CN117305687A

CN117305687A - Precipitation strengthening type tempered martensite high-reaming steel and manufacturing method thereof

Info

Publication number: CN117305687A
Application number: CN202210713421.3A
Authority: CN
Inventors: 王焕荣; 张晨; 杨阿娜; 庞厚君; 范佳杰
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2022-06-22
Filing date: 2022-06-22
Publication date: 2023-12-29

Abstract

A precipitation strengthening type tempered martensite high-reaming steel and a manufacturing method thereof comprise the following components in percentage by weight: 0.01 to 0.1 percent of C, 0.2 to 2.0 percent of Si, 1.5 to 3.0 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.005 percent of S, 0.01 to 0.08 percent of Al, less than or equal to 0.005 percent of N, 0.8 to 3.0 percent of Cu, less than or equal to 0.003 percent of O, and the balance of Fe and other unavoidable impurities. The yield strength of the high-reaming steel is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the elongation A50 is more than or equal to 15%, the reaming rate is more than or equal to 100%, and the high-reaming steel has high strength, high plasticity, high reaming rate and good matching of performance stability, and can be applied to various parts which are responsible for high-strength thinning such as chassis, brackets and the like.

Description

Precipitation strengthening type tempered martensite high-reaming steel and manufacturing method thereof

Technical Field

The invention belongs to the field of high-strength steel, and particularly relates to precipitation strengthening type tempered martensite high-reaming steel and a manufacturing method thereof.

Background

Automobiles occupy a very important position in national economy development. Many parts in passenger cars, especially chassis and parts of the car body, often require hot rolled pickled products. The weight reduction of passenger cars is not only a development trend of the automobile industry, but also a requirement of laws and regulations. The law and regulation prescribes oil consumption, and the actual requirement is that the weight of a vehicle body is reduced in a phase-change manner, and the requirement reflected on materials is that the vehicle body is high-strength, thin and light. High strength and weight reduction are the necessary requirements of the subsequent new vehicle, which tend to lead to higher steel grade, and the chassis structure also has the necessary change: if the parts are more complex, the requirements on material performance, surface and the like and the forming technology are improved, such as hydroforming, hot stamping, laser welding and the like, so that the performances of high strength, stamping, flanging, rebound, fatigue and the like of the materials are converted.

Compared with overseas, the development of the domestic high-strength high-reaming steel has relatively low strength level and poor performance stability. The high-hole-enlarging steel used by domestic automobile spare part enterprises is basically high-strength steel with the tensile strength of below 600MPa, and the high-hole-enlarging steel with the grade of below 540MPa competes for white heat. High-hole-expansion steel with tensile strength of 780MPa is gradually used in batches at home at present, but high requirements are also put forward on important indexes in two forming processes of elongation and hole expansion rate, and meanwhile, the requirements on performance stability are also stricter. In order to reduce the process cost, passenger car enterprises further improve the performance requirements of materials. For example, in the production of automobile chassis parts, in order to reduce the stamping process, the material is required to have high strength and high plasticity, and simultaneously, the hole expansion rate index is required to be higher. If the reaming rate of 780 MPa-level high reaming steel is required to be further improved to be more than or equal to 100 percent on the basis of ensuring the current capability to be more than or equal to 50 percent. The existing high-reaming steel, in particular 780MPa high-reaming steel, is mostly designed by adopting a high-titanium component, and is sometimes added with a high-aluminum or high-silicon combined medium-temperature coiling process, so that the temperature control precision and the structure uniformity are poor, the index fluctuation of the reaming rate and the like is large, and the punching cracking is easy to occur at a user end.

780 MPa-grade acid-washed high-reaming steel has more patents. Such as:

chinese patent CN103602895A relates to a low-carbon Nb-Ti microalloyed high-reaming steel, which is characterized in that the component design characteristics are that the low-carbon high-silicon Nb-Ti microalloyed high-reaming ratio is more than or equal to 50%, the coiling temperature interval required for forming bainite is about 500 ℃, the medium-temperature coiling control difficulty is high under the condition of the existing technological equipment, and the fluctuation of the overall length performance of a steel coil is easy to be caused.

