Steel for hot-rolled ultrahigh-strength welded pipe for automobile and production method of steel

Abstract

The invention provides steel for a hot-rolled ultrahigh-strength welded pipe for an automobile and a production method of the steel. The steel comprises the following main chemical components in percentage by weight: 0.04-0.08% of C, 0.2-0.35% of Si, 1.2-2.0% of Mn, 0.07-0.12% of Ti, 0.2-0.8% of Cr, 0.01-0.05% of Nb, less than or equal to 0.006% of S, less than or equal to 0.008% of N, less than or equal to 0.020% of P, more than or equal to 0.16 and less than or equal to 0.24% of C, more than or equal to 1.6 and less than or equal to 2.5% of Cr and Mn, and the balance of Fe and inevitable impurities. The production method comprises the following steps: heating: the heating temperature is 1230 to 1300 DEG C, and the heating time is 140 to 200 minutes; rough rolling is conducted, specifically, the casting blank is roughly rolled into an intermediate blank, and the rough rolling outlet temperature is 1030-1080 DEG C; finish rolling: the finish rolling temperature of finish rolling is 840-880 DEG C; coiling: the coiling temperature is 500 to 580 DEG C; wherein the heating temperature ranges from 1230 DEG C to 1270 DEG C, and the heating time ranges from 150 min to 180 min. The problem that the transverse and longitudinal strength fluctuation of the existing steel for the hot-rolled ultrahigh-strength welded pipe for the automobile is large can be solved.

Description

Technical Field

[0001] This invention relates to the field of hot-rolled ultra-high strength welded pipe steel, and particularly to a hot-rolled ultra-high strength welded pipe steel for automobiles and its production method. Background Technology

[0002] In today's automotive industry, energy conservation and emission reduction, along with improved safety performance, have become two core development trends. With increasing global environmental awareness and increasingly stringent emission standards imposed by various countries, reducing vehicle energy consumption has become an urgent task for automakers. Related research indicates that a 10% reduction in vehicle weight can save approximately 6-8% in fuel consumption. Against this backdrop, automotive steel needs to possess the advantages of high strength, high toughness, and lightweight design to meet the dual requirements of weight reduction and improved safety performance. High-strength steel plates, with their high weight reduction potential and high fatigue strength, have become the most economical and effective solution for simultaneously achieving weight reduction and safety. In automobile manufacturing, hot-rolled ultra-high-strength welded steel is widely used in key components such as driveshaft tubes and vehicle frames. Taking driveshaft tubes as an example, as a crucial component for transmitting power in the automotive transmission system, it has extremely stringent requirements for material performance. On the one hand, the inner surface of the driveshaft tube is in contact with lubricating oil, while the outer surface is in contact with external environmental factors such as atmosphere, water, and fog. Furthermore, the complex and variable driving environment, with different terrains and temperature conditions, poses challenges to the performance of driveshaft tubes, thus requiring them to possess excellent weldability, corrosion resistance, and impact resistance. On the other hand, in the pursuit of lightweight automobiles while prioritizing safety, the use of ultra-high-strength steel can reduce the amount of material used, promoting the lightweighting of components. For example, the wall thickness of the driveshaft tube can be reduced, achieving the goal of lightweighting the automotive driveshaft tube. However, the current application and production of hot-rolled ultra-high-strength welded steel for automotive applications still face numerous challenges. Regarding material properties, the highest domestic industry standard for welded steel pipes for drive shafts is 550 (yield strength ≥ 550 MPa). While some companies have developed ultra-high-strength hot-rolled steel strips for automotive drive shafts (tensile strength ≥ 750 MPa), these utilize high Nb and Mn content and incorporate rare earth elements, resulting in high costs and extremely complex production processes and controls. Internationally, countries such as the United States, Germany, and Japan have only focused on the forming and welding technologies for high-strength automotive drive shafts with yield strengths below 700 MPa, indicating a lack of research and development for materials with even higher strength levels. Furthermore, under the existing process, during the hot rolling and cooling process of alloyed ultra-high strength steel, the different cooling paths result in uneven structure and precipitation in both the longitudinal and transverse directions of the hot-rolled steel coil. This leads to performance fluctuations of 100-400MPa in the longitudinal direction and 60-200MPa in the transverse direction of the hot-rolled raw material, which makes it easy for quality problems such as poor coiling and cracking to occur during the processing. Summary of the Invention

[0003] The purpose of this invention is to provide a hot-rolled ultra-high strength welded pipe steel for automobiles and its production method, so as to solve the problem of large fluctuations in transverse and longitudinal strength of existing hot-rolled ultra-high strength welded pipe steel for automobiles.

