Patent Application
20150023835
The present invention relates to martensitic steel compositions and methods of production thereof. More specifically, the martensitic steels have tensile strengths ranging from 1700 to 2200 MPa. Most specifically, the invention relates to thin gage (thickness of ≦1 mm) ultra high strength steel with an ultimate tensile strength of 1700-2200 MPa and methods of production thereof.
Low-carbon steels with martensitic microstructure constitute a class of Advanced High Strength Steels (AHSS) with the highest strengths attainable in sheet steels. By varying the carbon content in the steel, ArcelorMittal has been producing martensitic steels with tensile strength ranging from 900 to 1500 MPa for two decades. Martensitic steels are increasingly being used in applications that require high strength for side impact and roll over vehicle protection, and have long been used for applications such as bumpers that can readily be rolled formed.
Currently, thin gage (thickness of ≦1 mm) ultra high strength steel with ultimate tensile strength of 1700-2200 MPa with good roll formability, weldability, punchability and delayed fracture resistance is in demand for the manufacture of hang on automotive parts such as bumper beams. Light gauge, high strength steels are required to fend off competitive challenges from alternative materials, such as lightweight 7xxx series of aluminum alloys. Carbon content has been the most important factor in determining the ultimate tensile strength of martensitic steels. The steel has to have sufficient hardenability so as to fully transform to martensite when quenched from a supercritical annealing temperature.
The present invention comprises a martensitic steel alloy that has an ultimate tensile strength of at least 1700 MPa. Preferably, the alloy may have an ultimate tensile strength of at least 1800 MPa, at least 1900 MPa, at least 2000 MPa or even at least 2100 MPa. The martensitic steel alloy may have an ultimate tensile strength between 1700 and 2200 MPa. The martensitic steel alloy may have a total elongation of at least 3.5% and more preferably at least 5%.
The martensitic steel alloy may be in the form of a cold rolled sheet, band or coil and may have a thickness of less than or equal to 1 mm. The martensitic steel alloy may have a carbon equivalent of less than 0.44 using the formula Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15, where Ceq is the carbon equivalent, and C, Mn, Cr, Mo, V, Ni, and Cu are in wt. % of the elements in the alloy.
The martensitic steel alloy may contain between 0.22 and 0.36 wt. % carbon. More specifically, the alloy may contain between 0.22 and 0.28 wt. % carbon or in the alternative the alloy may contain between 0.28 and 0.36 wt. % carbon. The martensitic steel alloy may further contain between 0.5 and 2.0 wt. % manganese. The alloy may also contain about 0.2 wt. % silicon. The optionally may contain one or more of Nb, Ti, B, Al, N, S, P.
A preferred embodiment of the present invention will be elucidated with references to the drawings in which:
a and 1b are schematic illustrations of annealing procedures for producing the alloys of the present invention;
a, 2b and 2c are SEM micrographs of experimental steels with 2.0% Mn-0.2% Si and various carbon contents (2a has 0.22% C; 2b has 0.25% C; and 2c has 0.28% C) after hot rolling and simulated coiling at 580° C.;
a and 4b are SEM micrographs of experimental steels with 0.22% C-0.2% Si-0.02% Nb and two different Mn contents (4a has 1.48% and 4b has 2.0%) after hot rolling and simulated coiling at 580° C.;
a and 6b are SEM micrographs of experimental steels with 0.22% C-2.0% Mn-0.2% Si and different Nb contents (6a has 0% and 6b has 0.018%) after hot rolling and simulated coiling at 580° C.;
a to 8f illustrate the effects of soaking temperature (830, 850 and 870° C.) and steel composition (
a to 9f show the effects of quenching temperature (780, 810 and 840° C.) and steel composition (
a and 10b are schematic depictions of the additional anneal cycles for producing alloys of the present invention;
a and 11b plot the tensile properties at room temperature of hot bands for producing steels of the present invention, after hot rolling and simulated coiling at 580° C.;
a to 12d are SEM micrographs at 1000× of the microstructure of hot band steels after hot rolling and simulated coiling at 660° C.;
a and 13b plot the tensile properties of experimental hot band steels at room temperature;
a to 14d represent the effects of soaking temperature (830° C., 850° C. and 870° C.), coiling temperature (580° C. and 660° C.), and alloy composition (Ti, B and Nb additions to the base steel) on the tensile properties of the steels after anneal simulation;
