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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12201—Width or thickness variation or marginal cuts repeating longitudinally

Definitions

• the invention relates to a strip of steel having a variable thickness in its length direction with at least thicker and thinner sections, the strip having been cold rolled to form the thicker and thinner sections, one thicker and one thinner section having a length of at most a few metres.
• a strip of steel having a variable thickness in its length direction is often made such that the strip has a repetitive thickness variation, wherein a thicker section of the strip is followed by a thinner section which is thereafter followed by a thicker section, and this is repeated over the length of the strip.
• the thinner sections all have approximately the same length, and so have the thicker sections.
• One thicker and one thinner section have a length of at most a few metres.
• One strip can have at least a few hundred thicker and thinner sections.
• the thicker and thinner sections have a thickness between a few tenths of a millimetre and a few millimetres.
• the strip is rolled into three or more different thicknesses which repeat along the length of the strip.
• a transitional section will be formed in which the thickness of the strip gradually changes from the thickness of one section to the thickness of the following section.
• the length of this transitional section is determined by the thickness change between the sections, the rolling speed and the speed with which the cold rolling mill can change the distance between the rolls, to mention the most important parameters.
• the length of the transitional section is of the same order as the length of the thicker and thinner sections or even shorter.
• the width of the strip can be from a few decimetre up to about two meter.
• the strip can be slit into two or more strips having a reduced width, but this is not always required.
• Such a strip is cut into pieces which are called tailor rolled blanks (TRBs), for instance for the automotive industry.
• TRBs tailor rolled blanks
• the blanks thus have at least two different thicknesses over their lengths, as required for the purpose and place they are used in.
• the steel strip has to be annealed to release the stresses in the strip and/or to recrystallise the strip. Annealing of a steel strip without thickness variations can be performed either by batch annealing or by continuous annealing.
• Annealing of steel strip having a variable thickness in its length direction is performed only by batch annealing, so as to provide the same temperature to both the thinner and the thicker sections.
• Batch annealing though is more expensive than continuous annealing, and it usually has a somewhat deteriorating effect on the strength of the steel. Due to the slow heating and cooling rate experienced in the case of batch annealing it is not attractive for all steel types, especially for steel types having a higher strength.
• At least one of these objects is reached using a strip of steel having a variable thickness in its length direction with at least thicker sections and thinner sections, the strip having been cold rolled to form the thicker and thinner sections, one thicker and one thinner section having a length of at most a few meter, which strip has been annealed, wherein the annealing is performed by continuous annealing.
• Continuous annealing has the advantage that it is a faster process and provides new and better tailor rolled blanks. Tailor rolled blanks produced using continuous annealing have better mechanical properties than tailor rolled blanks produced using batch annealing having the same composition and rolling history, such as a higher mechanical strength, and so have the strips of steel from which such tailor rolled blanks are produced.
• a strip having a variable thickness will have different mechanical properties in the different sections because of the variation in cold rolling reduction, whereas the annealing temperature and heating rate will be the same in all sections.
• a higher cold rolling reduction will produce different mechanical properties, for instance a higher yield strength.
• the advantage of continuous annealing over batch annealing is that with continuous annealing the sections with a variable thickness will also experience different temperatures and heating rates. In a thinner section the temperature will reach higher values than in a thicker section. The higher annealing temperature experienced in the thinner sections will reduce the strength, which partly or completely compensates the effect of the higher cold rolling reduction.
• the yield strength of the thicker sections is equal to or higher than the yield strength of the thinner sections. This is advantageous because the TRBs made from such strips are used for parts that need to have more strength in the thicker section than in the thinner section.
• the steel strip is a DP, TRIP or multi phase high strength steel.
• These high strength steels can not be produced using batch annealing, so continuous annealing makes the use of DP, TRIP and multi phase high strength steels possible for producing strip having a variable thickness and the TRBs made thereof.
• the steel strip is a HSLA steel or a low carbon steel.
• Using continuous annealing for these steel types provides strip having a variable thickness and TRBs made thereof that have better mechanical properties, such as a higher yield strength.
• the strip of steel is a HSLA steel or low carbon steel, preferably only the thinner sections are recrystallised and the difference in yield strength of the thicker and thinner sections is smaller than in the same HSLA or low carbon steel strip that has been batch annealed.
• the recrystallised thinner sections reach a higher temperature due to the continuous annealing, compared to batch annealing, and therefore the thinner sections have for instance a higher yield strength.
• the yield strengths of the thicker and thinner sections have values that are more near to each other than the corresponding values of batch annealed strip having the same composition.
• the composition of the steel has lower values of alloying elements than in a batch annealed HSLA or low carbon steel having the same yield strength of the thinner sections. Since the yield strength is better for continuous annealed strip having a variable thickness then for batch annealed strip with the same composition, it is possible to provide strip having a variable thickness with the same yield strength as batch annealed strip, using a continuous annealed strip having lower values of alloying elements (which strip, when batch annealed, would have a lower yield strength). Thus, the steel strip having a variable thickness is cheaper.
