Method for solving toughness reduction of automobile beam steel under continuous work hardening

Abstract

The invention relates to a method for solving the problem of toughness reduction of automobile beam steel under continuous work hardening, which comprises the following steps: step 1, component design: the components comprise carbon, silicon, manganese, phosphorus, sulfur, nitrogen, aluminum, titanium and niobium, and the balance is unavoidable impurity elements, step 2, molten iron, scrap steel and slagging materials are added into the designed components in a converter for smelting, and step 3, casting is carried out. The method comprises the following steps of 1, feeding converter molten steel obtained in the step 2 into an LF furnace for refining, and carrying out desulfurization treatment and sulfide removal in the process, 4, feeding the molten steel treated in the LF furnace into an RH vacuum furnace for treatment so as to remove oxides in the molten steel, and 5, feeding the molten steel subjected to duplex refining treatment in the step 4 into a continuous casting process to produce a plate blank, 6, the plate blank is heated in the heating furnace of the hot rolling plant; 7, the plate blank is subjected to rough rolling and finish rolling to form a plate coil; and 8, when the plate coil is coiled, the coiling temperature of the plate coil is set to be 590 DEG C + / -20 DEG C.

Description

Technical Field

[0001] The present invention relates to a method, specifically to a method for solving the problem of reduced toughness of automotive beam steel under continuous work hardening, belonging to the technical fields of steelmaking and hot rolling. Background Art

[0002] During the processing of automotive beam steel coils with a tensile strength of 500 - 630 MPa, stamping cracking problems occur. The reason is that during the processing of this steel grade, its processing characteristics belong to continuous processing, which causes the material to continuously work harden, resulting in an increase in material hardness and a decrease in toughness. This continuous processing leading to continuous work hardening of the material is the direct cause of material cracking.

[0003] For this problem, the existing solutions are as follows:

[0004] Patent: CN 111715755 "A stamping and forming die for automotive beam parts" discloses a stamping and forming die for automotive beam parts, which avoids wrinkling and cracking at the forming bending part through the optimized design of the die.

[0005] Patent: CN 109518074 "An economical high-toughness 800 MPa grade automotive beam steel and its production method". The present invention provides a production method for an economical high-toughness 800 MPa grade automotive beam steel, which can avoid cracking during processes such as shearing, stamping, bending, and welding when used as automotive structural parts. However, Mo strengthening is used in the production process, and the alloy cost is relatively high.

[0006] The solution of the present technology to the problem of reduced material toughness and resulting stamping cracking of this automotive beam steel coil under continuous work hardening is to solve this stamping cracking problem from three aspects: composition design, removal of inclusions in the steelmaking process, and homogenization of the material structure in the hot rolling process, which is different from the solutions to this problem in the above-mentioned existing technologies. Summary of the Invention

[0007] The present invention precisely aims at the problems existing in the prior art and provides a method for solving the problem of reduced toughness of automotive beam steel under continuous work hardening. This technical solution solves the problem of reduced toughness of this steel grade under continuous work hardening from aspects such as composition design, inclusion control in the steelmaking refining process, and design of the heating process of hot rolling slabs and the coiling temperature of coils.

[0008] To achieve the above object, the technical solution of the present invention is as follows. A method for solving the problem of reduced toughness of automotive beam steel under continuous work hardening, the method comprising the following steps:

[0009] Step 1: Composition design. The components designed in the technology of the present invention include carbon, silicon, manganese, phosphorus, sulfur, nitrogen, aluminum, titanium, and niobium, and the rest are inevitable impurity elements. Specific ranges: C: 0.06 - 0.09%, Si: 0.15 - 0.25%, Mn: 1.0 - 1.5%, P: ≤0.022%, S: ≤0.008%, N: ≤0.006%, Al: 0.01 - 0.045%, Ti: 0.008 - 0.018%, Nb: 0.015 - 0.035%. The reasons for the design of these elements are as follows:

[0010] 1) Carbon: It is the most economical solid-solution strengthening element and can effectively improve the strength of the material. If its content is less than 0.06%, the strength requirements of the material cannot be met. However, as the carbon content increases, the toughness of the material will decrease. Therefore, on the premise of ensuring strength, in order to reduce the yield point, increase the elongation rate, and improve toughness, the carbon content is controlled within the range of 0.06 - 0.09%.

