High-surface-quality ultrahigh reaming steel and manufacturing method thereof

Abstract

The invention discloses high-surface-quality ultrahigh reaming steel and a manufacturing method thereof. The high-surface-quality ultrahigh reaming steel comprises the following chemical components in percentage by weight: 0.02-0.08% of C, less than or equal to 0.1% of Si, 0.4-0.8% of Mn, 0.015-0.05% of Al, 0.03-0.08% of Ti, less than or equal to 0.007% of N and the balance of Fe. The steel comprises at least one of 0.001 to 0.05 percent of Nb, 0.002 to 0.06 percent of V, 0.002 to 0.6 percent of Cr, 0.0005 to 0.0050 percent of Ca, 0.0005 to 0.005 percent of RE and 0.0002 to 0.0030 percent of B, and the balance of Fe and other inevitable impurities, and the requirements of Tieffgt, Tieffgt, Tieffgt, Tieffgt, Tieffgt and Tieffgt are met at the same time. And Tieff is equal to Ti-3. 42 * N-3 * S. The invention also discloses a preparation method of the Tieff. The low-carbon-low-silicon-low-manganese component design is adopted, the hole expansion rate is increased while the strength of the steel plate is guaranteed, the high surface quality is guaranteed, and the yield strength Rel of the high-surface hole expansion steel is larger than or equal to 305 MPa, the tensile strength Rm is larger than or equal to 440 MPa, the ductility A is larger than or equal to 34%, and the hole expansion rate lambda is larger than or equal to 110%.

Description

Technical Field

[0001] The present invention belongs to the field of hole-expanding steel, and particularly relates to a super high hole-expanding steel with high surface quality and a manufacturing method thereof. Background Art

[0002] In recent years, with the rapid development of the automotive industry, the market and users have put forward higher requirements for the performance of automobiles, which requires the use of more excellent steel plates in the preparation of automobiles. For example, hot-rolled plates and hot-rolled pickled plates have been widely used in the preparation of chassis and wheel structure parts of passenger cars, accounting for one-fourth of the steel used in automobiles.

[0003] In the actual preparation of passenger car parts, it is often necessary to adopt hole-expanding and local flanging designs for steel to achieve the purpose of high-strength thinning and lightweight required by the automotive industry. It has been found that the hole-expanding performance of steel plates is related to the composition, strength and tissue uniformity of the steel plates. The traditional 440MPa steel types mainly use carbon-manganese solid solution strengthened steel and low-alloy precipitation strengthened steel, and their hole-expanding rates are between 50% and 70%.

[0004] With the increasing requirements of automotive design for the chassis structure, the current preparation of automotive part forming is more complex, and the requirements of automobile manufacturers for the flanging and hole-expanding performance of steel plates are also continuously increasing. The existing traditional carbon-manganese solid solution strengthened steel and low-alloy precipitation strengthened steel structure steel plates have difficulty meeting the current forming requirements of automotive chassis and cantilever parts. High hole-expanding steel has become one of the important varieties of automotive steel plates.

[0005] Currently, in response to the demand for high hole-expanding steel in the existing automotive field, many researchers have begun to study and develop high hole-expanding steel plates.

[0006] Chinese Patent CN103667880A discloses "a high hole-expanding steel plate with a tensile strength of 440MPa and a manufacturing method thereof", and its chemical composition by weight percentage is: C: 0.05 - 0.1%, Si: 0.1 - 0.6%, Mn: 0.9 - 1.8%, P≤0.02%, S≤0.005%, Al: 0.015 - 0.060%, Ca<0.0050%, and the rest are Fe and unavoidable impurities; among them, ([C]×[Mn])≤0.1; ([P]+10[S])≤0.04; the product of the strength and hole-expanding rate of the obtained steel plate TSxλ≥44000MPa%. However, in the chemical element composition design of this technical solution, there is more Si element added, which will cause poor surface quality of the steel plate and cannot meet the high-quality requirements of users.

[0007] Japanese Patent JP2006063394A discloses a hot-rolled high hole-expansion steel with C: 0.20 - 0.48%, Si: below 0.1%, Mn: 0.20 - 0.60%, P: below 0.02%, S: below 0.01%, Al: below 0.1%, N: below 0.005%, B: 0.001 - 0.005%, Cr: 0.05 - 0.3%. Its tensile strength is also ≥440 MPa, but its hole-expansion rate is only ≥70%, and it also needs to be annealed at 640 °C after hot rolling.

