AU717294B2 - Dual-phase high-strength steel sheet having excellent dynamic deformation properties and process for preparing the same - Google Patents
Dual-phase high-strength steel sheet having excellent dynamic deformation properties and process for preparing the same Download PDFInfo
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- AU717294B2 AU717294B2 AU63118/98A AU6311898A AU717294B2 AU 717294 B2 AU717294 B2 AU 717294B2 AU 63118/98 A AU63118/98 A AU 63118/98A AU 6311898 A AU6311898 A AU 6311898A AU 717294 B2 AU717294 B2 AU 717294B2
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- 229910000831 Steel Inorganic materials 0.000 title claims description 153
- 239000010959 steel Substances 0.000 title claims description 153
- 238000004519 manufacturing process Methods 0.000 title description 13
- 238000010521 absorption reaction Methods 0.000 claims description 81
- 229910000859 α-Fe Inorganic materials 0.000 claims description 78
- 229910000734 martensite Inorganic materials 0.000 claims description 57
- 238000001816 cooling Methods 0.000 claims description 49
- 238000005482 strain hardening Methods 0.000 claims description 36
- 238000000137 annealing Methods 0.000 claims description 32
- 238000005098 hot rolling Methods 0.000 claims description 26
- 238000000034 method Methods 0.000 claims description 25
- 230000003068 static effect Effects 0.000 claims description 24
- 239000000126 substance Substances 0.000 claims description 22
- 239000000203 mixture Substances 0.000 claims description 19
- 238000009864 tensile test Methods 0.000 claims description 13
- 238000005096 rolling process Methods 0.000 claims description 11
- 239000010960 cold rolled steel Substances 0.000 claims description 10
- 238000005272 metallurgy Methods 0.000 claims description 9
- 239000004033 plastic Substances 0.000 claims description 9
- 238000005496 tempering Methods 0.000 claims description 9
- 229910052782 aluminium Inorganic materials 0.000 claims description 8
- 229910052748 manganese Inorganic materials 0.000 claims description 8
- 239000000047 product Substances 0.000 claims description 8
- 229910052710 silicon Inorganic materials 0.000 claims description 8
- 239000002131 composite material Substances 0.000 claims description 7
- 229910052804 chromium Inorganic materials 0.000 claims description 6
- 238000005266 casting Methods 0.000 claims description 5
- 238000003303 reheating Methods 0.000 claims description 5
- 229910052802 copper Inorganic materials 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 229910052759 nickel Inorganic materials 0.000 claims description 4
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- 229910052719 titanium Inorganic materials 0.000 claims description 4
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 239000002253 acid Substances 0.000 claims description 3
- 238000005554 pickling Methods 0.000 claims description 3
- 239000012467 final product Substances 0.000 claims description 2
- 238000002360 preparation method Methods 0.000 claims description 2
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- 239000000463 material Substances 0.000 description 23
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- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
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- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
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- 239000006096 absorbing agent Substances 0.000 description 1
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Classifications
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- 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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- 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
-
- 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
-
- 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
-
- 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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- 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
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
- C23C2/0224—Two or more thermal pretreatments
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
NSC-F813 -1- DUAL-PHASE HICH-STRENGTH STEEL SHEET HAVING EXCELLENT DYNAMIC DEFORMATION PROPERTIES AND PROCESS FOR PREPARING THE SAME Technical Field The present invention relates to dual-phase type high-strength steel sheets, for automobiles use, which have excellent dynamic deformation properties and exhibit excellent impact absorption properties, and are intended to be used as structural members and reinforcing materials primarily for automobiles, as well as to a method of producing them.
Background Art The applications of high-strength steels have been increasing for the purpose of achieving lighter weight vehicle bodies in consideration of fuel consumption restrictions on automobiles and even more applications for high-strength steel are expected as domestic and foreign restrictions, relating to estimated impact absorption properties in automobile accidents, become rapidly more broad and strict. For example, for frontal collisions of passenger cars, the use of materials with high impact absorption properties for members known as "front side members" can allow impact energy to be absorbed through collapse of the member, thus lessening the impact experienced by passengers.
However, conventional high-strength steels have been developed with a main view toward improving press formability, and doubts exist as to their application in terms of impact absorption properties. Prior art techniques relating to automobile steel with excellent impact absorption properties and methods of producing it have been developed which result in increased yield strength of steel sheets under high deformation speeds as an indicator of impact absorption properties, as disclosed in Japanese Unexamined Patent Publication No.
7-18372, but because the members undergo deformation 2 during the shaping process or during collision deformation, it is necessary to include a work-hardening aspect to the yield strength as an indicator of impact resistance, and this is inadequate in terms of anticollision safety in the prior art described above.
In addition, since the strain rate undergone by each location upon automobile collision reaches about 103 consideration of the impact absorption properties of the materials requires an understanding of the dynamic deformation properties in such a high strain rate range.
Also, high-strength steel sheets with excellent dynamic deformation properties are understood to be important for achieving both lighter weight and improved impact absorption properties for automobiles, and recent reports have highlighted this fact. For example, the present inventors have reported on the high strain rate properties and impact energy absorption properties of high-strength thin steel sheets in CAMP-ISIJ Vol.9 (1966), pp.1112-111 5 wherein they explain that the dynamic strength at a high strain rate of 10 3 increases dramatically compared to the static strength at a low strain rate speed of 10 3 that absorption energy during crashes is increased by greater steel material strengths, that the strain rate dependency of materials depends on the structure of the steel, and that TRIP type steel (Transformation induced plasticity type steel) and dual-phase (hereunder, type steel exhibit both excellent press formability and high impact absorption properties. Also, the present inventors have already filed Japanese Patent Applications No.8-98000 and No.8-10922 4 relating to such a DP-type steel, among which there are proposed high-strength steel sheets with higher dynamic strength than static strength, which are suitable for achieving both lighter weights and improved impact absorption properties for automobiles, and a process for their production.
3 As mentioned above, although the dynamic deformation properties of high-strength steel sheets are understood at the high strain rates of automobile collisions, it is still unclear what properties should be maximized for automobile members with impact energy absorption properties, and on what criteria the selection of materials should be based. In addition, the automobile members are produced by press forming of steel sheets, and collision impacts are applied to these press formed members. However, high-strength steel sheets with excellent dynamic deformation properties as actual members, based on an understanding of the impact energy absorption properties after such press forming, are still unknown.
For press forming of members for collision safety, a combination of excellent shape fixability, excellent stretchability (tensile strength x total elongation 18,000) and excellent flangeability (hole expansion ratio 1.2) is desirable, but at the current time no material has provided both excellent impact absorption properties and excellent press formability.
Disclosure of the Invention The present invention has been proposed as a means of overcoming the problems described above, and provides dual-phase type high-strength steel sheets for automobiles use, which have excellent impact absorption properties and excellent dynamic deformation properties, as well as a method of producing them.
The invention further provides dual-phase type highstrength steel sheets, for automobiles, with excellent dynamic deformation properties, which are high-strength steel sheets used for automotive parts, such as front side members, and which are selected based on exact properties and standards for impact energy absorption during collisions and can reliably provide guaranteed safety, as well as a method of producing them.
A The invention still further provides dual-phase type 4 high-strength steel sheets for automobiles with excellent dynamic deformation properties, which exhibit all the properties suitable for press forming of members, including excellent shape fixability, excellent stretchability and excellent flangeability, as well as a method of producing them.
The invention was devised to achieve the objects stated above by the following concrete means.
A dual-phase type high-strength steel sheets having high impact energy absorption properties, characterized in that the final microstructure of the steel sheet is a composite microstructure wherein the dominating phase is ferrite, and the second phase is another low temperature product phase containing martensite at a volume fraction between 3% and 50% after deformation at 5% equivalent strain of the steel sheet, wherein the difference between the quasi-static deformation strength os when deformed in a strain rate range of 5 x 10- 4 5 x 10 3 (s
I
after pre-deformation of more than 0% and less than or equal to 10% of equivalent strain, and the dynamic deformation strength od when deformed in a strain rate range of 5 x 102 5 x 103 after the aforementioned pre-deformation, i.e. (od os), is at least 60 MPa, and the work hardening coefficient at 5-10% strain is at least 0.13.
A dual-phase type high-strength steel sheet having high impact energy absorption properties, characterized in that the final microstructure of the steel sheet is a composite microstructure wherein the dominating phase is ferrite, and the second phase is another low temperature product phase containing martensite at a volume fraction between 3% and 50% after deformation at 5% equivalent strain of the steel sheet, wherein the average value odyn (MPa) of the deformation stress in the range of 3-10% of equivalent strain when deformed in a strain rate range of 5 x 102 5 x 103 5 after pre-deformation of more than 0% and less than or equal to 10% of equivalent strain, satisfies the inequality: odyn 2 0.766 x TS 250 as expressed in terms of the tensile strength TS (MPa) in the quasi-static tensile test as measured in a strain rate range of 5 x 4 5 x 10- 3 (s prior to pre-deformation, and the work hardening coefficient at 5-10% strain is at least 0.13.
A dual-phase type high-strength steel sheet having high impact energy absorption properties according to or above, characterized in that the ratio between the yield strength YS(0) and the tensile strength in the tensile test after pre-deformation at 5% of equivalent strain or after further bake hardening treatment (BH treatment) satisfies the inequality 5 0.7, and also satisfies the inequality: yield strength YS(0) x work hardening coefficient 2 A dual-phase type high-strength steel sheet having high impact energy absorption properties according to any of or above, characterized in that the average grain size of the martensite is 5 pm or less, and the average grain size of the ferrite is 10 pm or less.
A dual-phase type high-strength steel sheet i 25 having high impact energy absorption properties according to any of or above, characterized by satisfying the inequality: tensile strength (MPa) x total elongation 2 18,000, and by satisfying the inequality: hole expansion ratio (d/do) 2 1.2.
A dual-phase type high-strength steel sheet having high impact energy absorption properties according to any of or above, characterized in that the plastic deformation by either or both a tempering rolling and a tension leveller satisfies the following inequality.
