A b-containing steel for building formwork pull tab and a production method thereof

Abstract

The application provides a kind of B-containing building template pull piece steel and its production method, belongs to the field of building steel.Its chemical composition by mass percent, in addition to containing Fe and other inevitable impurities, also contains: C=0.15~0.25%, Si=0.09~0.35%, Mn=0.25~1.35%, B=0.0005~0.005%, P=0.02~0.05%, S≤0.03%, and satisfies C+Si / 24+Mn / 6=0.20~0.50%.The B-containing building template pull piece steel provided by the application provides two kinds of production processes, the first is steelmaking, continuous casting, heating, hot rolling, coiling, offline heat treatment, the second is steelmaking, continuous casting, heating, hot rolling, online quenching, the microstructure of the final product of the two processes is more than 80% of martensite microstructure, the rest is less than 20% of second phase microstructure;yield strength Rel=845~1045MPa, tensile strength Rm=1095~1395MPa, fracture elongation A=5~11%, impact energy Akv (10mm, 20℃)=8~20J.Both element design and production process can effectively reduce production cost.

Description

Technical Field

[0001] This invention relates to the field of steel for construction, and more particularly to a steel for building formwork tie rods containing B and its production method. Background Technology

[0002] With the development of high-rise buildings in China, cast-in-place concrete structures have emerged and rapidly captured the market due to their advantages such as good earthquake and fire resistance and low cost. Correspondingly, formwork technology has also developed rapidly. In recent years, tie rod systems for fixing formwork have gradually become popular. The tie rods play the role of fixing the formwork, so they are required to have sufficient strength, while also having a certain degree of brittleness to ensure efficient demolition by breaking them.

[0003] Currently, the most commonly used steel grades for architectural tie rods on the market are alloy steels such as 40Mn and 50Mn. Their manufacturing process mainly involves multiple steps, including hot rolling, pickling, slitting, heat treatment, and cold rolling, which is not only complex but also costly. Therefore, developing architectural tie rod steel that is simple to manufacture, meets performance standards, and is inexpensive has practical value and significance.

[0004] In response, patent document 1 (CN109023115A) discloses a hot-rolled template tensioning steel and its manufacturing method. The chemical composition of this steel is: C: 0.22-0.75%, Si: 0.17-1.60%, Mn: 0.20-1.50%, C+Si / 3+Mn / 6=0.50-0.85%, P≤0.035%, S≤0.035%, Cr≤0.55%, Ni≤0.55%, Cu≤0.20%, with the remainder being iron and unavoidable impurities. It has the characteristics of high strength, good cold bending performance, and short production cycle. However, due to its microstructure being bainite + pearlite or pearlite + ferrite, although it has good toughness, the demolition speed is slow and the construction efficiency is low in actual use.

[0005] Patent document 2 (CN103911557B) discloses "A steel for building formwork tensioning and its production process". Its chemical composition is C: 0.20-0.30%, Si: 0.6-1.0%, Mn: 1.2-1.6%, P≤0.035%, S≤0.035%, Cr≤0.25%, Ni≤0.30%, Cu≤0.15%, with the remainder being iron and unavoidable impurities. After smelting, casting, hot rolling, cold rolling, slitting, and stamping, the final product has the characteristics of high strength and high elongation at break. However, this steel is cold-rolled steel, which has a complex process and high cost. Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] In view of this, the inventors have studied a high-performance steel for building formwork tensioning sheets containing B and its production method. Based on reasonable composition design and offline heat treatment / online quenching process, compared with the traditional process which requires a cold rolling process after hot rolling, the present invention adopts "hot instead of cold", which can simplify the production process and reduce production costs while meeting the performance requirements of building tensioning sheets having sufficient strength and a certain degree of brittleness.

[0008] Technical solutions to the problem

[0009] This invention, through compositional and heat treatment process design, ensures that the microstructure of the steel is over 80% martensitic with the remainder being less than 20% second-phase, meeting the performance requirements for steel used in architectural tie rods. The steel of this invention has a simple composition and low cost. Taking the heat treatment production process as an example, the process window is wide during the hot rolling stage; only a base plate needs to be rolled to meet the requirements of subsequent processes. Process method two, using direct quenching and coiling, further simplifies the process. The steel of this invention, water-quenched to form a martensitic microstructure, exhibits high strength and low toughness, meeting practical application needs.

