EP0016846B1 - Process for producing high-strength cold-rolled steel plate for press working - Google Patents

Process for producing high-strength cold-rolled steel plate for press working Download PDF

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Publication number
EP0016846B1
EP0016846B1 EP79901046A EP79901046A EP0016846B1 EP 0016846 B1 EP0016846 B1 EP 0016846B1 EP 79901046 A EP79901046 A EP 79901046A EP 79901046 A EP79901046 A EP 79901046A EP 0016846 B1 EP0016846 B1 EP 0016846B1
Authority
EP
European Patent Office
Prior art keywords
rolled steel
steel sheet
cold rolled
rolling
cold
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
EP79901046A
Other languages
German (de)
English (en)
French (fr)
Other versions
EP0016846A4 (en
EP0016846A1 (en
Inventor
Masashi Central Research Laboratories Takahashi
Atsuki Central Research Laboratories Okamoto
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Sumitomo Metal Industries Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sumitomo Metal Industries Ltd filed Critical Sumitomo Metal Industries Ltd
Publication of EP0016846A1 publication Critical patent/EP0016846A1/en
Publication of EP0016846A4 publication Critical patent/EP0016846A4/en
Application granted granted Critical
Publication of EP0016846B1 publication Critical patent/EP0016846B1/en
Expired legal-status Critical Current

