US20160208356A1 - Non quenched and tempered steel and manufacturing process thereof - Google Patents

Non quenched and tempered steel and manufacturing process thereof Download PDF

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US20160208356A1
US20160208356A1 US15/023,513 US201315023513A US2016208356A1 US 20160208356 A1 US20160208356 A1 US 20160208356A1 US 201315023513 A US201315023513 A US 201315023513A US 2016208356 A1 US2016208356 A1 US 2016208356A1
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cooling
steel
temperature
controlled
non quenched
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Donglin Liu
Xu Zhou
Yifeng XU
Zhiwei Zhou
Jie Yu
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JIANGSU SUZHOU STEEL GROUP CO Ltd
SUZHOU SUXIN SPECIAL STEEL CO Ltd
Peking University Founder Group Co Ltd
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JIANGSU SUZHOU STEEL GROUP CO Ltd
SUZHOU SUXIN SPECIAL STEEL CO Ltd
Peking University Founder Group Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific 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
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • 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
    • C21D8/02—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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper

Definitions

  • the present invention relates to a non quenched and tempered steel and its manufacturing process, and belongs to the technical field of ferrous metallurgy.
  • Non quenched and tempered steels means mechanical structural steels that can achieve performance requirements without quenching and tempering treatment. Using such steels to manufacture parts can omit quenching and tempering heat treatment process, and has advantages such as saving energy and materials, simple technology, etc., and can decrease environmental pollution and prevent oxidation, decarbonization, deformation and cracking.
  • the domestic traditional process for producing non quenched and tempered steels for cutting includes: electric furnace smelting ⁇ refining ⁇ mold casting ⁇ controlled rolling and controlled cooling.
  • the difficulty of such process in production is the control of steel performance.
  • the domestic and oversea manufacturers generally achieve the control of steel performance by improving chemical composition of non quenched and tempered steels.
  • Shougang Corporation has proposed a novel process for producing non quenched and tempered steels, which process comprises: converter smelting, slag cutoff tapping, deoxidation alloying in ladle, LF ladle refining, feeding S line, ladle bottom blowing argon to achieve full protection casting, slab temperature control, controlled cooling and rolling, etc.
  • the heating temperature is 1100-1180° C.
  • initial rolling temperature is 1020-1100° C.
  • finishing rolling temperature is 850-920° C.
  • the relative deformation is 15-35%.
  • the temperature is decreased to 600° C. and then slowly cooled to room temperature.
  • the non quenched and tempered steels produced by above process are difficult to ensure the core temperature and the surface temperature of the steels to become the same by slow cooling within a short time, and tend to cause the surface and core of the steels have large differences in strength and toughness, and seriously nonuniform mechanical properties.
  • the above process is used to produce large-sized non quenched and tempered steels (e.g. ⁇ 70 to ⁇ 145 mm bars), the phenomenon of nonuniform mechanical properties of the surface and the core of the bars is even more obvious.
  • the technical problem to be solved by the present invention is to overcome the defects that the surface mechanical properties and core mechanical properties of the steels produced by existing processes for producing non quenched and tempered steels are nonuniform. Therefore, the present invention provides a non quenched and tempered steel and its production process to ensure the uniformity of the surface mechanical properties and core mechanical properties of final products.
  • the present invention provides a non quenched and tempered steel consisting of following chemical components by weight percent: carbon 0.42 to 0.50, silicon 0.20 to 0.40, manganese 0.5 to 1.0, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.025, sulfur 0.00 to 0.025, and balance iron.
  • the non quenched and tempered steel according to present invention consists of following chemical components by weight percent: carbon 0.42 to 0.45, silicon 0.20 to 0.30, manganese 0.50 to 0.7, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.025, sulfur 0.00 to 0.025, and balance iron.
  • the non quenched and tempered steel according to present invention consists of following chemical components by weight percent: carbon 0.42 to 0.47, silicon 0.25 to 0.35, manganese 0.60 to 1.0, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.025, sulfur 0.00 to 0.025, and balance iron.
  • the non quenched and tempered steel according to present invention consists of following chemical components by weight percent: carbon 0.45 to 0.50, silicon 0.25 to 0.35, manganese 0.60 to 0.9, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.015, sulfur 0.00 to 0.015, and balance iron.