[0010] To achieve the above objectives, one embodiment of the present invention provides a steel for building formwork tie rods containing B, characterized in that, by mass percentage, in addition to containing Fe and other unavoidable impurities, it also contains: C = 0.15–0.25%, Si = 0.09–0.35%, Mn = 0.25–1.35%, B = 0.0005–0.005%, P = 0.02–0.05%, S ≤ 0.03%; and satisfies C + Si / 24 + Mn / 6 = 0.20–0.50%.

[0011] Another embodiment of the present invention is a steel for building formwork tie rods containing B, characterized in that the chemical composition by mass percentage is: C = 0.15-0.25%, Si = 0.09-0.35%, Mn = 0.25-1.35%, B = 0.0005-0.005%, P = 0.02-0.05%, S ≤ 0.03%, with the remainder being iron (Fe) and unavoidable impurities; and satisfying C + Si / 24 + Mn / 6 = 0.20-0.50%.

[0012] Preferably, the steel for the formwork tie rods of the present invention containing B further contains, by mass percentage, one or more of the following: Nb = 0.002-0.025%, V = 0.002-0.25%, Ca = 0.001-0.01%, Zr = 0.001-0.01%, and REM = 0.001-0.10%.

[0013] Preferably, in the steel for building formwork tie rods containing B of the present invention, as unavoidable impurities, it contains Cu≤0.40%, Ni≤0.40%, Cr≤0.40%, Mo≤0.40%, Co≤0.10%, Zn≤0.10%, Na≤0.05%, and K≤0.05%.

[0014] The roles of various elements in this invention:

[0015] C: Carbon is one of the important elements for ensuring the mechanical properties of steel. An appropriate amount of carbon can improve the strength of steel while reducing its toughness, meeting the performance requirements for fixing and dismantling in actual use of steel for tensioning sheets. In this invention, the carbon content is controlled at 0.15–0.25%, more preferably 0.15–0.20%.

[0016] Silicon (Si): Silicon strengthens steel through solid solution treatment, increasing its strength but also reducing its toughness, similar to its effect with carbon. In this invention, the silicon content is controlled at 0.09–0.35%, more preferably 0.15–0.20%.

[0017] Mn: The main role of manganese in steel is to improve the strength of steel, but too high a manganese content can easily lead to central segregation. Therefore, the manganese content in this invention is controlled at 0.25-1.35%, more preferably 0.65-1.35%.

[0018] In particular, C, Si, and Mn can all improve strength, and there is a certain synergistic effect among them. The effect achieved by the combination of the three can better meet the performance requirements of steel for building tie rods. Therefore, the content of C, Si, and Mn in this invention also needs to meet the requirement of C+Si / 24+Mn / 6=0.20~0.50%.

[0019] B: In this invention, boron plays a crucial role. During the quenching process, boron can effectively enhance the hardenability of steel, and its enrichment at the grain boundaries helps to reduce the toughness of the steel, which is more in line with the actual use requirements of the steel described in this invention. The content is controlled between 0.0005% and 0.005%.

[0020] P: In this invention, the phosphorus content is selected as P = 0.02-0.05%. The effect of P is to enrich at grain boundaries, thereby reducing toughness. In the steel grade described in this invention, since B has a stronger ability to enrich at grain boundaries than P, the effect of P enrichment at grain boundaries is relatively weakened. The P content is more preferably 0.02-0.03%.

[0021] S: Sulfur is an unavoidable harmful element in steel. Similar to P, it will affect the toughness of steel. However, Mn will combine with S to form MnS, which will reduce the influence of S to a certain extent. Therefore, the S content is selected to be ≤0.03%.

