CN113502440A - Nickel-saving type ultra-low temperature high-strength steel and heat treatment process thereof - Google Patents
Nickel-saving type ultra-low temperature high-strength steel and heat treatment process thereof Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/001—Heat treatment of ferrous alloys containing Ni
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
Abstract
The invention provides nickel-saving type ultra-low temperature high-strength steel which comprises the following elements in percentage by mass: c: 0.005-0.015%; ni: 3.5 to 6.5 percent; mn: 2.0 to 5.0 percent; cu: 0.5-2.5%; mo: 0.2 to 0.8 percent; s: less than or equal to 0.004 percent; p: less than or equal to 0.003 percent; n: 0.003-0.005%; o: 0.0005-0.001%; ca: 0.0005-0.005%; the balance being Fe. The invention further provides a heat treatment process of the nickel-saving ultralow-temperature high-strength steel. The nickel-saving type high-strength steel for ultralow temperature and the heat treatment process thereof provided by the invention can obtain nickel-saving type low-temperature steel with toughness at the low temperature of-196 ℃, do not need cold deformation treatment, have simple heat treatment process, can be used for manufacturing structural steel for low temperature, have strong economic applicability and have excellent application and development prospects.
Description
Technical Field
The invention belongs to the technical field of low-temperature steel, relates to nickel-saving type ultralow-temperature high-strength steel and a heat treatment process thereof, and particularly relates to nickel-saving type ultralow-temperature steel and a tempering-double-partitioning heat treatment process thereof.
Background
In recent years, Liquefied Natural Gas (LNG) has become an extremely important strategic reserve resource, and materials used in LNG carriers, large-sized storage tanks, and the like, which are closely related thereto, have attracted much attention from the industrial and academic circles. The LNG transport ship is a special ship for transporting liquefied gas at a low temperature of-162 ℃, so that the material for storing and transporting the liquefied gas is required to have excellent toughness and crack arrest capability at a low temperature, and is an internationally recognized three-high product with high technology, high difficulty and high added value. Researches show that the ferrite section steel can effectively reduce the ductile-brittle transition temperature under the condition of ensuring certain strength, thereby inhibiting the brittle fracture problem of materials under the low-temperature environment, and becoming one of the low-temperature steel types which are widely applied at present.
The mature 9Ni steel is the most practical low-temperature structural material, and is widely applied to manufacturing large containers for storing and transporting low-temperature liquid. The heat treatment process of the 9Ni steel mainly relates to multi-step critical annealing in a dual-phase region of austenite and ferrite, in the annealing process, a large amount of film-shaped austenite nucleates among laths in a martensite matrix, the austenite has good thermal stability, and the martensite phase transformation still cannot occur under the condition of cooling to-196 ℃, so that the crystal arrangement of the martensite laths is broken, the effects of inhibiting crack nucleation and propagation are achieved, and the ductile-brittle transition temperature of the material is effectively reduced. The addition of the Ni element can be enriched in the retained austenite to reduce the martensite phase transition temperature on one hand, and on the other hand, the Ni element which is partially gathered at the grain boundary improves the interfacial cohesion, thereby avoiding the intergranular fracture and the transgranular fracture at low temperature.
However, as it is known that the cost of Ni element is high and development difficulty is large, such low temperature steel is faced with the problem of improving performance and reducing cost at the present stage.
In particular, in terms of performance, in the prior art, in order to ensure good low-temperature plasticity, a certain amount of retained austenite needs to be introduced, and then a transformation induced plasticity (TRIP) effect is activated. The thermal and mechanical stability of the retained austenite is a key factor in maintaining the TRIP effect, and the conventional quench-partition (QP) or quench-partition-tempering (QPT) process is to achieve austenite stabilization by partitioning of C element between martensite and austenite. Meanwhile, it is well known that grain refinement and dislocation introduction by cold deformation are effective ways to improve strength. However, the above process brings at least the following technical problems:
1. austenite stabilized by C element tends to have low mechanical stability at low temperature, and an increase in hardenability caused by an excessive amount of C element further increases the heat-affected zone at the time of welding, and degrades the welding performance of the material.
