Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/021—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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
• • 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
  • C21D2211/002—Bainite
• • 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

Definitions

• the invention relates to a method for manufacturing a hot rolled steel strip product having a wall thickness of 2 to 12 mm using steel whose composition in percentage by weight is
• Ni ⁇ 0.5 and the rest iron and unavoidable impurities.
• Low carbon content is excellent for providing steel with good welding characteristics. Also the low carbon equivalent of steel has a positive effect to good weldability.
• the invention further relates to a steel product with a wall thickness of 2 to 12 mm and a composition as described above.
• EP 1319725 discloses a method for manufacturing a steel strip having the above composition.
• the strength of the steel strip thus manufactured is relatively high, its yield strength exceeding 690 MPa, combined with a relatively high percentage of elongation after fracture (12 to 21%).
• these mechanical properties are arrived at by subjecting the steel to a two-step cooling.
• an extremely fast cooling is carried out, the cooling rate being over 150°C/s after hot rolling, followed by a pause of 3 to 10 seconds without active cooling, after which a second cooling step is carried out to the coiling temperature of the steel strip to be manufactured, the temperature being chosen according to the desired strength.
• the recommended coiling temperature for yield strengths exceeding 690 MPa is 580 0 C.
• the high cooling rate of over 150°C/s at the first quenching step may be obtained only at low strip thicknesses, and the publication only discusses strip thicknesses lower than 4 mm.
• the cooling pause is meant to provide time for a phase change, during which the yield strength of the material in particular decreases and the yield strength/tensile strength value decreases compared with continued cooling.
• the publication does not disclose how a yield strength of over 690 MPa is obtained in the steel when the coiling temperature is below 580 0 C.
• the publication shows that the yield strength obtained in a coiling temperature of less than 580 0 C remains below 690 MPa.
• Said two-step cooling is in practice more complicated to carry out than a one-step cooling and requires more complex production equipment.
• the bendability of the steel strip obtained by two-step cooling is not particularly good, although the steel strip has relatively good values for percentage of elongation after fracture. Bendability means the ability of steel strip to bend to a small bending radius without surface damage emerging at the bending point.
• Two-step cooling has not succeeded in providing steel with particularly good impact strength values at low temperatures in combination with high strength.
• An object of the invention is to overcome said drawbacks of the prior art and to provide a method that is easy to implement for manufacturing strip steel product, typically a steel strip, of high strength and a particularly good bendability, the strip steel product having a chemical composition as mentioned above.
• the method of the invention is characterized by
• the invention has surprisingly shown that said steel composition is capable of producing high-strength steel which also has good bend- ability. Also surprisingly, it was discovered that the strength of the steel is isotropic, i.e. its yield strength does not vary considerably irrespective of whether it is measured lengthwise or crosswise in relation to the rolling direction.
• the direct quenching rate is preferably 120 °C/s at the most, because this enables to obtain a steel microstructure that provides the steel with particularly good mechanical properties, including good impact strength combined with good bendability.
• the end temperature in direct quenching is preferably 100 0 C at the most, because this enables to obtain a planar strip with also planar and even edges after the quenching.
• the steep strip is preferably direct quenched directly to the coiling temperature and coiled.
• the processing of the steel strip is preferably thermome- chanical, and thus no tempering is carried out after the direct quenching. It has been observed that a steel product manufactured with the method has good mechanical properties although no tempering adding the costs is required on the product. Tempering does not significantly improve the mechanical properties of the product, and it complicates the process.
• the major advantages of the method of the invention are that it allows a steel product with good mechanical properties, including bendability, and a predetermined composition to be manufactured in a simple and economical manner and with simple equipment.
• the invention further relates to a product manufactured in method steps of the invention.
• the steel strip product of the invention having a wall thickness of 2 to 12 mm and composed in percentage by weight of
• Ni: ⁇ 0.5, the rest being iron and inevitable impurities, is characterized in that the micro structure of the steel is substantially low carbon ferritic and/or low carbon bain- itic, that upon bending the steel sustains a bending radius of R ⁇ 0.75 t without fractures or cracks visible to the eye, t being the thickness of the steel product, that its yield strength is 650 - 800 MPa and that its percentage of elongation after fracture is at least 12%.
• High strength has been obtained although the microstructure of the steel mostly consists of a low carbon ferrite and/or bainite, without containing significant amounts of carbon-rich martensite or carbon-rich bainite.
• the dominant phase consists of ferrite with an almost fully ferritic microstructure, as recommended, and small amounts of bainite and/or martensite and/or residue austenite in extremely small islands of enriched carbon content.
