CN113755760B - In-situ nano reinforced and toughened steel for crankshafts - Google Patents

In-situ nano reinforced and toughened steel for crankshafts Download PDF

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CN113755760B
CN113755760B CN202111063047.9A CN202111063047A CN113755760B CN 113755760 B CN113755760 B CN 113755760B CN 202111063047 A CN202111063047 A CN 202111063047A CN 113755760 B CN113755760 B CN 113755760B
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molten steel
vanadium
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CN113755760A (en
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王艳林
张灵通
陈晓华
王自东
张博炜
郑志浩
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University of Science and Technology Beijing USTB
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    • B—PERFORMING OPERATIONS; TRANSPORTING
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Abstract

The in-situ nano reinforced and toughened crankshaft steel is characterized by comprising the following components in parts by weight: 0.40-0.42% of carbon, 0.23-0.27% of silicon, 0.70-0.74% of manganese, 1.08-1.18% of chromium, 0.21-0.23% of molybdenum, 0.005-0.015% of titanium, 0.02-0.04% of vanadium, 0.010-0.040% of aluminum, 0.014-0.025% of sulfur and less than or equal to 0.015% of phosphorus. According to the invention, through adding the trace alloy elements of vanadium and rare earth nanowires in the refining process and improving a series of operations of the refining step, a large number of in-situ nano particles which are dispersed and distributed are formed in the processes of steel melt, solidification and the like, the particle size is 2-30 nanometers, the distance between the nano particles is 20-100 nanometers, and 10 percent of the nano particles are contained in each crystal particle 9 ～10 12 The nanoparticles are dispersed in a large amount of in-situ nanoparticles, so that the generation of columnar crystals is inhibited, fine isometric crystals are formed, the macro-segregation is improved, the formation and the size of a brittle phase are reduced, and the strength and the plasticity of the crankshaft steel are improved; the in-situ nano particles and the matrix are in a coherent or semi-coherent relationship, dislocation cannot be easily accumulated, stress concentration is reduced, toughness is not reduced, and the problem of contradiction between reinforcement and toughening is solved.

Description

In-situ nano reinforced and toughened crankshaft steel
Technical Field
The invention belongs to the field of preparation of steel for crankshafts, and particularly relates to steel for in-situ nano reinforced and toughened crankshafts.
Background
The crankshaft is the most important part of the engine, the geometrical shape is complex, and the working condition is very bad. When the engine works, the periodically changed bending and torsional stress is generated, and the force transmitted by the connecting rod is borne and converted into torque which is output through the crankshaft and drives other accessories on the engine to work. In the actual operation process of the engine, the crankshaft is subjected to constantly changing gas pressure, inertia force and moment to bear bending and twisting composite loads, and particularly, the crankshaft of the diesel engine generates sliding friction with the bearing at a high relative speed under a high compression ratio, so that alternating stress such as bending, twisting, shearing, tension and compression and the like are generated at each part, and higher temperature and abrasion are generated. Because of the importance of crankshafts and the complexity of their operating conditions, crankshafts are required to have high tensile strength, fatigue strength, surface strength, and wear resistance, while the center must have some toughness. If the density of the core of the material is not good, the fatigue performance of the material can be reduced due to the existence of more defects such as loose segregation and the like, and the generation and the expansion of fatigue cracks are promoted to cause the broken shaft failure of the crankshaft.
42CrMo is used as a representative steel grade of alloy structural steel and is widely applied to high-end fields such as mechanical manufacturing, high-speed locomotives, automobiles and the like. However, compared with the foreign advanced steel enterprises, the problem of insufficient quality stability still exists, and particularly in the field of automobile crankshafts, the high-strength steel crankshaft can not be stably used as a raw material and needs to be improved again in the aspect of toughness.
Disclosure of Invention
The invention provides in-situ nano reinforced and toughened crankshaft steel, which is used for overcoming the defects in the prior art.
The invention is realized by the following technical scheme:
the in-situ nano reinforced and toughened crankshaft steel is characterized by comprising the following components in parts by weight: 0.40-0.42% of carbon, 0.23-0.27% of silicon, 0.70-0.74% of manganese, 1.08-1.18% of chromium, 0.21-0.23% of molybdenum, 0.005-0.015% of titanium, 0.02-0.04% of vanadium, 0.010-0.040% of aluminum, 0.014-0.025% of sulfur and less than or equal to 0.015% of phosphorus.
