Free-cutting steel with excellent plasticity and drilling performance and preparation method thereof

Abstract

The invention discloses free-cutting steel with excellent plasticity and drilling performance and a preparation method of the free-cutting steel. The preparation method comprises the following steps of converter smelting, LF refining, continuous casting, hot rolling and air cooling line cooling. In the continuous casting step, the water quantity of a crystallizer is 130-140 m < 3 > / h, and the secondary cooling specific water quantity is 0.8-1.0 L / kg; in the air cooling line cooling step, cooling is conducted at the speed of 1-1.5 DEG C / s in the range from the spinning temperature to the temperature larger than 650 DEG C, and cooling is conducted at the speed of 2-4 DEG C / s in the range from 650 DEG C to 500 DEG C; under the conditions that free-cutting elements are not added and the production cost is not increased, the plasticity and the drilling performance of the free-cutting steel are remarkably improved by controlling the size, the hardness and the structure of the rolled manganese sulfide inclusion.

Description

Technical Field

[0001] The invention belongs to the technical field of free-cutting steel, and in particular relates to free-cutting steel with excellent plasticity and drilling performance and a preparation method thereof. Background Art

[0002] Free-cutting steel is widely used in the processing of parts for instruments, automobiles, machine tools, etc. due to its excellent cutting performance. During the production and processing of free-cutting steel, it is easy to have quality problems such as drawing cracking and drilling cracking, which will cause the product to be scrapped and unusable.

[0003] At present, the existing technology pays more attention to the cutting performance and sulfide morphology control of free-cutting steel, but pays little attention to the plasticity and drilling performance of free-cutting steel.

[0004] Chinese patent CN 101003877A discloses a low-carbon free-cutting steel with good machinability and chip crushing properties. The steel does not contain lead and has machinability, drilling properties and chip handling properties similar to or even better than other free-cutting steels. However, the patent adds 0.02-0.30% Se or 0.01-0.20% Te, resulting in high alloy element costs and unstable recovery rates.

[0005] Chinese patent CN 111876689A discloses a low-carbon selenium-containing free-cutting steel for instrumentation and a manufacturing method thereof. The invention controls the key process parameters of converter smelting, LF refining, continuous casting, billet heating and slow cooling after rolling by using a certain ratio of manganese and selenium and a certain combination of selenium and sulfur, and obtains a free-cutting steel with low cutting resistance and low cutting heat during the cutting process, thereby protecting the tool and obtaining a good machining surface roughness. In the patent, LF refining requires white slag, which will reduce the yield of ferrosulphur alloy; the heating time requires high temperature and long time, that is, 1200℃~1300℃, heating for 2~3h, which will reduce production efficiency in the industrial production process and has high requirements for refractory materials in the heating furnace. Summary of the invention

[0006] The object of the present invention is to provide a free-cutting steel with excellent plasticity and drilling performance and a preparation method thereof, which significantly improves the plasticity and drilling performance of the free-cutting steel by controlling the size, hardness and structure of manganese sulfide inclusions in the rolled material without adding free-cutting elements and increasing production costs.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A method for preparing free-cutting steel with excellent plasticity and drilling performance, the method comprising the following steps: converter smelting, LF refining, continuous casting, hot rolling, and air cooling line cooling;

[0009] In the continuous casting step, the water volume of the crystallizer is 130-140m3 / h, secondary cooling water volume 0.8-1.0L / kg;

[0010] In the air-cooling line cooling step, cooling is performed at a rate of 1-1.5°C / s from the spinning temperature to a temperature above 650°C, and at a rate of 2-4°C / s in a temperature range of 650°C-500°C.

[0011] In the converter smelting step, the C content at the blowing end point is 0.04-0.06%, and the end point temperature is 1600-1650° C.; alloying is performed during the steel tapping process, and aluminum iron and lime are added during the steel tapping process for pre-deoxidation.

[0012] The addition amount of aluminum iron and lime is 1-2kg / t molten steel and 2-3kg / t molten steel respectively.

[0013] In the LF refining step, lime, calcium carbide and silicon carbide are used for slag making; before the LF refining leaves the station, the sulphur line is supplemented with the sulphur content target value, and steel is applied after soft blowing for ≥15 minutes.

[0014] The addition amounts of lime, calcium carbide and silicon carbide are 1-1.5kg / t molten steel, 0.5-1.0kg / t molten steel and 0.5-1.0kg / t molten steel respectively.

[0015] In the continuous casting step, the continuous casting speed is 1.5-2.0 m / min, and the superheat degree of the molten steel is 25-35°C.

[0016] In the hot rolling step, the soaking temperature of the heating furnace is 1180-1220° C., the soaking section is kept for ≥30 min, and the time in the furnace is ≥120 min.

[0017] In the hot rolling step, the starting rolling temperature is 1180±10°C, the finishing rolling temperature is 950±10°C, and the spinning temperature is 840±10°C.

[0018] The invention also provides free-cutting steel with excellent plasticity and drilling performance prepared by the preparation method.

[0019] Furthermore, the free-cutting steel comprises the following chemical components in weight percentage: C 0.05-0.15%, Si≤0.15%, Mn≤1.50%, P 0.40-0.90%, S 0.35-0.45%, Al≤0.005%, and the balance is Fe and unavoidable impurities.

[0020] The proportion of manganese sulfide inclusions with a size of ≤200 μm in the free-cutting steel reaches more than 90%, the hardness is 100-120 HBW, and the proportion of ferrite is 93-95%.

[0021] Aiming at the problems of drawing cracking and drilling cracking in the processing of free-cutting steel, the present invention improves the continuous casting cooling intensity, controls the size of manganese sulfide inclusions in the rolled material to be ≤200μm and the proportion to reach more than 90%; and controls the cooling rate of the air cooling line in stages, so that the hardness of the disc is stably controlled at 100-120HBW, and the ferrite proportion is 93-95%.

[0022] Free-cutting steel belongs to the high-oxygen and high-sulfur steel type. Manganese sulfide inclusions in high-sulfur free-cutting steel are mainly generated at the end of solidification of the ingot, and the continuous casting cooling rate determines the original size of the manganese sulfide inclusions in the ingot. As solidification continues, the manganese sulfide continues to grow, and the closer to the solidification end, the faster the size of the manganese sulfide grows. The present invention can shorten the growth time of manganese sulfide by increasing the continuous casting cooling intensity and shortening the solidification time at the end of solidification, thereby controlling the original size of manganese sulfide inclusions in the ingot, and the size of manganese sulfide of the rolled material after rolling is also controlled accordingly. In the process of drawing or drilling and tapping free-cutting steel, when the parts are subjected to lateral stress, due to the short size of manganese sulfide, it will not become a source of microcracks, thereby avoiding quality problems such as drawing cracking and drilling cracking.

[0023] The hardness of the material has a significant impact on the processing performance. Too high or too low hardness will lead to a decrease in processing performance. Too high hardness will lead to poor cutting processability, because materials with high hardness will increase the cutting force, causing faster tool wear, thus affecting processing quality and efficiency; too low hardness may cause tool sticking, and the surface roughness of the processed parts will be large and the precision will be poor, which is also not conducive to processing. The present invention controls the cooling rate of the air cooling line after rolling in stages, appropriately prolongs the ferrite structure formation time, controls the hardness of the rolled material within a moderate range, has excellent plasticity, and has good drawing and drilling performance during the processing.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention overcomes the problem of drawing cracking and drilling cracking that are easy to occur in the processing of free-cutting steel.

[0026] 2. The present invention significantly improves the processing performance of the product by controlling the size, hardness and structure of manganese sulfide inclusions in the rolled material without adding free-cutting elements and increasing production costs.

[0027] 3. The present invention is easy to operate, has a stable process and strong operability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The size and morphology of manganese sulfide in the free-cutting steel rolled product in Example 1; the size of manganese sulfide is within 200um, and the morphology is mostly spindle-shaped;

[0029] Figure 2This is the metallographic structure diagram of the free-cutting steel rolled product in Example 1; the metallographic structure is ferrite + pearlite, and the distribution ratio of the modified structure is statistically analyzed using Pro-Image software, and the ferrite ratio is 95%;

[0030] Figure 3 The hardness distribution diagram of multiple batches of free-cutting steel rolled products obtained by the method of the present invention, the hardness is stably controlled at 100-120 HBW;

[0031] Figure 4 Schematic diagram of the free-cutting steel drawing cracking in Comparative Example 1;

[0032] Figure 5 Schematic diagram of cracking in drilling of free-cutting steel in Comparative Example 1. DETAILED DESCRIPTION

[0033] The free-cutting steel with excellent plasticity and drilling performance provided by the invention has the following chemical components and weight percentages: C 0.05-0.15%, Si≤0.15%, Mn≤1.50%, P 0.40-0.90%, S 0.35-0.45%, Al≤0.005%, and the balance is Fe and unavoidable impurities.