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Classifications

    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing

Definitions

  • This invention relates to a method of producing a high strength cold rolled steel sheet having excellent press formability, comparable to the conventional cold rolled steel sheet for drawing.
  • the conventional cold rolled steel sheet for deep drawing has a tensile strength of 28-33 Kg/mm 2 and has been used as outer panels of automobile bodies after being press formed.
  • a tensile strength of 28-33 Kg/mm 2
  • more and more automobile bodies are using thinner outer panels.
  • car manufacturers demand cold rolled steel sheets whose formability is comparable to the conventional one and having a tensile strength higher than the latter.
  • the conventional high tensile steel has a high yielding point and experiences considerable spring back and as a result, it is difficult to provide a press formed article with a desired configuration.
  • the steel permits only too small elongation, it can easily develop cracks during press forming.
  • the dual phase steel of recent development has a low yielding point, but since its tensile strength is 50-70 kg/mm 2 , it undergoes too rapid work hardening and provides high yielding point after slight press forming, making it difficult to produce a desired configuration free from the effect of spring back.
  • 51-107 218 (1976) deals with a cold rolled steel plate having a tensile strength 40-60 kg/mm 2 and consisting of C 0,03-0,15%; Si ⁇ 0,20%; Mn 1,00-2,50%, P ⁇ 0,050%; S ⁇ 0,030%, sol. AI 0,010-0,100%, >1 of Nb, Ti and V0005­0,20% in total and the balance iron. It is disclosed in this reference a method of producing cold rolled steel plate which restricts the content of Si to not greater than 0,20% so as to prevent the occurrence of blueing during annealing. But, in this case, a nitrogen content lower than 0,005% causes in particular the effect of spring back.
  • the inventors of this invention have found, after carrying out intensive study to solve these problems, that a cold rolled steel sheet having a tensile strength of 35-45 kg/mm 2 , a yield ratio (yield strength/tensile strength) of 0.4-0.6, an r-value of more than 1.2 is most suitable for providing the desired steel sheet for use as outer panels of an automobile body.
  • the primary purpose of this invention is to provide a method of producing a cold rolled steel sheet having the above defined characteristics.
  • a cold rolled steel sheet which has press formability and form retention comparable to the conventional cold rolled steel sheet and yet has high strength and dent resistance higher than the conventional one, thus making great contribution to making of lighter cars.
  • dense resistance means the property of a cold rolled steel sheet that does not retain a depression (permanent deformation) resulting from an external force applied to a press formed article made of that sheet.
  • This invention resides in a method of producing a high strength cold rolled steel sheet for press formation which is characterized by hot rolling a steel consisting of 0.005-0.080% of C, less than 0.30% of Si, 1.6 ⁇ 3.5% of Mn, 0.02-0.08% of Sol. AI, 0.005-0.020% of N, the balance of Fe and incidental impurities, cold rolling the same with a reduction of at least 30%, and annealing the cold rolled steel by heating it at 660-750°C for a period longer than 30 minutes.
  • a hot rolled steel is desirably coiled at a temperature lower than 600°C.
  • Temper rolling is not necessarily performed, but if it is performed for a special purpose, a desired elongation ratio is 0.5% at maximum.
  • the characteristic feature of this invention consists in optimizing the Sol. AI content, N content and optionally the coiling temperature after hot rolling in the production of a low yield ratio steel sheet from high-Mn steel for the purposes of limiting the number of recrystallizing nuclei to form large grains in the process of annealing after cold rolling as well as promoting the growth through recrystallization of grains having an orientation [111] parallel to the plate surface, thereby providing a cold rolled steel sheet having a yield ratio less than 0.60 and an r-value higher than 1.2.
  • Fig. 1 is a photograph (x500) showing a microstructure of a cold rolled steel sheet produced in accordance with this invention.
  • Fig. 2 is a photograph (x500) showing a microstructure of a cold rolled steel sheet produced in accordance with the prior art.
  • Fig. 3 is diagrammatical view explaining the test of form retention, in which a steel sheet is bent in a U-shaped form.
  • Fig. 4 is a perspective view showing the measuring of residual indentation in dent resistance test.
  • Figs. 1 and 2 are photomicrographs (x500) showing, respectively, microstructures of our cold rolled steel sheet of Steel Nn. 1 and the conventional high strength and low yield ratio cold rolled steel sheet of Steel No. 10 in Table 1.
  • the grain size of steel sheet produced in accordance with this invention as shown in Fig. 1 is larger than that in Fig. 2. This is because, as mentioned hereinbefore, the formation of recrystallization nuclei has been restricted by optimizing the proportions of Sol. AI and nitrogen as well as coiling temperature after hot rolling.
  • the lower yield point and the higher r-value which are obtained in accordance with this invention are due to the microstructure shown in Fig. 1.
  • Carbon is effective for forming a dispersed phase of martensite in the cold rolled steel sheet to give a low yield ratio in the presence of manganese in an amount of more than 1.6%. A carbon content of less than 0.005% is not enough to give a thoroughly low yield ratio. On the other hand, more than 0.080% carbon raises tensile strength excessively and impairs spot weldability. The carbon content is restricted to from 0.005% to 0.080%.
  • Silicon may be used as a deoxidizer and is effective for making the formation of martensite easier.
  • Si may be used as a deoxidizer and is effective for making the formation of martensite easier.
  • a large amount of Si increases yield point and tensile strength, impairing formability and surface flatness of the cold rolled steel sheet.
  • the Si content is restricted to less than 0.30%. Si is not required if the addition of AI achieves thorough deoxidation.
  • Manganese is effective for forming a dispersed phase of martensite in the presence of carbon. A manganese content of less than 1.6% is not enough to give a low yield ratio. A manganese content of more than 3.5% is not desirable, since such a large amount of manganese increases tensile strength excessively.
  • Acid soluble aluminium (Sol. Al):
  • Sol. AI is effective for lowering yield ratio, optimizing crystal grain and improving recrystallization texture i.e. increasing an r-value in the presence of nitrogen in an amount of more than 0.005%.
  • the Sol. AI content of 0.02-0.08% is desirable for obtaining desired mechanical properties.
  • Nitrogen is necessary to optimize crystal grain and to improve recrystallization texture in the presence of a suitable amount of Sol. Al. For this purpose, a nitrogen content of 0.005-0.020% is necessary. A nitrogen content of less than 0.005% is not effective for this purpose.
  • the balance of the steel composition is Fe with incidental impurities, of which P and S are allowed to present in an amount of less than 0.05%, respectively.
  • a preferred composition of the steel to which this invention is applied is:
  • Coiling temperature after hot rolling determines the structure and arrangement of AIN and has an influence on the r-value of the resulting steel sheet containing Sol. Al, N and Mn in amounts specified in the above.
  • the coiling temperature may be above 600°C.
  • Annealing after cold rolling is necessary to obtain a cold rolled steel sheet which is softened due to removal of stress introduced during cold rolling and which has a dual phase of ferrite plus martensite, giving a tensile strength of 35-45 Kg/mm 2 , a yield ratio of 0.4-0.6 and an r-value of more than 1.2.
  • the desirable annealing temperature is within the range of 660-750°C.
  • the soaking time during annealing is also important. A soaking time of more than 30 minutes is necessary to effect concentration of C and Mn in the steel, and to form martensite during the period of cooling in annealing. A soaking time of less than 30 minutes is sometimes insufficient to form martensite.
  • temper rolling with a reduction of more than about 1% is applied to a cold rolled steel sheet so as to avoid the formation of stretcher strains and to improve flatness of the steel sheet.
  • temper rolling is not necessarily required. This is because the steel sheet produced in accordance with this invention does not show an elongation at yield point in the annealed state. Thus, it is not necessary to effect temper rolling in order to avoid the formation of stretcher strains.
  • skin pass rolling may be applied to the cold rolled steel sheet in order to improve the surface flatness.
  • skin pass rolling (temper rolling) with a reduction of more than 0.5% results in increase in yield point with deterioration of formability, when temper rolling is applied, it is advisable to restrict the reduction on temper rolling to not more than 0.5%.
  • the press formability of each steel sample was determined by the conventional Erichsen test and hole widening test.
  • a 700 mm ⁇ dent resistance test piece in the form of a disc (2) was cut from each steel sample and subjected to shallow drawing to form a 400 mmo dome (3) having a maximum depth of 45 mm as illustrated in Fig. 4.
  • a loop tester (1) was forced against the center of the domed test piece under a load of 20 kg, and the residual deflection 8 after removal of the load was measured. A greater residual deflection 8 is not desired since it provides an outer panel of an automobile body that easily forms a dent if it is pressed with a finger or hit by a bouncing pebble.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Steel (AREA)
EP79901046A 1978-08-26 1979-08-25 Process for producing high-strength cold-rolled steel plate for press working Expired EP0016846B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP10417278A JPS5531159A (en) 1978-08-26 1978-08-26 Manufacture of high strength cold rolled steel plate for press working
JP104172/78 1978-08-26