[0011] 2) Silicon: It can improve the strength of the material through solid-solution strengthening and can also inhibit the precipitation of carbides. If its content is less than 0.10%, such an effect cannot be exerted; if its content exceeds 0.25%, excessive scale defects will occur on the surface of the hot plate, and silicon forms a substitutional solid solution in the steel, significantly increasing the yield strength of the steel plate and reducing the elongation rate and deep drawing performance of the steel plate. Therefore, the Si content is controlled within the range of 0.15 - 0.25%. This is one of the highlights of the silicon composition design for solving the excessive scale on the surface of hot-rolled coils in this composition system during composition design.

[0012] 3) Manganese: It is an effective solid-solution strengthening element and can inhibit the hot brittleness of the steel plate caused by sulfur. However, the addition of manganese will lower the starting temperature of ferrite transformation, resulting in the refinement of ferrite grains, an increase in yield strength, and a decrease in elongation rate. Therefore, on the premise of ensuring the strength of the material, the manganese content is controlled within 1.0 - 1.5% to improve the toughness of the material of this steel grade.

[0013] 4) Phosphorus, sulfur: Phosphorus segregates at the grain boundaries, easily causing cold working brittleness and producing solid-solution strengthening, resulting in a decrease in the toughness of the material. Therefore, the P content in the steel should be minimized. Sulfur combines with titanium in the steel to form TiC 2 S 2 compounds, reducing the effect of titanium in fixing carbon and nitrogen interstitial atoms. At the same time, non-metallic inclusion sulfides have obvious directionality, which is unfavorable to the ductility of the steel and causes a decrease in toughness. Therefore, the sulfur content should be reduced. To sum up, the designed contents of phosphorus and sulfur for this steel grade in this technology are: P ≤ 0.022%, S ≤ 0.008%.

[0014] 5) Nitrogen: Nitrogen easily combines with aluminum and precipitates AlN during coiling, which hinders the growth of ferrite grains, increases the yield point (the yield ratio increases), reduces the elongation, and causes a decrease in toughness. Therefore, the nitrogen (N) is controlled below 0.006%.

[0015] 6) Aluminum: Aluminum element plays a deoxidizing role during the refining of molten steel, can improve the purity of the steel, and at the same time fixes nitrogen, prevents strain aging of the steel plate, and reduces the consumption of titanium. When the acid-soluble aluminum (Als) content is less than 0.01%, the deoxidation effect will be affected; when it exceeds 0.1%, its effect reaches saturation and resources are wasted. Moreover, excessive precipitation of AlN will also prevent the growth of ferrite grains, reduce the elongation, and is not conducive to the improvement of material toughness. Therefore, the Al content is controlled within the range of 0.01 - 0.045%.

[0016] 7) Titanium: Titanium is an element that forms carbides and nitrides (TiN, TiS, TiC2S2, Ti(CN)), which reduces the number of dissolved carbon and nitrogen atoms in the steel and can increase the recrystallization temperature of ferrite. At the same time, the precipitation temperature of TiN is high, and no solution and precipitation phenomena occur during the entire hot rolling process. Therefore, adding titanium element to fix nitrogen in the steel has the best effect and is beneficial to preventing the decrease in elongation. However, when the titanium content is greater than 0.2%, it is extremely unfavorable for the recrystallization, nucleation, and growth of hot-rolled thin plates. At this time, the precipitated carbonitrides are relatively coarse, and the probability of cracking is extremely high. Moreover, due to the hardness difference between the carbonitrides and the steel matrix, it becomes a crack source during the subsequent material deformation process, causing a decrease in elongation. According to the actual production situation, the control range of titanium is: 0.008 - 0.018%.