Chinese patent CN105821301A relates to 800MPa grade hot rolled high strength high hole expansion steel, and the component design characteristics are low carbon high silicon Nb-Ti microalloying, the Ti content is very high and is 0.15-0.18%, in the actual production process, the ultrahigh Ti component design is easy to form coarse TiN inclusion, and the hole expansion rate stability is unfavorable.

Chinese patent CN108570604A relates to 780MPa grade hot-rolled pickled high-reaming steel, which has the characteristics of low carbon, high aluminum and high chromium in component design, adopts a three-section cooling process in process design, is easy to cause the blockage of a casting nozzle in the actual production process, and has complex process, large control difficulty of the three-section cooling process and low hole-reaming rate.

Chinese patent CN114107789a relates to 780MPa grade hot rolled and pickled high-hole-enlarging steel, which is characterized by low carbon and high titanium, and adding a certain amount of molybdenum element, and the composition design contains no copper or a small amount of copper. The copper element is not added as a precipitation strengthening element.

The 780 MPa-grade high-hole-enlarging steel adopts high Ti as a precipitation strengthening element, and no patent document on the aspect of high-copper high-hole-enlarging steel is found according to the result of searching the existing 780 MPa-grade patent. The problems of difficult steelmaking, large control difficulty of the temperature uniformity of the whole strip steel and the like exist in the patents.

Disclosure of Invention

The invention aims to provide precipitation strengthening type tempered martensite high-reaming steel and a manufacturing method thereof, wherein the yield strength of the high-reaming steel is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the elongation A50 is more than or equal to 15 percent, and the reaming rate is more than or equal to 100 percent, and the high-reaming steel can be applied to various parts which are responsible for high-strength thinning such as chassis, brackets and the like.

In order to achieve the above purpose, the technical scheme of the invention is as follows:

in order to meet the requirements of users for better performance stability, higher strength, plasticity, hole expansibility matching and the like, subversion changes are required to be made on the traditional high-hole-expansion steel.

It is well known that in general, the elongation of a material is inversely related to the hole expansion ratio, i.e., the higher the elongation, the lower the hole expansion ratio; conversely, the lower the elongation, the higher the hole expansion ratio. The higher the strength of the material, the lower the hole expansion ratio under the same or similar strengthening mechanism. In order to obtain a steel product with good plasticity and reaming and flanging properties, the relationship between the two needs to be balanced better.

Therefore, the invention adopts a completely different design thought from the prior art in component design, and can obtain good matching of high strength, high plasticity, high hole expansion rate and performance stability by accurately controlling the organization and nano precipitated phase, thereby better meeting the requirements of users.

Specifically, the precipitation strengthening type tempered martensite high-reaming steel comprises the following components in percentage by weight: 0.01 to 0.1 percent of C, 0.2 to 2.0 percent of Si, 1.5 to 3.0 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.005 percent of S, 0.01 to 0.08 percent of Al, less than or equal to 0.005 percent of N, 0.8 to 3.0 percent of Cu, less than or equal to 0.003 percent of O, and the balance of Fe and other unavoidable impurities; the high-reaming steel has a structure of tempered martensite and nano precipitated copper contained in the tempered martensite.

Further, the alloy also comprises one or more than one of Mo which is less than or equal to 0.5%, V which is less than or equal to 0.5%, ni which is less than or equal to 0.5%, cr which is less than or equal to 0.5%, nb which is less than or equal to 0.1%, ti which is less than or equal to 0.2% and B which is less than or equal to 0.002%.

In the component design of the precipitation strengthening type tempered martensite high-reaming steel, the following components are included:

carbon, which is a basic element in steel, is one of the important elements in the present invention. Carbon expands the austenite phase region, stabilizing austenite. Carbon plays a very important role in improving the strength of steel as a interstitial atom in steel, and has the greatest influence on the yield strength and tensile strength of steel. In the invention, as the structure to be obtained in the hot rolling stage is low-carbon martensite, the carbon content is required to be ensured to be more than 0.01 percent in order to obtain the high-strength steel with the final tensile strength of 780 MPa; at the same time, the carbon content cannot be higher than 0.10%. The carbon content is too high, and medium-high carbon martensite is easy to form during low-temperature coiling, so that the strength is too high. Thus, the carbon content is controlled to be 0.01-0.10%, preferably 0.03-0.07%.

