Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab

Definitions

• This invention relates to a method for manufacturing high tensile steel, having more than 60 Kg/mm of tensile strength as rolled and at the same time having superior cold workability and excellent low temperature toughness, preferably from a hot rolled steel plate having more than 4.5 mm of thickness.
• High tensile steel having more than 60 Kg/mm' grade, tensile strength toughness, weldability, cold workability and the like are, and the usual high tensile steel plate may be classified into two types.
• One type is where the toughness and weldability are regarded as important, since they are mainly used for shipbuilding steel and structural steel.
• a thermal refined steel is used which is relatively thick and is mainly subjected to quenching and tempering and the like.
• Its most important characteristics are toughness and weldability, and it possesses little cold workability properties
• the other type is steel plate having a comparatively small thickness (for instance, less than 9 mm) which is used for truck frames and the like. In case, its most important characteristic is superior cold workability which is bearable under severe cold working, and particularly under bending working and hole expanding working, while toughness and the like are not as important.
• niobium added steel and vanadium added steel have been widely used as a high tensile steel for cold working. These steel are superior in toughness, however, they have such disadvantages that the strength of niobium steel is limited up to 60 Kg/mm grade and vanadium steel may be produced up to 70 Kg/mm grade but is expensive.
• titanium added high tensile steel has been investigated.
• the titanium added steel has the advantage of being cheaper than niobium steel or vanadium steel and is superior in cold workability. But on the other hand, it is inferior to such steels in toughness which is an essential characteristics of a high tensile steel of more than 4.5 mm of thickness.
• Method for manufacturing of high tensile steel having superior cold workability and excellent toughness characterized in that when Ti added Al killed steel, which contains 0.04 to 0.35% of Ti, under the condition of Ti/C 4, and 0.005 to 0.15% of So] Al, is finishing its hot rolling in a temperature range of 815 to 880C, at least more than one time of large reduction of more than 28% of draft percentage for one pass are given in a temperature range of 980 to ll00C.
• Method for manufacturing of high tensile steel having superior cold workability and excellent toughness characterized in that when Ti added Al killed steel, which contains 0.04 to 0.35% of Ti, under the condition of Ti/C 4, and 0.005 to 0.15% of S01 Al, is finishing its hot rolling in a temperature range of 730 to 815C, total draft percentage below 980C is made more than 50%.
• Method for manufacturing of high tensile steel having superior cold workability and excellent toughness according to the items (1) to (3), characterized in that the Ti added Al killed steel contains 0.03 to 0.20% of C, less than or equal to 0.8% of Si, 0.90 to 2.5% of Mn, less than or equal to 0.020% of S, less than or equal to 0.008% of N, the rest of Fe and inevitable elements.
• Method for manufacturing of high tensile steel having superior cold workability and excellent toughness characterized in that Ti added Al killed steel is heated above l200C in a heating furnace for slab and hot-rolled subject to predetermined conditions.
• the preferable range for the carbon is 0.05 to 0.15% C.
• Ti is a very important element for the invention, because it will remarkably increase the strength of the steel by the precipitation of fine Ti(CN), Ti(CN), and at the same time decrease the plasticity of the non-metallic inclusion and increase the cold workability and impact absorbed energy in the transverse direction. But, the object of the invention cannot be attained by less than 0.04% Ti, while its effect will be saturated with more than 0.35% Ti. It is preferable to add 0.05 to 0.30% Ti in order to realize the characteristics of the invention. Also, it is necessary to make Ti/C 4 to increase the strength efficiently, and when the ratio becomes more than 4, the strength of the steel will decrease.
• Si is effective to increase the strength, but when it becomes more than 0.8%, the toughness and weldability will become inferior, so that its upper limit is made as 0.8%. However, if Si is not necessary for the strength, it may not be added. However, normally it is preferable to add more than 0.10% Si in order to obtain the desired strength.
• Mn is an essential element to make the steel strong and tough.
• less than 0.9% Mn is effective, and the cold workability is decreased with more than 2.5% Mn, so that the upper limit is 2.5%, but it is preferable to add less than 2.0% Mn in order to minimize the appearance of the upper bainite structure, which is harmful to the toughness.