a to 15d show the effects of quenching temperature (780° C., 810° C. and 840° C.), coiling temperature (580° C. and 660° C.), and alloy composition (Ti, B and Nb additions to the base steel) on the tensile properties of the steels after anneal simulation;
a to 16c are even more schematic depictions of anneal cycles for producing the alloys of the present invention;
a to 17e are SEM micrographs at 1,000× of hot rolled steels (0.28 to 0.36% C) after hot rolling and simulated coiling at 580° C.;
a and 18b plot the corresponding tensile properties of the hot rolled steels of
a to 19e are SEM micrographs at 1,000× of hot rolled steels (0.28 to 0.36% C) after hot rolling and simulated coiling at 660° C.;
a and 20b plot the corresponding tensile properties of the hot rolled steels of
a to 21d represents the effects of soaking temperature (830° C., 850° C. and 870° C.), coiling temperature (580° C. and 660° C.), and alloy composition (C content and B addition to the base steel) on the tensile properties of the steels after annealing simulation;
a to 22d show the effects of quenching temperature (780° C., 810° C. and 840° C.), coiling temperature (580° C. and 660° C.), and alloy composition (C content and B addition to the base steel) on the tensile properties of the steels after annealing simulation;
a to 23d illustrates the effect of composition and annealing cycle on (23a-23b) tensile strength and (23c-23d) ductility;
a to 24l are micrographs of four alloys which were annealed using various soak/quenching temperature pairs; and
a to 25d show the tensile properties of the steels with 0.5% to 2.0° A Mn after coiling at 580° C., cold rolling (50% cold rolling reduction for the steel with 0.5 and 1.0% Mn and 75% cold rolling reduction for the steel with 2.0% Mn) and various annealing cycles.
The present invention provides a family of martensitic steels with tensile strength ranging from 1700 to 2200 MPa. The steel may be thin gauge (thickness of less than or equal to 1 mm) sheet steel. The present invention also includes the process for producing the very high tensile strength martensitic steels. Examples and embodiments of the present invention are presented below.
Table 1 shows the chemical compositions of some steels within the present invention, which includes a range of carbon content from 0.22 to 0.28 wt % (steels 2, 4 and 5), manganese content from 1.5 to 2.0 wt % (steels 1 and 3) and niobium content from 0 to 0.02 wt % (alloys 2 and 3). The remainder of the steel composition is iron and inevitable impurities.
Five 45 Kg slabs were cast in the laboratory. After reheating and austenitization at 1230° C. for 3 hours, the slabs were hot rolled from 63 mm to 20 mm in thickness on a laboratory mill. The finishing temperature was about 900° C. The plates were air cooled after hot rolling.
After shearing and reheating the pre-rolled 20 mm thick plates to 1230° C. for 2 hours, the plates were hot rolled from a thickness of 20 mm to 3.5 mm. The finish rolling temperature was about 900° C. After controlled cooling at an average cooling rate of about 45° C./s, the hot bands of each composition were held in a furnace at 580° C. for 1 hour, followed by a 24-hour furnace cooling to simulate the industrial coiling process.
Three JIS-T standard specimens were prepared from each hot band for room temperature tensile test. Microstructure characterization of hot bands was carried out by Scanning Electron Microscopy (SEM) at the quarter thickness location in the longitudinal cross-sections.
Both surfaces of the hot rolled bands were ground to remove any decarburized layer. They were then subjected to 75% lab cold rolling to obtain full hard steels with final thickness of 0.6 mm for further annealing simulations.
Annealing simulation was performed using two salt pots and one oil bath. The effects of soaking and quenching temperatures were analyzed for all of the steels. A schematic illustration of the heat treatment is shown in
To study the effect of soaking temperature, the annealing process included reheating the cold rolled strips (0.6 mm thick) to 870° C., 850° C. and 830° C. respectively followed by isothermal holding for 60 seconds. The samples were immediately transferred to the second salt pot maintained at a temperature of 810° C. and isothermally held for 25 s. This was followed by a water quench. The samples were then reheated to 200° C. for 60 s in an oil bath, followed by air cooling to room temperature to simulate overage treatment. The holding times at soaking, quenching and overaging temperatures were chosen to closely approximate industrial conditions for this gauge.