2. The strengthening by grain refinement and dislocation introduction by cold deformation is not suitable for thicker materials, thus greatly limiting the large-scale industrial application of the materials.
Disclosure of Invention
In view of the defects of the prior art, the invention aims to provide the nickel-saving type ultra-low temperature high-strength steel and the heat treatment process thereof, and the nickel-saving type ultra-low temperature high-strength steel is prepared by optimizing the process route based on the independent component design, thinning the size of film-shaped residual austenite, improving the thermal stability and the mechanical stability of austenite, and solving the problems that the prior art lacks the nickel-saving type low-temperature steel which has good low-temperature and high-toughness combination and can be used in the ultra-low temperature (-196 ℃) environment and the preparation method thereof.
In order to achieve the above objects and other related objects, a first aspect of the present invention provides a nickel-saving ultra-low temperature high-strength steel, which comprises the following elements by mass:
c (carbon): 0.005-0.015%; ni (nickel): 3.5 to 6.5 percent; mn (manganese): 2.0 to 5.0 percent; cu (copper): 0.5-2.5%; mo (molybdenum): 0.2 to 0.8 percent; s (sulfur): less than or equal to 0.004 percent; p (phosphorus): less than or equal to 0.003 percent; n (nitrogen): 0.003-0.005%; o (oxygen): 0.0005-0.001%; ca (calcium): 0.0005-0.005%; the balance being Fe (iron).
The precipitation-strengthened low-temperature steel further contains inevitable impurities. The inevitable impurities are trace impurity elements which are inevitably contaminated in the preparation process of the low-temperature steel.
Preferably, the precipitation-strengthened low-temperature steel consists of the following elements in percentage by mass:
c (carbon): 0.008-0.012%; ni (nickel): 4.0 to 6.0 percent; mn (manganese): 2.0-4.0%; cu (copper): 1.0-2.0%; mo (molybdenum): 0.3-0.7%; s (sulfur): less than or equal to 0.004 percent; p (phosphorus): less than or equal to 0.003 percent; n (nitrogen): 0.003-0.005%; o (oxygen): 0.0005-0.001%; ca (calcium): 0.0005-0.005%; the balance being Fe (iron).
The element composition of the precipitation-strengthened low-temperature steel can improve the economic benefit and the low-temperature toughness of the nickel-based ferrite low-temperature steel, wherein:
more than 2 percent of Mn element is added as austenite stabilizing element, thereby greatly reducing the contents of C element and Ni element; specifically, the reduction of the element C can reduce austenite stabilized by the element C, improve the stability of the austenite, and meanwhile, the reduction of the element C can also ensure the welding performance of the material; meanwhile, the heat treatment window of the material can be effectively optimized by adding Mn element to replace Ni element, and the reduction of Ni element can greatly reduce cost and improve economic benefit;
the Cu element is added to form a copper-rich phase to generate a precipitation strengthening effect and cause local segregation of the Mn element, so that the effect of improving the driving force of local inverse transformation is achieved, more importantly, the austenite memory effect based on local forward and inverse transformation is an effective way for optimizing the coherence of a dual-phase interface, and the effect of improving the mechanical stability of the retained austenite is achieved;
the low-temperature steel has extremely low austenite content, the content of Mn element is controlled below 5 percent to prevent the excessive austenite content caused by the large diffusion of Mn element, and a proper amount of Mo element is added to pin a grain boundary to reduce the content of the transformed austenite after two phases are isothermal between two phase interfaces. And the austenite stability is further good due to the extremely little austenite, so that the low-temperature toughness is improved.
The second aspect of the invention provides a heat treatment process of nickel-saving type ultra-low temperature high-strength steel, which comprises the steps of taking all the element components according to the proportion, mixing and smelting, then slab continuous casting into cast ingots, carrying out hot rolling treatment, tempering and water quenching on the obtained steel ingots, and then sequentially carrying out first two-phase isothermal zone water quenching and second two-phase isothermal zone water quenching to provide the low temperature steel.