The preparation method of the in-situ nano reinforced and toughened steel for the crankshaft comprises the following steps:
the method comprises the following steps: the raw materials comprise molten iron, pig iron blocks and scrap steel, wherein the proportion of the molten iron is more than or equal to 70 percent, and the scrap steel containing more than or equal to 0.20 percent of Ni and more than or equal to 0.20 percent of Cu cannot be added;
step two: 1000 kg of lime and 200 kg of high-alkalinity high-aluminum refining slag are adopted as slag materials, 180 kg of pure aluminum blocks and 200 kg of pure aluminum blocks are adopted as deoxidizing agents, and low-nitrogen recarburizing agents are adopted as recarburizing agents;
step three: when tapping, adding high-carbon ferromanganese, imported high-carbon ferrochrome, ferromolybdenum and other alloys into the steel ladle along with the steel flow;
step four: before the temperature of the converter molten steel is not lower than 1540 ℃, the converter molten steel is sent into an LF furnace for refining, vanadium is added in the refining process, and the refining time is controlled to be 50 minutes per furnace;
step five: refining in a VD furnace, wherein the vacuum degree is less than or equal to 67Pa, the vacuum retention time is more than or equal to 20min, and after the vacuum is finished, feeding a sulfur cored wire to adjust the sulfur content to the right position;
step six: carrying out light blowing after the adjustment of the sulfur content is finished, wherein the soft blowing time is required to be more than or equal to 25 min;
step seven: the ladle is hoisted to a continuous casting station, the ladle is connected with the ladle long nozzle through the steel tapping hole of the ladle to protect molten steel from being oxidized, the molten steel flows into a 40-ton tundish through the ladle long nozzle, and the molten steel flows into a tundish through an integral built-in nozzle of the tundish
Figure GDA0003721642700000021
Crystallizer with water quantity of 230m 3 H, M-EMS300A/2Hz, the shelling is realized by the lubrication of special covering slag, the casting blank forms a thin layer blank shell with the thickness of about 100-150mm, and the obtained product is cooled by gas mist with the secondary cooling specific water quantity of 0.18L/kg and matched with F-EMS 480A/5Hz to obtain the alloy
Figure GDA0003721642700000022
The continuous casting slab of (1).
According to the in-situ nano reinforced and toughened crankshaft steel, fluorite is added according to the slag condition and the proportion of 0.8-1.0Kg/t for slag adjustment in the process of putting the steel into an LF furnace and transmitting electricity for heating.
In the fourth step, silicon carbide or activated carbon powder with the weight percentage of 0.05 percent of molten steel is added for diffusion deoxidation operation, and aluminum particles with the weight percentage of 0.03 percent of molten steel can also be added for strengthening deoxidation.
The steel for the in-situ nano reinforced and toughened crankshaft is obtained by the steps of transmitting electricity for heating for 10 minutes, measuring the temperature after slagging and sampling, taking the first sample for full analysis when the temperature of molten steel is more than or equal to 1560 ℃, adjusting the components of the molten steel according to the content requirement of an internal control standard according to the analysis result, and adding vanadium according to the standard of 0.6kg/t for alloying.
According to the in-situ nano reinforced and toughened crankshaft steel, the white slag time is guaranteed to be more than or equal to 20 minutes in the refining process in the fourth step.
In the fourth step, after the components and the temperature of the molten steel meet the target requirements, the rare earth nanowires are firstly added into the molten steel according to the standard of 0.2kg/t for rare earth modification treatment to generate rare earth composite inclusions, and then Si-Ca wires with the specification of phi 13 are fed into the molten steel according to the standard of 0.5kg/t for final deoxidation and inclusion modification treatment.
In the fourth step, the outlet temperature of the LF furnace is controlled as follows: the first furnace-entering VD temperature is controlled within the range of 1620 ℃ to 1630 ℃, and the furnace-entering VD temperatures of other furnaces are slightly reduced and controlled within the range of 1610 ℃ to 1620 ℃.
