CN117626108A - High-strength steel based on direct quenching process and preparation method thereof - Google Patents
High-strength steel based on direct quenching process and preparation method thereof Download PDFInfo
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Abstract
The invention provides high-strength steel based on a direct quenching process, which comprises the following chemical elements in percentage by mass: c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%. The invention also provides a preparation method of the high-strength steel, which comprises the steps of slab heating, rough rolling, finish rolling and online quenching treatment, wherein the cooling rate is more than or equal to 100 ℃/s, and the final cooling temperature is less than or equal to (Ms-150); then coiling, water blowing treatment and tempering heat treatment are carried out. On the basis of lower alloy cost, the invention obtains the high-strength steel with the longitudinal yield strength of 900-1050MPa, the tensile strength of 950-1200MPa, the elongation rate of more than or equal to 14 percent and good plate shape.
Description
Technical Field
The invention belongs to the field of steel production, and particularly relates to high-strength steel based on a direct quenching process and a preparation method thereof.
Background
Under the large background of green low carbon, energy conservation and weight reduction, the fields of semitrailers, refitted vehicles and the like are increasingly in demand for high-strength steel. The high-strength steel is mainly applied to the fields of semitrailers, refitted vehicles and the like, and is respectively used for preparing bearing parts and structural parts, wherein the former is mainly represented by semitrailer girders, and the latter is represented by the upper parts of the semitrailers and the refitted vehicles. The girder steel needs to be subjected to welding and cold bending processing in the forming process and can bear static load and dynamic load of cargoes of the whole girder in the service process, so that the girder steel needs to have good welding performance, cold forming performance and fatigue performance under the condition of meeting the strength level. The forming process of the steel for upper mounting is simpler and does not bear direct load in the service process, so that the steel for upper mounting only needs to pay attention to basic mechanical properties, including strength, plasticity and low-temperature toughness indexes.
The production process of the hot-rolled high-strength steel mainly comprises three main processes of conventional hot continuous rolling, tempering heat treatment and direct quenching. For hot rolled high strength steel with yield strength of 900MPa, the direct quenching process is the most economical production method at present. Compared with the conventional hot continuous rolling process, the direct quenching process can greatly reduce the consumption of noble alloys such as Mo, nb, V and the like, and obviously reduces the alloy cost. Compared with the quenching and tempering heat treatment process, the direct quenching process does not need to carry out re-austenitizing and quenching processes, and the process cost is obviously reduced.
Chinese patent CN104561827A discloses a high-strength steel with 900-1000MPa grade yield strength and a production method thereof, wherein the steel is produced based on a direct quenching process, namely an on-line quenching-tempering process, and has a microstructure of tempered martensite, 900-1080MPa yield strength, 950-1200MPa tensile strength, elongation of more than or equal to 10 percent and impact energy of more than or equal to 27J at minus 40 ℃. However, the high-strength steel of the patent is steel for engineering machinery, and has high requirements on welding, fatigue and plate shape, so that a large amount of alloy elements including 0.10-0.45% of Mo element, 0.10-0.50% of Ni element, 0.01-0.03% of Nb element and 0.01-0.05% of V element by mass percentage are added into the patent; the cost of the steel alloy is greatly increased, and the cost advantage is not obvious.
Chinese patent CN103789655A discloses a method for producing Nb-alloyed NM500 high-strength wear-resistant steel plate by on-line quenching, wherein the tensile strength is 1700-2200MPa, the elongation is more than or equal to 10%, the impact energy at minus 20 ℃ is more than or equal to 30J, the Brinell hardness of the surface layer is more than or equal to HBW520, and a small amount of noble alloy Nb element (0.02-0.04%) is added in the method, so that the method has fine grain strengthening and precipitation strengthening effects, and can simultaneously improve the strength and toughness of the wear-resistant steel plate. However, in order to ensure higher hardness, the patent uses a lower tempering temperature, and the plasticity of the steel is relatively low and the problem of plate shape is more likely to occur compared to the high temperature tempered steel type.
Disclosure of Invention
The method aims to solve the problems of high cost or low plasticity of materials of the high-strength steel alloy based on the direct quenching process in the prior art.
The invention provides high-strength steel based on a direct quenching process, which comprises the following chemical elements in percentage by mass: c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%.
According to another specific embodiment of the invention, the high-strength steel based on the direct quenching process provided by the invention comprises the following chemical elements in percentage by mass: c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%; the balance being Fe and unavoidable impurities.
By adopting the scheme, the invention is based on a C-Mn-Ti-Cr low-cost alloy system, the alloy content ratio is reasonably designed, and the direct quenching process is adopted to obtain the low-cost high-strength steel with the yield strength of 900 MPa.
According to another specific embodiment of the invention, the high-strength steel based on the direct quenching process is provided, wherein P, S, O is controlled in the following mass percent among unavoidable impurities: less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, less than or equal to 0.008 percent of O.
By adopting the scheme, the content of the impurity elements is controlled, and the influence of the impurity elements on the plasticity and toughness of the steel is reduced.
According to another specific embodiment of the invention, the high-strength steel based on the direct quenching process provided by the invention has the following chemical elements: mn+2Cr is more than or equal to 2.0%, ti-3.5N is more than or equal to 0.08%, ca/S is more than or equal to 1.0 and less than or equal to 3.0.
By adopting the scheme, the Mn+2Cr is controlled to be more than or equal to 2.0%, the content of Mn and Cr is ensured to be enough, so that the high-strength steel is ensured to have enough hardenability, on-line quenching can be realized under the cooling condition provided by the conventional laminar cooling process, and the transformation process from unconverted austenite to martensite is fully completed. By controlling Ti-3.5N to be more than or equal to 0.08 percent, the generation of sufficient nano TiC precipitated phase is ensured, and the yield strength of tempered martensite is improved to 900 MPa. The low-temperature impact toughness of the steel is improved by controlling Ca/S to be more than or equal to 1.0 and less than or equal to 3.0 and limiting the content of Ca and S elements.
According to another specific embodiment of the invention, the microstructure of the high-strength steel based on the direct quenching process comprises tempered martensite, wherein TiC nano precipitated phases are dispersed and distributed; the grain size of TiC nano precipitated phase is less than or equal to 10nm.
By adopting the scheme, the tempered martensite structure is reinforced and TiC precipitation strengthening is overlapped, so that the high-strength steel has higher strength, relatively higher plasticity and good plate shape.
The invention also provides a preparation method of the high-strength steel based on the direct quenching process, which comprises the following steps:
s1: heating the plate blank, and performing rough rolling after heating to obtain strip steel;
s2: finish rolling is carried out on the strip steel;
s3: carrying out online quenching treatment on the strip steel, controlling the cooling rate to be more than or equal to 100 ℃/s and the final cooling temperature to be less than or equal to (Ms-150) DEG C, wherein Ms is the martensite transformation starting temperature, and Ms=539-423C-11.0 Si-30.4Mn-12.1Cr;
s4: strip steel enters a coiling machine to be coiled, blown with water and tempered; wherein the tempering heat treatment temperature is 550-700 ℃.
By adopting the scheme, the elongation of the strip steel is fully improved based on the rolling-on-line quenching-high temperature tempering treatment process, and the precipitation strengthening effect of the solid solution Ti element is exerted.
According to another specific embodiment of the invention, in the preparation method of the high-strength steel based on the direct quenching process, in the step S1, the high-strength steel is heated to 1250-1300 ℃ and the soaking time is 30-90min; the rough rolling outlet temperature is 1020-1060 ℃.
By adopting the scheme, the heating temperature ensures the full solid solution of Ti element, and the rough rolling outlet temperature ensures the strength of steel.
According to another embodiment of the present invention, in the method for preparing high-strength steel based on the direct quenching process, in step S2, the finish rolling finishing temperature is 840-880 ℃.