Preferably, the smelting is a conventional steel smelting process.
Preferably, the slab casting is a conventional steel casting process.
Preferably, the cast ingot needs to be derusted, deoiled and cleaned before hot rolling treatment. The phenomenon of uneven stress in the hot rolling treatment process is avoided.
Preferably, the hot rolling treatment is to perform multi-step hot rolling on the ingot from the initial rolling temperature to the final rolling temperature and then air cooling.
More preferably, the initial rolling temperature is 1150-1250 ℃.
More preferably, the finishing temperature is 700-.
More preferably, the multi-step hot rolling comprises the steps of:
the first step is as follows: hot rolling temperature: 1150-: 115 ℃ for 125 minutes;
the second step is that: hot rolling temperature: 880-: 65-75 minutes;
the third step: hot rolling temperature: 700 ℃ and 800 ℃, and the heat preservation time is as follows: 20-30 minutes.
More preferably, each reduction of the multi-step hot rolling is maintained at 20 to 30%.
The reduction ratio is a reduction ratio indicating a degree of deformation at a time of forging and rolling, which indicates a relative deformation. When the rolling reduction rate of the multiple steps is kept in a stable range as much as possible, the rolling effect is better.
Preferably, the tempering temperature is A1Temperature to A1At a temperature below 50-150 ℃, wherein A is1Is A1The critical point temperature is a temperature at which austenite, ferrite, and cementite coexist in equilibrium in the equilibrium state.
More preferably, the tempering temperature is A1Temperature to A1The temperature is 50-100 deg.C below.
Preferably, the tempering time is 0.5-1.5 h. More preferably, the tempering time is 0.5-1.0 h.
Preferably, the isothermal temperature of the first two-phase region is A1Temperature to A1The temperature is 50-100 ℃ above.
Further preferably, the isothermal temperature of the first two-phase region is A1Temperature to A1The temperature is 50-80 ℃ above.
A above1The temperature is 620 ℃ and 670 ℃.
Preferably, the isothermal temperature of the first two-phase zone is 50-150 ℃ higher than the isothermal temperature of the second two-phase zone. The higher isothermal temperature of the first two-phase region is to obtain more fresh martensite before the second two-phase region is isothermal, and research shows that the fresh martensite has an optimization effect on the subsequent coherent relationship between the reversed austenite and the matrix.
Preferably, the isothermal time of the first two-phase region isothermal time and the second two-phase region isothermal time is 0.5-1.5 h.
More preferably, the isothermal time of the first isothermal phase zone and the second isothermal phase zone is 0.5-1.0 h.
Preferably, the water quenching is to cool the tempered or isothermally treated steel ingot to room temperature by water. The room temperature is 20-25 ℃.
As described above, the nickel-saving type ultra-low temperature high-strength steel and the heat treatment process thereof provided by the invention adopt a multi-step isothermal quenching to achieve a tempering-double partitioning process, select a proper isothermal temperature and heat preservation time, obtain local segregation of a nano precipitated phase and Mn elements through tempering in the first step, obtain a small amount of residual austenite and fresh martensite through isothermal in a first two-phase region in the second step, realize reverse transformation of austenite and partitioning of Mn and Ni elements through isothermal in a second two-phase region in the third step, and obtain the nickel-saving type ultra-low temperature steel with both obdurability through the comprehensive effect of the nano precipitated phase and austenite.
The first-step tempering has the functions of precipitation strengthening by introducing a nanometer precipitated phase on one hand, and realizes local segregation of Mn element by virtue of the enrichment function of the Mn element near the precipitated phase on the other hand. In the invention, the precipitation of the copper-rich phase does not reduce the toughness while improving the strength, and the local segregation of the Mn element can be realized due to the higher bonding enthalpy of the Mn element, the Cu element and the Ni element. In addition, Mn replaces partial Ni to obviously improve the strong plasticity of the steel, reduce the cost and improve the economic benefit. Based on the local segregation of Mn element introduced in the first step, fresh martensite is further generated in the subsequent quenching process of part of the retained austenite obtained by the isothermal treatment of the first two-phase region in the second step due to the deficiency of austenite stabilizing elements, and the fresh martensite optimizes the interface congruency of the reverse austenite and the ferrite matrix in the isothermal process of the second two-phase region in the third step, thereby improving the mechanical stability of the retained austenite. The addition of trace Mo element plays a role in solid solution strengthening on one hand, and has an inhibiting effect on thermal activation migration of a grain boundary on the other hand, and the volume fraction of austenite generated by isothermal transformation can be reduced, so that the stability of the austenite is improved, and the low-temperature service performance of the material is optimized.
