Method for preparing mn-b non-quenched and tempered steel

Abstract

The present invention belongs to the technical field of the production of special steels. Disclosed is a method for preparing Mn-B non-quenched and tempered steel. The chemical composition of the Mn-B non-quenched and tempered steel is [C], [Si], [Mn], [Cr], [Ti], [Al], [P], [S], [B], and [N], and the preparation process therefor involves treatment in a primary steelmaking furnace, treatment in an LF refining furnace, treatment in a vacuum furnace, and continuous casting. In the present invention, by means of the control of tapping conditions, a wire feeding process after vacuum treatment, and the endpoint composition, the cleanliness and material performance of the Mn-B non-quenched and tempered steel are improved, and material performance requirements of quenched and tempered steel are met. In particular, by means of the strict control of the contents of the components Al, Ti, B, and N in molten steel, the generation of large-size Ti(C,N) inclusions during a solidification process is avoided, and the impact performance of the Mn-B non-quenched and tempered steel is improved and stabilized. In conjunction with an existing special steel smelting device and by means of the strict control of the composition and the optimization of the feeding sequence during a smelting process, the present invention can achieve the purpose of improving and stabilizing the material performance of the Mn-B non-quenched and tempered steel without adding additional devices and working procedures.

Description

A preparation method of Mn-B series non-adjustable steel Technical Field

[0001] The invention belongs to the technical field of special steel production and relates to a preparation method of Mn-B series non-adjustable steel. Background Art

[0002] Non-quenched and tempered steel refers to high-quality steel that achieves or approaches the properties of quenched and tempered steel through the use of microalloying, controlled rolling, and cooling methods during manufacturing and application. Microalloying elements dissolve in the austenite during heating, and nanometer-scale carbides, nitrides, or carbonitrides precipitate during cooling, strengthening the steel. The main features of non-quenched and tempered steel are energy saving, omission of heat treatment, short production cycle, and low cost. Microalloying elements in non-quenched and tempered steel include Cr, Mo, Ti, and B. Depending on their presence in the steel, their properties will vary. Mn-B-based non-quenched and tempered steels are widely used due to their relatively low alloying element prices.

[0003] Boron in non-tempered steel primarily increases the extent of the bainite transformation zone, resulting in exceptional strength and toughness under rolling and slow cooling conditions. Boron also enhances hardenability. During part processing, induction hardening can increase the depth of hardening, resulting in parts with a hard surface and a soft core, making the material more suitable for use.

[0004] Titanium in non-tempered steel is a strong nitride, carbide, or carbonitride-forming element. The lower the titanium and nitrogen content in the molten steel, the lower the temperature at which titanium-containing nitrides form, and the smaller their size. This not only provides secondary strengthening but also helps stabilize grain boundaries. However, if the titanium and nitrogen content is too low, the number density of nanoscale titanium-containing nitrides formed in the steel matrix will be significantly reduced, thus failing to achieve the desired material properties. However, titanium also readily combines with oxygen in the molten steel to form corresponding Ti-O compounds. These compounds not only reduce the beneficial effects of the titanium content but also reduce the cleanliness of the molten steel. In severe cases, they can even clog the nozzles, making casting difficult. Therefore, titanium-containing non-tempered steels generally utilize an Al deoxidation process. Controlling the Al content not only deoxidizes and improves the cleanliness of the molten steel, but also generates nanoscale AlN particles during the post-rolling slow cooling process, which also contribute to strengthening.

[0005] Controlling the nitrogen content in non-tuned steel is also crucial. Excessive nitrogen content will react with the boron in the non-tuned steel to form boron nitride (BN). This, on the one hand, reduces the dissolved boron content, thereby reducing the material's hardenability. Furthermore, this nitrogen consumption reduces the precipitation of nano-Ti(C,N), weakening the non-tuned steel's performance. Furthermore, given that hydrogen in molten steel can easily cause white spots, significantly reducing the material's performance and service life, high-end non-tuned steels are typically vacuum-treated, where nitrogen content is further reduced.

[0006] Based on the above analysis, it can be seen that the proper control of elements such as Al, Ti, B, and N in Mn-B non-tempered steels is crucial to their material properties. Improper control can easily lead to the formation of micron-scale Ti(C,N). These inclusions, on the one hand, reduce the precipitation of nano-scale Ti(C,N), thereby reducing the strengthening of the non-tempered steel; on the other hand, these inclusions can act as crack sources, causing the non-tempered steel's material properties to fall short of those of quenched and tempered steel. Therefore, developing a dedicated preparation method for Mn-B non-tempered steels is extremely important.

[0007] Summary of the Invention

[0008] In view of the above problems, the present invention aims to solve the defects in the prior art and proposes a method for preparing Mn-B series non-quenched and tempered steel, the cleanliness and material properties of the molten steel meet the requirements of replacing quenched and tempered steel.

[0009] The technical solution of the present invention is: a method for preparing a Mn-B series non-tempered steel according to the present invention, wherein the preparation process is as follows: primary refining furnace → LF refining furnace → vacuum refining furnace → continuous casting. By controlling the steel tapping conditions of the primary refining furnace, the wire feeding process after vacuum treatment, and the end composition, the cleanliness and material properties of the Mn-B series non-tempered steel are improved, and the material performance requirements for quenched and tempered steel are met, with the tensile strength of the rolled material being ≥750MPa, the cross-sectional shrinkage being ≥40%, the elongation being ≥12%, and the impact absorption energy being ≥50J.

[0010] Furthermore, the mass fractions of the elements in the non-adjustable steel are as follows: 0.19%≤ω[C]≤0.45%, 0.05%≤ω[Si]≤0.30%, 0.76%≤ω[Mn]≤1.50%, 0.08%≤ω[Cr]≤0.50%, 0.02%≤ω[Ti]≤0.035%, 0.008%≤ω[Al]≤0.05%, ω[P]≤0.020%, ω[S]≤0.015%, 0.001%≤ω[B]≤0.003%, and ω[N]≤0.01%.

[0011] Furthermore, the Ti / N mass fraction ratio is between 2-5.

[0012] Furthermore, the primary smelting furnace includes an electric furnace and a converter, the tapping conditions are: 0.08% ≤ ω[C], ω[P] ≤ 0.015%, temperature ≥ 1600°C, and the charging order during the tapping process is: deoxidizer → alloy → slag-forming agent, the deoxidizer is 1.0-1.5 kg / t, the slag-forming agent includes lime and pre-melted slag, the lime is 4-7 kg / t, and the pre-melted slag is 3-6 kg / t.

[0013] Furthermore, the vacuum furnace includes a VD vacuum furnace and an RH vacuum furnace, with an ultimate vacuum degree ≤ 80 Pa and an ultimate vacuum time ≥ 5 minutes.

[0014] Furthermore, after the vacuum furnace is broken, the Al content is adjusted first, the Ti content is adjusted after 2-5 minutes, the B content is adjusted after 2-5 minutes, and 0.5-1.5 m / ton Ca wire is fed after 2-5 minutes. The best way to adjust the element content is to feed the wire.

[0015] Furthermore, after rolling, the number density of micron-level Ti(C,N) in the rolled material is ≤10 pieces / mm2, and the number density of micron-level Ti(C,N) ≥5μm accounts for ≤5%.

[0016] The mechanism of the present invention is as follows:

[0017] First, through strict control of tapping conditions, charging sequence, deoxidizers, and slag systems, the generation of inclusions is reduced at the source, while the absorption of inclusions is accelerated, thereby improving the cleanliness of the molten steel.

[0018] Subsequently, through strict control of Al, Ti, B, and N elements, the formation of Ti-O oxides, BN nitrides, and large-sized Ti(C,N) was prevented, effectively inhibiting the harm of such inclusions to Mn-B non-tuning steels, thereby achieving the purpose of improving the performance of Mn-B non-tuning steel materials. Through the synergistic effect of the above processes, a preparation method for Mn-B non-tuning steel was obtained.

[0019] The beneficial effects of the present invention are as follows: the characteristics of the present invention are as follows: the present invention combines existing special steel smelting equipment, strictly controls the composition, and optimizes the charging sequence during the smelting process, so as to achieve the purpose of improving and stabilizing the performance of Mn-B series non-adjustable steel materials without adding additional equipment and processes; this method is simple, novel, and has wide applicability, and has an excellent guiding role in further improving product quality and developing new steel grades. DETAILED DESCRIPTION

[0020] The specific technical solutions of the present invention are further described in detail below with reference to specific examples.

[0021] The present invention discloses a method for preparing a Mn-B series non-tempered steel, comprising the following steps: a primary refining furnace → an LF refining furnace → a vacuum furnace → continuous casting. By controlling the steel tapping conditions of the primary refining furnace, the wire feeding process after vacuum treatment, and the control of the end-point composition, the cleanliness and material properties of the Mn-B series non-tempered steel are improved, thereby meeting the material performance requirements for quenched and tempered steel, with the rolled product exhibiting a tensile strength of ≥750 MPa, a cross-sectional reduction of ≥40%, an elongation of ≥12%, and an impact absorption energy of ≥50 J.