[0008] Thus, it can be seen that for the 440 MPa steel grade high hole-expansion steel plates developed and designed by the current existing technology, there are still problems of insufficient hole-expansion rate and poor surface quality. Summary of the Invention

[0009] The purpose of the present invention is to provide a high surface quality and ultra-high hole-expansion steel and its manufacturing method. The obtained steel has the characteristics of high surface quality and high hole-expansion rate. Its yield strength Rel≥305 MPa, tensile strength Rm≥440 MPa, elongation A≥34%, and hole-expansion rate λ≥110%, which can meet the requirements of the performance of steel for application scenarios such as automotive chassis and complex stamping parts, and has a very good prospect of popularization and application.

[0010] To achieve the above purpose, different from the existing high hole-expansion mainly using Si addition and high Mn design, the technical solution of the present invention adopts a low-carbon composition design to improve the hole-expansion rate, avoids poor painting caused by red iron scale through low Si design, and effectively guarantees the strength through low Mn combined with microalloying element design.

[0011] Specifically, for the high surface quality and ultra-high hole-expansion steel described in the present invention, its chemical composition by weight percentage is: C: 0.02 - 0.08%, Si≤0.1%, Mn: 0.4 - 0.8%, Al: 0.015 - 0.05%, Ti: 0.03 - 0.08%, N≤0.007%; and at least one of Nb: 0.001 - 0.05%, V: 0.002 - 0.06%, Cr: 0.002 - 0.6%, Ca: 0.0005 - 0.0050%, RE: 0.0005 - 0.005%, B: 0.0002 - 0.0030%, and the rest includes Fe and other inevitable impurities, and it is required to simultaneously satisfy Ti eff >0.015%, Ti eff =Ti - 3.42×N - 3×S.

[0012] Preferably, N≤0.0065%.

[0013] Furthermore, the balance is Fe and other inevitable impurities.

[0014] Preferably, in the inevitable impurities, P ≤ 0.025% and S ≤ 0.006%, preferably S ≤ 0.003%.

[0015] The microstructure of the high surface quality ultra-high hole-expanding steel of the present invention is ferrite + bainite, wherein the volume ratio of ferrite is ≥ 90%.

[0016] For the high surface quality ultra-high hole-expanding steel of the present invention, the yield strength Rel ≥ 305 MPa, the tensile strength Rm ≥ 440 MPa, the elongation A ≥ 34%, and the hole-expanding rate λ ≥ 110%.

[0017] In the composition design of the high surface ultra-high hole-expanding steel of the present invention:

[0018] C: In the high surface quality ultra-high hole-expanding steel of the present invention, adding an appropriate amount of C can be used to form sufficient carbide strengthening phases to ensure the strength level of the steel. When the content of C element in the steel is too low, the strength of the steel cannot meet the requirements; while when the content of C element in the steel is too high, the precipitated carbide particles are large and it is not conducive to the hole-expanding performance of the steel plate. In addition, when the carbon and manganese content in the steel is too high, it is easy to produce coarse pearlite phase and pearlite banded structure, which will affect the performance of the steel. Therefore, the present invention controls the content of C element at 0.02 - 0.08%.

[0019] Si: In the high surface quality ultra-high hole-expanding steel of the present invention, Si element can play a solid solution strengthening role in carbon manganese steel, but the content of Si element in the steel should not be too high. Too high content of Si is likely to cause surface defects such as red scale on the steel plate surface and result in its surface quality not meeting the high requirements of users. Therefore, the present invention controls the content of Si element ≤ 0.1%.

[0020] Mn: In the high surface quality ultra-high hole-expanding steel of the present invention, Mn is a solid solution strengthening element. When the content of Mn element in the steel is lower than 0.4%, it will lead to insufficient strength of the steel; similarly, the content of Mn element should not be too high. Adding too high content of Mn in the steel will increase the proportion of pearlite phase and cause manganese composition segregation in the slab, which is one of the main reasons for the banded structure of the steel plate, and the banded structure will deteriorate the hole-expanding performance of the steel plate. At the same time, the alloy cost of Mn element is relatively high. Therefore, in order to ensure that the steel can obtain excellent hole-expanding performance, the present invention controls the content of Mn element in the steel at 0.4 - 0.8%.

