JPH0445223A - Production of thick tough steel plate free from segregation - Google Patents

Abstract

PURPOSE:To provide a uniform solidified structure and to efficiently produce a thick tough steel plate free from segregation by forming oxides in a molten Mn-containing steel with a specific composition at the time of horizontal continuous casting and then carrying out controlled cooling of the transformation temp. range. CONSTITUTION:A steel having a composition consisting of, by weight, 0.02-0.25% C, 0.05-0.6% Si, 0.30-2.0% Mn, <=0.01% Al, 0.005-1.0% of at least one element selected from Zr, Hf, Y, La, Ce, and Ti, 0.001-0.02% S, and the balance Fe with inevitable impurities is refined. The resulting molten steel is continuously cast horizontally or at an angle of <=10 deg. from the horizontal line in an open top state at <=25 deg.C/min cooling rate between the liquidus temp. and the solidus temp. The resulting cast slab is cooled from a temp. not lower than the Ar3 point down to <=550 deg.C at 1-50 deg.C/sec cooling rate. By this method, the oxides previously formed in the molten steel acts as the nuclei of intragranular transformed ferrite and the intragranular transformed ferrite after transformation can be dispersed finely and uniformly and, as a result, the thick steel plate having superior material characteristics can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for casting steel and a subsequent direct rolling method.

(Prior Art) It is known that it is effective to make the metal structure fine in order to obtain a high-strength steel material without impairing its properties such as toughness and weldability. On the other hand, with the intention of reducing energy costs and improving productivity, attempts have been made in recent years to use a direct rolling method in which steel is rolled as it is without cooling after casting. Such techniques include, for example, Japanese Patent Laid-Open Nos. 59-208018 and 61-146072.

However, when such direct rolling is performed, the coarse solidified structure is difficult to recrystallize during subsequent rolling, and as a result, the final metal structure is relatively coarse or partially formed. Recrystallization results in a mixed grain state of coarse grains and fine grains, which has the disadvantage that it is difficult to obtain one with good strength and toughness.

In addition, if an attempt is made to force recrystallization, strong reduction at high temperatures is required, which has the drawback of creating significant restrictions on the positional relationship between the casting equipment and the rolling mill, the capacity of the rolling mill, etc. In addition, there is so-called macro segregation of carbon, manganese, etc. in the center of the thickness of the slab, which inevitably results in poor toughness.

On the other hand, as a method for refining metallographic structure, JP-A-61-23
There is a method, as disclosed in Japanese Patent No. 8940, in which fine intragranular ferrite is generated within austenite grains using inclusions dispersed in steel as transformation nuclei. However, even with this method, if the austenite before transformation is coarse like a normal casting structure, the expected high toughness cannot be obtained in many cases.

(Problems to be Solved by the Invention) The present invention provides a method for manufacturing strong thick steel plates by horizontal continuous casting in which fine isometric islands are generated over the entire surface of a slab in a solidified state.

(Means for Solving the Problems) The present invention is a steel casting method and a subsequent direct rolling method that can advantageously eliminate the drawbacks of the conventional methods as described above, and the gist thereof is as follows. be.

(1) C in weight%: 0.02% to 0.25%, Si
: 0.05% to 0.6%, Mn: 0, 30
%~2.0%, AI≦0.01%, Zr, Hr, Y, L
At least one selected from the group of a, Ce and T1:
0.005% to 1.0%, S2 0.001 to 0.02%
, the cooling rate between the liquidus temperature and the solidus temperature is 25℃ when the molten steel, the balance of which is Fe and unavoidable impurities, is placed horizontally or with the upper surface of the molten steel open at an angle of 10 degrees or less from the horizontal line.
Continuous casting is carried out at a cooling rate of 1°C/s or more and 50°C/s or less from the temperature of the A ra point or more.
A method for manufacturing a strong thick steel plate without segregation, characterized by cooling to 0°C or less, and (2) C: 0.02% by weight
~0.25%, Si: 0.05%~0.6%, M
n: 0, 30% to 2.0%, Al≦0.01%
, Zr, Hf, Y, La, Ce and Ti: 0.005% to 1.0%, S: 0
．． 001~0.02%, furthermore, Nb50610%, C
u≦1.0%, Ni≦2.0%, C “51.0%, Mo
S2.0%, Co≦1.0%, WS2.0%, ■≦0.
10%, B≦0.0025%, Ca≦0.005%
Cooling rate between liquidus temperature and solidus temperature of molten steel containing one or more species, with the remainder consisting of Fe and unavoidable impurities, with the top surface of the molten steel open horizontally or at an angle of 10 degrees or less from the horizontal line. is continuously cast at a rate of 25°C/min or more, and the slab is cast at a temperature of 1°C/s or more at 50°C from a temperature of A ra point or higher.
This is a method for producing a strong thick steel plate without segregation, which is characterized by cooling to 550° C. or less at a cooling rate of 100° C./s or less.

