CN117344205A

CN117344205A - Non-quenched and tempered steel, connecting rod manufactured by same and manufacturing method of non-quenched and tempered steel

Info

Publication number: CN117344205A
Application number: CN202210745298.3A
Authority: CN
Inventors: 余大江; 赵四新; 黄宗泽; 黄瑞; 蒋锡军; 柳向椿
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2022-06-27
Filing date: 2022-06-27
Publication date: 2024-01-05

Abstract

The present disclosure relates to a non-quenched and tempered steel, a method of manufacturing the same, and a connecting rod, particularly a broken connecting rod, manufactured from the non-quenched and tempered steel. The non-quenched and tempered steel of the present disclosure comprises the following chemical elements in mass percent: c:0.40-0.50%, si:0.45-0.80%, mn:1.35-1.75%, P: less than or equal to 0.025 percent, S:0.040-0.060%, cr:0.10-0.30%, ni:0.05-0.20%, mo:0.01-0.07%, V:0.10-0.20%, nb:0.020-0.035%, N:0.014-0.022%, al: less than or equal to 0.025 percent, ca:0.0008-0.0018%, the balance being Fe and other unavoidable impurities; wherein the content of Mn and S satisfies Mn/S not less than 30. The expansion fracture connecting rod made of the non-quenched and tempered steel has high strength, uniform structure and flat expansion fracture surface.

Description

Non-quenched and tempered steel, connecting rod manufactured by same and manufacturing method of non-quenched and tempered steel

Technical Field

The disclosure belongs to the technical field of metallurgy, and relates to non-quenched and tempered steel, a manufacturing method thereof, and a connecting rod, particularly a broken connecting rod, manufactured by the non-quenched and tempered steel.

Background

Non-quenched and tempered steel originates from a second energy crisis and has subsequently been rapidly developed in europe and even worldwide. In use, 90% of crankshafts and 75% of connecting rods in the japanese automotive industry are manufactured from non-quenched and tempered steel. As carbon peaking and carbon neutralization are globally advanced, the demand for non-quenched and tempered steel will increase. The connecting rod is used as a core stress part of the engine and bears strong alternating load. In order to meet the requirements of high strength, weight reduction, energy conservation and emission reduction of vehicles, the steel for the connecting rod is developed towards the direction of improving the strength and high fatigue.

The expanding fracture processing technology developed in the 90 th century of the 20 th century fundamentally changes the processing method of the traditional connecting rod. The expansion fracture processing is to process a cracking groove at the inner side of a large end hole of the connecting rod, and the cracking groove is cracked and rapidly expanded by pressure by using notch sensitivity, so that the separation of the connecting rod body and the connecting rod cover is realized, and the working procedures are reduced. However, the problem of uneven fracture, slag falling and the like easily occurs in the processing process of the connecting rod. The expansion fracture processing technology requires that the material hardly generates plastic deformation after expansion fracture, namely, the toughness of the connecting rod is limited on the premise of ensuring comprehensive use performance, and the brittle expansion fracture is ensured. For the expansion connecting rod manufactured by expansion processing, the expansion connecting rod is required to have high strength and toughness from the point of product use, and is required to have the characteristic of easy expansion from the point of processing and manufacturing. Non-quenched and tempered steel is successfully applied to the expansion joint rod by virtue of its excellent comprehensive properties.

Chinese patent application publication No. CN111218614a, entitled "steel for free-cutting connecting rod and method for manufacturing same", discloses a steel for free-cutting connecting rod, comprising the following chemical components in mass percent: c:0.43-0.47%, si:0.20-0.35%, mn:1.10-1.40%, cr:0.30-0.40%, P: less than or equal to 0.020 percent, S:0.060-0.090%, ni: less than or equal to 0.10 percent, cu: less than or equal to 0.20 percent, mo: less than or equal to 0.10 percent, ti: less than or equal to 0.015 percent of Al:0.010-0.040 percent, less than or equal to 0.0015 percent of [ O ] and the balance of Fe and unavoidable impurities. The grain size of the steel for the free-cutting connecting rod is 6-9 grade, the yield strength of the steel after normalizing is more than or equal to 470MPa, the tensile strength is more than or equal to 730MPa, the elongation is more than or equal to 17%, and the hardness is 217-245HB. However, from the mechanical property point of view, the strength of the steel for the free-cutting connecting rod is low, and the requirement of a high-explosion-pressure engine for high strength cannot be met.

Chinese patent application publication No. CN110343954a, entitled "steel for automotive engine connecting rod and method of manufacturing same", discloses a steel for automotive engine connecting rod, which comprises the following chemical elements in weight percentage: c:0.32% -0.40%, si:0.40% -0.70%, mn:0.80% -0.95%, S:0.025% -0.040%, P:0.030% -0.045%, cr:0.10% -0.25%, mo is less than or equal to 0.10%, nb:0.08% -0.20%, ti:0.015% -0.035%, ni is less than or equal to 0.10%, N:0.012-0.020%, O is less than or equal to 15X 10 ^-6 The balance of Fe and unavoidable impurities. However, the steel for connecting rod of automobile engine contains a certain amount of Ti element, which is liable to generate angular liquid-out TiN and is unfavorable for fatigue performance.

