CN114645217A - Free-cutting steel and preparation method thereof - Google Patents
Free-cutting steel and preparation method thereof Download PDFInfo
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- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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Abstract
The invention relates to the technical field of free-cutting steel, in particular to free-cutting steel and a preparation method thereof; the method comprises the steps of controlling the composition ratio of the free-cutting steel, adding bismuth wires, heating in sections, controlling the cooling water flow in the continuous casting process, controlling the rolling reduction rate and the cooling water flow, and controlling the surface cracking problem of the free-cutting steel, thereby improving the surface quality of the free-cutting steel.
Description
Technical Field
The invention relates to the technical field of free-cutting steel, in particular to free-cutting steel and a preparation method thereof.
Background
The free-cutting steel is alloy steel which is added with a certain amount of one or more free-cutting elements such as sulfur, phosphorus, lead, calcium, selenium, tellurium and the like to improve the machinability.
Because the sulfur content of the free-cutting steel is high, and the free oxygen content in the smelting process is high, the surface quality control difficulty of the free-cutting steel is very high, and the surface quality of the free-cutting steel is poor.
Disclosure of Invention
The invention aims to provide free-cutting steel and a preparation method thereof.
The invention is realized by the following steps:
in a first aspect, the present invention provides a free-cutting steel comprising, in weight percent:
C：0.04-0.09％，
Si：0.005-0.05％，
Mn：0.90-1.20％，
S：0.27-0.34％，
P：0.050-0.070％，
Cr≤0.20％，
Cu：0.01-0.05％，
Ti≤0.010％，
Nb≤0.010％，
V≤0.010％，
Mo：0.001-0.011％，
Bi：0.08-0.18％，
Al：0.002-0.007％，
the content of O is 61-109ppm,
the balance being Fe and impurities.
In an alternative embodiment, the composition comprises, in weight percent: c: 0.06-0.08%, Mn: 1.11-1.18%, Cr: 0.11 to 0.18 percent;
the yield strength Rel of the free-cutting steel is more than or equal to 200 MPa; the tensile strength Rm of the free-cutting steel is more than or equal to 378 MPa; the elongation A of the free-cutting steel is more than or equal to 25 percent; wherein the content of the first and second substances,
the weight percentage content of C and Mn is as follows: yield strength Rel ═ (20 × C weight percent +0.70 × Mn weight percent) × 100.
In a second aspect, the present invention provides a method for producing a free-cutting steel according to the foregoing embodiment, comprising:
refining: ferrosilicon is used for deoxidation so as to control the content of free oxygen to be 30-50 ppm;
adding bismuth wires after refining;
continuous casting: the flow rate of cooling water for cooling the casting blank crystallizer is controlled to 2200 +/-100L/min, the amplitude of the crystallizer is 5mm, and the vibration frequency is 190 +/-4 opm;
heating: the continuous casting billet is carried out according to a preheating section, a first heating section, a second heating section and a soaking section, wherein the second heating section and the soaking section are both high-temperature sections, the temperature of the high-temperature sections is 1200 +/-15 ℃, and the time of the high-temperature sections is determined by the thickness of the continuous casting billet;
rolling: controlling the single-pass reduction rate of rough rolling to be 10-14%, and controlling the flow of cooling water to be 60 +/-10 m3/h。
In an alternative embodiment, the amount of ferrophosphorus added per 70kg of molten steel in the refining step is 70 × (0.06% -converter out-of-service content of phosphorus); wherein the content of P in the ferrophosphorus is 50-60% by mass percent.
In an alternative embodiment, in the step of adding bismuth wire, the amount of bismuth wire added per 130t of molten steel is 400m ± 30 m.
In an optional embodiment, in the step of continuous casting, the low-carbon high-sulfur mold flux is used for casting, and the low-carbon high-sulfur mold flux contains Al2O3(SiO) in an amount of 5. + -. 1%2+ CaO) content of 60. + -. 5%.