  • the present invention provides a process for producing non quenched and tempered steel, comprising a cooling step at least carried out after a finishing step; in said cooling step, an intense cooling and a moderate cooling are carried out alternately to allow the steel to undergo at least two stages of water cooling, so that the core temperature and the surface temperature of the steel become the same within a specified time.
  • the steel in said cooling step, is subjected to three stages of water cooling, wherein the first stage of water cooling uses intense cooling, the second stage of water cooling uses moderate cooling, and the third stage of water cooling uses intense cooling.
  • the cooling intensity is controlled by controlling the opening degree of valve(s) of a water cooling unit.
  • the temperature of the steel decreases 100° C.-400° C. in 4-7 seconds after water cooling, and the temperature of the steel further decreases 50° C.-100° C. after temperature reversion.
  • the first stage valve opening degree is controlled to be 30%-40%
  • the second stage valve opening degree is controlled to be 20%
  • the third stage valve opening degree is controlled to be 30%-40%, to ensure the surface temperature of the steel to decrease 100-400° C. in 4-7 seconds.
  • the steel is subjected to temperature-dropping cooling by means of spray cooling after temperature reversion.
  • the steel is separately disposed on a cold bed and subjected to air cooling for 10-12 minutes.
  • the steel is stacked and subjected to shield cooling.
  • the process for producing non quenched and tempered steel according to present invention further comprises a finish rolling step before the cooling step, in said finish rolling step, the temperature of the steel is controlled at ⁇ 950° C. at the entry into the finish rolling step; the steel is subjected to low temperature rolling when the steel temperature is 850° C.-900° C.
  • the process for producing non quenched and tempered steel according to present invention further comprises a smelting step before the finish rolling step, the smelting step comprises electric furnace smelting step, ladle furnace smelting step and refining step carried out sequentially.
  • a molten iron smelting is utilized in the electric furnace smelting, wherein the final phosphorus content is ⁇ 0.015%, the final carbon content is 0.03% to 0.10%, and the final temperature is 1620-1700° C.
  • silicon carbide, ferrosilicon powder are used to deoxidize.
  • the ladle furnace smelting step comprises making white slag, and holding the white slag for 20 minutes or more.
  • the refining time is 45 minutes or more, and the hydrogen content is controlled at 1.5 ppm or less.
  • the process for producing non quenched and tempered steel of present invention further comprises a continuous casting step after the refining step, in said continuous casting step, a overheat is controlled at 20-35° C., a pulling speed is controlled at 0.5 m/min-0.6 m/min.
  • the process for producing non quenched and tempered steel of present invention further comprises a heating step after the continuous casting step, in said heating step, the steel billet is placed in a heating furnace to be heated, wherein the preheating stage temperature is controlled at 850 ⁇ 30° C., the heating stage temperature is controlled at 1100 ⁇ 30° C., the soaking stage temperature is controlled at 1130 ⁇ 30° C., and the total time of the soaking stage is not less than 2 hours.
  • the present invention provides a process for producing a non quenched and tempered steel, the process comprises following steps, successively:
  • the process for the production of non quenched and tempered steel according to present invention alters the cooling mode before finish rolling in previous production of non quenched and tempered steel; the process at least has a cooling step after the finish rolling step; in contrast to the cooling mode in the prior art which employs the cooling mode of single water cooling or consistent air cooling, the present process utilizes alternate intense cooling and moderate cooling.
  • the intense cooling can ensure the surface temperature of the steel to decrease rapidly; and the moderate cooling allows the core temperature of the steel to dissipate gradually to the surface; a further intense cooling is carried out to allow rapid heat dissipation.
  • the intense cooling and the moderate cooling can be carried out alternately several times according to practical requirement.
  • a water cooling mode combining intense cooling and moderate cooling allows the core temperature and the surface temperature of the steel to become the same within a short time, and thus ensures the uniformity of the mechanical properties of the steel and improves the production efficiency.
  • the steel is subjected to three stages of water cooling, wherein the first stage of water cooling uses intense cooling, the second stage of water cooling uses moderate cooling, and the third stage of water cooling uses intense cooling.
  • the steel Upon the completion of the finish rolling, the steel has a relatively high temperature.
  • An intense cooling is utilized in the first stage of water cooling so that the surface temperature of the steel decreases rapidly. Due to the heat transfer effect, after the surface temperature decreases, the heat of the core gradually transfers to the surface.