Publications (3)

Publication Number Publication Date
EP0016846A1 EP0016846A1 (en) 1980-10-15
EP0016846A4 EP0016846A4 (en) 1980-11-14
EP0016846B1 true EP0016846B1 (en) 1983-08-17

Family

ID=14373606

Family Applications (1)

Application Number Title Priority Date Filing Date
EP79901046A Expired EP0016846B1 (en) 1978-08-26 1979-08-25 Process for producing high-strength cold-rolled steel plate for press working

Country Status (6)

Country Link
EP (1) EP0016846B1 (it)
JP (1) JPS5531159A (it)
DE (1) DE2953072C1 (it)
GB (1) GB2039951B (it)
IT (1) IT1121469B (it)
WO (1) WO1980000456A1 (it)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR101147975B1 (ko) * 2010-04-27 2012-05-24 태성정밀공업 주식회사 악기용 멜빵 고정구

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0033600A3 (en) * 1980-01-18 1981-11-25 British Steel Corporation Process for producing a steel with dual-phase structure
JPS579831A (en) * 1980-05-21 1982-01-19 British Steel Corp Steel production
JPS62139848A (ja) * 1985-12-11 1987-06-23 Kobe Steel Ltd 自動車補強部材用高強度高延性冷延鋼板
AR045563A1 (es) 2003-09-10 2005-11-02 Warner Lambert Co Anticuerpos dirigidos a m-csf

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS4918713A (it) * 1972-06-13 1974-02-19
JPS51107218A (en) * 1975-03-19 1976-09-22 Nippon Kokan Kk Buruuinguoshojinai kochoryokureienkohanno seizohoho

Family Cites Families (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3709744A (en) * 1970-02-27 1973-01-09 United States Steel Corp Method for producing low carbon steel with exceptionally high drawability
JPS4837565A (it) * 1971-09-20 1973-06-02
GB1376266A (en) * 1971-12-27 1974-12-04 Nippon Steel Corp Cold-rolled steel sheet for pressforming
JPS5627583B2 (it) * 1973-12-10 1981-06-25
JPS5178730A (en) * 1974-12-30 1976-07-08 Nippon Steel Corp Fueraitosoto kyureihentaisoyorinaru fukugososhikikohanno seizohoho
JPS5943531B2 (ja) * 1976-08-17 1984-10-23 新日本製鐵株式会社 加工性に優れた高強度冷延鋼板の製造法
JPS5381544A (en) * 1976-12-16 1978-07-19 Shinto Paint Co Ltd Continuous powder electrodeposition painting method
JPS5425948A (en) * 1977-07-28 1979-02-27 Kansai Paint Co Ltd Supplying of cationic electrodeposit coating compound

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS4918713A (it) * 1972-06-13 1974-02-19
JPS51107218A (en) * 1975-03-19 1976-09-22 Nippon Kokan Kk Buruuinguoshojinai kochoryokureienkohanno seizohoho

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR101147975B1 (ko) * 2010-04-27 2012-05-24 태성정밀공업 주식회사 악기용 멜빵 고정구

Also Published As

Publication number Publication date
EP0016846A4 (en) 1980-11-14
GB2039951A (en) 1980-08-20
EP0016846A1 (en) 1980-10-15
IT7968708A0 (it) 1979-08-24
JPS5531159A (en) 1980-03-05
WO1980000456A1 (en) 1980-03-20
GB2039951B (en) 1982-10-06
DE2953072C1 (de) 1985-03-21
IT1121469B (it) 1986-04-02

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