Furthermore, in the present invention, the slab is directly hot-rolled as it is at a temperature above the A ra point, and then it is rolled at a temperature above the A ra point.
It includes cooling to 550°C or less at a cooling rate of 50°C/s or more and tempering the thick steel plate.

The present invention will be explained in detail below.

The technical idea underlying the present invention is as follows.

So-called macro-segregation of carbon, manganese, etc. exists in the center of normal slab thickness and deteriorates toughness.

However, the present inventors have discovered new facts that make it possible to overcome the above-mentioned drawbacks, and based on these discoveries, a new method for casting steel and a subsequent direct rolling method have been developed.

In the conventional continuous casting method, cooling starts from the four peripheries of the mold, which causes various additive elements to concentrate in the unsolidified molten steel, eventually forming macro-segregation in the center of the thickness of the slab, causing deterioration in dynamics. . However, if the molten steel is continuously cast horizontally or at an angle close to horizontal with the top surface of the molten steel open, the final solidification position will be on the top surface of the slab, thereby preventing the formation of macro-segregation. However, the solidified structure of such a slab consists of coarse columnar crystals, and the metal structure when transformed as it is is still coarse and poor in toughness.

In addition, even if direct rolling is performed, it is extremely difficult to recrystallize.
The metal structure after transformation becomes coarse or becomes a mixed structure of a fine structure and a coarse structure, deteriorating the material quality. It is not possible to obtain thick steel plates of good quality using conventional manufacturing methods that involve rolling and cooling.

However, as in the present invention, if oxides are generated in the molten steel in advance and the subsequent transformation temperature range is cooled at a predetermined cooling rate after casting, these oxides can act as nuclei of intragranular transformed ferrite. After the transformation, the intragranular transformed ferrite is finely and uniformly dispersed, resulting in good material properties. By paying attention to the predetermined thickness and width during casting and cooling the slab under predetermined conditions such that intragranular transformed ferrite is produced, it is possible to produce a thick steel plate that is sufficiently durable for use.

Further, by continuing hot rolling after casting and performing cooling or a combination of cooling and tempering under predetermined conditions after rolling, it becomes possible to manufacture a thick steel plate with even better material properties. Based on these new discoveries, the chemical composition of steel, casting method, rolling method, and post-rolling heat treatment conditions were investigated in detail, and as a result, the method for producing strong thick steel plates of the present invention was developed.

The reasons for the limitations of the present invention will be explained in detail below.

First, the reasons for limiting the components of the starting materials in the present invention will be described.

C is an effective element for strengthening the rope, and contains 0.02%
If it is less than that, sufficient strength cannot be obtained. On the other hand, if the content exceeds 0.25%, weldability will deteriorate.

Although Sl is effective as a strengthening element for steel, it has no effect at a content of less than 0.05%. On the other hand, if it exceeds 0.6%, the surface quality of the steel will be impaired.

Mn is an effective element for strengthening steel, and if it is less than 0.30%, sufficient effects cannot be obtained. On the other hand, its content is 2.0
%, the workability of steel deteriorates.

Since A and Q act as deoxidizing elements, their content is reduced to 0 in order to inhibit the formation of other oxides that become the nucleus of intragranular transformation.
．． 01% or less.

Zr', Hr, Y, La, Ce, and Ti are a group of elements that generate oxides that become the core of intragranular transformation, and their effects are exerted when the sum of each is 0.005% or more, but 1%
If it exceeds this, inclusions will increase and the toughness of the steel will deteriorate.

