EP1870172A1

EP1870172A1 - A continuous casting and rolling method for medium plate

Info

Publication number: EP1870172A1
Application number: EP06722170A
Authority: EP
Inventors: Jie Liu; Hao Dong Huang; Long Zhen Li; Xu Yang; Leng Zhang; Yong Gang Ma; Xiao Chun Sha; Yi Kang Sun; Ming Ren Wang; Jie Fu Na; Heng Lu Zhang; Fu Xing Jin
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2005-03-28
Filing date: 2006-03-27
Publication date: 2007-12-26
Anticipated expiration: 2026-03-27
Also published as: AU2006228889A1; KR20070112879A; EP1870172A4; CN1840252A; AU2006228889B2; WO2006102835A1; ATE515334T1; EP1870172B1; KR100960096B1

Abstract

Disclosed herein is a continuous casting and rolling process for medium plate comprising following steps: continuous casting, during molten steel passing through molds, to form casting billets having section thickness of 100mm-170mm and width of 900-2000mm by hydraulic oscillating, continuously bending and continuously straightening; direct rolling, hot feeding and hot charging by two casters with four streams and four-streams-merging-one technique, achieving rapid hot feeding and hot charging or direct rolling for continuous casting billets; roughing by a rough rolling mill with front and back vertical rolls; finishing rolling using speed-up rolling by finishing rolling mill, each stand of the rolling mill is provided with a device for axial shifting and bending device working rollers, wherein the maximum speed is 22.8m/s and the speed of roller table is up to 5.89m/s; stream cooling using three-stage stream cooling control technique; coiling with hydraulic coiler controlled by stepping-controlled technique. The invention provides improvements on layout, device, production process etc., improving production capacity, development of new grades, product quality and production organization.

Description

Field of the invention

The application belongs to metallurgical metal processing field, more specifically relates to a continuous casting and rolling process for medium plate.

Description of the related art

Now, the producing technique of continuous hot rolling plate is mainly divided into traditional process and CSP process. The traditional process comprise continuous casting thick billet with thickness of 200mm~250mm, flame cutting, heating by walking-beam furnace, descaling, rolling by rough rolling group, shearing, descaling, rolling by finishing rolling train, stream cooling, coiling. Although this process has advantages of full categories, high output, flexible production organization and high reduction ratio and so on, this process still has many disadvantages such as heavy casting device, long rolling line, high workshop, large space requirement, large device investment, large billet storeroom, high energy consuming and so on. The CSP process comprises continuous casting thin billet with thickness of 50mm~70mm, flame cutting, heating by tunnel furnace, descaling, rolling by finishing rolling train, stream cooling, coiling. Although this process has advantages of lowering investment and production cost, omitting rough rolling train, strengthening finishing rolling train, strengthening reduction capability and so on, this process has many disadvantages such as high casting speed, high breakout ratio, too much restriction to production kinds, difficult developing new grades, high investment of tunnel furnace, complicated devices, hard to operate and maintain, rigid production organization, huge cost of accident treatment, low output and so on. Chinese Patent No. 02132970.2 of the same applicant disclosed a continuous casting and rolling process for medium/thin plate (ASP process). The ASP process comprises converter steel-making, continuous casting thick billet with thickness of 100mm~150mm, flame cutting, heating by walking-beam furnace, descaling, rough rolling by 3+1 rolling train, hot coil box, shearing, descaling, rolling by finishing rolling train, stream cooling, coiling. The ASP process has advantages of low cost, low investment, flexible production organization, simple maintaining, and low accident rate. But it still has disadvantages, for example, it uses too many old devices, the power of the motor of finishing rolling train is low, the speed of the roller table is low, the capacity of water supply is not enough, the strength of rolling mills' transmission system is not enough, the configuration of the power supply system is aging and so on. Therefore, the development of the technology is more or less limited, resulting that output can't reach requirement and the production development also be limited, and further the width precision, surface quality and shape control have certain limitation.