The nickel-saving ultra-low temperature high-strength steel and the heat treatment process thereof provided by the invention do not need cold deformation treatment, adopt a tempering-double distribution process, utilize TRIP effect of residual austenite and precipitation strengthening of nano precipitated phase in the stretching process, and can prepare strong plasticity with yield strength of 1150-1400MPa, tensile strength of 1200-1450MPa and elongation of 20-30% at the temperature of-196 ℃, and V-notch impact toughness of 220-260J/cm at the temperature of-196 DEG C2And the nickel-saving type ultralow-temperature steel with high strength and toughness is realized.
The nickel-saving type high-strength steel for ultralow temperature and the heat treatment process thereof provided by the invention adopt a multi-step tempering-double partitioning process, utilize TRIP effect of residual austenite and strengthening effect of copper-rich phase particles to prepare and obtain the steel for ultralow temperature with both obdurability and toughness, and have the advantages of low application cost, simple heat treatment process and strong economic applicability.
Drawings
FIG. 1 is a graph showing comparison of electron back scattering diffraction patterns of a tempered and untempered double-portioned low-temperature steel in the present invention with FIGS. 1a and 1b, wherein FIG. 1a is an electron back scattering diffraction pattern of a tempered-double-portioned low-temperature steel in example 1; FIG. 1b is an electron back-scattered diffraction pattern of the untempered double-portioned low-temperature steel of comparative example 1.
FIG. 2 is a graph showing the oscillometric shock at room temperature and at ultra low temperature of the low temperature steel obtained after the tempering and non-tempering double partition heat treatment in the present invention, FIGS. 2a and 2b, wherein FIG. 2a is the oscillometric shock at room temperature and at ultra low temperature of the tempered-double partition low temperature steel in example 1; FIG. 2b is an oscillographic impact diagram of the untempered double-portioned cryogenic steel of comparative example 1 at room temperature and at ultra-low temperature.
Detailed Description
The following description of the embodiments of the present invention is provided for illustrative purposes, and other advantages and effects of the present invention will become apparent to those skilled in the art from the present disclosure.
Please refer to fig. 1-2. It should be understood that the structures, ratios, sizes, and the like shown in the drawings and described in the specification are only used for matching with the disclosure of the specification, so as to be understood and read by those skilled in the art, and are not used to limit the conditions under which the present invention can be implemented, so that the present invention has no technical significance, and any structural modification, ratio relationship change, or size adjustment should still fall within the scope of the present invention without affecting the efficacy and the achievable purpose of the present invention. In addition, the terms "upper", "lower", "left", "right", "middle" and "one" used in the present specification are for clarity of description, and are not intended to limit the scope of the present invention, and the relative relationship between the terms and the terms is not to be construed as a scope of the present invention.
It should be understood that the processing equipment or devices not specifically mentioned in the following examples are conventional in the art; all pressure values and ranges refer to relative pressures.
Furthermore, it is to be understood that one or more method steps mentioned in the present invention does not exclude that other method steps may also be present before or after the combined steps or that other method steps may also be inserted between these explicitly mentioned steps, unless otherwise indicated; it is also to be understood that a combined connection between one or more devices/apparatus as referred to in the present application does not exclude that further devices/apparatus may be present before or after the combined device/apparatus or that further devices/apparatus may be interposed between two devices/apparatus explicitly referred to, unless otherwise indicated. Moreover, unless otherwise indicated, the numbering of the various method steps is merely a convenient tool for identifying the various method steps, and is not intended to limit the order in which the method steps are arranged or the scope of the invention in which the invention may be practiced, and changes or modifications in the relative relationship may be made without substantially changing the technical content.
