TWI876779B - High flatness and ultra-high strength steel plates and manufacturing method the same - Google Patents

Abstract

A high flatness and ultra-high strength steel and a manufacturing method the same are provided. The manufacturing method includes the following steps: providing a steel billet, wherein based on total 100 wt% of the steel billet, the steel billet has a composition of: carbon content ranging from 0.30 to 0.45 wt%, silicon content below 0.4 wt%, manganese content ranges from 0.1 to 0.75 wt%, phosphorus content below 0.01 wt%, sulfur content below 0.005 wt%, chromium content below 1.5 wt%, molybdenum content below 0.5 wt%, nickel content ranging from 0.5 to 5.5 wt%, copper content below 0.5 wt%, vanadium content below 0.30 wt%, aluminum content ranging from 0.01 to 0.05 wt%, calcium content below 0.05 wt%, nitrogen content below 0.01 wt%, and the other are iron and unavoidable impurities; performing a re-heating step to the steel billet; performing a rolling step to the steel billet to form a steel sheet; performing a coiling step to the steel sheet; performing a cooling step and a normalization step to the steel sheet; performing a hot stamping step to the steel sheet to form a steel plate.

Description

High flatness and ultra-high strength steel and its manufacturing method

The present invention relates to a high-flatness and ultra-high-strength steel and a manufacturing method thereof, and in particular to a high-flatness and ultra-high-strength steel and a manufacturing method thereof that combines hot rolling and hot stamping processes and matches a specific alloy ratio.

Traditional ultra-high hardness steel plates are usually designed with medium carbon components, combined with alloying elements such as manganese (Mn), chromium (Cr), molybdenum (Mo), nickel (Ni), and boron (B) to improve the hardening ability of the material and produce a matte iron structure during the quenching process, giving the steel sufficient hardness.

Generally speaking, this type of steel is produced by reheat quenching (RQ) followed by low-temperature tempering (T). The tempering treatment prevents the steel from being too hard and brittle to be used.

However, the production of steel by reheating, quenching and tempering consumes a lot of energy. Moreover, the flatness of steel plates produced by reheating, quenching and tempering can only meet the EN 10029 class N Steel Type L standard (< 9 mm/1000 mm), while the products of advanced steel plants can reach the level of 3 mm/1000 mm.

Therefore, it is necessary to provide a high flatness and ultra-high strength steel and a manufacturing method thereof to solve the problems existing in the conventional technology.

One of the purposes of the present invention is to provide a high flatness and ultra-high strength steel material and a manufacturing method thereof, which is produced by hot rolling and hot stamping die quenching. By using appropriate alloy design and analyzing and studying the hot rolling and hot stamping parameters, the most appropriate production conditions are found, and ultra-high hardness steel plates with a hardness of 450 HBW or above, high strength and good low-temperature toughness can be produced, and the flatness can reach less than 1 mm/1000 mm.

Another object of the present invention is to provide a high flatness and ultra-high strength steel and a manufacturing method thereof. Compared with the traditional steel that adopts reheat quenching and tempering, the present invention has high hardness, good low-temperature toughness and elongation after hot stamping, and good flatness, which is more valuable for application. In addition, since the temperature drop process of hot stamping can complete the tempering operation of the steel plate through the self-tempering method, it can achieve the effect of energy saving and carbon reduction compared with the traditional production method.

To achieve the above-mentioned object, the present invention provides a method for manufacturing a high-flatness and ultra-high-strength steel, comprising the following steps: providing a steel billet, wherein the steel billet has the following components: based on the weight percentage of the steel billet as 100 wt%, a carbon content of 0.30 to 0.45 wt%, a silicon content of 0.4 wt% or less, a manganese content of 0.1 wt% or more and less than 0.75 wt%, a phosphorus content of 0.01 wt% or less, a sulfur content of 0.005 wt% or less, a chromium content of 1.5 wt% or less, a molybdenum content of 0.5 wt% or less, a nickel content of 0.5 to 5.5 wt%, a copper content of 0.5 wt% or less, a vanadium content of 0.30 wt% or less, an aluminum content of 0.01 to 0.05 wt%, and a calcium content of 0.05 wt% or less, the nitrogen content is 0.01 wt% or less, and the remainder is iron and inevitable impurities; performing a reheating step on the steel blank; performing a rolling step on the steel blank after the reheating step to form a steel material; performing a coiling step on the steel material after the rolling step; performing a cooling step and a normalizing step on the steel material after the coiling step; performing a hot stamping step on the steel material after the cooling step and the normalizing step to form a steel plate.