• a significant reason for the obtained high strength is the use of niobium and titanium as micro-alloy elements in the steel produced with the method. Both niobium and titanium must be used.
• the major advantages of the steel product of the invention are its excellent mechanical properties, including bendability and shear characteristics and impact strength values, in relation to its composition.
• the steel is also well applicable in arctic conditions.
• the steel of the invention is extremely useful due to its properties related to engineering works, because its weldabil- ity is good and its isotropic strength properties allow a very efficient optimization of its use.
• the small bending radius particularly facilitates the work of bent product designers.
• the steel strip product of the invention is specifically well suited for use as strong structure steel. BRIEF DISCLOSURE OF THE FIGURES
• FIG 1 shows the method steps of the invention
• Figure 2 is a schematic view of a V-bending in bending tests
• Figure 3 illustrates an example of a successful bending test result
• Figure 4 illustrates an example of a failed bending test result
• Figure 5 presents transition curves for charpy-V obtained with the steel of the invention and a reference steel
• Figure 6 illustrates the connection between yield strength isotropy and strip rolling
• Figure 7 illustrates the connection between yield strength isotropy and coiling temperature.
• Figure 1 shows the method steps of the invention for producing a steel strip product with a wall thickness of 2 to 12mm.
• the manufacture starts with a work piece of steel whose composition in percentage by weight is
• Ni ⁇ 0.5 and the rest iron and unavoidable impurities.
• the steel has a low carbon content C of 0.04 to 0.08%, which is advantageous in view of the impact strength, bendability and weldabil- ity of the material.
• Silicon, Si may be used in an amount of 0 to 0.50% as a killing agent (in addition to aluminium) and for ferrite reinforcement. If a particularly good surface quality is to be aimed at, the silicon content must be limited to below 0.25%.
• the alloying content of manganese, Mn is 1.0 to 2.2%. Because of the low carbon content, steel is not prone to manganese and carbon segregation during casting, which enhances the homogeneity of the micro- structure also at relatively high contents of Mn. Preferably at least 1.3% of manganese is alloyed to achieve a high strength and to ensure a weldability of 2.0% at the most.
• the steel of the invention may be cut into precisely dimensioned pieces both thermally (e.g. by laser and plasma) and mechanically. It has been observed that a piece with a relatively smooth cutting surface is obtained. This has an advantageous effect on fatigue strength. In addition, low carbon content prevents the cutting surface from becoming rough during thermal cutting and reduces maximum hardness, the cutting surface being less prone to brittle and crack during forming of the piece and in the conditions of use thereof.
• the cutting gap may be set at a value of 10 to 15% of the sheet thickness, the cutting result being still smooth and non- fractured and hence separate grinding of the cutting surface or thermal cutting is not needed, which significantly reduces working allowances and decreases the number of manufacturing steps, thereby enhancing the manufacturing process.
• the amounts of phosphor, P, (0.03% at the most) and sulphur, S, (0.015% at the most) present as impurities are to be restricted.
• the maximum amount of P is preferably 0.015% and that of S preferably 0.005%.
• the properties may be improved, when necessary, by treatment with molten Ca or CaSi.
• As a killing agent aluminium Al 0.01 - 0.15% is used.
• the amount of aluminium used is preferably 0.05% at the most.
• the amount of nitrogen, N, used is 0.01% at the most, because when present in steel containing titanium, nitrogen forms hard titanium nitride particles that impair the bendability of the steel.
• the preferred amount of nitrogen used is 0.006% at the most.
• the content of copper, Cu is to be reduced to 0.3% at the most to ensure excellent surface quality for a hot-rolled strip. If the copper con- tent exceeds 0.3%, it is recommended to alloy also nickel, Ni, in an amount equal to at least 0.25 times the Cu content. Although steel achieves its good properties also without copper, it may be used, when necessary, to slightly increase strength.
• the Cu content is 0.5% at the most.
• An alloy 0.3 to 0.5% of copper and at least 0.1% of nickel is preferably used particularly for thick strips of 8 to 12 mm, for example.
• Ni is restricted to 0.5% at the most. Although steel achieves its excellent strength properties also without the blending of Ni, it may slightly increase strength, when necessary.