Although S is an impurity element, it is necessary to contain 0.001% or more in order to generate MnS and promote intragranular transformation, or if it exceeds 0.02%, the toughness of the steel deteriorates.

Since Nb effectively functions in terms of crystal grain refinement and precipitation hardening when added in a small amount, it may be added within a range that does not deteriorate the toughness of the weld zone. From this point of view, the upper limit of the amount added is set at 0.
10%.

Cu, Ni, Cr, and Mo are all elements that improve the hardenability of the steel. In the case of the present invention, the strength of the steel can be increased by adding it, but since excessive addition impairs the weldability of the steel, Cu≦1.0%, Ni≦2
．． 0%, Cr≦1.0%, and MnS 1.0%.

Co and W increase the high temperature strength of steel. However, excessive addition of Co≦1.0%, W
! Limited to 1.0%.

■ is effective in increasing the strength of steel through precipitation hardening, but excessive addition impairs the toughness of steel, so its upper limit is reduced to 0.
．． 10%.

B is an element that improves the hardenability of steel. In the case of the present invention, the addition of B can increase the strength of the steel, but excessive addition increases B precipitates and impairs the toughness of the steel, so the upper limit of its content is set to 0.0025%.

Ca is effective in preventing hydrogen-induced cracking, but excessive addition deteriorates the toughness of steel, so the upper limit of its content should be reduced to 0.
．． 005%.

Next, the process conditions in the present invention will be described (first
(see figure).

The reason why molten steel is continuously cast horizontally or at an angle of 10 degrees or less from the horizontal with the top surface of the molten steel open is mainly to prevent the formation of macro-segregation within the slab.

The cooling rate between liquidus temperature and solidus temperature during casting is 25℃.
The reason why the solidification rate is set to 1/min or more is to make the solidification rate relatively high and to finely and uniformly disperse oxides, which become the nucleus of intragranular transformation, in the molten steel.

The slab obtained in this way is subsequently cooled at a predetermined cooling rate, and intragranular ferrite is produced using oxides in the steel as transformation nuclei. If the cooling rate is 1°C/s or less, intragranular ferrite will not be produced, and if it is 50"C/s or more, martensitic transformation will occur, and no intragranular ferrite will be produced.

The thick steel plate can be used as it is, but a stronger material can be obtained by directly hot rolling it after casting and cooling it at a predetermined cooling rate. Further, in order to adjust the strength to a predetermined strength, a tempering treatment may be performed after cooling.

(Example) Figure 1 shows a horizontal continuous casting machine. In this method, a fixed weir (rear layer) is placed on an endless belt running in the casting direction, a pool is formed downstream from the fixed weir in the casting direction, and the molten metal poured into the pool is cooled.・This is a single-belt continuous casting machine that continuously casts steel billets by solidifying them.

First, steel having the composition shown in Table 1 is cast by applying the method of the present invention and comparative method shown in Table 2, and then the slab is rolled by applying the method of the present invention and comparative method shown in Table 2. In the case of heat treatment, the macro segregation rating of the solidified structure as shown in Table 2 shows the intragranular ferrite percentage and strength.
It became toughness.

It is clear that the present invention reduces macro-segregation and further improves the quality of the material, so the present invention is effective.

(Note 1) Casting method = 1. Horizontal continuous casting (top open) Casting angle 4 degrees 2, horizontal continuous casting (top open) Casting angle 0 degrees 3, horizontal continuous casting (top open) Casting angle 9 degrees (comparison) 41 Double-sided cooling continuous casting mold part vertical → Curvature (comparison)5. Steel ingot casting (effects of the invention) In horizontal continuous casting, the present invention generates oxides in the molten steel in advance and cools the molten steel while controlling the transformation temperature range. Therefore, it is possible to efficiently create a uniform solidified structure, and it is possible to Transformed ferrite can be dispersed finely and uniformly to produce high-toughness thick steel plates.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the present invention.

Claims (1)

[Claims] 1. C: 0.02% to 0.25%, Si: 0.05% to 0.6%, Mn: 0.30% to 2.0%, Al≦0 in weight%. .01%, at least one selected from the group of Zr, Hf, