KR101735336B1

KR101735336B1 - Method of controlled rolling and slow cooling of steels for lowering hardness suitable for omission of heat treatment

Info

Publication number: KR101735336B1
Application number: KR1020150177208A
Authority: KR
Inventors: 최수조; 동현우
Original assignee: 주식회사 세아베스틸
Priority date: 2015-12-11
Filing date: 2015-12-11
Publication date: 2017-05-15

Abstract

A method of performing controlled rolling and controlled cooling of a rolling steel material to reduce hardness suitable for omission of heat treatment comprises: a step of rolling a steel material including 0.30-0.37 wt% of C, 0.40 wt% or less of Si (excluding 0 wt%), 0.60-0.90 wt% of Mn, 0.035 wt% or less of P (excluding 0 wt%), 0.035 wt% or less of S (excluding 0 wt%), 0.90-1.20 wt% of Cr, 0.15-0.30 wt% of Mo, the remaining consisting of iron and unavoidable impurities, water-cooling the steel material using a water-cooling rod at a temperature of 900-950C, and making a temperature of the rolled steel material after water-cooling to 820-870C; transferring the water-cooled steel material to a cooling rod, and making a temperature of the steel material to 780-830C when the water-cooled steel material arrives at the cooling rod; and slowly cooling the same at a cooling speed of 0.10-0.15 C/s using a slowly cooling facility through the cooling rod.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a control rolling method and a control cooling method for a rolling steel for reducing hardness suitable for omission of a heat treatment,

The present invention relates to a control rolling method and a control cooling method of a rolled steel material capable of omitting a heat treatment by reducing the hardness of a rolled product.

The present invention relates to a technique for omitting a heat treatment by reducing hardness through controlled rolling and controlled cooling methods, and more particularly to a method for heating and holding a bar (or a slab) into an austenite region, followed by rolling and cooling.

Generally, the plastic deformation of a metal is referred to as plastic deformation, and machining is performed by mechanical cutting and plastic deformation. In the plastic working, the temperature during processing is lower than the recrystallization temperature, and it is classified into cold working and hot working depending on the high temperature. Hot working includes rolling, press forging, and the like. Rolling is performed by heating and holding the material (BLOOM, BILLET, SLAB, INGOT, etc.) which has been initially solidified at a temperature higher than the recrystallization temperature (about 1000 to 1250 ° C) As a method, rods with appropriate shapes can be used to make rods, slabs, rails, and the like. Rolled products are produced by end-products or parts by various processes.

On the other hand, the hardness standards required for rolled products differ depending on the cutting method in the cutting process at the customer. Cutting methods include Cold Shearing, Band Saw or Circular Saw. In the case of cold shearing, it is generally required to have a hardness of less than 235HB for the rolled products in a moment cutting manner. Band sawing or circular saw cutting is not an instantaneous cutting method, but some customers require hardness values of less than 300HB or 280HB for rolled products in order to improve tool life. For relatively high hardness rolled products, the tool life is faster than for low hardness rolled products. Therefore, there are cases where it is difficult to secure the required hardness of the customer by the general rolling method according to the cutting process of the customer company. Therefore, annealing heat treatment is necessarily required to secure the required hardness of the product having high hardness after rolling.

In the case of steel for pressure vessels, a hardness value of 280HB or less is required for the rolled product by the customer, and since the required hardness can not be secured by the general rolling method as described above, the annealing heat treatment is inevitably required, .

An object of the present invention is to provide a control rolling and control cooling method capable of securing a required hardness value of a steel material for a pressure vessel. In particular, it aims to omit annealing through controlled rolling and controlled cooling.

The above-mentioned object is achieved by a process for cooling a bar steel using a water-cooled bar after rolling (control rolling); And a step of slow cooling of the pit (control cooling).

Preferably, the above object is achieved by a process for the production of a ferritic stainless steel comprising the steps of: 0.30 to 0.37 wt.% Of C, 0.40 wt.% Or less of Si, 0.60 to 0.90 wt.% Of Mn, 0.035 wt.% Or less of P, , 0.90 to 1.20% by weight of Cr, 0.15 to 0.30% by weight of Mo, and the balance consisting of iron and unavoidable impurities is rolled at a temperature of 900 to 950 캜 using a water bath to measure the temperature of the rolled steel after water- 820 to 870 캜; Transferring the water-cooled steel material to the cooling zone to make the temperature of the steel material at 780 to 830 ° C upon arrival of the cooling zone; And a step of gradually cooling the steel sheet to a cooling rate of 0.10 to 0.15 DEG C / s by using a slow cooling facility through the cooling table, thereby achieving controllability of the rolling and control cooling of the rolled steel material for reducing hardness.

