EP0179952B1 - Method for producing high strength steel with good ductility - Google Patents
Method for producing high strength steel with good ductility Download PDFInfo
- Publication number
- EP0179952B1 EP0179952B1 EP84201584A EP84201584A EP0179952B1 EP 0179952 B1 EP0179952 B1 EP 0179952B1 EP 84201584 A EP84201584 A EP 84201584A EP 84201584 A EP84201584 A EP 84201584A EP 0179952 B1 EP0179952 B1 EP 0179952B1
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- EP
- European Patent Office
- Prior art keywords
- steel
- sequence
- rolling
- temperature
- range
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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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
- C21D7/00—Modifying the physical properties of iron or steel by deformation
- C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
Definitions
- the present invention relates generally to a method for fabricating structural steel which has high strength and good ductility. '
- the former structure is obtained by making the finishing rolling passes with a high degree of reduction, the latter structure by effecting the finishing rolling passes at a low temperature i.e., at a temperature lower than about 900°C. It is the fine grain of the recrystallized austenite, and in the alternative, the high degree of reduction of the non-recrystallized austenite that provide the basis for forming a fine-grained polygonal ferrite during the subsequent cooling, and thus a basis for the combination of the high strength and good ductility characteristics of the finished steel.
- the present invention provides a method for producing hot rolled steel products having high strength and ductility.
- the method consists in a process for manufacturing hot rolled steel products having high strength and good ductility as hot rolled, the method comprising heating and low-alloy steel having a C content within the range of about 0.02 to 0.15 wt% a Nb content within the range of about 0.015 to 0.100 wt% and a titanium content within the range of about 0.005 to 0.040 wt% to a temperature in the range of about 1150 to 1250°C, hot rolling the steel in one or more passes during a first rolling sequence such that the average austenite grain size is reduced to below 50 J.lm at the end of the first sequence, the first sequence being completed with, the steel at a temperature in the range of about 950 to 1100°C, directly thereafter hot rolling the steel at about 9000 to 1100°C in a second rolling sequence of one or more passes, the reduction in each pass being below about 15%, and cooling the steel to ambient temperature.
- the steel should contain a certain amount of Nb, e.g., about 0.015 to about 0.100%, the lower amount applicable to higher carbon contents, e.g., about .15% C and the higher amount applicable to a carbon content of about 0.05% and lower.
- the steels particularly suitable for the method contain minor amounts, e.g., less than about 2%, of one or more of Si, Mn, Cr and Mo.
- the rolling of the steel is divided into two sequences.
- the first sequence is conducted such that the steel has a uniform, relatively fine grained recrystallized austenite (average less than 50 J.lm measured on a sample section which has been cut out and water quenched) at the end of the sequence.
- the area reduction oftentimes exceeds about 25%, preferably about 30%.
- the second rolling sequence includes one or more light passes, each pass producing less than about 15% in area reduction. This second sequence can immediately following the first sequence without any time waiting for the steel to cool if the temperature of the heat at the end of the first sequence is below about 1100°C, and for lower amounts of Nb, below about 1070°C.
- the procedure of the invention is well adapted to be combined with accelerated cooling by water spraying and the like as an extra strength increasing step performed after the second rolling sequence.
- the steel produced by the method of the invention is compared to steel produced by methods outside the scope of the invention in the following example.
- a number of steel blooms with the approximate analysis of 0.09% C, 0.33% Si, 1.41% Mn, 0.013% P, 0.011 % S, 0.010% N, 0.027% Al, 0.007% Ti and 0.027% Nb were heated to about 1200°C and hot rolled from a thickness of about 120 mm to a thickness of about 20 mm as per the different pass schedules set forth in the following table.
- the temperature of the plate bar was determinsd and sample sections were cut and cooled in water. In each of the sample sections, the austenite grain size existing before the cooling was determined.
- the plates were allowed to cool in air. The mechanical properties of the plates were determined from test bars that were cut normal to the rolling direction. The test results are set forth in the upper portion of the Table.
- the plates with low reductions in the second rolling sequence show a better combination of yield point and transition temperature characteristics than the plates produced with high reductions, such as 35% in the finishing pass. It should also be noted that, with a reduction of 17% in the finishing pass (plate 0), a lower ductility is obtained than in the plates where finishing reductions are limited to below about 15%. A lower ductility is also obtained after the normal rolling with high finishing reduction of 35% (plates D and K).
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Crystallography & Structural Chemistry (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Heat Treatment Of Steel (AREA)
Abstract
Description
- The present invention relates generally to a method for fabricating structural steel which has high strength and good ductility. '
- In the conventional fabrication of structural steel such that the steel has both high strength and good ductility (low transition temperature), small additions of the so-called grain refining element (Nb, V, Ti, B) have been used to a great extent along with a method of hot rolling the steel where the reductions in the individual passes as well as the deformation temperatures are regulated with respect to the structure desired. This method is often termed controlled rolling. The objective of the controlled rolling method is that when the rolling is completed, either a fine-grained recrystallized austenite or a heavy deformed unrecrystallized austenite structure has been produced in the steel.
- The former structure is obtained by making the finishing rolling passes with a high degree of reduction, the latter structure by effecting the finishing rolling passes at a low temperature i.e., at a temperature lower than about 900°C. It is the fine grain of the recrystallized austenite, and in the alternative, the high degree of reduction of the non-recrystallized austenite that provide the basis for forming a fine-grained polygonal ferrite during the subsequent cooling, and thus a basis for the combination of the high strength and good ductility characteristics of the finished steel.
- To obtain a fine grained ferrite after the first alternative of controlled rolling where a fine grained recrystallized austenite is produced after the last pass, it is necessary not only that the steel contains a suitable amount of grain-refining elements, e.g. Nb, but, also that at least in this last pass, reduction is relatively high, i.e., with an area reduction of 30% or more, as the recrystallized grain size of the austenite which is within the range of interest here, uniformly decreases with increasing reduction. On the other hand, in the last stage of the rolling, it is advisable to avoid reductions about and below about 20% as at this point, according to known laws for recrystallization, as there is a risk of producing enlarged recrystallized grains when approaching the so-called critical draft (degree of reduction). At even lower reductions, grain-coarsening has appeared even in grains without recrystallization by the so-called strain induced grain growth. With several such light passes, there also appears a risk for partial recrystallization and the known phenomenon of non-uniform grain size, which means a large loss of ductility and strength for the steel.
- The other method mentioned above, where the rolling is conducted such that recrystallization does not occur, requires that, after a certain degree of roughening, the material must be cooled to a temperature below about 900°C, normally below about 850°C, before the finish rolling begins which causes retained deformation of the austenite. This method is often characterized as a thermomechanical treatment.
- Both methods set forth above have inherent disadvantages with respect to the technical portion of the rolling procedure. High reductions during the finishing pass according to the first method require high roll forces which means that the capacity of the rolling mill can be a limiting factor. In addition, it is more difficult to maintain narrow thickness tolerances with high reductions. Another aggravating circumstance is that, to obtain good results in the form of fine grain in the final structure, the finishing temperature must be kept fairly low, as other conditions being equal, the recrystallized grain size becomes finer, the lower the deformation temperature is, and a low rolling temperature also contributes to an increase in the roll load. Conducting the rolling within the temperature range for non-recrystallization also results in extra loads in the rolling mill and is also an impediment for material flow since the material has to cool between roughening and the finishing rolling which increases the fabrication time and also decreases rolling capacity.
- In accordance with the method of the present invention, it has now been shown, however, that, even with small finishing reductions under certain circumstances, it is possible to obtain steel with equal or better properties as compared with a high finishing reduction and this method can be accomplished without delaying the rolling by utilizing the low finishing temperatures which are required in the second rolling sequence as described above. The finish rolling can, on the contrary, be conducted directly after roughening in the temperature range of about 900 to 1100°C and with low reductions, i.e., below about 15%.
- Thus, the present invention provides a method for producing hot rolled steel products having high strength and ductility. The method consists in a process for manufacturing hot rolled steel products having high strength and good ductility as hot rolled, the method comprising heating and low-alloy steel having a C content within the range of about 0.02 to 0.15 wt% a Nb content within the range of about 0.015 to 0.100 wt% and a titanium content within the range of about 0.005 to 0.040 wt% to a temperature in the range of about 1150 to 1250°C, hot rolling the steel in one or more passes during a first rolling sequence such that the average austenite grain size is reduced to below 50 J.lm at the end of the first sequence, the first sequence being completed with, the steel at a temperature in the range of about 950 to 1100°C, directly thereafter hot rolling the steel at about 9000 to 1100°C in a second rolling sequence of one or more passes, the reduction in each pass being below about 15%, and cooling the steel to ambient temperature.
- The method of the invention is explained in further detail in the following with reference to the drawing in which the sole figure is a graphical representation of the mechanical properties of various steels, some of the steels having been produced by the method of the invention and others by methods outside the scope of the invention.
- More particularly, in the method of the invention, the steel should contain a certain amount of Nb, e.g., about 0.015 to about 0.100%, the lower amount applicable to higher carbon contents, e.g., about .15% C and the higher amount applicable to a carbon content of about 0.05% and lower. Generally, the steels particularly suitable for the method, contain minor amounts, e.g., less than about 2%, of one or more of Si, Mn, Cr and Mo.
- In the present invention, the rolling of the steel is divided into two sequences. The first sequence is conducted such that the steel has a uniform, relatively fine grained recrystallized austenite (average less than 50 J.lm measured on a sample section which has been cut out and water quenched) at the end of the sequence. To achieve this structure, at least in the last pass in this first sequence, the area reduction oftentimes exceeds about 25%, preferably about 30%. The second rolling sequence includes one or more light passes, each pass producing less than about 15% in area reduction. This second sequence can immediately following the first sequence without any time waiting for the steel to cool if the temperature of the heat at the end of the first sequence is below about 1100°C, and for lower amounts of Nb, below about 1070°C.
