A Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel and its manufacturing method

Abstract

This application discloses a Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel and its manufacturing method, relating to the field of metallic materials technology. The chemical composition of the steel is: C: 0.10~0.16%, Si: 0.20~0.40%, Mn: 1.40~1.70%, Cr: 0.25~0.60%, Ti: 0.014~0.025%, P≤0.015%, S≤0.005%, N: 0.0025~0.0045%. This application does not add precious alloys such as Mo, Nb, and V. By adding appropriate amounts of Cr and Ti and precisely controlling the N content, the following conditions are met: 0.008≥(Ti-3.42N)(Cr+Mn / 4)≥0.005, N / (Ti / 3.42)≤0.78. A short-process manufacturing technique is used to prepare a structural steel with ultra-high strength, excellent low-temperature toughness, and good economic efficiency.

Description

Technical Field

[0001] This application relates to the field of metallic materials technology, specifically to a Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel and its manufacturing method. Background Technology

[0002] The production of low-alloy high-strength structural steel currently faces enormous cost pressures. Eliminating expensive microalloying elements such as Mo, Nb, and V is the most direct way to reduce costs. However, this places extremely high demands on the precise control of rolling and cooling processes. The development of economical high-strength steel has long faced a triangular contradiction between strength, toughness, and cost. Adding Cr alone can improve hardenability, but its improvement on toughness is limited. Adding Ti alone can achieve strengthening and toughening through the precipitation of fine TiC or TiN, but its precipitation behavior is extremely sensitive to the process and lacks stability. Theoretically, combining Cr with the Ti-N system can produce a multiple composite effect of "hardenability strengthening + precipitation strengthening + grain refinement strengthening." However, existing technologies mostly focus on the simple addition of single elements, lacking a composition design criterion that can quantify and maximize this composite effect. How to precisely control the content ratio of Cr, Ti, and N to produce a "1+1+1>3" effect during subsequent rolling and cooling processes is the key to overcoming the current technological bottlenecks. Summary of the Invention

[0003] In view of this, this application provides a Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel. By adding chromium (Cr) and titanium (Ti) and strictly controlling the nitrogen (N) content, and establishing a composite relationship formula between their contents, a structural steel plate with ultra-high strength, excellent low-temperature toughness and good economy can be produced through a short process.

[0004] Another objective of this application is to provide a method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel.

[0005] The specific technical solution of this application is as follows: A Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel, wherein the chemical composition of the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel by mass percentage is: C: 0.10~0.16%, Si: 0.20~0.40%, Mn: 1.40~1.70%, Cr: 0.25~0.60%, Ti: 0.014~0.025%, P≤0.015%, S≤0.005%, N: 0.0025~0.0045%.

[0006] In some embodiments, the chemical composition of the Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel satisfies the following relationship: 0.008 ≥ (Ti - 3.42N). (Cr+Mn / 4)≥0.005; and N / (Ti / 3.42)≤0.78.

[0007] In some embodiments, the mechanical properties of the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel meet the following requirements: yield strength ≥ 550 MPa, tensile strength 690~820 MPa, yield ratio ≤ 0.80, elongation A ≥ 20%, and Charpy V-notch impact energy ≥ 100 J at -40℃.

[0008] The alloying elements and their mass percentage design principles for the Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel provided in this application are as follows: Carbon (C): Carbon is the most effective element for increasing the strength of steel, contributing to strength through solid solution strengthening and the formation of pearlite. However, excessively high carbon content can significantly deteriorate weldability (increasing carbon equivalent), reduce plasticity and low-temperature toughness, and increase the yield strength ratio. This invention controls the C content at 0.10~0.16%, aiming to ensure basic strength while laying the foundation for obtaining excellent low-temperature toughness and high elongation.

[0009] Si (silicon): Silicon is an essential deoxidizer in the steelmaking process and can also improve strength through solid solution strengthening. In this invention, the Si content is controlled at 0.20~0.40% to make full use of its solid solution strengthening effect while avoiding its potential negative impact on surface quality and toughness.

[0010] Manganese (Mn): Manganese is one of the key alloying elements in this invention. It enhances strength and hardness through significant solid solution strengthening and is the most important element for improving the hardenability of steel, contributing to the formation of bainitic structure during controlled cooling. Mn also refines the interlamellar spacing of pearlite, improving toughness. However, excessive Mn content increases the tendency for center segregation. This invention sets the Mn content in the range of 1.40~1.70% to ensure sufficient hardenability and matrix strength to support the final high strength and toughness indicators.