Chinese patent application publication No. CN106939391a, entitled "a Ca microalloyed free-cutting high strength steel for expansion joint rod and method of manufacturing", discloses a Ca microalloyed free-cutting high strength steel for expansion joint rod having the following chemical composition (in mass%): c:0.25-0.60%, si:0.10-1.20%, mn:0.40-1.50%, cr:0.05-0.50%, S:0.02-0.15%, P:0.02-0.15%, V:0.03-0.55%, ca:0.0005-0.0080%, N:0.002-0.035%, al:0.002-0.080%, ti: less than or equal to 0.02 percent, the balance being Fe and unavoidable impurity elements, and S/Ca: and is more than or equal to 15. The yield strength of the steel is 550-650Mpa, the tensile strength is 800-950Mpa, and the elongation is less than or equal to 20%. However, the tensile strength of the steel is below 950MPa, and the requirement of a high-explosion-pressure engine for high strength cannot be met.

Disclosure of Invention

In view of the above-described drawbacks and deficiencies of the prior art, the present inventors have obtained a non-quenched and tempered steel having high strength and good machinability, while satisfying the requirements of high strength and easy expansion processing of steel for connecting rods, by optimizing alloy compositions.

In a first aspect, the present disclosure provides a non-quenched and tempered steel comprising, in addition to Fe and unavoidable impurities in an amount of 90% or more, the following chemical elements in mass percent: c:0.40-0.50%, si:0.45-0.80%, mn:1.35-1.75%, P: less than or equal to 0.025 percent, S:0.040-0.060%, cr:0.10-0.30%, ni:0.05-0.20%, mo:0.01-0.07%, V:0.10-0.20%, nb:0.020-0.035%, N:0.014-0.022%, al: less than or equal to 0.025 percent, ca:0.0008-0.0018%; and the content of Mn and S satisfies Mn/S not less than 30.

In a second aspect, the present disclosure provides a non-quenched and tempered steel comprising, in mass percent: c:0.40-0.50%, si:0.45-0.80%, mn:1.35-1.75%, P: less than or equal to 0.025 percent, S:0.040-0.060%, cr:0.10-0.30%, ni:0.05-0.20%, mo:0.01-0.07%, V:0.10-0.20%, nb:0.020-0.035%, N:0.014-0.022%, al: less than or equal to 0.025 percent, ca:0.0008-0.0018%, the balance being Fe and other unavoidable impurities; and the content of Mn and S satisfies Mn/S not less than 30.

In one embodiment, the non-quenched and tempered steel of the present disclosure further comprises a gas element [ O ] and/or [ H ], the content of [ O ] being 0.0020% or less and the content of [ H ] being 0.0002% or less.

In a preferred embodiment, the non-quenched and tempered steel of the present disclosure has a V content of 0.14 to 0.18%.

In a preferred embodiment, the content of Nb in the non-quenched and tempered steel of the present disclosure is 0.023 to 0.032%.

In a preferred embodiment, the content of S in the non-quenched and tempered steel of the present disclosure is 0.040 to 0.056%.

In a preferred embodiment, the non-quenched and tempered steel of the present disclosure does not add Ti, cu or Mg elements.

In one embodiment, the metallurgical structure of the non-quenched and tempered steel is ferrite + pearlite.

In a preferred embodiment, the ferrite content in the non-quenched and tempered steel is 20 to 30 area%.

In a preferred embodiment, the non-quenched and tempered steel of the present disclosure is used to make a tie rod.

The non-quenched and tempered steel realizes high strength and free cutting property by controlling the mass percentage content of each element and controlling the ratio of Mn to S element content to be more than or equal to 30 Mn/S. The mechanical properties of the non-quenched and tempered steel of the present disclosure satisfy: the tensile strength Rm is more than or equal to 950MPa, the regulated non-proportional extension strength Rp0.2 is more than or equal to 650MPa, the elongation after fracture A is more than or equal to 15%, and the area shrinkage Z is more than or equal to 40%.

In a third aspect, the present disclosure provides a connecting rod made of the non-quenched and tempered steel according to the first or second aspect.

In one embodiment, the grain size of the connecting rod is 6±1 grade.

In one embodiment, the metallographic structure of the connecting rod is ferrite + pearlite.

In a preferred embodiment, the ferrite content of the connecting rod is 5-15 area%.

In a preferred embodiment, the average lamellar spacing of the pearlite in the connecting rod is 160-210nm.

In a preferred embodiment, the connecting rod of the present disclosure is a broken connecting rod.

The connecting rod disclosed by the invention has good mechanical properties, and meets the following conditions: rm is more than or equal to 1000MPa, rp0.2 is more than or equal to 800MPa, A is more than or equal to 10%, and Z is more than or equal to 20%. In addition, the connecting rod disclosed by the invention has uniform structure, flat fracture surface, no plastic deformation, no slag falling and the like.

In a fourth aspect, the present disclosure provides a method of manufacturing non-quenched and tempered steel according to the first or second aspect, comprising the steps of: smelting, refining, vacuum degassing and continuous casting are sequentially carried out on molten steel to prepare a continuous casting blank; and hot rolling the continuous casting blank to obtain the non-quenched and tempered steel.

In the manufacturing method of the present disclosure, smelting, refining, and vacuum degassing may be performed using a manner commonly used in the art, such as electric furnace smelting, LF refining, or VD vacuum degassing, and the like.