[0017] 8) Niobium: Nb is the most commonly used alloying element in the controlled rolling of microalloyed steel. During hot rolling, it can inhibit the recrystallization of austenite, expand the range of the non-recrystallized austenite zone, and can perform austenite rolling at low temperatures, thereby being able to refine grains while also changing the type and strength of the recrystallization and phase transformation textures. When the grain size is small, the metal is easy to deform, and its elongation index is also high. When the niobium content is greater than 0.05%, its strengthening effect is close to saturation, and the alloy cost increases. Therefore, the Nb content is controlled within the range of 0.015 - 0.035%.

[0018] It is found through research that in the above composition design, adopting the combination of niobium and titanium can improve the toughness of the material. Niobium element is a fine grain strengthening element, which can effectively improve the toughness and strength of the material; titanium element is an inexpensive strengthening element, which will greatly increase the strength of the material, but will weaken the toughness of the material to a certain extent.

[0019] The research findings show that if these two elements are used simultaneously and their contents have the following relationship: Nb% = 2 * Ti%, that is, the niobium content is twice the titanium content, the improvement of the material toughness can reach the best effect. The mechanism is to control the content of the strengthening element titanium while increasing the niobium element that can improve the material toughness. In this way, since the strength of the material is controlled, the toughness is improved.

[0020] This invention demonstrates that when two measures (increasing the niobium content and reducing the titanium content) are carried out simultaneously, in addition to improving the strength, there is also a combined effect, that is, the purpose of improving the material toughness can be achieved. This is the second highlight reflected in the design of the niobium element and titanium element contents.

[0021] As for the insufficient strength of the material, increasing the manganese content can solve this problem. The research finds that controlling the Mn content in the range of 1.0 - 1.5% can not only meet the strength requirements but also improve the material toughness.

[0022] Step 2: Smelt the designed composition in a converter by adding hot metal, scrap steel, and slag-making materials.

[0023] Step 3: Send the converter molten steel above into the LF furnace for refining, and carry out desulfurization treatment during this process to remove sulfides. The invention content of this step is combined with the invention content of the following RH vacuum treatment step to establish a relational expression for the molten steel treatment time to remove inclusions.

[0024] Step 4: Send the molten steel treated in the LF furnace into the RH vacuum furnace for treatment. The treatment purpose is to remove the oxides in the molten steel. The research finds that the inclusions affecting the toughness of this material are mainly oxides. Since the desulfurization efficiency of the LF furnace is high, the influence of sulfides on the material toughness is no longer significant. Therefore, in these two combined refining processes of the LF furnace and the RH vacuum treatment, in order to effectively remove the oxides in the molten steel without delaying the production rhythm, it is necessary to balance the treatment time of the LF furnace and the RH vacuum furnace. After research, it is found that the treatment time T 1 of the LF furnace and the treatment time T 2 of the RH vacuum treatment have a ratio of 3:2, that is, when T 1 : T 2 = 3:2, it can not only effectively remove the oxides in the molten steel, but also not delay the production rhythm, and will not significantly increase the RH vacuum treatment cost, thus contributing to the improvement of the material toughness. The ratio of the treatment times of the combined refining process here is the third highlight of this invention.

[0025] Step 5: Send the molten steel that has undergone combined refining treatment above into the continuous casting process to produce slab billets and send them to the heating furnace of the hot rolling mill for heating.

[0026] Step 6: The slab is heated in the heating furnace of the hot rolling mill. From the perspective of improving the material toughness, one of the functions of heating is to homogenize the material structure (measured by the grain size distribution). Since the uniformity of the material structure has a significant impact on the toughness of the material. Only when the uniformity of the material structure is improved can the toughness of the material be improved. To improve the uniformity of the material structure, the slab heating is carried out in two heating sections. The first stage is the temperature rising stage, and the temperature reaches 1210 °C. The second stage is the uniform heating stage, with a temperature of 1220 °C and a uniform heating time of 30 minutes. The reason for setting this uniform heating temperature to 1220 °C is as follows: Due to the low titanium element content and weak temperature sensitivity, for energy conservation, the temperature is set to 1220 °C (usually 1230 °C or higher).