To study the effect of quenching temperature, the analysis includes reheating of cold rolled strips to 870° C. for 60 seconds, followed by immediate cooling to 840° C., 810° C. and 780° C. After a 25 second isothermal hold at the quenching temperature, the specimens were quenched in water. The steels were then reheated to 200° C. for 60 seconds followed by air cooling to simulate the overage treatment. Three ASTM-T standard specimens were prepared from each annealed blank for tensile testing at room temperature.
The samples processed at 870° C. soaking temperature and quenched from 810° C. were selected for bend testing. A 90° free V-bend with the bending axis in the rolling direction was employed for bendability characterization. A dedicated Instron mechanical testing system with 90° die block and punches was utilized for this test. A series of interchangeable punches with different die radius facilitated the determination of minimum die radius at which the samples could be bent without microcracks. The test was run at a constant stroke of 15 mm/sec until the sample was bent by 90°. A 80 KN force and 5 second dwell time was deployed at the maximum bend angle after which the load was released and the specimen was allowed to spring back. In the present test, the range of die radius varied from 1.75 to 2.75 mm with 0.25 mm incremental increase. The sample surface after bend testing was observed under 10× magnification. A crack length on the sample bending surface that is smaller than 0.5 mm is considered to be a “micro crack”, and any that is larger than 0.5 mm is recognized as a crack and the test marked as a failure. Samples with no visible crack are identified as “passed test”.
a, 2b and 2c are SEM micrographs of experimental steels with 2.0% Mn-0.2% Si and various carbon contents (2a has 0.22% C; 2b has 0.25% C; and 2c has 0.28% C) after hot rolling and simulated coiling at 580° C.
The increase in carbon content resulted in an increase in the volume fraction and the colony size of pearlite. The corresponding tensile properties at room temperature of the experimental steels are plotted in
a and 4b are SEM micrographs of experimental steels with 0.22% C-0.2% Si-0.02% Nb and two different Mn contents (4a has 1.48% and 4b has 2.0%) after hot rolling and simulated coiling at 580° C. An increase in the Mn content resulted in an increase in the volume fraction and in size of pearlite colony. The large grain size in the higher Mn steel can be attributed to grain coarsening during finish rolling and subsequent cooling. The hot rolling finish temperature was about 900° C., which is in the austenite region for both of the experimental steels but it is much higher than the Ar3 temperature for the higher Mn steel. Thus, during and after finish rolling, the austenite in the higher Mn steel had a greater opportunity to coarsen, resulting in a coarser ferrite-pearlite microstructure after phase transformation.
The corresponding tensile properties of the experimental steels with 0.22% C-2.0% Mn at room temperature are plotted in
a and 6b are SEM micrographs of experimental steels with 0.22% C-2.0% Mn-0.2% Si and different Nb contents (6a has 0% and 6b has 0.018%) after hot rolling and simulated coiling at 580° C. An increase in the Nb content resulted in an increase in the volume fraction and colony size of pearlite, which can be explained by higher hardenability of the steel with Nb and lower temperature of pearlite formation.
The corresponding tensile properties of the compared steels with 0.22% C-2.0% Mn are illustrated in
Tensile Properties of the Investigated Steels after Cold Rolling and Annealing Simulation
a to 8f illustrate the effects of soaking temperature (830, 850 and 870° C.) and steel composition (
a to 9f show the effects of quenching temperature (780, 810 and 840° C.) and steel composition (
a, 8b, 9a, and 9b show that an increase in the C content resulted in a significant increase in tensile strength but had little effect on ductility. Taking the annealing cycle of 830° C. (soaking temperature)-810° C. (quenching temperature) as an example, the increase in YS and UTS is 163 and 233 MPa, respectively, when C content is increased from 0.22 to 0.28 wt %. The increase in Mn content from 1.5 to 2.0 wt % has barely any effect on strength and ductility (see
Bendability of the Investigated Steels
Table 2 summarizes the effects of C, Mn and Nb on tensile properties and bendability of the experimental steels after 75% cold rolling and annealing. The annealing cycle included: heating the cold rolled bands (about 0.6 mm thick) to 870° C., isothermal hold for 60 seconds at soaking temperature, immediate cooling to 810° C., 25 seconds isothermal holding at that temperature, followed by rapid water quench. The panels were then reheated to 200° C. in an oil bath and held for 60 seconds, followed by air cooling to simulate overage treatment. The data shows that carbon has the strongest effect on strength and a slight effect on bendability. The addition of Nb increases yield strength and improves bendability. The improvement in bendability is achieved in spite of marginally inferior elongation. An increase in the Mn content from 1.5 to 2.0% in the Nb bearing steel has no significant effect on tensile properties but results in a big improvement in bendability.
