A smelting and rolling method of rare earth hot-rolled round steel for forging

Abstract

The application discloses a smelting and rolling method of rare earth hot-rolled round steel for forging, which comprises two parts of smelting and rolling. The smelting comprises the following steps in sequence: hot metal pretreatment, top and bottom combined blowing of a converter, LF (Ladle Furnace) external refining, VD (Vacuum Degassing) vacuum degassing and continuous casting. The rolling comprises the following steps in sequence: acceptance of the cast blank, step beam segmented heating, high-pressure water descaling, blooming rolling and continuous rolling mill set rolling. The smelting component control is as follows: C 0.15-0.21%, Si 0.15-0.30%, Mn 0.88-1.35%, P≤0.020%, S≤0.015%, Als 0.015-0.050%; the carbon equivalent CE≤0.42%; and 30 kg of rare earth iron alloy with a content of 30% is added to each furnace in the later stage of LF refining. The application focuses on strengthening rare earth micro-alloying and control of gas, inclusions and macrostructure, and meets the demand of high-end forging steel.

Description

Technical Field

[0001] This invention belongs to the field of high-quality forging round steel production technology, and particularly relates to a smelting and rolling method for rare earth hot-rolled round steel for forging. Background Technology

[0002] High-end forged round steel often needs to meet requirements such as low segregation, high density, controllable inclusions, and excellent hot working properties. Traditional production methods have the following problems:

[0003] Insufficient purity of molten steel, with high levels of hydrogen, oxygen, and nitrogen gases, can easily lead to white spots and cracks.

[0004] Unstable control over the morphology and grade of inclusions affects forging and fatigue performance.

[0005] Improper timing and process of rare earth addition result in low recovery rate and poor modification effect;

[0006] Insufficient matching between heating and rolling makes it difficult to simultaneously address low-magnification defects and microstructure uniformity.

[0007] The connection between continuous casting and hot rolling is not reasonable, resulting in insufficient product stability.

[0008] This invention provides a complete process for hot metal pretreatment, combined blowing converter, LF refining, VD vacuum, continuous casting, walking beam heating, high-pressure descaling, and continuous rolling. It focuses on strengthening rare earth microalloying and controlling gas, inclusions, and low-magnification microstructure to meet the needs of high-end forging steel. Summary of the Invention

[0009] The purpose of this invention is to provide a smelting and rolling method for rare earth hot-rolled round steel for forging, focusing on strengthening rare earth microalloying and controlling gas, inclusions, and low-magnification microstructure to meet the needs of high-end forging steel.

[0010] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0011] This invention discloses a smelting and rolling method for rare earth hot-rolled round steel for forging, comprising two parts: smelting and rolling. The smelting process includes: hot metal pretreatment, converter top and bottom reblowing, LF ladle refining, VD vacuum degassing, and continuous casting. The rolling process includes: billet acceptance, walking beam segmented heating, high-pressure water descaling, billet rolling, and continuous rolling mill rolling. The smelting composition is controlled as follows: C 0.15~0.21%, Si 0.15~0.30%, Mn 0.88~1.35%, P≤0.020%, S≤0.015%, Als 0.015~0.050%; residual Ni≤0.40%, Cr≤0.40%, Cu≤0.20%, Mo≤0.12%, V≤0.02%, Nb≤0.02%; carbon equivalent CE≤0.42%. During the later stage of LF refining, 30 kg of rare earth ferroalloy with a content of 30% is added to each furnace.

[0012] Converter endpoint control: C≥0.06%, P≤0.010%, tapping temperature≥1550℃, final slag basicity 3.0, strict slag blocking during tapping;

[0013] VD vacuum degassing: vacuum degree ≤0.10KPa, deep vacuum time ≥13min, soft blowing time >13min; gas control [H] ≤2.0ppm, [O] ≤25ppm, [N] ≤80ppm; SiCa wire is fed after vacuum breaking;

[0014] Continuous casting superheat ≤30℃, constant casting speed; billet low magnification: center porosity ≤1.5 grade, general porosity ≤1.5 grade, ingot segregation ≤1.5 grade, no point segregation or harmful defects;

[0015] The rolling process uses Φ850mm roughing + Φ700mm×3 + Φ550mm×4 continuous rolling; roughing temperature is 1100~1150℃, and final rolling temperature is 850~930℃; descaling pressure is 18~24MPa.