In an alternative embodiment, the secondary cooling water flow is reduced during the step of continuous casting.
In an optional embodiment, in the continuous casting step, the continuous casting slab is cooled in a pit entering and slow cooling mode, the temperature of the pit entering is greater than or equal to 600 ℃, and the temperature of the pit leaving is less than or equal to 220 ℃.
In an alternative embodiment, the time of the high temperature period is determined by the thickness of the slab, and the heating is controlled to be 0.30T/Thk 0.55, wherein Thk is the thickness of the slab.
In an alternative embodiment, the temperature of the preheating section is less than or equal to 850 ℃ and the temperature of one heating section is 850-.
The invention has the following beneficial effects:
the free-cutting steel can improve the surface quality of the free-cutting steel by adjusting the component proportion.
The preparation method of the free-cutting steel controls the surface cracking problem of the free-cutting steel by controlling the free oxygen content, the addition of bismuth wires, the segmented heating treatment, and the control of cooling water flow, rolling reduction rate and cooling water flow in the continuous casting process, thereby improving the surface quality of the free-cutting steel; particularly, the problem of poor surface quality caused by the generation of surface bubbles due to too high oxygen content can be solved by strictly controlling the amount of free oxygen; if the cooling speed is too high in the continuous casting process, cracks or fissures are easily generated on the surface of the casting blank, and the cooling speed of the casting blank can be reasonably controlled by controlling the flow rate of cooling water so as to solve the problem that the surface is easy to crack; bismuth is in a free state in steel, is insoluble in a steel matrix and is easy to enrich, so that the temperature of a high-temperature section in the heating process is strictly controlled, and surface cracks caused by bismuth can be effectively improved, namely the generation of the surface cracks is effectively reduced; the rolling reduction rate and the flow rate of cooling water are controlled, so that the stress crack sensitivity can be reduced, and the good surface quality of the free-cutting steel can be ensured.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.
The invention provides free-cutting steel which comprises the following components in percentage by weight:
c: 0.04-0.09% (e.g., 0.05%, 0.055%, 0.06%, 0.07%, 0.08%, 0.085%, etc.), Si: 0.005-0.05% (e.g., 0.006%, 0.007%, 0.008%, 0.010%, 0.013%, 0.022%, 0.027%, 0.032%, 0.037%, 0.040%, 0.041%, 0.045%, etc.), Mn: 0.90-1.20% (e.g., 0.91%, 0.95%, 0.99%, 1.00%, 1.05%, 1.12%, 1.18%, etc.), S: 0.27-0.34% (e.g., 0.28%, 0.285%, 0.29%, 0.30%, 0.31%, 0.32%, 0.33%, etc.), P: 0.050-0.070% (e.g., 0.052%, 0.055%, 0.058%, 0.060%, 0.062%, 0.066%, 0.069%, etc.), Cr ≦ 0.20% (e.g., 0.19%, 0.17%, 0.15%, 0.13%, 0.09%, 0.07%, 0.055%, etc.), Cu: 0.01 to 0.05% (e.g., 0.015%, 0.018%, 0.02%, 0.025%, 0.03%, 0.04%, 0.045%, 0.05%, etc.), Ti ≦ 0.010% (e.g., 0.008%, 0.007%, 0.006%, 0.004%, 0.003%, etc.), Nb ≦ 0.010% (e.g., 0.009%, 0.007%, 0.005%, 0.003%, 0.001%, etc.), V ≦ 0.010% (e.g., 0.009%, 0.008%, 0.007%, 0.005%, 0.003%, 0.002%, etc.), Mo: 0.001 to 0.011% (e.g., 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.010%, etc.), Bi: 0.08-0.18% (e.g., 0.09%, 0.11%, 0.12%, 0.135%, 0.14%, 0.155%, 0.16%, 0.175%, etc.), Al: 0.002-0.007% (e.g., 0.003%, 0.004%, 0.005%, 0.006%), an O content of 61-109ppm (e.g., 65ppm, 72ppm, 80ppm, 88ppm, 91ppm, 95ppm, 101ppm, 104ppm, etc.), and the balance of Fe and impurities.