[0004] To solve the above-mentioned technical problems, the present invention provides a hot-rolled ultra-high strength welded pipe steel for automobiles, the main chemical composition and its weight percentage are as follows: C: 0.04~0.08%, Si: 0.2~0.35%, Mn: 1.2~2.0%, Ti: 0.07~0.12%, Cr: 0.2~0.8%, Nb: 0.01~0.05%, S≤0.006%, N≤0.008%, P≤0.020%, and 0.16≤C+Nb+Ti≤0.24, 1.6≤Cr+Mn≤2.5, and the balance being Fe and unavoidable impurities.

[0005] Optionally, the weight percentage of C is 0.06-0.08%.

[0006] Optionally, the weight percentage of Mn is 1.6% to 2.0%.

[0007] Optionally, the weight percentage of Ti is 0.07~0.10%.

[0008] Optionally, the weight percentage of Cr is 0.5% to 0.8%.

[0009] Optionally, the weight percentage of Nb is 0.01~0.03%.

[0010] The present invention also provides a method for producing the above-mentioned hot-rolled ultra-high strength welded pipe steel for automobiles, comprising: Heating: The heating temperature is 1230~1300℃, and the heating time is 140~200min; Rough rolling: The slab is rough rolled to the intermediate slab, and the exit temperature of the rough rolling is 1030~1080℃; Finishing rolling: The finishing rolling temperature is 840~880℃; Winding: Winding temperature is 500~580℃.

[0011] Optionally, the heating temperature is 1230~1270℃ and the heating time is 150~180min.

[0012] Optionally, the finishing rolling temperature is 840~860℃.

[0013] Optionally, the winding temperature is 500~540℃.

[0014] The present invention provides a hot-rolled ultra-high strength welded pipe steel for automobiles and its production method, which has the following beneficial effects: The steel used for hot-rolled ultra-high strength welded pipes for automobiles in this invention has a yield strength ≥700MPa, tensile strength ≥780MPa, elongation ≥16%, and transverse and longitudinal strength fluctuations ≤80MPa. The expansion rate of the steel used to make precision welded pipes is ≥20%, and the flattening rate can reach more than 60%. It has excellent comprehensive performance and good processing performance.

[0015] By constructing a low-carbon + Nb / Ti / Cr composite microalloying low-cost composition system and precisely controlling the content and interaction of elements such as C, Nb, Ti, and Cr through a "multi-element synergistic proportioning rule," and by implementing refined control of the entire process parameters, the total amount of solid solution strengthening, precipitation strengthening, and grain refinement strengthening is matched and synergistically enhanced. While ensuring ultra-high strength performance of steel with a yield strength ≥700MPa and tensile strength ≥780MPa, the low-carbon design reduces hardening tendency, and microalloying elements refine the grains in the weld heat-affected zone and suppress cold crack initiation. Simultaneously, it meets the plasticity requirements of elongation ≥16% and the formability requirements of tube expansion rate ≥20% and flattening rate ≥60%. This successfully breaks through the traditional bottleneck of "high strength and weldability / formability mutually restricting each other" in ultra-high strength steel, and the composition system avoids dependence on high-priced alloys, combining cost advantages and processing feasibility. Attached Figure Description

[0016] Figure 1 The image shows the metallographic structure of the hot-rolled ultra-high strength welded pipe steel for automobiles obtained in Example 1 of this invention. Detailed Implementation

[0017] The technical solution and the technical problem solved by the present invention will be described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0018] This invention provides a hot-rolled ultra-high strength welded pipe steel for automobiles, the main chemical composition and their weight percentages of which are: C: 0.04~0.08%, Si: 0.2~0.35%, Mn: 1.2~2.0%, Ti: 0.07~0.12%, Cr: 0.2~0.8%, Nb: 0.01~0.05%, S≤0.006%, N≤0.008%, P≤0.020%, and 0.16≤C+Nb+Ti≤0.24, 1.6≤Cr+Mn≤2.5, and the balance being Fe and unavoidable impurities.

[0019] The hot-rolled ultra-high strength welded pipe steel for automobiles in this embodiment of the invention has a yield strength ≥700MPa, tensile strength ≥780MPa, elongation ≥16%, transverse and longitudinal strength fluctuation ≤80MPa, and the expansion rate of the precision welded pipe is ≥20%, and the flattening rate can reach more than 60%. It has excellent comprehensive performance and good processing performance.

[0020] The alloy types and their contents are selected based on the following reasons: C: Solid solution C will increase the yield strength and tensile strength of the base metal and weld area, but too high carbon content will lead to a decrease in plasticity and increase the hardening tendency during welding, resulting in cold cracks in the heat-affected zone of the weld. Taking all factors into consideration, the C content should be controlled at 0.04~0.08%.