The steel material for pressure vessels produced by the controlled rolling and controlled cooling of the present invention can secure a hardness value of not more than 280HB required by the customer in rolling products without heat treatment. The present invention relates to a control rolling and control cooling method for securing a required hardness in a rolled product by forming soft tissues by microstructure and slow cooling of a low hardness (ferrite) in comparison with general rolling and general cooling after controlled rolling and control cooling .

1 is a schematic diagram of a manufacturing process according to the present invention.
Fig. 2 is a view showing microstructure by rolling and cooling conditions according to the present invention.
3 is a schematic diagram of a time temperature transformation (TTT) curve according to rolling conditions according to the present invention.

Unless defined otherwise, all technical terms used in the present invention have the following definitions and are consistent with the meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Also, preferred methods or samples are described in this specification, but similar or equivalent ones are also included in the scope of the present invention. The contents of all publications referred to herein are incorporated herein by reference. The term " drug " is used in reference to a reference amount, level, value, number, frequency, percentage, dimension, size, amount, weight, or length of 30, 25, 20, 25, 10, 9, 8, 7, 6, Level, value, number, frequency, percentage, dimension, size, quantity, weight or length of a sample,

Throughout this specification, the words " comprising " and " comprising ", unless the context clearly requires otherwise, include the steps or components, or groups of steps or elements, And that they are not excluded.

The steel material for a pressure vessel used in the present invention is characterized by comprising 0.30 to 0.37 wt% of C, 0.40 wt% or less of Si (not including 0), 0.60 to 0.90 wt% of Mn, 0.035 wt% or less of P (not including 0) 0), 0.90 to 1.20% by weight of Cr, 0.15 to 0.30% by weight of Mo, and the balance of iron and unavoidable impurities.

Table 1 shows the chemical composition of the steel material for a pressure vessel described above.

(Unit: wt%) C Si Mn P S Cr Mo 0.30-0.37 max.0.40 0.60-0.90 max.0.035 max.0.035 0.90-1.20 0.15-0.30

The bars made of the composition shown in Table 1 are produced in the following manner. First, BLOOM prepared by the above composition is reheated and hot-rolled, and finally cooled in the air. The product hardness value required for the steel for cylinder is 280HB or less.

In the present invention, the temperature of the rolled product is reduced immediately before reaching the final cooling point after rolling in order to secure the required hardness value for the rolled steel product for pressure vessels. According to a preferred embodiment of the present invention, a water cooling system can be introduced after rolling to reduce the temperature of the rolled product. In addition, a slow cooling step using a slow cooling facility can be added to the rolled product that has reached the cooling point.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a schematic diagram of controlled rolling and controlled cooling according to the present invention. Referring to FIG. 1, the production process according to the present invention comprises: water-cooling a secondary rolled product; Reaching a cooling zone for the water-cooled product; And cooled by slow cooling using a slow cooling facility through a cooling stage. Table 2 shows the water cooling application conditions and cooling conditions by conditions. Condition 1 generally indicates rolling and cooling conditions, Condition 2 indicates air cooling after rolling and water cooling, and Condition 3 indicates slow cooling after rolling and water cooling.

(Water cooling and cooling conditions after rolling) Condition size Water-cooled application Cooling conditions #One
218 × 218 mm ²
Unapplied Air cooling #2 apply Air cooling # 3 apply Slow cooling

In Condition 1, cooling was performed in the air after cooling without cooling the water after the rolling. Conditions 2 to 3 were passed through a rolled product under a condition of 250 m < ³ > / h in a water jacket after rolling. The rolled product thus passed was moved to the cooling, Condition 2 was cooled in the same air as Condition 1, and Condition 3 was subjected to slow cooling using the slow cooling facility. Table 3 shows the product temperature and cooling rate for each condition.

(Product temperature and cooling rate by condition) Condition Before water cooling Temperature after water cooling Cooling zone arrival temperature Cooling rate #One 900 ~ 950 ℃ No water cooling 850 to 900 ° C 0.13 to 0.17 ° C / S #2 900 ~ 950 ℃ 820 ~ 870 ℃ 780 ~ 830 ℃ 0.13 to 0.17 ° C / S # 3 900 ~ 950 ℃ 820 ~ 870 ℃ 780 ~ 830 ℃ 0.10 to 0.15 ° C / S

Condition 1 was not water cooled after rolling, and the arrival temperature of the cooling zone was about 850 ~ 900 ℃, and the cooling rate was cooled to 0.13 ~ 0.17 ℃ / S by air cooling. Conditions 2 and 3 were water cooling after rolling, and the arrival temperature of the cooling zone was about 780 to 830 ° C. The cooling rates of the conditions 2 and 3 were 0.13 to 0.17 ° C./s and 0.10 to 0.15 ° C./s respectively .