Summary of the invention

The invention is aimed at eliminating the disadvantages of the above plate production process. The objects of the invention is to enhancing some crucial technique so as to save energy, not only improving the heating quality of billet but also improving the uniformity of plate thickness and quality of plate shape. The invention has advantage of high width precision, improves the work rate and output of the rolling mill, and makes the continuous casting and rolling process for medium plate meet the requirement of modem manufacturing.
The production process of the invention substantially employs that of 02132970.2, and also makes some improvements basing on the former process, so as to improve the production capacity of the invention, broaden the range of the production categories, and meanwhile gain new revolution in the fields of shape control, width precision control and surface quality control. The invention also can realize direct rolling of some steel grades.
To achieve the above objects, the solution of the invention is a continuous casting and rolling process for medium plate comprising the following steps:

continuous casting, during molten steel passing through mold, to form continuous casting billet having section thickness of 100mm-170mm and width of 900mm-2000mm by hydraulic oscillating, continuously bending and continuously straightening;
direct rolling, hot feeding and hot charging by two casters with four streams and four-streams-merging-one technique, in which each of the two continuous casters' knockout roller tables respectively connects with entering roller table and exiting roller table of heating furnace, achieving rapid hot feeding and hot charging or direct rolling for continuous casting billet; continuous casting billet is fed to entering roller table of heating furnace through convey roller table, then fed into walking-beam furnace by long-course steel loader of the heating furnace in order to be heated; the exiting roller table of the continuous caster connects with the entering roller table of the heating furnace so that the continuous casting billet is directly fed into the heating furnace by the roller table for hot charging, the hot charging temperature averagely is no less than 700°C; and one exiting roller table of the continuous caster connects with the exiting roller table of the heating furnace;
rough rolling by rough rolling mill with front and back vertical rollers, in which the maximum single-pass side-pressing amount of the vertical rollers is 50mm, the maximum rolling pressure is 3800kN, and the rolling speed is 0~5.89m/s; when rolling, in each pass, acting of front vertical rollers and opening of back vertical rollers are automatically completed by computer, and the automatic control of width is realized through hydraulic automatic width control (AWC);
finishing rolling using speed-up rolling by finishing rolling train of F1~F7 with a maximum speed of 22.8m/s, in which vertical rollers are provided before F1; when thickness of the intermediate billet is 50mm, the maximum pressing amount is 10mm, and the maximum rolling pressure is 1000kN; F1~F7 finishing rolling train all are provided with a device for axial shifting and bending working rollers with a maximum axial shifting course of ±200mm and a maximum bending force of 1500kN, so as to improve the control of plate section's thickness difference and plate shape, and realize free rolling; the maximum speed of the roller table before the finishing rolling train may be up to 5.89m/s; edge heater is provided before the finishing rolling train;
stream cooling using three-stage stream cooling technique, in which roughly adjusting valve set and precisely adjusting valve set as well as online high-level water tank are provided, so as to stabilize water amount and water pressure of each cooling stage; the side water-jet pressure is 1Mpa~1.2Mpa so as to assure the plate's lengthwise temperature uniformity;
coiling with stepping-controlled full hydraulic coiler, in which automatic stepping control is performed by hydraulic-controlled side guide, pinch roller, wrapper roller's collapsible reels and high-responsivity wrapper roller to realize precise centering and micro-tension coiling of plate, reduce towered-shape, avoid the impaction of plate's head on wrapper roller and eliminate impression of plate's head and wrapper roller on plate.