0.5} 15 2 T 2 6 0.5} The dual-phase type high-strength steel sheet having high impact energy absorption properties according to the invention is also a dual-phase type high-strength steel sheet with excellent dynamic deformation properties according to to above, characterized in that the chemical compositions, in terms of weight percentage, C at 0.02-0.25%, either or both Mn and Cr at a total of 0.15-3.5%, one or more from among Si, Al and P at a total of 0.02-4.0%, if necessary one or more from among Ni, Cu and Mo at a total of no more than one or more from among Nb, Ti and V at no more than 0.30%, and either or both Ca and REM at 0.0005-0.01% for Ca and 0.005-0.05% for REM, with the remainder Fe as the primary component.
The dual-phase type high-strength steel sheet having high impact energy absorption properties according to the invention is also a dual-phase type high-strength steel sheet with excellent dynamic deformation properties according to to above, characterized in that one or more from among B 0.01), S 0.01%) and N (50.02%) are further added if necessary to the steel.
The method of producing a dual-phase type highstrength hot-rolled steel sheet having high impact energy absorption properties according to the invention is a method of producing a dual-phase type high strength hotrolled steel sheet with excellent dynamic deformation properties according to to above, characterized in that after a continuous casting slab is fed directly from casting to a hot rolling step, or is hot rolled upon reheating after momentary cooling, it is subjected to hot rolling at a finishing temperature of Ar 3 50 0 C to Ar 3 120 0 C, cooled at an average cooling rate of more than 0 C/sec in a run-out table, and then coiled at a temperature of no greater than 350 0 C; and (10) a method of producing a dual-phase highstrength hot-rolled steel sheet having high impact energy 7 absorption properties according to above, characterized in that at the finishing temperature for hot rolling in a range of Ar 3 50 0 C to Ar 3 120 0 C, the hot rolling is carried out so that the metallurgy parameter A satisfies inequalities and below, the subsequent average cooling rate in the run-out table is at least 5 0 C/sec, and the coiling is accomplished so that the relationship between the above-mentioned metallurgy parameter A and the coiling temperature (CT) satisfies inequality below.
9 logA 18 (1) AT 21 x logA 6 1 (2) CT 6 x logA 242 (3) (11) The method of producing a dual-phase type highstrength cold rolled steel sheet having high impact energy abosorption properties according to the invention is a method of producing a dual-phase type high-strength cold rolled steel sheet with excellent dynamic deformation properties according to to above, characterized in that after a continuous cast slab is fed directly from casting to a hot rolling step, or is hot rolled upon reheating after momentary cooling, it is hot rolled, the hot-rolled and subsequently coiled steel sheet is cold-rolled after acid pickling, and during annealing in a continuous annealing step for preparation of the final product, it is heated to a temperature between Ac, and Ac 3 and subjected to the annealing while held in this temperature range for at least 10 seconds, and then cooled at a cooling rate of more than 5 0 C/sec; and (12) a method according to (11) above for producing a dual-phase type high-strength cold rolled steel sheet having high impact energy absorption properties according to to above, characterized in that in the continuous annealing step, the cold rolled steel sheet is heated to a temperature (To) between Ac, and Ac 3 and 8 subjected to the annealing while held in this temperature range for at least 10 seconds, and for subsequent cooling, it is cooled to a secondary cooling start, temperature (Tq) in the range of 550C-To at a primary cooling rate of l-10 0 C/sec and then cooled to a secondary cooling end temperature (Te) which is no higher than Tem determined by the chemical compositions and annealing temperature at a secondary cooling rate of 10~200OC/sec.
Brief Description of the Drawings Fig. 1 is a graph showing the relationship between the absorption energy (Eab) of a shaped member during collision and the material strength according to the invention.
Fig. 2 is a perspective view of a shaped member for measurement of impact absorption energy for Fig. 1.
Fig. 3 is a graph showing the relationship between the work hardening coefficient and dynamic energy absorption for a steel sheet.
Fig. 4 is a graph showing the relationship between the yield strength x work hardening coefficient and the dynamic energy absorption for a steel sheet.
Fig. 5 is a general view of a "hat model" used in the impact crush test method relating to Figs. 3 and 4.
Fig. 6 is a cross-sectional view of the shape of the test piece of Fig. Fig. 7 is a schematic view of the impact crush test method relating to Figs. 3-6.
Fig. 8 is a graph showing the relationship between TS and the difference between the average value odyn of the deformation stress in the range of 3-10% of equivalent strain when deformed in a strain rate range of x 102 5 x 10' and TS, as an index of the impact energy absorption property upon collision, according to the invention.
Fig. 9 is a graph showing the change in the 9 static/dynamic ratio with tempered rolling for an example of the invention and a comparative example.
Fig. 10 is a graph showing the relationship between AT and the metallurgy parameter A for a hot-rolling step according to the invention.
Fig. 11 is a graph showing the relationship between the coiling temperature and the metallurgy parameter A for a hot-rolling step according to the invention.
Fig. 12 is a graph showing the annealing cycle for continuous annealing according to the invention.
Best Mode for Carrying Out the Invention Impact absorbing members such as front side members of automobiles are produced by bending and press forming of steel sheets. Because impacts during automobile collisions are absorbed by such members which have undergone press forming, they must have high impact absorption properties even after having undergone the pre-deformation corresponding to the press forming. At the current time, however, no attempt has been made to obtain high-strength steel sheets with excellent impact absorption properties as actual members, with consideration of both the increase in the deformation stress by press forming and the increase in deformation stress due to a higher strain rate, as was mentioned above.
As a result of much experimentation and research with the aim of achieving this purpose, the present inventors have found that steel sheets with a dual-phase (DP) structure are ideal as high-strength steel sheets with excellent impact absorption properties for actual members which are press formed as described above. It was demonstrated that such steel sheets with a dual-phase microstructure, which is a composite microstructure wherein the dominating phase is a ferrite phase responsible for the increase in deformation resistance by an increased strain rate, and the second phase includes a Shard martensite phase, have excellent dynamic deformation properties. That is, it was found that high dynamic deformation properties are exhibited when the microstructure of the final steel sheets is a composite structure wherein the dominating phase is ferrite and another low temperature product phase includes a hard martensite phase at a volume fraction of 3~50% after deformation at 5% equivalent strain of the steel sheet.
Concerning the volume fraction of 3-50% for the hard martensite phase, since high-strength steel sheets and even steel sheets with high dynamic deformation properties cannot be obtained if the martensite phase is less than the volume fraction of the martensite phase must be at least Also, if the martensite phase exceeds 50%, this results in a smaller volume fraction of the ferrite phase responsible for greater deformation resistance due to increased deformation speed, making it impossible to obtain steel sheets with excellent dynamic deformation properties compared to static deformation strength while also hindering press formability, and therefore it was found that the volume fraction of the martensite phase must be 3~50%.
The present inventors then pursued experimentation and research based on these findings and, as a result, found that although the degree of pre-deformation corresponding to press forming of impact absorbing members such as front side members sometimes reaches a maximum of over 20%, depending on the location, the majority are locations with 0%~10% of equivalent strain, and that by understanding the effect of pre-deformation in this range, it is possible to estimate the behavior of the member as a whole after pre-deformation.
Consequently, according to the invention, a deformation of from 0% to 10% of equivalent strain was selected as the amount of pre-deformation applied to members during press forming.
Fig. 1 is a graph showing the relationship between the absorption energy (Eab) of a press formed member 11 during collision and the material strength for the different steel types shown in Table 5, according to an example to be described later. The material strength S is the tensile strength (TS) according to the common tensile test. The member absorption energy (Eab) is the absorption energy in the lengthwise direction (direction of the arrow) along a press formed member such as shown in Fig. 2, upon collision with a 400 kg mass weight at a speed of 15 m/sec, to a crushing degree of 100 mm. The shaped member in Fig. 2 consists of a 2.0 mm-thick steel sheet formed into a hat-shaped section 1 with a steel sheet 2 of the same thickness and the same type of steel, joined together by spot welding, the hat-shaped section 1 having a corner radius of 2 mm, and with spot welding points indicated by 3.
From Fig. 1 it is seen that the member absorption energy (Eab) tends to increase with the strength of materials under normal tensile testing, though with considerable variation. Here, the materials in Fig. 1 were subjected to pre-deformation of more than 0% and less than or equal to 10% of equivalent strain, and then the static deformation strength as when deformed in a strain rate range of 5 x 10 4 5 x 10 3 (s and the dynamic deformation strength ad when deformed in a strain rate range of 5 x 102 5 x 103 (s after the predeformation, were measured. As a result, a classification was possible based on (ad as). The symbols plotted in Fig. 1 were as follows: o: (ad as) 60 MPa with any pre-deformation of more than 0% and less than or equal to 60 MPa (ad as) with any pre-deformation in the above range, and 60 MPa (ad as) 80 MPa with pre-deformation of 0: 60 MPa (ad as) with any pre-deformation in the above range, and 80 MPa (ad as) 100 MPa with pre-deformation of 12 A: 60 MPa (ad os) with any pre-deformation in the above range, and 100 MPa (ad os) with pre-deformation of Also, when 60 MPa 5 (od as) with any predeformation in the range of more than 0% and less than or equal to 10% of equivalent strain, the values for member absorption energy (Eab) during collision was equal to or greater than the values predicted from the material strength S, thus indicating steel sheets with excellent dynamic deformation properties as impact absorbing members for collision. These predicted values are those shown in the curve in Fig. i, represented by Eab 0.062S 0.8. Consequently, (od os) must be at least MPa.
For improved impact absorption properties, it is basically important to increase the work hardening coefficient, specifically to at least 0.13, and preferably at least 0.16; by controlling the yield strength and the work hardening coefficient to specified ranges it is possible to achieve excellent impact absorption properties, and for improved press formability it is effective to design the volume percentage and particle size of the martensite to within a specified range.
Fig. 3 shows the relationship between the work hardening coefficient of a steel sheet and the dynamic energy absorption which indicates the member impact absorption properties, for a class of materials with the same yield strength. Here it is shown that increased work hardening coefficients of the steel sheets result in improved member impact absorption properties (dynamic energy absorption), and that the work hardening coefficient of a steel sheet can properly indicate the member impact absorption properties so long as the yield strength class is the same. Also, when the yield strengths differ, as shown in Fig. 4, the yield strength 13x work hardening coefficient can be an indicator of the member impact absorption properties. While the work hardening coefficient was expressed in terms of an n value of 5%-10% strain in consideration of the strain undergone by members during press forming, from the viewpoint of improving the dynamic energy absorption, work hardening coefficients of under 5% strain or work hardening coefficients of even more than 10% strain may be preferred.