[0022] Furthermore, the mass fractions of the elements in the non-adjusted steel are as follows: 0.19%≤ω[C]≤0.45%, 0.05%≤ω[Si]≤0.30%, 0.76%≤ω[Mn]≤1.50%, 0.08%≤ω[Cr]≤0.50%, 0.02%≤ω[Ti]≤0.035%, 0.008%≤ω[Al]≤0.05%, ω[P]≤0.020%, ω[S]≤0.015%, 0.001%≤ω[B]≤0.003%, ω[N]≤0.01%;

[0023] Among them, Al element mainly plays the role of deoxidation. The strict requirement of Al content is also to inhibit the formation of Ti-O in the molten steel and prevent the ineffective loss of Ti element.

[0024] The Ti and N elements are mainly used to form nano-scale Ti(C, N) during the cooling process to play a secondary strengthening role, and the B element is mainly used to increase the hardenability of the material.

[0025] Furthermore, the Ti / N mass fraction ratio is between 2 and 5. Strict requirements are imposed on the Ti / N ratio in order to maximize the effect of the B element in promoting hardenability, and on the other hand to inhibit the formation of large-sized micron-level Ti(C, N) during the solidification process, thereby preventing large-sized micron-level Ti(C, N) from deteriorating the material properties of non-quenched and tempered steel and preventing it from achieving the purpose of replacing quenched and tempered steel.

[0026] Furthermore, the primary melting furnace includes an electric furnace and a converter, the tapping conditions are: 0.08% ≤ ω[C], ω[P] ≤ 0.015%, and the temperature is ≥ 1600°C. The charging order during the tapping process is: deoxidizer → alloy → slag-forming agent, the deoxidizer is 1.0-1.5 kg / t, the slag-forming agent includes lime and pre-melted slag, the lime is 4-7 kg / t, and the pre-melted slag is 3-6 kg / t;

[0027] The temperature requirement is mainly to accelerate the melting of the alloy and the formation of refined slag; the control of the feeding sequence and the amount of deoxidizer added is mainly to reduce the formation of deoxidation products and improve the cleanliness of the molten steel.

[0028] Furthermore, the vacuum furnace includes a VD vacuum furnace and an RH vacuum furnace, with an ultimate vacuum degree ≤ 80 Pa and an ultimate vacuum time ≥ 5 minutes;

[0029] The vacuum limit pressure and time are set to better remove the H element in the molten steel, prevent the occurrence of white spot defects in hot-rolled materials, and thus reduce the material properties of non-adjusted steel.

[0030] Furthermore, after the vacuum furnace is broken, the Al wire is first fed, and after 2-5 minutes the Ti-containing wire is fed, followed by the B-containing alloy, and after 2-5 minutes the Ca-containing wire is fed;

[0031] The strict regulations on the adjustment order of Al, Ti, B and Ca elements are mainly to maximize the role of each element. Compared with Ti element, Al element and O element have stronger binding force. Al element is added first to reduce the O element in the molten steel and avoid the formation of Ti-O oxide when Ti element is added later; compared with B element, Ti and N element have stronger binding force, so Ti element is added first, mainly to control the N element in the molten steel and prevent the formation of BN; the addition of Ca element is mainly to completely transform the inclusions in the molten steel into liquid inclusions, prevent nozzle blockage, and ensure smooth casting.

[0032] Furthermore, after rolling, the number density of micron-level Ti(C,N) in the rolled material is ≤10 pieces / mm2, and the number density of micron-level Ti(C,N) ≥5μm accounts for ≤5%.

[0033] Taking 15B36Cr bar as an example, the smelting and processing methods of the four embodiments are basically the same. There are certain differences in the control of Ti and N content, and the smelting methods are described uniformly.

[0034] The tapping conditions for 15B36Cr are: 0.08% ≤ ω[C], ω[P] ≤ 0.015%, and a temperature ≥ 1600°C. The order of adding materials during tapping is: deoxidizer → alloy → slag-forming agent. The deoxidizer is 1.0-1.5 kg / t. The slag-forming agents include lime and pre-melted slag, with lime 4-7 kg / t and pre-melted slag 3-6 kg / t.

[0035] Vacuum furnaces include VD vacuum furnaces and RH vacuum furnaces, with ultimate vacuum degree ≤80Pa and ultimate vacuum time ≥5 minutes;

[0036] After the vacuum furnace is broken, Al wire is fed first, followed by Ti-containing wire after 2-5 minutes, followed by B-containing alloy, and then Ca-containing wire after 2-5 minutes.

[0037] The chemical compositions of the four examples are shown in Table 1. The Ti content and Ti / N ratio of Examples 1 and 2 do not meet the requirements of the present invention, while the Ti content and Ti / N ratio of Examples 3 and 4 meet the requirements of the present invention. After rolling bars of different sizes, the corresponding material properties are shown in Table 2.