- To achieve the desired uniform, relatively fine grained austenite at the end of the first rolling sequence, it has been found to be advantageous to include Ti in amounts between about 0.005 and about 0.04%.
- Furthermore, when particularly good ductility in the steel is required, it has proven important that at least one of the light finishing passes be conducted at a sufficiently high temperature (about 950° to 1100°C) so that the precipitation of NbC from the austenite is favored as much as possible. Since the austenite obtained by the procedure of light finishing passes tends to transform to ferrite more readily as compared with the same austenite rolled by conventional methods, the procedure of the invention is well adapted to be combined with accelerated cooling by water spraying and the like as an extra strength increasing step performed after the second rolling sequence.
- The steel produced by the method of the invention is compared to steel produced by methods outside the scope of the invention in the following example.
- A number of steel blooms with the approximate analysis of 0.09% C, 0.33% Si, 1.41% Mn, 0.013% P, 0.011 % S, 0.010% N, 0.027% Al, 0.007% Ti and 0.027% Nb were heated to about 1200°C and hot rolled from a thickness of about 120 mm to a thickness of about 20 mm as per the different pass schedules set forth in the following table. When passing from the first rolling sequence with high degrees of reduction to the second rolling sequence with ligh passes, the temperature of the plate bar was determinsd and sample sections were cut and cooled in water. In each of the sample sections, the austenite grain size existing before the cooling was determined. After the finish rolling of the second sequence, the plates were allowed to cool in air. The mechanical properties of the plates were determined from test bars that were cut normal to the rolling direction. The test results are set forth in the upper portion of the Table.
- In another rolling series, blooms from the same heat as in earlier series were heated to about 1170°C. Hot rolling was performed and, as was done previously, the reductions for the particular passes were varied. The test results from this series are set forth in the lower portion of the Table. For greater clarity, the most important property values, i.c., the yield point and the transition temperature, are graphically presented in the drawing.
- From the results set forth in the Table and the Drawing it can be noted that the plates with low reductions in the second rolling sequence show a better combination of yield point and transition temperature characteristics than the plates produced with high reductions, such as 35% in the finishing pass. It should also be noted that, with a reduction of 17% in the finishing pass (plate 0), a lower ductility is obtained than in the plates where finishing reductions are limited to below about 15%. A lower ductility is also obtained after the normal rolling with high finishing reduction of 35% (plates D and K).
-
Claims (6)
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
DE8484201584T DE3474042D1 (en) | 1984-10-30 | 1984-10-30 | Method for producing high strength steel with good ductility |
AT84201584T ATE37202T1 (en) | 1984-10-30 | 1984-10-30 | PROCESS FOR PRODUCTION OF HIGH STRENGTH AND DUCTILE STEEL. |
EP84201584A EP0179952B1 (en) | 1984-10-30 | 1984-10-30 | Method for producing high strength steel with good ductility |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP84201584A EP0179952B1 (en) | 1984-10-30 | 1984-10-30 | Method for producing high strength steel with good ductility |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0179952A1 EP0179952A1 (en) | 1986-05-07 |
EP0179952B1 true EP0179952B1 (en) | 1988-09-14 |
Family
ID=8192493
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP84201584A Expired EP0179952B1 (en) | 1984-10-30 | 1984-10-30 | Method for producing high strength steel with good ductility |
Country Status (3)
Country | Link |
---|---|
EP (1) | EP0179952B1 (en) |
AT (1) | ATE37202T1 (en) |
DE (1) | DE3474042D1 (en) |
Family Cites Families (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
ZA705046B (en) * | 1969-07-30 | 1971-04-28 | Armco Steel Corp | Process for production of high strength low alloy steel |
US3849209A (en) * | 1972-02-01 | 1974-11-19 | Nippon Steel Corp | Manufacturing method of high tension, high toughness steel |
US4400223A (en) * | 1981-08-21 | 1983-08-23 | Inland Steel Company | Hot rolled steel product and method for producing same |
CS330783A2 (en) * | 1982-07-09 | 1984-06-18 | Mannesmann Ag | Zpusob vyroby plechu s jemnozrnnou strukturou z nizce legovane oceli pro vyrobu trub velkeho prumeru |
-
1984
- 1984-10-30 EP EP84201584A patent/EP0179952B1/en not_active Expired
- 1984-10-30 DE DE8484201584T patent/DE3474042D1/en not_active Expired
- 1984-10-30 AT AT84201584T patent/ATE37202T1/en not_active IP Right Cessation
Also Published As
Publication number | Publication date |
---|---|
ATE37202T1 (en) | 1988-09-15 |
EP0179952A1 (en) | 1986-05-07 |
DE3474042D1 (en) | 1988-10-20 |
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