The dynamic energy absorptions for members shown in Fig. 3 and Fig. 4 were determined in the following manner. Specifically, the steel sheet was shaped into the member shape shown in Fig. 6 (corner R 5 mm) and spot welded at 35 mm pitch using an electrode with a tip radius of 5.5 mm at a current of 0.9 times the expulsion current, and then after baking and painting treatment at 170 0 C x 20 minutes, an approximately 150 Kg falling weight was dropped from a height of about 10 m to crush the member in its lengthwise direction, and the displacement work where displacement 0-150 mm is calculated from the area of the corresponding load displacement diagram to determine the dynamic energy absorption. A schematic illustration of this test method is shown in Fig. 7. In Fig. 5, 4 is a worktop, 5 is a test piece and 6 is a spot welding section.
In Fig. 6, 7 is a hat-shaped test piece and 8 is a spot welding section. In Fig. 7, 9 is a worktop, 10 is a test piece, 11 is a falling weight (150 kg), 12 is a frame, and 13 is a shock absorber. The work hardening coefficient and yield strength of each steel sheet was determined in the following manner. The steel sheet was shaped into a JIS-#5 test piece (gauge length: 50 mm, parallel width: 25 mm), subjected to tensile test at a strain rate of 0.001 (s to determine the yield strength and work hardening coefficient (n value at 5%-10% strain). The steel sheet used had a sheet 14thickness of 1.2 mm and the steel sheet composition contained C at 0.02-0.25 wt%, either or both Mn and Cr at a total of 0.15-3.5 wt% and one or more of Si, Al and P at a total of 0.02-4.0 wt%, with the remainder Fe as the main component.
Fig. 8 is a graph showing the relationship between the average value odyn of the deformation stress in the range of 3-10% of equivalent strain when deformed in a strain rate range of 5 x 102 5 x 103 and the static material strength as an index of the impact energy absorption property upon collision according to the invention, where the static material strength (TS) is the tensile strength (TS: MPa) in the static tensile test as measured in a strain rate range of 5 x 10- 4 5 x 10 3 (s As mentioned above, impact absorbing members such as front side members have a hat-shaped cross-sectional shape, and as a result of analysis of deformation of such members upon crushing by high-speed collision, the present inventors have found that despite deformation proceeding up to a high maximum strain of over 40%, at least 70% of the total absorption energy is absorbed in a strain range of 10% or lower in a high-speed stressstrain diagram. Therefore, the dynamic deformation resistance with high-speed deformation at 10% or lower was used as the index of the high-speed collision energy absorption property. In particular, since the amount of strain in the range of 3-10% is most important, the index used for the impact energy absorption property was the average stress: adyn in the range of 3-10% of equivalent strain when deformed in a strain rate range of 5 x 102 x 103 (s 1) high-speed tensile deformation.
The average stress: odyn of 3-10% upon high-speed deformation generally increases with increasing static tensile strength {maximum stress (TS: MPa) in a static tensile test measured in a stress rate range of 5 x 15 4 5 x 10- 3 of the steel material prior to predeformation or baking treatment. Consequently, increasing the static tensile strength (which is synonymous with the static material strength) of the steel material directly contributes to an improved impact energy absorption property of the member. However, increased strength of the steel results in poorer press formability into members, making it difficult to obtain members with the necessary shapes. Consequently, steels having a high odyn with the same tensile strength TS are preferred. It was found that, based on this relationship, steel sheets wherein the average value odyn (MPa) of the deformation stress in the range of 3-10% of equivalent strain when deformed in a strain rate range of 5 x 102 5 x l03 after pre-deformation of more than 0% and less than or equal to 10% of equivalent strain satisfies the inequality: odyn 0.766 x TS 250 as expressed in terms of the tensile strength (TS: MPa) in the static tensile test as measured in a strain rate range of 5 x 10 4 5 x 10 3 (s 1) prior to predeformation, have higher impact energy absorption properties as actual members compared to other steels, and that the impact energy absorption property is improved without increasing the overall weight of the member, making it possible to provide high-strength steel sheets with high dynamic deformation resistance.
Also, although the details are still unclear, it has been discovered that steel sheets with excellent dynamic deformation properties can be obtained when, as shown in Fig. 9, YS(0)/TS'(5) is no greater than 0.7, which amount is dependent on the initial microstructure, the amount of solid solution elements in the low temperature product phase other than the martensite phase and the main ferrite phase, and the deposited state of carbides, nitrides and carbonitrides. Here, YS(0) is the yield strength, and TS'(5) is the tensile strength in the 16 static tensile test with pre-deformation at 5% of equivalent strain or after further bake hardening treatment (BH treatment). It was also demonstrated that steel sheets with even more excellent dynamic deformation properties can be obtained when the yield strength: YS(0) x work hardening coefficient is at least Furthermore, it is known that dynamic deformation strength is usually expressed in the form of the power of the static tensile strength, and as the static tensile strength increases, the difference between the dynamic deformation strength and the static deformation strength decreases. However, a small difference between the dynamic deformation strength and the static deformation strength will mean that no greater improvement in the impact absorption properties can be expected. From this standpoint, it is preferred for the value of (od as) to be in a range which satisfies the following inequality, 0.8 (ad os) 4.1 x os -os.
The microstructure of a steel sheet according to the invention will now be described in detail. As already mentioned, the martensite is at a volume fraction of 3-50%, and preferably 3-30%. The average grain size of the martensite is preferably no greater than 5 Im, and the average grain size of the ferrite is preferably no greater than 10 tm. That is, the martensite is hard, and contributes to a decrease in the yield ratio and an improvement in the work hardening coefficient, by producing a mobile dislocations primarily in adjacent ferrite grains; however, by satisfying the restrictions mentioned above it is possible to disperse fine martensite in the steel, so that the improvement in the properties spreads throughout the entire steel sheet. In addition, this dispersion of fine martensite in the steel can help to avoid deterioration in the hole expansion ratio and tensile strength x total elongation, which is an adverse effect of the hard martensite. Also, because 17 it is possible to reliably achieve work hardening coefficient 0.130, tensile strength x total elongation S18,000 and hole expansion ratio 2 1.2, it is thereby possible to improve the impact absorption properties and press formability.
With a martensite volume fraction of less than 3%, the yield ratio becomes larger while the press formed member cannot exhibit an excellent work hardening property (work hardening coefficient 2 0.130) after it has undergone collision deformation, and since the deformation resistance (load) stays at a low level, and the dynamic energy absorption is low preventing improvement in the impact absorption properties. On the other hand, with a martensite volume fraction of greater than 50%, the yield ratio becomes larger while work hardening coefficient is reduced, and deterioration also occurs in the tensile strength x total elongation and the hole expansion ratio. From the standpoint of press formability, the volume fraction of the martensite is preferred to be no greater than Also, the ferrite is present at a volume fraction of preferably at least 50%, and more preferably at least and its average grain size (mean circle equivalent diameter) is preferably no greater than 10 lm, and more preferably no greater than 5 jim, with the martensite preferably adjacent to the ferrite. This aids the fine dispersion of the martensite in the ferrite matrix, while effectively extending the property-improving effect, beyond simply a local effect, to the entire steel sheet, favorably acting to prevent the adverse effects of the martensite. The structure of the remainder present with the martensite and ferrite may be a mixed structure comprising a combination of one or more from among pearlite, bainite, retained y, etc., and although primarily bainite is preferred in cases which require hole expansion properties, since retained y undergoes work-induced transformation into martensite by press 18 forming, experimental results have shown that including retained austenite prior to press forming has an-effect even in preferred small amounts or less).
Also, from the standpoint of impact absorption properties and press formability it is preferred for the ratio of the martensite and ferrite particle sizes to be no greater than 0.6, and the ratio of the hardnesses to be at least The restrictions on the values for the chemical components of dual-phase type high-strength steel sheets with excellent dynamic deformation properties according to the invention, and the reasons for those restrictions, will now be explained.
Dual-phase type high-strength steel sheets with excellent dynamic deformation properties which are used according to the invention are steel sheets containing the following chemical compositions, in terms of weight percentage: C at 0.02-0.25%, either or both Mn and Cr at a total of 0.15-3.5%, one or more from among Si, Al and P at a total of 0.02-4.0%, if necessary also one or more from among Ni, Cu and Mo at a total of no more than one or more from among Nb, Ti and V at no more than 0.30%, and either or both Ca and REM at 0.0005-0.01% for Ca and 0.005-0.05% for REM, with the remainder Fe as the primary component. They are also dual-phase type high strength steel sheets with excellent dynamic deformation properties which contain, if necessary, one or more from among B (50.01), S 0.01%) and N These chemical components and their contents (percent by weight) will now be discussed.
C: C is the element which most strongly affects the microstructure of the steel sheet, and if its content is too low it will become difficult to obtain martensite with the desired amount and strength. Addition in too great an amount leads to unwanted carbide precipitation, inhibited increase in deformation resistance at higher strain rates and overly high strength, as well as poor 19press formability and weldability; the content is therefore 0.02-0.25 wt%.
Mn, Cr: Mn and Cr have an effect of stabilizing austenite and guaranteeing sufficient martensite, and are also solid solution hardening elements; they must therefore be added in a minimum amount of 0.15 wt%, but if added in too much the aforementioned effect becomes saturated thus producing adverse effects such as preventing ferrite transformation, and thus they are added in the maximum amount of 3.5 wt%.
Si, Al, P: Si and Al are useful elements for producing martensite, and they promote production of ferrite and suppress precipitation of carbides, thus having the effect of guaranteeing sufficient martensite, as well as a solid solution hardening effect and a deoxidization effect. P can also promote martensite formation and solid solution hardening, similar to Al and Si. From this standpoint, the minimum amount of Si Al P added must be at least 0.02 wt%. On the other hand, excessive addition will saturate this effect and result instead in brittleness, and therefore the maximum amount of addition is no more than 4.0 wt%. In particular, when an excellent surface condition is required, Si scales can be avoided by adding Si at no greater than 0.1 wt%, and conversely by adding it at 1.0 wt% or greater Si scales can be produced over the entire surface so that they are not conspicuous. Also, when excellent secondary workability, toughness, spot weldability and recycling properties are required, the P content may be kept at no greater than 0.05%, and preferably no greater than 0.02%.
