • a steel sheet which has a combination of a high paint-bake hardenability and non-aging properties at room temperature and is also excellent in formability in respect of average r value (deep drawability) and elongation (punch stretchability).
  • a BH property on a high level of not less than 5 kgf/mm2 can be stably imparted according to need, and it is possible to provide a cold-rolled steel sheet which also has non-aging properties at room temperature.
  • Steels having compositions specified in Table 1 were prepared by a melt process and hot-rolled under conditions of a slab heating temperature of 1,200°C, a finishing temperature of 920°C and a coiling temperature of 700°C to form steel strips having a thickness of 4.0 mm. After pickling, the steel strips were cold-rolled with a reduction ratio of 80% to form cold-rolled sheets having a thickness of 0.8 mm and then subjected to continuous annealing under conditions of a heating rate of 10°C/sec, a soaking of 860 to 980°C for 50 sec, an average rate of cooling to 650°C of 3°C/sec and an average rate of cooling from 650°C to room temperature of 80°C/sec.
  • the annealed sheet was subjected to temper rolling with a reduction ratio of 1.0%, and a JIS No. 5 tensile specimen was extracted therefrom and subjected to a tensile test.
  • the results of the tensile test are summarized in Table 2.
  • the WH value is the level of work hardening when a 2% tensile strain is applied in the rolling direction. This value is determined by subtracting the yield stress (YP) from a 2% deformation stress.
  • the BH property is the level of an increment of the stress when the tensile test is again effected after a 2% prestrained material is subjected to a heat treatment corresponding to painting baking at 170°C for 20 min (that is, a value determined by subtracting 2% deformation stress from lower yield stress in the retensile test).
  • the fabrication embrittlement transition temperature is a ductility-embrittlement transition temperature determined by punching a blank having a diameter of 50 mm from a temper-rolled steel sheet, molding a cup using a punch having a diameter of 33 mm and subjecting the cup to a drop weight test at various temperatures.
  • the annealing temperature necessary for the formation of a single-phase structure of a low-temperature transformation product is considerably lower than that in the case of the comparative steels. Therefore, the steels can be produced without applying an excessive burden on continuous annealing equipment.
  • the steels of the present invention have a higher BH property than the conventional steel sheets having a tensile strength on the same level as the steel sheets of the present invention, and additionally have a very excellent non-aging properties at room temperature.
  • This advantage is considered largely attributable to a better dislocation density of the steel sheet of which the single-phase structure of a low-temperature transformation product has been formed using Mn or Cr as compared with other steel sheets.
  • Another feature of the present invention is that substantially no deterioration in r value occurs despite annealing in a ⁇ single-phase temperature region.
  • the steels of the present invention have a low yield strength, an excellent retention of face shape and a high WH value. Therefore, the steels of the present invention are suitable as a material, for example, for an outer or inner plate panel of automobiles.
  • the influence of soaking temperature on continuous annealing was studied using steel No. 2-2 specified in Table 1. Conditions for hot rolling and cold rolling were the same as those of Example 1. Thereafter, the cold-rolled steel sheet was subjected to continuous annealing as follows. It was heated at a rate of 10°C/sec, held at a temperature in the range of from 840 to 940°C for 50 sec, cooled to 650°C at an average rate of 60°C/sec and then cooled from 650°C to room temperature at an average rate of 80°C/sec. Further, the annealed sheet was subjected to temper rolling with a reduction ratio of 1.0%, and a JIS No. 5 tensile specimen was extracted therefrom and subjected to a tensile test. The results of the tensile test are summarized in Table 3.
  • the appearance of plating was evaluated based on the following criteria.
  • the plated sheet was bent at 180°C for close overlapping, and an adhesive tape was adhered to the bent portion and then peeled off to measure the amount of peeled plating to evaluate the peeling of the galvanized coating.
  • the evaluation was made based on the following five grades.
  • the concentration of Fe in the plating was determined by X-ray diffractometry.
  • the alloyed galvanized steel sheets of the present invention had good plating appearance and powdering resistance. Further, the concentration of Fe in the alloy layer corresponds to that of ⁇ 1 phase considered as a desired phase. In the present invention, the above properties are considered to be attained by reducing the amount of P and Si, which deteriorates plating adhesion and delays alloying reaction, and adding Mn or Cr. Further, it is apparent that, when Mn or Cr is added, the platability is not deteriorated even though P and Si are contained in a certain amount. Table 4 Sample No.