[0011] Cr (chromium): Chromium is the core microalloying element for achieving the performance objectives of this invention. Cr significantly improves the hardenability of steel, ensuring a bainite-dominated microstructure at a relatively economical cooling rate. It forms fine carbides with C, providing a secondary hardening effect. More importantly, Cr lowers the austenite-to-ferrite transformation temperature, contributing to the refinement of ferrite grains. This invention controls the Cr content at 0.25~0.60%, aiming to combine it with Mn to effectively regulate the phase transformation process and microstructure type without excessively increasing costs.

[0012] Ti (Ti): Titanium is the key element in this invention for achieving grain refinement and precipitation strengthening. Ti has a very strong affinity for N and C. The fine, dispersed TiN particles formed at high temperatures can effectively pin the austenite grain boundaries, inhibiting the growth of austenite grains during billet heating and rolling, thereby achieving grain refinement strengthening. This is the only way to simultaneously improve strength and toughness. Furthermore, during rolling and cooling, supersaturated Ti (“effective titanium”) precipitates as nanoscale TiC particles, producing a strong precipitation strengthening effect. The addition of Ti is crucial for achieving high strength. This invention precisely controls the Ti content to 0.014~0.025% to ensure that there is sufficient TiN to refine the original grains and sufficient “effective titanium” for precipitation strengthening.

[0013] Nitrogen (N): In this invention, nitrogen is not an impurity element, but rather a beneficial element used in proportion to Ti. Nitrogen is an essential element for the formation of TiN particles. Controlling the appropriate N content and maintaining a specific ratio with the Ti content is a prerequisite for generating fine, stable TiN particles to control the austenite grain size. However, excessive free N will deteriorate the aging properties of steel. This invention controls the N content at 0.0025~0.0045% to provide a stable and precise nitrogen source for TiN formation.

[0014] Phosphorus (P) and sulfur (S): Phosphorus and sulfur are harmful impurity elements in steel. P significantly increases the brittle-cold transition temperature of steel, impairs low-temperature toughness, and exacerbates segregation. S forms MnS inclusions, reducing the steel's transverse toughness, ductility, and resistance to lamellar tearing. This invention utilizes advanced smelting technology to strictly limit the P and S contents to ultra-low levels of P ≤ 0.015% and S ≤ 0.005%, respectively, to maximize the purity of the steel, thereby achieving superior low-temperature toughness and weldability.

[0015] In addition to the above requirements for chemical element content, the key is that the content of each element must simultaneously satisfy the following two composite relationships: 0.008 ≥ (Ti - 3.42N) (Cr+Mn / 4)≥0.005; and N / (Ti / 3.42)≤0.78. Where Ti, N, Cr, and Mn all represent their mass percentage content.

[0016] Specifically, 0.008 ≥ (Ti - 3.42) N) (Cr+Mn / 4)≥0.005: (Ti-3.42N) represents the "effective titanium" content. 3.42 is the atomic weight ratio of Ti to N, representing the remaining Ti content after deducting the Ti consumed in forming high-melting-point TiN (whose size and distribution are crucial), which can be used to form nanoscale TiC precipitates during rolling and cooling. This is the main source of precipitation strengthening. (Cr+Mn / 4) is a hardenability enhancement factor. Both Cr and Mn can significantly improve the stability of supercooled austenite, ensuring a strong and tough bainitic structure when combined with high-speed cooling processes. Adding the effect of Mn to Cr to quantify their contribution to phase transformation strengthening, extensive empirical data shows that only when 0.008≥(Ti-3.42N) is effective titanium content sufficient. N) A balance between precipitation strengthening and phase transformation strengthening mechanisms can only be achieved when (Cr+Mn / 4) ≥ 0.005. This requires sufficient "effective titanium" and adequate hardenability, and both must work in combination to reach the threshold. This avoids the situation where simply increasing the Ti or Cr content yields poor results, achieving a combined effect of properties, which is key to obtaining ultra-high strength.

[0017] N / (Ti / 3.42)≤0.78: This formula ensures a slight excess of Ti, meaning the N content in the steel is insufficient to completely fix all the Ti. The above design serves two purposes: first, to ensure sufficient "effective titanium" for TiC precipitation; and second, to ensure that the formed TiN particles are "undersaturated," making them more stable and less prone to dissolution and growth at high temperatures, thus more effectively pinning austenite grain boundaries and achieving fine-grain strengthening. This is crucial for ensuring toughness.

[0018] This application provides a method for manufacturing the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel, the process flow of which includes: converter top and bottom composite smelting → LF furnace → vacuum treatment → continuous casting → billet heating → rolling → cooling → finishing.