• Titanium, T may be alloyed to achieve a desired strength level. Typically 0.06 to 0.16%, although higher Ti levels could be used as well, but in that case its strength increasing effect is extremely small and may complicate the casting of the work piece. Lower Ti percentages are not used, because then high strength is difficult to obtain without using a more expensive alloying or increasing the carbon content to over 0.08%. Surprisingly the invention has shown that even at low temperatures, such as -40 0 C and -60 0 C, titanium does not lower significantly the impact strength of the basic agent, as shown by the measurement results of Table 3.
• Chromium, Cr, and molybdene, Mo do not need to be alloyed. They are elements that increase hardening and have a disadvantageous effect on weldability, at least in higher amounts. For this reason Cr is restricted to a maximum content of 0.2% and, similarly, Mo to a maximum content of 0.2%.
• the amount of chromium is preferably less than 0.1%.
• Molybdene is preferably allowed in an amount of 0.10%, and most preferably 0.5%, at the most, because the mechanical properties of the steel of the invention are most preferably achieved by alloying titanium which provides more affordable alloying element costs than molybdene. Molybdene may even be harmful for strength in a direct quenched steel strip product of the invention. In any case, added molybdene does not significantly improve the strength of the steel strip product of the invention, when the product is produced by thermomechanical treatment.
• Vanadine, V does not need to be alloyed. In addition, it increases unnecessary hardening and has a disadvantageous effect on weld- ability at least in high concentrations. For this reason, V is restricted to a maximum content of 0.1%.
• Nb and Ti concentrations are restricted as follows: Nb: 0.04 - 0.06% and Ti: 0.06 - 0.10% for reducing the rolling forces and, at the same time, a vanadium concentration V of 0.06 - 0.10% may be selected to obtain high strength.
• silicon may be advantageously added in an amount of Si: 0.30 - 0.50% to increase strength, as shown in Table 1 of the tests run with an experimental composition E1.
• the sum of the niobium, titanium and vanadium concentrations is greater than 0.15%, i.e. Ti + Nb + V > 0.15%, the steel strip product being applicable as a particularly strong structural steel.
• the steel strip product of the invention is excellent to bend (fold) and to weld e.g. by autogenous high frequency (HF) welding into a tube or a tube beam.
• HF high frequency
• the work piece of steel is 210 mm thick, for example, and heated to an austenizing temperature of 1280 0 C, where it is kept for about 3 hours.
• the thickness of the steel work piece may differ from the one disclosed here and the austenizing temperature may be differently chosen, but a range of 1200 - 1350 0 C is recommended. If the austenizing temperature is below the lower limit given, there is a risk that the microalloying elements do not dissolve into the austenite, i.e. a homogenous austenite is not obtained.
• the annealing time varies within a range of 2 to 4 hours.
• the carbon equivalent C + Mn/6 + (Cr + Mo +V)/5 + Ni + Cu)/15 for steel is preferably not higher than 0.45, which guarantees a good weldability of the steel.
• the steel work piece After austenizing, the steel work piece is hot-rolled at a temperature of 950 - 1250 0 C to a thickness which is typically 25 - 50 mm and then immediately transferred to a strip rolling mill to be rolled into a strip with a final thickness of 2 - 12 mm.
• the recommended final thickness of the steel strip is at least 4 mm. It also recommended that the final thickness does not exceed 10 mm.
• the number of passes in the strip roll mill is typically 5 to 7.
• the last pass in the strip roll mill is carried out at a temperature range of 760 - 960 0 C, the recommendation being 780 - 850 0 C.
• the direct quenching of the steel strip starts within 15 seconds.
• the temperature of the steel strip must be at least 700 0 C.
• the direct quenching is carried out as a water quenching at a quenching rate of 30 - 150°C/s, the recommended upper limit being 12O 0 CVs at the most.
• the direct quenching continues up to a temperature of 300 0 C at the most, the recommended temperature being 100 0 C.
• the steel is coiled.
• the coiling temperature may fall within a temperature range of 30 - 300 0 C.
• a recommended initial coiling temperature is 100 0 C at the most, because when steel is coiled at a temperature exceeding 100 0 C, a discontinuous steam cushion complicating the process may form onto the steel surface.
• the micro- structure of the steel becomes homogenous and consists of a dominant phase, which is preferably low carbon ferrite and/or low carbon bainite.
• the amount of the dominant phase is typically over 90%.
• extremely low amounts of high carbon bainite and/or residual austenite and/or martensite is present in extremely high carbon groups.
• the average grain size in the micro- structure is small, preferably approximately 2 - 4 micrometers. It is also essential that the microstructure does not contain big grains in the first place and therefore the steel has particularly good bending characteristics taking into consideration the strength of the steel.