The raw materials containing elements such as carbon, nickel, manganese, copper, molybdenum, sulfur, phosphorus, nitrogen, oxygen, calcium, iron, etc. used in the following examples are commercially available, and facilities for carrying out processes such as smelting, slab casting, hot rolling, tempering, isothermal treatment, water quenching, etc. are also commercially available.
Example 1
Taking the components containing various elements according to the proportion, mixing, smelting, and then continuously casting a plate blank into an ingot, wherein the components comprise the following elements in percentage by mass: c: 0.008-0.012%; ni: 5.0 to 6.0 percent; mn: 2.0-4.0%; cu: 1.5 to 2.0 percent; mo: 0.4-0.6%; s: less than or equal to 0.004 percent; p: less than or equal to 0.003 percent; n: 0.003-0.005%; o: 0.0005-0.001%; ca: 0.0005-0.005%; the balance being Fe. The cast ingot is derusted, deoiled and cleaned, and the phenomenon of uneven stress in the hot rolling treatment process is avoided.
And carrying out hot rolling treatment on the cast ingot, carrying out multi-step hot rolling from the initial rolling temperature of 1200 ℃ to the final rolling temperature of 750 ℃, and then cooling in air. The multi-step hot rolling comprises the following steps: respectively continuously at 1200, 900 and 750 ℃, the pressing rates are respectively 25%, 25% and 25%, and the heat preservation time is respectively 120, 70 and 30 minutes.
The steel ingot after hot rolling treatment is processed at A1Keeping the temperature below 50-150 ℃ for 0.5-1.0h for tempering, and then cooling to room temperature by water for water quenching. Then A1Performing isothermal heat preservation in a first two-phase region for 0.5-1.0h at 50-80 ℃, and then performing water quenching after water cooling to room temperature. And finally, carrying out isothermal heat preservation in a second two-phase region for 0.5-1.5h, wherein the isothermal temperature in the second two-phase region is 50-150 ℃ lower than that in the first two-phase region, and then carrying out water quenching after water cooling to room temperature to obtain a steel sample for low temperature No. 1. A above1The temperature is 620 ℃ and 670 ℃. The room temperature is 20-25 ℃.
Example 2
Taking the components containing various elements according to the proportion, mixing, smelting, and then continuously casting a plate blank into an ingot, wherein the components comprise the following elements in percentage by mass: c: 0.008 percent; ni: 5.0 percent; mn: 3.0 percent; cu: 1.5 percent; mo: 0.4 percent; s: 0.004%; p: 0.003%; n: 0.003%; o: 0.0005%; ca: 0.0005%; the balance being Fe. The cast ingot is derusted, deoiled and cleaned, and the phenomenon of uneven stress in the hot rolling treatment process is avoided.
And carrying out hot rolling treatment on the cast ingot, carrying out multi-step hot rolling from the initial rolling temperature of 1200 ℃ to the final rolling temperature of 750 ℃, and then cooling in air. The multi-step hot rolling comprises the following steps: the pressing rates are respectively 30%, 25% and 25% at 1150, 850 and 770 ℃ continuously, and the holding time is respectively 115 minutes, 65 minutes and 25 minutes.
The steel ingot after hot rolling treatment is processed at A1Keeping the temperature below 100 ℃ for 1.0h for tempering, and then cooling to room temperature by water for water quenching. Then A1Carrying out isothermal heat preservation in a first two-phase region for 0.5h at the temperature of 80 ℃ above, and then carrying out water quenching after water cooling to room temperature. And finally, carrying out isothermal heat preservation for 1.0h in a second two-phase region, wherein the isothermal temperature of the second two-phase region is 120 ℃ lower than that of the first two-phase region, and then carrying out water quenching by cooling to room temperature to obtain a steel sample 2# for low temperature. A above1The temperature was 620 ℃. The room temperature is 20-25 deg.C。
Example 3
Taking the components containing various elements according to the proportion, mixing, smelting, and then continuously casting a plate blank into an ingot, wherein the components comprise the following elements in percentage by mass: c: 0.008 percent; ni: 5.5 percent; mn: 3.0 percent; cu: 1.5 percent; mo: 0.4 percent; s: 0.004%; p: 0.003%; n: 0.003%; o: 0.0005%; ca: 0.0005%; the balance being Fe. The cast ingot is derusted, deoiled and cleaned, and the phenomenon of uneven stress in the hot rolling treatment process is avoided.