The invention has the advantages that: according to the invention, a series of operations of adding trace alloying elements vanadium and rare earth nanowires and improving the refining step are performed in the refining process, so that a large number of in-situ nano particles which are dispersed and distributed are formed in the processes of steel melt, solidification and the like, the particle size is 2-30 nanometers, and the nano particles are arranged among the nano particlesThe distance is 20-100 nm, and each crystal grain contains 10 9 ～10 12 The nanoparticles are dispersed in a large amount of in-situ nanoparticles, so that the generation of columnar crystals is inhibited, fine isometric crystals are formed, the macro-segregation is improved, the formation and the size of a brittle phase are reduced, and the strength and the plasticity of the crankshaft steel are improved; the in-situ nano particles and the matrix are in a coherent or semi-coherent relationship, dislocation cannot be easily accumulated, stress concentration is reduced, and therefore toughness is not reduced, and the contradiction problem of strengthening and toughening is solved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a microscopic representation of the grain size of a sample strengthened with vanadium in example 1 of the present invention;
FIG. 2 is a microscopic representation of the grain size of samples not strengthened with vanadium in example 1 of the present invention;
FIG. 3 is a schematic diagram of nanoparticles in a sample strengthened by vanadium-doped rare earth nanowires in example 2 of the invention;
FIG. 4 is a schematic diagram of the in-situ nano-reinforcement mechanism in example 2 of the present invention;
FIG. 5 is a schematic view of rare earth nanowires in example 2 of the present invention;
FIG. 6 is a schematic view showing the feeding of rare earth nanowires into molten steel in example 2.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without inventive step based on the embodiments of the present invention, are within the scope of protection of the present invention.
Example 1
Research on strengthening of vanadium to crankshaft steel 42CrMoA
The trace element vanadium has the highest solubility and is one of the most commonly used and most effective strengthening elements for microalloyed steel. The role of vanadium is to influence the structure and properties of steel by forming V (C, N). It is mainly precipitated in ferrite of austenite grain boundary, thereby refining ferrite grains and improving the strength of steel. Theoretical studies have shown that adding 0.10% vanadium to steel can increase strength by more than 200 MPa. The strengthening effect generated by the massive precipitation of the trace element V in the ferrite is obvious, and compared with other microalloying modes, the V-containing steel has lower yield ratio. For the requirement of strong plastic matching of steel, the V-containing microalloyed steel has unique product performance advantages.
(1) Chemical composition of 42CrMoA steel for engine crankshaft
Vanadium addition strengthening is carried out according to a design scheme, and the detection results of chemical components are shown in the following table 1.
TABLE 142CrMoA chemistry
Figure GDA0003721642700000051
The grain size and the impact property of 42CrMo steel for crankshafts in industrial tests are detected by microalloying the component V, the influence on the grain size and the impact property of the steel before and after the V is added is contrastively analyzed, and various research results are as follows.
(2) Influence of V microalloying on austenite grain size of 42CrMoA steel for engine crankshaft
In the blank heating process, the heating time is long, the heating temperature is high, austenite grains are easy to become coarse, and after V is added, the coarsening temperature can be increased, and the growth of the austenite grains is effectively prevented. And the foreign particles which are highly dispersed and distributed in the matrix play a role in preventing dislocation movement, so that the strength of the material is improved. The grain size of 42CrMo steel without V addition and 42CrMoA steel for a crankshaft after V microalloying were measured, and the specific grain size grades are shown in table 2. Therefore, the grain size of the 42CrMoA is stabilized at 7.5 grade and above after vanadium is added, and is improved by about 1 grade compared with 42CrMo without vanadium, and the effect of refining the grains of the vanadium is verified.
TABLE 2 grain size test results
Figure GDA0003721642700000061
The grain size of the sample strengthened by adding vanadium is shown in figure 1, the grain size of the sample not strengthened by adding vanadium is shown in figure 2, and from figures 1 and 2, the grain size of the sample strengthened by adding vanadium is grade 8, while the grain size of the sample not strengthened by adding vanadium is grade 7, so that the grain size grade of the grain strengthened by adding vanadium can be improved from grade 7 to grade 8.
(3) Influence of V microalloying on impact toughness of 42CrMoA steel for engine crankshaft
The normal temperature impact toughness test is carried out on the 42CrMo steel without V and the 42CrMoA steel for the crankshaft after V microalloying, and the specific test results are shown in tables 3 and 4. It can be seen that after vanadium strengthening, the impact energy is increased from 65.3J to 68.9, and is increased by 5.5%. The performance improvement verifies that the addition of V refines the crystal grains of the steel, so that more crystal boundaries are generated, the directions of all interfaces are staggered to form interface energy, and the impact toughness of the crankshaft steel is effectively improved.
