JPH0380848B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for obtaining a steel material of a desired quality by controlling the transformed structure during cooling, and is particularly suitable for cooling control after continuous hot rolling. (Prior art) The type of transformation structure of a steel whose material quality is determined by transformation depends on the composition of the steel and various manufacturing conditions (in the case of hot-rolled steel, slab heating temperature, rolling conditions,
Among them, the influence of cooling conditions is particularly large, and its control is important for adjusting the material quality. In the hot rolling process, the cooling conditions are controlled by controlling the exit temperature of the final rolling stage and the coil winding temperature, but the following two methods are used to set the cooling conditions. It is being one is
This method uses CCT diagrams and TTT diagrams to set a cooling pattern to obtain the desired transformed structure.
However, actual cooling does not take simple temperature changes such as the CCT diagram corresponding to linear cooling or the TTT diagram corresponding to constant temperature maintenance, but more complex temperature changes (especially changes in the cooling curve due to transformation heat generation). Therefore, it is impossible to accurately set cooling conditions using this method. Another method, as seen in Japanese Patent Publication No. 58-2246 and Japanese Patent Application Laid-Open No. 56-119741, is to create a model to predict the progression of transformation during cooling and find the optimal cooling pattern to obtain the desired material quality. It's a method. (Problems to be Solved by the Invention) The method described above is a control method by presetting cooling conditions, and since the actual amount of transformation during cooling is not known, it is difficult to control by feedback or feedback. Therefore, if the cooling conditions vary from the set conditions, it becomes impossible to obtain the desired tissue. In addition, the manufacturing conditions before cooling (slab heating temperature, rolling conditions) also affect the final transformed structure through changes in the austenite structure before transformation, but even if the manufacturing conditions before cooling change, the objective It will not be possible to obtain the desired organization. In the present invention, changes in the progress of transformation during cooling due to variations in manufacturing conditions are actually measured using a transformation amount meter set in the middle of the cooling zone, and based on this, subsequent cooling conditions are corrected to maintain a constant final material quality. This was done for the purpose of preserving. (Means and effects for solving the problem) The gist of the present invention is to control the temperature and amount of transformation of the steel material after the start of transformation of the steel material during cooling, when controlling the cooling of the steel material after continuous hot rolling. a step of actually measuring the actual value, a step of predicting the subsequent progress of transformation of the steel material using the following formulas (1) to (3), and a step of predicting the predicted final structure and the target material structure. This is a steel cooling control method characterized by comprising a step of comparing the cooling conditions, and a step of correcting the cooling conditions based on the comparison results. dx/dt=α(1-X) ……(1) α=A・1/Dγ・GR・(1n1/1−X) ^2/3 ……(2) α=B・1/Dγ・GR _P ...(3) However, α in equation (2) for ferrite transformation and bainite transformation, and (3) for pearlite transformation.
Substituting α in the formula into (1), where: X: Measured amount of transformation of the steel material during cooling α: Composition of steel material, initial austenite grain size, actual measured value of transformation amount during cooling, and actual measured value of temperature Determining function Dγ: Grain size of γ before transformation GR: Growth rate of ferrite and bainite GR _P : Growth rate of pearlite A, B: Constant. As a result of repeated basic heating (austenitization)-cooling experiments and actual field experiments, the present inventors found that when a steel material is cooled, the transformation rate dX/dt due to normal diffusion generally depends on the transformation rate X at that time. We found that it is expressed as dX/dt=α(1-X)...(1). Here α is the component,
This is a function determined by the austenite structure before transformation and X. Since this equation does not include time, the transformation rate at that time can be predicted regardless of the progress of cooling or transformation up to that point. Also,
α depends on the type of transformation; specifically, it can be described as equation (2) for phallite transformation and bainite transformation, and as equation (3) for pearlite transformation. α=A・1/Dγ・GR・(1n1/1−X) ^2/3 ……(2) α=B・1/Dγ・GR _P ……(3) However, X is the metamorphosis rate and Dγ is the metamorphosis The particle size of the previous γ,
GR is the growth rate of ferrite and bainite;
GR _P is the growth rate of pearlite. Also A, B
is a constant determined from actual measurements, and can be determined from an isothermal curve or a constant velocity cooling curve. Here, X is a value obtained from actual measurements, and Dγ is the austenite grain size before transformation. This value is determined by calculating the progress of crystallization. In addition, GR and GR _P are values determined from the sample components and humidity, and here they are expressed by equations (4) and (5) (C. Zener:
Trans.AIME, 167 (1946), P.550 and M.
Hillert: Jernkonl. Ann., 141 (1957), p. 757). GR=1/r・Cγα−Cγ/Cγ−Cα・exp(−Q ₀ /T)
...(4) GR _P = (TA ₁ −T)・(Cγα −Cγβ)・exp(−Q ₀ /T) ...(5) However, Q ₀ is a constant, and T is the value on the runout table. The steel temperature, TA ₁ , is the A ₁ transformation temperature, which is determined by the composition of the steel. Cγα, Cα, Cγβ,
r is a value determined by ordinary thermodynamic calculations based on the composition and temperature of the steel material, and the amount of carbon at the interface of each phase that determines the growth rate (varies depending on added elements other than carbon). carbon content). Furthermore, Cγ is a value calculated from the amount of added carbon in the steel material using equation (6). Cγ=(Co−X _F Cα)/(1−X _F ) (6) where Co is the amount of added carbon and X _F is the fraction transformed to ferrite. Generally, when hypoeutectoid steel is cooled from a high-temperature austenite state, it first transforms into ferrite, and then untransformed austenite transforms into pearlite, bainite, or martensite. The state of transformation during the above cooling process is shown as an example in FIG. 3, with the horizontal axis representing the temperature of the steel material and the vertical axis representing the transformation rate of the steel material. In the figure, Bs and Ms are the bainite transformation start temperature (Bs) and martensitic transformation temperature (Ms), which will be described later. The progress of metamorphosis at this time is calculated as follows. The temperature change during cooling is divided into steps and considered as a collection of constant temperature maintenance during a minute time Δt. The start of ferrite transformation is considered to be the Ae ₃ temperature determined from thermodynamic data, and the calculation of the transformation progress is started when the temperature during cooling reaches the Ae ₃ temperature. The subsequent metamorphosis progress is expressed as the amount of metamorphosis in a minute time Δt as dx/dt・Δt
It is calculated using equation (1) and calculated from the accumulation from the start of metamorphosis. At this time, first of all, the ferrite metamorphosis progresses (1),
Calculations are made using equations (2) and (4), but considering the concentration of solid solute carbon in untransformed austenite as the transformation progresses, the average amount of solid solute carbon in untransformed austenite is calculated using equation (6). It is considered that pearlite transformation begins when the amount of solid solute carbon reaches a critical value determined from thermodynamic calculations based on the components and temperature. For the subsequent transformation progress, use equations (1), (3), and (5) to calculate the amount of transformation progress in a minute time Δt, as in the case of ferrite transformation.
Calculated by finding and accumulating dx/dt・Δt. Bainite transformation starts when the temperature during cooling reaches the bainite transformation start temperature Bs determined by the components determined from equation (7), and the subsequent transformation progress is determined using equations (1), (2), and (4). Calculate using the same procedure as ferrite metamorphosis. Bs (℃) = 727.5-425 [wt%C] -42.5[wt%Mn] ...(7) Here, [wt%C] is the amount of added carbon, [wt%
Mn] is the amount of added Mn. Regarding martensitic transformation, it is calculated assuming that all remaining untransformed austenite transforms into martensite when the temperature during cooling reaches the martensitic transformation temperature (Ms) determined by the components. In this method, final structure control is performed, but since the aim is to control the final steel strength (TS), it is necessary to correlate the structure and steel strength. For this purpose, the following equation is used in this method. TS=k ₁ {f _F・(H _F +k ₂ Dα ^-1/2 ) +f _P・H _P +f _B・H _B +f _M・H _M } ...(8) Here, k ₁ and k ₂ depend on the steel type. is a determined constant, f is the ratio of each phase and is a value obtained from the above calculation, and H is the intensity of each phase. However, F, P, B, and M represent ferrite, pearlite, bainite, and martensite, respectively. Further, Dα indicates the ferrite grain size after cooling, and this value can be found from the following equation. Dα={5.51×10 ¹⁰・Dγ ^1.75・exp(−21430/T ₅ )・f _F } ^1/3 …(9) Here, Dγ is the austenite grain size before transformation, and as mentioned above, for example, iron and Steel 70 (1984),
This is a value determined by the method shown in P2112. T ₅ is the temperature at which the transformation rate is considered to have completed the generation of ferrite nuclei, which determines the final ferrite particle size in the calculation of the transformation progress described above, and f _F is the ferrite transformation rate. It is. The transformation prediction formula that is the basis of this prediction method uses the transformation rate at a certain time as a function that does not include the time from the start of cooling, in other words, as a function of the components, temperature, and transformation rate. ) The subsequent progress of transformation can be easily predicted by actually measuring the temperature and transformation rate. The present invention is based on the above groundbreaking discovery, and by actually measuring the temperature and transformation rate during transformation, it is possible to compensate for deviations from predictions due to variations in various manufacturing conditions by adjusting subsequent cooling conditions. This makes it possible to suppress variations in the final material as much as possible. A specific example of the invention is described above with respect to a continuous hot rolling mill. Figure 1 shows the entire system. A cooling pattern to obtain a structure corresponding to the desired material quality is determined in advance from the composition of the steel material and the austenite grain size after finish rolling using the above-mentioned prediction method, and the cooling conditions are set to obtain this temperature pattern ( →→). Next, the temperature and transformation rate are measured using a thermometer and a transformation rate meter installed in the middle of the cooling zone. If the measured temperature and transformation rate are not different from the predicted temperature and transformation rate, subsequent cooling is performed according to the initially set cooling conditions. If the measured temperature and transformation rate are different from the predicted temperature and transformation rate, use formula (1) based on the measured temperature and transformation rate to obtain the structure ratio or phallite grain size corresponding to the desired material. Find the cooling conditions again and change the cooling conditions (→→). Various methods can be considered as a metamorphosis meter, but
For example, it is possible to measure the amount of transformation of a steel plate with a thickness of approximately 10 mm or more online using a transmission eddy current method. This method measures the amount of transformation by sensing changes in the magnetic properties of steel. The device consists of an oscillating coil and a receiving sensor, and is installed with a steel plate sandwiched between the two devices. The features of this device include that it is resistant to flapping when steel material is threaded, that the steel sheet can be wet with cooling water, and that the device itself is simple. With this method, measurements can only be made at temperatures below the Curie point of iron (approximately 770°C), so it is best to install the transformation meter at a location where the temperature of the steel material is below this temperature. In the case of continuous hot rolling, it is best to install it somewhere in the middle of the run-out table (ROT).
A thermometer is usually installed at this position, and a transformation meter using the transmission eddy current method is most suitable for applying the method of the present invention. (Example) An example of manufacturing a hot rolled steel plate with a tensile strength of 55 Kg/mm ² by applying the method of the present invention to steel having the components shown in Table 1 will be explained in comparison with a case where a conventional method was applied. do. Using equations (1) to (7) to calculate the structure and material properties in various cooling patterns, the final structure preset as the conditions for obtaining the above material has a ferrite percentage of 80%, a pearlite percentage of 17%, and a bainite percentage of 3. %, and the ferrite particle size was 10 μm. In addition, the preset values of the rolling conditions are the final rolling exit temperature of 870℃,
Metamorphosis rate 70% in the middle of the runout table, temperature 670℃ in the middle of the runout table, temperature at the time of surrounding
It was 600℃. Table 2 shows the results of field experiments, and as is clear from this table, the method of the present invention reduces the variation in final material properties to a greater extent than when the conventional method is applied. FIG. 2 shows examples of changes in structure and properties according to the present method and the conventional method. FIG. 2a shows an example of a part that has been rolled and cooled under preset conditions, and the material is the same as planned. Figure b shows an example of the conventional method and corresponds to the top part of the rolling process. In this part, the rolling speed was low and the final rolling exit temperature was high, so the austenite grain size became large, and the amount of ferrite in the final structure decreased due to subsequent cooling, resulting in high strength. In contrast, in an example where this method is applied, the temperature and transformation amount are actually measured using a thermometer and transformation amount meter installed in the middle of the cooling zone, and all the actual measurements up to this point are combined.
The final structure when the subsequent cooling conditions proceed under the preset conditions is calculated using equations (1) to (7) and compared with the preset structure. In this case, the predicted final structure of the Top part is 61% ferrite, 0% pearlite, 39% bainite, and 8 μm ferrite grain size.
, which is significantly different from the preset value,
In order to correct the difference, cooling conditions (cooling rate, coiling temperature) from the time when the temperature and transformation amount were actually measured.
We calculated the final structure when cooling with different conditions using equations (1) to (7), selected cooling conditions that gave a final structure close to the preset final structure, and adjusted the amount of water to give this cooling condition ( (decrease in cooling rate and increase in coiling temperature), the desired structure as shown in Fig. 2c was obtained at the top part of the rolling process, and as a result, the material had the desired steel strength.