Ni, Cu, Mo: These elements are added when necessary, and are austenite-stabilizing elements similar to Mn, which increase the hardenability of the steel, and are effective for adjustment of the strength. From the standpoint of weldability and chemical treatment, they can be used when the amounts of C, Si, Al and Mn are restricted, but if the total amount of these elements added exceeds 3.5 wt% the dominant ferrite phase will tend to be hardened, thus inhibiting the increase in deformation resistance by a greater strain rate, as well as raising the cost of the steel sheet; the amount of these elements added is therefore 3.50 wt% or lower.
Nb, Ti, V: These elements are added when necessary, and are effective for strengthening the steel sheet through formation of carbides, nitrides and carbonitrides. However, when added at greater than 0.3 wt% they are deposited in large amounts in the dominant ferrite phase or at the grain boundaries as carbides, nitrides and carbonitrides, becoming a source of the mobile dislocation during high speed deformation, and inhibiting the increase in deformation resistance by greater strain rates. In addition, the deformation resistance of the dominant phase becomes higher than necessary, thus wasting the C and leading to higher costs; the maximum amount to be added is therefore 0.3 wt%.
B: B is an element which is effective for strengthening since it improves the hardenability of the steel by suppressing production of ferrite, but if it is added at greater than 0.01 wt% its effect will be saturated, and therefore B is added at a maximum of 0.01 wt%.
Ca, REM: Ca is added to at least 0.0005 wt% for improved press formability (especially hole expansion ratio) by shape control (spheroidization) of sulfidebased inclusions, and the maximum amount thereof to be added is 0.01 wt% in consideration of effect saturation and the adverse effect due to increase in the aforementioned inclusions (reduced hole expansion ratio).
For the same reasons, REM is added in an amount of from 0.005% to 0.05 wt%.
S: The amount of S is no greater than 0.01 wt%, and preferably no greater than 0.003 wt%, from the standpoint of press formability (especially hole expansion ratio) by 21sulfide-based inclusions, and reduced spot weldability.
The method of applying the pre-deformation according to the invention will now be explained. The predeformation may be press forming for member shaping, or it may be working with a tempering rolling or tension leveler which applied to the steel sheet material prior to its press forming. In this case, either or both a tempering roller and tension leveler may be used. That is, the means used may include a tempering rolling, a tension leveler, or a tempering roller and tension leveler. The steel sheet material may also be subjected to press forming after being worked with a tempering rolling or tension leveler. The amount of predeformation applied with the tempering rolling and/or tension leveler, i.e. the degree of plastic deformation will differ depending on the initial dislocation density, and T should be small if the initial density is large. Also, with few solid solution elements the introduced dislocations cannot be fixed, and high dynamic deformation properties cannot be guaranteed.
Consequently, it was found that the plastic deformation is determined based on the ratio between the yield strength YS(0) and the tensile strength TS'(5) in the static tensile test with pre-deformation at 5% of equivalent strain or after further bake hardening treatment (BH treatment), or That is, is an indicator of the sum of the initial dislocation density and the dislocation density introduced by 5% deformation, and the amount of the solid solution elements; it may be concluded that a smaller means a higher initial dislocation density and more of the solid solution elements. YS(0)/TS'(5) is therefore no greater than 0.7, and is preferably provided according to the following equation: 2.5 0.5} 15 2 T 2 0.5} wherein the upper limit for T is determined from the 22 standpoint of press formability including impact absorption property and flexibility.
A method of producing a dual-phase type high strength hot rolled steel sheet and a cold rolled steel sheet with excellent dynamic deformation properties according to the invention will now be explained. In this production method, a continuous cast slab is fed directly from casting to a hot rolling step, or is hot rolled upon reheating after momentary cooling. Thin gauge continuous casting and continuous hot rolling techniques (endless hot rolling) may be applied for the hot rolling in addition to normal continuous casting, but in order to avoid a lower ferrite volume fraction and a coarser average grain size of the thin steel sheet microstructure, the bar (cast strip) thickness at the hot rolling approach side (the initial steel bar thickness) is preferred to be at least 25 mm. At less than 25 mm, the mean circle equivalent size of ferrite of the steel sheet is made coarser, while it is also a disadvantage against obtaining the desired martensite. The final pass rolling speed for the hot rolling is preferred to be at least 500 mpm and more preferably at least 600 mpm, in light of the problems described above. At less than 500 mpm, the mean circle equivalent diameter of ferrite of the steel sheet is made coarser, while it is also a disadvantage against obtaining the desired martensite.
The finishing temperature for the hot rolling is from Ar 3 50 0 C to Ar 3 120 0 C. At lower than Ar 3 50 0
C,
deformed ferrite is produced, with inferior work hardening property and press formability. At higher than Ar 3 120 0 C, and the mean circle equivalent size of ferrite of the steel sheet is made coarser, while it is also becomes difficult to obtain the desired martensite.
The average cooling rate for cooling in the run-out table is at least 5 0 C/sec. At less than 5 0 C/sec it becomes difficult to obtain the desired martensite.
23 The coiling temperature is no higher.than 350 0 C. At higher than 350 0 C it becomes difficult to obtain the desired martensite.
According to the invention, it was found particularly that a correlation exists between the finishing temperature in the hot rolling step, the finishing approach temperature and the coiling temperature. That is, as shown in Fig. 10 and Fig. 11, specific conditions exist which are determined primarily between the finishing temperature, finishing approach temperature and the coiling temperature. Specifically, the hot rolling is carried out so that when the finishing temperature for hot rolling is in the range of Ar 3 50 0
C
to Ar 3 120 0 C, the metallurgy parameter A satisfies inequalities and The above-mentioned metallurgy parameter A may be expressed by the following equation.
A e x exp{(75282 42745 x Ceq)/[1.
9 7 8 x (FT 273)]} where FT: finishing temperature Ceq: carbon equivalents C Mneq/ 6 Mneq: manganese equivalents Mn (Ni Cr Cu Mo)/2 final pass strain rate (v/IRxhl) x x In hi: final pass approach sheet thickness h 2 final pass exit sheet thickness r (hi h2)/hi R roll radius v final pass exit speed AT: finishing temperature (finishing final pass exit temperature) finishing approach temperature (finishing first pass approach temperature) Ar 3 901 325 C% 33 Si% 92 Mneq Thereafter, it is preferred for the average cooling rate on the run-out table to be at least 5°C/sec, and the S coiling to be carried out under conditions such that the 24 relationship between the metallurgy parameter A and the coiling temperature (CT) satisfies inequality 9 logA 18 (1) AT 21 x logA 61 (2) CT 6 x logA 242 (3) In inequality above, a log A of less than 9 is unacceptable from the viewpoint of production of retained martensite and refinement of the microstructure, while it will also result in an inferior dynamic deformation resistance odyn and 5-10% work hardening property. Also, if log A is to be greater than 18, massive equipment will be required to achieve it. With inequality if the condition of inequality is not satisfied it will be impossible to obtain the desired martensite, and the dynamic deformation resistance odyn and 5-10% work hardening property, etc. will be inferior. The lower limit for AT is more flexible with a lower log A as indicated by inequality Furthermore, if the relationship with the coiling temperature in inequality is not satisfied, there will be an adverse effect on ensuring the amount of martensite, while the retained y will be excessively stable even if retained y can be obtained, it will be impossible to obtain the desired martensite during deformation, and the dynamic deformation resistance adyn and 5-10% work hardening property, etc. will be inferior. The limit for the coiling temperature is more flexible with a higher log A.
The cold rolled sheet according to the invention is then subjected to the different steps following hotrolling and coiling and is cold rolled and subjected to annealing. The annealing is ideally continuous annealing through an annealing cycle such as shown in Fig. 12, and during the annealing of the continuous annealing step, it must be kept for at least 10 seconds in the temperature range of Ac, Ac 3 At less than Ac, austenite will not be produced and it will therefore be impossible to obtain 25 martensite thereafter, while at greater than Ac 3 the austenite monophase structure will be coarse, and it will therefore be impossible to obtain the desired average grain size for the martensite. Also, at less than seconds the austenite production will be insufficient, making it impossible to obtain the desired martensite thereafter. The maximum residence time is preferably no greater than 200 seconds, from the standpoint of avoiding addition to the equipment and coarsening of the microstructure. The cooling after this annealing must be at an average cooling rate of at least 5 0 C/sec. At less than 5 0 C/sec the desired space factor for the martensite cannot be achieved. Although there is no particular upper limit here, it is preferably 300 0 C/sec when considering temperature control during the cooling.
According to the invention, the cooled steel sheet is heated to a temperature To from Ac, Ac 3 in the continuous annealing cycle shown in Fig. 12, and cooled under cooling conditions provided by a method wherein cooling to a secondary cooling start temperature Tq in the range of 550 0 C-To at the primary cooling rate of 1-10°C/sec is followed by cooling to a secondary cooling end temperature Te which is no higher than a temperature Tem which is determined by the chemical compositions of the steel and annealing temperature To, at a secondary cooling rate of 10-20 0 °C/sec. This is a method whereby the cooling end temperature Te in the continuous annealing cycle shown in Fig. 12 is represented as a function of the chemical compositions and annealing temperature, and is kept under a given critical value.
After cooling to Te, the temperature is preferably held in a range of Te 500C to 400 0 C for up to 20 minutes prior to cooling to room temperature.
Here, Tem is the martensite transformation start temperature for the retained austenite at the quenching start point Tq. That is, Tem is defined by Tem T1 26 T2, or the difference between the value excluding the effect of the C concentration in the austenite (TI) and the value indicating the effect of the C concentration Here, Tl is the temperature calculated from the solid solution element concentration excluding C, and T2 is the temperature calculated from the C concentration in the retained austenite at Ac, and Ac 3 determined by the chemical compositions of the steel and Tq determined by the annealing temperature To. Ceq represents the carbon equivalents in the retained austenite at the annealing temperature To. Thus, T1 is expressed as: Tl 561 33 x {Mn% (Ni Cr Cu Mo)/2} and T2 is expressed in terms of: Ac, 723 0.7 x Mn% 16.9 x Ni% 29.1 x Si% 16.9 x Cr%, Ac 3 910 203 x 1 2 15.2 x Ni% 44.7 x Si% 104 x V% 31.5 x Mo% 30 x Mn% 11 x Cr% 20 x Cu% 70 x P% 40 x Al% 400 x Ti%, and the annealing temperature To, and when Ceq (Ac 3 Ac 1 x C/(To Acl) (Mn Si/4 Ni/7 Cr Cu 1.5 Mo)/6 is greater than 0.6, T2 474 x (Ac 3 Ac 1 x C/(To Ac 1 and when it is 0.6 or less, T2 474 x (Ac 3 Acl) x C/{3 x (Ac 3 Acl) x C [(Mn Si/4 Ni/7 Cr Cu Mo)/2 0.85)] x (To Acl).