[0027] After the above slab heating temperature measures are implemented, and making the first-stage temperature rising time (t 1 ) and the second-stage soaking time (t 2 ) maintain such a time ratio: t 1 : t 2 = 2:1, then an unexpected effect occurs, that is, when the slab is rolled at such heating temperature and time ratio, the shape of the rolled sheet coil is significantly improved. The mechanism is that when the temperature rising time is longer than the soaking time, due to the slow temperature rise, there is sufficient time for the uniformity of the slab structure to be improved. The structure uniformity of the rolled sheet coil is good, and the stress distribution uniformity in the sheet coil structure is improved, so it is beneficial to the improvement of the sheet coil shape. However, if the temperature rising time is too long (i.e., the temperature rise is too slow), it will delay production, cause fuel waste, and also cause coarse grain structure. If the ratio of the temperature rising time to the soaking time of the slab is lower than this technology, that is, the temperature rising time is short (i.e., rapid temperature rise), then due to the uneven temperature distribution of each part of the slab, the uniformity of the structure cannot be fully improved. The structure uniformity of the rolled sheet coil is poor, the stress distribution in the sheet coil structure is uneven, stress concentration occurs, and the shape of the sheet coil is poor. If the soaking temperature of the slab is lower than the design value of this technology (1220 °C), then the driving force for the uniformity of the slab and sheet coil structure is insufficient, and the structure uniformity is poor. If the soaking temperature of the slab is higher than the design value of this technology, although the structure uniformity of the slab and the rolled sheet coil is good, the structure is coarse, which will deteriorate the performance and reduce the toughness.

[0028] After the improvement of the sheet coil shape, it is also beneficial to the improvement of the material toughness. If the shape of the sheet coil is poor, the user needs to strongly straighten the sheet coil material, which will cause an increase in the material hardness and a decrease in toughness. But now that the sheet coil shape is good, the user does not need to strongly straighten the sheet coil material, thus maintaining the toughness of the material.

[0029] The design of the slab heating temperature, and the resulting ratio of the temperature rising time to the soaking time, producing the above unexpected effects, is the fourth highlight of this technical invention.

[0030] Step 7: The slab is rough rolled and finish rolled to form a coil. There is no content of the present invention in these two stages.

[0031] Step 8: When the coil is coiled, the coiling temperature is set at 590°C ± 20°C. Compared with the coiling temperature in the prior art (550°C ± 20°C), the coiling temperature of this technology belongs to a high coiling temperature. The mechanism of adopting a high coiling temperature is that a high temperature is beneficial to the homogenization of the material structure, thereby improving the toughness of the material. If the coiling temperature is lower than this temperature range, the coil cools quickly and the structure uniformity is poor; if the coiling temperature is higher than this temperature range, although the structure uniformity of the coil is good, the crystal grains are coarse, which instead deteriorates the toughness of the material.

[0032] In addition, after these two measures of designing the coiling temperature and the slab heating temperature are implemented simultaneously, it is found that there is a superimposed effect, that is, both increase the uniformity of the material structure, thus playing a significant role in improving the material structure uniformity.

[0033] Compared with the prior art, the present invention has the following advantages. The present technology invention has the following technical advantages:

[0034] 1. The design of the silicon component content reflects the solution to the problem of excessive scale on the surface of the hot-rolled coil of this component system. Therefore, the silicon component design of this technology is one of the highlights.

[0035] 2. By adopting the combination of niobium and titanium elements to form a high-niobium and low-titanium alloy component design mode, it is found that after implementing the two measures of increasing the niobium content and reducing the titanium content, in addition to improving the strength, there is also a common effect, that is, the purpose of improving the toughness of the material can be achieved. The designed relationship between the niobium and titanium element contents: Nb% = 2 * Ti%, which is the second highlight of the present technology invention.