• Al is necessary with 0.005 to 0.15% thereof being present as acid soluble Al to assure the effectiveness of to Ti.
• the preferable range is 0.01 to 0.10% sol Al.
• S and N are deleterious elements in the steel of the invention, because they will combine with Ti and decrease the amount of effective Ti which will act on the reinforcement of the steel.
• Reduction of S and N is extremely effective to toughen the steel, and it is necessary to make the S less than 0.020% and the N less than 0.0080%, respectively, to stabilize the characteristics of the invention.
• S and N contents are as small as possible, i.e., S is less than 0.010% and N is less than 0.0060%. Additionally, one or more of the undermentioned elements may be added as occasion demands for the purposes indicated.
• Zr will fix the N and S in the steel to increase the effectiveness of the Ti and to improve the characteristics of the invention, so that it is a very important element. However, less than 0.01% Zr is not effective and its effect will be saturated with more than 0.15% Zr. The optimum content of Zr is 0.03 to 0.06%.
• Mischmetal will also fix the S in the steel and improve the effectiveness of the Ti as well as the cold workability and toughness.
• the mischmetal is present, no effect is observed if (La Ce) is less than 0.005%, and the effect is saturated if (La Ce) is more than 0.10%.
• the optimum content thereof is (La Ce) 0.005-0.05%.
• Nb and V will toughen steel by way of grain refining and stabilize the effect of the invention, and also V will fix N which is deleterious to the invention, so that both of these are effective elements in the steel of the invention. But neither has any effect in the range of less than 0.01%, and in the range of more than 0.10%, not only is the effect is, but the steel becomes very expensive. The optimum content thereof is 0.01 to 0.05%, respectively.
• Ni, Cr and Cu will lower the transformation temperature (Ar of steel and are effective for toughening the steel as well as improving the corrosion resistance thereof, so that less than 0.5% of each may be added if needed.
• B will fix N which is deleterious to the invention and increase the strength of steel, so that less than 0,0030% thereof may be added if needed.
• the heating temperature for the slab in order to obtain high strength, it is necessary to elevate the heating temperature so as to maximize the solid solution of Ti in the austenite structure, and to assure a tensile strength of more than 60 Kglmm which being the object of the invention. Particularly, it is necessary to make the heating temperature more than 1200C, and, it is preferable to make the temperature more than l260C for Kg/mm grade and more than 1280 C for Kg/mm grade.
• the heating temperature for slab is made high to obtain the highest strength in the invention, but under the condition of ordinary hot rolling, the low temperature toughness of steel is remarkably lowered. This is because the austenite grains become remarkably coarse, and in titanium contained steel as well as niobium steel, the low temperature toughness will decrease if the heating temperature is raised to improve the strength. Conversely, if low temperature heating is effected to improve the low temperature toughness, the strength of steel will be lowered. Moreover, the titanium containing steel shows a more remarkable deterioration in toughness from high temperature heating compared with the niobium containing steel.
• the inventors succeeded in developing of high tensile steel plate which has high strength, superior toughness and cold workability at the same time by specifying the rolling condition, as the result of various researches about methods for improving toughness for titanium containing steel which is heated at a temperature of more than 1200C.
• the object is attained by two kinds of rolling conditions, i.e., the high temperature finishing type and low temperature finishing type.
• the characteristic of this high temperature finishing type resides in that said specific rolling of large reduction is effected more than one time between ll00980C, and of course the more times of said large reduction are effected, the more preferable.
• the rolling conditions over ll0OC are not specified, because when the rolling at higher than ll0OC is effected according to the conventional schedule (normally a total draft percentage of more than 50%) and in succession said specific large reduction is effected between ll00980C, the structure will recrystallize into finer austenite grains.
• steel having very superior cold workability and low temperature toughness can be obtained by omitting entirely the rolling at higher than 1 100C and by effecting a number of specific large reductions between I l00-980C.
• the rolling condition in the low temperature region is particularly important. Namely, since the finely recrystallized austenite grains cannot be obtained as in the high temperature finishing type, the structure after transformation is liable to become a ferrite structure of mixed grains or mixed with an upper bainite structure, and steel having such a structure is remarkably inferior in low temperature toughness and cold workability. To solve such a problem, it is necessary to effect a larger reduction in a lower temperature region than the high temperature finishing type in order to make the ferrite grains, after transformation, fine and uniform.