The content of P in the free-cutting steel can be controlled to improve the cutting performance, but the content of P needs to be noticed not to be too high, if the content of P is too high, the billet is easy to crack in the production, and the production difficulty is increased; the content of P is controlled in a proper range, so that the cutting performance is improved, and simultaneously, the billet is ensured not to crack easily.
Bi forms a composite inclusion in MnS in steel and can contribute to improving the cutting performance of the steel, but the Bi content is low in the steel, and if the Bi is added too much, the Bi exists in the steel in the form of a Bi simple substance, so that pores are easily formed around the Bi during heating, and finally cracks are formed; therefore, the content of Bi is controlled within the adaptive range, so that the cutting performance is improved, the steel is not easy to form cracks, and the good surface quality of the free-cutting steel is ensured.
O can also improve the free-cutting property of the steel, and the higher O content can increase the surface tension of the sulfide and promote the sulfide to be spherical or spindle-shaped, thereby improving the cutting property; if the content of O is too high, surface bubbles can be formed in continuous casting, so that casting blanks are scrapped; therefore, the content of O is controlled at an appropriate position, so that the machinability is ensured, the good surface quality is ensured, and the problem that the casting blank is easy to scrap is solved.
The yield strength Rel of the free-cutting steel is more than or equal to 200 MPa; the tensile strength Rm of the free-cutting steel is more than or equal to 378 MPa; the elongation A of the free-cutting steel is more than or equal to 25 percent.
It should be noted that, free-cutting steel needs to ensure the strength and plasticity of the free-cutting steel besides controlling the surface quality of the steel and reducing cracks; in some embodiments, the composition comprises, in weight percent: c: 0.06-0.08%, Mn: 1.11-1.18%, Cr: 0.11 to 0.18 percent; wherein, the weight percentage content of C and Mn satisfies: yield strength Rel ═ (20 × C weight percent +0.70 × Mn weight percent) × 100.
It should be noted that the yield strength Rel is a theoretical value, i.e. the weight percentages of C and Mn are such that the theoretical yield strength Rel is greater than or equal to 200MPa when calculated by the formula (20 × C +0.70 × Mn) × 100).
It should be further noted that there may be a certain error between the theoretical value of the yield strength Rel calculated in the above manner and the actually detected value.
The preparation method of the free-cutting steel provided by the invention comprises the following steps: refining, continuous casting, heating and rolling.
In the refining process, after the molten steel arrives at the station, the component detection can be carried out, and ferrophosphorus can be added, wherein the content of P in the steel is calculated and controlled according to the addition of 0.01 percent P in 70kg of the molten steel, and the content of the added ferrophosphorus is 70 times (0.06 percent of the converter-leaving content of phosphorus), namely the amount of the ferrophosphorus added per 70kg of the molten steel is 70 times (0.06 percent of the converter-leaving content of phosphorus); wherein the content of P in the ferrophosphorus is 50-60% by mass percent.
Phosphorus is added into molten steel, so that the prepared steel meets the design requirements, the content of P is adaptively controlled, the hot brittleness of the steel can be reasonably controlled, the surface quality deterioration is avoided, and the good surface quality is ensured.
In the refining process, ferrosilicon is adopted for deoxidation so as to control the content of free oxygen to be 30-50 ppm; because the oxygen content is increased continuously in the later stage of refining, the content of free oxygen is reasonably controlled in the early stage, so that the total oxygen content of the final free-cutting steel is convenient to ensure to be 61-109 ppm; in order to ensure both the oxygen content and the silicon content, ferrosilicon deoxidation may be performed only in the early stage of refining, and may not be performed until about 10min before the end of refining.
It should be noted that, the oxygen content is controlled in the refining process, and the inclusion can be synchronously controlled, which is beneficial to ensuring the quality of the free-cutting steel.