In one embodiment, in the manufacturing process of the present disclosure, after refining, the sulfur line is fed to an upper limit of sulfur content, and/or the calcium wire is fed 40-60m after the break, prior to vacuum degassing treatment.

In one embodiment, in the manufacturing method of the present disclosure, a casting temperature of 1500-1520 ℃ and/or a constant pull rate of 0.62-0.68m/min is employed during continuous casting.

In one embodiment, in the manufacturing method of the present disclosure, electromagnetic stirring is performed during continuous casting, including crystallizer electromagnetic stirring and solidification end electromagnetic stirring.

In one embodiment, in the manufacturing method of the present disclosure, the electromagnetic intensity of the electromagnetic stirring of the crystallizer is 300A/2.5Hz, and the electromagnetic intensity of the electromagnetic stirring of the solidification end is 800A/8Hz.

In one embodiment, in the manufacturing method of the present disclosure, a continuous casting slab obtained by continuous casting is slowly cooled to 500 ℃ or lower and then enters a heating furnace to be hot rolled, the rolling heating and heat preserving temperature is 1200-1220 ℃, and the final rolling temperature is 900-950 ℃.

In one embodiment, the manufacturing method of the present disclosure further comprises: after hot rolling, the non-quenched and tempered steel is cooled to 330-380 ℃ in a cooling bed, and then is slowly cooled in a slow cooling pit at a cooling speed of 0.15-0.25 ℃/min.

In the manufacturing method disclosed by the disclosure, the manufacturing process parameters of the non-quenched and tempered steel are optimally designed, so that the structural uniformity in the steel is further improved.

Drawings

FIG. 1 shows the metallographic structure of the non-quenched and tempered steel of example 1.

FIG. 2 shows the fracture surface of a broken connecting rod forged from the non-quenched and tempered steel of example 1.

FIG. 3 shows the metallographic structure of a broken connecting rod forged from the non-quenched and tempered steel of example 1.

Fig. 4 shows pearlite lamellae in the metallographic structure of the expansion joint rod forged from the non-quenched and tempered steel of example 1.

FIG. 5 shows the surface quality of the non-quenched and tempered steel of comparative example 2.

Fig. 6 shows grain size and distribution in a metallographic structure of a broken connecting rod forged from non-quenched and tempered steel of comparative example 3.

Detailed Description

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

In this context, the tensile strength Rm is the critical value for the transition of a metal from uniform plastic deformation to localized concentrated plastic deformation, and is also the maximum load-bearing capacity of a metal under static tensile conditions. Tensile strength is the resistance characterizing the material to maximum uniform plastic deformation.

Herein, the specified non-proportional elongation strength Rp is the stress at which the non-proportional elongation is equal to the specified extensometer gauge length percentage, rp0.2 is the stress at which the specified extensometer gauge length is 0.2%.

The elongation after break A refers to the percentage of the elongation of the test bar to the original length when the metal material breaks under the action of external force (tensile force). The elongation after break is an important index reflecting the plasticity of the metal material, and the larger the value is, the better the plasticity of the metal material is.

In this context, the reduction of area Z is the percentage of the maximum reduction in cross-sectional area at the reduced diameter of the test specimen after it has been pulled off, to the original cross-sectional area.

Herein, the tensile strength Rm, the prescribed non-proportional elongation Rp0.2, the elongation after break A and the reduction of area Z are according to GB/T228.1-2010 section 1 of the metallic Material tensile test: room temperature test method.

As used herein, "crystallizer electromagnetic stirring (M-EMS)" means that an electromagnetic stirrer is placed at the periphery of the crystallizer.

As used herein, "solidification end electromagnetic stirring (F-EMS)" refers to an electromagnetic stirrer placed in a section of casting core having an equivalent diameter of less than 40 mm.

In this context, the higher the number of grades of grain size, the smaller the grain size. The grain sizes of the 6+ -1 grade and 7-8 grade are 31-63 μm and 20-31 μm, respectively.

In this context, the grain size is determined according to GB/T6394-2017 method for determination of mean grain size of metals.

Herein, the ferrite content is measured by statistical calculation of the ferrite area ratio in metallographic photographs using image analysis software of a metallographic microscope itself in%.

Herein, the average lamellar spacing of pearlite is measured as follows: on the pearlite lamellar photograph taken by a scanning electron microscope, an average value of the single lamellar spacing is calculated according to the number of the cutting points where the line segments of known length intersect lamellar. For example, a line is drawn along the diagonal line of fig. 4, the number of intersections of the line with the sheet is counted, and the average sheet pitch is calculated by the line segment length/(the number of intersections-1).

Aiming at the requirements of a high-explosion-pressure engine on high-strength non-quenched and tempered steel for a connecting rod and the problem that fracture defects easily occur in the connecting rod expanding and breaking processing process, the non-quenched and tempered steel is obtained by optimizing alloy components. Specifically, certain amounts of V element and Nb element are added into the non-quenched and tempered steel, the strength of the steel is improved through precipitation strengthening of V (N, C), and grain boundaries are pinned in the hot working process through uniform Nb (N, C) precipitation phases; meanwhile, a certain amount of S element is added to improve the cutting processing performance; and the production of hot brittle FeS is avoided by controlling Mn/S to be more than or equal to 30.