[0019] In some embodiments, during the vacuum treatment step, the nitrogen content is controlled to be 0.0025~0.0045%.

[0020] In some embodiments, the continuous casting step involves creating an inert gas protective atmosphere, which includes the following steps: (1) Argon gas is used to seal the connection between the ladle drain and the long water outlet; an argon gas sealing ring is used to seal the connection and prevent air from being sucked in.

[0021] (2) An alkaline covering agent is used on the liquid surface of the intermediate bag, with a thickness of ≥65mm; to prevent air intake and absorb floating impurities.

[0022] (3) Before pouring and throughout the entire casting process, inert gas is continuously introduced into the tundish through the vent hole to completely prevent the molten steel from coming into contact with air.

[0023] In some embodiments, the heating temperature in the billet heating step is 1200~1250℃. The above design allows most of the TiN particles to dissolve in solid solution, while retaining a small number of extremely fine TiN particles to pin the grain boundaries and prevent excessive growth of austenite grains.

[0024] In some embodiments, the rolling process is carried out in two stages: recrystallization zone rolling and non-recrystallization zone rolling. The initial rolling temperature in the recrystallization zone is 1100~1160℃, the final rolling temperature is ≥1020℃, and the intermediate billet thickness is 2.2~2.8 times the finished product thickness. The initial rolling temperature in the non-recrystallization zone is 890~920℃, and the final rolling temperature is 860~900℃.

[0025] In some embodiments, the recrystallization zone rolling employs a multi-pass, medium-reduction process, with a single-pass reduction of 11-28% and a total of ≥9 rolling passes. Through repeated deformation and recrystallization in multiple passes, the austenite grains are sufficiently refined, providing a uniform and fine original microstructure for subsequent non-recrystallization zone rolling and phase transformation. The non-recrystallization zone rolling employs a high-reduction rolling process, with a single-pass reduction of ≥16% and a total of 5-7 passes. At least three passes have a single-pass reduction of ≥18%, and the distortion energy accumulated from the large deformation provides a significant driving force and nucleation sites for the subsequent strain-induced precipitation of TiC.

[0026] In some embodiments, the cooling rate in the cooling step is 20~35℃ / s, and the final cooling temperature is 350~420℃. Specifically, the cooling step employs the ACC cooling process, which uses high-rate cooling. The high cooling rate, combined with the hardenability-enhancing effect of Cr, ensures the formation of a fine bainite matrix; simultaneously, the low-temperature final cooling ensures that TiC is fully dispersed and precipitated during and after the phase transformation.

[0027] Compared with existing technologies, the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel provided in this application does not require the addition of precious metal alloys such as Nb and V. It adopts a low-cost composition design system and can produce an ultra-high strength and toughness economic steel with a thickness of 10~100mm, yield strength ≥550MPa, tensile strength of 690~820MPa, yield ratio ≤0.80, elongation A ≥20%, and Charpy V-notch impact energy ≥100J at -40℃.

[0028] Compared with the prior art, this application has the following beneficial effects: (1) On the basis of C, Si and Mn, add appropriate amounts of Cr and Ti, strictly control N within a certain range, and accurately control the content ratio of Cr, Ti and N so that it produces the effect of "1+1+1>3" in the subsequent rolling and cooling process. (2) To accurately control nitrogen content and prevent nitrogen increase during continuous casting, an inert gas protective atmosphere is constructed throughout the process; (3) One-stage low-temperature rolling, while increasing the number of passes in the first stage (≥9 passes) and controlling the single-pass reduction rate below 30% (11~28%), this process design is superior to simple high-temperature high-reduction rolling. Through more cycles of "deformation-recrystallization", it can refine the austenite grains more thoroughly and uniformly, effectively avoiding the uneven deformation and local microstructure inheritance problems that may be caused by excessive single-pass reduction, and providing better initial conditions for obtaining the final uniform and fine microstructure; (4) Excellent comprehensive performance: With the precise guidance of the above composite formula and the highly matched TMCP process, the present invention achieves the best coupling of "fine grain strengthening + precipitation strengthening + phase transformation strengthening", and the final steel plate strength reaches 550MPa or even higher, while maintaining excellent low temperature toughness (-40℃KV2≥100J). (5) Significant economic benefits: Although Cr and Ti are added, their total amount is controlled to be low, far lower than the cost of traditional alloys with added Mo, Nb, V, etc., achieving economic benefits under high performance. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some implementation examples of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 This is a metallographic diagram of an ultra-high strength and toughness economical steel with Cr-Ti-N composite microalloying according to this application. Detailed Implementation

[0030] It should be noted that, in this application, the terms "comprising," "including," or any other variations are intended to cover exclusive inclusion, such that a process, method, article, or system that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or system. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or system that includes that element.