• the grain size must be as uniform and fine as possible, which is achieved by the method of the invention.
• Tables 1 to 3 below provide examples of the concentrations and manufacturing parameters of the steel of the invention and of the strength and toughness values obtained with them. For the sake of comparison, Tables 2 and 3 also contain manufacturing parameters not belonging to the scope of the method of the invention, i.e. treatments not corresponding to the method of the invention. In Table 2 on the manufacturing parameters and in the table on the mechanical strength properties reference tests have been indicated with R.
• a further topic of examination are the bending characteristics obtained with the treatments of the invention, these being compared with the bending characteristics obtained by manufacturing parameters remaining outside the scope of the method, see Tables 3 and 4, Steel B3Q23 (bending test a) according to the invention)and Steel A3M33 (bending test b) outside the invention).
• Indication T_f in Table 2 denotes the temperature at the last rolling pass, indication T_c denoting the temperature at the start of the coiling, indication Th denoting the thickness of the steel strip and indication Wi denoting the width of the steel strip.
• T denotes a sample whose strength and toughness have been determined in a direction transverse to the roll direction.
• Ending L denotes a sample whose strength and toughness have been determined in the rolling direction.
• Tables 2 and 3 show that the impact strength values are good and the strength is isotropically high, when direct quenching is carried out to a low temperature (50 0 C).
• the yield strength of steels according to the invention is 635 - 829 MPa.
• the percentage of elongation after fracture A5 is at least 12% and typically at least 15%.
• the yield ratio (yield strength/break strength) of the steels is about 0.8 - 0.95.
• said uniform quality is present in steels in which the final rolling temperature is low (below 890 0 C) and/or coiled at a low temperature (coiling temperatures 50 0 C).
• Uniform quality is advantageous, because when a steel strip for different purposes is being designed, there is no need to take into account the fact that the steel strip has a higher strength in rolling direction than in the direction that is transverse to the rolling direction. Hence it is possible to take advantage of the high strength of the steel strip in all situations, i.e. also when cutting blanks that are processed into products which in use receive their greatest load in the direction that corresponds to the rolling direction of the steel strip. Further, the use of the steel strip may be optimized, because varia- tions in strength in relation to the loading direction do not need to be taken into account.
• isotropic strength properties probably contribute to the formation of bends of uniform quality irrespective of the bending direction (longitudinal/transverse), which further improves the applicability of the steel strip product of the invention.
• Table 4 shows that bendability in longitudinal bending, which is known to be problematic, is excellent.
• Steel sample B5Q3 for example, in longitudinal bending allows an RfT value of 1.3 to be reached. Transverse bending of this steel still succeeds at R7t value 0.3.
• the bending has been carried out by a prior art method as a V-bend between the upper and the lower tool, Figure 3 illustrating the principle.
• the bending method used is free bending with a V-opening width V of 100 mm.
• the test pieces were bent in both directions, whereby they were bent into Z-shapes.
• steel B3Q23 (bending test a in Table 2) has a far better bendability than steel A3M33 (bending test b in Table 2).
• the ratio of the bending radius to material strength (R/t) may be even 0.4, whereas the ratio achieved by conventionally manufactured reference steel is only about 1.6.
• the conclusion drawn from Tables 1 to 4 and Figure 5 is that the in the method of the invention direct quenching is performed to a temperature of 300 0 C at the most.
• the impact strength of the steel of the invention at a temperature of -20 0 C is at least 200 J/cm 2 and/or at a temperature of -40 0 C at least 190 J/cm 2 and/or at a temperature of -60 0 C at least 180 J/cm 2 .
• Example 1 A strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (A1) of Table 1.
• the roll parameters (A1M33) are shown in Table 2.
• the results (A1M33) are shown in Table 3.
• the results show that when the strip is coiled at a coiling temperature of 600 0 C, an excellent strength is achieved, but impact strength, however, remains on a normal level only.
• yield strengths are clearly different in different testing directions, which is normal for micro-alloy steels conventionally rolled thermomechanically. Elongation level is normal.
• a strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (A1) of Table 1.
• the roll parameters (A1 M63) are shown in Table 2.
• the results are shown in Table 3.
• the results show that coiling at a relatively low coiling temperature (about 480 0 C) produces steel (A1M63) of a low strength but improved impact energy. Elongation level is normal. Cooling the strip to a still lower coiling temperature (about 250 0 C) increases the strength of steel (A1Q61) close to the normal level with clearly improved impact energy. Elongation remains below the normal level.