  • a moderate cooling is utilized in the second stage of water cooling. After the moderate cooling, heat transfer allows the surface temperature to increase, and then the surface is cooled rapidly again by intense cooling mode so that the surface heat is removed quickly. At this time, the heat transfer allows the surface temperature and the core temperature become the same, thus ensuring the uniformity of mechanical properties. 3.
  • the cooling is controlled by controlling the opening degree of valves of a water cooling unit, in particular, the first stage valve opening degree is controlled to be 30%-40%, the second stage valve opening degree is controlled to be 20%, and the third stage valve opening degree is controlled to be 30%-40% to ensure the surface temperature of the steel to decrease 100° C.-400° C. in 4-7 seconds.
  • the flow rate of water can be controlled by controlling the opening degree of valve, such that the intensity of water cooling can be controlled. This control manner is very simple. After the valve opens a certain length, the steel is introduced into water to perform water treatment. During the water treatment, the surface of the steel is cooled in all directions to ensure the uniformity of surface cooling. 4.
  • the steel is subjected to temperature-dropping cooling by means of spray cooling after temperature reversion.
  • Spray cooling is an advantageous supplement of water cooling.
  • the heat of the core further spreads to the surface, which further ensures the core temperature and the surface temperature become the same.
  • the steel is separately disposed on a cold bed and subjected to air cooling for 10-12 minutes. After spray cooling, the steel is separately disposed on a cold bed to perform air cooling.
  • a spray cooling can be additionally provided to further dissipate surface heat.
  • the steel is stacked to perform shield cooling.
  • Shield cooling is a manner of slow cooling.
  • the steel is stacked and subjected to shield cooling.
  • the cooling scheme including water cooling, spray cooling and air cooling, the surface temperature and the core temperature of the steel has essentially become the same.
  • shield cooling is used to slow down the cooling speed, thereby improving the structure and property of the steel. 7.
  • white slag is made and held for 20 minutes or more.
  • the white slag holding time is controlled strictly to make the effect of deoxidation, desulfurization and inclusion removal of the white slag more obvious, thereby improving the purity of steel.
  • the refining time is not less than 45 minutes, and the hydrogen content is controlled at 1.5 ppm or less.
  • the refining process effectively controls the hydrogen content and can better solve the risk of subsequent hydrogen induced cracking of steel; there is an adequate time to make a more uniform composition; and the process provides an adequate time for inclusions to float upward, thereby effectively solving the problem of controlling the inclusions, such that the finished product will be purer.
  • the overheat is strictly controlled at 20-35° C.; the pulling speed is controlled at 0.5 m/min-0.6 m/min.
  • a low overheat and a low pulling speed ensure the quality of casting blanks.
  • the metallographic structure at a magnification of 500 ⁇ of the non quenched and tempered steel provided in present invention comprises ferrite and pearlite, and has an actual grain size (100 ⁇ ) rated to 10 to 11 according to GB/T6394; the grains are fine and uniform; and the rate difference from the core to the edge is not greater than 1.5.
  • the mechanical properties of the surface and the core of steel are uniform.
  • the strength and the toughness from the core to the edge change very little, thus effectively avoiding the defects that the mechanical properties of general materials cannot satisfy application demands after subjecting to large surface processing.
  • the hardness difference from the core to the edge is less than 30 HB, thus effectively avoiding the disadvantageous effect of large hardness changes on cutting tools and processing.
  • the content of inclusions is low, and the purity of the steel is high.
  • the keypoint of present invention is to make the performance of the surface and core of steels substantially consistent by controlling the rolling and by controlling the cooling step after the rolling, thereby improving the quality of steels.
  • Specific cooling control includes:
  • the steel is subjected to cooling control through above manner (especially water cooling), which alters the cooling mode before finish rolling in previous production of non quenched and tempered steel; the process at least has a cooling step after the finish rolling step; in contrast to the cooling mode in the prior art which employs the cooling mode of single water cooling or consistent air cooling, the present process utilizes alternate intense cooling and moderate cooling.
  • the intense cooling can ensure the surface temperature of the steel to decrease rapidly; and the moderate cooling allows the core temperature of the steel to dissipate gradually to the surface; a further intense cooling is carried out to allow rapid heat dissipation.
  • the intense cooling and the moderate cooling can be carried out alternately several times according to practical requirement.