[0016] Furthermore, non-metallic inclusions are evaluated according to GB / T 10561 Method A: A fine ≤ 2.0, A coarse ≤ 1.5; B, C, and D fine ≤ 1.5, coarse ≤ 1.0.

[0017] Furthermore, a deep desulfurization pretreatment of blast furnace hot metal is adopted; after pretreatment, the sulfur content of the hot metal is ≤ 0.005%.

[0018] Furthermore, the converter undergoes top and bottom combined blowing smelting:

[0019] Process mode: Single slag / double slag operation, final slag basicity 3.0.

[0020] Furthermore, the temperature inside the steel ladle is ≥ 1550℃.

[0021] Furthermore, a silicon-manganese alloy + ferromanganese pre-deoxidation method is adopted; aluminum is used for final deoxidation to ensure the stability of acid-soluble aluminum.

[0022] Furthermore, segmented heating using a walking beam is employed:

[0023] Preheating section: 800-950℃

[0024] Heating range: 1150-1220℃

[0025] Heat spreader: 1130-1180℃, hold for ≥40 minutes

[0026] Total heating time: 2.0-3.5h.

[0027] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0028] 1. Low-magnification structure:

[0029] Central looseness ≤ 1.5 grade;

[0030] Generally loose, grade ≤1.5;

[0031] Ingot segregation ≤ 1.5 grade;

[0032] No pinpoint segregation, no shrinkage cavities, no bubbles, no cracks, no peeling, and no white spots.

[0033] 2. Non-metallic inclusions (GB / T 10561 Method A):

[0034] A Fine ≤ 2.0; A Coarse ≤ 1.5;

[0035] Bfine ≤ 1.5; Bcoarse ≤ 1.0;

[0036] Cfine ≤ 1.5; Ccoarse ≤ 1.0;

[0037] Dfine ≤ 1.5; Dcoarse ≤ 1.0;

[0038] 3. Surface quality:

[0039] No cracks, scars, folds, or inclusions;

[0040] Grinding is permitted, but the cleaning width must be at least 5 times the depth, and only one such cleaning point is allowed on the same cross section.

[0041] Grinding depth ≤ 1 / 2 of dimensional tolerance;

[0042] The depth of minor defects is allowed to be ≤0.3mm. Detailed Implementation

[0043] A method for smelting and rolling rare earth hot-rolled round steel for forging includes core smelting and rolling processes, and the parameters can be directly implemented industrially.

[0044] 1. Smelting process

[0045] 1.1 Hot metal pretreatment

[0046] Deep desulfurization pretreatment of blast furnace hot metal is adopted;

[0047] After pretreatment, the sulfur content (S) in the molten iron is ≤ 0.005%.

[0048] Objective: To lay the foundation for low sulfur content in the converter and reduce the refining load;

[0049] 1.2 Converter top and bottom combined blowing smelting

[0050] Process mode: Single-slag / double-slag operation, final slag basicity 3.0;

[0051] Endpoint control:

[0052] C ≥ 0.06%;

[0053] P ≤ 0.010%;

[0054] The temperature inside the tapping ladle is ≥ 1550℃;

[0055] Deoxidation alloying:

[0056] It adopts silicon-manganese alloy + ferromanganese pre-deoxidation;

[0057] Aluminum is used for final deoxidation to ensure the stability of acid-soluble aluminum;

[0058] Steel tapping requirements: Strict slag control to prevent slag from increasing sulfur and phosphorus levels;

[0059] 1.3 LF Out-of-furnace refining

[0060] Argon stirring is carried out throughout the process, and the intensity of the soft blowing is based on the slight movement of the slag surface and the absence of exposed molten steel.

[0061] Precisely fine-tune the composition of C, Si, Mn, and Al to control the target range;

[0062] Desulfurization treatment ensures that the sulfur content (S) in the finished product is ≤ 0.015%.