• Example 3 A strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (A1) of Table 1.
• the roll parameters (A1 M83) are shown in Table 2.
• the results (A1M83) are shown in Table 3.
• the results show that coiling at a very low coiling temperature (about 50°C) raises strength to a good level close to normal with impact energy still clearly better than the normal level. Elongation remains below normal level.
• IO0631 Example 4 A strip rolling mill was used to roll a hot strip having a thickness of 8 mm and composition (B4) of Table 1.
• the roll parameters (B4Q23) are shown in Table 2 and the corresponding results in Table 3.
• the results show that coiling at a very low coiling temperature (about 50 0 C) increases strength to a normal level and provides an impact energy that is clearly better than normal. Again, it is noteworthy that yield strengths in the rolling direction are nearly the same both crosswise and lengthwise. Elongation is slightly below normal.
• IO0641 Example 5 A strip rolling mill was used to roll a hot strip having a thickness of 10 mm and composition (B2) of Table 1.
• the roll parameters (B2L13) are shown in Table 2, the corresponding results in Table 3.
• Example 6 A strip rolling mill was used to roll a hot strip having a thickness of 10 mm and composition (B3) of Table 1.
• the roll parameters (B3Q25) are shown in Table 2, the corresponding results in Table 3.
• the results show that at a very low rolling temperature (about 800 0 C) and a very low coiling temperature (about 50 0 C) yield strength increases to a normal level also for a thick strip, with impact energy still on a good level.
• yield strengths in relation to the rolling direction are the same both crosswise and lengthwise. Elongation is slightly below the normal level.
• Example 7 A strip rolling mill was used to roll a hot strip having a thickness of 5 mm and composition (D1) of Table 1.
• the roll parameters (D1Q63) are shown in Table 2, the corresponding results in Table 3.
• the results show that reduction of alloying elements (Ti, Nb in particular) decrease strength heavily when the steel is rapidly cooled to a temperature of 50 0 C. Elongation and impact strength are at a good level.
• r00671 Example 8.
• a strip rolling mill was used to roll a hot strip having a thickness of 7.7 mm and a composition (C1) of Table 1 , the strip being then used for manufacturing a HF-welded quadratic tube beam with dimensions of 100mm x 250mm.
• the rolling parameters (C1Q35) are shown in Table 2 and the results measured from the tube beam in Table 3.
• the measured strength values are strengths obtained after the formation of the tube beams. Because of cold forming taking place in the manufacture of tube beams, impact strength values typically drop a little. The results show that the steel according to the method is well suited also for manufacturing high-strength tube beams.
• Example 9 A strip rolling mill was used to roll a hot strip having a thickness of 8 mm and compositions (A3 and B4) of Table 1.
• the rolling parameters (A3M33 and B3Q23) are shown in Table 2 and the corresponding test results measured from the strip in Table 3.
• Table 4 shows a comparison of the bending of these steels (A3M33 and B3Q23), whereby it is noted that a direct quenched steel B3Q23 sustains bending well even at R/t value 0.4.
• Steel A3M33 cooled to a temperature of about 600 0 C can be successfully bent to R/t value 1.6.
• Figure 5 compares the impact strength values of steels A2M33 and B4Q23 at different testing temperatures by a Charpy V impact test.
• the compositions and manufacturing parameters of steels A2M33 and B4Q23 are shown in Tables 1 and 2.
• Direct quenched steel B4Q33 proves to be clearly better, maintaining its strength also at extremely low temperatures.
• Example 11 A strip rolling mill was used to roll hot strip having thicknesses of 5 and 6mm and composition (E 1) of Table 1.
• the rolling parameters (E1Q11 and E1Q33) are shown in Table 2 and the corresponding test results measured from the strip in Table 3.
• the results show that a steel strip product of the invention may be manufactured also in small thicknesses, for example by selecting the niobium, titan and vanadine contents of the steel as follows: Nb: 0.04 - 0.06%, Ti: 0.06 - 0.10% and V: 0.06 - 0.1%.
• Example 12 A strip rolling mill was used to roll a hot strip having a thickness of 12 mm and compositions (F1 and F2) of Table 1.
• the rolling parameters (F1Q23 and F2Q43) are shown in Table 2 and the corresponding test results measured from the strip in Table 3. The results show that a steel strip product of the invention can be manufactured also with thick dimensions. In addition, this example further proves that uniform quality is obtained by direct quenching carried out to a temperature below 100°C and/or by using a strip rolling end temperature that is below 890 0 C.

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