Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-04-18
2003-11-25
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C148S541000, C148S547000, C148S602000, C148S661000, C148S654000
Reexamination Certificate
active
06652670
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a steel sheet such as hot-rolled steel sheets and cold-rolled steel sheets, and to a method for manufacturing the same.
BACKGROUND OF THE INVENTION
Steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets are used in wide fields including automobiles, household electric appliances, and industrial machines. Since these steel sheets are subjected to some processing before use, they are requested to have various kinds of workability.
Recently, the request of manufacturers of automobiles, household electric appliances, industrial machines, and the like relating to rationalization becomes severer than ever, particularly in the request for improvement in production yield. To cope with the requirement, the materials thereof are requested to have particularly high homogeneity and high workability level.
Regarding the workability requested to the hot-rolled steel sheets and cold-rolled steel sheets, high tension materials (high tensile strength hot-rolled steel sheets) having strengths of 340 MPa or higher class and for the uses other than deep drawing, for example, are required to have high stretch flanging performance during burring. The cold-rolled steel sheets having strengths of 440 MPa or lower and for the drawing uses are requested to have high r value and high breaking elongation.
In recent years, the quality requirement of the consumers to the steel sheets has continuously been increasing, so that not only further improvement in the above-described workability but also homogeneity in mechanical properties in coiled products are strongly requested.
Responding to these requirements of consumers, several measures have been studied. For example, in view of the homogeneity of material quality, JP-A-9-241742, (the term “JP-A” referred herein signifies “Unexamined Japanese Patent Publication”), discloses a method for improving the homogeneity of mechanical properties in a hot-rolled coil by adopting continuous hot-rolling. The method is a technology that uses a process of continuous hot-rolling to improve the material quality of the rolled steel sheet at front end thereof and at rear end thereof, and to eliminate the dispersion in material quality within a coil.
As for the improvement in workability of high tension materials, JP-B-61-15929 and JP-B-63-67524, (the term “JP-B-” referred herein signifies “Examined Japanese Patent Publication”), disclose a method to improve the workability of high tension hot-rolled steel sheet by controlling the cooling speed after the hot-rolling and controlling the coiling temperature.
For the improvement in workability of IF steels (Interstitial-Free Steels), JP-A-5-112831 discloses a method to apply strong drafting during hot-rolling and to apply rapid cooling. The technology intends to improve the r value of cold-rolled steel sheet by applying final reduction in thickness of hot-rolling to 30% or more and by applying rapid cooling immediately after completed the rolling, thus reducing the grain size in the hot-rolled steel sheet.
All the above-described technologies, however, could not obtain steel sheet that is superior in both the workability and the homogeneity in mechanical properties. For example, the material properties (determined at center portion of coil width) obtained by the technology described in JP-A-9-241742 aiming at elimination of dispersion of material quality in a coil gave variations of tensile strength (TS) in an approximate range of from 4.5 to 6.3 kg/mm
2
for the steel sheets of 30 to 70 K class, which range is not satisfactory for users' requirement.
The technology described in JP-B-61-15929 aiming at the improvement in workability of high tension materials improves the balance of strength and ductility compared with conventional steel sheets, but fails to substantially solve the stretch flanging performance. Furthermore, the technology cannot improve the surface defects. Similarly, the high tension hot-rolled steel sheets manufactured by the method of JP-B-63-67524 cannot substantially solve the stretch flanging performance, though the breaking elongation and the toughness of steel sheets are improved.
Also the method described in JP-A-5-112831 aiming at the improvement in workability of IF steels cannot reduce the dispersion of material quantity to a satisfactory level. That is, according to the description of Examples of JP-A-5-112831, the average cooling speed immediately after the rolling, which average cooling speed is a feature of the invention, is in a range of from 90 to 105° C./sec during 1 second after starting the cooling, and from 65 to 80° C./sec during 3 seconds after starting the cooling. With that level of cooling speed, however, it was found that, under the hot-rolling condition in commercial apparatuses, the all grains in the steel sheet, particularly those in rolling top portion, cannot be refined.
The cause is presumably that the cooling cannot be started immediately after completed the finish-rolling, and there needs a time to start cooling. Since the cooling unit cannot be installed at directly adjacent to the exit of the final rolling stand owing to the necessity of installing finish thermometer and instruments to the exit of the final stand of finish-rolling mill, the cooling cannot be performed within, for example, 0.1 second after the completion of the finish-rolling. Particularly at the rolling top portion, high speed travel is not available and the rolling speed is slow, which results in long time before starting the cooling. Thus, the cooling at a cooling speed described in the patent disclosure cannot prevent the formation of coarse austenitic grains.
As described above, the top portion of the steel strip after the hot-rolling is difficult to be rapidly cooled, thus the grains cannot be fully reduced in their size, which fails to obtain superior mechanical properties and homogeneity thereof. Increased reduction in thickness in the final pass of hot-rolling is favorable for reducing the size of austenitic grains. However, increase of the reduction in thickness to 30% or more as in the technology described in JP-A-5-112831 is difficult to be actually implemented because the insufficient shape of steel sheet likely occurs.
The automobile industry has a strong need of weight reduction. Accordingly, the use rate of high strength steel sheets has been increased. To this point, the high tension materials are inferior in workability to the mild materials of 270 MPa class, thus there occur problems of production yield (cracks generated during press-working) and of quality dispersion. Consequently, the improvement in workability which is a basic characteristic of material quality is requested.
Regarding the workability, high tension materials having 340 MPa or higher tensile strength, for example, are requested for hot-rolled steel sheets and cold-rolled steel sheets to have high stretch flanging performance during burring. In addition, in recent years, the automobile application is requested to satisfy the collision safety as one of the critical characteristics, thus the steel sheets are requested to have excellent shock resistance (high shock absorption energy as an evaluation item of collision safety).
As for the improvement in workability of high tension materials, there is a prior art, Japanese Patent No. 2555436. According to the disclosure of the patent, a Ti base precipitation hardening steel is processed at cooling speeds of from 30 to 150° C./sec after the finish-rolling, at coiling temperatures of from 250 to 540° C., thus improving the stretch flanging performance of high tension steels of 50 to 60 K class utilizing the formed (ferrite+bainite) structure. However, the cooling speeds of from 30 to 150° C./sec after the finish-rolling cannot be said to substantially improve the stretch flanging performance, and, there is a problem of low breaking elongation owing to the low temperature level of coiling.
JP-B-7-56053 discloses a method to improve the stretch flanging performance of hot dip zinc-coated steel shee
Imada Sadanori
Inazumi Toru
Inoue Tadashi
Ishiguro Yasuhide
Kikuchi Hiroyasu
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