In order to reduce carbon equivalent, thus to improve the weldability of the steels of Example 1, steels containing 0.28 wt % carbon and reduced manganese content (about 1.0 wt % vs. 2.0 wt % of Example 1) along with were produced. The alloys were cast into slabs, hot rolled, cold rolled, annealed (simulated) and over age treated. In addition, the effect of Mn content (1.0 and 2.0% Mn) on the properties of hot rolled bands and annealed products are described in detail.
Table 3 shows the chemical compositions of investigated steels. The alloy design analyzed the effects of incorporated Ti (steels 1 and 2), B (steels 2 and 3) and Nb (alloys 3 and 4).
Four 45 Kg slabs (one of each alloy) were cast in the laboratory. After reheating and austenitization at 1230° C. for 3 hours, the slabs were hot rolled from 63 mm to 20 mm in thickness on a laboratory mill. The finishing temperature was about 900° C. The plates were air cooled after hot rolling.
After shearing and reheating the pre-rolled 20 mm thick plates to 1230° C. for 2 hours, the plates were hot rolled from a thickness of 20 mm to 3.5 mm. The finish rolling temperature was about 900° C. After controlled cooling at an average cooling rate of about 45° C./s, the hot bands of each composition were held in a furnace at 580° C. and 660° C. respectively for 1 hour, followed by a 24-hour furnace cooling to simulate the industrial coiling process. The use of two different coiling temperatures was designed to understand the available process window during hot rolling for the manufacture of this product.
A recheck of hot band compositions was performed by inductively coupled plasma (ICP). In comparison with ingot derived data, a carbon loss is generally observed in the hot bands. Three JIS-T standard specimens were prepared from each hot band for room temperature tensile tests. Microstructure characterization of hot bands was carried out by Scanning Electron Microscopy (SEM) at the quarter thickness location of longitudinal cross-sections.
After grinding both surfaces of the hot rolled bands to remove any decarburized layer, the steels were cold rolled in the laboratory by 50% to obtain full hard steels with final thickness of 1.0 mm for further annealing simulations.
The effects of soaking and quenching temperatures during annealing on the mechanical properties of the steels were investigated for all of the experimental steels. A schematic of the anneal cycles is shown in
The annealing process includes reheating the cold band (about 1.0 mm thick) to 870° C., 850° C. and 830° C. for 100 s, respectively, to investigate the effect of soaking temperature on final properties. After immediate cooling to 810° C. and isothermal holding for 40 s, water quench was applied. The steels were then reheated to 200° C. for 100 s, and followed by air cooling to simulate overaging treatment.
The annealing process includes reheating the cold band to 870° C. for 100 s and immediate cooling to 840° C., 810° C. and 780° C. respectively to investigate the effect of quenching temperature on the mechanical properties of the steels. Water quench was employed after 40 s isothermal hold at the quenching temperature. The steels were then reheated to 200° C. for 100 s, and followed by air cooling to simulate the overaging treatment.
Three ASTM-T standard tensile specimens were prepared from each annealed band for room temperature tensile test. Samples processed by one annealing cycle were selected for bend testing. This annealing cycle involved the reheating of the cold band (about 1.0 mm thick) to 850° C. for 100 s, immediate cooling to 810° C., 40 s isothermal hold at quench temperature, followed by water quench. The steels were then reheated to 200° C. for 100 s, and followed by air cooling to simulate the overaging treatment. A 90° free V-bend testing along the rolling direction was employed for bendability characterization. In the present study, the range of die radius varied from 2.75 to 4.00 mm at 0.25 mm increments. The sample surface after bend testing was observed under 10× magnification. When the crack length on the sample at the outer bend surface is smaller than 0.5 mm the crack is deemed a “micro crack”. A crack larger than 0.5 mm is recognized as a failure. Samples without any visible crack are identified as “passed test”.