• the cold workability and the low temperature toughness are remarkably improved by making the total draft percentage below 980C more than 50% and by finishing the rolling at a temperature lower than 815C and higher than 730C, and this is the specified rolling conditions of the low temperature finishing method in the present invention.
• the total draft percentage below 850C more than 20%, the cold workability and the low temperature toughness can be improved further.
• the rolling conditions at higher than 980C are not specified, because the object of the invention can be attained so long as the rolling schedule at above 980C which has been effected conventionally (normally the total draft percentage at higher than 980C is effected with more than 60%) is performed and the rolling conditions in the low temperature region are satisfied according to the invention.
• the characteristics of the invention may be improved by minimizing the rolling at higher than 1 C and by increasing the total draft percentage below 1 100C.
• this is not a preferred method, because the productivity will be remarkably decreased and improvement of the quality of steel is not great.
• the difference between the two types of rolling described above is that the former is available for higher rolling finishing temperatures and, therefore has high productivity and, as shown in some examples, the higher the rolling finishing temperature the higher strength can be obtained, so that it the high temperature finishing type is preferred.
• the object of the invention can be attained by'adopting the low temperature fin ishing type, even though the strength will somewhat decrease.
• the low temperature toughness particularly the transition temperature
• the rolling finishing temperature is below 880 and above 730C.
• the steel After the rolling is finished the steel may be aircooled in plate shape as is carried out in the ordinary thick plate process, or taken up in the coil shape. However, higher strength may be obtained when watercooled and coiled.
• the characteristics of the invention may be displayed to the utmost, by making the coiling temperature 500 to 680C and by making the cooling velocity from the finish of rolling to the coiling (hereinafter simply called the cooling velocity) between 3 and 40C/sec.
• Example 1 shows various values of the characteristics of hot rolled steel plate having thickness of 8mm, which is obtained by rolling of the steel shown in Table 1 subject to the rolling condition shown in Table 2, i.e. mechanical properties of the hot rolled steel plate; minimum radius capable of bending 180 in the transverse direction (width of test piece 200mm) and hole expanding limit (initial hole diameter 20mm); vE-20 after 5% tensile strain representing low temperature toughness after cold working and vE-20 without pre-strain.
• Table 2 shows various values of the characteristics of hot rolled steel plate having thickness of 8mm, which is obtained by rolling of the steel shown in Table 1 subject to the rolling condition shown in Table 2, i.e. mechanical properties of the hot rolled steel plate; minimum radius capable of bending 180 in the transverse direction (width of test piece 200mm) and hole expanding limit (initial hole diameter 20mm); vE-20 after 5% tensile strain representing low temperature toughness after cold working and vE-20 without pre-strain.
• EXAMPLE 2 High Temperature Finishing Type Chemical compositions of the steel according to the invention and the comparative steel are shown in Table 3. These steels are molten and produced in converters, and after they are formed as slabs by way of ordinary process, they are rolled subject to the rolling condition shown in Table 4 into hot rolled steel plates of 8.0 mm thickness, of which various values of characteristics are shown in Table 4. And, coiling temperatures were 580-620C, and cooling velocity 7l 1C/sec.
• the rolled products having rolling numbers l7, 18, and 21 according to the high temperature finishing type of the invention have higher strength than the rolled products from the comparative steel. Moreover cold workability of the invention steels is remarkably more superior than those from the comparative steel and also their low temperature toughness is more than or equal to those from the comparative steel plate.
• EXAMPLE 3 High Temperature Finishing Type Chemical compositions of the steel according to the invention and the comparative steel are shown in Table 5. These steels are molten and produced in converters, and after they are formed as slabs by way of ordinary process, they are rolled subject to the rolling condition shown in Table 6 into hot rolled steel plates of 6.0l0.0mm thickness, of which various values of characteristics are shown in Table 7.
• the hot rolled steel plates produced according to the high temperature finishing type of the invention have higher strength and superior cold workability to any comparative steel as well as superior low temperature toughness more than or equal to the comparative steel.