[0021] Al: In the high surface quality ultra-high hole-expanding steel of the present invention, Al is a deoxidizing element in the steel. Adding an appropriate amount of Al element in the steel can effectively reduce the oxide inclusions in the steel and purify the steel quality. Therefore, the present invention controls the content of Al element at 0.015 - 0.05%.

[0022] Ti: In the high surface quality and ultra-high hole-expanding steel of the present invention, the Ti element has a very strong affinity with nitrogen, oxygen, and carbon. Ti is a good deoxidizer and degassing agent and an effective element for fixing nitrogen and carbon. When Ti exists in a solid solution state in ferrite, it can effectively strengthen the strength of ferrite. Therefore, the content of the Ti element in the present invention is controlled to be 0.03 - 0.08%.

[0023] It should be noted that in the high surface quality and ultra-high hole-expanding steel designed in the present invention, in addition to the above-mentioned C, Si, Mn, and Al, according to specific requirements, at least one element of Nb, V, Cr, Ca, RE, and B can be further added to the steel to improve the properties of the steel.

[0024] Nb: In the high surface quality and ultra-high hole-expanding steel of the present invention, the strengthening effect of Nb on the steel is mainly grain refinement strengthening and dispersion strengthening. Niobium can form stable carbides and carbonitrides with carbon and nitrogen in the steel, and can also disperse the carbides and form a steel with refined grains. Therefore, the content of the Nb element in the present invention is controlled to be 0.001 - 0.05%.

[0025] V: In the high surface quality and ultra-high hole-expanding steel of the present invention, adding the V element to the steel can effectively refine the structure and grains of the steel. It can increase the grain coarsening temperature, thereby reducing the overheating sensitivity of the steel and effectively improving the strength and toughness of the steel. Therefore, the content of the V element in the present invention is controlled to be 0.002 - 0.06%.

[0026] Cr: In the high surface quality and ultra-high hole-expanding steel of the present invention, chromium can form a continuous solid solution with iron and improve the strength and hardness of the steel. Therefore, the content of the Cr element in the present invention is controlled to be 0.002 - 0.6%.

[0027] Ca: In the high surface quality and ultra-high hole-expanding steel of the present invention, the Ca element can change the morphology of sulfides in the steel and improve the plasticity and toughness of the steel plate. Therefore, the molten steel designed in the present invention can be subjected to calcium treatment, and the content of the Ca element is controlled to be 0.0005 - 0.0050%.

[0028] RE: In the high surface quality and ultra-high hole-expanding steel of the present invention, it can modify inclusions. When rare earth is added to the steel, spherical rare earth sulfides or sulfur oxides are formed, replacing long strip-shaped manganese sulfide inclusions, so that the morphology of sulfides can be completely controlled and the toughness and plasticity of the steel are improved. Therefore, the content of the RE element in the present invention is controlled to be 0.0005 - 0.005%.

[0029] B: In the high surface quality and ultra-high hole-expanding steel of the present invention, adding an appropriate amount of the B element to the steel can effectively increase the hardenability of the steel and inhibit the recrystallization of austenite. Therefore, the content of the B element in the present invention is controlled to be between 0.0002 - 0.0030%.

[0030] Furthermore, in the ultra-high hole-expanding steel with high surface quality according to the present invention, among the inevitable impurities, P ≤ 0.025% and S ≤ 0.006%; preferably, S ≤ 0.003%.

[0031] In the above technical solution, both P element and S element are impurity elements in steel. Under the allowable technical conditions, in order to obtain steel with better performance and higher quality, the content of impurity elements in the material should be reduced as much as possible.

[0032] P: In the present invention, the P element will exacerbate the segregation of slab composition during continuous casting, resulting in non-uniform structure. Therefore, the present invention controls the content of P element ≤ 0.025%.