Table 4 shows the chemical compositions of the steels with different Ti, B and Nb contents after hot rolling. Compared with the compositions of ingots (Table 3), there was about 0.03% carbon and 0.001% B loss after hot rolling.
a and 11b show the tensile properties (JIS-T standard) of experimental steels (of Table 4) at room temperature, after hot rolling and simulated coiling at 580° C. The base composition consists of 0.28% C-1.0% Mn-0.2% Si.
As shown in
The corresponding tensile properties of the experimental steels at room temperature are shown in
Comparing the tensile properties in
Tensile Properties of the Steels after Annealing Simulation
a to 14d represent the effects of soaking temperature (830° C., 850° C. and 870° C.), coiling temperature (580° C. and 660° C.), and alloy composition (Ti, B and Nb additions to the base steel) on the tensile properties of the steels after anneal simulation.
a to 15d show the effects of quenching temperature (780° C., 810° C. and 840° C.), coiling temperature (580° C. and 660° C.), and alloy composition (Ti, B and Nb additions to the base steel) on the tensile properties of the steels after anneal simulation.
Comparing
As shown in
Bendability of the Steels after Anneal Simulation
Table 5 summarizes the effect of Ti, B and Nb on the tensile properties and bendability of the steels after 50% cold rolling and annealing after simulated coiling at 580° C. The annealing process consisted of reheating the cold band (about 1.0 mm thick) to 850° C. for 100 seconds, immediate cooling to 810° C., 40 seconds isothermal hold at “quench” temperature, followed by water quench. The steels were then reheated to 200° C. for 100 seconds followed by air cooling to simulate overaging treatment (OA). As shown, it was possible to produce steels with ultimate tensile strength between 1850 and 2000 MPa by varying alloy composition. The steel with only C, Mn and Si demonstrated the best bendability. The addition of Nb increased strength with a slight deterioration of bendability. Bendability pass defined as “micro crack length smaller than 0.5 mm at 10× magnification.
Comparison with Example 1—Effect of Manganese
The steel with 0.28% C-2.0% Mn-0.2% Si was presented in Example 1 above. We can compare its behavior with the steel of Example 2 containing 0.28% C-1.0% Mn-0.2% Si to investigate the effect of Mn (1.0 and 2.0%) on tensile properties. The detailed chemical compositions of both steels are shown in Table 6.
Tensile Properties of Hot Rolled Bands with 1.0 and 2.0% Mn
Table 7 displays the tensile properties of the steels with 1.0% and 2.0% Mn respectively after hot rolling and simulated coiling at 580° C. For the tensile properties of hot rolled bands, the steel with the lower Mn content showed a lower strength than the steel with the higher Mn content (51 MPa lower in YS and 61 MPa lower in UTS). This may facilitate a higher extent of cold rolling for the low Mn steel.
Table 8 shows the tensile properties of the steels with 1.0% and 2.0% Mn respectively after cold rolling (50% cold rolling reduction for the steel with 1.0% Mn and 75% cold rolling reduction for the steel with 2.0% Mn) and various annealing cycles. It can be seen that at the same annealing treatment of 870° C. (soaking), 840° C. (quench) and 200° C. (overaging), Mn content had no significant effect on strength. At the same quenching temperature of 810° C., the decrease in soaking temperature from 870 to 830° C. did not affect the strength of the steel with 1.0% Mn, but significantly increased the strength of the steel with 2.0% Mn by about 90 MPa. This indicates that the steel with 1.0% Mn is quite stable in strength regardless soaking temperature (870 to 830° C.), and the steel with 2.0% Mn is more sensitive to the soaking temperature, perhaps due to grain coarsening at higher anneal temperatures. The steel with 1.0% Mn will be relatively easier to process during manufacturing due to the wider process windows.
Bendability of Annealed Steels with 1.0 and 2.0% Mn
Table 9 lists the tensile properties and bendability of the steels with 1.0% and 2.0% Mn after anneal simulation. The steel with 1.0% Mn demonstrated a better bendability (3.5t compared to 4.0t) at a comparable strength level. Bendability pass is defined as micro crack length smaller than 0.5 mm at 10× magnification.
To ensure good weldability of the steels, the carbon equivalent (Ceq) should be less than 0.44. The carbon equivalent for the present steels is defined as:
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15.