[0022] In addition, as unavoidable impurities, this invention also contains Nb, V, Ca, Zr, and REM. Niobium and vanadium form NbC and VC carbides, which have a certain precipitation strengthening effect. The specific content is 0, 1, or 2 of Nb = 0.002-0.025% and V = 0.002-0.25%. Ca, Zr, and REM are beneficial to controlling the morphology of sulfide inclusions. The specific content is 0, 1, or more of Ca = 0.001-0.01%, Zr = 0.001-0.01%, and REM = 0.001-0.10%. REM refers to elements with atomic coefficients of 21, 39, 57-71. Furthermore, as unavoidable impurities, the presence of, for example, Cu≤0.40%, Ni≤0.40%, Cr≤0.40%, Mo≤0.40%, Co≤0.10%, Zn≤0.10%, Na≤0.05%, and K≤0.05% is also within the scope of this invention.

[0023] Preferably, the metallographic structure of the steel for building formwork tie rods containing B of the present invention contains more than 80% martensite and less than 20% second-phase structure. The second-phase structure may be a small amount of ferrite, bainite, pearlite, retained austenite, precipitates, etc., that appear during the rolling process. The ideal metallographic structure of the present invention tends to be entirely martensite.

[0024] Preferably, the steel for the B-containing building formwork tie rod of the present invention has a yield strength Rel = 845~1045MPa, a tensile strength Rm = 1095~1395MPa, a breaking elongation A = 5~11%, and an impact absorption energy Akv (10mm, 20℃) = 8~20J.

[0025] This invention provides a method for manufacturing steel containing B for building formwork tie rods, wherein the first embodiment of the manufacturing method includes steelmaking, continuous casting, hot rolling, coiling, and heat treatment stages.

[0026] Preferably, the method for manufacturing B-containing steel for building formwork tie rods of the present invention is used to prepare a continuously cast billet having the chemical composition described in the above embodiments.

[0027] The continuous casting temperature is 1495-1545℃.

[0028] During the hot rolling stage, the heating temperature of the continuously cast billet is 1099-1200℃, and the time in the furnace is 15-200min;

[0029] During the hot rolling stage, the temperature of the final roughing pass is 950-1099℃, and the reduction rate per pass is 10-50%.

[0030] During the hot rolling stage, the finishing rolling temperature is 800-950℃, and the reduction rate per pass is 10-50%.

[0031] Preferably, the steel after the hot rolling stage is subjected to laminar flow cooling before being coiled.

[0032] The cooling rate of the laminar flow cooling is 30-200℃ / s;

[0033] The winding temperature during the winding stage is 500-660℃.

[0034] Preferably, after winding, further heat treatment is carried out. The heat treatment stage adopts offline heat treatment, with a heating temperature of 900-940℃, and then cooling to below 100℃ at a rate of 70-200℃ / s.

[0035] Meanwhile, the second embodiment of the manufacturing method of the steel for building formwork tie rods containing B of the present invention includes steelmaking, continuous casting, hot rolling and coiling stages.

[0036] Preferably, the method for manufacturing B-containing steel for building formwork tie rods of the present invention is used to prepare a continuously cast billet having the chemical composition described in the above embodiments.

[0037] The continuous casting temperature is 1495-1545℃.

[0038] During the hot rolling stage, the heating temperature of the continuously cast billet is 1099-1200℃, and the time in the furnace is 15-200min;

[0039] During the hot rolling stage, the temperature of the final roughing pass is 950-1099℃, and the reduction rate per pass is 10-50%.

[0040] During the hot rolling stage, the finishing rolling temperature is 800-950℃, and the reduction rate per pass is 10-50%.

[0041] Preferably, the coiling is performed after online quenching following the hot rolling stage.

[0042] The cooling rate for online quenching is 70-200℃ / s, cooling down to below 100℃;

[0043] The winding temperature during the winding stage is below 100°C.

[0044] The beneficial effects of this invention are as follows:

[0045] 1. From a chemical composition perspective, the steel for building formwork tie rods described in this invention has a simple composition design and effectively reduces costs. While ensuring specific amounts of C, Si, and Mn in the raw materials, the content of trace components such as B and P is adjusted to give the steel for building formwork tie rods sufficient strength, while simultaneously maintaining low toughness and a certain degree of brittleness to ensure efficient demolition by breaking. In practical use, high strength ensures a secure fixing effect, while low toughness facilitates formwork removal.