[0031] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0032] In this application, the use of terms such as "first," "second," and "third," etc., does not indicate any order. These words can be interpreted as names. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit this application.

[0033] In practical applications, depending on the steel plate production specifications and batches, there are different component contents, specific process control conditions, and corresponding mechanical property indicators within a controlled range. To better illustrate and explain this application, Tables 1 to 3 list the components, process parameters, and mechanical properties of the examples and comparative examples for comparison. Among them, Examples 1 to 9 are the steel grades involved in this application, and Comparative Examples 1 and 2 are existing steel grades.

[0034] Examples 1-9 and Comparative Examples 1-2 show a Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel. The chemical composition of the ultra-high strength and toughness economic steel by mass percentage is shown in Table 1 below.

[0035] Table 1. Chemical composition (wt%) of ultra-high strength and toughness economical steels in Examples 1-9 and Comparative Examples 1-2

[0036] The manufacturing methods for Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel described in Examples 1-9 and Comparative Examples 1-2 include the following process flow: converter top and bottom composite smelting → LF furnace → vacuum treatment → continuous casting → billet heating → rolling → cooling → finishing.

[0037] 1) Vacuum treatment: During the vacuum degassing process, the nitrogen content is controlled to 0.0025~0.0045%.

[0038] 2) Continuous casting: Construct an inert gas protective atmosphere throughout the smelting and continuous casting process to prevent nitrogen accumulation. This includes the following steps: a) Use an argon gas sealing ring at the connection between the ladle's bottom nozzle and the long nozzle to seal and prevent air intake; b) Apply an alkaline covering agent to the tundish surface with a thickness ≥65mm to isolate air intake and absorb floating inclusions; c) Before pouring and throughout the entire casting process, continuously introduce inert gas into the tundish through the vent holes to completely prevent the molten steel surface from contacting air.

[0039] 3) Billet heating: The billet heating temperature is 1200~1250℃.

[0040] 4) Rolling Process: The rolling process consists of two stages: recrystallization zone rolling and non-recrystallization zone rolling. Stage 1, Recrystallization Zone Rolling: The initial rolling temperature is 1100~1160℃. A low-temperature initial rolling process with multiple passes and a medium reduction rate is used, with a single-pass reduction rate of 11~28%, and a total of ≥9 rolling passes; the final rolling temperature is ≥1020℃, and the intermediate billet thickness is 2.2~2.8 times the finished product thickness. Stage 2, Non-Recrystallization Zone Rolling: The initial rolling temperature is 890~920℃, using a high reduction rolling process, with a single-pass reduction rate ≥16%, a total of 5~7 passes, of which at least three passes have a single-pass reduction rate ≥18%, and the final rolling temperature is 860~900℃.

[0041] 5) Cooling: High-speed cooling is achieved using the ACC cooling process, with a cooling rate of 20~35℃ / s and a final cooling temperature of 350~420℃.

[0042] The specific process parameters for the production of Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel described in Examples 1 to 9 are shown in Table 2 below. The mechanical properties of the ultra-high strength and toughness economic steel prepared by the manufacturing methods of Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel described in Examples 1 to 9 are shown in Table 3 below. Processes and parameters not mentioned shall be carried out in accordance with existing technologies.

[0043] Table 2. Process parameters of ultra-high strength and toughness economical steels in Examples 1-9 and Comparative Examples 1-2.

[0044] Table 3 Mechanical properties of ultra-high strength and toughness economical steels in Examples 1-9 and Comparative Examples 1-2

[0045] Figure 1 The microstructure of the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel produced in the embodiments of this application is shown under a scanning electron microscope: the microstructure of the ultra-high strength and toughness economic steel produced in this application is a fine ferrite + bainite structure. As can be seen from Table 3, using the composition and production process designed in the embodiments of this application, the produced 10~100nm thick steel plates have a yield strength of 562~587MPa, a tensile strength of 719~753MPa, an elongation of 21.5~25%, an impact energy at -40℃ of 148~214J, and a yield strength ratio of 0.753~0.79. In contrast, the existing steel grades in Comparative Examples 1~2 have a yield strength of 455~546MPa, a tensile strength of 536~651MPa, an elongation of 14.5~19%, an impact energy at -40℃ of 9~87J, and a yield strength ratio of 0.84~0.85.

[0046] This application relates to the field of metallic materials technology, specifically to a Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel and its manufacturing method. This involves adding chromium (Cr) and titanium (Ti) while strictly controlling the nitrogen (N) content, and establishing a composite relationship formula between their contents: 0.008 ≥ (Ti - 3.42N). (Cr+Mn / 4)≥0.005, N / (Ti / 3.42)≤0.78. Regarding the manufacturing method, this application employs a short-process rolling technology, specifically including two key stages: the first stage uses multiple passes, with a total of ≥9 rolling passes and a moderate reduction rate controlled between 11% and 28%. This stage aims to fully refine the austenite grains through moderate deformation. The second stage involves low-temperature rolling in the non-recrystallization zone, followed by rapid accelerated cooling (ACC), with a cooling rate controlled between 20 and 35°C / s, and a final cooling temperature range of 350 to 420°C. Through synergistic control of composition and process, this application achieves a yield strength ≥550MPa and a Charpy V-notch impact energy ≥100J at -40°C without the large addition of precious alloys such as Mo, Nb, and V. The resulting steel possesses ultra-high strength, excellent low-temperature toughness, and good economic efficiency, making it of significant promotional value.

[0047] Those skilled in the art will understand that the steps, measures, and solutions in the various operations, methods, and processes discussed in this application can be alternated, modified, combined, or deleted. Furthermore, other steps, measures, and solutions in the various operations, methods, and processes discussed in this application can also be alternated, modified, rearranged, decomposed, combined, or deleted. Furthermore, steps, measures, and solutions in the prior art that are similar to those disclosed in this application can also be alternated, modified, rearranged, decomposed, combined, or deleted.

[0048] The above description is only a partial embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel, characterized in that, The chemical composition of the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel, by mass percentage, is as follows: C: 0.10~0.16%, Si: 0.20~0.40%, Mn: 1.40~1.70%, Cr: 0.25~0.60%, Ti: 0.014~0.025%, P≤0.015%, S≤0.005%, N: 0.0025~0.0045%; the chemical composition of the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel satisfies the following relationship: 0.008≥(Ti-3.42N) (Cr+Mn / 4)≥0.005; and N / (Ti / 3.42)≤0.

78.

2. The Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to claim 1, characterized in that, The mechanical properties of the Cr-Ti-N composite microalloyed ultra-high strength and toughness economic steel are as follows: yield strength ≥ 550 MPa, tensile strength 690~820 MPa, yield ratio ≤ 0.80, elongation A ≥ 20%, and Charpy V-notch impact energy ≥ 100 J at -40℃.

3. The method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to any one of claims 1 to 2, characterized in that, The manufacturing process includes: converter top and bottom composite smelting → LF furnace → vacuum treatment → continuous casting → billet heating → rolling → cooling → finishing.

4. The method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to claim 3, characterized in that, In the vacuum treatment step, the nitrogen content is controlled to be 0.0025~0.0045%.

5. The method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to claim 4, characterized in that, The continuous casting step involves creating an inert gas protective atmosphere, which includes the following steps: (1) Argon gas is used to seal the connection between the ladle drain and the long water outlet; (2) An alkaline covering agent is used on the liquid surface of the tundish, with a thickness ≥65mm; (3) Before pouring and throughout the entire casting process, inert gas is continuously introduced into the tundish through the vent hole.

6. The method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to claim 3, characterized in that, In the billet heating step, the heating temperature is 1200~1250℃.

7. The method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to claim 3, characterized in that, The rolling process is carried out in two stages: rolling in the recrystallization zone and rolling in the non-recrystallization zone. The initial rolling temperature in the recrystallization zone is 1100~1160℃, the final rolling temperature is ≥1020℃, and the thickness of the intermediate billet is 2.2~2.8 times the thickness of the finished product. The initial rolling temperature for the non-recrystallization zone is 890~920℃, and the final rolling temperature is 860~900℃.

8. The method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to claim 7, characterized in that, The recrystallization zone rolling adopts a multi-pass, medium reduction process, with a single-pass reduction rate of 11-28% and a total of ≥9 rolling passes; the non-recrystallization zone rolling adopts a high reduction rolling process, with a single-pass reduction rate of ≥16% and a total of 5-7 passes.

9. The method for manufacturing Cr-Ti-N composite microalloyed ultra-high strength and toughness economical steel according to claim 3, characterized in that, In the cooling step, the cooling rate is 20~35℃ / s, and the final cooling temperature is 350~420℃.

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