  • Steels having compositions specified in Table 5 were prepared by a melt process and hot-rolled under conditions of a slab heating temperature of 1,180°C, a finishing temperature of 910°C and a coiling temperature of 600°C to form steel strips having a thickness of 4.0 mm. After pickling, the steel strips were cold-rolled with a reduction ratio of 80% to form cold-rolled sheets having a thickness of 0.8 mm and then subjected to continuous annealing under conditions of a heating rate of 10°C/sec, a soaking of 830 to 980°C for 50 sec, an average rate of cooling to 650°C of 5°C/sec and an average rate of cooling from 650°C of 80°C/sec.
  • the annealed sheet was subjected to temper rolling with a reduction ratio of 0.5%, and a JIS No. 5 tensile specimen was extracted therefrom and subjected to a tensile test.
  • the results of the tensile test are summarized in Table 6.
  • the BH value is the level of an increment of the stress when the tensile test is again effected after a 2% prestrained material is subjected to a heat treatment corresponding to painting baking at 170°C for 20 min (that is, a value determined by subtracting 2% deformation stress from lower yield stress in the retensile test).
  • the fabrication embrittlement transition temperature is a ductility-embrittlement transition temperature determined by punching a blank having a diameter of 50 mm from a temper-rolled steel sheet, molding a cup using a punch having a diameter of 33 mm and subjecting the cup to a drop weight test at various temperatures.
  • the steels of the present invention have a higher BH property than the conventional steel sheets having a tensile strength on the same level as the steel sheets of the present invention, and additionally have a excellent non-aging properties at room temperature.
  • This advantage is considered largely attributable to a better dislocation density of the steel sheet having a single-phase structure of a low-temperature transformation product as compared with other steel sheets.
  • the steels of the present invention are excellent also in r value. Therefore, the steels of the present invention are suitable as a material for, an outer or inner plate panel of automobiles, for example.
  • the structure was brought to a single phase of a low-temperature transformation product by annealing in a ⁇ single-phase region, whereas in the comparative examples, the annealing was effected in an ⁇ single-phase or ( ⁇ + ⁇ ) two-phase region, so that the structure was not a single-phase structure of a low-temperature transformation product.
  • Example 2 The influence of soaking temperature in continuous annealing was studied using steel No. 3-2 specified in Table 5. Conditions for hot rolling and cold rolling were the same as those of Example 1. Thereafter, the cold-rolled steel sheet was subjected to continuous annealing as follows. It was heated at a rate of 10°C/sec, held at a temperature in the range of from 840 to 930°C for 50 sec, cooled at an average rate of 60°C/sec.
  • the annealed sheet was subjected to temper rolling with a reduction ratio of 0.5%, and a JIS No. 5 tensile specimen was extracted therefrom and subjected to a tensile test.
  • the results of the tensile test are summarized in Table 7.
  • Sample Nos. 3-1 to 3-4 and 4-1 to 4-4 specified in Table 5 were hot-rolled under conditions of a slab heating temperature of 1220°C, a finishing temperature of 900°C and a coiling temperature of 500°C to form steel sheets having a thickness of 3.8 mm.
  • the steel sheets were cold-rolled to form cold-rolled sheets having a thickness of 0.75 mm, heated at a heating rate of 15°C/sec to a maximum heating temperature in the range of from 840 to 980°C, cooled at a rate of about 70°C/sec, subjected to conventional hot-dip galvanizing at 460°C (Al concentration of bath: 0.11%), further heated at 520°C for 20 sec to effect alloying and then cooled to room temperature at about 20°C/sec.
  • the resultant alloyed galvanized steel sheets were subjected to measurement of appearance of plating, powdering resistance and concentration of Fe in plating. The results are summarized in Table 8.
  • the alloyed galvanized steel sheets of the present invention had good plating appearance and powdering resistance. Further, the concentration of Fe in the alloy layer corresponds to that of ⁇ 1 phase considered as a desired phase.
  • the present invention it is possible to provide a cold-rolled steel sheet having a combination of BH property with non-aging properties at room temperature unattainable by the prior art techniques.
  • the steel of the present invention has an excellent press moldability and is excellent also in platability in hot-dip galvanizing, so that it can exhibit also a rust preventive property. Therefore, use of the steel of the present invention in bodies or frames of automobiles enables the thickness of the sheet, that is, the weight of the automobile bodies, to be reduced, which can greatly contribute to environmental protection which has attracted attention in recent years. Thus, the present invention is very valuable from the viewpoint of industry.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