[0036] 3. The main type of inclusions in the steel that affect the toughness of this steel grade is oxides. When carrying out the double-refining process of LF furnace and RH vacuum furnace, in order to effectively remove the oxides in the molten steel without delaying the production rhythm and without significantly increasing the RH vacuum treatment cost, it is necessary to balance the treatment time of LF furnace and RH vacuum furnace. Through research, it is found that the ratio of the treatment time T 1 of LF furnace to the treatment time T 2 of RH vacuum treatment is 3:2, that is, when T 1 : T 2 = 3:2, it can not only effectively remove the oxides in the molten steel, but also not delay the production rhythm and not significantly increase the RH vacuum treatment cost, thus contributing to the improvement of the toughness of the material. The ratio of the treatment time of the double-refining process here is the third highlight of the present invention.

[0037] 4. When the slab heating temperature of this steel grade adopts the design value of this technology and the ratio of heating-up time to soaking time formed by this technology, an unexpected discovery is made. That is, when the slab is under such heating temperature and time ratio and is rolled, the shape of the rolled coil is significantly improved, which can reduce the problems that users need to strongly straighten due to poor shape, resulting in increased material hardness and decreased material toughness. The mechanism has been described in step six of the invention content. The design value of the slab heating temperature and the time ratio, as well as this unexpected effect, are the fourth highlight of this technological invention.

[0038] 5. When the coiling temperature of the coil of this steel grade adopts the temperature designed by this technology, it can not only improve the uniformity of the material structure but also prevent the material grains from becoming coarse and deteriorating the toughness of the material. In addition, after these two measures of the design of the coiling temperature and the design of the slab heating temperature are implemented simultaneously, it is found that there is a superimposed effect, that is, both increase the uniformity of the material structure, thus playing a significant role in improving the uniformity of the material structure. This is the fifth highlight of this technological invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a schematic diagram of the metallographic structure of the present invention;

[0040] Figure 2 is a schematic diagram of the metallographic structure of the prior art. DETAILED DESCRIPTION OF THE INVENTION

[0041] To deepen the understanding of the present invention, the following detailed description of this embodiment will be given with reference to the drawings.

[0042] Example 1: Refer to Figure 1 , a method for solving the reduction of toughness of automotive beam steel under continuous work hardening, the method comprising the following steps:

[0043] Step 1: Chemical composition example, see Table 1.

[0044] Table 1. Chemical composition example (unit: %)

[0045]

[0046]

[0047] As can be seen from Table 1, the contents of niobium and titanium maintain a 2-fold relationship.

[0048] Step 2: Add scrap steel, hot metal, slag-making materials and other furnace charges into the converter, smelt the molten steel, smelt for a specified time, and tap the steel at a specified temperature.

[0049] Step 3: The molten steel from the converter is refined in the LF furnace and the RH vacuum furnace to remove sulfides and oxides. Among them, the desulfurization effect of the LF furnace is measured by the sulfur content of the molten steel, and the removal of oxides in the RH vacuum furnace is measured by the free oxygen content in the molten steel. The specific implementation data are shown in Table 2.

[0050] Table 2. Inclusion removal in the LF furnace and the RH vacuum furnace

[0051]

[0052] As can be seen from Table 2, according to the LF furnace desulfurization treatment time T 1 and the RH vacuum treatment time T 2 with a ratio of 3:2 for LF furnace refining desulfurization and RH vacuum removal of oxides and other inclusions, the sulfur content of the molten steel is much lower than the standard requirements, and the free oxygen content of the molten steel is also lower than the standard requirements; at the same time, it does not delay the production rhythm and does not significantly increase the cost of treating molten steel in the RH vacuum.

[0053] Step 4: The refined molten steel enters the continuous casting process to produce slabs.

