Production method for reducing banded structures of Q345R (R-HIC) medium-thickness plate

Abstract

A production method for reducing the banded structure of a Q345R (R-HIC) medium-thickness plate comprises the following steps that (1) heating is conducted, specifically, a continuous casting blank is heated in a heating furnace, the heating temperature ranges from 1180 DEG C to 1230 DEG C, and the heating time is determined according to the plate thickness and ranges from 0.5 min / mm to 1.0 min / mm; (2) hot rolling is conducted, specifically, the heated continuous casting billet is subjected to 7-9 passes of rough rolling and 7-9 passes of finish rolling to be hot-rolled into a medium-thickness plate, the initial rolling temperature of finish rolling is 930-960 DEG C, and the final rolling temperature of finish rolling is 810-850 DEG C; and (3) cooling is conducted, specifically, the hot-rolled medium-thickness plate is cooled to 500 DEG C or below through a two-stage cooling process, the cooling speed of the first stage is larger than or equal to 30 DEG C / s, the corresponding cooling final temperature ranges from 600 DEG C to 650 DEG C, the cooling speed of the second stage is 2-5 DEG C / s, and the corresponding cooling final temperature ranges from 500 DEG C or below. On the basis that the HIC resistance is met, the grade of the banded structure in the hot-rolled Q345R (R-HIC) steel is controlled to be 2.5 or below, the technological process is short, the production cost is low, and the production method is quite economical and effective.

Description

Technical Field

[0001] The invention belongs to the field of hot-rolled medium and thick plate preparation, and relates to a production method for reducing the banded structure of a Q345R (R-HIC) medium and thick plate. Background Art

[0002] With the vigorous development of my country's petrochemical industry, the demand for pressure vessel steel represented by hot-rolled Q345R (R-HIC) steel has increased dramatically. In the production process of medium and thick Q345R (R-HIC) steel, due to the existence of element segregation bands (mainly C and Mn segregation), a banded structure with alternating ferrite and pearlite is often formed in the hot-rolled plate. This banded structure not only causes the anisotropy of the mechanical properties of the steel plate, but also increases the tendency of hydrogen-induced cracking. Therefore, it is of great significance to reduce or even eliminate the influence of banded structure on hot-rolled Q345R (R-HIC) medium and thick plates.

[0003] In actual industrial production, post-rolling offline heat treatment such as normalizing, modulation and other new processes are usually used to control the banded structure inside the hot-rolled medium and thick plates. In comparison, the use of online hot rolling control including rolling system and temperature system to achieve banded structure control can not only simplify the process flow and reduce production costs, but also help to improve the surface quality of hot-rolled finished plates. At present, the relevant technologies for reducing and eliminating the banded structure of hot-rolled medium and thick plates using online hot rolling control include: Chinese patent CN110016616A discloses a production method for improving the banded structure of steel plates. This method can effectively reduce the banded structure by reducing the start-rolling temperature of the finishing rolling and controlling the final rolling temperature and water entry temperature, thereby improving the rapid diffusion of alloy elements. However, this method also uses ultra-fast cooling technology to cool to room temperature, which makes it easy to generate hard phase martensite structure, resulting in a significant deterioration in the resistance to hydrogen induced cracking (HIC).

[0004] Chinese patent CN111334721A discloses a normalizing rolling method for medium-thick ship plate steel and a method for eliminating banded structure. The production method adds microalloying elements such as Nb, V, and Ti to the steel, and performs high-reduction finishing rolling in the non-recrystallized zone to obtain a refined structure. However, the addition of microalloying elements such as Nb, V, and Ti will significantly deteriorate the hydrogen-induced cracking (HIC) resistance.

[0005] In summary, for hot-rolled Q345R (R-HIC) medium and thick plates, the existing online hot rolling process including rolling system and temperature system is difficult to take into account both the effective elimination of banded structure and the requirements of anti-HIC performance. Summary of the invention

[0006] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and to provide an online hot rolling production method with a short process flow and low production cost for Q345R (R-HIC) medium and thick plates, thereby improving the hot-rolled strip structure while meeting the HIC resistance performance.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions: A production method for reducing the banded structure of Q345R (R-HIC) medium and thick plates, comprising the following steps: 1) Heating: Heat the continuous casting billet in a heating furnace at a temperature of 1180-1230°C and a heating time of 0.5-1.0 min / mm for the plate thickness (mm). 2) Hot rolling: The heated continuous casting billet is hot rolled into medium and thick plates by 7 to 9 passes of rough rolling and 7 to 9 passes of finishing rolling. The starting rolling temperature of finishing rolling is 930 to 960°C, and the final rolling temperature of finishing rolling is 810 to 850°C. 3) Cooling: After hot rolling, the medium and thick plates adopt a two-stage cooling process, in which the cooling rate in the first stage is ≥30℃ / s, corresponding to the cooling end temperature of 600~650℃, and the cooling rate in the second stage is 2~5℃ / s, corresponding to the cooling end temperature below 500℃.

[0008] Furthermore, the chemical composition mass percentage of the Q345R (R-HIC) steel is C=0.16%~0.20%, Mn=1.10%~1.30%, Si=0.1%~0.5%, Cu=0.10%~0.30%, P≤0.015%, S≤0.005%, and the remainder is Fe and unavoidable inclusions.