【table】

[Table] (Effects of the Invention) Since the present invention can perform cooling while controlling the transformed structure during cooling, a steel material of the desired quality can be properly obtained, and its industrial effects are enormous.

[Brief explanation of drawings]

Figure 1 is a diagram showing an example of the entire system of the method of the present invention, Figure 2 is a photograph showing the structures of the method of the present invention and the conventional method, and Figure 3 is a graph showing an example of the transformation of steel during the cooling process. It is.

[Claims]

1. When producing sintered ore by adding water to powdered sintering raw materials consisting of iron ore, auxiliary raw materials, miscellaneous raw materials, and fuel, and loading this into a sintering machine to produce sintered ore, each powder The saturated moisture value Wi of each powdery substance is determined in advance, and the weighted average (ΣWi×Mi) of the saturated moisture value of the sintering raw material is calculated from each saturated moisture value Wi and the blending ratio Mi of each powdery substance. A method for producing sintered ore, characterized in that the sintered raw material contains water in an amount of 50±2% of the calculated weighted average saturated moisture value and is granulated.

Claims (1)

However, α in equation (2) is used for ferrite transformation and bainite transformation, and (3) is used for pearlite transformation.
Substituting α in the formula into (1), where Determining function Dγ: Grain size of γ before transformation GR: Growth rate of ferrite and bainite GR _P : Growth rate of pearlite A, B: Constant.