In some embodiments, the reheating step is to reheat the steel billet to 1050 to 1200° C. before performing the rolling step.

In some embodiments, the completion temperature of the rolling step is above the Ar3 point of the material + 50°C.

In some embodiments, the coiling temperature of the coiling step is between the Ar1 point of the material and Ar1 point-200°C.

In some embodiments, the cooling step is to cool the steel material to room temperature and perform an uncoiling and slitting step.

In some embodiments, after the coiling step, a box annealing step is further included, and the temperature range of the box annealing step is between Ac1 point - 50°C and Ac1 point - 100°C.

In some embodiments, the temperature of the normalization step is between the Ac3 point of the material + 80°C and the Ac3 point + 150°C, and the temperature is averaged for 6 to 15 minutes.

In some embodiments, after the hot stamping step, the steel plate is removed from the mold after the temperature reaches below the Mf point -50°C.

In some embodiments, after the hot stamping step, a tempering step is further performed, wherein the steel plate is placed in a heating furnace at a temperature of Ms point -80°C to Ms point -120°C for 30 to 90 minutes for tempering.

A high flatness and ultra-high strength steel manufactured by the manufacturing method as described above, wherein the hardness of the steel is 450 HBW or more, the flatness is less than 1 mm/1000 mm, the -40°C LT impact value is 34 J/ ^cm2 or more, the -40°C TL impact value is 33 J/ ^cm2 or more, and the elongation is 10% or more.

As used herein, reference to a numerical range for a variable is intended to mean that the variable is equal to any value within the range. Thus, for a variable that is itself discontinuous, the variable is equal to any integer value within the numerical range, including the endpoints of the range. Similarly, for a variable that is itself continuous, the variable is equal to any real value within the numerical range, including the endpoints of the range. As an example, and not a limitation, a variable described as having a value between 0-2 takes on a value of 0, 1, or 2 if the variable is itself discontinuous, and takes on a value of 0.0, 0.1, 0.01, 0.001, or any other real value ≧0 and ≦2 if the variable is itself continuous.

In order to make it easier to understand the design principle of the present invention, the details and implementation principle of the high flatness and ultra-high strength steel and its manufacturing method of the present invention are explained below.

The term "Ac1 point" used in this article refers to the temperature at which pulex (ferrous iron + carbides) begins to transform into austenite when the steel is heated.

The term "Ac3 point" used herein refers to the temperature at which ferrous iron completely transforms into austenite when steel is heated.

The term "Ar1 point" used in this article refers to the temperature at which austenite begins to transform into pulex (ferrous iron + carbides) when the steel is cooled.

The term "Ar3 point" used herein refers to the temperature at which austenite begins to transform into ferrous iron when the temperature of the steel decreases.

The term "Ms point" used herein refers to the starting temperature of the Martensitic phase transformation reaction.

The term "Mf point" used herein refers to the termination temperature of the Martensitic phase transformation reaction.

Referring to FIG. 1 , FIG. 1 shows a schematic flow chart of a manufacturing method according to an embodiment of the present invention. The manufacturing method S10 of the present invention comprises the following steps: (step S11) providing a steel billet, wherein the steel billet comprises, based on the weight percentage of the steel billet as 100 wt%, a carbon content of 0.30 to 0.45 wt%, a silicon content of 0.4 wt% or less, a manganese content of 0.1 wt% or more and less than 0.75 wt%, a phosphorus content of 0.01 wt% or less, a sulfur content of 0.005 wt% or less, a chromium content of 1.5 wt% or less, a molybdenum content of 0.5 wt% or less, a nickel content of 0.5 to 5.5 wt%, a copper content of 0.5 wt% or less, a vanadium content of 0.30 wt% or less, an aluminum content of 0.01 to 0.05 wt%, a calcium content of 0.05 wt% or less, and a nitrogen content of 0.01 wt% or less and the remainder being iron and inevitable impurities; (step S12) performing a reheating step on the steel blank; (step S13) performing a rolling step on the steel blank after the reheating step to form a steel material; (step S14) performing a coiling step on the steel material after the rolling step; (step S15) performing a cooling step and a normalizing step on the steel material after the coiling step; (step S16) performing a hot stamping step on the steel material after the cooling step and the normalizing step to form a steel plate. The steel billet can be prepared by steelmaking or electric furnace.