[0054] Step 5: The actual heating of the slabs in the hot rolling mill is shown in Table 3.

[0055] Table 3. Implementation of slab heating temperature

[0056]

[0057] As can be seen from Table 3, after the slabs are heated at the designed temperature, the slab shape is improved, while the prior art cannot guarantee this requirement.

[0058] Step 6: The slabs are rough rolled and finish rolled.

[0059] Step 7: The actual implementation of the coiling temperature of the strip coils is shown in Table 4.

[0060] Table 4. Actual implementation of the coiling temperature of the strip coils

[0061] Coiling temperature (°C) Example 1 592 Example 2 593 Example 3 589 Example 4 590 Example 5 592

[0062] Step 8: Take strip coil samples to detect the uniformity of the microstructure (measured by grain size distribution), toughness (measured by elongation), tensile strength, and yield strength. The situation is shown in Table 5.

[0063] Table 5. Microstructure and properties of the steel grade

[0064]

[0065] The metallographic structure of this technology is shown in Figure 1 , and the metallographic structure of the prior art is shown in Figure 2 .

[0066] Comparing the metallographic structure of the present technology with that of the prior art, it can be seen that the metallographic structure of the present technology is fine and evenly distributed, while the metallographic structure of the prior art is coarser and uneven in size.

[0067] Through practical use, when continuously processing the steel coil for the automobile girder, problems such as cracking during continuous processing such as stamping no longer occur.

[0068] It should be noted that the above embodiments are not intended to limit the protection scope of the present invention, and equivalent transformations or substitutions made on the basis of the above technical solutions all fall within the protection scope of the claims of the present invention.

Claims

1. A method for solving the reduction of toughness of automotive beam steel under continuous work hardening, characterized in that, the method comprises the following steps: Step 1: Composition design, the components include carbon, silicon, manganese, phosphorus, sulfur, nitrogen, aluminum, titanium and niobium, and the rest are inevitable impurity elements, Step 2: Smelt the designed components in a converter by adding hot metal, scrap steel and slag-making materials, Step 3: Send the converter molten steel in Step 2 into an LF furnace for refining, and carry out desulfurization treatment during this process to remove sulfides, Step 4: Send the molten steel treated by the LF furnace into an RH vacuum furnace for treatment, and the purpose of the treatment is to remove oxides in the molten steel, Step 5: Send the molten steel treated by the double ladle refining in Step 4 into the continuous casting process to produce slab billets, and send them to the heating furnace of the hot rolling mill for heating, Step 6: Heat the slab billets in the heating furnace of the hot rolling mill, Step 7: Carry out rough rolling and finish rolling on the slab billets to form coil sheets, Step 8: When coiling the coil sheets, set the coiling temperature to 590°C ± 20°C.

2. The method for solving the reduction of toughness of automotive beam steel under continuous work hardening according to claim 1, characterized in that, in Step 1, according to mass fraction, the specific ranges are: C: 0.06 - 0.09%, Si: 0.15 - 0.25%, Mn: 1.0 - 1.5%, P: ≤0.022%, S: ≤0.008%, N: ≤0.006%, Al: 0.01 - 0.045%, Ti: 0.008 - 0.018%, Nb: 0.015 - 0.035%.

3. The method for solving the reduction of toughness of automotive beam steel under continuous work hardening according to claim 2, characterized in that, In step 4, the molten steel treated by the LF furnace is fed into the RH vacuum furnace for treatment. The treatment time T of the LF furnace 1 and the treatment time T 2 of the RH vacuum treatment are in the ratio of 3:

2.

4. The method for solving the reduction of toughness of automotive beam steel under continuous work hardening according to claim 3, characterized in that, in Step 6, the heating of the slab billets is carried out in two heating sections. The first stage is the temperature rising stage, and the temperature reaches 1210°C; the second stage is the uniform heating stage, and the temperature is 1220°C, and the uniform heating time is maintained for 30 minutes.

Images