TABLE 3 impact test results after no vanadium reinforcement
Figure GDA0003721642700000071
TABLE 4 impact test results after vanadium addition strengthening
Figure GDA0003721642700000072
Example 2
In-situ nano reinforced and toughened engine crankshaft steel
In-situ nanoparticles induce grain refinement, and a large number of nanoparticles are distributed in fine grains to form a nano microstructure structure, so that the toughness is improved; the in-situ nano particles absorb solute atoms, refine crystal grains and provide diffusion channels to eliminate segregation and relieve the deterioration of brittle segregation relative to plasticity and toughness; the interaction of in-situ nano particles, fine grains and dislocation is utilized to generate strain hardening, dissipate energy, delay material instability and simultaneously improve obdurability. The strengthening mechanism is shown in FIG. 4.
Nanowires are fed through the LF external refining process, as shown in figures 5 and 6, and precipitation strengthening is carried out.
Before the temperature of the converter molten steel is not lower than 1540 ℃, the converter molten steel reaches an LF furnace for refining, the refining time is controlled to be 50 minutes per furnace, the LF furnace is stirred by adopting argon, and the argon stirring adopts a blowing mode of firstly blowing large gas quantity and then blowing small gas quantity, so that the effects of desulfurization and alloy component uniformity are enhanced. In the refining period, the diffusion deoxidation is mainly carried out by taking silicon carbide or carbon powder, and an appropriate amount of aluminum particles can be added for strengthening deoxidation. The LF refining steps are as follows:
(1) in the process of putting the ladle into an LF furnace for power transmission and heating, fluorite can be properly added according to the slag condition for slag adjustment.
(2) In the refining period, the diffusion deoxidation is mainly carried out by taking silicon carbide or carbon powder, and an appropriate amount of aluminum particles can be added for strengthening deoxidation.
(3) And (3) transmitting electricity for about 10 minutes, measuring the temperature after slagging, sampling, taking the first sample for full analysis when the temperature of the molten steel is more than or equal to 1560 ℃, adjusting the components of the molten steel according to the analysis result and the content requirement of the internal control standard, and adding vanadium according to the standard of 0.6kg/t for alloying.
(4) Ensuring that the white slag time is more than or equal to 20 minutes, ensuring good fluidity of refining slag, strengthening diffusion deoxidation and controlling the LF total oxygen to be less than 15 ppm. After the second sample is analyzed, the components are adjusted to target values.
(5) After the components and the temperature of the molten steel meet the target requirements, firstly, the rare earth nanowires are added into the molten steel according to the standard of 0.5m/t for rare earth modification treatment to generate rare earth composite inclusions, then Si-Ca wires with the specification of phi 13 are fed into the molten steel according to the standard of 3m/t for final deoxidation and inclusion modification treatment to ensure that the dissolved calcium and Al meet the target requirements 2 O 3 Reaction is carried out to convert the CaO into CaO-Al with lower melting point 2 O 3 The composite inclusion-like substance is preferably 12 CaO.7Al having a melting point of 1450 ℃ as a target product 2 O 3 The substance is completely liquid at the steelmaking temperature, and is more beneficial to floating removal.
(6) Controlling the outlet temperature of the LF furnace, controlling the temperature of the first ladle VD furnace within 1620-1630 ℃, and controlling the temperature of the other furnace VD furnaces to be slightly reduced to 1610-1620 ℃.
The analysis of a scanning electron microscope and the analysis of mechanical properties are carried out on the 42CrMoA engine crankshaft steel, and the analysis specifically comprises the following steps:
(1) through detecting the reinforced engine crankshaft steel 42CrMoA, a large amount of nano-micron microstructure structures exist in the steel, so that the strengthening and toughening effects are achieved.
(2) The changes of the mechanical properties of the crankshaft steel before and after the in-situ nano-reinforcing and toughening are contrastively analyzed and shown in tables 5 and 6. After the trace element V and the rare earth nanowires are added, the strength and the plasticity of the steel are well improved, the tensile strength is improved by more than 90MPa, the yield strength is improved by more than 60MPa, and the use performance of the crankshaft is improved.