And carrying out hot rolling treatment on the cast ingot, carrying out multi-step hot rolling from the initial rolling temperature of 1200 ℃ to the final rolling temperature of 750 ℃, and then cooling in air. The multi-step hot rolling comprises the following steps: the pressing rates are respectively 25%, 25% and 30% under 1180, 880 and 760 ℃, and the holding time is respectively 125, 65 and 30 minutes.
The steel ingot after hot rolling treatment is processed at A1Keeping the temperature below 150 ℃ for 1.0h for tempering, and then cooling to room temperature by water for water quenching. Then A1Carrying out isothermal heat preservation in a first two-phase region for 0.5h at the temperature of 80 ℃ above, and then carrying out water quenching after water cooling to room temperature. And finally, carrying out isothermal heat preservation for 1.0h in a second two-phase region, wherein the isothermal temperature of the second two-phase region is 120 ℃ lower than that of the first two-phase region, and then carrying out water quenching by cooling to room temperature to obtain a steel sample 3# for low temperature. A above1The temperature was 620 ℃. The room temperature is 20-25 ℃.
Comparative example 1
Taking the components containing various elements according to the proportion, mixing, smelting, and then continuously casting a plate blank into an ingot, wherein the components comprise the following elements in percentage by mass: c: 0.008-0.012%; ni: 5.0 to 6.0 percent; mn: 2.0-4.0%; cu: 1.5 to 2.0 percent; mo: 0.4-0.6%; s: less than or equal to 0.004 percent; p: less than or equal to 0.003 percent; n: 0.003-0.005%; o: 0.0005-0.001%; ca: 0.0005-0.005%; the balance being Fe. The cast ingot is derusted, deoiled and cleaned, and the phenomenon of uneven stress in the hot rolling treatment process is avoided.
And carrying out hot rolling treatment on the cast ingot, carrying out multi-step hot rolling from the initial rolling temperature of 1200 ℃ to the final rolling temperature of 750 ℃, and then cooling in air. The multi-step hot rolling comprises the following steps: respectively continuously at 1200, 900 and 750 ℃, the pressing rates are respectively 25%, 25% and 25%, and the heat preservation time is respectively 120, 60 and 30 minutes.
The steel ingot after hot rolling treatment is processed at A1Performing isothermal heat preservation in a first two-phase region for 0.5-1.0h at 50-80 ℃, and then performing water quenching after water cooling to room temperature. And finally, carrying out isothermal heat preservation of the second two-phase region for 0.5-1.5h, wherein the isothermal temperature of the second two-phase region is 50-150 ℃ lower than that of the first two-phase region, and then carrying out water quenching after water cooling to room temperature to obtain a low-temperature steel comparison sample 1. A above1The temperature is 620 ℃ and 670 ℃. The room temperature is 20-25 ℃.
Comparative example 2
Taking the components containing various elements according to the proportion, mixing, smelting, and then continuously casting a plate blank into an ingot, wherein the components comprise the following elements in percentage by mass: c: 0.008-0.012%; ni: 5.0 to 6.0 percent; mn: 2.0-4.0%; cu: 1.5 to 2.0 percent; mo: 0.4-0.6%; s: less than or equal to 0.004 percent; p: less than or equal to 0.003 percent; n: 0.003-0.005%; o: 0.0005-0.001%; ca: 0.0005-0.005%; the balance being Fe. The cast ingot is derusted, deoiled and cleaned, and the phenomenon of uneven stress in the hot rolling treatment process is avoided.