  • a water cooling mode combining intense cooling and moderate cooling allows the core temperature and the surface temperature of the steel to become the same within a short time, and thus ensures the uniformity of the mechanical properties of the steel and improves the production efficiency.
  • air cooling and shield cooling allows the core temperature to continuously dissipate to the surface and the surface temperature to be taken away constantly; moreover, the combination of said cooling manners results in an appropriate cooling speed.
  • Use of shield cooling after air cooling allows the cooling speed not so fast in the case that the surface temperature and the core temperature of steel are consistent, thus improving overall mechanical properties.
  • FIG. 1 is a metallograph at a magnification of 500 ⁇ of a non quenched and tempered steel produced by the manufacturing process of present invention
  • FIG. 2 is an image showing the grain size of a non quenched and tempered steel produced by the manufacturing process of present invention
  • FIG. 3 is an image showing inclusions of a non quenched and tempered steel produced by the manufacturing process of present invention.
  • This example provides a process of producing non quenched and tempered steel, comprising a finish rolling step and a cooling step after the finish rolling, wherein, in the finish rolling step, the temperature of rods at the entry into the finish rolling step was controlled at ⁇ 950° C., the rods were subjected to low temperature rolling when the rods temperature is 850° C.-900° C.
  • the steel was subjected to three stages of water cooling by means of a specialized controllable water cooling unit, wherein the first stage of water cooling employed intense cooling, the second stage of water cooling employed moderate cooling, and the third stage of water cooling employed intense cooling.
  • the water flow was controlled by controlling the opening degree of the valve(s) of water cooling unit so as to control water cooling strength.
  • the first stage valve opening degree was controlled to be 30%-40%
  • the second stage valve opening degree was controlled to be 20%
  • the third stage valve opening degree was controlled to be 30%-40% to ensure the surface temperature of the rods to decrease 100° C.-400° C. in 5 seconds.
  • the rods were subjected to spray cooling to decrease the rods temperature by 50-100° C., such that the heat quickly dissipates, then the rods were separately disposed on a cold bed and subjected to air cooling for 10-12 minutes, finally the rods were removed from the cold bed and stacked to undergo shield cooling.
  • the rods were subjected to three stages of water cooling, wherein the first stage of water cooling employed intense cooling, the second stage of water cooling employed moderate cooling, and the third stage of water cooling employed intense cooling. Upon the completion of finish rolling, the rods had a relatively high temperature. The rods were subjected to the first stage of water cooling using intense cooling so that the surface temperature of the steel decreased rapidly. After the surface temperature decreased, the heat of the core gradually transferred to the surface, due to the heat transfer effect.
  • the second stage of water cooling employed moderate cooling so that more time was left for heat transfer of the core during cooling.
  • the heat transfer allowed the surface temperature increasing, and then the surface was cooled rapidly again by intense cooling manner so that the heat of the surface was removed quickly.
  • the heat transfer allowed the surface temperature and the core temperature become the same, thus ensuring the uniformity of mechanical properties.
  • This example provides a process of producing non quenched and tempered steel, which was a further improvement over Example 1.
  • this process further comprises a smelting step before the finish rolling step, the smelting step comprising electric furnace smelting step, ladle furnace smelting and refining step carried out sequentially.
  • the electric furnace smelting step molten iron smelting was employed, the phosphorus content before steel tapping was strictly controlled at ⁇ 0.015%, the final carbon content was 0.03% to 0.10%, and the final temperature was 1620° C.-1700° C.
  • the electric furnace smelting can better control deslagging operation than traditional converter smelting.
  • VD refining furnace
  • This example provides a process for producing non quenched and tempered steel, which was a further improvement over Example 2.
  • the continuous casting step and heating step were improved.
  • the continuous casting step and the heating step were arranged after the refining step, and before the rolling step and the water cooling step.
  • molten iron in a tundish was introduced into a crystallizer by a submerged nozzle, thus the problem that air tends to be brought in when using traditional nozzles is avoided. Furthermore, argon gas was blow to the joint site of the submerged nozzle and the tundish, thus preventing air from entering into the tundish.
  • the overheat was strictly controlled at 20-35° C., and the pulling speed was controlled at 0.5 m/min-0.6 m/min. In the continuous casting, low overheat and low casting speed ensured the quality of casting blank.
  • the temperature at cut place was controlled at ⁇ 820° C. After cutting, the surface of the casting blank needed to be manually checked to ensure no obvious defects.