In some embodiments, the reheating step is to reheat the steel billet to 1050 to 1200° C. before performing the rolling step.

In addition, the finishing temperature of the rolling step is above the Ar3 point of the material + 50°C. In some embodiments, the finishing temperature of the steel blank is 850 to 1000°C. The coiling temperature of the coiling step is controlled to be between the Ar1 point of the material and Ar1 point - 200°C. The temperature of the normalization step is between the Ac3 point of the material + 80°C and Ac3 point + 150°C, and the temperature is averaged for 6 to 15 minutes. After the hot stamping step, the steel plate is demolded after it reaches below the Mf point - 50°C.

In addition, the cooling step is to cool the steel material to room temperature, and an unwinding and slitting step may be performed.

Optionally, after the coiling step, a box sealing annealing step is further included, and the temperature range of the box sealing annealing step is between Ac1 point - 50°C and Ac1 point - 100°C.

Furthermore, optionally, after the hot stamping step, a tempering step is further included, wherein the steel plate is placed in a heating furnace at a temperature of Ms point -80°C to Ms point -120°C for 30 to 90 minutes for tempering treatment.

The following will illustrate the embodiments of the present invention by way of example. It should be noted that the examples shown herein illustrate preferred embodiments of the present invention in a form, and such examples should not be construed as limiting the scope of the present invention in any way.

Embodiment 1

The steel billet of Example 1 is an alloy of the present application, that is, the steel billet composition is: based on the weight percentage of the steel billet as 100 wt%, the carbon content is 0.30 to 0.45 wt%, the silicon content is 0.4 wt% or less, the manganese content is 0.1 wt% or more and less than 0.75 wt%, the phosphorus content is 0.01 wt% or less, the sulfur content is 0.005 wt% or less, the chromium content is 1.5 wt% or less, the molybdenum content is 0.5 wt% or less, the nickel content is 0.5 to 5.5 wt%, the copper content is 0.5 wt% or less, the vanadium content is 0.30 wt% or less, the aluminum content is 0.01 to 0.05 wt%, the calcium content is 0.05 wt% or less, and the nitrogen content is 0.01 The following wt% and the rest are iron and unavoidable impurities.

The steel blank containing the alloy composition of the present invention is subjected to reheating, rolling, coiling, cooling, normalization, hot stamping, etc., wherein the processing steps can be made into different shapes depending on the subsequent use. The preparation steps are shown in (a)-(d) below:

(a) The obtained steel billet is heated to 1050°C to 1200°C and hot rolled.

(b) The final rolling temperature in the hot rolling stage is 850℃ to 1000℃, which is above the Ar3 point of the material + 50℃.

(c) The coil temperature is controlled between Ar1 point and Ar1 point - 200°C, and then unwinding and slitting are carried out after it drops to room temperature.

(d) The slit steel is normalized at a temperature of Ac3 point + 80°C to Ac3 point + 150°C, and after averaging the temperature for 6 to 15 minutes, hot stamping is performed. When the temperature of the steel plate drops to below Mf point -50°C, it is removed from the mold, which is the steel of Example 1.

Embodiment 2

The steel blank with the same alloy composition as that of Example 1 is subjected to reheating, rolling, coiling, cooling, normalization, hot stamping, etc., wherein the processing steps can be used to produce different shapes depending on the subsequent use. The preparation steps are as follows (a), (b), (c), (c1), (d), (e):

(a) The obtained steel blank is heated to 1050 to 1200°C and hot rolled.