In other words, when Te is equal to or greater than Tem, the desired martensite cannot be obtained. Also, if Toa is 400 0 C or higher, the martensite obtained by cooling is tempered, making it impossible to achieve satisfactory dynamic properties and press formability.
On the other hand, if Toa is less than Te 50 0
C,
additional cooling equipment is necessary, and greater variation will result in the material due to the difference between the temperature of the continuous annealing furnace and the temperature of the steel sheet; 27 this temperature was therefore determined as the lower limit. Also, the upper limit for the holding time was determined to be 20 minutes, because when it is longer than 20 minutes it becomes necessary to expand the equipment.
By employing the chemical composition and production method described above, it is possible to produce a dualphase type high-strength steel sheet with excellent dynamic deformation properties, wherein the microstructure of the steel sheet is a composite microstructure wherein the dominating phase is ferrite, and the second phase is another low temperature product phase containing martensite at a volume fraction from 3%~50% after shaping and working at 5% equivalent strain, and wherein the difference between the quasi-static deformation strength as when deformed in a strain rate range of 5 x 10 4 5 x 10- 3 after pre-deformation of more than 0% and less than or equal to 10% of equivalent strain, and the dynamic deformation strength ad measured in a strain rate range of 5 x 102 5 x 103 after the aforementioned pre-deformation, i.e.
(od as), is at least 60 MPa, and the work hardening coefficient at 5-10% strain is at least 0.13. The steel sheets according to the invention may be made into any desired product by annealing, tempering rolling, electronic coating or hot-dip coating.
Examples The present invention will now be explained by way of examples.
(Example 1) The 26 steel materials listed in Table 1 (steel nos.
1-26) were heated to 1050-1250 0 C and subjected to hot rolling, cooling and coiling under the production conditions listed in Table 2, to produce hot rolled steel sheets. As shown in Table 3, the steel sheets satisfying the chemical composition conditions and production 28 conditions according to the invention have a dual-phase structure with a martensite volume fraction of at least 3% and no greater than 50%, and as .shown in Fig. 4, the mechanical properties of the hot rolled steel sheets indicated excellent impact absorption properties as represented by a work hardening coefficient of at least 0.13 at 5-10% strain, ad as 60 MPa, and adyn 2 0.766 x TS 250, while also having suitable press formability and weldability.
Table 1 Chemical compositions c steels Steel INo. CI Si Mn P -1 S [A1I N [Si+AIA Chemical compositions (wtZ) Cu ua KLLVI ru I Ni Cr Cu Ca REM Mn+Cr Ceq M'neq
P
P 1 0.08 2 0.08 3 0.05 4 0.15 0.08 6 0.07 7 0.07 8 0.08 9 0.08 0.07 11 0.10 12 0.08 13 0.08 14 0.08 1.0 1. 0 1.2 1.5 0.
0.
0.
0.
0.
-i.
1.( 1.
1.
1 cN n9'n AA3 AN-N310.003 1.06 0.10 1.30! 0.2911.
20 0 3 .0 .0413 02 )1 1.20 0.02 0.003 0.03 0.002 1.06 0.10 0.004 1.30 0.29 1.25 20 1.30 0.01 0.0010.010.003 1.22 1.30 0.27 1.30 1 1.01 0.02 0.002 0.02 0.002 1.55 1.01 0.32 1.01 01 1.30 0.02 0.0031.200.003 1.23 1.30 0.30 1.30 01 1.60 0.08 0.003 0.030.003 0.12 1.60 0.34 1.60 50 1.10 0.07 0.00210.0110.002 0.58 1.10 0.25 1.10 80 1.20 0.02 0.0030.500.003 1.32 1.20 0.28 1.20 A 00 011 02 00 01 01 1.] A An.)In 'lAin AA'~ N ?q .26 1 A A. 0.20 0002 0.80 1.80 1.20 1.40 0.01 0.02 0.02 .1 0.0
V~
'II
A n'lI AAO9 O N& 0.40 24 00 .J.IO 002.j~ 05 0.40- -1 i I A'l A Anlt N' 1 0N& 1.80 0.401 1.80
V
0 1 01 002 02 0 0 2 04 A AA'~ A flqI An') 1.06 0.10 U0.UU051.301 0.29 1.2 0_ 4 I I F -0t 1 AA 0 02J~ 0 0 00 9 A nn')I nAn1A003 1.06 0.10 1.501 0.321 0 21f 0 0310 03 .0 A A' 9 A AA9I A31AAN.03 1.06 0.10 1.301 0.291.5
U
20-6~ 0201003'10 I 001 IA nn-,In nun AAS O Nq
.V
.181 1 00 I0.01 0 0.10 1 O51 0 r "J nnnuln AlA An0'9 O qq 0.5 .39 1 ib U.U1 U.0U 0. J u.VL u 0.90 0.26.1.15 17 0.07 0.90 0.90 0.02 0.001 0.01 0.003 0.93 0.5 0.90 0.26 1.15 18 0.09 2.16 1.91 0.01 0.003 0.03 0.003 2.20 0.09 0.02 2.00 0.42 1.96 19 0.06 0.80 1.10 0.02 0.002 0.01.0.002 0.83 0.05 1.10 0.24 1.10 0.07 0.90 1.20 0.01 0.001 0.0110.003 0.92 0.003 1.20 0.27 1.20 21 0.05 1.00 1.36 0.01 0.0030.020.003 1.03 0.90 2.26 0.35 1.81 22 0.07 0.02 1.80 0.08 0.003 0.02 0.002 0.12 1.80 0.37 1.80 23 0.10 0.20 0.81 0.08 0.003 0.02 0.002 0.30 0.81 0.24 0.81 24 0.09 0.20 1.15 0.01 0.005 0.05 0.003 0.26 1.15 0.281 1.1 0.08 1.61 1.71 0.01 0.003 0.03 0.003 1.65 0.12 1.71 0.37 1.71 26 0.40 0.0021 0.10 0.005 0.010 0.01 0.003 0.017 0.10 0.42 0.10 ld id f th ran out g e s e o U Table 1 cont.) Chemical compositions of steels Steel Transformation Type No. temperature 0
C
Acl Ac3 Ar3 1 741 863 793 present invention 2 741 863 793 present invention 3 744 880 805 present invention 4 756 871 809 present invention 709 863 756 present invention 6 706 816 731 present invention 7 726 851 794 present invention 8 733 874 791 present invention 9 712 834 774 present invention 722 830 787 present invention 11 733 839 736 present invention 12 741 863 793 present invention 13 739 857 775 comparative example 14 741 863 793 comparative example 713 861 806 comparative example 16 728 839 732 present invention 17 740 887 802 present invention 18 767 889 763 present invention 19 735 870 807 present invention 736 862 798 present invention 21 753 860 751 present invention 22 704 810 713 present invention 23 720 837 801 present invention 24 717 826 773 present invention 752 923 771 present invention 26 722 779 762 comparative example ±a1J~L~
I.
Steel Hot rolling conditions Cooling Coiling No. conditions conditions Finishing Initial steel Final Final Strain log A AT Inequal- Aver. Note Coiling Inequaltemp. strip thick- pass sheet rate calculated ity cooling temp. ity (3) 0C ness (mm) rolling thickness (/sec) 0C rate 0C speed (mm) 0 C/sec) (mpm) 1 880 50 1000 1.2 300 14.4 140 0 120 #1 100 0 2 780 26 500 2.9 90 15.0 150 0 30 #1 300 0 3 830 30 600 2.9 140 14.8 160 0 60 200 0 4 820 28 700 1.4 190 14.6 155 0 70 310 0 840 35 500 2.3 95 14.3 120 0 50 150 0 6 845 40 600 2.3 145 14.1 140 0 60 150 0 7 830 35 650 2.3 150 14.9 150 0 50 150 0 8 825 38 750 1.6 190 14.9 60 0 60 150 0 9 850 36 600 1.8 150 14.6 170 0 40 150 0 840 40 600 1.8 150 15.0 130 0 50 150 0 11 800 26 550 1.8 145 14.0 110 0 40 200 0 12 845 32 600 1.8 150 14.5 135 0 50 100 0 13 930 20 500 1.8 135 13.3 100 0 15 500 x 14 700 26 300 1.8 100 16.1 125 0 15 320 0 850 30 600 1.8 150 15.4 150 0 4 310 0 16 840 28 500 1.4 150 13.7 80 0 30 150 0 17 830 28 500 1.4 145 14.9 100 0 30 150. 0 18 860 30 700 1.4 190 13.4 50 0 35 100 0 19 840 30 700 1.4 180 15.0 180 0 30 200 0 830 30 700 1.4 190 14.9 130 0 30 200 0 21 840 30 700 1.4 190 14.1 140 0 30 200 0 22 780 30 700 1.4 190 14.6 90 0 25 200 0 23 800 30 700 1.4 190 15.6 110 0 25 200 0 24 810 30 700 1.4 190 15.0 120 0 25 200 0 820 30 700 1.4 190 14.2 40 0 25 200 0 26 880 30 700 1.4 190. 13.2 220 x 15 6 320 0 4 I outi nt thp rang inven tion. 750OC00C at 15 Csec.
Under ned daLa n e va I, 4-i,-i C TaD-i± e l J- u LmL uc u e L o Steel Dominant phase Ferrite Martensite No. Phase Circle equivalent Volume Circle equivalent Volume fraction after diameter (pm) fraction diameter (jIm) 5Z working (Z) 1 ferrite 5.5 80 2.5 2 ferrite 4.0 90 1.8 8 3 ferrite 5.0 85 2.2 4 ferrite 4.0 80 1.8 4 ferrite 4.5 80 2.0 6 ferrite 5.0 85 2.2 7 ferrite 4.5 90 2 8 ferrite 4.5 90 2 9 ferrite 5.0 90 2.2 ferrite 5.0 90 2.2 11 ferrite 4.0 80 1.7 12 ferrite 5.0 90 2.2 13 ferrite 11.0 50 0 14 ferrite Worked structure 90 0 ferrite 10.0 95 0 16 ferrite 4.4 90 1.9 17 ferrite 4.5 91 2 9 18 ferrite 3.4 78 1.4 22 19 ferrite 4.4 91 1.9 9 ferrite 4.3 88 1.8 12 21 ferrite 4.5 85 2 13 22 ferrite 4.4 84 1.9 11 23 ferrite 4.4 85 1.9 8 24 ferrite 4.4 85 1.8 12 ferrite 2.4 80 1 26 bainite 10.5 30 0 r ine data indicate values outside of the range of the invention.