In the non-quenched and tempered steel of the present disclosure, the design principle of each chemical element is specifically as follows:

c (carbon): the C element has a larger influence on the strong plasticity of the steel. The strength of the steel increases with the content of C element. Therefore, the higher the content of C element in the steel, the higher the strength of the steel. However, it is also desirable for the connecting rod to have a good toughness to avoid brittle failure of the connecting rod under alternating stresses during use. When the content of the C element in the steel is too low, the strength of the steel is insufficient. When the content of C element in steel is too high, the toughness of the steel is insufficient, and the use requirement under alternating stress is difficult to meet. Therefore, in order to ensure that the non-quenched and tempered steel has higher strength and meets the use requirement of bearing alternating stress, the content of the C element in the non-quenched and tempered steel is controlled to be between 0.40 and 0.50 percent.

Si (silicon): the Si element is a deoxidizing element and can effectively improve the strength of the steel. In the case where other elements are certain, the strength of the steel can be improved to some extent by increasing the content of Si element in the steel. However, it should be noted that as the Si element content in the steel further increases, a bainitic or martensitic structure is easily formed in the steel. Therefore, in the non-quenched and tempered steel of the present disclosure, the content of Si element is controlled to be between 0.45 and 0.80%.

Mn (manganese): mn element is used as alloy element, and can effectively improve the strength of steel. When the content of Mn element in steel is controlled within a certain range, the method can also help the plasticity and toughness of the steel to a certain extent. Meanwhile, for sulfur-containing free-cutting steel, sufficient Mn element is added to the steel, so that MnS inclusions can be formed with S, thereby improving the cutting performance of the steel and avoiding S and Fe from generating hot-brittle FeS. However, when the Mn element content in the steel is too high, not only localized segregation is likely to be formed, but also the contribution to strength is weak. Based on this, in the non-quenched and tempered steel of the present disclosure, the content of Mn element is controlled to be between 1.35 and 1.75%.

P (phosphorus): the P element is an unavoidable impurity element in the steel material. Theoretically, the lower the content of the impurity element in the steel, the better. In the non-quenched and tempered steel of the present disclosure, the content of the P element is controlled to 0.025% or less in consideration of factors such as low toughness requirements of the steel for connecting rods and cost control.

S (sulfur): s is a hot shortness and free cutting machinability element. Machinability of steel improves with an increase in the content of S element in the steel. However, the hot workability of steel deteriorates with increasing sulfur content. Therefore, the content of S element in the steel is not excessively large for the non-quenched and tempered steel for connecting rod requiring hot working. However, when the content of S element in steel is too low, for example, less than 0.040%, chip breaking thereof becomes poor, and a winding tool often occurs, and drilling becomes difficult. Based on this, in the non-quenched and tempered steel of the present disclosure, the content of S element is controlled to be between 0.040 and 0.060%, preferably between 0.040 and 0.056%.

Cr (chromium): the Cr element can effectively improve the strength and hardenability of the steel, and is favorable for refining the interlayer spacing of pearlite sheets in the structure, thereby improving the strength of the non-quenched and tempered steel. It should be noted that when the Cr element content in the steel is less than 0.10%, the effect thereof is not significant. However, when the Cr element content in the steel is too high, for example, more than 0.30%, more alloy carbide is formed. Based on this, in the non-quenched and tempered steel of the present disclosure, the content of Cr element is controlled to be between 0.10 and 0.30%.

Ni (nickel): the Ni element can effectively improve the strength of the steel without reducing the toughness of the steel, and is particularly beneficial to the low-temperature performance of the steel. It should be noted that when the Ni element content in the steel is less than 0.05%, its contribution to the strength is weak. However, when the Ni element content in the steel is high, for example, higher than 0.20%, bainite occurs in the structure. Based on this, in the non-quenched and tempered steel of the present disclosure, the content of Ni element is controlled to be between 0.05 and 0.20%.

Mo (molybdenum): the Mo element can obviously improve the hardenability and strength of the steel. When the content of Mo element in the steel is less than 0.01%, the action effect of the Mo element is not obvious. However, when the Mo element content in the steel exceeds 0.07%, hardenability of the steel is greatly improved, and a bainitic structure is easily formed. Based on this, in the non-quenched and tempered steel of the present disclosure, the content of Mo element is controlled to be between 0.01 and 0.07%.

V (vanadium): v is an important precipitation strengthening element. The addition of a proper amount of V element in the steel can form V (C, N) precipitated phase in ferrite without affecting the plasticity and toughness of the material, and the strength of the material can be greatly improved. It should be noted that if the content of V element added to steel is too low, the strengthening effect is not obvious. However, when the content of the V element in the steel is too high, the cost is greatly increased. Based on this, considering the production cost and the beneficial effects of the V element in combination, in the non-quenched and tempered steel of the present disclosure, the content of the V element is controlled to be between 0.10 and 0.20%, preferably between 0.14 and 0.18%.

N (nitrogen): the N element may form nitrides or nitrogen carbides with the alloy element V, nb and the like in the steel, and these nitrides or nitrogen carbides may control grains and improve the toughness of the steel by precipitation. However, when the content of N element in the steel is too high, nitrogen remaining after forming a precipitated phase is solid-dissolved in the steel, reducing the economical efficiency thereof. Based on this, in the non-quenched and tempered steel of the present disclosure, the content of N element is controlled to be between 0.014 and 0.022%.