[0046] 2. From the perspective of production process, the steel for building formwork tie rods containing B described in this invention has a wide production process window, low control difficulty, and fewer process steps (currently used tie rods are mostly cold-rolled, while the steel grade described in this invention is delivered in hot-rolled condition, which effectively improves production efficiency), and the cost can be effectively reduced.

[0047] 3. Through the synergistic effect of composition design and heat treatment process, this invention, under the composition system set by this invention, uses two different quenching processes to control the pre-quenching temperature, cooling rate, and post-quenching temperature. This ensures that the microstructure of the steel product used for building formwork tie rods described in this invention is over 80% martensite + second phase, thereby obtaining high-strength, low-toughness steel for building formwork tie rods. Its yield strength Rel = 845–1045 MPa, tensile strength Rm = 1095–1395 MPa, elongation at break A = 5–11%, and impact absorption energy Akv (10mm, 20℃) = 8–20 J. In actual use, the high strength ensures the fixing effect, while the low toughness facilitates formwork removal. Attached Figure Description

[0048] 【 Figure 1 Metallographic image of steel after nitric acid alcohol etching as observed under an optical microscope in Example 1 of this invention.

[0049] 【 Figure 2 [Image 1] is a metallographic image observed under an optical microscope after being etched with nitric acid and alcohol, as shown in Comparative Example 4.

[0050] 【 Figure 3 Metallographic image of steel after nitric acid alcohol etching as observed under an optical microscope in Example 7 of this invention. Detailed Implementation

[0051] The following specific embodiments further illustrate the steel for high-performance building formwork tie rods containing B and its production method according to the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention will be presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to this embodiment. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, without conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0052] Table 1 lists the mass percentage of the chemical composition of the high-performance building formwork tie rods containing B in Examples 1-9 and Comparative Examples 1-5 (the balance being Fe and unavoidable impurities). Table 2 lists the production process parameters of the high-performance building formwork tie rods containing B in Examples 1-9 and Comparative Examples 1-5. Table 3 lists the performance test results of the high-performance building formwork tie rods containing B in Examples 1-9 and Comparative Examples 1-5.

[0053] Example 1

[0054] The steel was smelted in a converter according to the chemical composition shown in Table 1 below. The resulting molten steel was continuously cast into billets at 1534°C. The billets were then heated to 1187°C in a reheating furnace and held for 98 minutes before hot rolling. The final roughing pass temperature was controlled at 1038°C, with a reduction rate of 40% per pass for a total of 4 passes. The finishing pass temperature was controlled at 850°C, with a reduction rate of 30% per pass for a total of 7 passes. After hot rolling, the steel was laminar cooled at a cooling rate of 60°C / s to 540°C and then coiled. Subsequently, it underwent offline heat treatment, heating to 900°C until austenitization, and then cooling to below 100°C at a cooling rate of 150°C / s to obtain the steel sample.

[0055] Examples 2-6 and Comparative Examples 1-4

[0056] The steel was smelted in a converter according to the chemical composition shown in Table 1 below, and the resulting molten steel was processed in the same manner as in Example 1 under the conditions shown in Table 2.

[0057] Example 7

[0058] The steel was smelted in a converter according to the chemical composition shown in Table 1 below. The resulting molten steel was continuously cast into billets at 1509°C. The billets were then heated to 1176°C in a reheating furnace and held for 104 min before hot rolling. The final roughing pass temperature was controlled at 1039°C, with a reduction rate of 40% per pass for a total of 4 passes. The final finishing pass temperature was controlled at 938°C, with a reduction rate of 30% per pass for a total of 7 passes. After hot rolling, the steel was quenched online at a cooling rate of 150°C / s, and after cooling to 40°C, it was coiled to obtain the steel sample.

[0059] Examples 8-9, Comparative Example 5

[0060] The steel was smelted in a converter according to the chemical composition shown in Table 1 below, and the resulting molten steel was processed in the same manner as in Example 7 under the conditions shown in Table 2 to obtain the steel sample.