By adopting the scheme, austenite grains are refined by adopting a lower finish rolling temperature.
According to another specific embodiment of the invention, in the preparation method of the high-strength steel based on the direct quenching process, in the step S4, tempering heat treatment is performed by adopting a hot coil continuous heat treatment process, and the heat preservation time is 30-120S.
By adopting the scheme, the high-temperature tempering is performed for a short time, and the TiC precipitation phase is prevented from growing up while the TiC is fully precipitated, so that better obdurability matching is obtained.
According to another specific embodiment of the invention, the preparation method of the high-strength steel based on the direct quenching process provided by the invention is used for carrying out leveling treatment on strip steel simultaneously when carrying out water blowing treatment.
By adopting the scheme, the strip shape of the strip steel is optimized.
The invention has the beneficial effects that:
the invention is based on a C-Mn-Ti-Cr low-cost alloy system, reasonably designs alloy content ratio, and prepares low-cost high-strength steel based on a direct quenching process. The preparation process of rolling, on-line quenching and high-temperature tempering treatment provided by the invention is particularly used for precisely controlling the on-line quenching cooling speed and the final cooling temperature, and adopting higher temperature in tempering heat treatment, fully improving the elongation of strip steel, and performing precipitation strengthening effect of solid solution Ti element, so that the obtained high-strength steel has higher strength and reaches 900 MPa-level strength and plasticity index.
The high-strength steel prepared by the method has the longitudinal yield strength of 900-1050MPa, the tensile strength of 950-1200MPa, the elongation of more than or equal to 14 percent, the impact energy of more than or equal to 30J at minus 20 ℃, and good plate shape.
Drawings
FIG. 1 is a microstructure of a high-strength steel obtained in example 1 of the present invention;
FIG. 2 is a microstructure of the high-strength steel obtained in example 3 of the present invention.
Detailed Description
For the purposes of the following detailed description, it is to be understood that, except in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present application. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
The terms "comprises" or "comprising," when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
As used herein, "about" or "approximately" includes the values described and means, for example, within an acceptable deviation of a particular value as determined by one of ordinary skill in the art in view of the measurements in question and the errors associated with the measurement of a particular quantity (i.e., limitations of the measurement system). Unless otherwise indicated, all ranges of parameters disclosed include the endpoints and all values therebetween.
In the description of the present invention, unless otherwise defined, terms have the same meaning as commonly understood by one of ordinary skill in the art, but are defined according to the present invention; unless otherwise specified, the test methods are all conventional; the raw materials and test materials used in the present invention are all commercially available unless otherwise specified.
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in further detail below.
The invention provides high-strength steel based on a direct quenching process, which comprises the following chemical elements in percentage by mass: c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%.
According to one specific embodiment of the invention, the high-strength steel consists of the following chemical elements in percentage by mass: c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%; the balance being Fe and unavoidable impurities.
The design principle of the content of the high-strength steel chemical elements is as follows:
carbon (C): the element C has important influence on the phase change process and the product of the steel; the increase of the C content can reduce the critical cooling rate of the generated martensite, and the martensite is more easily generated under the same cooling condition. The C element exists in a solid solution form in the martensite, so that the strength of the martensite can be obviously improved, but a brittle twin-crystal type martensite structure can be generated due to the excessively high C content, and the low-temperature impact toughness of the steel is not facilitated; in the subsequent tempering process, the excessive content of C easily causes the generation of coarse carbides and the reduction of plasticity and impact toughness; however, on the other hand, too low a content of C tends to produce a large amount of low strength structure such as ferrite, and the strength requirement of the present invention cannot be satisfied. Therefore, the content of the C element of the present invention is in the range of 0.06-0.20%.
Silicon (Si): the Si element is a solid solution element in the steel, and has good deoxidizing effect; when the content of Si element is high, surface red iron scales are easily generated in the hot rolling process, and the toughness and welding performance of martensite are deteriorated. Therefore, the Si content of the present invention ranges from 0.05 to 0.30%.
Manganese (Mn): mn element is an important alloy element in the invention; the addition of Mn element can inhibit the generation of proeutectoid ferrite and improve the hardenability of steel. In addition, the addition of Mn element can refine microstructure, so that the microstructure has good toughness matching, but excessive Mn element addition easily generates remarkable center segregation, and a high-carbon twin-crystal type martensite structure is generated at a corresponding position, so that the toughness of the martensite high-strength steel is not facilitated. Thus, the Mn content of the present invention ranges from 0.5 to 1.8%.
Chromium (Cr): cr element is an important alloy element in the invention; the addition of Cr element can obviously improve the hardenability of steel and is beneficial to generating a full martensitic structure in the quenching process. In addition, cr element can generate carbide precipitated phase in the tempering process, and has certain tempering softening resistance effect; too high a Cr element content tends to cause a decrease in toughness of the high-strength steel. Thus, the Cr content of the present invention is in the range of 0.1 to 0.8%.
Titanium (Ti): the Ti element is an important microalloy element in the invention; the Ti element can form fine dispersed phase particles with C and N, and can pin austenite grain boundaries in the hot rolling process to refine austenite structures; in the subsequent tempering process, ti element can generate nano TiC precipitated phase, so that the yield strength and tensile strength of tempered martensite are improved, and the tempering softening resistance effect is obvious; in order to compensate for tempering softening caused by no use of noble elements such as Mo, nb and the like, the Ti content of the invention is relatively high, and the range is 0.10-0.20%.
Aluminum (Al): al element is a deoxidizer of steel, and a small amount of Al element can refine grains and improve impact toughness; the aluminum oxide inclusion is easily generated when the content of the Al element is too high, and the generation of proeutectoid ferrite is promoted. Therefore, the present invention controls the Al content to be in the range of 0.015 to 0.06%.
Calcium (Ca): the Ca element is a purifying agent in the steel smelting process, so that the form of sulfide can be improved, and the impact toughness of steel can be improved; too high a Ca content tends to form Ca compounds of a large size, which in turn causes deterioration of toughness. Therefore, the Ca content of the present invention ranges from 0.001 to 0.004%.
Nitrogen (N): the invention strictly controls the content of N element; n and Ti can form stable indissoluble TiN precipitates, and the size of TiN is obviously coarsened due to the fact that the content of N is too high, so that the toughness is reduced; furthermore, too high an N content will result in a decrease in the content of effective Ti element in the steel and in a decrease in strength. Therefore, the invention controls the N content to be less than or equal to 0.005 percent.
The invention is based on a C-Mn-Ti-Cr low-cost alloy system, reasonably designs alloy content ratio, and adopts a direct quenching process to obtain the low-cost high-strength steel with the yield strength of 900 MPa.
According to one embodiment of the invention, P, S, O among the unavoidable impurities is controlled in the following mass percentages: p is less than or equal to 0.02%, S is less than or equal to 0.01%, and O is less than or equal to 0.008%.
Phosphorus (P), sulfur (S) and oxygen (O): p, S and O are impurity elements in steel, and when the content is too high, the plasticity and toughness of the steel are obviously affected, so that the range of the invention is controlled to be less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S and less than or equal to 0.008 percent of O respectively.
According to one embodiment of the invention, the composition of the chemical elements also satisfies: mn+2Cr is more than or equal to 2.0%, ti-3.5N is more than or equal to 0.08%, ca/S is more than or equal to 1.0 and less than or equal to 3.0.
According to the invention, the Mn+2Cr is controlled to be more than or equal to 2.0%, so that the content of Mn and Cr is ensured to be enough, the high-strength steel is ensured to have enough hardenability, the online quenching can be realized under the cooling condition provided by the conventional laminar cooling process, and the transformation process from the non-transformed austenite to the martensite is fully completed. When Mn+2Cr is less than 2.0%, hardenability is insufficient, ferrite is easily generated during cooling, and strength is low.
By controlling Ti-3.5N to be more than or equal to 0.08 percent, the steel is ensured to have enough effective Ti element, and austenite grain boundaries can be pinned and austenite tissues can be refined in the hot rolling process; in the subsequent tempering process, the generation of enough nano TiC precipitated phases is ensured, and the yield strength of tempered martensite is improved to 900 MPa.
The content of Ca and S elements is limited by controlling Ca/S to be less than or equal to 1.0 and less than or equal to 3.0, so that sulfides in the steel are spheroidized, the generation of MnS inclusions is reduced, the low-temperature impact toughness, particularly the transverse impact toughness, of the steel is improved, and the safety in the service process is improved.
According to one embodiment of the invention, the microstructure of the high-strength steel comprises tempered martensite, wherein TiC nano precipitated phases are dispersed and distributed; the grain size of TiC nano precipitated phase is less than or equal to 10nm.
The invention is based on the chemical element composition and on-line quenching to obtain microstructure mainly of tempered martensite, wherein the tempered martensite still maintains a finer lath martensite structure, and a large amount of TiC nano precipitated phases below 10nm are dispersed and distributed in the tempered martensite. The tempered martensite structure is reinforced and TiC precipitation strengthening is overlapped, so that the high-strength steel has higher strength, the yield strength of the high-strength steel reaches 900-1050MPa, and the tensile strength reaches 950-1200MPa; and the martensite tempered at high temperature has relatively high plasticity and good plate shape.
The invention also provides a preparation method of the high-strength steel based on the direct quenching process, which mainly comprises the steps of obtaining a plate blank through steelmaking continuous casting, heating the plate blank through a heating furnace, carrying out rough rolling, finish rolling and layer cooling on-line quenching, and then coiling to obtain hot rolled strip steel, and then carrying out tempering treatment. The preparation method comprises the following steps:
s1: adopting a converter or an electric furnace to make steel and refine, and obtaining a plate blank through a continuous casting process; and heating the plate blank, discharging the heated plate blank from a heating furnace, fixing the width of the heated plate blank, and feeding the heated plate blank into rough rolling to obtain the strip steel.
The alloy comprises the following chemical elements in percentage by mass during steelmaking: c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%. Further, the alloy consists of the following chemical elements in percentage by mass: c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%; the balance being Fe and unavoidable impurities. Further, among unavoidable impurities, P, S, O is controlled in the following mass percent: p is less than or equal to 0.02%, S is less than or equal to 0.01%, and O is less than or equal to 0.008%. Still further, the composition of the chemical elements also satisfies: mn+2Cr is more than or equal to 2.0%, ti-3.5N is more than or equal to 0.08%, ca/S is more than or equal to 1.0 and less than or equal to 3.0.
According to one specific embodiment of the invention, in order to ensure the sufficient solid solution of Ti element, the slab is heated to 1250-1300 ℃ and the soaking time is controlled to be 30-90min, wherein the soaking time is calculated from the center of the slab to the temperature.
The rough rolling stage should ensure a sufficiently high descaling pressure to obtain a good descaling effect; the rough rolling outlet temperature is an important index, and the excessive rough rolling outlet temperature is unfavorable for refining austenite grains, so that the impact toughness of the high-strength steel is deteriorated; if the temperature of the rough rolling outlet is too low, a large amount of TiC particles which are induced to be precipitated in austenite are generated, and because the precipitation temperature is too high, the size of the TiC precipitation phase is too large, the precipitation strengthening effect is weak, the effective Ti content in the subsequent tempering process is reduced, and the strength of the high-strength steel is reduced. According to one embodiment of the invention, the rough rolling outlet temperature is controlled between 1020 and 1060 ℃ in consideration of the two factors.
S2: and (5) after rough rolling, the strip steel enters a finishing mill group to finish rolling the strip steel. And adopting a multi-frame continuous rolling process, and selecting proper finishing temperature according to different thickness specifications of rolled strip steel. The steel of the present invention requires a lower finishing temperature than in the usual rolling process, with the aim of refining the austenite grains. On the other hand, the finishing temperature is not too low in order to suppress strain-induced precipitation of TiC second phase in austenite. According to one embodiment of the present invention, the finishing temperature is controlled to 840-880 ℃ in consideration of the above two factors.
S3: the strip steel after finish rolling enters a laminar flow cooling unit for online quenching treatment, and in order to obtain a full martensitic structure after quenching, the invention provides that the laminar flow cooling rate is more than or equal to 100 ℃/s, and the final cooling temperature is less than or equal to (Ms-150), wherein Ms is the martensite transformation starting temperature, and Ms=539-423C-11.0 Si-30.4Mn-12.1Cr. Insufficient cooling rate easily causes ferrite formation, bainite and retained austenite are easily formed when the final cooling temperature is high, and both insufficient cooling rate and high final cooling temperature may cause strength reduction.
Note that C, si, mn, cr in the above calculation formula represents the mass percentages of the corresponding chemical components, respectively.
S4: after the layer cooling is finished, the strip steel enters a coiling machine to be coiled to obtain a steel coil; because the temperature of the steel coil is lower after online quenching, the residual cooling water between the coil layers of the steel coil cannot be evaporated in time, and the surface of the strip steel is easy to be rusted, the steel coil should be subjected to water blowing treatment on a flat line in time after coiling, so that the surface of the strip steel is prevented from being rusted; further, according to one specific embodiment of the invention, in order to optimize the strip shape of the strip steel, a certain flattening force can be applied to perform flattening treatment on the strip steel during water blowing treatment.
Tempering the steel coil after water blowing, wherein the tempering heat treatment is performed at a higher temperature in the range of 550-700 ℃ in order to improve the elongation of the strip steel and fully exert the precipitation strengthening effect of solid solution Ti element; and after the core part of the strip steel reaches the tempering temperature, starting to preserve heat, wherein the heat preservation time is determined based on the tempering process. According to different tempering production lines, tempering can be performed by adopting a hot coil continuous heat treatment process or a cutting plate heat treatment process, and according to one specific embodiment of the invention, tempering heat treatment is performed by adopting the hot coil continuous heat treatment process, wherein the heat preservation time is 30-120s; the continuous heat treatment device for the hot coil performs induction heating tempering, performs short-time high-temperature tempering by utilizing the advantage of high temperature rising speed of induction heating, and prevents TiC precipitation phase growth while TiC is sufficiently precipitated, so that better toughness matching is obtained.
It should be noted that the tempering heat treatment can also be performed by adopting a cutting plate heat treatment process, and the heat preservation time is 10-40min.
Based on the control rolling-on-line quenching-high temperature tempering treatment process, particularly, the on-line quenching cooling speed and the final cooling temperature are precisely controlled, and the higher temperature is adopted in tempering heat treatment, so that the elongation of the strip steel is fully improved, and the precipitation strengthening effect of solid solution Ti element is exerted. By adopting the chemical element composition and the preparation method of the high-strength steel provided by the invention, the microstructure of the obtained high-strength steel is mainly tempered martensite, the tempered martensite still maintains a finer lath martensite structure, and a large amount of TiC nano precipitated phases below 10nm are dispersed and distributed in the tempered martensite; the tempered martensite structure is reinforced and TiC precipitation strengthening is overlapped, so that the high-strength steel has higher strength and reaches 900 MPa-level strength and plasticity indexes.
The high-strength steel prepared by the invention realizes that on the basis of lower alloy cost, the full-martensitic microstructure is obtained by the optimized design of alloy components and the combination of a direct quenching process, the longitudinal yield strength of the high-strength steel is 900-1050MPa, the tensile strength is 950-1200MPa, the elongation is more than or equal to 14 percent, the impact energy at minus 20 ℃ is more than or equal to 30J, and the high-strength steel has good plate shape.