Nb (niobium): nb element can form Nb (C, N) precipitated phase with N, C element, so that thermal defects of non-quenched and tempered steel for connecting rods, which affect material performance, caused by overheating and the like in a high-temperature heating process can be effectively avoided, abnormal growth of crystal grains in a connecting rod forging heating process can be restrained, uniform growth and uniform size of crystal grains after connecting rod forging are ensured, and accordingly a relatively flat expansion fracture is formed after expansion fracture processing of a forged connecting rod, and problems of slag falling and the like caused by larger local crystal grain difference are avoided. The Nb element content in the steel is required to be not less than 0.020% to obtain the above effects. However, when the Nb element content in the steel exceeds 0.035%, the precipitated phase Nb (C, N) tends to accumulate and grow, which is disadvantageous for the fatigue performance of the connecting rod. Thus, in the non-quenched and tempered steel of the present disclosure, the content of Nb element is controlled to be between 0.020 and 0.035%, preferably between 0.023 and 0.032%.

Al (aluminum): the Al element plays a role in deoxidizing in the steelmaking process. However, sulfur-containing steels have a relatively high Al content and are prone to nodulation. Therefore, in the non-quenched and tempered steel of the present disclosure, the content of Al element is controlled to 0.025% or less.

Ca (calcium): for sulfur-containing steels, ca element can improve the morphology of MnS inclusions. The addition of 0.0008% or more of Ca makes it possible to make MnS inclusions finer and more uniform. However, ca combines with S in steel to form high melting CaS, which tends to accumulate at the nozzle during casting, causing nozzle clogging. Therefore, in order to reduce CaS, the Ca content in steel must be controlled to be 0.0018% or less. Therefore, in the non-quenched and tempered steel of the present disclosure, the content of Ca element is controlled to 0.0008 to 0.0018%.

[ O ] (oxygen): oxygen dissolved in steel is easy to form inclusion with Al, ca, casting powder and the like, and the control capacity of oxygen content represents the level of producing clean steel. Thus, the lower the oxygen content control in the steel, the better. Based on the use requirement of the product and the production control cost, the content of the [ O ] element in the non-quenched and tempered steel is controlled below 0.002%.

[H] (hydrogen): hydrogen in steel tends to accumulate to produce white spot defects, and in particular in sulfur-containing steel, residual hydrogen tends to accumulate around MnS inclusions, thereby producing stress cracks. Therefore, it is necessary to minimize the hydrogen content by the degassing process. Based on the study of the hydrogen content of stress cracking generated on the steel, the content of the [ H ] element is controlled below 0.0002% in the non-quenched and tempered steel disclosed by the invention.

In the non-quenched and tempered steel of the present disclosure, while controlling the contents of each element, in order to suppress the formation of hot shortness FeS, the ratio of the contents of Mn and S elements is also controlled so as to satisfy Mn/S not less than 30 (wherein Mn and S each represent the chemical mass percentage content of the corresponding element). The inventor finds that for sulfur-containing steel, the improvement of Mn/S ratio, the reduction of FeS generation and the improvement of the surface quality of hot rolled steel are critical. When Mn/S is not less than 30, steel having good surface quality can be obtained. Therefore, in the non-quenched and tempered steel of the present disclosure, the Mn/S ratio is controlled to 30 or more.

In addition, the composition differences of the non-quenched and tempered steels of the present disclosure from the steels disclosed in CN111218614A, CN110343954a and CN106939391a are compared in table 1 below.

TABLE 1 chemical composition (wt.%) of related patent

Element (wt%)	CN111218614A	CN110343954A	CN106939391A	The present disclosure
					C	0.43-0.47	0.32-0.40	0.25-0.60	0.40-0.50
Si	0.20-0.35	0.40-0.70	0.10-1.20	0.45-0.80
					Mn	1.10-1.40	0.80-0.95	0.40-1.50	1.35-1.75
P	≤0.02	0.030-0.045	0.02-0.15	≤0.025
					S	0.060-0.090	0.025-0.040	0.02-0.15	0.040-0.060
Cr	0.30-0.40	0.10-0.25	0.05-0.5	0.10-0.30
					Ni	≤0.1	≤0.1	-	0.05-0.20
Cu	≤0.20％	-	-	-
					Mo	≤0.1	≤0.1	-	0.01-0.07
V	-	-	0.03-0.55	0.10-0.20
					Nb	-	0.08-0.20	-	0.020-0.035
Ti	≤0.015	0.015-0.035	≤0.02	-
					Al	0.010-0.040	-	0.002-0.080	≤0.025
O	≤0.0015％	≤15×10 ^-6	-	-
					N	-	0.012-0.020	0.002-0.035	0.014-0.022
Ca	-	-	0.0005-0.0080	0.0008-0.0018

It can be seen that the non-quenched and tempered steel of the present disclosure is different from the steels of CN111218614A, CN110343954a and CN106939391a in both element types and element contents.

In one embodiment, the non-quenched and tempered steel of the present disclosure is manufactured by a method comprising the steps of: smelting molten steel, LF refining, vacuum Degassing (VD) and continuous casting to obtain a continuous casting blank; and hot rolling the continuous casting blank to obtain the non-quenched and tempered steel.

Since the steel of the present disclosure is sulfur-containing steel, the addition of a certain amount of Ca can improve the morphology of MnS inclusions. However, when the Ca content is high, caS with a high melting point tends to be formed and accumulated in the nozzle, resulting in clogging of the nozzle. Thus, preferably, in the present disclosure, mnS inclusions are improved and CaS production is reduced by micro Ca treatment.

Accordingly, in a preferred embodiment, the method of manufacturing non-quenched and tempered steel of the present disclosure comprises: after refining, before vacuum degassing treatment, sulfur wires are fed to the upper limit of sulfur content, and calcium wires are fed for 40-60m after the sulfur wires are broken.

The present inventors have also found through studies that by controlling the process parameters in the production method of non-quenched and tempered steel, in particular, the degree of superheat (difference between the casting temperature and the liquidus line (melting point)), the drawing speed, and the electromagnetic stirring, the segregation of the non-quenched and tempered steel for connecting rod can be improved, and the uniform distribution of each element component and Nb (C, N) precipitate phase in the steel can be improved. The uniform distribution of the element components and the Nb (C, N) precipitated phases in the non-quenched and tempered steel can improve the structural uniformity of the broken connecting rod manufactured by the non-quenched and tempered steel, and improve the flatness of the fracture surface of the broken connecting rod, so that the broken connecting rod is less prone to adverse conditions such as plastic deformation and slag drop.

Thus, in a particularly preferred embodiment, the method of manufacturing non-quenched and tempered steel of the present disclosure comprises: in the continuous casting process, the casting temperature of 1500-1520 ℃ and the constant pulling speed of 0.62-0.68m/min are adopted, the electromagnetic intensity of the electromagnetic stirring of the crystallizer is controlled at 300A/2.5Hz, and the electromagnetic intensity of the electromagnetic stirring at the solidification end is controlled at 800A/8Hz.

In one embodiment, the method of manufacturing non-quenched and tempered steel of the present disclosure further comprises: after hot rolling, the hot rolled steel is cooled to 330-380 ℃ in an air cooling bed, and then is slowly cooled in a slow cooling pit at a cooling speed of 0.15-0.25 ℃/min.

The non-quenched and tempered steel and the manufacturing method thereof and the expansion connecting rod manufactured by the non-quenched and tempered steel have the following advantages and beneficial effects:

1) The non-quenched and tempered steel of the present disclosure improves ferrite strength by precipitation strengthening of V (N, C), and refines pearlite colony spacing by using strong hardenability elements Cr and Mo to further improve strength. Meanwhile, the grains of the connecting rod in the hot working process are controlled through Nb (N, C) precipitated phases, so that the grains are not thinned and are prevented from growing abnormally, the fracture surface of the broken connecting rod manufactured by the non-quenched and tempered steel is smooth, and plastic deformation, slag falling and the like are avoided.

2) A certain amount of S element is added into the non-quenched and tempered steel so as to improve the cutting processability of the non-quenched and tempered steel, and the generation of hot-brittle FeS is avoided by controlling Mn/S to be more than or equal to 30.

3) The metallographic structure of the non-quenched and tempered steel after rolling is ferrite and pearlite, the ferrite content is 20-30 area percent, the structure is uniform, and the grain size is 7-8 grades.

4) The Rm of the expansion fracture connecting rod forged by the non-quenched and tempered steel is more than or equal to 1000MPa, rp0.2 is more than or equal to 800MPa, A is more than or equal to 10%, Z is more than or equal to 20%, the grain size is 6+/-1 grade, the metallographic structure is ferrite and pearlite, the ferrite content is 5-15 area percent, the average lamellar spacing of the pearlite is 160-210nm, the structure is uniform, and the expansion fracture of the connecting rod is smooth.

5) In the manufacturing method of the present disclosure, a micro-calcium treatment is used to change the morphology of MnS inclusions and reduce CaS production.

6) In the manufacturing method of the present disclosure, the distribution uniformity of each element component and Nb (N, C) precipitated phase in steel is further improved by adopting a segregation control technique such as a suitable degree of superheat, drawing speed, electromagnetic stirring, and the like.

The present disclosure is described in further detail below with reference to the drawings and examples. The following examples are merely illustrative of the present disclosure and are not intended to limit the scope of the present disclosure.

Examples

Examples 1 to 7 and comparative examples 1 to 3

The non-quenched and tempered steels of examples 1 to 7 and comparative examples 1 to 3 were prepared by the following steps:

according to the formula shown in the following table 2, carrying out electric furnace smelting, LF refining, vacuum Degassing (VD) and continuous casting on molten steel in sequence to obtain a continuous casting blank; and then carrying out hot rolling on the continuous casting billet to obtain the non-quenched and tempered steel. The tapping temperature is about 1600-1630 ℃ during smelting. The sulfur line is fed to the upper limit of sulfur content before Vacuum Degassing (VD) treatment, and the calcium wire is fed for 40-60m after the breaking. In the continuous casting process, the casting temperature is about 1500-1520 ℃, constant pulling speed of 0.62-0.68m/min is adopted, and the electromagnetic stirring of the crystallizer and the electromagnetic stirring of the solidification end are carried out in the continuous casting process, wherein the electromagnetic intensity of the crystallizer is controlled at 300A/2.5Hz, and the electromagnetic intensity of the electromagnetic stirring of the solidification end is controlled at 800A/8Hz. Slowly cooling the continuous casting blank to below 500 ℃, entering a heating furnace, rolling, heating and preserving at 1200-1220 ℃, wherein the final rolling temperature is 900-950 ℃ when the casting blank is hot rolled, and after hot rolling, air cooling the non-quenched and tempered steel to 330-380 ℃ on a cooling bed, and slowly cooling in a slow cooling pit at a cooling speed of 0.15-0.25 ℃/min.