[0063] Rare earth iron alloy is added during the later stages of refining:

[0064] Amount added per batch: 30kg;

[0065] Rare earth content: 30%;

[0066] Timing for addition: Late stage of refining, after temperature has become uniform;

[0067] Functions: Modifies inclusions, refines grains, and improves forging plasticity

[0068] 1.4 VD Vacuum Degassing (Critical Degassing)

[0069] Vacuum degree: ≤ 0.10 KPa;

[0070] Deep vacuum holding time: ≥ 13 min;

[0071] After breaking the vacuum, SiCa wire is fed to modify the inclusions;

[0072] Soft blowing argon time: > 13 min, molten steel is not exposed throughout the process;

[0073] Gas control objectives:

[0074] [H] ≤ 2.0 ppm;

[0075] [O] ≤ 25 ppm;

[0076] [N] ≤ 80 ppm;

[0077] 1.5 Continuous casting

[0078] Constant casting speed operation, superheat control: ≤ 30℃ (except for the first casting batch);

[0079] Low-magnification control of continuously cast billets:

[0080] Central looseness ≤ Grade 1.5;

[0081] Generally loose (≤ 1.5 grade);

[0082] Ingot segregation ≤ 1.5 grade;

[0083] No pinpoint segregation, no bright white bands, no shrinkage cavities, no bubbles, and no cracks;

[0084] After passing the sulfur stamping and hot acid inspection, the billet is sent for hot rolling.

[0085] 2. Rolling process

[0086] 2.1 Acceptance of Cast Billets and Furnace Loading

[0087] The surface and internal quality of the cast billet are inspected according to the company's internal control standards.

[0088] Unqualified billets are reclassified or rejected and are not allowed to proceed with rolling.

[0089] 2.2 Segmented Heating of Walking Beam

[0090] Implement a special low-alloy steel heating process;

[0091] Preheating section: 800-950℃;

[0092] Heating section: 1150-1220℃;

[0093] Heat spreader: 1130-1180℃, heat preservation ≥40min;

[0094] Total heating time: 2.0-3.5 hours;

[0095] Objective: To homogenize austenite without overheating or burning;

[0096] 2.3 High-pressure water descaling

[0097] Descaling pressure: 18-24 MPa;

[0098] Two descaling processes are performed before rolling;

[0099] Objective: To thoroughly remove primary iron oxide scale and prevent oxidation from pressing in and forming surface defects;

[0100] 2.4 Billet preparation + continuous rolling mill rolling

[0101] Billet milling unit: Φ850mm billet mill;

[0102] Continuous rolling mill: Φ700mm×3 + Φ550mm×4 continuous rolling;

[0103] Rough rolling temperature: 1100-1150℃;

[0104] Finishing rolling temperature: 850-930℃;

[0105] Reasonable distribution of pressure during each pass ensures deformation penetration and uniform structure;

[0106] After rolling, samples are cut and taken online for low-magnification, inclusion, and grain size testing.

[0107] 3. Composition control range (melting composition, no grade)

[0108] Main element:

[0109] C: 0.15-0.21%;

[0110] Si: 0.15-0.30%;

[0111] Mn: 0.88-1.35%;

[0112] P ≤ 0.020%;

[0113] S ≤ 0.015%;

[0114] Acid-soluble Al: 0.015-0.050% (internal control 0.020-0.040%);

[0115] Residual elements:

[0116] Ni ≤ 0.40%;

[0117] Cr ≤ 0.40%;

[0118] Cu ≤ 0.20%;

[0119] Mo ≤ 0.12%;

[0120] V ≤ 0.02%;

[0121] Nb ≤ 0.02%;

[0122] Carbon equivalent: CE ≤ 0.42%;

[0123] 4. Quality Control Indicators

[0124] 4.1 Low-magnification structure

[0125] Central looseness ≤ 1.5 grade;

[0126] Generally loose, grade ≤1.5;

[0127] Ingot segregation ≤ 1.5 grade;

[0128] No pinpoint segregation, no shrinkage cavities, no bubbles, no cracks, no peeling, no white spots;