[0021] Si: Si has a significant solid solution strengthening effect, but too high a content will lead to a decrease in plasticity. At the same time, increasing the Si content will increase the oxidizability of steel, making it easy to form SiO2 inclusions during welding, which will worsen the weld toughness. Taking all factors into consideration, the Si content should be controlled at 0.2~0.35%.

[0022] Mn: Mn dissolves in ferrite to produce solid solution strengthening, while inhibiting austenite grain growth and refining the microstructure. However, excessive carbon content in Mn can lead to decreased plasticity and increase the tendency to harden during welding. Considering all factors, the Mn content should be controlled at 1.2~2.0%.

[0023] Ti combines with C and N to form TiC and TiN particles, inhibiting austenite grain growth and improving strength through precipitation strengthening. It is an effective strengthening element to compensate for the decrease in carbon equivalent. Ti can also refine the grains in the weld heat-affected zone and reduce hardening tendency. However, excessively high Ti content can easily lead to oxide inclusions in the weld. Considering all factors, the Ti content should be controlled between 0.07% and 0.12%.

[0024] Nb: Nb can inhibit austenite grain growth by forming NbC precipitates, and also improve strength through precipitation strengthening. However, excessive Nb content may lead to lower strength in the welded area compared to the base metal, resulting in softening and reducing the fatigue life of the welded pipe. Considering all factors, the Nb content should be 0.01~0.05%.

[0025] P: P tends to segregate at grain boundaries, causing cold brittleness in steel. To reduce the risk of cold brittle cracking, P should be controlled below 0.020%.

[0026] S: S easily forms low-melting-point FeS inclusions, causing hot brittleness and reducing impact toughness. To reduce the incidence of weld cracks, S needs to be controlled below 0.006%.

[0027] Nitrogen (N): During the welding process, nitrogen can easily enter the weld, forming porosity or aging embrittlement. Nitrogen content must be controlled below 0.008%.

[0028] To ensure that both strength and elongation meet the requirements, it is necessary to control 0.16≤C+Nb+Ti≤0.24 and 1.6≤Cr+Mn≤2.5.

[0029] This invention also provides a method for producing hot-rolled ultra-high strength welded pipe steel for automobiles, comprising: Heating: The heating temperature is 1230~1300℃, and the heating time is 140~200min; Rough rolling: The slab is rough rolled to the intermediate slab, and the exit temperature of the rough rolling is 1030~1080℃; Finishing rolling: The finishing rolling temperature is 840~880℃; Winding: Winding temperature is 500~580℃.

[0030] The present invention relates to a method for producing hot-rolled ultra-high-strength welded pipe steel for automobiles, which has the following technical features: The heating temperature is set to no less than 1230℃, and the heating time is no less than 140 minutes. The core purpose of this setting is to promote the full integration of microalloying elements such as Nb and Ti into the austenite lattice. During the subsequent rolling and cooling process, these elements will be dispersed in the form of nanoscale precipitates, refining the grains by pinning grain boundaries and improving the strength and toughness of the steel through precipitation strengthening mechanisms. However, the heating parameters must be strictly controlled. If the temperature is too high or the time is too long, a dense and thick iron oxide scale will form on the surface of the billet. This scale is difficult to completely remove during the rolling process, and the residue will embed into the surface of the steel, resulting in a decrease in surface quality.

[0031] Strictly limiting the roughing mill exit temperature to the range of 1030-1080℃ promotes the formation of an iron oxide scale structure dominated by FeO on the billet surface. The loose FeO layer is easy to peel off, which is beneficial for subsequent descaling processes. If the exit temperature is too low, the iron oxide scale will be dominated by Fe3O4, which has a dense structure and is firmly bonded to the matrix, significantly increasing the difficulty of descaling. If the temperature is too high, the oxidation rate accelerates, and the total thickness of the iron oxide scale increases significantly. Even if descaling is completed, pitting or color differences may be left on the steel surface. In addition, high-temperature rolling can also cause uneven metal flow on the steel surface, forming rough surface defects similar to orange peel.

[0032] The final rolling temperature is strictly controlled within the range of 840-880℃. This temperature range allows for precise regulation of the austenite state, fully leveraging the strengthening effect of microalloying elements. Rolling at this temperature effectively refines the austenite grains, while simultaneously maximizing the precipitation strengthening effect of microalloying elements. This ultimately achieves a synergistic effect of fine-grain strengthening and precipitation strengthening, endowing the steel with an excellent balance of high strength and toughness. If the final rolling temperature is too high, the austenite grains will rapidly grow and coarsen, leading to a significant decrease in the steel's strength and toughness. If the temperature is too low, the austenite plasticity decreases drastically, and the deformation resistance increases sharply, not only increasing rolling energy consumption but also making it difficult to ensure high-precision thickness control of the steel.