After refining, the bismuth wire is fed before soft blowing without calcium treatment, and the amount of the bismuth wire added is 400m +/-30 m per 130t of molten steel. The effect of the subsequent feeding of the bismuth wire and the sulfide cannot be influenced because the calcium wire is not added, namely the shape of the sulfide is changed if the calcium wire is fed firstly, the composite inclusion formed by Bi and MnS is influenced, the cutting performance is influenced, and the cutting performance cannot be influenced if the calcium wire is not added.
In the step of continuous casting, because the free-cutting steel is high-sulfur low-carbon steel with strong oxidizability, the performance of the crystallizer covering slag is easy to change in the casting process, and the effect of the covering slag is influenced, the low-carbon high-sulfur type covering slag is used for casting in the casting process, and Al in the low-carbon high-sulfur type covering slag2The content of O3 is 5 +/-1%, (SiO)2+ CaO) content of 60 + -5%; moreover, the protective cover needs to be continuously supplemented by adopting the principle of small quantity and multiple times.
The protective slag plays a role in lubricating between the copper wall of the crystallizer and the solidified blank shell, prevents blank drawing cracking or microcracks from being generated in the continuous casting process, and prevents cracking from being generated in the rolling process. The choice of the mold flux has an influence on the surface quality of the steel. Steel of low carbon steel with [ C ]]<0.09%, no serious phase change volume change exists in the solidification process, the internal stress is small, and the production is generally carried out at a high throwing speed so as to improve the productivity. The casting powder is required to be low in crystallization temperature and moderate in solidification temperature so as to ensure that the low-carbon steel crystallizer casting powder is in an amorphous state at the temperature of more than 950 ℃ and minimize the possibility of steel leakage adhesion. The casting blank and the crystallizer can be prevented from cracking due to bonding and affecting the surface quality when the viscosity of the casting powder is required to be low. In addition, the initial A1 in the slag2O3The lower the content is, the lower the viscosity of the covering slag is, and the stronger the covering slag has the capability of absorbing impurities, which is beneficial to pure molten steel.
Generally, steel containing Bi has high crack sensitivity, and Bi is easy to enrich, so that the probability of steel leakage caused by cracking in the casting blank casting process and the surface cracking tendency of the casting blank are increased; if the casting blank is cooled too fast, particularly if the corner of the casting blank is cooled too fast, surface cracks or corner cracks are easy to generate, so that the cooling water flow rate in the casting blank cooling crystallizer is controlled within 2200 +/-100L/min, the amplitude of the crystallizer is 5mm, and the vibration frequency is 190 +/-4 opm, the problem that the surface is easy to crack can be solved by reasonably controlling the cooling speed of the casting blank.
Further, in the step of continuous casting, the secondary cooling water flow is reduced, the continuous casting blank is cooled in a pit entering and slow cooling mode, the temperature of entering the pit is greater than or equal to 600 ℃, the temperature of leaving the pit is less than or equal to 220 ℃, so that the cooling rate is reduced, and the surface quality of the steel can be further ensured.
The reduction of the secondary cooling water flow rate may be: the cooling water flow during the secondary cooling of continuous casting is reduced, namely the flow of the cooling water is reduced when the casting blank is forcibly and uniformly cooled in the length interval from the outlet of the crystallizer to the withdrawal and straightening unit in the continuous casting and steelmaking process.
It should be further noted that the flow rate of the secondary cooling water may be reduced, and specifically, the flow rate of the cooling water may be reduced to be less than about 2100L/min during the secondary cooling, which is not particularly limited herein.