• the steel has the following composition in wt%:
• This is a normal composition for a low carbon steel, wherein the steel can contain one or more of the optional alloying elements Si, P, Nb, V and Ti.
• the steel contains C, Mn, and optionally Si, P, Nb, V, and Ti, the remainder being iron and inevitable impurities, and is characterised by the equation:
• YS > 250 + 225(Mn/6 + Si/24) + 716P + 2938Nb + 600V + 2000Ti [MPa] with Mn, Si, P, Nb, V, Ti in wt% and YS being the yield strength in the thinner sections of the strip.
• This equation shows that by using continuous annealing a high yield strength can be achieved in the thinner sections of the strip with less alloying elements than would be needed when such a strip had been batch annealed.
• the steel is characterised by the equation YS > 270 + 225(Mn/6 + Si/24) + 716P + 2938Nb + 600V + 2000Ti [MPa]. Due to optimised process conditions for the continuous annealing, the steel strip having a variable thickness will reach the higher yield strength according to this equation.
• the strip of steel is characterised by the equation A80 > -0.05*YS + 40 with A80 being the total elongation in the thinner sections of the strip and YS being the yield strength in the thinner sections of the strip.
• A80 being the total elongation in the thinner sections of the strip
• YS being the yield strength in the thinner sections of the strip.
• This equation shows that continuous annealed strip having a variable thickness will have product properties that are often required, that is a high total elongation combined with a high yield strength. A high total elongation is for instance required for stamping parts.
• the steel in the thinner sections has a tensile strength above 600 MPa and a yield strength below 400 MPa.
• the steel of this strip is for instance a dual phase steel that has been temper rolled.
• the steel in the thinner sections has a tensile strength above 600 MPa and a yield strength below 300 MPa.
• the lower yield strength is reached by an optimised rolling schedule before and/or after the continuous annealing of the strip.
• the steel in the thinner sections has a tensile strength above 800 MPa and a yield strength below 550 MPa.
• the steel of this strip can be a dual phase steel as well, having a composition with higher amounts of alloying elements, which has been temper rolled.
• the steel in the thinner sections has a tensile strength above 800 MPa and a yield strength below 450 MPa.
• the lower yield strength is reached by an optimised rolling schedule before and/or after the continuous annealing of the strip.
• the steel in the thinner sections has a tensile strength above 980 MPa and a yield strength below 750 MPa.
• the steel can be a dual phase steel, having a composition having still higher amounts of alloying elements, which has been temper rolled.
• the steel in the thinner sections has a tensile strength above 980 MPa and a yield strength below 650 MPa. Again, the lower yield strength is reached by an optimised rolling schedule before and/or after the continuous annealing of the strip.
• a tailor rolled blank produced from a strip of steel according to the description above.
• the tailor rolled blanks are cut from the strip having a variable thickness, and these tailor rolled blanks are used in the automotive industry, for instance.
• the method according to the invention will be elucidated referring to the figures and examples below.
• Figure 1 shows a schematic representation of a continuous annealing time- temperature cycle
• Figure 2 shows a schematic representation of the differences in temperature, heating and cooling rates between thin and thick sections of the TRB;
• Figure 3 shows a schematic representation of the use of selective heating to adjust the differences in temperature, heating and cooling rates between thin and thick sections of the TRB.
• Figure 4 shows a comparison between the yield strength measured for a number of steel types that are batch annealed and continuous annealed.
• the temperature T is presented along the vertical axis and time t along the horizontal axis.
• FIG 1 a typical continuous annealing time-temperature curve is presented.
• the process in a continuous annealing line for steel strip often consists of a sequential of different heating and cooling sections.
• a fast heating section Hl
• a slow heating section H2
• This maximum temperature is normally higher than the recrystallisation temperature to ensure complete recrystallisation of the microstructure of the steel.
• the maximum temperature In the case of high strength steels such as DP, TRIP and multi-phase high- strength steels the maximum temperature must be higher than 720 0 C to bring the material in the two-phase region of austenite and ferrite.
• FIG. 2 the effect of continuous annealing on TRB is illustrated.
• the sections with variation in thickness will show a difference in heating and cooling rates, and as a result will follow different time-temperature cycles.
• the line Sl indicates the time- temperature cycle for the thinner sections of the TRB
• the line S2 indicates the time- temperature cycle for the thicker sections of the TRB.
• the exact time- temperature profile depends on many parameters, such as the thickness profile of the strip, line speed, width of the strip, heating and cooling capacity of individual sections in the continuous annealing line.
• Noteworthy in figure 2 is the relatively large difference in temperature at the end of the fast heating section ( ⁇ T1).
• the difference ⁇ T1 can in some cases reach values of more than 100 0 C.