In said continuous casting, direct rolling, hot feeding and hot charging, and rough rolling, the exiting roller tables are covered by insulating covers, such that the temperature drop of the casting billet and the temperature difference between the intermediate billet's head and tail are reduced and the temperature of the billet when entering the finishing rolling mill can be increased by 30 °C ~50 °C and the temperature difference between the billet's head and tail can be decreased by 30°C ~50°C..
In said rough rolling, the vertical roller of the front and back vertical rollers provided in the rough rolling mill is top-attaching driven type, the size of rollers is φ 1200mm/φ 1100mm×430mm, and the power of main motor is AC1200kW X 2X 200/400r/min.
In said finishing rolling, the vertical roller of the vertical rollers provided before F1 is top-driven attached type, the size of rollers is φ 750mm/ φ 600mm × 350mm, and the main drive motor is 2-AC 370kW X 550r/min.
In said finishing rolling, hydraulic low-inertia kinks are provided between the finishing rolling stands in order to control tension between finishing rolling train and improve plate quality.
In said stream cooling, the cooling device is provided with 12 sets of online high-level water tank with a water amount of 1700m³/h and a water pressure of 0.05Mpa~0.08Mpa.
Said mold is a mold for billet continuous casting which has a cavity whose four corners are in form of right angle or round angle or straight bevel angle, the right angle mold is traditional billet mold which is composed by two wide sidewalls and two narrow sidewalls with a right angle being formed between respective wide sidewall and narrow sidewall; the round angle or straight bevel angle mold for billet continuous casting is composed by two wide sidewalls and two narrow sidewalls, at both sides of each narrow sidewall, namely the inner sides connected with the wide sidewalls, respectively have a row of right-triangle shaped flange, the bevel of which directs to inside, one right-angle surface joins to the inner surface of the narrow sidewalls, and another right-angle surface is in the same plane with the side face of the narrow sidewalls, namely the another right-angle surface is an extension of the side face of the narrow sidewalls.
By carrying out three kinds of timing control, namely billet merging timing control, billet passing through logistics bottleneck timing control and billet exiting timing control, and comparing cushioning ability, heating ability and rolling ability, said four-streams-merging-one technique optimizes logistics in three walking-beam furnaces with cushioning zone into which the medium plate in single roller table of two casters with four streams is directly fed, so as to exert the producing capacity of caster, furnace and rolling line and realize half-rigid connection of rolling line, furnace and caster.
Said three kinds of timing control are:

Billet converge timing control: which one passes through firstly when two streams from caster A converge; which one passes through firstly when two streams from caster B converge; which one passes through firstly when streams from caster A and caster B converge; wherein such principle is followed that "when the billets converge that the stream which reaches convergence point firstly passes through firstly and if the two streams reach simultaneously then the straight stream passes through firstly";
Billet passing through logistics bottleneck timing control: determining the timing of No. 1 billet stream charging #1 larry; determining the timing of No. 4 billet stream charging #3 larry; determining the timing of converging No. 1 and No. 2 billet streams charging #2 larry; determining the timing when the billet charging #1 furnace exits A₁ roller table; determining the timing of the billet entering the furnace, wherein such principle is followed that "when the billets converge that the stream which reaches convergence point firstly passes through firstly and if the two streams reach simultaneously then the straight stream passes through firstly";
Billet exiting timing control: by tandem superimpose-calculating billet's values at each critical timing as well as checking the cushion capacity and heating capacity of the furnace and the rolling capacity of the rolling line, finding out billet's values at each critical timing of each condition, realizing semi-rigid connection of caster, furnace and rolling line, wherein such principle is followed that "when the billets converge that the stream which reaches convergence point firstly passes through firstly and if the two streams reach simultaneously then the straight stream passes through firstly".

Compared with prior arts, the invention has the following advantages:

1. Since new mold is used in the present invention, the corner cooling during molten steel solidifying into billet shell in the mold is adjusted from two-dimensional cooling to nearly one-dimensional cooling, so as to make billet shell solidification relatively uniform to avoid surface defect caused by nonuniform billet corner's cooling solidification and crack caused by too low edge temperature while direct rolling.
2. The long-course steel loader used in the present invention not only makes the furnace having the ability of heating billet but also has the cushion ability for matching continuous casting and rolling pace. If the rolling mill temporarily halts for a short while, it at most can store nine cushioning billets, therefore the casting production is not affected.
3. The device for axial shifting and bending working rollers provided in all finishing rolling trains not only can control thickness difference of plate section (crown and wedge) and plate flatness, but also can realize free rolling so as to increase rolling yield. The hydraulic low-inertia kinks used in the production line helps to control tension between finishing rolling train and improve plate quality. The speed-up rolling technique used in the finishing rolling trains shortens the feeding time of roller table, reduces temperature drop, improves thickness uniformity and shape quality and increase mill yield and output.
4. The edge heater helps to improve uniformity of billet temperature, reduce edge crack, improve edge microstructure, and stabilize operation of the finishing rolling mill.
5. The use of three-stage stream cooling control technique and the provision of roughly adjusting valve set and precisely adjusting valve set stabilize water amount and water pressure of each cooling stage and assure the plate's lengthwise temperature uniformity, so as to assure uniformity of the plate's microstructure and overcome the production defect caused by water-supply inefficiency in prior arts.