TTnrd Table 4 Mechanical properties or steels teel Static tension (strain rate 0.001/s) Pre-deformation and BH treatment o. TS YS T.El TS' 5-102 n YS x n YS/TS' TS x T.E1 Form of pre-deformation Equivalent BH Mpa Mpa Mpa value Mpa-2 strain Z treatment 1 680 475 27 720 0.17 81 0.66 18360 C-directional uniaxial tension 5% yes 2 630 440 35 665 0.19 84 0.66 22050 C-directional uniaxial tension 5% no 3 580 405 32 610 0.18 73 0.66 18560 L-directional uniaxial tension 5% yes 4 780 545 28 815 0.19 104 0.67 21840 C-directional uniaxial tension 2% yes 700 455 29 740 0.20 91 0.61 20300 C-directional uniaxial tension 5% yes 6 660 430 30 695 0.19 82 0.62 19800 C-directional uniaxial tension 10% yes 7 650 420 31 685 0.21 88 0.61 20150 C-directional uniaxial tension 5% yes 8 620 400 32 645 0.21 84 0.62 19840 C-directional uniaxial tension 5% yes 9 640 415 31 675 0.18 75 0.61 19840 C-directional uniaxial tension 5% yes 670 435 30 710 0.19 83 0.61 20100 C-directional uniaxial tension 5% yes 11 850 510 24 885 0.21 107 0.58 20400 C-directional uniaxial tension 5% yes 12 650 420 31 685 0.20 84 0.61 20150 Equal biaxial tension 5% yes 13 520 494 31 530 0.12 59 0.93 16120 C-directional uniaxial tension 5% yes 14 500 375 28 515 0.10 38 0.73 14000 Equal biaxial tension 5% yes 420 310 36 430 0.12 37 0.72 15120 L-directional uniaxial tension 5Z yes 16 650 390 31 685 0.20 78 0.57 20150 L-directional uniaxial tension 5% yes 17 640 380 30 670 0.21 80 0.57 19200 L-directional uniaxial tension 5Z yes 18 840 505 27 870 0.16 81 0.58 22680 L-directional uniaxial tension 5% yes 19 660 405 28 695 0.20 81 0.58 18480 L-directional uniaxial tension 5Z yes 660 400 29 690 0.20 80 0.58 19140 L-directional uniaxial tension 5% yes 21 660 480 28 695 0.15 72 0.69 18480 L-directional uniaxial tension 5%2 yes 22 620 290 31 650 0.25 73 0.45 19220 L-directional uniaxial tension 5% yes 23 640 420 29 670 0.18 76 0.63 18560 L-directional uniaxial tension 5% yes 24 640 380 29 675 0.19 72 0.56 18560 L-directional uniaxial tension 5% yes 820 615 23 875 0.15 92 0.70 18860 L-directional uniaxial tension 5% yes 26 400 290 30 410 0.10 29 0.71 12000 L-directional uniaxial tension 5% yes Underlined data indicate values outside of the range of the invention.
*3 udyn (0.766 x TS 250) *4 2.5(YS/TS' 15 2 plastic deformation T 2 2.5(YS/TS' 0v$ j -L If ,I r F aC+-Q Mg C: -lTabIe 4 (cont. iiBidcui.Lc-L opc-,j .c Steei Static/dynamic tension after pre-deformation-BH treatment Plastic deformation Other JNo. (strain rate 1000/s) properties WH*I AYS*2 as ud od-us udyn Inequality *3 T Inequality *4 Weldability d/d 0 Mpa Mpa Mpa Mpa Mpa Mpa z satisfied 1 140 205 720 841 121 825 54.1 1.0 0 OK 1.31 2 150 215 665 788 123 771 38.4 1.0 o OK 1.45 3 145 210 610 732 122 718 23.7 1.0 0 OK 1.54 4 80 150 800 923 123 903 55.5 1.0 0 OK 1.21 140 210 740 864 124 847 60.8 0.9 0 OK 1.27 6 155 260 710 833 123 830 74.4 0.9 o OK 1.35 7 145 215 685 810 125 794 46.1 0.9 0 OK 1.37 8 140 215 645 770 125 755 30.1 0.9 0 OK 1.44 9 140 210 675 797 122 781 40.8 0.9 0 OK 1.39 140 210 710 833 123 818 54.8 0.9 0 OK 1.33 11 150 220 885 1010 125 992 90.9 0.8 o OK 1.20 12 140 210 685 809 124 793 45.1 0.9 0 OK 1.40 13 15 20 530 575 45 565 -83.3 1.6 0 OK 1.71 14 20 25 515 565 50 555 -78.0 0.1 x OK 1.18 25 35 430 485 5_5 476 -95.7 1.2 0 OK 1.90 16 140 210 685 809 124, 793 45.1 0.7 0 OK 1.37 17 1 140 215 670 795 125 780 39.8 0.7 0 OK 1.39 18 145 210 870 989 119 972 78.6 0.8 0 OK 1.20 19 140 210 695 819 124 804 48.4 0.8 0 OK 1.35 140 210 690 814 124 798 42.4 0.8 0 OK 1.35 21 125 195 695 813 118 799 43.4 1.0 0 OK 1.35 22 145 215 650 779 129 765 40.1 0.4 0 OK 1.44 23 130 200 670 792 122 777 36.8 0.9 0 OK 1.39 24 140 210 675 798 123 784 43.8 0.7 0 OK 1.35 120 190 875 993 118 977 98.9 1.1 0 OK 1.20 26 25 35 410 445 35 437 -119.4 1.1 0 oor 1.50 Underlined data indicate values outside of the range of the invention.
cudyn (0.766 x TS 250) *4:2.5{YS/TS' 15 2! plastic deformation T 2.5{YS/TS' 35 (Example 2) The 22 steel materials listed in Table 5 (steel nos.
27-48) were heated to 1050-1250 0 C and subjected to hot rolling, cooling and coiling, followed by acid pickling and then cold rolling under the conditions listed in Table 6 to produce cold rolled steel sheets.
Temperatures Ac, and Ac 3 were then calculated from the chemical compositions for each steel, and the sheets were subjected to heating, cooling and holding under the annealing conditions listed in Table 6, prior to cooling to room temperature. As shown in Table 7, the steel sheets satisfying the chemical composition conditions and production conditions according to the invention have a dual-phase structure with a martensite volume fraction of at least 3% and no greater than 50% and, as shown in Fig.
8, the mechanical properties of the hot-rolled steel sheets indicated excellent impact absorption properties as represented by a work hardening coefficient of at least 0.13 at 5-10% strain, od os 60 MPa, and adyn 2 0.766 x TS 250, while also having suitable press formability and weldability.
able 5 Chemical compositions steel rable 5 Chemical compositions o^ steel
L
I I I aii~ L A. J k'~
-I
o teel Chemical compos No.
C Si Mn P S Al N Al+Si Ni
+P
27 0.01 1.00 0.10 0.01 0.003 0.04 0.003 1.05 28 0.05 0.90 1.20 0.01 0.005 0.05 0.002 0.96 29 0.05 0.90 1.20 0.01 0.005 0.05 0.002 0.96 0.05 0.90 1.20 0.01 0.005 0.05 0.002 0.96 31 0.05 1.30 1.20 0.01 0.002 0.04 0.003 1.35 32 0.08 1.00 1.20 0.01 0.003 0.05 0.002 1.06 33 0.08 0.20 0.70 0.01 0.003 0.04 0.003 0.25 34 0.10 0.02 2.10 0.01 0.002 0.03 0.003 0.06 0.08 0.80 1.00 0.01 0.003 1.20 0.002 2.01 36 0.05 0.50 1.00 0.01 0.005 0.80 0.003 1.31 37 0.05 1.00 1.00 0.01 0.003 0.05 0.002 1.06 0.9 38 0.05 1.00 1.00 0.01 0.005 0.04 0.003 1.05 39 0.05 1.00 1.00 0.01 0.003 0.04 0.003 1.05 0.05 1.00 1.50 0.01 0.005 0.05 0.002 1.06 41 0.10 1.0011.50 0.01 0.003 0.46 0.002 1.47 42 0.10 1.00 1.50 0.01 0.005 0.04 0.002 1.05 43 0.10 1.00 1.50 0.10 0.003 0.05 0.003 1.15 44 0.10 1.00 1.50 0.01 0.003 0.04 0.003 1.05 0.10 0.01 1.50 0.003 0.003 0.002 0.003 0.015 46 0.35 1.00 1.20 0.01 0.002 0.05 0.003 1.06 47 0.05 0.80 2.50 0.01 0.003 0.04 0.002 0.85 48 0.08 3.50 1.50 0.25 0.003 0.80 0.003 4.55 Underlined data indicate values outside of the range i tion rw-) mation temperature T e °c I I z I llLj .r 1 C IM M Acl i Ac3 I A r MO N D 1 L nul r., eqqi ,l~l El. J. J[ J 10 75 1%9f/.rllonm n 03 0 p. ex.
2- I 1.2 0.2 1.2 73 87 80 op e.
1invention 1.20 0.25 1.20 736 872 1804 comp. ex.
1.2010.251.20. 736 872 804 comp. ex.