  • a macrostructure sample of the casting blank was taken to ensure that the casting blank had no cracks or shrinkage.
  • the central looseness was not more than level three; this requirement was to ensure the quality of the surface and macrostructure of the rods that are subsequently rolled.
  • the casting blank was sent to a heating furnace to be heated, wherein the preheating stage temperature was 850 ⁇ 30° C., the heating stage temperature was 1100 ⁇ 30° C., the soaking stage temperature was 1130 ⁇ 30° C., and the total time of the soaking stage was not less than 2 hours.
  • This example provides a non quenched and tempered steel produced by the process described in above Example 1, which consists of following chemical components by weight percent: carbon 0.42, silicon 0.20, manganese 0.50, chromium 0.30, aluminum 0.010, nickel 0.10, copper 0.20, phosphorus 0.010, sulfur 0.015, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 1, which consists of following chemical components by weight percent: carbon 0.50, silicon 0.35, manganese 1.0, chromium 0.30, aluminum 0.030, nickel 0.10, copper 0.20, phosphorus 0.015, sulfur 0.020, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 1, which consists of following chemical components by weight percent: carbon 0.45, silicon 0.30, manganese 0.70, chromium 0.20, aluminum 0.020, nickel 0.05, copper 0.10, phosphorus 0.010, sulfur 0.025, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 2, which consists of following chemical components by weight percent: carbon 0.47, silicon 0.25, manganese 0.60, chromium 0.30, aluminum 0.010, nickel 0.10, copper 0.20, phosphorus 0.010, sulfur 0.10, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 2, which consists of following chemical components by weight percent: carbon 0.48, silicon 0.28, manganese 0.8, chromium 0.30, aluminum 0.025, nickel 0.08, copper 0.10, phosphorus 0.010, sulfur 0.015, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 2, which consists of following chemical components by weight percent: carbon 0.43, silicon 0.30, manganese 0.55, chromium 0.25, aluminum 0.020, nickel 0.10, copper 0.20, sulfur 0.012, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 3, which consists of following chemical components by weight percent: carbon 0.49, silicon 0.32, manganese 0.92, chromium 0.25, aluminum 0.018, nickel 0.20, phosphorus 0.025, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 3, which consists of following chemical components by weight percent: carbon 0.48, silicon 0.29, manganese 0.70, chromium 0.30, aluminum 0.028, nickel 0.15, phosphorus 0.010, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 3, which consists of following chemical components by weight percent: carbon 0.50, silicon 0.40, manganese 1.0, chromium 0.30, aluminum 0.018, nickel 0.20, phosphorus 0.010, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 3, which consists of following chemical components by weight percent: carbon 0.48, silicon 0.30, manganese 0.60, chromium 0.30, aluminum 0.015, phosphorus 0.010, sulfur 0.010, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 3, which consists of following chemical components by weight percent: carbon 0.49, silicon 0.28, manganese 0.75, aluminum 0.018, nickel 0.10, phosphorus 0.010, and balance iron.
  • This example provides a non quenched and tempered steel produced by the process described in Example 3, which consists of following chemical components by weight percent: carbon 0.47, silicon 0.28, manganese 0.75, aluminum 0.018, nickel 0.10, sulfur 0.010, phosphorus 0.010, and balance iron.
  • the metallographic structures at a magnification of 500 ⁇ of the core of the non quenched and tempered steel produced in Examples 4-15 comprise ferrite and pearlite (as shown in FIG. 1 ), and have actual grain size (100 ⁇ ) rated to level 10 to 11 according to GB/T6394 (as shown in FIG. 2 ); the grains are fine and uniform; and the rate difference from the core to the edge is not greater than 1.5.
  • the mechanical properties of the surface and the core of steel are uniform. The strength and the toughness from the core to the edge change very little, thus effectively avoiding the defects that the mechanical properties of general materials cannot satisfy application demands after subjecting to large surface processing.
  • the hardness difference from the core to the edge is less than 30 HB, thus effectively avoiding the disadvantageous effect of large hardness changes on cutting tools and processing. Moreover, the content of inclusions is low, and the purity of the steel is high (as shown in FIG. 3 ).