The invention improves the continuous casting and rolling process for medium/thin plate in terms of layout, devices and production process, so that the production ability, development of new grades, production quality are improved with the present invention, production capacity reaches 5000,000t per year, width precision within 0~10mm reaches 98.5%, the adjustment ability for shape control and crown control reaches advanced world standard, control ability of surface quality fully reaches the quality standard requirements of automobile's high grade panel (05), and the proportion of thin plate reaches advanced world standard.

Description of embodiment

The invention applies to a continuous casting and rolling process for 2150 medium plate.
Molten steel passes through three 250t top-bottom blowing converter, two molten iron desulphurizing and slag skimming station, two LF ladle furnace and one RH-TB vacuum processing device, and before being cast, it may be refined by LF ladle furnace or RH-TB vacuum processing device.
Continuous casting: molten steel passes through mold, to form continuous casting billet having section thickness of 100mm-170mm, width of 900-2000mm and length of 11m~18m by hydraulic oscillating, continuously bending and continuously straightening, and then the billet is subjected to flame cutting. The billet has a maximum single weight of 38t, the average number of continuously casting furnaces is 30, and the average casting time is 37.3 minute per one furnace. Because of employing the technique of mold hydraulic oscillating, the quality of casting billet can be improved and also the billet surface defect can be eliminated.
Direct rolling, hot feeding and hot charging by two casters with four streams and four-streams-merging-one technique, in which each of the two continuous casters' knockout roller tables respectively connects with entering roller table and exiting roller table of heating furnace, achieving rapid hot feeding and hot charging or direct rolling for continuous casting billet; continuous casting billet is fed to entering roller table of heating furnace through convey roller table, then fed into walking-beam furnace by long-course steel loader of the heating furnace in order to be heated. Furthermore, the invention uses direct rolling technique for some kinds of carbon steel and certain kinds of lower alloy steel. The exiting roller table of the continuous caster connects with the entering roller table of the heating furnace so that the continuous casting billet is directly fed into the heating furnace by the roller table for hot charging. The hot charging temperature averagely is no less than 700°C. One exiting roller table of the continuous caster connects with the exiting roller table of the heating furnace. Upon billet being on connecting roller table, its length is automatically measured and also its temperature is measured by pyrometer, in order that the billet can be centered according the layout and then be heated. A computer is used to optimize the heating control based on the temperature of the billet entering the furnace. The billets are heated to 1150°C~21250°C in the furnace, carried out of the furnace by the extractor of the furnace, and placed on the exiting roller table. This can not only save energy, but also shorten heating time and improve heating quality of the billet. Some kinds of carbon steel and certain kinds of lower alloy steel, for example silicon steel, may apply "lower temperature exiting" rolling technique, realizing rolling of different steel grades.
The long-course steel loader used in the present invention not only makes the furnace having the ability of heating billet but also has the cushion ability for matching continuous casting and rolling pace. If the rolling mill temporarily halts for a short while, it at most can store nine cushioning billets, therefore the casting production is not affected.
By carrying out three kinds of timing control, namely billet merging timing control, billet passing through logistics bottleneck timing control and billet exiting timing control, and comparing cushioning ability, heating ability and rolling ability, the four-streams-merging-one technique optimizes logistics in three walking-beam furnaces with cushioning zone into which the medium plate in single roller table of two casters with four streams is directly fed, so as to exert the producing capacity of caster, furnace and rolling line and realize half-rigid connection of rolling line, furnace and caster.
Said three kinds of timing control are:

Rough rolling: the heated or direct rolled billet is conveyed by roller tables and passes through high-pressure water descaling box after the furnace so as to remove scale, and then enters into four-roller reversible rough rolling mill so as to be rolled. The four-roller reversible rough rolling mill has attached front and back vertical rollers with a type of top-attaching driven. The size of rollers is φ 1200/φ 1100mm X 430mm, the maximum single-pass side-pressing amount is 50mm, the maximum rolling pressure is 3800kN, the rolling speed is 0~5.89m/s, and the power of main motor is AC1200kW X 2 X 200/400r/min. when rolling, in each pass, acting of front vertical rollers and opening of back vertical rollers are automatically completed by computer, and the automatic control of width is realized through hydraulic automatic width control (AWC) in which the front vertical rollers carry out odd pass rolling and the back vertical rollers carry out even pass rolling. The maximum side-pressing amount of each pass is 50mm. After being rolled 3~5 passes in the four-roller reversible rough rolling mill, the billet is rolled into intermediate billet with a thickness of 30~60mm. During being rolled by the four-roller reversible rough rolling mill, high-pressure water before and after the four-roller reversible rough rolling mill are used to descale regenerated scales of the billet. The front and back vertical rollers of the four-roller reversible rough rolling mill are provided with automatic width control system (AWC), its main purpose being to correct error of billet width caused by rolling pressure change and correct the width difference between the billet's head and tail. Width gage and pyrometer are provided after the four-roller reversible rough rolling mill, so as to monitor width and temperature of billet and provide correction data for setting of finishing rolling mill. The four-roller reversible rough rolling mill is also provided with dust removal device.
Insulating cover is provided at the exiting roller table in order to reduce temperature drop of intermediate billet and temperature difference between the billet's head and tail. Intermediate billet is conveyed to truncate scissor via insulating cover so as to truncate its head and tail. Detector is provided in front of the truncate scissor in order to detect the shape of the billet's head, so as to reduce temperature drop of intermediate billet and temperature difference between the billet's head and tail, and also the temperature at the time of entering finishing rolling mill can be improved by 30°C~50°C and the temperature difference between the billet's head and tail can be shorten by 30°C~50°C. Therefore, it can realize the optimal truncation of the scissor, in order to reduce truncation loss and increase yield.
Edge heater is provided in front of the scissor. The heater's parameters are as follows: the power of the main motor is 2 X 3000kW, the heating depth is 40mm~60mm, the heating width is 100mm~150mm, and the temperature rise is 60 °C~100°C. This helps to improve temperature uniformity of the billet, reduce edge crack, and improve mischcrystal of edge microstucture. Also this helps to stabilize operation of the finishing rolling mill, and simultaneously this can satisfy the requirement of dual phase steel development.
After being truncated head and tail, the billet enters into the high-pressure water descaling box before finishing rolling mill in order to remove regeneration scale.
Finishing Rolling: billet passes through the small vertical roller device F₁E before finishing rolling mill F₁ so as to be rolled. When thickness of the intermediate billet is 50mm, the maximum pressing amount is 10mm, the maximum rolling pressure is 1000kN, the main driving motor is 2-AC 370kW X 550r/min. Thereafter the billet enters finishing rolling train. Passing through F1~F7 four-roller finishing rolling train, the billet is rolled into 1.8mm~25.4mm final plate.
F1~F7 finishing rolling train all employ a device for axial shifting and bending working roller, the maximum shifting course is ±200mm, and the bending force is 1500kN. (Only F3~F6 in ASP have the device for axial shifting and bending working roller, and bending force only has positive value, no negative value.) This not only can control thickness difference of plate section (crown and wedge) and plate flatness, but also can realize free rolling and increase rolling yield.
The production line uses hydraulic low-inertia kink in order to control tension between finishing rolling train and improve plate quality.
Stream cooling: it uses three-stage stream cooling technique, in which roughly adjusting valve set and precisely adjusting valve set as well as 12 sets of online high-level water tank are provided, so as to stabilize water amount and water pressure of each cooling stage. The water amount is 1700m³/h, the water pressure is 0.05Mpa~0.08Mpa, and the side water-jet pressure is 1Mpa~1.2Mpa, so as to assure the plate's lengthwise temperature uniformity.
Coiling: it uses stepping-controlled full hydraulic coiler to coil plate, in which automatic stepping control is performed by hydraulic-controlled side guide, pinch roller, wrapper roller's collapsible reels and high-responsivity wrapper roller to realize precise centering and micro-tension coiling of plate, reduce towered-shape, avoid the impaction of plate's head on wrapper roller and eliminate impression of plate's head and wrapper roller on plate
The automatic system of the production line is three-grade computer control system, the first grade being basic automatic system, the second grade being procedure control computer system and the third grade being production control computer system.