Silicon is a basic element in steel. As mentioned above, in order to meet the requirements of high strength, high plasticity and high hole expansion ratio, which are set forth by users, more silicon is generally added in the composition design. In the invention, the addition of higher silicon can play a role in improving hardenability and inhibiting excessive cementite formation in the process of annealing besides conventional deoxidation; the silicon content of more than 0.2 percent can play a role in better deoxidization and improving hardenability, further improves the silicon content to more than 0.8 percent, and can also play a role in inhibiting cementite formation; when the silicon content exceeds 2.0%, the effect of silicon on suppressing cementite formation becomes saturated, and the addition of excessive silicon content is detrimental to impact toughness. Therefore, the silicon content of the steel of the present invention should be controlled between 0.2 and 2.0%.

Manganese is also the most basic element in steel and is one of the most important elements in the present invention. Mn is known to be an important element for enlarging the austenite phase region, and can reduce the critical quenching speed of steel, improve hardenability, stabilize austenite, refine grains, and retard transformation of austenite to pearlite. In the invention, in order to ensure that the steel plate obtains low-carbon martensite in the cooling process, the Mn content is generally controlled to be more than 1.5 percent; meanwhile, the Mn content is generally not more than 3.0%, otherwise, the slab has obvious Mn segregation. Therefore, the Mn content in the steel of the present invention is controlled to 1.5-3.0%.

Phosphorus is an impurity element in steel. P is easily biased to grain boundary, and Fe is formed when the content of P in steel is higher (more than or equal to 0.1 percent) ₂ P is precipitated around crystal grains, and the plasticity and toughness of steel are reduced, so that the lower the phosphorus content is, the better the phosphorus content is controlled within 0.02 percent, and the steelmaking cost is not increased.

Sulfur is an impurity element in steel. S in steel is usually combined with Mn to form MnS inclusion, more MnS is formed in the steel especially when the contents of S and Mn are high, the MnS has certain plasticity, and the MnS deforms along the rolling direction in the subsequent rolling process, so that the transverse plasticity of the steel is reduced, the tissue anisotropy is increased, and the reaming performance is unfavorable. Therefore, the lower the S content in the steel, the more preferably the S content is controlled to be within 0.005%, preferably 0.003% or less, in order to reduce the MnS content.

The role of aluminum in steel is mainly deoxidation and nitrogen fixation. In the presence of strong carbide forming elements such as Ti and the like, al has the main functions of deoxidizing and refining grains. In the invention, al is used as a common deoxidizing element and an element for refining grains, and the content of the Al is usually controlled to be 0.01-0.08%; al content is less than 0.01%, and the effect of refining grains is not achieved; also, when the Al content is higher than 0.08%, the effect of refining the crystal grains is saturated. Therefore, the Al content in the steel of the present invention is controlled to be 0.01 to 0.08%, preferably 0.02 to 0.05%.

Nitrogen, which is an impurity element in the present invention, is preferably contained in a lower amount. Nitrogen is an inevitable element in the steelmaking process. Although the content thereof is small, the formed VN particles, in combination with strong carbide forming elements such as V and the like, have a detrimental effect on the properties of the steel, especially on the reaming properties. Because VN is square, there is very big stress concentration between its closed angle and the base member, and in the reaming deformation's in-process, the stress concentration between VN and the base member easily forms the crack source to greatly reduced the reaming performance of material. Because the invention adopts the high vanadium design on the component system, the adverse effect on reaming caused by VN is reduced as much as possible. Therefore, the nitrogen content is controlled to be 0.005% or less, preferably 0.004% or less.

Copper is an important element in the present invention. Copper is added into the steel to improve the corrosion resistance of the steel, and when the copper and the phosphorus are added together, the corrosion resistance effect is better; when the addition amount of copper exceeds 0.8%, under certain heat treatment conditions, epsilon-Cu copper-rich nano precipitated phases can be formed in a body-centered cubic structure type structure such as ferrite, and obvious precipitation strengthening effect is achieved; when the copper content exceeds 3.0%, the reinforcing effect of the epsilon-Cu copper-rich nano precipitated phase reaches saturation. The optimal process window of the epsilon-Cu copper-rich nano precipitated phase can be obtained through the temperature and time regulation of the heat treatment. Copper embrittlement is easy to occur when more copper is added into the steel, so that a certain nickel element is mostly added into the copper-containing steel to inhibit copper embrittlement.

Molybdenum is one of the additive elements in the present invention. The addition of molybdenum to steel can greatly delay ferrite and pearlite transformation, which is beneficial to obtaining martensitic structure. In addition, molybdenum has strong weld softening resistance. Since the main purpose of the invention is to obtain a low-carbon martensitic structure, and the low-carbon martensite is easy to soften after welding, the addition of a certain amount of molybdenum can effectively reduce the welding softening degree. Thus, the present invention controls the molybdenum content to be 0.1 to 0.5%, preferably 0.20 to 0.40%.

Niobium is one of the additive elements of the present invention. Niobium is similar to titanium and is a strong carbide element in steel, the unrecrystallized temperature of the steel can be greatly increased by adding the niobium into the steel, deformed austenite with higher dislocation density can be obtained in the finish rolling stage, and the final phase transformation structure can be refined in the subsequent transformation process. However, the amount of niobium added is not too large, and on the one hand, the amount of niobium added exceeds 0.10%, so that relatively coarse niobium carbonitrides are easily formed in the structure, part of carbon atoms are consumed, and the precipitation strengthening effect of carbides is reduced. Meanwhile, the niobium content is high, anisotropy of a hot rolled austenitic structure is easy to cause, and the hot rolled austenitic structure is inherited to a final structure in a subsequent cooling phase transformation process, so that the reaming performance is not good. Therefore, the niobium content in the steel of the present invention is controlled to be 0.10% or less, preferably 0.06% or less.

Titanium is an additive element in the present invention. A small amount of Ti is added into the steel, so that on one hand, the Ti can be combined with N to form TiN in a high-temperature stage, the effect of nitrogen fixation is achieved, and VN formation can be reduced in the subsequent covering and annealing process; on the other hand, the excessive Ti combined with N can be combined with carbon to form nano TiC in the subsequent annealing process, and the nano TiC and the nano VC can jointly improve the performance of the steel. When the titanium content is higher than 0.20%, more coarse TiN tends to be formed at a high temperature stage and the impact toughness of the steel is deteriorated. Therefore, the titanium content in the steel of the present invention is controlled to be within 0.20%, preferably within 0.10%.

Vanadium is one of the additizable elements in the present invention. Vanadium, like titanium, niobium, is also a strong carbide forming element. However, the vanadium carbide has a low solution or precipitation temperature, and is usually entirely dissolved in austenite in the finish rolling stage. Vanadium starts to form in ferrite only when the temperature decrease starts to change phase. When the heat treatment temperature reaches more than 550 ℃, vanadium is not added, and mainly because the carbide of the vanadium coarsens in the temperature range, the obvious precipitation strengthening effect cannot be achieved; when the heat treatment temperature is below 550 ℃, certain vanadium element can be added according to the requirement, so that the strength of the steel can be further improved. Therefore, the addition amount of vanadium in the steel of the invention is controlled to be less than or equal to 0.5 percent.

Nickel is an additive element in the invention. The nickel added into the steel has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper, and the nickel added into the steel has little influence on the tensile property of the steel, but can refine the structure and the precipitated phase of the steel, so that the low-temperature toughness of the steel is greatly improved; meanwhile, in the steel added with more copper elements, the occurrence of copper embrittlement can be restrained by adding a small amount of nickel. The addition of higher nickel has no adverse effect on the properties of the steel itself. If copper and nickel are added at the same time, not only the corrosion resistance can be improved, but also the structure and the precipitated phase of the steel are refined, and the low-temperature toughness is greatly improved. But since copper and nickel are both relatively noble alloying elements. Therefore, in order to reduce the cost of alloy design as much as possible, the addition amount of nickel is controlled to be less than or equal to 0.5%, preferably less than or equal to 0.3%.

Chromium is an additive element in the present invention. Chromium is added into steel to improve the strength of the steel mainly through solid solution strengthening, tissue refining and other modes. Because the structure is fine martensitic ferrite plus nano precipitated carbide and the movable dislocation in the structure is reduced after the high-temperature shield annealing process is carried out, the ratio of the yield strength to the tensile strength of the steel, namely the yield ratio, is higher and is generally more than 0.90. The addition of a small amount of chromium element can properly reduce the yield strength of steel, thereby reducing the yield ratio. In addition, the addition of a small amount of chromium may also function to improve corrosion resistance, and the addition amount of chromium is usually not more than 0.5%, preferably not more than 0.3%.

Boron is an additive element in the present invention. Boron can greatly improve the hardenability of steel and is beneficial to obtaining a martensitic structure. Considering that the structure expected to be obtained in the hot rolling stage is low-carbon martensite rather than martensite, a proper amount of boron element can be added into steel, the addition amount of boron in the steel is generally controlled to be less than or equal to 0.002 percent, and the addition of excessive boron element can lead supersaturated boron to be biased at grain boundaries, so that the low-temperature impact toughness of the steel is not good. Therefore, the boron content in the steel of the present invention is controlled to be less than or equal to 0.002%.

Oxygen is an impurity element in the invention, and in order to obtain a steel coil with more excellent performance, the lower the oxygen content in steel is, the better, but the lower the oxidation amount is, the steelmaking cost is increased, and the oxygen content in the steel is controlled within 0.003%, preferably within 0.002% under the condition of ensuring the performance of strip steel.

From the aspect of component design, the traditional high-reaming steel adopts a component design thought with high titanium as a main component, and the invention adopts a high-copper component design. The main purpose of adding alloy element copper in the existing high-strength steel is to improve the corrosion resistance of the steel, the addition amount of the alloy element copper is generally within 0.5 percent, and the purpose of adding higher copper content in the component design of the invention is to form nano precipitated copper.

From the structural point of view, the traditional high titanium type high hole-enlarging steel mainly comprises ferrite and nano titanium carbide, while the invention mainly comprises tempered bainite and nano precipitated copper, and the related patent literature on the aspect of the high hole-enlarging steel designed by adopting high copper components is not searched.

The invention relates to a manufacturing method of precipitation strengthening type tempered martensite high-reaming steel, which comprises the following steps:

1) Smelting and casting

Smelting the components by adopting a converter or an electric furnace, secondarily refining by adopting a vacuum furnace, and casting into a casting blank or an ingot;

2) Reheating of billets or ingots

Heating temperature is more than or equal to 1100 ℃, and heat preservation time is as follows: 1 to 2 hours;

3) Hot rolling and cooling

The initial rolling temperature is 1000-1100 ℃, the rolling is carried out at 3-5 times of high-pressure rolling above 1000 ℃ and the accumulated deformation is more than or equal to 80%, then the intermediate billet is heated to 950-1000 ℃, then the rolling is carried out at the last 3-7 times and the accumulated deformation is more than or equal to 80%, and the final rolling temperature is 800-950 ℃; after finishing rolling, cooling the steel plate to the temperature of less than or equal to 300 ℃ at the cooling rate of more than or equal to 10 ℃/s, coiling, and then cooling to room temperature;

4) Annealing

And (3) adopting hood annealing, heating to 400-600 ℃ at a heating speed of more than or equal to 20 ℃/h, preserving heat for 8-24 h, and cooling to less than or equal to 300 ℃ at a cooling speed of less than or equal to 50 ℃/h and discharging.

Further, step 5) pickling, wherein the running speed of strip steel pickling is 30-140 m/min, the pickling temperature is controlled at 75-85 ℃, the withdrawal and straightening rate is controlled at less than or equal to 3%, rinsing is carried out at a temperature range of 35-50 ℃, surface drying is carried out at a temperature range of 120-140 ℃, and oiling is carried out, so that the pickling precipitation strengthening type tempered martensite high-reaming steel is obtained.

In the method for manufacturing precipitation strengthening type tempered martensitic high-hole-enlarging steel, according to the invention:

the invention adopts a combination process of on-line quenching and a shield annealing process and is combined with an innovative low-carbon high-copper component design, so that the high-strength high-reaming steel with excellent performance stability can be obtained.

The martensite with uniform structure is obtained by on-line quenching, after the quenching is carried out at medium and high temperature, the quenched martensite is converted into tempered martensite with uniform structure, and meanwhile, nano copper is precipitated and uniformly distributed in the tempered martensite, so that the strength of the steel is improved, and the reaming performance is also improved. By combining the innovative components and the process, the high-strength high-reaming steel with excellent performance stability can be obtained.

The traditional high titanium type high reaming steel mostly adopts a high temperature coiling process. The invention adopts the on-line quenching and the cover annealing process. The hot rolling start temperature is 1000-1100 ℃, the accumulated deformation is more than or equal to 80% under 3-5 times of high pressure above 1000 ℃, and the main purpose is to refine austenite grains and retain more solid solution copper at the same time; then the intermediate blank is heated to 950-1000 ℃, and then is rolled for the last 3-7 passes, and the accumulated deformation is more than or equal to 80%; after finishing the finish rolling at 800-950 ℃, cooling the steel plate to below 300 ℃ at a cooling rate of more than or equal to 10 ℃/s, and slowly cooling to room temperature after coiling.

In the rough rolling and finish rolling stages, the rolling rhythm should be completed as fast as possible, so as to ensure that more copper is dissolved in austenite. After finishing the finish rolling, the strip steel is cooled to below 300 ℃ on line at a cooling speed of more than or equal to 10 ℃/s so as to obtain a low-carbon martensitic structure.

The steel coil obtained by hot rolling on-line quenching is annealed at 400-600 ℃ for a long time, so that the formation of nano precipitated Cu in the martensite lath is promoted, and the structure and precipitation are more uniform.

In the hot rolling stage, coiling at low temperature to form low-carbon martensite with uniform and fine structure; in the annealing stage, a dispersed fine nano copper-rich phase is separated out from the martensite lath, so that the strength is improved, and the plasticity and the hole expansion rate are improved. The cover annealing temperature and the time are in inverse proportion, namely, the lower the cover annealing temperature is, the longer the cover annealing time is; conversely, the higher the mask annealing temperature, the shorter the mask annealing time. If the annealing temperature is lower than 400 ℃, the nano copper-rich phase is not sufficiently separated out; if the annealing temperature is higher than 600 ℃, the tissue and nano copper-rich compatibility is easy to grow and coarsen, and the strength is reduced. Therefore, the annealing temperature is selected to be 400-600 ℃.

In the annealing process, the low-carbon martensite with uniform and fine structure obtained by low-temperature coiling in the hot rolling stage is subjected to medium-high temperature annealing process to promote the precipitation of the nanoscale copper-rich phase in the martensite lath, and meanwhile, the strength can be further improved and the uniformity of the structure can be improved.

Compared with the prior art, the invention has the advantages that:

compared with the Chinese patent CN105821301A, the composition design is low-carbon high-silicon Nb-Ti microalloying, the Ti content is very high and is 0.15-0.18%, and in the actual production process, the ultrahigh Ti composition design is easy to form coarse TiN inclusions and has adverse effect on the stability of the hole expansion rate.

The invention is designed with low carbon and high copper, and is different from the high titanium design, and the invention adopts on-line quenching and shield annealing treatment in the process, so that the performance stability is better.

The 780 MPa-grade hot-rolled pickled high-reaming steel related to the Chinese patent CN108570604A is designed to be low-carbon, high-aluminum and high-chromium, and adopts a three-section cooling process in process design, so that the control difficulty of the three-section cooling process is high, and the actual reaming rate is not high. The composition design of the hot rolling material does not contain copper, and is completely different from the composition design of the hot rolling material, the conventional low-temperature coiling and covering-backing process is adopted in the process route, a complex three-stage cooling process is not needed, and the process stability is better.

The invention adopts an innovative design thought of low carbon and high copper components, can obtain the high-performance stability high-reaming steel with excellent surface, strength, plasticity and reaming performance, has the yield strength of more than or equal to 700MPa and the tensile strength of more than or equal to 780MPa, has good elongation (transverse A50 of more than or equal to 15%) and reaming performance (reaming rate of more than or equal to 100%), can be applied to manufacturing complex parts such as automobile chassis, auxiliary frames and the like which need high-strength thinning and reaming flanging, and has very wide application prospect.

Drawings

FIG. 1 is a schematic diagram of hot rolling and cooling processes of precipitation strengthened tempered martensitic high-reaming steel according to the present invention;

fig. 2 is a schematic diagram of a process for annealing the precipitation strengthening type tempered martensitic high-reaming steel according to the present invention.

Detailed Description

The invention is further described below with reference to examples and figures.

The composition of the inventive example steel is shown in table 1, the balance of the composition comprising Fe and unavoidable impurities.

The process route of the embodiment of the invention is as follows: smelting, casting, reheating of cast billets or ingots, hot rolling, cooling, and annealing, as shown in fig. 1 and 2. Table 2 shows the production process parameters of the steel according to the example of the present invention, and Table 3 shows the performance parameters of the steel according to the example of the present invention.

As can be seen from Table 1, comparative examples 1 to 4 each have no copper element added in the composition design, and comparative examples 1 to 3 use a high aluminum composition design, which is greatly different from the composition design of the present invention.

As is clear from Table 3, while comparative examples 1 to 3 are similar to the present invention in terms of yield strength, tensile strength, elongation, etc., the index of the hole expansion ratio of comparative examples 1 to 3 is significantly lower than that of the examples of the present invention. Comparative example 4 has similar properties to the examples of the present invention, but the structure of comparative example 4 is ferrite and nano titanium carbide, and the structure of the examples of the present invention is tempered martensite and nano precipitated copper.

It can be seen from table 3 that the yield strength of the high-reaming steel is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the elongation A50 is more than or equal to 15%, the reaming rate is more than or equal to 100%, and the high-reaming steel has good strength, plasticity and reaming performance matching, is particularly suitable for parts needing high-strength thinning and reaming flanging forming such as control arms and the like of automobile chassis structures, can also be used for complex parts needing hole flanging such as wheels and the like, and has wide application prospect.

Claims

1. The precipitation strengthening type tempered martensite high-reaming steel comprises the following components in percentage by weight: 0.01 to 0.1 percent of C, 0.2 to 2.0 percent of Si, 1.5 to 3.0 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.005 percent of S, 0.01 to 0.08 percent of Al, less than or equal to 0.005 percent of N, 0.8 to 3.0 percent of Cu, less than or equal to 0.003 percent of O, and the balance of Fe and other unavoidable impurities; the high-reaming steel has a structure of tempered martensite and nano precipitated copper contained in the tempered martensite.

2. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 1, further comprising one or more of Mo not more than 0.5%, V not more than 0.5%, ni not more than 0.5%, cr not more than 0.5%, nb not more than 0.1%, ti not more than 0.2%, and B not more than 0.002%.

3. The precipitation strengthening type tempered martensitic high-hole-enlarging steel according to claim 1, wherein C is 0.03-0.07%.

4. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 1, wherein S is less than or equal to 0.003%.

5. The precipitation strengthening type tempered martensitic high-hole-enlarging steel according to claim 1, wherein Al is 0.02-0.05%.

6. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 1, wherein N is not more than 0.004%.

7. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 1, wherein O is less than or equal to 0.002%.

8. The precipitation strengthening type tempered martensitic high-hole-enlarging steel according to claim 2, wherein Mo is 0.2-0.4%.

9. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 2, wherein V is less than or equal to 0.30%.

10. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 2, wherein Nb is less than or equal to 0.06%.

11. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 2, wherein Ti is less than or equal to 0.10%.

12. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 2, wherein Ni is less than or equal to 0.3%.

13. The precipitation strengthened tempered martensitic high-hole-enlarging steel according to claim 2, wherein Cr is less than or equal to 0.3%.

14. The precipitation-strengthened tempered martensitic high-reamed steel as claimed in any one of claims 1 to 13, wherein said precipitation-strengthened tempered martensitic high-reamed steel has a yield strength of at least 700MPa, a tensile strength of at least 780MPa, an elongation a50 of at least 15% and a hole expansion ratio of at least 100%.

15. The method for producing a precipitation-strengthened tempered martensitic high-reamed steel as claimed in any one of claims 1 to 14, comprising the steps of:

1) Smelting and casting

Smelting the components according to any one of claims 1 to 13 by using a converter or an electric furnace, secondarily refining by using a vacuum furnace, and casting into a casting blank or an ingot;

2) Reheating of billets or ingots

3) Hot rolling and cooling

4) Annealing

16. The method for producing precipitation strengthening type tempered martensitic high-reamed steel as claimed in claim 15, wherein the step 5) is to perform acid washing, the operation speed of the acid washing of the strip steel is 30-140 m/min, the acid washing temperature is controlled to be 75-85 ℃, the withdrawal rate is controlled to be less than or equal to 3%, rinsing is performed at a temperature range of 35-50 ℃, and surface drying and oiling are performed at a temperature range of 120-140 ℃ to obtain the acid washing precipitation strengthening type tempered martensitic high-reamed steel.