Thus, at a C content of 0.28 wt % and Mn content of 1 or 2 wt %, the weld integrity is determined to be unacceptable. The present examples are designed to reduce the Ceq and still meet the strength and ductility needs. High carbon content is beneficial for increasing strength but deteriorates weldability. According to the carbon equivalent formula, Mn is another element which deteriorates weldability. Thus, the motivation is to maintain a certain amount of carbon content (at least 0.28%) to achieve sufficient ultra-high strength and to study the effect of Mn content on UTS. The inventors look to reduce Mn content to improve the weldability but at maintain an ultra-high strength level.
Table 10 shows the chemical compositions of investigated steels in Example 3. The alloy design incorporated the understanding of the effect of C content and B addition on tensile properties in the final annealed products.
Five 45 Kg slabs (one of each alloy) were cast in the laboratory. After reheating and austenitization at 1230° C. for 3 hours, the slabs were hot rolled from 63 mm to 20 mm in thickness on a laboratory mill. The finishing temperature was about 900° C. The plates were air cooled after hot rolling.
After shearing and reheating the pre-rolled 20 mm thick plates to 1230° C. for 2 hours, the plates were hot rolled from a thickness of 20 mm to 3.5 mm. The finish rolling temperature was about 900° C. After controlled cooling at an average cooling rate of about 45° C./s, the hot bands of each composition were held in a furnace at 580° C. and 660° C. respectively for 1 hour, followed by a 24-hour furnace cooling to simulate industrial coiling process. The use of two different coiling temperatures was designed to understand the available process window during hot rolling for the manufacture of this product.
Three JIS-T standard specimens were prepared from each hot rolled steel (also known as a “hot band”) for room temperature tensile tests. Microstructure characterization of hot bands was carried out by Scanning Electron Microscopy (SEM) at the quarter thickness location of longitudinal cross-sections.
After grinding both surfaces of the hot rolled bands to remove any decarburized layer, the steels were cold rolled in the laboratory by 50% to obtain full hard steels with final thickness of 1.0 mm for further annealing simulations.
The effects of soaking, quenching temperatures and a comparison of different combination of soaking and quenching temperatures during annealing on the mechanical properties of the steels were investigated for all of the experimental steels. A schematic of the anneal cycles is shown in
The annealing process includes reheating the cold band (about 1.0 mm thick) to 870° C., 850° C. and 830° C. for 100 seconds, respectively, to investigate the effect of soaking temperature on the final properties. After immediate cooling to 810° C. and isothermal holding for 40 seconds, water quench was applied. The steels were then reheated to 200° C. for 100 seconds, followed by air cooling to simulate overaging treatment.
The annealing process includes reheating the cold band to 870° C. for 100 seconds and immediate cooling to 840° C., 810° C. and 780° C. respectively to investigate the effect of quenching temperature on the mechanical properties of the steels. Water quench was employed after 40 seconds of isothermal hold at the quenching temperature. The steels were then reheated to 200° C. for 100 seconds, followed by air cooling to simulate overaging treatment.
The annealing cycle includes reheating the cold rolled steels to 790° C., 810° C. and 830° C. for 100 seconds respectively, immediate cooling to various quench temperatures (770° C., 790° C. and 810° C. respectively), isothermal holding for 40 seconds, followed by water quench. The steels were then reheated to 200° C. for 100 seconds, followed by air cooling to simulate overaging treatment.
ASTM-T standard tensile specimens were prepared from each annealed band for room temperature tensile test. The samples processed by one annealing cycle were selected for bend testing. This annealing cycle involved the reheating of the cold band (about 1.0 mm thick) to 850° C. for 100 seconds, immediate cooling to 810° C., 40 seconds isothermal hold at the quench temperature, followed by water quench. The steels were then reheated to 200° C. for 100 seconds, followed by air cooling to simulate overaging treatment. A 90° free V-bend test along the rolling direction was employed for bendability characterization. In the present study, the range of die radius varied from 2.75 to 4.00 mm at 0.25 mm increments. The sample surface after bend testing was observed under 10× magnification. A crack length on the sample at the outer bend surface that is smaller than 0.5 mm is considered to be a “micro crack”, and a crack larger than 0.5 mm is recognized as a failure. A sample without any length of visible crack is identified as “passed the test”.
a to 17e are SEM micrographs at 1,000× of hot rolled steels (0.28 to 0.36% C) after hot rolling and simulated coiling at 580° C. The increase in carbon content and the addition of boron led to an increase in martensite volume fraction, which can be attributed to the role of C and B in increasing hardenability.