• the difference in temperature at maximum temperature is a critical parameter for successfully producing continuous annealed TRB. If ⁇ T2 becomes too big the mechanical properties of the thicker and/or thinner sections become unstable. If the temperature of the thicker sections becomes too low than the material is not fully recrystallised and the mechanical properties, especially the elongation, are not fully developed and extremely sensitive to small fluctuations of the maximum temperature. On the other hand, if the temperature of the thinner sections becomes too high, higher than 800 0 C, the mechanical properties of especially high strength steels will deteriorate. The deterioration is caused by the fact that the grain size will increase with the maximum temperature, because the fine grain size after cold rolling and recrystallisation will be eliminated by transformation.
• the difference in temperature between the thicker en thinner sections of the TRB during cooling ( ⁇ T3 or ⁇ T4) is also of importance. Especially if a metal coating process like hot dip galvanising is applied.
• a metal coating process like hot dip galvanising is applied.
• the zinc will not make good contact with the strip surface and problems with zinc adherence and surface quality will arise.
• the zinc only starts to solidify below a temperature of 420 °C.
• the temperature of the strip entering the zinc bath is too high, the amount of iron dissolving in the zinc increases and thus the amount of metallic dross formation in the zinc bath. This can lead to a bad surface quality of the material.
• a high strip temperature can cause increased alloying between the zinc layer and the substrate.
• the temperature differences between the thick en thin sections of the TRB can be reduced by selective heating. This is illustrated in figure 3. At some point during heating of the strip the temperature of the thicker sections is increased (H3). The temperature of the thicker sections can be increased to a temperature level reaching that of the thin section, or even above. In this way the difference in maximum temperature ( ⁇ T2) can be reduced significantly.
• a steel strip is formed by hot rolling. After hot rolling, a steel strip having a variable thickness in length direction is formed by cold rolling both the thicker sections and the thinner sections with a reduction of at least 15%. As a result, both the thicker and the thinner sections will recrystallise during annealing.
• a steel strip is formed by hot rolling. After hot rolling, a steel strip having a variable thickness in length direction is formed by cold rolling the thicker sections with a reduction of less than 15%, usually approximately 5%, and by cold rolling the thinner sections with a reduction of at least 15%, usually between 20 and 50%.
• This rolling type has the advantage that in the thicker sections the hot rolled yield strength is increased by a small cold rolling reduction, which improves the yield strength, which is to a large extend retained during subsequent annealing. Another advantage is that cold rolling of the thinner sections is more easy because only the thinner sections have to be reduced.
• the yield strength of the continuous annealed strip in the thinner sections is 73 MPa higher than for the batch annealed product. Also the yield strength in the thicker sections is higher after continuous annealing.
• Producing TRB by only applying a large reduction to the thinner sections is a production route that has many economical advantages.
• the inhomogeneity of the mechanical properties between the thinner en thicker sections is a problem.
• the advantage of a high yield strength in the thicker sections, based on the mechanical properties in hot rolled condition can not be utilised fully in case of batch annealing because the yield strength in the thinner sections will always be much lower.
• the yield strength in the thinner sections will come much closer to the yield strength in the thicker sections, with as result a TRB with better and more homogeneous mechanical properties.
• Line speed in a continuous annealing line is important economical parameter. If line speed is low than cooling devices like gas jet cooling have to be operated at minimum capacity, outside the normal operation modus, making it more difficult to control the strip temperature before hot dip galvanising. Producing TRB with a normal line speed is both for economical and practical reasons beneficial. Selective heating is an effective method to enable the producer to increase line speed and at the same time improve the mechanical properties of the TRB.
• a dual phase steel is presented.
• Essential for producing dual phase kind of steel types is a high annealing temperature (in two phase region) and relatively high cooling rate to promote transformation from austenite to martensite, bainite and/or retained austenite.
• a low line speed is a disadvantage because also the cooling rate will be slow.
• Figure 4 shows a comparison between the batch annealing and the continuous annealing for a number of low carbon steel types, of which the composition is given in table 3.
• the Yield Strength (YS) in the sections that are significantly reduced by cold rolling is given on the vertical axis, on the horizontal axis the different steel types are indicated.
• Such steel types are normal steel types that are produced and on the market. From Figure 4 it is clear that the yield strength of continuous annealed steel is significantly higher than the yield strength of the same steel types that are batch annealed.
• Such improved yield strengths are also reached in the thinner sections of a strip of steel having a variable thickness when it is continuous annealed instead of batch annealed, as elucidated in the examples above.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Metal Rolling (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)

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• 2008
  • 2008-03-19 HU HUE08718031A patent/HUE037337T2/hu unknown
  • 2008-03-19 EP EP08718031.1A patent/EP2171102B1/en active Active
  • 2008-03-19 US US12/668,855 patent/US20100304174A1/en not_active Abandoned
  • 2008-03-19 JP JP2010516432A patent/JP5425770B2/ja active Active
  • 2008-03-19 PL PL08718031T patent/PL2171102T3/pl unknown
  • 2008-03-19 WO PCT/EP2008/053310 patent/WO2008068352A2/en not_active Ceased
  • 2008-03-19 CN CN200880025214.7A patent/CN101802230B/zh active Active

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