Une feuille laminée à froid, éventuellement galvanisée à chaud, présente de bonnes qualités de séchage de peinture à chaud et de non-vieillissement à froid et d'excellentes caractéristiques de formage telle que sa valeur r moyenne (caractéristique d'emboutissage profond) et son allongement (caractéristique d'aplatissement). On la produit par recuit d'un acier à teneur en carbone ultra-faible, ou contenant au moins du titane ou du niobium, au-dessus du point critique Ac3, pour obtenir un produit à basse température de transformation et à structure mono-phase. Pour ce qui est des qualités de séchage de peinture à chaud, on peut obtenir une valeur de dureté Brinell allant jusqu'à 10 kgf/mm2 environ.
EP93919599A 1992-08-31 1993-08-31 Feuille laminee a froid, eventuellement galvanisee a chaud, aux qualites de sechage de peinture a chaud, de non-vieillissement a froid et de formage excellentes, et procede de production Expired - Lifetime EP0620288B1 (fr)

Applications Claiming Priority (10)

Application Number Priority Date Filing Date Title
JP23230192 1992-08-31
JP232319/92 1992-08-31
JP23231992 1992-08-31
JP232301/92 1992-08-31
JP23230192 1992-08-31
JP23231992 1992-08-31
JP4410893 1993-03-04
JP5044108A JP2980785B2 (ja) 1992-08-31 1993-03-04 焼付硬化性と成形性とに優れた冷延鋼板あるいは溶融亜鉛メッキ冷延鋼板およびそれらの製造方法
JP44108/93 1993-03-04
PCT/JP1993/001231 WO1994005823A1 (fr) 1992-08-31 1993-08-31 Feuille laminee a froid, eventuellement galvanisee a chaud, aux qualites de sechage de peinture a chaud, de non-vieillissement a froid et de formage excellentes, et procede de production

Publications (3)

Publication Number Publication Date
EP0620288A1 true EP0620288A1 (fr) 1994-10-19
EP0620288A4 EP0620288A4 (fr) 1995-01-25
EP0620288B1 EP0620288B1 (fr) 2000-11-22

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EP93919599A Expired - Lifetime EP0620288B1 (fr) 1992-08-31 1993-08-31 Feuille laminee a froid, eventuellement galvanisee a chaud, aux qualites de sechage de peinture a chaud, de non-vieillissement a froid et de formage excellentes, et procede de production

Country Status (4)

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EP (1) EP0620288B1 (fr)
KR (1) KR0121737B1 (fr)
DE (1) DE69329696T2 (fr)
WO (1) WO1994005823A1 (fr)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0769565A1 (fr) * 1995-03-27 1997-04-23 Nippon Steel Corporation Tole laminee a froid, a teneur en carbone ultra-faible, et tole galvanisee, excellentes par leurs caracteristiques de fatigue, et procede de production
DE19622164C1 (de) * 1996-06-01 1997-05-07 Thyssen Stahl Ag Verfahren zur Erzeugung eines kaltgewalzten Stahlbleches oder -bandes mit guter Umformbarkeit
EP0918098A1 (fr) * 1997-04-09 1999-05-26 Kawasaki Steel Corporation Tole d'acier a froid mince revetue de type trempe presentant une excellente resistance au vieillissement, et procede de production
US6143100A (en) * 1998-09-29 2000-11-07 National Steel Corporation Bake-hardenable cold rolled steel sheet and method of producing same
EP1306456A1 (fr) * 2000-08-04 2003-05-02 Nippon Steel Corporation Feuilles d'acier laminees a froid et a chaud presentant une excellente trempabilite et une excellente resistance au vieillissement a la temperature ordinaire et procede de fabrication associe
US9777350B2 (en) 2012-04-11 2017-10-03 Tata Steel Nederland Technology B.V. High strength interstitial free low density steel and method for producing said steel
CN107995931A (zh) * 2015-06-05 2018-05-04 Posco公司 拉拔性及烘烤硬化性优异的高强度薄钢板及其制造方法
EP3327164A4 (fr) * 2015-07-24 2018-05-30 Posco Tôle d'acier galvanisée par immersion à chaud et tôle d'acier recuite par galvanisation par immersion à chaud présentant d'excellentes durée de conservation et aptitude au durcissement après cuisson et procédé de fabrication associé
EP3733906A4 (fr) * 2017-12-26 2020-11-04 Posco Tôle d'acier laminée à froid possédant d'excellentes propriétés à température élevée et une excellente aptitude au façonnage à température ambiante, et son procédé de fabrication

