Cold-formed high-strength steel S700MC with good coiling performance and production method thereof

Abstract

The invention discloses a cold-formed high-strength steel S700MC with good through-coiling performance and a production method thereof. The chemical composition and mass percentage of the cold-formed high-strength steel S700MC with good through-coiling performance are as follows: C: 0.055-0.078%, Si: 0.12-0.22%, Mn: 1.65-2.00%, P≤0.013%, S≤0.005%, Nb: 0.045-0.055%, V: 0.040-0.052%, Ti: 0.10-0.13%, N≤0.045%, and the balance is Fe and impurities within the allowable range; the production method includes converter smelting, LF+RH refining, continuous casting, controlled rolling and controlled cooling, coiling, and storage stacking and slow cooling processes. The cold-formed high-strength steel S700MC provided by the invention has good mechanical properties and good through-coiling performance, and the through-coiling performance fluctuation range is within 50MPa.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a cold-formed high-strength steel S700MC with good coiling performance and a production method thereof. Background Art

[0002] High-strength steel S700MC is mainly used in the field of engineering machinery, such as heavy-duty vehicle beams, crane booms and other key parts. Due to the large load-bearing capacity of the material, the safety performance requirements are high, so the stability of the strip coiling performance is also put forward. At present, the alloys used to produce high-strength steel are mainly Nb, V, Ti, Mo, etc., and Ti alloy has become the preferred alloy for steel strengthening due to its low price advantage. However, Ti is very active in chemical properties. At high temperatures, it will react with the N element in steel to produce TiN. Large-sized TiN inclusions will affect the material properties. At low temperatures, nano-scale TiC will precipitate to achieve precipitation strengthening effects. Secondly, the temperature of the steel strip is uneven during the rolling process and the offline cooling process, resulting in large fluctuations in the strip coiling performance, generally above 60MPa, and even some strips have fluctuations of more than 100MPa, which not only brings difficulties to customers' processing, but also increases the risk of materials during use. Summary of the invention

[0003] The object of the present invention is to provide a cold-formed high-strength steel S700MC with good coiling performance, good mechanical properties, and a coiling performance fluctuation range within 50MPa.

[0004] To achieve the above purpose, the technical solution provided by the present invention is as follows:

[0005] A cold-formed high-strength steel S700MC with good through-coiling performance has the following chemical compositions and their mass percentages: C: 0.055-0.078%, Si: 0.12-0.22%, Mn: 1.65-2.0%, P≤0.013%, S≤0.005%, Nb: 0.045-0.055%, V: 0.040-0.052%, Ti: 0.10-0.13%, N≤0.045%, and the remainder is Fe and impurities within a permissible range.

[0006] The cold-formed high-strength steel S700MC of the present invention has a thickness of 6.0-12.0 mm; a metallographic structure of ferrite, a small amount of pearlite and nano-level TiC precipitates, a grain size of 11.5-12.5; and a fluctuation range of through-coiling performance within 50 MPa.

[0007] The present invention also provides a production method of cold-formed high-strength steel S700MC with good coiling performance, which includes converter smelting, LF+RH refining, continuous casting, controlled rolling and controlled cooling, and warehousing, stacking and slow cooling processes.

[0008] The converter smelting process of the present invention adopts a 250t converter for blowing, the slag basicity in the furnace is controlled at 3.0-4.5, the oxygen blowing speed is controlled at 10-12m³ / s, the total oxygen blowing time is controlled at 13.5-16.5min; the converter terminal temperature is 1610-1680°C.

[0009] In the LF+RH refining process of the present invention, the LF inlet temperature is 1550-1600°C, the refining time is 30-60min, and the outlet temperature is controlled at 1615-1645°C; the RH ultimate vacuum degree is less than 0.5bar, the cycle time is 20-30min, and the outlet temperature is 1565-1575°C.

[0010] The continuous casting process of the present invention adopts dynamic light pressure reduction, increases the pressure reduction at the end of solidification to improve the center quality of the casting billet, adopts constant drawing speed casting, and controls the billet drawing speed at 1.0-1.3m / min.

[0011] The controlled rolling and controlled cooling process of the present invention comprises the following steps: the first stage is rough rolling, which adopts 3+3 rolling passes and a cumulative deformation of 50-70%; the second stage is finishing rolling, which is rolled by 7 rolling passes of the F1-F7 finishing rolling units and a cumulative deformation of >80%, the finishing rolling entrance temperature is controlled at 960-1080°C, and the final rolling temperature is controlled at 850-900°C; the third stage is laminar cooling, and in order to reduce the high temperature residence time, the laminar cooling stage adopts the front-stage concentrated mode, the cooling water temperature is controlled at 26-31°C, the cooling rate is controlled at 40-60°C / s, and the steel is rapidly cooled to 590-650°C for coiling.

[0012] In the storage stacking slow cooling process of the present invention, in order to increase the low-temperature residence time and improve the uniformity of TiC precipitation, the steel coil stack is placed in a special slow cooling pit for slow cooling for more than 36 hours.