• Steel 2 of the invention made by adding Zr to steel 1 of the invention and steel 3 of the invention made by adding Nb and V to steel 1 are more improved than steel 1 of the invention in any of strength, cold workability and low temperature toughness, thus it is understood that the effect of the invention is still more displayed by the addition of Zr, Nb and V,
• Table 8 shows various values of the characteristics of hot rolled steel plate having thickness of 8mm, which is obtained by rolling of the steel shown in Table 1 subject to the rolling condition shown in Table 8, i.e. mechanical properties of the hot rolled steel plate; minimum radius capable of bending 180 in the transverse direction I 8 and hole expanding limit representing cold workability of high tensile steel plate; and vE-2O after 5% tensile strain representing low temperature toughness after cold working and vE-20 without pre-strain.
• EXAMPLE 5 Low Temperature Finishing Type Steel according to the invention and comparative steel having chemical compositions shown in Table 3 are rolled subject to the rolling condition shown in Table 9 into hot rolled steel plate of 8.0mm thickness, of which various kinds of characteristic values are shown in Table 9. And, coiling temperature therefor are 580-620C and cooling velocity from the end of rolling to the coiling are 7-l 1C/sec.
• the steel plate having rolling number 37 which does not satisfy the rolling condition of the invention, has equal cold workability to comparative steel plate (rolling number 22, 23), but the low temperature is inferior to the comparative steel plate.
• the steel plate having rolling number 38 obtained according to the low temperature finishing type of the method of the invention has higher strength than com parative steel, nevertheless cold workability of the invention steel plate is remarkably superior and low temperature toughness thereof has characteristics of more than or equal to comparative steel plate.
• EXAMPLE 6 Low Temperature Finishing Type Chemical compositions of the steel within the range of the invention and of the comparative steel are shown in Table 5. These steels are rolled subject to rolling condition shown in Table 10 into hot rolled steel plate having thickness of 6.0lO. Omm, of which various kinds of characteristic values are shown in Table l 1.
• the hot rolled steel plate formed according to the low temperature finishing type of the invention has higher strength and superior cold workability to the comparative steel as well as superior low temperature toughness more than or equal to the comparative steel Steel 2 of the invention made by adding Zr to steel 1 of the invention and steel 3 of the invention made by adding Nb and V to steel 1 are more improved than steel 1 of the invention in any of strength, cold workability and low temperature toughness, thus it isunderstood that the effect of the invention is still more displayed by the addition of Zr, Nb and V.
• the high tensile steel plate produced according to the invention has strength of more than Kg/mm having superior cold workability and low temperature toughness at the same time compared with usual high strength steel plates, and its carbon equivalent is low so that its weldability is very good.
• drawability and stretchability are more than or equal to those of usual steel, so that the high tensile steel of the invention is not only adapted for automobile frame and the like but also best suited for making line-pipe because of good low temperature toughness as V 2.1 kg-m and V C according to and good weldability.
• the present steel .118 No. 4, 5mm sub size specimen.
• Example 1 Chemical Compositions of Examples 1 and 4 (wt%) C Si Mn 7 P S Ti Sol. N Ti/C Rest 0.10 0.33 1.27 0.012 0.006 0.12 0.031 0.0040 1.2 Fe and inevitable I elements Table 2 Rolling conditions and various kinds of characteristic values of Example 1 Rolling Condition Roll- Ro1l Heating Total Rolling Total Rolling ing ing temp.of draft between draft finishpattern No. slab percen- 1 C/ percening (C) tage 980C tage temp.

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• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
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• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
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• Heat Treatment Of Sheet Steel (AREA)

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Citations (5)

• 1973
  • 1973-12-28 US US429274A patent/US3925111A/en not_active Expired - Lifetime
  • 1973-12-28 FR FR7347037A patent/FR2212434B1/fr not_active Expired
  • 1973-12-28 IT IT7332331A patent/IT1002387B/it active
  • 1973-12-28 CA CA189,115A patent/CA1003311A/en not_active Expired
  • 1973-12-29 DE DE2365156A patent/DE2365156B2/de not_active Ceased
  • 1973-12-31 GB GB6003573A patent/GB1455440A/en not_active Expired
  • 1973-12-31 SE SE7317631A patent/SE417985C/xx unknown
• 1982
  • 1982-09-17 SE SE8205332A patent/SE452339B/sv unknown

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