1.2010.251.201 748 889817 invention 1.20 0.28 1.20 739 864 798 invention 0. /U U.2UU. 7l 843 811 nven ion -2.10 0.452.10701 785 676 invention 1.00 0.251.00736 907 809 invention 1.00 0.22 1.00727 890809 invention 1.00 0.291.45726 869 784 invention S1.0- 1.00 0.30 1.50741 862 780 invention 1 0.1 1.00 0.231.05741 885 821 invention "I O 1 856 1734 2.50J 30|J.U U J JU SLIrnLtion 50 86 7 0.04 1..50U 0.J- 5 or,. I I) I .LULVC., I. lLIV* J 0.04 M 0.04 1 .5 0} -35 1L. 5'I ou I tJontl V 1 U. U J-J 1 1 J I"7" 0.U1 U.U4 1..50 |0.35| 1.-U />U U U J I L .L LLV CLL I J l 0.2U -II UL U of heinvnto.o U.UUZ 1..U u.JJ L.Ju /3U I UHt| V[.l nLLt--l l 0 =0 1 0/.0 i7641i 50 707 1 Rnq 7311comp. ex.
50 0 35 1.
01.J4. ~I iU~I...UIIIj) CA 1.zu U.55 1.U /39 o0 op eUxI.u A nI ni 50 753A 806 5891Qnmn.
50 0 63 3. ex n n "5Th';5 809 11014 q53 comn. ex.
50 10 33I of the invention.
Table 6 Production conditions Steel Cold rolling Annealing conditions No. conditions Rolling Sheet Anneal- Anneal- Primary Rapid Second- Rapid Calcul- Calcul- Calcul- Calcul- Holding Holding reduc- thick- ing ing cooling cooling ary cooling ated Ti ated ated T2 ated tempera- time tion. ness tempera- time start cooling end Ceq* Tern ture ture m nm To 'C sec 0 C/sec Tq 0 C 0 C/sec Te 0 C C 0 C a Toa 0 C sec 27 80 0.8 780 90 5 680 100 350 558 0.12 -62 619 350 180 28 80 0.8 780 90 5 680 100 230 521 0.39 224 297 230 270 29 80 0.8 780 90 5 680 100 320 521 0.39 224 297 320 270 80 0.8 780 90 5 500 100 230 521 0.39 224 297 230 270 31 80 0.8 780 90 5 700 100 270 521 0.47 182 339 270 300 32 80 0.8 780 90 5 680 80 270 521 0.49 190 331 270 250 33 80 0.8 750 120 8 680 100 200 538 0.46 297 241 200 300 34 80 0.8 750 120 8 680 100 270 492 0.52 114 378 270 300 80 0.8 800 90 5 680 100 270 528 0.41 259 269 270 300 36 80 0.8 750 90 5 650 130 200 528 0.54 217 311 250 300 37 80 0.8 750 90 5 650 130 250 513 0.53 192 321 240 300 38 80 0.8 800 90 5 650 100 270 512 0.48 83 428 270 300 39 80 0.8 780 90 5 650 100 250 526 0.42 216 310 250 300 80 0.8 780 90 5 680 100 270 495 0.65 90 405 270 300 41 80 0.8 780 90 8 680 100 250 512 .0.58 154 358 250 300 42 68 1.2 780 90 8 680 100 250 512 0.58 154 358 270 300 43 68 1.2 780 90 5 630 150 250 512 0.58 154 358 250 300 44 68 11.2 780 90 5 680 100 250 512 0.55 153 359 250 300 80 0.8 750 90 5 680 100 250 512 0.47 186 326 250 300 46 80 0.8 780 90 5 680 100 200 521 0.77 252 269 200 300 47 80 0.8 770 90 5 680 100 270 446 0.94 74 31 270 300 48 80 0.8 850 90 5 680 10 lf 250 512 0.79 188 323 250 300 Underlined data indicate values outside of the range of the invention.
Table 7 Microstructure ot steels Steel Dominant phase Ferrite Martensite No. Phase Circle equivalent Volume Circle equivalent Volume fraction after diameter fraction diameter (pm) 5Z working (Z) 27 ferrite 9.8 100 0 28 ferrite 6.4 86 3.2 12 29 ferrite 6.4 95 1 ferrite 6.4 94 0 31 ferrite 5.3 89 3.1 11 32 ferrite 4.8 82 2.8 33 ferrite 5.1 84 2.9 12 34 ferrite 4.8 75 2.2 18 ferrite 5.1 90 2.3 36 ferrite 5.5 90 2.8 8 37 ferrite 6.2 89 3.1 11 38 ferrite 5.8 81 3.0 16 39 ferrite 5.6 78 3.2 18 ferrite 5.6 87 3.2 13 41 ferrite 4.2 80 1.7 16 42 ferrite 4.5 78 2.1 18 43 ferrite 4.3 79 2.2 19 44 ferrite 5.0 79 2.3 13 ferrite 4.9 81 2.1 1 46 ferrite 4.1 42 2.4 47 ferrite 4.6 51 2.6 48 ferrite 5.6 88 2.6 12 ned data indicate values outside of the range of the invention.
TTnderli Underli~- Tahble Mrchahnica nroDerties o steels ;teel Static tension (strain rate 0.001/s) Pre-deformation and BH treatment TS YS T.E1 TS' 5-10% n YS x n YS/TS' TS x T.E1 Form of pre-deformation Equivalent BH Mpa Mpa Z Mpa value Mpa Z strain Z treatment 27 357 243 48 357 0.28 68 0.68 17136 C-directional uniaxial tension 5 yes 28 592 349 34 630 0.24 84 0.55 20128 C-directional uniaxial tension 5 no 29 603 457 32 612 0.20 91 0.75 19296 C-directional uniaxial tension 5 no 583 472 26 591 0.15 71 0.80 15158 C-directional uniaxial tension 5 no 31 599 341 33 621 0.23 79 0.55 19767 L-directional uniaxial tension 10 yes 32 641 359 34 690 0.23 83 0.52 21794 C-directional uniaxial tension 5 yes 33 558 340 36 597 0.26 88 0.57 20088 C-directional uniaxial tension 5 yes 34 640 397 33 650 0.22 87 0.61 21120 C-directional surface strain 3 yes tension 611 354 35 633 0.23 82 0.56 21385 C-directional uniaxial tension 5 yes 36 589 324 36 600 0.24 78 0.54 21204 C-directional uniaxial tension 5 yes 37 634 361 33 657 0.21 76 0.55 20922 Equal biaxial tension 10 yes 38 625 388 31 668 0.19 74 0.58 19375 C-directional uniaxial tension 5 yes 39 689 434 29 712 0.18 78 0.61 19981 L-directional uniaxial tension 1 no 623 368 32 623 0.20 74 0.59 19936 C-directional uniaxial tension 1 yes 41 709 425 26 721 0.17 72 0.59 18434 C-directional uniaxial tension 5 yes 42 722 448 25 734 0.17 76 0.61 18050 C-directional uniaxial tension 5 no 43 731 468 25 755 0.16 75 0.62 18275 Equal biaxial tension 5 yes 44 715 465 26 726 0.16 74 0.64 18590 C-directional uniaxial tension 5 yes 648 531 25 681 0.12 64 0.78 16200 C-directional uniaxial tension 5 yes 46 1075 742 10 1075 0.08 59 0.69 10750 C-directional uniaxial tension 5. yes 47 712 484 21 712 0.11 53 0.68 14952 C-directional uniaxial tension 5 yes 48 792 475 22 792 0.14 67 0.60 17424 C-directional uniaxial tension 5 yes nderlined data indicate values outside of the range of tie invention.
i Table 8 (cont.) Mechanical properties of steels l Steel Static/dynamic tension after pre-deformation.BH treatment Plastic deformation Weldability No. (strain rate WH*1 AYS*2 us ad ad-us udyn Inequality T Inequality *4 Mpa Mpa Mpa Mpa Mpa Mpa *3 Z satisfied 27 90 116 390 438 48 412 -111.5 1.0 0 OK 28 182 260 630 734 104 721 17.5 1.0 0 OK 29 102 127 620 662 42 645 -66.9 1.5 0 OK 97 115 593 626 33 599 -97.6 1.5 0 OK 31 240 297 740 846 106 798 89.2 1.0 0 OK 32 212 294 757 818 61 786 45.0 1.0 0o OK 33 143 212 607 686 79 680 2.6 1.0 0 OK 34 191 229 660 748 88 745 4.8 1.0 0 OK 201 275 658 756 98 742 24.0 1.0 0 OK 36 212 289 612 733 121 722 20.8 1.0 0 OK 37 257 318 703 790 87 776 40.4 1.0 0 OK 38 182 244 678 762 84 748 19.3 1.0 0o OK 39 199 239 712 798 86 789 11.2 4.0 0 OK 201 181 625 748 123 740 12.8 1.0 x OK 41 209 280 729 824 95 819 25.9 1.0 0 OK 42 201 260 734 833 99 818 14.9 1.0 0O OK 43 182 246 767 829 62 820 10.1 1.0 0 OK 44 177 253 739 835 96 824 26.3 1.0 0 OK 58 92 696 744 48 712 -34.4 1.0 x OK 46 299 341 1076 1088 12 1052 -21.5 1.0 0 poor 47 181 229 715 757 42 740 -55.4 1.0 0 poor 48 265 332 806 856 50 832 -24.7 1.0 0 OK Underinea data indicate values outside or tne range or ihne WH -represents increase in YS with 5Z pre-deformation at invenLien.
equivalent strain listed in the table.
treatment as paint baking treatment at AYS represents increase in YS after pre-deformation in the table and heat 170°C x 20 min.
*3 adyn (0.766 x TS 250) *4 2.5{YS/TS' 15 plastic deformation T 2.5{YS/TS' 41- The microstructure was evaluated by the following method.
Identification of the ferrite, bainite, martensite and residual structure, observation of the location and measurement of the average grain size (mean circle equivalent-diameter) was accomplished using a 1000 magnification optical micrograph with the thin steel sheet rolling direction cross-section etched with a nital and the reagent disclosed in Japanese Unexamined Patent Publication No. 59-219473.
The properties were evaluated by the following methods.
A tensile test was conducted according to (gauge mark distance: 50 mm, parallel part width: 25 mm) with a strain rate of 0.001/s and, upon determining the tensile strength yield strength total elongation El) and work hardening coefficient (n value for strain), the YS x work hardening coefficient and TS x T. El. were calculated.
The stretch flanging property was measured by expanding a 20 mm punched hole from the burrless side with a 300 cone punch, and determining the hole expansion ratio (d/d 0 between the hole diameter at the moment at which the crack penetrated the plate thickness and the original hollow diameter (d o 20 mm) The spot weldability was judged to be unsuitable if a spot welding test piece bonded at a current of 0.9 times the expulsion current using an electrode with a tip radius of 5 times the square root of the steel sheet thickness underwent peel fracture when ruptured with a chisel.