[0013] The technical solution of the present invention has the following beneficial technical effects:

[0014] (1) The present invention adopts the double refining LF-RH process for production, and uses a small amount of Nb, V and low-alloy cost Ti as the main strengthening elements, so the product has a greater low-alloy cost advantage.

[0015] (2) The present invention adopts controlled rolling and controlled cooling, adopts concentrated rapid cooling in the front stage, reduces the residence time of steel in the high temperature stage, stacks and slowly cools after coiling, increases the low-temperature residence stage, improves the uniformity of nano-TiC precipitation, and ensures the stability of the material's coiling performance.

[0016] (3) The cold-formed high-strength steel S700MC provided by the present invention has good mechanical properties, a yield strength of 750-800 MPa, a tensile strength of 820-870 MPa, an elongation of 17-22%, and good coiling performance, with a fluctuation range of coiling performance within 50 MPa.

[0017] (4) The metallographic structure of the cold-formed high-strength steel S700MC provided by the present invention is ferrite, a small amount of pearlite and nano-scale TiC precipitates, and the grain size is 11.5 to 12.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a typical metallographic photograph of the steel of Example 1;

[0019] Figure 2 is a typical metallographic photograph of the steel of Example 2;

[0020] Figure 3 is a typical metallographic photograph of the steel of Example 3;

[0021] Figure 4 is a typical metallographic photograph of the steel of Example 4;

[0022] Figure 5 is a typical metallographic photograph of the steel of Example 5;

[0023] Figure 6 This is a typical metallographic photograph of the steel in Example 6. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] The invention provides a cold-formed high-strength steel S700MC with good through-coiling performance, wherein the chemical composition and the mass percentage thereof are as follows: C: 0.055-0.078%, Si: 0.12-0.22%, Mn: 1.65-2.0%, P≤0.013%, S≤0.005%, Nb: 0.045-0.055%, V: 0.040-0.052%, Ti: 0.10-0.13%, N≤0.045%, and the balance is Fe and impurities within a permissible range.

[0026] The present invention also provides a production method of cold-formed high-strength steel S700MC with good coiling performance, including converter smelting, LF+RH refining, continuous casting, controlled rolling and controlled cooling, and warehousing, stacking and slow cooling processes, which are specifically as follows:

[0027] Converter smelting process: 250t converter is used for blowing, the slag basicity in the furnace is controlled at 3.0-4.5, the oxygen blowing speed is controlled at 10-12m³ / s, and the total oxygen blowing time is controlled at 13.5-16.5min; the converter terminal temperature is 1610-1680℃;

[0028] LF+RH refining process: LF inlet temperature is 1550-1600℃, refining time is 30-60min, outlet temperature is controlled at 1615-1645℃; RH ultimate vacuum degree is less than 0.5bar, cycle time is 20-30min, outlet temperature is 1565-1575℃;

[0029] Continuous casting process: dynamic light pressure is adopted to increase the pressure reduction at the end of solidification to improve the center quality of the casting billet, and constant drawing speed casting is adopted, and the drawing speed is controlled at 1.0-1.3m / min.

[0030] Controlled rolling and controlled cooling process: The first stage is rough rolling, which adopts 3+3 rolling passes with a cumulative deformation of 50-70%; the second stage is finishing rolling, which is rolled by F1-F7 finishing mills for 7 passes with a cumulative deformation of >80%, the finishing inlet temperature is controlled at 960-1080℃, and the final rolling temperature is controlled at 850-900℃; the third stage is laminar cooling. In order to reduce the high temperature residence time, the laminar cooling stage adopts the front-stage concentrated mode, the cooling water temperature is controlled at 26-31℃, the cooling rate is controlled at 40-60℃ / s, and it is quickly cooled to 590-650℃ for coiling.

[0031] Warehouse stacking slow cooling process: In order to increase the low-temperature residence time and improve the uniformity of TiC precipitation, the steel coil stack is placed in a special slow cooling pit for more than 36 hours of slow cooling.

[0032] Embodiment 1-6:

[0033] The composition of the steel plates in each example is shown in Table 1.

[0034] The smelting process parameters of each embodiment are shown in Table 2.

[0035] The parameters of the LF+RH refining process and continuous casting process of each embodiment are shown in Table 3

[0036] The parameters of the rolling process and the storage, stacking and slow cooling process of each embodiment are shown in Table 4.

[0037] The properties of the cold-formed high-strength steel S700MC of each embodiment are shown in Table 5.

[0038] Table 1 Composition of steel plates in various examples (wt%)

[0039]

[0040] Table 2 Smelting process parameters of each embodiment

[0041]

[0042] Table 3 LF+RH refining process and continuous casting process parameters of each embodiment

[0043]

[0044] Table 4 Parameters of the rolling process and the storage stacking slow cooling process of each embodiment

[0045]

[0046] Table 5 Properties of cold-formed high-strength steel S700MC in various examples

[0047]

[0048] It can be seen from Table 5 that the mechanical properties of the high-strength steel S700MC provided by the present invention meet the standard requirements, the overall performance is good, and the fluctuation range of the through-coiling performance is within 50 MPa.