(b) The final rolling temperature in the hot rolling stage is 850 to 1000°C, which is above the material's Ar3 point + 50°C.

(c) The coiling temperature is controlled between Ar1 point and Ar1 point - 200°C.

(c1) Carry out box sealing annealing treatment in the temperature range of Ac1 point -50℃ to Ac1 point -100℃, and unwind and slit the steel coil after it cools to room temperature.

(d) The slit steel plate is normalized at a temperature of Ac3 point + 80°C to Ac3 point + 150°C. After averaging the temperature for 6 to 15 minutes, hot stamping is performed. The steel plate is demolded when the temperature drops to below Ms point - 150°C.

(e) After cooling, the steel plate is placed in a heating furnace at a temperature of Ms point -80°C to Ms point -120°C for 30 to 90 minutes for tempering treatment, which is Example 2.

Comparison Example 1

Different from Examples 1 and 2, Comparative Example 1 uses a steel billet of SCM440 composition specification, that is, based on the weight percentage of the steel billet as 100 wt%, the carbon content is 0.40 wt%, the silicon content is 0.20 wt%, the manganese content is 0.80 wt%, the phosphorus content is less than 0.01 wt%, the sulfur content is less than 0.005 wt%, the chromium content is 1.0 wt%, the molybdenum content is less than 0.2 wt%, the nickel content is less than 0.25 wt%, the copper content is less than 0.3 wt%, the aluminum content is 0.03 wt%, the calcium content is less than 0.05 wt%, the nitrogen content is less than 0.01 wt%, and there is substantial iron and insignificant impurities. The steel blank of the composition of Comparative Example 1 is subjected to the steps of reheating, rolling, coiling, cooling, normalization, hot stamping, etc. of the present invention, wherein the processing steps can be made into different shapes depending on the subsequent use. The steel preparation steps of Comparative Example 1 are shown as follows (a), (b), (c), and (d):

(a) The obtained steel blank is heated to 1050 to 1200°C and hot rolled.

(b) The final rolling temperature in the hot rolling stage is 850℃ to 1000℃, which is above the Ar3 point of the material + 50℃.

(c) The coil temperature is controlled between Ar1 point and Ar1 point - 200°C, and then unwinding and slitting are carried out after it drops to room temperature.

(d) The slit steel is normalized at a temperature of Ac3 point + 80°C to Ac3 point + 150°C, and after 6 to 15 minutes of temperature balancing, hot stamping is performed. When the temperature of the steel plate drops to below Mf point -50°C, it is removed from the mold, which is the steel of Comparative Example 1.

Next, the flatness measurement, hardness test, low temperature impact test and microstructure observation were performed on Examples 1, 2 and Comparative Example 1. The testing method is as follows.

Flatness measurement: The steel materials of Example 1, Example 2, and Comparative Example 1 were placed on a measuring platform, and the flatness of the steel plates was measured using a gap gauge.

Hardness test: After grinding away the rust and decarburized layer from the steel surfaces of Example 1, Example 2, and Comparative Example 1, the steel surfaces were tested using a Brinell hardness tester with a load of 3000 kgf and a tungsten carbide indenter. In addition, a Rockwell hardness tester was used to perform a hardness test.

Low temperature impact test: The steel materials of Example 1, Example 2, and Comparative Example 1 were processed into Charpy impact test pieces with v-shaped grooves in the (L-T) and (T-L) directions, and then cooled to -40°C for impact test. The impact absorption values in the (L-T) and (T-L) directions were measured respectively.

Microstructure observation: The steel samples of Example 1, Example 2, and Comparative Example 1 were observed using an electron microscope.

result

The flatness measurement results show that the flatness of Example 1, Example 2, and Comparative Example 1 are 0 mm/1000 mm, 0 mm/1000 mm, and 0.35 mm/1000 mm, respectively.