[0034] The method for preparing the free-cutting steel with excellent plasticity and drilling performance comprises the following steps: converter smelting, LF refining, continuous casting, hot rolling, and air cooling line cooling, specifically:

[0035] In the converter smelting step, the C content at the blowing end point is 0.04-0.06%, and the end point temperature is 1600-1650°C; alloying is performed during the steel tapping process, and 1-2kg / t of molten steel aluminum iron and 2-3kg / t of molten steel white ash are added during the steel tapping process for pre-deoxidation.

[0036] In the LF refining step, 1-1.5kg / t lime, 0.5-1.0kg / t calcium carbide and 0.5-1.0kg / t silicon carbide are used for slag making. Before the LF refining leaves the station, the sulfur line is fed according to the sulfur content target value, and steel is loaded after soft blowing for ≥15min.

[0037] During the continuous casting step, the water volume in the crystallizer is 130-140m 3 / h, secondary cooling water volume 0.8-1.0L / kg; continuous casting speed 1.5-2.0m / min, molten steel superheat 25-35℃.

[0038] In the hot rolling step, the heating furnace soaking temperature is 1180-1220°C, the soaking section holding time is ≥30min, and the furnace time is ≥120min; the starting rolling temperature is 1180±10°C, the finishing rolling temperature is 950±10°C, and the wire laying temperature is 840±10°C.

[0039] In the air cooling line cooling step, cooling is performed at a rate of 1-1.5°C / s from the spinning temperature to a temperature above 650°C, and at a rate of 2-4°C / s in a temperature range of 650°C-500°C.

[0040] The present invention is described in detail below with reference to the embodiments.

[0041] Example 1

[0042] The invention discloses a free-cutting steel with excellent plasticity and drilling performance, wherein the chemical composition and weight percentage thereof are: C 0.08%, Si 0.05%, Mn 1.23%, P 0.55%, S 0.38%, Al 0.003%, and the balance is Fe and inevitable impurities.

[0043] The method for preparing the free-cutting steel with excellent plasticity and drilling performance comprises the following steps: converter smelting, LF refining, continuous casting, hot rolling, and air cooling line cooling, specifically:

[0044] In the converter smelting step, the C content at the end of blowing is 0.05% and the end temperature is 1620°C; alloying is performed during the steel tapping process, and 1.5 kg / t of molten steel aluminum iron and 2.5 kg / t of molten steel white ash are added during the steel tapping process for pre-deoxidation.

[0045] In the LF refining step, 1.2kg / t lime, 0.6kg / t calcium carbide and 0.6kg / t silicon carbide are used for slag making. Before the LF refining leaves the station, the sulfur line is fed according to the sulfur content target value, and steel is loaded after 18 minutes of soft blowing.

[0046] During the continuous casting step, the water volume in the crystallizer is 135m 3 / h, secondary cooling water volume 0.8L / kg; continuous casting speed 1.6m / min, molten steel superheat 32℃.

[0047] In the hot rolling step, the soaking temperature of the heating furnace is 1200°C, the holding time in the soaking section is 35 minutes, and the time in the furnace is 125 minutes; the starting rolling temperature is 1180°C, the finishing rolling temperature is 950°C, and the wire laying temperature is 845°C.

[0048] In the air cooling line cooling step, cooling is performed at a rate of 1.2°C / s in the range from 845°C to greater than 650°C, and cooling is performed at a rate of 2.5°C / s in the range from 650°C to 500°C.

[0049] Example 2

[0050] The invention discloses a free-cutting steel with excellent plasticity and drilling performance, wherein the chemical composition and weight percentage thereof are: C 0.011%, Si 0.08%, Mn 1.35%, P 0.62%, S 0.42%, Al 0.002%, and the balance is Fe and inevitable impurities.

[0051] The method for preparing the free-cutting steel with excellent plasticity and drilling performance comprises the following steps: converter smelting, LF refining, continuous casting, hot rolling, and air cooling line cooling, specifically:

[0052] In the converter smelting step, the C content at the end of blowing is 0.055%, and the end temperature is 1645°C; alloying is performed during the steel tapping process, and 1.6 kg / t of aluminum iron and 3 kg / t of white ash are added during the steel tapping process for pre-deoxidation.

[0053] In the LF refining step, 1.4kg / t lime, 0.8kg / t calcium carbide and 0.7kg / t silicon carbide are used for slag making. Before the LF refining leaves the station, the sulfur line is fed according to the sulfur content target value, and steel is loaded after 16 minutes of soft blowing.