The hardness and microstructure of the rolled product under the above three conditions are shown in Table 4 and FIG.

division Condition 1 Condition 2 Condition 3 Hardness 290 ~ 300HB 280 ~ 290HB 270 ~ 280HB

Based on the product hardness result value, condition 1 to condition 2 exceed the reference value of 280HB or less required hardness value, and hardness value is less than reference value only in condition 3 in which both water cooling and slow cooling are applied. The hardness value in the condition 2 in which the water-cooling is performed when the conditions 1 and 2 are compared has a relatively low value. In Table 4, condition 2 is relatively low in the arrival temperature of the cooling zone, and the TTT curve of FIG. 3 shows that the condition 2 has a high ferrite ratio with a low hardness and a relatively low veneite ratio , It can be seen that the ratio of ferrite in Condition 2 to Condition 1 is high in the microstructure of FIG. 2, and generally, when the ferrite ratio is high, the hardness is low.

Both Condition 2 and Condition 3 were water cooling, Condition 2 was air cooling, Condition 3 was slow cooling. As shown in FIG. 2, the temperature was lowered to an initial low temperature by water cooling, and then the temperature rises again due to the internal temperature of the rolled product. As shown in Table 3, both of the two conditions are similar to those of the cooling zone, but the cooling rate differs depending on whether the cooling is performed after the cooling zone arrives. Condition 3, in which the slow cooling was performed, exhibited a relatively slow cooling rate of 0.10 to 0.15 DEG C / S as compared with the condition 2. As shown in Table 4, the condition 3 shows a relatively low hardness value compared to the condition 2, and even in the microstructure of FIG. 2, the ferrite and pearlite structures are relatively large and the veneite ratio is low. Generally, the hardness of the veneite structure is higher than that of ferrite and pearlite. That is, differences in hardness and texture are caused finely by the difference in cooling rate.

In the case of steel for pressure vessels, the customer manufactures the final product for the rolled product by cutting (Circular Saw) → heating and hot forging → roughing → finishing and tempering heat treatment → finishing → inspection process. In order to achieve the required mechanical properties after the heat treatment of normal manufacturing and tempering of the customer, it is necessary to control the chemical composition of 34CrMo4 in Table 1 to the upper limit in the required range. While it is possible to lower the hardness in the final rolled product when the chemical composition is controlled to the lower limit within the required range, it is difficult to achieve the mechanical property after the heat treatment of the customer for the rolling product manufactured with the lower limit value of chemical composition. However, it is possible to obtain the hardness value required by the customer through softening treatment such as annealing for the high rolling product which is controlled by the upper limit. However, there is a portion where the heat treatment must be performed, so that the manufacturing cost increases.

The present invention overcomes these problems by introducing a water cooling application and a slow cooling facility which can secure the required hardness of the customer. By securing the hardness value of the rolled products required by the customer, it is possible to prevent the reduction of the tool life in the cutting process of the customer.

The present invention has been described with reference to the preferred embodiments. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the disclosed embodiments should be considered in an illustrative rather than a restrictive sense. The scope of the present invention is defined by the appended claims rather than by the foregoing description, and all differences within the scope of equivalents thereof should be construed as being included in the present invention.

Claims

0.30 to 0.37% by weight of C, 0.40% by weight or less of Si (not including 0), 0.60 to 0.90% by weight of Mn, 0.035% by weight or less of P (not including 0) , 0.90 to 1.20% by weight of Cr, 0.15 to 0.30% by weight of Mo, and the remaining amount of iron and inevitable impurities is rolled at a temperature of 900 to 950 캜 using a water bath to cool the rolled steel after water- Making the temperature 820 - 870 캜; Transferring the water-cooled steel material to the cooling zone to make the temperature of the steel material at 780 to 830 ° C upon arrival of the cooling zone; And a step of slowly cooling the steel sheet at a cooling rate of 0.10 to 0.15 DEG C / s by using a slow cooling facility through the cooling table, to control hardness reduction of the rolled steel material suitable for omission of heat treatment.