The compositions and specific process parameters of the steels in examples 1-7 and comparative examples 1-3 are shown in tables 2 and 3.

TABLE 3 specific process parameters for non-quenched and tempered steels (round steels) of examples 1 to 7 and comparative examples 1 to 3

For the determination of the mechanical properties, the steels of examples 1 to 7 and comparative example 1 were sampled respectively and subjected to tensile property test. The results of the performance tests are shown in Table 4. The related mechanical property testing method is as follows: tensile Properties according to GB/T228.1-2010 section 1 of Metal Material tensile test: room temperature test method.

TABLE 4 tensile Properties of non-quenched and tempered steels (round steels) of examples 1 to 7 and comparative example 1

Numbering device	R _P 0.2/MPa	Rm/MPa	A/％	Z/％
					Example 1	697	1020	17.5	49
Example 2	722	1040	15.0	40
					Example 3	678	978	18.0	51
Example 4	718	1040	15.5	42
					Example 5	685	979	19.0	52
Example 6	718	1030	16.5	41
					Example 7	702	1020	16.5	42
Comparative example 1	550	850	17	45

As shown in Table 4, the mechanical properties of the non-quenched and tempered steels of examples 1 to 7 satisfy: rm is greater than or equal to 950MPa, rp0.2 is greater than or equal to 650MPa, A is greater than or equal to 15%, and Z is greater than or equal to 40%. The metallurgical structure of the non-quenched and tempered steel of examples 1 to 7 was ferrite + pearlite, the ferrite content was 20 to 30 area%, and the grain size was 7 to 8 grades. The metallurgical structure of the non-quenched and tempered steel of example 1 is shown in FIG. 1.

In addition, the surface qualities of the non-quenched and tempered steels (round steels) of examples 1 to 7 and comparative example 2 were also compared, as shown in the following table 5.

TABLE 5 surface quality of non-quenched and tempered steels (round steels) of examples 1 to 7 and comparative example 2

Numbering device	Evaluation
		Example 1	Surface crackingLess and shallow
Example 2	Less and shallow surface cracks
		Example 3	Less and shallow surface cracks
Example 4	Less and shallow surface cracks
		Example 5	Less and shallow surface cracks
Example 6	Less and shallow surface cracks
		Example 7	Less and shallow surface cracks
Comparative example 2	Long and deep surface crack

As shown in fig. 5, it can be seen that the round steel of comparative example 2 had a long crack in the longitudinal direction. In contrast, the steels of examples 1 to 7 were good in surface quality, and showed substantially no surface cracks or only a small amount of and shallow surface cracks (not shown).

Then, the non-quenched and tempered steels of each of the examples and comparative examples were sent to a connecting rod processing enterprise to perform forging and expansion processing of the connecting rod.

The tensile properties of the forged connecting rods of examples 1 to 7 and comparative example 1 are shown in Table 6.

TABLE 6 tensile Properties of the forged connecting rods of examples 1-7 and comparative example 1

Numbering device	RP0.2/MP	Rm/MPa	A/％	Z/％
					Example 1	825	1162	13.0	36
Example 2	855	1185	12.0	34
					Example 3	803	1114	15.0	37
Example 4	850	1185	12.0	33
					Example 5	811	1115	16.0	37
Example 6	850	1173	13.5	36
					Example 7	831	1162	13.5	35
Comparative example 1	700	950	13	38

As shown in table 6, the mechanical properties of the connecting rods forged from the non-quenched and tempered steels of examples 1 to 7 satisfy: rm is more than or equal to 1000MPa, RP0.2 is more than or equal to 800MPa, A is more than or equal to 10%, and Z is more than or equal to 20%. The morphology of the expansion fracture of the connecting rod forged from the non-quenched and tempered steel of example 1 is shown in fig. 2, the photograph of the metallographic structure is shown in fig. 3, and the pearlite lamellar structure is shown in fig. 4. It can be seen that the forged connecting rods of examples 1 to 7 are uniform in structure, have a grain size of 6.+ -.1 grade, have a ferrite content of 5 to 15%, and have a pearlite lamellar spacing of 160 to 210nm; and the fracture surface of the broken connecting rod is flat, and plastic deformation and slag falling do not occur.

In addition, the grain sizes of the forged connecting rods of examples 1 to 7 and comparative example 3 were also compared, as shown in Table 7 below.

TABLE 7 grain size of the forged connecting rods of examples 1-7 and comparative example 3

Numbering device	Grain size rating
		Example 1	5.5
Example 2	6
		Example 3	5
Example 4	6.5
		Example 5	7
Example 6	6.5
		Example 7	5.5
Comparative example 3	5-8 grade mixed crystal

As shown in fig. 6, the metallographic structure of comparative example 3 had inconsistent grain sizes, and the mixed crystal phenomenon occurred. In contrast, the forged connecting rods of examples 1 to 7 were uniform in grain size distribution and stabilized in grain size at 6±1 grade.