[0054] During the continuous casting step, the water volume in the crystallizer is 140m 3 / h, secondary cooling water volume 0.9L / kg; continuous casting speed 1.8m / min, molten steel superheat 26℃.

[0055] In the hot rolling step, the soaking temperature of the heating furnace is 1220°C, the holding time in the soaking section is 40 minutes, and the time in the furnace is 140 minutes; the starting rolling temperature is 1170°C, the finishing rolling temperature is 940°C, and the wire laying temperature is 830°C.

[0056] In the air cooling line cooling step, cooling is performed at a rate of 1.4°C / s in the range from 830°C to greater than 650°C, and cooling is performed at a rate of 3°C / s in the range from 650°C to 500°C.

[0057] Comparative Example 1

[0058] The rest is the same as in Example 1, except that in the continuous casting step, the water volume of the crystallizer is 120m 3 / h, the secondary cooling water volume is 0.5L / kg. In the air cooling line cooling step, cooling is performed from 845°C to 500°C at a rate of 0.8°C / s.

[0059] Comparative Example 2

[0060] The rest is the same as in Example 1, except that in the air cooling line cooling step, cooling is performed from 845°C to 500°C at a speed of 4.5°C / s.

[0061] The hardness, the proportion of manganese sulfide inclusions with a size of ≤200 μm, and the ferrite ratio of the free-cutting steels prepared in the above-mentioned embodiments and comparative examples are shown in Table 1.

[0062] Table 1

[0063]

[0064]

[0065] The detailed description of a free-cutting steel with excellent plasticity and drilling performance and a preparation method thereof with reference to the above-mentioned embodiments is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for preparing free-cutting steel with excellent plasticity and drilling performance, characterized in that: The preparation method comprises the following steps: converter smelting, LF refining, continuous casting, hot rolling, and air cooling line cooling; In the continuous casting step, the water volume of the crystallizer is 130-140m 3 / h, secondary cooling water volume 0.8-1.0L / kg; In the air-cooling line cooling step, cooling is performed at a rate of 1-1.5°C / s from the spinning temperature to a temperature above 650°C, and at a rate of 2-4°C / s in a temperature range of 650°C-500°C.

2. The preparation method according to claim 1, characterized in that: In the converter smelting step, the C content at the blowing end point is 0.04-0.06%, and the end point temperature is 1600-1650° C.; alloying is performed during the steel tapping process, and aluminum iron and lime are added during the steel tapping process for pre-deoxidation.

3. The preparation method according to claim 2, characterized in that: The addition amount of aluminum iron and lime is 1-2kg / t molten steel and 2-3kg / t molten steel respectively.

4. The preparation method according to claim 1, characterized in that: In the LF refining step, lime, calcium carbide and silicon carbide are used for slag making; before the LF refining leaves the station, the sulphur line is supplemented with the sulphur content target value, and steel is applied after soft blowing for ≥15 minutes.

5. The preparation method according to claim 4, characterized in that: The addition amounts of lime, calcium carbide and silicon carbide are 1-1.5kg / t molten steel, 0.5-1.0kg / t molten steel and 0.5-1.0kg / t molten steel respectively.

6. The preparation method according to claim 1, characterized in that: In the continuous casting step, the continuous casting speed is 1.5-2.0 m / min, and the superheat degree of the molten steel is 25-35°C.

7. The preparation method according to claim 1, characterized in that: In the hot rolling step, the soaking temperature of the heating furnace is 1180-1220°C, the soaking section holding time is ≥30min, and the time in the furnace is ≥120min; the starting rolling temperature is 1180±10°C, the finishing rolling temperature is 950±10°C, and the spinning temperature is 840±10°C.

8. Free-cutting steel with excellent plasticity and drilling performance prepared by the preparation method according to any one of claims 1 to 7.

9. The free-cutting steel with excellent plasticity and drilling performance according to claim 8, characterized in that: The free-cutting steel comprises the following chemical components in weight percentage: C 0.05-0.15%, Si≤0.15%, Mn≤1.50%, P 0.40-0.90%, S 0.35-0.45%, Al≤0.005%, and the balance is Fe and unavoidable impurities.

10. The free-cutting steel with excellent plasticity and drilling performance according to claim 8, characterized in that: The manganese sulfide inclusions in the free-cutting steel have a size of ≤200um and a proportion of more than 90%; the hardness is 100-120 HBW; and the ferrite proportion is 93-95%.

Images