Claims

A continuous casting and rolling process for medium plate, characterized in that said process comprises the following steps:
continuous casting, during molten steel passing through mold, to form continuous casting billet having section thickness of 100mm~170mm and width of 900mm-2000mm by hydraulic oscillating, continuously bending and continuously straightening;

direct rolling, hot feeding and hot charging by two casters with four streams and four-streams-merging-one technique, in which each of the two continuous casters' knockout roller tables respectively connects with entering roller table and exiting roller table of heating furnace, achieving rapid hot feeding and hot charging or direct rolling for continuous casting billet; continuous casting billet is fed to entering roller table of heating furnace through convey roller table, then fed into walking-beam furnace by long-course steel loader of the heating furnace in order to be heated; the exiting roller table of the continuous caster connects with the entering roller table of the heating furnace so that the continuous casting billet is directly fed into the heating furnace by the roller table for hot charging, the hot charging temperature averagely is no less than 700°C; and one exiting roller table of the continuous caster connects with the exiting roller table of the heating furnace;

rough rolling by rough rolling mill with front and back vertical rollers, in which the maximum single-pass side-pressing amount of the vertical rollers is 50mm, the maximum rolling pressure is 3800kN, and the rolling speed is 0~5.89m/s; when rolling, in each pass, acting of front vertical rollers and opening of back vertical rollers are automatically completed by computer, and the automatic control of width is realized through hydraulic automatic width control (AWC);

finishing rolling using speed-up rolling by finishing rolling train of F1~F7 with a maximum speed of 22.8m/s, in which vertical rollers are provided before F1; when thickness of the intermediate billet is 50mm, the maximum pressing amount is 10mm, and the maximum rolling pressure is 1000kN; F1~F7 finishing rolling train all are provided with a device for axial shifting and bending working rollers with a maximum axial shifting course of ±200mm and a maximum bending force of 1500kN, so as to improve the control of plate section's thickness difference and plate shape, and realize free rolling; the maximum speed of the roller table before the finishing rolling train may be up to 5.89m/s; edge heater is provided before the finishing rolling train;

stream cooling using three-stage stream cooling technique, in which roughly adjusting valve set and precisely adjusting valve set as well as online high-level water tank are provided, so as to stabilize water amount and water pressure of each cooling stage; the side water-jet pressure is 1Mpa~1.2Mpa so as to assure the plate's lengthwise temperature uniformity;