All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated herein by reference in their entirety.

While the present disclosure has been shown and described with respect to certain preferred embodiments thereof, it will be understood by those of ordinary skill in the art that the foregoing is a further detailed description of the present disclosure with reference to specific embodiments and is not intended to limit the practice of the present disclosure to such descriptions. Various changes in form and detail may be made therein by those skilled in the art, including a few simple inferences or alternatives, without departing from the spirit and scope of the present disclosure.

Claims

1. A non-quenched and tempered steel characterized by comprising, in addition to Fe and unavoidable impurities in an amount of 90% or more, the following chemical elements in mass percent:

C：0.40-0.50％，Si：0.45-0.80％，Mn：1.35-1.75％，P：≤0.025％，S：0.040-0.060％，Cr：0.10-0.30％，Ni：0.05-0.20％，Mo：0.01-0.07％，V：0.10-0.20％，Nb：0.020-0.035％，N：0.014-0.022％，Al：≤0.025％，Ca：0.0008-0.0018％；

wherein the content of Mn and S satisfies Mn/S not less than 30.

2. A non-quenched and tempered steel, characterized in that it comprises the following chemical elements in mass percent:

c:0.40-0.50%, si:0.45-0.80%, mn:1.35-1.75%, P: less than or equal to 0.025 percent, S:0.040-0.060%, cr:0.10-0.30%, ni:0.05-0.20%, mo:0.01-0.07%, V:0.10-0.20%, nb:0.020-0.035%, N:0.014-0.022%, al: less than or equal to 0.025 percent, ca:0.0008-0.0018%, the balance being Fe and other unavoidable impurities;

wherein the content of Mn and S satisfies Mn/S not less than 30.

3. The non-quenched and tempered steel according to claim 1 or 2, further comprising a gaseous element [ O ] and/or [ H ], wherein the content of [ O ] is 0.0020% or less and the content of [ H ] is 0.0002% or less.

4. The non-quenched and tempered steel according to claim 1 or 2, wherein V is 0.14-0.18%, nb is 0.023-0.032% and/or S is 0.040-0.056%.

5. The non-quenched and tempered steel according to claim 1 or 2, wherein the non-quenched and tempered steel is free of Ti, cu or Mg elements.

6. The non-quenched and tempered steel according to any of claims 1 to 5, wherein the metallurgical structure of the non-quenched and tempered steel is ferrite + pearlite; preferably, the ferrite content is 20-30 area%; preferably, the non-quenched and tempered steel has the following properties: the tensile strength Rm is more than or equal to 950MPa, the regulated non-proportional extension strength Rp0.2 is more than or equal to 650MPa, the elongation after fracture A is more than or equal to 15%, and the area shrinkage Z is more than or equal to 40%.

7. A connecting rod made of the non-quenched and tempered steel of any one of claims 1 to 6.

8. The connecting rod of claim 7, wherein the connecting rod has a grain size of 6±1 grade.

9. The connecting rod of claim 7, wherein the connecting rod has the following properties: the tensile strength Rm is more than or equal to 1000MPa, the regulated non-proportional extension strength Rp0.2 is more than or equal to 800MPa, the elongation after fracture A is more than or equal to 10%, and the area shrinkage Z is more than or equal to 20%.

10. The connecting rod of claim 7, wherein the metallurgical structure of the connecting rod is ferrite + pearlite; preferably, the ferrite content is 5-15 area%; preferably, the average lamellar spacing of the pearlite is 160-210nm.

11. A connecting rod according to any one of claims 7 to 10, wherein the connecting rod is a broken connecting rod.

12. The method for producing non-quenched and tempered steel as claimed in any one of claims 1 to 6, wherein the method comprises the steps of: smelting, refining, vacuum degassing and continuous casting are sequentially carried out on molten steel to prepare a continuous casting blank; and hot rolling the continuous casting blank to obtain the non-quenched and tempered steel.

13. The method of claim 12, wherein after refining, the sulfur line is fed to an upper limit of sulfur content and/or the calcium wire is fed 40-60m after the break before vacuum degassing.

14. The method of claim 12, wherein a casting temperature of 1500-1520 ℃ and/or a constant pull rate of 0.62-0.68m/min is used in the continuous casting process.

15. The production method according to claim 12, wherein electromagnetic stirring is performed during continuous casting, the electromagnetic stirring including mold electromagnetic stirring and solidification end electromagnetic stirring; preferably, the electromagnetic intensity of the electromagnetic stirring of the crystallizer is 300A/2.5Hz, and the electromagnetic intensity of the electromagnetic stirring of the solidification end is 800A/8Hz.

16. The production method according to any one of claims 12 to 15, wherein the continuously cast slab obtained by continuous casting is cooled down to 500 ℃ or lower and then hot rolled in a heating furnace, the rolling heat retaining temperature is 1200 to 1220 ℃ and the finishing temperature is 900 to 950 ℃; preferably, after hot rolling, the non-quenched and tempered steel is air cooled to 330-380 ℃ on a cooling bed and then slowly cooled in a slow cooling pit at a cooling rate of 0.15-0.25 ℃/min.