Table 1 shows the hardness, Charpy low-temperature impact absorption energy, and tensile test results of Example 1, Example 2, and Comparative Example 1. From the data in Table 1, it can be seen that the Brinell hardness of Comparative Example 1 and Example 1 can reach 558 HBW and 578 HBW respectively, while the hardness of Example 2 is 515 HBW. In terms of low-temperature impact absorption energy, both Example 1 and Example 2 are more advantageous than Comparative Example 1. The impact absorption energy in the LT direction of Example 1 can reach 34.1 J/cm ² and the impact absorption energy in the TL direction can reach 33.1 J/cm ^2. The impact absorption energy in the LT direction of Example 1 can reach 49.5 J/cm ² and the impact absorption energy in the TL direction can reach 41.5 J/cm ² . The impact absorption energy of Comparative Example 1 is only 9 J/cm ² . In terms of tensile properties, Example 1 also exhibits better elongation than Comparative Example 1. [Table 1] hardness Impact absorption Pull body (T direction) Hc HBW LT direction (J/cm ² ) TL direction (J/cm ² ) YS (MPa) TS (MPa) EL (%) Embodiment 1 56.1 578 34.1 33.1 1363.2 2110.8 10.2 Embodiment 2 52.8 515 49.5 41.5 1373.8 1895.0 11.2 Comparison Example 1 54.9 558 9.0 9.0 1250.0 1972.0 3.2

The steel materials of Example 1 and Example 2 are obtained by hot rolling, coiling and hot stamping. The structure of Example 1 (Figure 2) is composed of self-tempered ferrous metal, and is filled with a large number of dispersed Fe3C and VC carbides. The long axis size of Fe3C carbides is about 50 nm to 100 nm, and VC is spherical with a diameter of about 20 nm. The structure of Example 2 (Figure 3) is composed of tempered ferrous metal, and contains a large number of dispersed Fe3C carbides. The long axis size of Fe3C carbides is about 50 nm to 200 nm. The structure of Comparative Example 1 (Figure 4) is mainly composed of self-tempered Fe3C and Fe3C. The carbides in the self-tempered Fe3C are larger, and the long axis size of Fe3C carbides is between 50 nm and 150 nm.

By observing the surface morphology of the tensile fracture surfaces of Comparative Example 1 (FIG. 5) and Example 1 (FIG. 6), it is found that the fracture surface of Example 1 is characterized by a dimple shape and is ductile failure. However, due to the high manganese (Mn) content in the composition of Comparative Example 1, a lot of MnS can be seen on the fracture surface. These MnS easily cause hydrogen to invade the material, which easily causes the material to become hydrogen brittle, resulting in poor impact toughness and elongation.

The present invention is a production technology for high-flatness ultra-high-strength steel plates, which combines hot rolling and hot stamping processes. Through the high uniform cooling characteristics of in-die quenching, a high-speed cooling rate of more than 10°C per second is used to avoid the occurrence of phase transformation reactions between granular iron and ductile iron. The steel is quenched in the hot stamping die to below 100°C, and a structure composed of tempered Matan loose iron (TM) can be obtained, with a hardness of more than 450 HBW. In addition to high hardness and excellent low-temperature toughness, the steel with this structure has good overall flatness and can be applied to mechanical structures that require ultra-high hardness and low-temperature toughness. Compared to the traditional ultra-high hardness steel plates produced by off-line quenching and tempering, the present invention is produced by hot stamping in-die quenching. Due to the good temperature uniformity of the quenching process, a steel plate with excellent flatness can be obtained after the phase transformation, and the flatness can reach less than 1 mm/1000 mm. And through the alloy design and the characteristics of the in-die quenching cooling rate that is first fast and then slow, the hemp iron generated by the phase transformation will undergo self-tempering, and there is no need for off-line tempering operations, thereby greatly reducing energy loss, and forming a uniform self-tempered hemp iron structure, which can significantly improve the low-temperature toughness of the steel, making the performance of the steel of the present invention superior to traditional off-line quenched and tempered steel.

Therefore, the present invention also provides a high flatness and ultra-high strength steel manufactured by the manufacturing method as described above, wherein the hardness of the steel is 450 HBW or more, the flatness is less than 1 mm/1000 mm, the -40°C LT impact value is 34 J/cm2 or ^more , the -40°C TL impact value is 33 J/ ^cm2 or more, and the elongation is 10% or more.