[0033] Setting the coiling temperature between 500-580℃ is crucial. Below 480℃, the atomic diffusion rate is too low, resulting in insufficient bonding reaction between Ti and C and a precipitation rate of less than 60%, leading to a lack of precipitation strengthening effect. Above 580℃, the atomic diffusion rate accelerates significantly, causing the already formed TiC / TiN to coarsen rapidly, also affecting the precipitation strengthening effect. Furthermore, when the coiling temperature exceeds 580℃, the large temperature difference between the inner and outer rings and along the width direction during the cooling process after the steel coil is unwound leads to fluctuations in transverse and longitudinal properties. Additionally, 480-580℃ promotes uniform precipitation of bainitic ferrite, preventing the formation of large bainite clusters. Below 480℃, bainitic ferrite precipitation is slow, easily resulting in "blocky bainite" and decreased toughness. Above 580℃, the bainite cluster size increases, grain boundary bonding decreases, and cold cracking is more likely to occur in the weld heat-affected zone.

[0034] The present invention will be described in detail below through specific embodiments 1 to 10 and comparative examples 1 to 7.

[0035] The hot-rolled ultra-high strength welded pipe steel for automobiles described in Examples 1-10 has its chemical composition and weight percentage designed according to Table 1 below. Table 1 also shows the chemical composition and weight percentage of the steel in Comparative Examples 1-7.

[0036] Table 1 Chemical composition (wt%) of each example The production methods for hot-rolled ultra-high strength welded pipe steel for automobiles described in Examples 1-10 sequentially include heating, primary rolling, finish rolling, and coiling. The difference lies in the different process parameter designs involved in each process, as shown in Table 2 below. Table 2 also provides the specific process parameters for Comparative Examples 1-7.

[0037] Table 2 Rolling process parameters for each embodiment The mechanical properties of the hot-rolled ultra-high strength welded pipe steel for automobiles produced in Examples 1-10 are shown in Table 3, where the yield strength is 700-794 MPa, the tensile strength is 780-918 MPa, and the elongation (A) is... 50mm The cross-sectional area of ​​the precision welded pipe is 16.2%~21.6%, the transverse and longitudinal strength fluctuates 58~80MPa, the expansion rate of the precision welded pipe is 20.3%~29.6%, and the flattening rate of the precision welded pipe is 60.2%~78.3%.

[0038] Table 3 also provides the performance indicators for comparative examples 1 to 7.

[0039] Among them, the yield strength of Comparative Example 3 was 682 MPa, which did not reach the minimum value of 700 MPa in Examples 1-10 of the present invention; the tensile strength of Comparative Examples 3, 4 and 5 did not reach the minimum value of 780 MPa in Examples 1-10 of the present invention; the elongation (A) of Comparative Examples 1, 2 and 6 was... 50mm The longitudinal strength fluctuation of Comparative Example 5 did not reach the lowest value of 16.2% in Examples 1-10 of the present invention; the expansion rate of the precision welded pipes of Comparative Examples 1, 2 and 3 did not reach the lowest value of 20.3% in Examples 1-10 of the present invention; and the flattening rate of the precision welded pipes of Comparative Examples 1, 2, 3, 5 and 7 did not reach the lowest value of 60.2% in Examples 1-10 of the present invention.

[0040] In addition, from an overall perspective, the elongation of Comparative Example 1 (A) 50mm The expansion rate and flattening rate of the precision welded pipe did not meet the requirements for hot-rolled ultra-high strength welded pipe steel for automobiles in this embodiment; the elongation (A) of Comparative Example 2 was also insufficient. 50mm The expansion rate and flattening rate of the precision welded pipe did not meet the requirements for hot-rolled ultra-high strength welded pipe steel for automobiles in this embodiment; the yield strength, tensile strength, expansion rate, and flattening rate of the precision welded pipe in Comparative Example 3 did not meet the requirements for hot-rolled ultra-high strength welded pipe steel for automobiles in this embodiment; the tensile strength in Comparative Example 4 did not meet the requirements for hot-rolled ultra-high strength welded pipe steel for automobiles in this embodiment; the tensile strength, longitudinal strength fluctuation, and flattening rate of the precision welded pipe in Comparative Example 5 did not meet the requirements for hot-rolled ultra-high strength welded pipe steel for automobiles in this embodiment; the elongation (A) of Comparative Example 6... 50mm The steel used in this embodiment for hot-rolled ultra-high strength welded pipes for automobiles did not meet the requirements. The flattening ratio of the precision welded pipe in Comparative Example 7 did not meet the requirements for the steel used in this embodiment for hot-rolled ultra-high strength welded pipes for automobiles.