The method for preparing the high-strength steel according to the present invention will be further described with reference to specific examples and drawings.
Examples 1 to 4
Examples 1-4 high strength steels were produced based on the above chemical element design requirements and manufacturing process, wherein the mass percentages of the respective chemical elements are specifically shown in table 1.
TABLE 1 chemical component content (wt.%) of examples 1 to 4, balance Fe and other unavoidable impurities except P, S
| Numbering device | C | Si | Mn | P | S | Cr | Ti | Al | Ca | N | Mn+2Cr | Ti-3.5N | Ca/S |
| Example 1 | 0.082 | 0.11 | 1.73 | 0.005 | 0.002 | 0.45 | 0.109 | 0.028 | 0.003 | 0.004 | 2.63 | 0.095 | 1.5 |
| Example 2 | 0.082 | 0.11 | 1.73 | 0.005 | 0.002 | 0.45 | 0.109 | 0.028 | 0.003 | 0.004 | 2.63 | 0.095 | 1.5 |
| Example 3 | 0.077 | 0.15 | 1.58 | 0.004 | 0.002 | 0.40 | 0.112 | 0.034 | 0.004 | 0.005 | 2.38 | 0.0945 | 2 |
| Example 4 | 0.077 | 0.15 | 1.58 | 0.004 | 0.002 | 0.40 | 0.112 | 0.034 | 0.004 | 0.005 | 2.38 | 0.0945 | 2 |
Further, a plate blank is obtained by adopting the working procedures of converter smelting, refining and continuous casting, after the plate blank is heated by a heating furnace, the plate blank is subjected to rough rolling, finish rolling, online quenching, coiling and water blowing treatment, the strip steel is subjected to leveling treatment during the water blowing treatment, and tempering heat treatment is carried out after the water blowing treatment, wherein the tempering heat treatment is carried out by adopting a hot coil continuous heat treatment process, and the corresponding production process parameters are shown in table 2.
TABLE 2 production process parameters for examples 1-4
And (3) performance detection:
the high-strength steels obtained in examples 1 to 4 were subjected to performance test.
(1) Room temperature tensile properties
The yield strength, tensile strength and elongation at break of the high-strength steel of each embodiment are measured by testing a high-strength steel sample of each embodiment according to the national standard GB/T228.1-2010 "method for tensile test of metallic Material at room temperature" by adopting a plate-shaped tensile sample taken from the strip steel at room temperature.
(2) Toughness at low temperature
The half-size test pieces of the steel strips of each example were tested for Charpy impact energy at-20℃by impact testing of test steel plates according to the national standard GB/T229-2007 Charpy impact pendulum test method for metallic materials.
(3) Unevenness of the surface of the steel sheet
The strip steel samples of each example were tested according to the national standard GB/T709-2019 size, shape, weight and allowable deviation of Hot rolled Steel sheet and Steel strip, and the unevenness of the examples was measured as a representative shape index.
The results of the performance measurements for examples 1-4 are shown in Table 3.
TABLE 3 Properties of the high-strength steels produced in examples 1 to 4 of the present invention
From Table 3, it can be seen that the invention realizes the production of high-strength steel with yield strength of over 900MPa and tensile strength of over 950MPa by using a low-cost component system, and has the advantages of high plasticity and low-temperature toughness, unevenness of 14-20mm, and good plate shape characteristics, wherein the elongation is more than or equal to 14%, the impact energy of a half-size sample at minus 20 ℃ is more than or equal to 18J (the impact energy of a full-size sample at minus 20 ℃ is more than or equal to 36J).
In addition, the Mn+2Cr is more than or equal to 2.0%, the Ti-3.5N is more than or equal to 0.08%, the Ca/S is more than or equal to 1.0 and less than or equal to 3.0, and the direct quenching and high-temperature tempering processes are combined, so that the required strength and plasticity indexes are more favorably achieved; therefore, the yield strength of the high-strength steel of the embodiment 1-4 is more than or equal to 947MPa, the tensile strength is more than or equal to 968MPa, and the elongation is more than or equal to 15%.