[0129] 4.2 Non-metallic inclusions (GB / T 10561 Method A)

[0130] A Fine ≤ 2.0; A Coarse ≤ 1.5;

[0131] Bfine ≤ 1.5; Bcoarse ≤ 1.0;

[0132] Cfine ≤ 1.5; Ccoarse ≤ 1.0;

[0133] Dfine ≤ 1.5; Dcoarse ≤ 1.0;

[0134] 4.3 Surface Quality

[0135] No cracks, scars, folds, or inclusions;

[0136] Grinding is permitted, with the cleaning width ≥ 5 times the depth, and only one location allowed on the same cross section;

[0137] Grinding depth ≤ 1 / 2 of dimensional tolerance;

[0138] The depth of minor defects is allowed to be ≤0.3mm.

[0139] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for smelting and rolling rare earth hot-rolled round steel for forging, characterized in that, The process consists of two parts: smelting and rolling. The smelting process is as follows: hot metal pretreatment, converter top and bottom reblowing, LF ladle refining, VD vacuum degassing, and continuous casting. The rolling process is as follows: billet acceptance, walking beam segmented heating, high-pressure water descaling, billet rolling, and continuous rolling mill rolling. The smelting composition is controlled as follows: C 0.15~0.21%, Si 0.15~0.30%, Mn 0.88~1.35%, P≤0.020%, S≤0.015%, Als 0.015~0.050%; residual Ni≤0.40%, Cr≤0.40%, Cu≤0.20%, Mo≤0.12%, V≤0.02%, Nb≤0.02%; carbon equivalent CE≤0.42%; 30 kg of rare earth ferroalloy with a content of 30% is added to each furnace during the later stage of LF refining. Converter endpoint control: C≥0.06%, P≤0.010%, tapping temperature≥1550℃, final slag basicity 3.0, strict slag blocking during tapping; VD vacuum degassing: vacuum degree ≤0.10KPa, deep vacuum time ≥13min, soft blowing time >13min; gas control [H] ≤2.0ppm, [O] ≤25ppm, [N] ≤80ppm; SiCa wire is fed after vacuum breaking; Continuous casting superheat ≤30℃, constant casting speed; billet low magnification: center porosity ≤1.5 grade, general porosity ≤1.5 grade, ingot segregation ≤1.5 grade, no point segregation or harmful defects; The rolling process uses Φ850mm roughing + Φ700mm×3 + Φ550mm×4 continuous rolling; roughing temperature is 1100~1150℃, and final rolling temperature is 850~930℃; descaling pressure is 18~24MPa.

2. The smelting and rolling method for rare earth hot-rolled round steel for forging according to claim 1, characterized in that, Non-metallic inclusions are evaluated according to GB / T 10561 Method A: A fine ≤ 2.0, A coarse ≤ 1.5; B, C, D fine ≤ 1.5, coarse ≤ 1.

0.

3. The smelting and rolling method for rare earth hot-rolled round steel for forging according to claim 1, characterized in that, Deep desulfurization pretreatment of blast furnace hot metal was adopted; after pretreatment, the sulfur content of the hot metal was ≤0.005%.

4. The smelting and rolling method for rare earth hot-rolled round steel for forging according to claim 1, characterized in that, Converter top and bottom blowing smelting: Process mode: Single slag / Double slag operation.

5. The smelting and rolling method for rare earth hot-rolled round steel for forging according to claim 1, characterized in that, The temperature inside the steel ladle is ≥1550℃.

6. The smelting and rolling method for rare earth hot-rolled round steel for forging according to claim 1, characterized in that, Pre-deoxidation is performed using a silicon-manganese alloy and ferromanganese; final deoxidation is performed using aluminum to ensure the stability of acid-soluble aluminum.

7. The smelting and rolling method for rare earth hot-rolled round steel for forging according to claim 1, characterized in that, Segmented heating using a walking beam: Preheating section: 800-950℃ Heating range: 1150-1220℃ Heat spreader: 1130-1180℃, hold for ≥40 minutes Total heating time: 2.0-3.5h.