  • the mechanical property data of the steel produced in Examples 4-15 were as shown in Table 1. It is clear from Table 1 that the non quenched and tempered steels produced by the process of present invention have excellent overall mechanical properties in terms of yield strength, tensile strength, elongation, reduction of area, impact absorbing energy and so on. Moreover, it can be seen from the performance data in Table 1 that, the Example in which the process of Example 3 is used, and the steel consisting of carbon 0.50, silicon 0.35, manganese 1.0, chromium 0.30, aluminum 0.018, nickel 0.20, phosphorus 0.010, and balance iron, exhibited the best overall mechanical properties; i.e. Example 12 exhibited the best overall mechanical properties.
  • the non quenched and tempered steels produced by the process provided in present invention can completely replace common quenched and tempered 45 steel directly for cutting processing, and exhibit excellent overall mechanical properties.
  • This example provides a general process for producing non quenched and tempered steels, the process begins from smelting step.
  • the smelting step comprising electric furnace smelting step, ladle furnace smelting and refining step carried out sequentially.
  • molten iron smelting was employed, the phosphorus content before steel tapping was strictly controlled at ⁇ 0.015%, the final carbon content was 0.03% to 0.10%, and the final temperature was 1620° C.-1700° C.
  • the electric furnace smelting can better control deslagging operation than traditional converter smelting.
  • LF ladle furnace
  • VD refining furnace
  • the refining step was followed by a continuous casting step.
  • molten iron in a tundish was introduced into a crystallizer by a submerged nozzle, thus the problem that air tends to be brought in when using traditional nozzles is avoided.
  • argon gas was blow to the joint site of the submerged nozzle and the tundish, thus preventing air from entering into the tundish.
  • the overheat was strictly controlled at 20-35° C., and the pulling speed was controlled at 0.5 m/min-0.6 m/min. In the continuous casting, low overheat and low casting speed ensured the quality of casting blank.
  • the temperature at cut place was controlled at ⁇ 820° C.
  • the surface of the casting blank needed to be manually checked to ensure no obvious defects.
  • a macrostructure sample of the casting blank was taken to ensure that the casting blank had no cracks or shrinkage. The central looseness was not more than level three; this requirement was to ensure the quality of the surface and macrostructure of the rods that are subsequently rolled.
  • the casting blank was sent to a heating furnace to be heated, wherein the preheating stage temperature was 850 ⁇ 30° C., the heating stage temperature was 1100 ⁇ 30° C., the soaking stage temperature was 1130 ⁇ 30° C., and the total time of the soaking stage was not less than 2 hours.
  • the heating step was followed by a finish rolling step and a cooling step.
  • the temperature of rods at the entry into the finish rolling step was controlled at ⁇ 950° C.
  • the rods were subjected to low temperature rolling when the rods temperature is 850° C.-900° C.
  • the steel was subjected to three stages of water cooling by means of a specialized controllable water cooling unit, wherein the first stage of water cooling employed intense cooling, the second stage of water cooling employed moderate cooling, and the third stage of water cooling employed intense cooling.
  • the water flow was controlled by controlling the opening degree of the valve(s) of water cooling unit so as to control water cooling strength.
  • the first stage valve opening degree was controlled to be 30%-40%
  • the second stage valve opening degree was controlled to be 20%
  • the third stage valve opening degree was controlled to be 30%-40% to ensure the surface temperature of the rods to decrease 100° C.-400° C. in 5 seconds.
  • the rods were subjected to spray cooling to decrease the rods temperature by 50-100° C., such that the heat quickly dissipates, then the rods were separately disposed on a cold bed and subjected to air cooling for 10-12 minutes, finally the rods were removed from the cold bed and stacked to undergo shield cooling.
  • the rods were subjected to three stages of water cooling, wherein the first stage of water cooling employed intense cooling, the second stage of water cooling employed moderate cooling, and the third stage of water cooling employed intense cooling.
  • the rods had a relatively high temperature.
  • the rods were subjected to the first stage of water cooling using intense cooling so that the surface temperature of the steel decreased rapidly. After the surface temperature decreased, the heat of the core gradually transferred to the surface, due to the heat transfer effect.
  • the second stage of water cooling employed moderate cooling so that more time was left for heat transfer of the core during cooling.
  • the heat transfer allowed the surface temperature increasing, and then the surface was cooled rapidly again by intense cooling manner so that the heat of the surface was removed quickly. At this time, the heat transfer allowed the surface temperature and the core temperature become the same, thus ensuring the uniformity of mechanical properties.
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