[0049] Metallographic photos of cold-formed high-strength steel S700MC provided in each embodiment are shown in Figures 1 to 6 .from Figures 1 to 6 It can be seen that the metallographic structure is ferrite, a small amount of pearlite (nanoscale TiC precipitates cannot be distinguished in the metallographic photograph), and the grain size is 11.5 to 12.5.

Claims

1. A cold-formed high-strength steel S700MC with good coiling performance, characterized in that: Its chemical composition and its mass percentage are: C: 0.055-0.078%, Si: 0.12-0.22%, Mn: 1.65-2.00%, P≤0.013%, S≤0.005%, Nb: 0.045-0.055%, V: 0.042-0.052%, Ti: 0.10-0.13%, N≤0.045%, the balance is Fe and impurities within the allowable range; The cold-formed high-strength steel S700MC has a yield strength of 750-800 MPa, a tensile strength of 820-870 MPa, an elongation of 17-22%, and a coiling performance fluctuation range of less than 50 MPa; The production method of the cold-formed high-strength steel S700MC includes converter smelting, LF+RH refining, continuous casting, controlled rolling and controlled cooling, and warehousing, stacking and slow cooling processes; In the LF+RH refining process, the LF inlet temperature is 1550-1600°C, the refining time is 30-60min, and the outlet temperature is controlled at 1615-1645°C; the RH ultimate vacuum degree is less than 0.5bar, the cycle time is 20-30min, and the outlet temperature is 1565-1575°C; The controlled rolling and controlled cooling process comprises the following steps: the first stage is rough rolling, which adopts 3+3 rolling passes and a cumulative deformation of 50-70%; the second stage is finishing rolling, which is rolled by the F1-F7 finishing mills for 7 passes, with a cumulative deformation of >80%, and the finishing inlet temperature is controlled at 960-1080°C, and the final rolling temperature is controlled at 850-900°C; the third stage is laminar cooling, and in order to reduce the high temperature residence time, the laminar cooling stage adopts the front-stage concentrated mode, the cooling water temperature is controlled at 26-31°C, the cooling rate is controlled at 47.2-60°C / s, and the steel is rapidly cooled to 590-650°C for coiling; In the storage stacking slow cooling process, the steel coil stack is placed in a special slow cooling pit and slow cooled for more than 36 hours.

2. The cold-formed high-strength steel S700MC with good coiling performance according to claim 1 is characterized in that: The cold-formed high-strength steel S700MC has a thickness of 6.0 to 12.0 mm.

3. The cold-formed high-strength steel S700MC with good coiling performance according to claim 1 is characterized in that: The cold-formed high-strength steel S700MC has a metallographic structure of ferrite, a small amount of pearlite and nano-scale TiC precipitates, and a grain size of 11.5 to 12.

5.

4. The method for producing cold-formed high-strength steel S700MC with good coiling performance as claimed in claim 1, characterized in that: Including converter smelting, LF+RH refining, continuous casting, controlled rolling and controlled cooling, warehousing and stacking for slow cooling; In the LF+RH refining process, the LF inlet temperature is 1550-1600°C, the refining time is 30-60min, and the outlet temperature is controlled at 1615-1645°C; the RH ultimate vacuum degree is less than 0.5bar, the cycle time is 20-30min, and the outlet temperature is 1565-1575°C; The controlled rolling and controlled cooling process comprises the following steps: the first stage is rough rolling, which adopts 3+3 rolling passes and a cumulative deformation of 50-70%; the second stage is finishing rolling, which is rolled by the F1-F7 finishing mills for 7 passes, with a cumulative deformation of >80%, and the finishing inlet temperature is controlled at 960-1080°C, and the final rolling temperature is controlled at 850-900°C; the third stage is laminar cooling, and in order to reduce the high temperature residence time, the laminar cooling stage adopts the front-stage concentrated mode, the cooling water temperature is controlled at 26-31°C, the cooling rate is controlled at 40-60°C / s, and the steel is rapidly cooled to 590-650°C for coiling; In the storage stacking slow cooling process, the steel coil stack is placed in a special slow cooling pit and slow cooled for more than 36 hours.

5. The method for producing cold-formed high-strength steel S700MC with good coiling performance according to claim 4, characterized in that: The converter smelting process adopts a 250t converter for blowing, the slag basicity in the furnace is controlled at 3.0-4.5, the oxygen blowing speed is controlled at 10-12m³ / s, the total oxygen blowing time is controlled at 13.5-16.5min; the converter terminal temperature is 1610-1680°C.

6. The method for producing cold-formed high-strength steel S700MC with good coiling performance according to claim 4, characterized in that: The continuous casting process adopts dynamic light pressure reduction, increases the pressure reduction at the end of solidification to improve the center quality of the casting billet, adopts constant drawing speed casting, and controls the billet drawing speed at 1.0-1.3 m / min.

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