Families Citing this family (10)

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Publication number Priority date Publication date Assignee Title
KR100544538B1 (ko) * 2001-12-21 2006-01-24 주식회사 포스코 가공성이 우수한 비시효 소부경화형 냉연고장력강판과 그제조방법
KR20030083276A (ko) * 2002-04-20 2003-10-30 현대자동차주식회사 소부경화형 고장력강판 및 이것의 제조방법
KR101105025B1 (ko) * 2003-12-29 2012-01-16 주식회사 포스코 면내이방성이 작은 소부경화형 고강도 냉연강판과 그제조방법
KR101105132B1 (ko) * 2003-12-23 2012-01-16 주식회사 포스코 소부경화형 고강도 냉연강판과 그 제조방법
KR101105007B1 (ko) * 2003-12-23 2012-01-16 주식회사 포스코 소부경화형 냉연강판과 그 제조방법
KR101105098B1 (ko) * 2003-12-29 2012-01-16 주식회사 포스코 가공성이 우수한 소부경화형 고강도 냉연강판과 그 제조방법
KR101104981B1 (ko) * 2003-12-23 2012-01-16 주식회사 포스코 내2차가공취성이 우수한 소부경화형 고강도 냉연강판과 그제조방법
US20150027597A1 (en) 2012-02-20 2015-01-29 Tata Steel Nederland Technology Bv High strength bake-hardenable low density steel and method for producing said steel
KR101505293B1 (ko) * 2013-05-31 2015-03-23 현대제철 주식회사 강판
KR101505269B1 (ko) * 2013-03-28 2015-03-23 현대제철 주식회사 강판 및 그 제조 방법

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JPH04214820A (ja) * 1990-12-14 1992-08-05 Nippon Steel Corp 焼付硬化性に優れる自動車用鋼板の製造方法
WO1992016668A1 (fr) * 1991-03-15 1992-10-01 Nippon Steel Corporation Tole d'acier laminee a froid, a haute resistance et presentant une excellente aptitude au formage, tole d'acier laminee a froid, a haute resistance et zinguee a chaud, et procede de fabrication desdites toles
JPH0578784A (ja) * 1991-09-12 1993-03-30 Nippon Steel Corp 成形性の良好な高強度冷延鋼板
JPH0578783A (ja) * 1991-09-12 1993-03-30 Nippon Steel Corp 成形性の良好な高強度冷延鋼板

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JPH0757892B2 (ja) * 1983-01-28 1995-06-21 新日本製鐵株式会社 ２次加工性と表面処理性の優れた深絞り用冷延鋼板の製造方法
JPS59143027A (ja) * 1983-02-07 1984-08-16 Kawasaki Steel Corp 延性および加工性の良好な高強度鋼板の製造方法
JPS60103128A (ja) * 1983-11-11 1985-06-07 Kawasaki Steel Corp 複合組織冷延鋼板の製造方法
JPS60174852A (ja) * 1984-02-18 1985-09-09 Kawasaki Steel Corp 深絞り性に優れる複合組織冷延鋼板とその製造方法
JPS61281852A (ja) * 1985-06-07 1986-12-12 Kawasaki Steel Corp 焼付硬化性に富む遅時効性深絞り用冷延鋼板
JPH06102816B2 (ja) * 1990-03-28 1994-12-14 川崎製鉄株式会社 加工性、常温非時効性及び焼付け硬化性に優れる複合組織冷延鋼板とその製造方法