The corresponding tensile properties of the experimental steels at room temperature (after hot rolling and simulated coiling at 580° C.) are shown in
a to 19e are SEM micrographs at 1,000× of hot rolled steels (0.28 to 0.36% C) after hot rolling and simulated coiling at 660° C.
The corresponding tensile properties at room temperature (after hot rolling and simulated coiling at 660° C.) are represented in
Comparing the tensile properties in
Tensile Properties of the Steels after Annealing Simulation Effect of Soaking Temperature (830° C., 850° C. and 870° C.)
a to 21d represents the effects of soaking temperature (830° C., 850° C. and 870° C.), coiling temperature (580° C. and 660° C.), and alloy composition (C content and B addition to the base steel) on the tensile properties of the steels after annealing simulation.
a to 22d show the effects of quenching temperature (780° C., 810° C. and 840° C.), coiling temperature (580° C. and 660° C.), and alloy composition (C content and B addition to the base steel) on the tensile properties of the steels after annealing simulation.
a to 23d illustrates the effect of composition and annealing cycle on (23a-23b) tensile strength and (23c-23d) ductility.
Bendability of the Steels after Anneal Simulation
Table 11 summarizes the effects of C and B on the tensile properties and bendability of the steels after 50% cold rolling and annealing after simulated coiling at 580° C. The annealing process consisted of reheating the cold band (about 1.0 mm thick) to 850° C. for 100 seconds, immediate cooling to 810° C., 40 seconds isothermal hold at “quench” temperature, followed by water quench. The steels were then reheated to 200° C. for 100 seconds, followed by air cooling to simulate overaging treatment (OA). As shown in Table 11, it was possible to produce steels with ultimate tensile strength between 1830 and 2080 MPa by varying alloy composition.
Comparison with Examples 1 and 2—Effect of Manganese for the Steels with 0.28% C
The steels with 0.28% C and 1.0%/2.0% Mn were presented above in Examples 1 and 2. We now compare those steels with the steel containing 0.28% C and 0.5% Mn to investigate the effect of Mn (0.5% to 2.0%) on tensile properties. The detailed chemical compositions of the steels are shown in Table 12.
Table 13 displays the tensile properties of the steels with 0.5% to 2.0% Mn and the additions of Ti and B after hot rolling and simulated coiling at 580° C. For the steels with Ti addition, the increase in Mn content from 0.5% to 1.0% led to an increase in both yield and tensile strengths and yield ratio but no significant effect on ductility. The addition of B in Ti added steels with 0.5% to 1.0% Mn resulted in an increase in strength. Compared to the steel “28C-1.0Mn”, the addition of Ti was beneficial for increasing both strength and yield ratio, which may be attributed to the effect of Ti precipitation hardening. The steels with the lower Mn content showed a lower strength than the steel with the higher Mn content. This may facilitate a higher extent of cold rolling for the low Mn steel.
a to 25d show the tensile properties of the steels with 0.5% to 2.0% Mn after coiling at 580° C., cold rolling (50% cold rolling reduction for the steel with 0.5 and 1.0% Mn and 75% cold rolling reduction for the steel with 2.0% Mn) and various annealing cycles. The X-axis of
Bendability of Annealed Steels with 0.5 to 2.0% Mn (0.28% C)
Table 14 lists the tensile properties and bendability of the steels with 0.5% to 2.0% Mn after anneal simulation, which were previously coiled at 580° C. The steel “28C-0.5Mn—Ti” demonstrated a better bendability than the steel “28C-1.0Mn—Ti” (3.5t compared to 4.0t) at a comparable UTS level of 1900 MPa.
It is to be understood that the disclosure set forth herein is presented in the form of detailed embodiments described for the purpose of making a full and complete disclosure of the present invention, and that such details are not to be interpreted as limiting the true scope of this invention as set forth and defined in the appended claims.
This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61/629,762 filed Nov. 28, 2011, the entire disclosure of which is hereby incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2012/066895 | 11/28/2012 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61629762 | Nov 2011 | US |