[0041] In summary, the hot-rolled ultra-high-strength welded pipe steel for automobiles in Examples 1-10 of this invention possesses high strength, high weldability, and high formability, while the hot-rolled ultra-high-strength welded pipe steel for automobiles in Comparative Examples 1-7 does not possess these three characteristics simultaneously. Examples 1-10 of this invention, while ensuring ultra-high strength performance with a yield strength ≥700MPa and tensile strength ≥780MPa, reduce hardening tendency through low-carbon design, refine grain size in the weld heat-affected zone using microalloying elements, and suppress cold crack initiation. Simultaneously, they meet the plasticity requirement of elongation ≥16% and the formability requirements of pipe expansion ≥20% and flattening ≥60%, successfully overcoming the traditional bottleneck of "mutual constraint between high strength and weldability / formability" in ultra-high-strength steel. Furthermore, the composition system avoids dependence on high-priced alloys, combining cost advantages with processing feasibility.

[0042] Table 3 Typical performance of each embodiment The metallographic structures of the finished products in each embodiment are similar. The observation results of Example 1 are as follows: Figure 1 As shown, Figure 1 The image shows the metallographic structure of the hot-rolled ultra-high strength welded pipe steel for automobiles obtained in Example 1 of this invention. The microstructure of the hot-rolled ultra-high strength welded pipe steel is mainly bainite, with dispersed carbides distributed in an irregular ferrite matrix. The ferrite matrix itself has a high dislocation density and fine substructure. The dispersed carbides exert a strong pinning effect on dislocation movement, resulting in a significant hardening effect, making the product's yield strength ≥700MPa and tensile strength ≥780MPa. Dislocations in the ferrite can be activated, multiplied, and moved under stress, allowing for continuous plastic deformation and exhibiting excellent plasticity (elongation ≥16%). The bainitic structure has an extremely high work hardening index. During deformation, its high dislocation density further increases sharply, leading to a rapid rise in flow stress. This allows deformation to be evenly distributed to a larger portion of the pipe body, avoiding deformation concentration at the flared edge and achieving a high flaring rate (≥20%). After cooling, bainitic structure easily forms fine-grained bainite with good toughness, rather than coarse and brittle structure, which makes the performance of the entire steel pipe section uniform and the deformation coordinated during flattening, achieving a flattening ratio of ≥60%.

[0043] The above description is merely a description of preferred embodiments of the present invention and is not intended to limit the scope of the present invention in any way. Any changes or modifications made by those skilled in the art based on the above disclosure shall fall within the protection scope of the claims.

Claims

1. A type of hot-rolled ultra-high strength welded pipe steel for automobiles, characterized in that, Its main chemical components and their weight percentages are as follows: C: 0.04~0.08%, Si: 0.2~0.35%, Mn: 1.2~2.0%, Ti: 0.07~0.12%, Cr: 0.2~0.8%, Nb: 0.01~0.05%, S≤0.006%, N≤0.008%, P≤0.020%, and 0.16≤C+Nb+Ti≤0.24, 1.6≤Cr+Mn≤2.5, with the balance being Fe and unavoidable impurities.

2. The hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 1, characterized in that, The weight percentage of C is 0.06-0.08%.

3. The hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 1, characterized in that, The weight percentage of Mn is 1.6~2.0%.

4. The hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 1, characterized in that, The weight percentage of Ti is 0.07~0.10%.

5. The hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 1, characterized in that, The weight percentage of Cr is 0.5-0.8%.

6. The hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 1, characterized in that, The weight percentage of Nb is 0.01~0.03%.

7. A method for producing hot-rolled ultra-high strength welded pipe steel for automobiles as described in any one of claims 1-6, characterized in that, include: Heating: The heating temperature is 1230~1300℃, and the heating time is 140~200min; Rough rolling: The slab is rough rolled to the intermediate slab, and the exit temperature of the rough rolling is 1030~1080℃; Finishing rolling: The finishing rolling temperature is 840~880℃; Winding: Winding temperature is 500~580℃.

8. The method for producing hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 7, characterized in that, The heating temperature is 1230~1270℃, and the heating time is 150~180min.

9. The method for producing hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 7, characterized in that, The finishing rolling temperature is 840~860℃.

10. The method for producing hot-rolled ultra-high strength welded pipe steel for automobiles as described in claim 7, characterized in that, The winding temperature is 500~540℃.

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