[0033] S: In the present invention, sulfur is an impurity element in steel. S is likely to form MnS in steel, and the quantity and morphology of sulfides in steel directly affect the hole-expanding rate of the steel plate; moreover, the quantity and morphology of inclusion elements have a great influence on the hole-expanding performance of the steel plate, especially the strip-shaped sulfide inclusions are prone to cause cracks during deformation. Therefore, the present invention controls the content of S element must ≤ 0.006%, further, preferably, controls the content of S element ≤ 0.003%.

[0034] N: The influence of nitrogen element on the performance of steel is similar to that of carbon and phosphorus. With the increase of nitrogen content, the strength of steel can be significantly improved, but the plasticity, especially the toughness, is also significantly reduced, the weldability becomes poor, and the cold brittleness is exacerbated; at the same time, it increases the aging tendency, cold brittleness and hot brittleness, and damages the welding performance, cold bending performance and hole-expanding performance of steel. Therefore, the present invention controls the content of N element must ≤ 0.0070%, further, preferably, controls the content of N element ≤ 0.0065%.

[0035] Ti eff > 0.015%, Ti eff = Ti - 3.42×N - 3×S. This formula is based on the atomic ratio of elements forming compounds between Ti and N, and the atomic ratio of elements forming compounds between Ti and S, to control Ti eff > 0.015%. The requirement for the Ti content to form an effective precipitation strengthening effect. Ensure that on the basis of low-carbon composition design, the strength and hole-expanding rate are improved.

[0036] The manufacturing method of the ultra-high hole-expanding steel with high surface quality according to the present invention includes the following steps:

[0037] 1) Smelting and casting

[0038] Smelt and refine according to the above composition, control the S content ≤ 0.006%, and cast into billets;

[0039] 2) Heating;

[0040] 3) Hot rolling

[0041] The rough rolling temperature is 990 - 1100 °C; the finish rolling temperature is 820 - 900 °C; then laminar cooling is carried out, adopting two-stage cooling: the cooling rate of the first stage is 6 - 20 °C / s, the cooling time is 3 - 8 s, the second stage is 60 - 120 °C / s, the cooling time is 2 - 4 s, and then coiling is carried out;

[0042] 4) Pickling

[0043] Cool naturally to below 50 °C and then pickle.

[0044] Preferably, in step 1), the S content is controlled to be ≤0.003%.

[0045] Preferably, in step 2), the heating temperature is 1100 - 1260 °C, and the heating time in the furnace is 160 - 250 min

[0046] Preferably, in step 3), the coiling temperature is 500 - 580 °C.

[0047] In the manufacturing method of the present invention:

[0048] In the smelting process, the steelmaking composition needs to meet the chemical composition design requirements of the present invention, and during smelting, deep desulfurization treatment of a conventional LF furnace can be specifically adopted to control the sulfur content of the molten steel to be ≤0.006%, preferably the sulfur content is ≤0.003%. After smelting is completed, for the molten steel after tapping, slab can be obtained by continuous casting accordingly.

[0049] In step 4), the steel coil obtained by coiling after hot rolling can be naturally cooled to below 50 °C, and then the scale on the steel plate can be removed by the pickling unit in a hot hydrochloric acid solution; and after pickling is completed, the pickled steel plate can be further subjected to rinsing, squeezing, drying and then oiling treatment.

[0050] In step 2), the heating temperature is controlled to be 1100 - 1260 °C, and the heating time in the furnace is 160 - 250 min to realize the austenitization of the slab and the effective solid solution of alloying elements.

[0051] In step 3), the laminar cooling adopts two-stage cooling, that is, the control of the two-stage cooling rate:

[0052] The cooling rate of the first stage is 6 - 20 °C / s, the cooling time is 3 - 8 s, and the low cooling rate in the first stage realizes the transformation of ferrite structure and the precipitation strengthening of Ti element, and controls the content of ferrite structure to be above 90%.

[0053] The cooling rate of the second stage is 60 - 120 °C / s, the cooling time is 2 - 4 s, and the remaining austenite is transformed into bainite under the high cooling rate to avoid the formation of structures such as pearlite that deteriorate the hole expansion rate.

[0054] In step 3), the coiling temperature is controlled at 500 - 580 °C to control the retained austenite to achieve bainite transformation.