And carrying out hot rolling treatment on the cast ingot, carrying out multi-step hot rolling from the initial rolling temperature of 1200 ℃ to the final rolling temperature of 750 ℃, and then cooling in air. The multi-step hot rolling comprises the following steps: respectively continuously at 1200, 900 and 750 ℃, the pressing rates are respectively 25%, 25% and 25%, and the heat preservation time is respectively 120, 60 and 30 minutes.
The steel ingot after hot rolling treatment is processed at A1And (3) carrying out isothermal heat preservation for 0.5-1.0h in a first two-phase region at the temperature of 50-80 ℃, then carrying out water cooling to room temperature for water quenching, and obtaining a low-temperature steel comparison sample 2. A above1The temperature is 620 ℃ and 670 ℃. The room temperature is 20-25 ℃.
Test example 1
The cold and cold drawing tests were carried out on the steel for low temperature sample # 1 obtained in example 1, the steel for low temperature comparative sample # 1 obtained in comparative example 1, and the steel for low temperature comparative sample # 2 obtained in comparative example 2, respectively, and the specific results are shown in table 1. As can be seen from Table 1, the ultra-low temperature toughness of the low temperature steel after the tempering-double partitioning treatment is compared with that of the low temperature steel (1# and 1 x comparison) which is not temperedFurther improved, can prepare strong plasticity with yield strength of 1150-1400MPa, tensile strength of 1200-1450MPa and elongation of 20-30% at-196 ℃, and the V-notch impact toughness of 220-260J/cm at-196 DEG2High strength and toughness nickel-saving low temperature steel. The strength and impact toughness of the 2 x samples were lower compared to the low temperature steel without repartition (1# vs. 2), indicating the importance of repartition for toughness improvement. Comparing the 1 and 2 samples, the 1 sample, although not subjected to the first tempering treatment, had better toughness due to the redistribution treatment, further illustrating the importance of the redistribution treatment in this process.
TABLE 1 comparison table of mechanical properties before and after tempering and double partitioning of low-temperature steel
Note: wherein √ denotes treated,/denotes untreated
Test example 2
The steel sample # 1 for low temperature use obtained in example 1 was compared with the steel comparative sample # 1 for low temperature use obtained in comparative example 1, and the two samples were subjected to X-ray diffraction experiments, respectively, to measure the residual austenite content, and the specific results are shown in table 2. And performing tissue characterization on the two samples by adopting electron back scattering diffraction, wherein specific results are shown in figures 1a and 1 b. As can be seen from Table 2, the residual austenite content of the low temperature steel after the tempering-double partitioning process was maintained at 3.9-5.6%, and the residual austenite content of the low temperature steel without the tempering process was 12.5-14.7%.
From the viewpoint of mechanical properties, the impact toughness of the 1 × sample without tempering treatment is equivalent to that of the 1# sample after tempering treatment, but the strength of the 1# sample is higher because the austenite content of the two samples is different, the austenite in the 1 × sample bears partial flow stress during deformation, so that compared with the 1# sample, yield effect occurs earlier, the TRIP effect induced by the austenite in the 1 × sample during transformation is beneficial to improving plasticity, and the austenite in the 1# sample after tempering-double partitioning treatment has higher mechanical stability, so that the toughness is not lost at low temperature.
TABLE 2 comparison table of the residual austenite content before and after tempering and double partitioning of low-temperature steel
| Sample (I) | Tempering | Dispensing of | Redistributing | Austenite content (%) |
| 1# | √ | √ | √ | 3.9-5.6 |
| 1* | / | √ | √ | 12.5-14.7 |
Note: wherein √ denotes treated,/denotes untreated
As shown in fig. 1a and 1b, both samples are dual-phase structures of tempered martensite and trace retained austenite, and the structure treated by the tempering-double partitioning process hardly sees retained austenite, because the step size of electron back scattering diffraction is limited (greater than or equal to 30nm), a small amount of nano-sized retained austenite in 1# cannot be detected, while the sample 1 without tempering can see some retained austenite distributed in the matrix due to the large volume fraction and relatively large size of austenite.