[0038] The tensile strength, cross-sectional shrinkage, and elongation of Examples 1 and 2 meet the requirements of the present invention, but the room-temperature impact absorption energy fluctuates greatly, ranging only from 12 to 22 J, which cannot meet the impact absorption energy requirements for quenched and tempered steel.

[0039] Subsequently, by optimizing the Ti line and vacuum time after breaking the air, the Ti content and Ti / N meet the requirements of the present invention, and the room temperature impact absorption energy stably reaches more than 60J.

[0040] Further electron microscopy observation was performed on the samples that failed the room temperature impact absorption energy test. The fracture surface was a typical dissociation fracture. Combining the crack propagation pattern, the crack initiation source was deduced. Large-sized Ti (C, N) at the micron level was found at the initiation source. The number density of micron-sized Ti (C, N) was further counted, and the number density reached 16.8 / mm. 2 , large-size Ti(C,N) above 5μm accounts for more than 20%;

[0041] After composition optimization, no large-sized Ti(C, N) was found in the impact fractures of Examples 3 and 4. Analysis using an automatic inclusion scanning system revealed that the number density of micron-level Ti(C, N) was only 6.8 per mm. 2 .

[0042] Table 1 Chemical composition of Mn-B series non-adjustable steel (%)

[0043] Table 2 Material properties of Mn-B series non-adjustable steel

[0044] The present invention combines existing special steel smelting equipment, strictly controls the composition, and optimizes the charging sequence during the smelting process to achieve the purpose of improving and stabilizing the performance of Mn-B series non-adjustable steel materials without adding additional equipment and processes. This method is simple, novel, and has wide applicability, and has an excellent guiding role in further improving product quality and developing new steel grades.

Claims

1. A Mn-B series non-adjustable steel, characterized in that: The chemical composition of the Mn-B series non-adjustable steel is calculated as follows by mass percentage: 0.19%≤ω[C]≤0.45%, 0.05%≤ω[Si]≤0.30%, 0.76%≤ω[Mn]≤1.50%, 0.08%≤ω[Cr]≤0.50%, 0.02%≤ω[Ti]≤0.035%, 0.008%≤ω[Al]≤0.05%, ω[P]≤0.020%, ω[S]≤0.015%, 0.001%≤ω[B]≤0.003%, ω[N]≤0.01%.

2. The Mn-B non-adjustable steel according to claim 1, characterized in that: The preparation steps are as follows: the Ti / N mass fraction ratio is between 2-5.

3. A method for preparing a Mn-B series non-adjustable steel as claimed in any one of claims 1 to 2, characterized in that: The preparation steps are as follows: primary refining furnace → LF refining furnace → vacuum refining furnace → continuous casting and rolling; By controlling the steel-making conditions of the primary furnace, the wire feeding process after vacuum treatment, and the final composition, the cleanliness and material properties of the Mn-B series non-quenched and tempered steel are improved, and the material performance requirements for quenched and tempered steel are achieved.

4. The method for preparing a Mn-B series non-adjustable steel according to claim 3, characterized in that: The tensile strength of the rolled material is ≥750MPa, the cross-sectional shrinkage is ≥40%, the elongation is ≥12%, and the impact absorption energy is ≥50J.

5. The method for preparing a Mn-B series non-adjustable steel according to claim 3, characterized in that: The specific process of the primary smelting furnace is: the primary smelting furnace includes an electric furnace and a converter, and the steel-making conditions are: 0.08%≤ω[C], ω[P]≤0.015%, and temperature≥1600°C.

6. The method for preparing a Mn-B series non-adjustable steel according to claim 5, characterized in that: The order of adding materials in the steel-making process is: deoxidizer → alloy → slag-forming agent, deoxidizer 1.0-1.5kg / t.

7. The method for preparing a Mn-B series non-adjustable steel according to claim 5, characterized in that: The slag-forming agent comprises lime and pre-melted slag, with the lime content being 4-7 kg / t and the pre-melted slag content being 3-6 kg / t.

8. The method for preparing a Mn-B series non-adjustable steel according to claim 3, characterized in that: The vacuum refining furnace includes a VD vacuum furnace and an RH vacuum furnace, with a limit vacuum degree of ≤80Pa and a limit vacuum time of ≥5 minutes.

9. The method for preparing a Mn-B series non-adjustable steel according to claim 8, characterized in that: After the vacuum furnace is broken, the Al content is adjusted first, the Ti content is adjusted after 2-5 minutes, the B content is adjusted after 2-5 minutes, and 0.5-1.5 m / ton Ca wire is fed in after 2-5 minutes.

10. The method for preparing a Mn-B series non-adjustable steel according to claim 3, characterized in that: After continuous casting and rolling, the number density of micron-level Ti in the rolled material is ≤10 / mm2, and the number density of micron-level Ti ≥5μm accounts for ≤5%.

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