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JPH04214820A (ja) * 1990-12-14 1992-08-05 Nippon Steel Corp 焼付硬化性に優れる自動車用鋼板の製造方法
WO1992016668A1 (fr) * 1991-03-15 1992-10-01 Nippon Steel Corporation Tole d'acier laminee a froid, a haute resistance et presentant une excellente aptitude au formage, tole d'acier laminee a froid, a haute resistance et zinguee a chaud, et procede de fabrication desdites toles
JPH0578784A (ja) * 1991-09-12 1993-03-30 Nippon Steel Corp 成形性の良好な高強度冷延鋼板
JPH0578783A (ja) * 1991-09-12 1993-03-30 Nippon Steel Corp 成形性の良好な高強度冷延鋼板

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Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0769565A4 (fr) * 1995-03-27 1999-01-20 Nippon Steel Corp Tole laminee a froid, a teneur en carbone ultra-faible, et tole galvanisee, excellentes par leurs caracteristiques de fatigue, et procede de production
EP0769565A1 (fr) * 1995-03-27 1997-04-23 Nippon Steel Corporation Tole laminee a froid, a teneur en carbone ultra-faible, et tole galvanisee, excellentes par leurs caracteristiques de fatigue, et procede de production
DE19622164C1 (de) * 1996-06-01 1997-05-07 Thyssen Stahl Ag Verfahren zur Erzeugung eines kaltgewalzten Stahlbleches oder -bandes mit guter Umformbarkeit
WO1997046720A1 (fr) * 1996-06-01 1997-12-11 Thyssen Stahl Ag Procede de production d'une tole ou d'un feuillard d'acier lamine(e) a froid, presentant une bonne aptitude au formage
EP0918098A4 (fr) * 1997-04-09 2005-09-14 Jfe Steel Corp Tole d'acier a froid mince revetue de type trempe presentant une excellente resistance au vieillissement, et procede de production
EP0918098A1 (fr) * 1997-04-09 1999-05-26 Kawasaki Steel Corporation Tole d'acier a froid mince revetue de type trempe presentant une excellente resistance au vieillissement, et procede de production
US6143100A (en) * 1998-09-29 2000-11-07 National Steel Corporation Bake-hardenable cold rolled steel sheet and method of producing same
EP1306456A1 (fr) * 2000-08-04 2003-05-02 Nippon Steel Corporation Feuilles d'acier laminees a froid et a chaud presentant une excellente trempabilite et une excellente resistance au vieillissement a la temperature ordinaire et procede de fabrication associe
EP1306456A4 (fr) * 2000-08-04 2005-02-16 Nippon Steel Corp Feuilles d'acier laminees a froid et a chaud presentant une excellente trempabilite et une excellente resistance au vieillissement a la temperature ordinaire et procede de fabrication associe
US9777350B2 (en) 2012-04-11 2017-10-03 Tata Steel Nederland Technology B.V. High strength interstitial free low density steel and method for producing said steel
CN107995931A (zh) * 2015-06-05 2018-05-04 Posco公司 拉拔性及烘烤硬化性优异的高强度薄钢板及其制造方法
EP3327164A4 (fr) * 2015-07-24 2018-05-30 Posco Tôle d'acier galvanisée par immersion à chaud et tôle d'acier recuite par galvanisation par immersion à chaud présentant d'excellentes durée de conservation et aptitude au durcissement après cuisson et procédé de fabrication associé
US10907233B2 (en) 2015-07-24 2021-02-02 Posco Hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet with excellent aging resistance properties and bake hardenability, and method for manufacturing same
EP3733906A4 (fr) * 2017-12-26 2020-11-04 Posco Tôle d'acier laminée à froid possédant d'excellentes propriétés à température élevée et une excellente aptitude au façonnage à température ambiante, et son procédé de fabrication
US11578379B2 (en) 2017-12-26 2023-02-14 Posco Cold-rolled steel sheet having excellent high-temperature properties and room-temperature workability

Also Published As

Publication number Publication date
EP0620288A4 (fr) 1995-01-25
DE69329696T2 (de) 2001-06-13
EP0620288B1 (fr) 2000-11-22
WO1994005823A1 (fr) 1994-03-17
KR0121737B1 (ko) 1997-12-04
DE69329696D1 (de) 2000-12-28

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