TABLE 5 mechanical Properties of 42CrMo crankshaft steel before strengthening
Figure GDA0003721642700000091
TABLE 6 mechanical properties of 42CrMoA reinforced rear crankshaft steel
Figure GDA0003721642700000101
According to the invention, by preparing the nano microstructure structure, a large number of grain boundaries and nano particle/matrix interfaces are provided, the strain hardening rate is improved, and the nano vanadium carbonitride particles are formed by adding the trace element V, so that the performance of the crankshaft steel is comprehensively improved, and the crankshaft steel is enhanced and toughened.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (1)

1. The in-situ nano reinforced and toughened crank shaft steel is characterized in that: 0.40-0.42% of carbon, 0.23-0.27% of silicon, 0.70-0.74% of manganese, 1.08-1.18% of chromium, 0.21-0.23% of molybdenum, 0.005-0.015% of titanium, 0.02-0.04% of vanadium, 0.010-0.040% of aluminum, 0.014-0.025% of sulfur, less than or equal to 0.015% of phosphorus, and the balance of iron and unavoidable impurities;
the preparation method comprises the following steps:
the method comprises the following steps: the raw materials comprise molten iron, pig iron blocks and scrap steel, wherein the proportion of the molten iron is more than or equal to 70 percent, and the scrap steel containing more than or equal to 0.20 percent of Ni and more than or equal to 0.20 percent of Cu cannot be added;
step two: 1000 kg lime/furnace and 200 kg high-alkalinity high-aluminum refining slag/furnace are adopted as slag materials, 180 kg-200 kg pure aluminum blocks are used as deoxidizing agents per furnace, and low-nitrogen recarburizing agents are used as recarburizing agents;
step three: adding high-carbon ferromanganese, imported high-carbon ferrochromium and ferromolybdenum into a ladle along with the steel flow during tapping;
step four: before the temperature of the converter molten steel is not lower than 1540 ℃, the converter molten steel is sent into an LF furnace for refining, vanadium is added in the refining process, and the refining time is controlled to be 50 minutes per furnace;
step five: refining in a VD furnace, wherein the vacuum degree is less than or equal to 67Pa, the vacuum retention time is more than or equal to 20min, and after the vacuum is finished, feeding a sulfur cored wire to adjust the sulfur content to the right position;
step six: after the adjustment of the sulfur content is finished, soft blowing is carried out, and the soft blowing time is required to be more than or equal to 25 min;
step seven: the ladle is hoisted to a continuous casting station, the ladle is connected with the ladle long nozzle through the steel tapping hole of the ladle to protect molten steel from being oxidized, the molten steel flows into a 40-ton tundish through the ladle long nozzle, and the molten steel flows into a tundish through an integral built-in nozzle of the tundishPhi 500mm crystallizer with water amount of 230m 3 M-EMS300A/2Hz, the unshelling is realized through the lubrication of special covering slag, the casting blank forms a 100-150mm thin-layer blank shell, and the phi 500mm continuous casting blank is obtained through the secondary cooling of aerosol with the specific water amount of 0.18L/Kg and the matching of F-EMS 480A/5 Hz;
in the process of putting the steel ladle into an LF furnace and transmitting electricity to heat, fluorite is added according to the slag condition and the proportion of 0.8-1.0Kg/t of steel per ton to carry out slag mixing;
in the fourth step, silicon carbide is added according to 0.5Kg/t to carry out diffusion deoxidation operation;
after the electric heating is carried out for 10 minutes in the fourth step, the temperature is measured and the sample is taken after the slag is melted, when the temperature of the molten steel is more than or equal to 1560 ℃, the first sample is taken for full analysis, the components of the molten steel are adjusted according to the requirement of the content of the internal control standard according to the analysis result, and at the moment, vanadium is added according to the standard of 0.6Kg/t for alloying;
in the fourth step, the refining process ensures that the white slag time is more than or equal to 20 minutes;
after the components and the temperature of the molten steel meet the target requirements in the fourth step, firstly, adding the rare earth nanowires into the molten steel according to the standard of 0.2Kg/t, performing rare earth denaturation treatment to generate rare earth composite inclusions, then, feeding Si-Ca wires with the specification of phi 13 into the molten steel according to the standard of 0.5Kg/t, and performing final deoxidation and inclusion denaturation treatment;
in the fourth step, the LF furnace outbound temperature is controlled as follows: the temperature of the first furnace entering VD is controlled within the range of 1620 ℃ to 1630 ℃, and the temperature of the other furnace entering VD is slightly reduced and controlled within the range of 1610 ℃ to 1620 ℃.
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