[0055] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0056] Different from the conventional existing high hole-expanding steel which mainly adopts the design of adding Si and high Mn, in the composition design of the present invention, a low-carbon composition design is adopted to improve the hole-expanding rate. At the same time, the addition of Ti element is combined, and Ti eff > 0.015% is controlled to form an effective precipitation strengthening effect, thereby avoiding the reduction of mechanical properties caused by the decrease of carbon content. The poor coating caused by red scale is avoided through the low-Si design, and the effective guarantee of strength is achieved through the low-Mn combined with microalloying element design. The steel has a good match of better mechanical properties, high hole-expanding rate and high surface quality, with its yield strength Rel ≥ 305 MPa, tensile strength Rm ≥ 440 MPa, elongation A ≥ 34%, and hole-expanding rate λ ≥ 110%, which can meet the requirements of the steel performance for application scenarios such as the automotive industry chassis and complex stamping parts, and has a very good promotion and application prospect.

[0057] Based on the composition design of the present invention, the hot rolling process window is broadened, combined with the control of subsequent two-stage laminar cooling, so that the microstructure in the steel is more than 90% ferrite structure, and the remaining is less than 10% second-phase bainite, to ensure that the prepared steel has high mechanical properties and high hole-expanding properties. Description of the Drawings

[0058] Figure 1 It is a metallographic structure photo observed under an optical microscope after the steel in Example 1 of the present invention is corroded by nitric acid alcohol. Detailed Embodiments

[0059] The present invention will be further described below in conjunction with the embodiments and the drawings.

[0060] The composition of the steel in the embodiment of the present invention is shown in Table 1, Table 2 shows the production process parameters of the steel in the embodiment of the present invention; Table 3 shows the mechanical properties of the steel plate in the embodiment of the present invention.

[0061] The steel in the embodiment and the comparative example of the present invention are respectively sampled, and their surface qualities are observed. It is found that there are no coating defects such as red iron oxide on the surface of the embodiment, and their microstructures are measured. The corresponding yield strength, tensile strength, elongation and hole-expanding rate of the steel plate are detected. The relevant observation and measurement results are shown in Table 3 in detail.

[0062] Tensile test: At room temperature of 20°C, the strain rate during yielding should be controlled within 0.00025 - 0.0025 / s, and the test should be carried out according to the conditions of GB / T 228. Thus, the yield strength Rel, tensile strength Rm, and elongation A of the sample steel plates of Examples 1 - 9 and Comparative Examples 1 - 3 were measured.

[0063] Hole expansion rate detection test: Take the sample steel plates of each example and comparative example with dimensions of 150×150 mm as the initial hole diameter d[1], punch a hole with a diameter of 10 mm, and expand the hole using a conical punch with a vertex angle of 60°. At the same time, measure the hole diameter d[2] when cracks penetrate the plate thickness in the punched part. Combining the above hole diameters d[1] and d[2], calculate the hole expansion rate from the following formula:

[0064] Hole expansion rate (λ) = [(d[2] - d[1]) / d[1]] × 100%.

[0065] Figure 1 The figure shows the metallographic structure photograph of the high-surface and high-hole-expansion steel of Example 1 observed under an optical microscope after being etched with nitric acid alcohol.

[0066] As Figure 1 shown, the microstructure of the high-surface and high-hole-expansion steel prepared in Example 1 is ferrite + bainite, and the volume phase ratio of ferrite ≥ 90%.

[0067] It can be seen from Table 3 that the comprehensive performance of the high-surface and high-hole-expansion steels of Examples 1 - 9 of the present invention is significantly better than that of the comparative steels of Comparative Examples 1 - 3. The surface of the steel plate in the example of the present invention has no coating defects such as red iron oxide, the surface quality is high, there is no coating defect caused by red scale, and at the same time, its microstructure is all ferrite + bainite, and the volume phase ratio of ferrite ≥ 90%. In Comparative Examples 1 - 3, the volume phase ratio of ferrite in Comparative Example 1 and Comparative Example 3 does not meet the hole expansion rate requirements; the surface quality of Comparative Example 2 is poor, and its microstructure does not meet the design requirements, and its microstructure is only ferrite.