Microstructure:
the microstructure diagrams of the high-strength steels obtained in examples 1 and 3 are shown in FIGS. 1 to 2, respectively. As can be seen from the graph, the microstructure of the high-strength steel provided by the invention is mainly tempered martensite, and a large amount of TiC nano precipitated phases below 10nm are dispersed and distributed in the microstructure, wherein the tempered martensite still maintains a finer lath martensite structure.
The foregoing is a further detailed description of the invention in connection with specific embodiments, and it is not intended that the invention be limited to such description. It will be apparent to those skilled in the art that several simple deductions or substitutions may be made without departing from the spirit of the invention, and these should be considered to be within the scope of the invention.
Claims (10)
1. The high-strength steel based on the direct quenching process is characterized by comprising the following chemical elements in percentage by mass:
C：0.06-0.20％，Si：0.05-0.30％，Mn：0.5-1.8％，Cr：0.1-0.8％，Ti：0.10-0.20％，Al：0.015-0.06％，Ca：0.001-0.004％，N≤0.005％。
2. the high-strength steel based on the direct quenching process as claimed in claim 1, wherein the high-strength steel is composed of the following chemical elements in mass percent:
c:0.06-0.20%, si:0.05-0.30%, mn:0.5-1.8%, cr:0.1-0.8%, ti:0.10-0.20%, al:0.015-0.06%, ca:0.001-0.004%, N is less than or equal to 0.005%; the balance being Fe and unavoidable impurities.
3. The high-strength steel based on a direct quenching process as claimed in claim 2, wherein P, S, O among the unavoidable impurities is controlled in the following mass percent: p is less than or equal to 0.02%, S is less than or equal to 0.01%, and O is less than or equal to 0.008%.
4. The high-strength steel based on a direct quenching process as claimed in claim 2, wherein the composition of the chemical elements further satisfies: mn+2Cr is more than or equal to 2.0%, ti-3.5N is more than or equal to 0.08%, ca/S is more than or equal to 1.0 and less than or equal to 3.0.
5. The high-strength steel based on a direct quenching process as claimed in any one of claims 1 to 4, wherein the microstructure of the high-strength steel comprises tempered martensite in which TiC nano precipitated phases are dispersed; the grain size of the TiC nano precipitated phase is less than or equal to 10nm.
6. A method for producing high-strength steel based on a direct quenching process according to any one of claims 1 to 5, characterized in that it comprises the steps of:
s1: heating the plate blank, and performing rough rolling after heating to obtain strip steel;
s2: finish rolling the strip steel;
s3: carrying out online quenching treatment on the strip steel, controlling the cooling rate to be more than or equal to 100 ℃/s and the final cooling temperature to be less than or equal to (Ms-150) DEG C, wherein Ms is the martensite transformation starting temperature, and Ms=539-423C-11.0 Si-30.4Mn-12.1Cr;
s4: the strip steel enters a coiling machine to be coiled, blown with water and tempered; wherein the tempering heat treatment temperature is 550-700 ℃.
7. The method for producing high-strength steel based on a direct quenching process as claimed in claim 6, wherein in the step S1, heating is performed to a temperature of 1250-1300 ℃ for a soaking time of 30-90min; the rough rolling outlet temperature is 1020-1060 ℃.
8. The method for producing high-strength steel according to claim 6, wherein in the step S2, the finish rolling temperature is 840 to 880 ℃.
9. The method for producing high-strength steel according to claim 6, wherein in step S4, tempering heat treatment is performed by a continuous heat treatment process for hot coil, and the heat preservation time is 30-120S.
10. The method for preparing high-strength steel based on a direct quenching process as claimed in any one of claims 6 to 9, wherein in said step S4: and when the water blowing treatment is carried out, carrying out leveling treatment on the strip steel.
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