Test example 3
The oscillometric shock analysis was performed on the steel for low temperature sample # 1 obtained in example 1 and the steel for low temperature comparative sample # 1 obtained in comparative example 1, and the specific results are shown in fig. 2a and 2 b. As can be seen from fig. 2a and 2b, after the tempering-double partitioning process, although the strength was improved compared to the samples without tempering, the toughness was not lost because the retained austenite size was smaller and the thermal and mechanical stability was higher in the sample # 1. Comparing 1 and 2 samples, the final redistribution treatment further diffuses the solid solution atoms in 1 sample into the residual austenite, and the stability of the austenite is further improved, so that the impact toughness is higher compared with that of 2 samples.
In conclusion, the nickel-saving type ultra-low temperature high-strength steel and the heat treatment process thereof provided by the invention can obtain the nickel-saving type ultra-low temperature steel with low temperature toughness, low application cost, no need of cold deformation treatment, simple heat treatment process and strong economic applicability. Therefore, the invention effectively overcomes various defects in the prior art and has high industrial utilization value.
The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.
Claims (10)
1. The low-temperature steel consists of the following elements in percentage by mass:
c: 0.005-0.015%; ni: 3.5 to 6.5 percent; mn: 2.0 to 5.0 percent; cu: 0.5-2.5%; mo: 0.2 to 0.8 percent; s: less than or equal to 0.004 percent; p: less than or equal to 0.003 percent; n: 0.003-0.005%; o: 0.0005-0.001%; ca: 0.0005-0.005%; the balance being Fe.
2. The steel for low temperature according to claim 1, wherein the steel for low temperature comprises the following elements in percentage by mass:
c: 0.008-0.012%; ni: 4.0 to 6.0 percent; mn: 2.0-4.0%; cu: 1.0-2.0%; mo: 0.3-0.7%; s: less than or equal to 0.004 percent; p: less than or equal to 0.003 percent; n: 0.003-0.005%; o: 0.0005-0.001%; ca: 0.0005-0.005%; the balance being Fe.
3. The heat treatment process of a low-temperature steel according to any one of claims 1 to 2, wherein the low-temperature steel is provided by mixing and smelting the element components according to the proportion, continuously casting the mixture into an ingot by using a slab, performing hot rolling treatment, tempering and water quenching the obtained steel ingot, and sequentially performing first two-phase isothermal zone water quenching and second two-phase isothermal zone water quenching.
4. The heat treatment process of the low-temperature steel as claimed in claim 3, wherein the hot rolling treatment comprises the steps of performing multi-step hot rolling on the ingot from the initial rolling temperature to the final rolling temperature and then cooling the ingot by air; the initial rolling temperature is 1150-1250 ℃; the finishing temperature is 700-800 ℃.
5. The heat treatment process of a steel for low temperature use according to claim 4, wherein the multi-step hot rolling comprises the steps of:
the first step is as follows: hot rolling temperature: 1150-: 115 ℃ for 125 minutes;
the second step is that: hot rolling temperature: 880-: 65-75 minutes;
the third step: hot rolling temperature: 700 ℃ and 800 ℃, and the heat preservation time is as follows: 20-30 minutes.
6. The heat treatment process for a steel for low temperature use according to claim 3, wherein the tempering temperature is A1Temperature to A150-15 deg.C belowIn the 0 ℃ interval.
7. The heat treatment process for a steel for low temperature according to claim 3, wherein the tempering time is 0.5 to 1.5 hours.
8. The heat treatment process for low-temperature steel as claimed in claim 3, wherein the isothermal temperature of the first two-phase zone is A1Temperature to A1The temperature is 50-100 ℃ above.
9. The heat treatment process for a low-temperature steel as claimed in claim 3, wherein the isothermal temperature of the first two-phase isothermal zone is 50-150 ℃ higher than the isothermal temperature of the second two-phase isothermal zone.
10. The heat treatment process of low-temperature steel as claimed in claim 3, wherein the isothermal time of the first isothermal zone and the second isothermal zone is 0.5-1.5 h.
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