coiling with stepping-controlled full hydraulic coiler, in which automatic stepping control is performed by hydraulic-controlled side guide, pinch roller, wrapper roller's collapsible reels and high-responsivity wrapper roller to realize precise centering and micro-tension coiling of plate, reduce towered-shape, avoid the impaction of plate's head on wrapper roller and eliminate impression of plate's head and wrapper roller on plate.
The continuous casting and rolling process for medium plate as claimed in claim 1, characterized in that in said continuous casting, direct rolling, hot feeding and hot charging, and rough rolling, the exiting roller tables are covered by insulating covers, such that the temperature drop of the casting billet and the temperature difference between the intermediate billet's head and tail are reduced and the temperature of the billet when entering the finishing rolling mill can be increased by 30°C~50°C and the temperature difference between the billet's head and tail can be decreased by 30°C~50°C.
The continuous casting and rolling process for medium plate as claimed in claim 1, characterized in that in said rough rolling, the vertical roller of the front and back vertical rollers provided in the rough rolling mill is top-attaching driven type, the size of rollers is φ 1200mm/ φ 1100mm × 430mm, and the power of main motor is AC1200kW × 2 × 200/400r/min.
The continuous casting and rolling process for medium plate as claimed in claim 1, characterized in that in said finishing rolling, the vertical roller of the vertical rollers provided before F1 is top-driven attached type, the size of rollers is φ 750mm/ φ 600mm × 350mm, and the main drive motor is 2-AC 370kW × 550r/min.
The continuous casting and rolling process for medium plate as claimed in claim 1, characterized in that in said finishing rolling, hydraulic low-inertia kinks are provided between the finishing rolling stands in order to control tension between finishing rolling train and improve plate quality.
The continuous casting and rolling process for medium plate as claimed in claim 1, characterized in that in said stream cooling, the cooling device is provided with 12 sets of online high-level water tank with a water amount of 1700m³/h and a water pressure of 0.05Mpa~0.08Mpa.
The continuous casting and rolling process for medium plate as claimed in claim 1, characterized in that the structure of said mold is as follows:
said mold is a mold for billet continuous casting which has a cavity whose four corners are in form of right angle or round angle or straight bevel angle, the right angle mold is traditional billet mold which is composed by two wide sidewalls and two narrow sidewalls with a right angle being formed between respective wide sidewall and narrow sidewall; the round angle or straight bevel angle mold for billet continuous casting is composed by two wide sidewalls and two narrow sidewalls, at both sides of each narrow sidewall, namely the inner sides connected with the wide sidewalls, respectively have a row of right-triangle shaped flange, the bevel of which directs to inside, one right-angle surface joins to the inner surface of the narrow sidewalls, and another right-angle surface is in the same plane with the side face of the narrow sidewalls, namely the another right-angle surface is an extension of the side face of the narrow sidewalls.
The continuous casting and rolling process for medium plate as claimed in claim 1, characterized in that by carrying out three kinds of timing control, namely billet merging timing control, billet passing through logistics bottleneck timing control and billet exiting timing control, and comparing cushioning ability, heating ability and rolling ability, said four-streams-merging-one technique optimizes logistics in three walking-beam furnaces with cushioning zone into which the medium plate in single roller table of two casters with four streams is directly fed, so as to exert the producing capacity of caster, furnace and rolling line and realize half-rigid connection of rolling line, furnace and caster.
The continuous casting and rolling process for medium plate as claimed in claim 8, characterized in that said three kinds of timing control are:
billet converge timing control: which one passes through firstly when two streams from caster A converge; which one passes through firstly when two streams from caster B converge; which one passes through firstly when streams from caster A and caster B converge; wherein such principle is followed that "when the billets converge that the stream which reaches convergence point firstly passes through firstly and if the two streams reach simultaneously then the straight stream passes through firstly";

billet passing through logistics bottleneck timing control: determining the timing of No. 1 billet stream charging #1 larry; determining the timing of No. 4 billet stream charging #3 larry; determining the timing of converging No. 1 and No. 2 billet streams charging #2 larry; determining the timing when the billet charging #1 furnace exits A₁ roller table; determining the timing of the billet entering the furnace, wherein such principle is followed that "when the billets converge that the stream which reaches convergence point firstly passes through firstly and if the two streams reach simultaneously then the straight stream passes through firstly";

billet exiting timing control: by tandem superimpose-calculating billet's values at each critical timing as well as checking the cushion capacity and heating capacity of the furnace and the rolling capacity of the rolling line, finding out billet's values at each critical timing of each condition, realizing semi-rigid connection of caster, furnace and rolling line, wherein such principle is followed that "when the billets converge that the stream which reaches convergence point firstly passes through firstly and if the two streams reach simultaneously then the straight stream passes through firstly".