S10: Manufacturing method of the present invention S11~S16: Steps

[Figure 1]: Schematic diagram of the manufacturing method of the embodiment of the present invention. [Figure 2]: Schematic diagram of the secondary electron image of the steel structure of the embodiment 1 of the present invention. [Figure 3]: Schematic diagram of the secondary electron image of the steel structure of the embodiment 2 of the present invention. [Figure 4]: Schematic diagram of the secondary electron image of the steel structure of the comparative example 1 of the present invention. [Figure 5]: Schematic diagram of the fracture surface morphology of the tensile test piece of the comparative example 1 of the present invention. [Figure 6]: Schematic diagram of the fracture surface morphology of the tensile test piece of the embodiment 1 of the present invention.

S10: Manufacturing method of the present invention

S11~S16: Steps

Claims (4)

A method for manufacturing high-flatness and ultra-high-strength steel comprises the following steps: Providing a steel billet, the steel billet composition is: based on the weight percentage of the steel billet as 100 wt%, the carbon content is 0.30 to 0.45 wt%, the silicon content is less than 0.4 wt%, the manganese content is more than 0.1 wt% and less than 0.75 wt%, the phosphorus content is less than 0.01 wt%, the sulfur content is less than 0.005 wt%, the chromium content is less than 1.5 wt%, the molybdenum content is less than 0.5 wt%, the nickel content is 0.5 to 5.5 wt%, the copper content is less than 0.5 wt%, the vanadium content is less than 0.30 wt%, the aluminum content is 0.01 to 0.05 wt%, the calcium content is less than 0.05 wt%, the nitrogen content is 0.01 wt% or less and the rest is iron and inevitable impurities; The steel blank is subjected to a reheating step, wherein the reheating step is to reheat the steel blank to 1050 to 1200°C; The steel blank after the reheating step is subjected to a rolling step to form a steel material, wherein the completion temperature of the rolling step is above the Ar3 point of the steel blank + 50°C, wherein the Ar3 point is the temperature at which austenite begins to transform into granular iron when the steel material is cooled; The steel material after the rolling step is subjected to a coiling step, wherein the coiling temperature of the coiling step is between the Ar1 point of the material and Ar1 point-200°C, wherein the Ar1 point is the temperature at which austenite begins to transform into pulex (ferrous iron + carbide) when the steel material is cooled; The steel material after the coiling step is subjected to a cooling step and a normalizing step, wherein the cooling step is to cool the steel material to room temperature, and then to an uncoiling and slitting step, and the temperature of the normalizing step is between the Ac3 point + 80°C and the Ac3 point + 150°C of the material, and the temperature is averaged for 6 to 15 minutes, wherein the Ac3 point is the temperature at which the granular iron is completely transformed into austenitic iron when the steel material is heated; The steel material after the cooling step and the normalization step is subjected to a hot stamping step to form a steel plate, wherein after the hot stamping step, the steel plate is removed from the mold after the temperature reaches below the Mf point -50°C, wherein the Mf point is the termination temperature of the phase transformation reaction of the Martensitic phase. The manufacturing method as described in claim 1, wherein after the coiling step, a box annealing step is further included, and the temperature range of the box annealing step is between Ac1 point - 50°C and Ac1 point - 100°C, wherein the Ac1 point is the temperature at which ferrite (ferrous iron + carbide) begins to transform into austenite when the steel is heated. The manufacturing method as described in claim 1, wherein after the hot stamping step, a tempering step is further performed, wherein the steel plate is placed in a heating furnace at a temperature of Ms point - 80°C to Ms point - 120°C for 30 to 90 minutes for tempering treatment, wherein the Ms point is the starting temperature of the Martensite phase transformation reaction. A high flatness and ultra-high strength steel manufactured by the manufacturing method as described in any one of claims 1 to 3, wherein the hardness of the steel is 450 HBW or more, the flatness is less than 1 mm/1000 mm, the -40°C LT impact value is 34 J/ ^cm2 or more, the -40°C TL impact value is 33 J/ ^cm2 or more, and the elongation is 10% or more.