Bi is in a free state in steel, is insoluble in a steel matrix, and is easily enriched. The defects on the surface of the steel are not only related to the heating temperature, but also related to the time, and when the defects and the time are both in a proper range, the cracks on the surface of the steel can be avoided. The continuous casting billet is heated according to the following steps: the preheating section, the first heating section, the second heating section and the soaking section are carried out in four sections, wherein the second heating section and the soaking section are high-temperature sections, the temperature of the high-temperature sections is 1200 +/-15 ℃, and the time of the high-temperature sections is determined by the thickness of a continuous casting billet and specifically comprises the following steps: the heating time is controlled to be 0.30-0.55 of T (min)/Thk, wherein Thk is the thickness (mm) of the continuous casting slab. If the heating temperature of the high-temperature section is too high, cracks are easy to appear on the contact surface of the sulfide and the bismuth, the cracks are easy to expand in the rolling process, and further cracks are formed, so that the temperature of the high-temperature section is controlled in an adaptive range, the condition that the cracks appear on the contact surface of the sulfide and the bismuth can be reduced, and the problem of crack expansion in the rolling process is solved.
Optionally, the temperature of the preheating section is less than or equal to 850 ℃, and the temperature of the first heating section is 850-.
A rolling process for reducing stress crack sensitivity to ensure good surface quality of the steel; controlling the rolling pass reduction rate of 10-14% of a single pass in a 1-6 frame of a rough rolling stage; simultaneously, the cooling water of the rolling mill is reduced, and the water flow is adjusted to 60 +/-10 m3And h control.
Alternatively, after the rolling is cooled to room temperature in air, the steel can be straightened one by one, and the surface quality of the steel can be inspected by performing surface leakage flux flaw detection according to the defect depth of 0.2 mm.
The features and properties of the present invention are described in further detail below with reference to examples.
Examples 1 to 3 and comparative examples 1 to 4 were set in accordance with tables 1 and 2, and the free-cutting steels of examples 1 to 3 and comparative examples 1 to 4 were examined, and the results of the examination are shown in Table 3.
TABLE 1 ingredients of examples and comparative examples
| Smelting composition | Example 1 | Example 2 | Example 3 | Comparative example 1 | Comparative example 2 | Comparative example 3 | Comparison ofExample 4 |
| C | 0.06 | 0.08 | 0.07 | 0.06 | 0.08 | 0.07 | 0.07 |
| Si | 0.011 | 0.007 | 0.023 | 0.015 | 0.032 | 0.046 | 0.038 |
| Mn | 1.17 | 1.16 | 1.14 | 1.13 | 1.16 | 1.12 | 1.17 |
| S | 0.28 | 0.31 | 0.32 | 0.29 | 0.30 | 0.33 | 0.31 |
| P | 0.056 | 0.061 | 0.067 | 0.054 | 0.063 | 0.061 | 0.059 |
| Cr | 0.12 | 0.17 | 0.16 | 0.15 | 0.14 | 0.13 | 0.16 |
| Cu | 0.01 | 0.03 | 0.04 | 0.02 | 0.03 | 0.03 | 0.04 |
| Mo | 0.006 | 0.009 | 0.005 | 0.003 | 0.008 | 0.010 | 0.005 |
| Bi | 0.12 | 0.16 | 0.10 | 0.11 | 0.09 | 0.17 | 0.14 |
| Al | 0.006 | 0.003 | 0.004 | 0.002 | 0.004 | 0.003 | 0.005 |
| O/ppm | 89 | 102 | 72 | 84 | 73 | 77 | 81 |
TABLE 2 Process parameters for the examples and comparative examples
TABLE 3 examination results of examples and comparative examples
As can be seen from the descriptions in tables 1 to 3, the strength of the mold cooling water, the heating temperature and time in the high temperature range, and the flow rate of the mill cooling water are controlled within the appropriate ranges, so that the surface quality of steel can be effectively improved, and the pass rate of magnetic leakage of 90% or more can be ensured.
And the weight percentage content of C and Mn satisfies: the yield strength Rel is (20 × C weight percentage +0.70 × Mn weight percentage) x 100, wherein the calculated theoretical yield strength Rel is more than or equal to 200MPa, the surface quality of the steel can be ensured, and the magnetic leakage qualified rate is improved.
Examples 4-8 were set up as per tables 4 and 5.
TABLE 4 compositions of the examples
TABLE 5 Process parameters for the examples
In conclusion, the free-cutting steel of the invention can effectively improve the surface quality by adjusting the component proportion and the process.
The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes will occur to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. The free-cutting steel is characterized by comprising the following components in percentage by weight:
C：0.04-0.09％，
Si：0.005-0.05％，
Mn：0.90-1.20％，
S：0.27-0.34％，
P：0.050-0.070％，
Cr≤0.20％，
Cu：0.01-0.05％，
Ti≤0.010％，
Nb≤0.010％，
V≤0.010％，
Mo：0.001-0.011％，
Bi：0.08-0.18％，
Al：0.002-0.007％，
the content of O is 61-109ppm,
the balance being Fe and impurities.
2. The free-cutting steel as claimed in claim 1, comprising, in weight percent: c: 0.06-0.08%, Mn: 1.11-1.18%, Cr: 0.11 to 0.18 percent;
the yield strength Rel of the free-cutting steel is more than or equal to 200 MPa; the tensile strength Rm of the free-cutting steel is more than or equal to 378 MPa; the elongation A of the free-cutting steel is more than or equal to 25 percent; wherein the content of the first and second substances,
the weight percentage content of C and Mn is as follows: yield strength Rel ═ 20 × C by weight +0.70 × Mn by weight × 100.
3. The method for producing the free-cutting steel as claimed in claim 1 or 2, comprising:
refining: ferrosilicon is used for deoxidation so as to control the content of free oxygen to be 30-50 ppm;
adding bismuth wires after refining;
continuous casting: the water flow of cooling water for cooling the casting blank crystallizer is controlled to be 2200 +/-100L/min, the amplitude of the crystallizer is 5mm, and the vibration frequency is 190 +/-4 opm;
heating: the method comprises the following steps that a continuous casting blank is subjected to a preheating section, a first heating section, a second heating section and a soaking section, wherein the second heating section and the soaking section are both high-temperature sections, the temperature of the high-temperature sections is 1200 +/-15 ℃, and the time of the high-temperature sections is determined by the thickness of the continuous casting blank;
rolling: controlling the single-pass reduction rate of rough rolling to be 10-14%, and controlling the flow of cooling water to be 60 +/-10 m3/h。
4. The method of producing a free-cutting steel as claimed in claim 3,
in the refining step, the amount of ferrophosphorus added per 70kg of the molten steel is 70 × (0.06% -converter out-of-service content of phosphorus); wherein the content of P in the ferrophosphorus is 50-60% by mass.
5. The method of producing a free-cutting steel as claimed in claim 3,
in the step of adding bismuth wire, the amount of bismuth wire added per 130t of the molten steel is 400m +/-30 m.
6. The method of producing a free-cutting steel as claimed in claim 3,
in the step of continuous casting, low-carbon high-sulfur type covering slag is used for casting, and Al in the low-carbon high-sulfur type covering slag2O3Is 5 +/-1%, (SiO)2+ CaO) content of 60. + -. 5%.
7. The method for producing free-cutting steel according to claim 3, wherein in the step of continuous casting, secondary cooling water flow rate is reduced.
8. The method for producing free-cutting steel as claimed in claim 3, wherein in the step of continuous casting, the slab is cooled by pit entry and slow cooling, the pit entry temperature is 600 ℃ or higher, and the pit exit temperature is 220 ℃ or lower.
9. The method for manufacturing a free-cutting steel as claimed in claim 3, wherein the time of the high temperature section is determined by the thickness of the slab, and the heating time is controlled to be 0.30T/Thk 0.55, wherein Thk is the thickness of the slab.
10. The method for producing free-cutting steel as claimed in claim 3, wherein the temperature of said preheating section is 850 ℃ or less and the temperature of said one heating section is 850-.
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