[0068] In the present invention, for the high-surface-quality and ultra-high-hole-expansion steels of Examples 1 - 9, the yield strength Rel ≥ 305 MPa, the tensile strength Rm ≥ 440 MPa, the elongation A ≥ 34%, and the hole expansion rate λ ≥ 110%. It not only has good strength and plasticity, but also has a very high hole expansion rate and excellent surface quality.

[0069] In the chemical composition design of Comparative Example 1, the element content is not within the range required by the present invention, and the content of C element is on the high side. Finally, the obtained microstructure is ferrite + bainite. Although the yield strength and tensile strength of the prepared comparative steel plate can meet the design requirements, its hole expansion rate does not meet the requirements.

[0070] In the design of chemical composition of Comparative Example 2, the element content is not within the scope required by the present invention, and its Si content is on the high side. The finally obtained microstructure is ferrite. Although the yield strength, tensile strength, and hole expansion rate of the prepared comparative steel plate can meet the design requirements, the excessive Si content in the steel easily leads to red scale, and high surface requirements cannot be achieved.

[0071] In the design of chemical composition of Comparative Example 3, the element content is not within the scope required by the present invention, and its C element content is on the high side. The finally obtained microstructure is ferrite + bainite, but the elongation and hole expansion rate of the prepared comparative steel plate are on the low side and do not meet the design requirements of the present invention.

[0072]

[0073]

[0074]

Claims

1. A super high hole-expanding steel with high surface quality, the chemical composition of which is by weight percentage: C: 0.02 - 0.08%, Si ≤ 0.1%, Mn: 0.4 - 0.8%, Al: 0.015 - 0.05%, Ti: 0.03 - 0.08%, N ≤ 0.007%; and at least one of Nb: 0.001 - 0.05%, V: 0.002 - 0.06%, Cr: 0.002 - 0.6%, Ca: 0.0005 - 0.0050%, RE: 0.0005 - 0.005%, B: 0.0002 - 0.0030%, the rest includes Fe and other inevitable impurities, and it needs to satisfy simultaneously: Ti eff > 0.015%, Ti eff = Ti - 3.42 × N - 3 × S.

2. The super high hole-expanding steel with high surface quality according to claim 1, characterized in that, N≤0.0065％。 3. The super high hole-expanding steel with high surface quality according to claim 1 or 2, characterized in that, The balance is Fe and other inevitable impurities.

4. The super high hole-expanding steel with high surface quality according to claim 1 or 2 or 3, characterized in that, In the inevitable impurities, P ≤ 0.025%, S ≤ 0.006%, preferably S ≤ 0.003%.

5. The super high hole-expanding steel with high surface quality according to claim 1 or 2 or 3 or 4, characterized in that, The microstructure of the hole-expanding steel is ferrite + bainite, and the volume ratio of ferrite is ≥ 90%.

6. The super high hole-expanding steel with high surface quality according to claim 1 or 2 or 3 or 4 or 5, characterized in that, The yield strength Rel of the hole-expanding steel ≥ 305 MPa, the tensile strength Rm ≥ 440 MPa, the elongation A ≥ 34%, and the hole-expanding rate λ ≥ 110%.

7. The manufacturing method of the super high hole-expanding steel with high surface quality according to any one of claims 1 - 6, characterized in that, comprises the following steps: 1) Smelting and casting Smelt and refine according to the composition described in claim 1 or 2 or 3, control the S content ≤ 0.006%, and cast into billets; 2) Heating; 3) Hot rolling The rough rolling temperature is 990 - 1100 °C; the finishing rolling final rolling temperature is 820 - 900 °C; then carry out laminar cooling, adopting two-stage cooling: the cooling rate of the first stage is 6 - 20 °C / s, the cooling time is 3 - 8 s, the second stage is 60 - 120 °C / s, the cooling time is 2 - 4 s, and then coil; 4) Pickling Cool naturally to below 50 °C and pickle.

8. The manufacturing method according to claim 7, characterized in that, In step 1), control the S content ≤ 0.003%.

9. The manufacturing method according to claim 7, characterized in that, In step 2), the heating temperature: 1100 - 1260 °C, and the heating time in the furnace is 160 - 250 min.

10. The manufacturing method according to claim 7, characterized in that, In step 3), the coiling temperature is 500 - 580 °C.

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