[0061] <Yield Strength Determination>

[0062] The test samples prepared in the above embodiments and comparative examples were used as yield strength test pieces. Tensile strength was determined using JIS 5# tensile specimens along the longitudinal direction, and yield strength was determined based on the tensile test performed according to GB / T228.1-2010 standard. Yield strength generally reflects the maximum stress a material can withstand during plastic deformation; in this invention, a higher value indicates better strength of the steel.

[0063] <Tensile Strength Determination>

[0064] The test samples prepared according to the above embodiments and comparative examples were used as tensile strength test pieces. Tensile strength was determined using JIS 5# tensile specimens along the longitudinal direction, and yield strength was determined based on the tensile test performed according to GB / T228.1-2010 standard. Tensile strength generally reflects the maximum stress a material can withstand before fracture or failure; in this invention, a higher value indicates better strength of the steel.

[0065] <Determination of Elongation at Break>

[0066] Elongation at break, also known as post-fracture elongation, is calculated by carefully fitting the fractured parts of the tensile specimen together after it has been broken, according to the GB / T228.1-2010 standard, ensuring that the axes are aligned and that the fractured parts of the specimen are in proper contact. The original gauge length Lo and the measured post-fracture gauge length are then substituted into the following formula for calculation.

[0067] A = (Lu - Lo) / Lo × 100%

[0068] Compared to manual testing, an extensometer can also be used for measurement. When using an extensometer to measure elongation at break, the extensometer gauge length is equal to the original gauge length of the specimen, and there is no need to mark the original gauge length of the specimen. The total elongation at break is used as the elongation measurement, and the elastic elongation should be subtracted from the total elongation to obtain the elongation after break.

[0069] In this invention, a higher elongation at break indicates better toughness of the metallic material.

[0070] <Impact Absorption Energy Measurement>

[0071] The impact absorption energy in this invention refers to the Charpy impact absorption energy. Using a pendulum impact testing machine, the test samples prepared in the above embodiments and comparative examples were made into specimens (small specimens) with a cross-sectional thickness of 2.5 mm and a V-notch. The test was conducted at 20°C based on the standard GB / T 229-2020, Charpy Pendulum Impact Test Method for Metallic Materials, and the results were converted to the impact energy of a 10 mm standard specimen. Generally, Charpy impact absorption energy reflects the impact performance of a material; in this invention, a higher value indicates better toughness of the steel.

[0072]

[0073]

[0074]

[0075] As shown in the table above, Examples 1-9 all meet the relevant requirements of the present invention, and the steel coils obtained by both production methods meet the performance requirements for steel used in building tie rods. Figure 1 and Figure 3 Metallographic images of the steels from Examples 1 and 7 of this invention after being etched with nitric acid and alcohol, observed under an optical microscope, show a microstructure of martensite plus a small amount of carbides.

[0076] The elemental content of Comparative Example 1 is not within the range required by this invention. The C and Si contents are too low, and the C+Si / 24+Mn / 6 is below the required range. The offline heat treatment process of Implementation Method 1 is used. The final microstructure is martensite + second phase, but the strength is too low, the elongation at break is too high, and the impact performance is too high, which does not meet the requirements for steel used in building tie rods.

[0077] The elemental content of Comparative Example 2 is not within the range required by this invention. The C, Si, Mn, and C+Si / 24+Mn / 6 are higher than the required range. Using the offline heat treatment process of Implementation Method 1, the final microstructure is martensite + second phase, but the strength is too high and does not meet the requirements for steel used in building tie rods.

[0078] The element content of Comparative Example 3 is not within the range required by this invention. The content of B and P is lower than the required value, so they cannot play a role at the grain boundaries. Using the manufacturing method of Implementation Method 1, the final microstructure is martensite + second phase. The impact performance is too high, which is not conducive to the demolition process in actual use and does not meet the requirements for steel used in building tie rods.

[0079] The element content of Comparative Example 4 meets the requirements of this invention, but the cooling rate is lower than the required value in the heat treatment process, resulting in the final product having a ferrite + pearlite microstructure, low overall strength, and excessively high elongation at break and impact energy, which does not meet the requirements for steel used in building tie rods.