[0009] Furthermore, in step 3), the cooled continuous casting billet is cold loaded into the furnace.

[0010] Furthermore, in step 3), the first stage of cooling adopts DQ ultra-fast cooling; Furthermore, the second stage cooling method of step 3) cooling adopts ACC laminar flow cooling.

[0011] Technical principle of the present invention: The present invention reduces the banded structure through the "online hot rolling control + two-stage cooling" process. The technical principle is: first, finish rolling is performed at a lower temperature in the recrystallization zone to obtain fine and equiaxed recrystallized grains. Subsequently, through the first stage of rapid cooling to the ferrite low-temperature phase transformation zone, the large undercooling causes the ferrite phase transformation to occur simultaneously in the Mn-poor and Mn-rich zones, destroying the influence of the element segregation band on the cooling phase transformation. At the same time, the fine austenite grains provide a large number of nucleation points for the ferrite phase transformation, thus eventually forming dispersed ferrite and pearlite particles. In addition, the second stage of slow cooling treatment avoids the formation of hard phase martensite, which is conducive to the maintenance of hydrogen-induced cracking (HIC) resistance.

[0012] The present invention has the following beneficial effects: the offline heat treatment process is omitted through the "online hot rolling regulation + two-stage cooling" process, the heat treatment time is greatly saved, and the effective control of the banded structure is achieved while ensuring the anti-HIC performance. The grade of the banded structure after regulation is below grade 2.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is the metallographic structure diagram of the steel plate rolled in Example 1.

[0014] Figure 2 This is the metallographic structure diagram of the steel plate rolled in Example 2. DETAILED DESCRIPTION

[0015] The present invention is further described below with reference to the embodiments. Example 1

[0016] The chemical composition of the Q345R (R-HIC) steel used in this embodiment is shown in Table 1. The specific process steps are as follows: 1) Heating: The continuous casting billet with a thickness of 264 mm is cold loaded into the furnace, the heating temperature is 1200°C, and the heating time is 4 hours; 2) Hot rolling: The heated continuous casting billet is subjected to 7 passes of rough rolling and 7 passes of finishing rolling (start rolling temperature 946°C, final rolling temperature 838°C) and then hot rolled into a steel plate with a thickness of 36 mm; 3) Cooling: After hot rolling, the medium and thick plates are first cooled to 643°C by DQ method (cooling rate ≥ 30°C / s), and then slowly cooled to 492°C by ACC (cooling rate 2~5°C / s). Finally, the metallographic structure of this embodiment is as follows Figure 1 As shown, the banded tissue grade is 2.5. Example 2

[0017] This embodiment also uses the Q345R (R-HIC) steel in Table 1, and the specific process steps are as follows: 1) Heating: The continuous casting billet with a thickness of 264 mm is cold loaded into the furnace, the heating temperature is 1200°C, and the heating time is 4 hours; 2) Hot rolling: The heated continuous casting billet is subjected to 7 passes of rough rolling and 9 passes of finishing rolling (start rolling temperature 929°C, final rolling temperature 820°C) and then hot rolled into a steel plate with a thickness of 25 mm; 3) Cooling: After hot rolling, the medium and thick plates are first cooled to 635°C by DQ method (cooling rate ≥ 30°C / s), and then air-cooled to 487°C (cooling rate 2-5°C / s). Finally, the metallographic structure of this embodiment is as follows Figure 2 As shown, the banded tissue grade is 1.5.

[0018] Table 1 Chemical composition used in the examples (wt%) 。

Claims

1. A production method for reducing the banded structure of Q345R (R-HIC) medium and thick plates, characterized in that The process steps include: 1) Heating: Heat the continuous casting billet in a heating furnace at a temperature of 1180-1230°C and a heating time of 0.5-1.0 min / mm for the plate thickness (mm). 2) Hot rolling: The heated continuous casting billet is hot rolled into medium and thick plates by 7 to 9 passes of rough rolling and 7 to 9 passes of finishing rolling. The starting rolling temperature of finishing rolling is 930 to 960°C, and the final rolling temperature of finishing rolling is 810 to 850°C. 3) Cooling: After hot rolling, the medium and thick plates adopt a two-stage cooling process, in which the cooling rate in the first stage is ≥30℃ / s, corresponding to the cooling end temperature of 600~650℃, and the cooling rate in the second stage is 2~5℃ / s, corresponding to the cooling end temperature below 500℃.

2. A production method for reducing banded structure of Q345R (R-HIC) medium and thick plates according to claim 1, characterized in that: The chemical composition mass percentage of the Q345R (R-HIC) steel is C=0.16%~0.20%, Mn=1.10%~1.30%, Si=0.1%~0.5%, Cu=0.10%~0.30%, P≤0.015%, S≤0.005%, and the remainder is Fe and unavoidable inclusions.

3. A production method for reducing banded structure of Q345R (R-HIC) medium and thick plates according to claim 1, characterized in that: Step 2) The hot-rolled continuous casting billet is placed into the furnace by cold charging.

4. A production method for reducing banded structure of Q345R (R-HIC) medium and thick plates according to claim 1, characterized in that: Step 3) The first stage of cooling uses DQ ultra-fast cooling.

5. The method for reducing the banded structure of Q345R (R-HIC) medium and thick plates according to claim 1, characterized in that: Step 3) The second stage of cooling uses ACC laminar cooling.

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