Industrial Applicability As explained above, the present invention makes it possible to provide, in an economical and stable manner, high-strength hot rolled steel sheets and cold rolled steel sheets for automobiles which provide previously 42 unobtainable excellent impact absorption properties and press formability and thus offers a markedly wider range of objects and conditions for uses of high-strength steel sheets.
Claims (13)
1. A dual-phase type high-strength steel sheets having high impact energy absorption.properties, characterized in that the final microstructure of the steel sheet is a composite microstructure wherein the dominating phase is ferrite, and the second phase is another low temperature product phase containing martensite at a volume fraction between 3% and 50% after deformation at 5% equivalent strain of the steel sheet, wherein the difference between the quasi-static deformation strength as when deformed in a strain rate range of 5 x 10 4 5 x 10- 3 (s after pre-deformation of more than 0% and less than or equal to 10% of equivalent strain, and the dynamic deformation strength ad when deformed in a strain rate range of 5 x 102 5 x 10' (s after said pre-deformation, i.e. (od os), is at least MPa, and the work hardening coefficient at' 5-10% strain is at least 0.13.
2. A dual-phase type high-strength steel sheet having high impact energy absorption properties, characterized in that the final microstructure of the steel sheet is a composite microstructure wherein the dominating phase is ferrite, and the second phase is another low temperature product phase containing martensite at a volume fraction between 3% and 50% after deformation at 5% equivalent strain of the steel sheet, wherein the average value odyn (MPa) of the deformation stress in the range of 3-10% of equivalent strain when 23 deformed in a strain rate range of 5 x 10 5 x 103 after pre-deformation of more than 0% and less than or equal to 10% of equivalent strain, satisfies the inequality: odyn 0.766 x TS 250 as expressed in terms of the tensile strength TS (MPa) in the quasi-static tensile test as measured in a strain rate range of 5 x 10 5 x 10 3 (s prior to pre-deformation, and the work hardening coefficient at 5-10% strain is at least 0.13. 44
3. A dual-phase type high-strength steel sheet having high impact energy absorption properties according to claim 1 or 2, characterized in that the ratio between the yield strength YS(0) and the tensile strength in the static tensile test after pre-deformation at 5% of equivalent strain or after further bake hardening treatment (BH treatment) satisfies the inequality 0.7, and also satisfies the inequality: yield strength YS(0) x work hardening coefficient
4. A dual-phase type high-strength steel sheet having high impact energy absorption properties according to any of claims 1, 2 or 3, characterized in that the average grain size of martensite is 5 im or less, and the average grain size of ferrite is 10 im or less.
5. A dual-phase type high-strength steel sheet with excellent dynamic deformation properties according to any of claims 1 to 4, characterized by satisfying the inequality: tensile strength (MPa) x total elongation 2 18,000, and by satisfying the inequality: hole expansion ratio (d/d 0 1.2.
6. A dual-phase type high-strength steel sheet having high impact energy absorption properties according to any of claims 1 to 5, characterized in that the plastic deformation by either or both a tempering rolling and a tension leveller satisfies the following inequality: 0.5} 15 2 T 0.5}
7. A dual-phase type high-strength steel sheet having high impact energy absorption properties according to any of claims 1 to 6, characterized in that the chemical compositions of the dual-phase type high- strength steel sheet with excellent dynamic deformation properties contains, in terms of weight percentage, C at 0.02-0.25%, either or both Mn and Cr at a total of 0.15-3.5%, one or more from among Si, Al and P at a total 45 of 0.02-4.0%, if necessary one or more from among Ni, Cu and Mo at a total of no more than one or more from among Nb, Ti and V at no more than 0.30%, and either or both Ca and REM at 0.0005-0.01% for Ca and 0.005-0.05% for REM, with the remainder Fe as the primary component.
8. A dual-phase type high-strength steel sheet having high impact energy absorption properties according to any of claims 1 to 7, characterized in that one or more from among B (50.01), S and N (50.02%) are further added if necessary to the chemical compositions of the steel of the dual-phase type high-strength steel sheet with excellent dynamic deformation properties.
9. A method of producing a dual-phase type high strength hot rolled steel sheet having high impact energy absorption properties according to any of claims 1 to 8, characterized in that after a continuous cast slab is fed directly from casting to a hot rolling step, or is hot rolled upon reheating after momentary cooling, it is subjected to hot rolling at a finishing temperature of Ar 3 50 0 C to Ar 3 120 0 C, cooled at an average cooling rate of more than 5 0 C/sec in a run-out table, and then coiled at a temperature of no greater than 350 0 C. A method of producing a dual-phase high- strength hot rolled steel sheet having high impact energy absorption properties according to claim 9, characterized in that at finishing temperature for hot rolling in a range of Ar 3 50 0 C to Ar 3 120 0 C, the hot rolling is carried out so that the metallurgy parameter A satisfies inequalities and below, the subsequent average cooling rate in the run-out table is at least 5 0 C/sec, and the coiling is accomplished so that the relationship between said metallurgy parameter A and the coiling temperature (CT) satisfies inequality below. 9 logA 18 (1) AT 21 x logA 61 (2) CT 6 x logA 242 (3)
11. A method of producing a dual-phase type high-strength cold rolled steel sheet having high impact energy absorption properties according to any of claims 1 to 8, characterised in that after a continuous cast slab is fed directly from casting to a hot rolling step, or is hot rolled upon reheating after momentary cooling, it is hot rolled, the hot rolled and subsequently coiled steel sheet is cold rolled after acid pickling, and during annealing in a continuous annealing step for preparation of the final product, it is heated to a temperature between Acl and Ac 3 and subjected to the annealing while held in this temperature range for at least 10 seconds, and then cooled at a cooling rate of more than
12. A method of producing a dual-phase type high-strength cold rolled steel sheet 0o having high impact energy absorption properties according to any of claims 1 to 8, characterised in that in said continuous annealing step, the cold rolled steel sheet is heated to a temperature between Ac, and Ac 3 and subjected to the annealing while held in this temperature range for at least 10 seconds, and for subsequent cooling, it is cooled to a secondary cooling start temperature (Tq) in the range of 550°C-To at a primary cooling rate of 1-10°C/sec and then cooled to a secondary cooling end temperature (Te) which is no higher than Tem determined by the chemical compositions and annealing temperature (To), at a secondary cooling rate of 10-200 0 C/sec.
13. A dual-phase type high-strength steel sheet having high impact energy absorption properties, said sheet being substantially as hereinbefore described with 20 reference to any one of the examples.
14. A method of producing a dual-phase type high strength hot rolled steel sheet having high impact energy absorption properties, said method being substantially as Shereinbefore described with reference to any one of the examples. Dated 15 October, 1999 Nippon Steel Corporation Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON 9 [RA:LIBC]07294.doc:bav
Applications Claiming Priority (11)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP9/82434 | 1997-03-17 | ||
| JP8243497 | 1997-03-17 | ||
| JP19029997 | 1997-07-15 | ||
| JP9/190299 | 1997-07-15 | ||
| JP19029797A JP3530347B2 (en) | 1997-07-15 | 1997-07-15 | How to select a high-strength steel sheet with excellent dynamic deformation characteristics |
| JP9/190297 | 1997-07-15 | ||
| JP22300897A JP3936440B2 (en) | 1997-08-06 | 1997-08-06 | High-strength steel sheet for automobiles with excellent collision safety and formability and its manufacturing method |
| JP9/223008 | 1997-08-06 | ||
| JP9/258938 | 1997-09-24 | ||
| JP25893897A JP3839928B2 (en) | 1997-07-15 | 1997-09-24 | Dual phase type high strength steel plate with excellent dynamic deformation characteristics |
| PCT/JP1998/001101 WO1998041664A1 (en) | 1997-03-17 | 1998-03-16 | Dual-phase high-strength steel sheet having excellent dynamic deformation properties and process for preparing the same |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| AU6311898A AU6311898A (en) | 1998-10-12 |
| AU717294B2 true AU717294B2 (en) | 2000-03-23 |
Family
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| AU63118/98A Expired AU717294B2 (en) | 1997-03-17 | 1998-03-16 | Dual-phase high-strength steel sheet having excellent dynamic deformation properties and process for preparing the same |
Country Status (7)
| Country | Link |
|---|---|
| EP (2) | EP0969112B2 (en) |
| KR (1) | KR100334949B1 (en) |
| CN (1) | CN1080321C (en) |
| AU (1) | AU717294B2 (en) |
| CA (1) | CA2283924C (en) |
| TW (1) | TW426742B (en) |
| WO (1) | WO1998041664A1 (en) |
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1998
- 1998-03-16 CN CN98803465A patent/CN1080321C/en not_active Expired - Lifetime
- 1998-03-16 EP EP98907247.5A patent/EP0969112B2/en not_active Expired - Lifetime
- 1998-03-16 CA CA002283924A patent/CA2283924C/en not_active Expired - Lifetime
- 1998-03-16 KR KR1019997008474A patent/KR100334949B1/en not_active IP Right Cessation
- 1998-03-16 WO PCT/JP1998/001101 patent/WO1998041664A1/en active IP Right Grant
- 1998-03-16 TW TW087103834A patent/TW426742B/en not_active IP Right Cessation
- 1998-03-16 AU AU63118/98A patent/AU717294B2/en not_active Expired
- 1998-03-16 EP EP10181225.3A patent/EP2314729B2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| AU6311898A (en) | 1998-10-12 |
| CA2283924A1 (en) | 1998-09-24 |
| EP2314729B2 (en) | 2017-03-08 |
| EP0969112A1 (en) | 2000-01-05 |
| TW426742B (en) | 2001-03-21 |
| CA2283924C (en) | 2006-11-28 |
| EP2314729A1 (en) | 2011-04-27 |
| CN1080321C (en) | 2002-03-06 |
| CN1251140A (en) | 2000-04-19 |
| EP0969112B1 (en) | 2011-08-17 |
| EP0969112B2 (en) | 2017-03-08 |
| WO1998041664A1 (en) | 1998-09-24 |
| KR100334949B1 (en) | 2002-05-04 |
| EP2314729B1 (en) | 2012-02-08 |
| KR20000076372A (en) | 2000-12-26 |
| EP0969112A4 (en) | 2003-05-21 |
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