[0080] Comparative Example 5 met the requirements of this invention in terms of element content and adopted the wire quenching process of Embodiment 2 of the manufacturing method. However, in the direct quenching stage, the cooling rate was too slow and the coiling temperature was too high, resulting in the formation of a microstructure of ferrite + bainite + a small amount of martensite. Figure 2 As shown, the overall strength is too low, and the elongation at break and impact energy are too high, which does not meet the requirements for steel used in building tie rods.

[0081] By comparing Examples 1-9 with Comparative Examples 1, 2, and 3, it can be seen that in this invention, the contents of C, Si, B, and P, as well as C+Si / 24+Mn / 6, have a significant impact on product performance. Meeting the component content characteristics of this invention is beneficial to obtaining products that meet the requirements for use in building tie rods.

[0082] By comparing Examples 1-9 with Comparative Examples 4 and 5, it can be seen that the production process parameters have a significant impact on product performance in this invention, such as the cooling rate during offline heat treatment, the cooling rate during online quenching, and the coiling temperature. Meeting the manufacturing process characteristics of this invention is beneficial for obtaining products that meet the requirements for use in building tie rods.

Claims

1. A type of steel for building formwork tie rods containing B, characterized in that, The chemical composition by mass percentage is: C=0.15~0.218%, Si=0.09~0.35%, Mn=0.25~1.35%, B=0.0005-0.005%, P=0.02~0.05%, S≤0.03%, with the remainder being iron (Fe) and unavoidable impurities; and satisfying C+Si / 24+Mn / 6=0.20~0.50%, and the metallographic structure of the steel is more than 80% martensite.

2. The steel for building formwork tie rods containing B according to claim 1, characterized in that, It further contains, by mass percentage, one or more of the following: Nb=0.002~0.025%, V=0.002~0.25%, Ca=0.001~0.01%, Zr=0.001~0.01%, REM=0.001~0.10%.

3. The steel for B-containing building formwork tie rods according to claim 1 or 2, characterized in that, As an unavoidable impurity, it contains Cu≤0.40%, Ni≤0.40%, Cr≤0.40%, Mo≤0.40%, Co≤0.10%, Zn≤0.10%, Na≤0.05%, and K≤0.05%.

4. The steel for B-containing building formwork tie rods according to claim 1 or 2, characterized in that, Yield strength Rel=845~1045MPa, tensile strength Rm=1095~1395MPa, elongation at break A=5~11%, impact absorption energy Akv=8~20J at 20℃ and 10mm plate thickness.

5. A method for manufacturing steel for building formwork tie rods containing B, as described in any one of claims 1 to 4, characterized in that, Its production process includes steelmaking, continuous casting, hot rolling, and coiling stages.

6. The method for manufacturing steel for B-containing building formwork tie rods according to claim 5, characterized in that, Prepare a continuously cast billet with the chemical composition of any one of claims 1 to 3. The continuous casting temperature is 1495-1545℃. During the hot rolling stage, the heating temperature of the continuously cast billet is 1099-1200℃, and the time in the furnace is 15-200 min. During the hot rolling stage, the temperature of the final roughing pass is 950-1099℃, and the reduction rate per pass is 10-50%. During the hot rolling stage, the finishing rolling temperature is 800-950℃, and the reduction rate per pass is 10-50%.

7. The method for manufacturing steel for B-containing building formwork tie rods according to claim 5 or 6, characterized in that, The steel after the hot rolling stage is cooled by laminar flow and then coiled. The cooling rate of the laminar flow cooling is 30-200℃ / s; The winding temperature during the winding stage is 500-660℃.

8. The method for manufacturing steel for B-containing building formwork tie rods according to claim 7, characterized in that, After winding, further heat treatment is carried out. The heat treatment stage adopts offline heat treatment, with a heating temperature of 900-940℃, and then cooling to below 100℃ at a rate of 70-200℃ / s.

9. The method for manufacturing steel for B-containing building formwork tie rods according to claim 5 or 6, characterized in that, After the hot rolling stage, the coil is subjected to online quenching and then coiled. The cooling rate for online quenching is 70-200℃ / s, cooling down to below 100℃; The winding temperature during the winding stage is below 100°C.

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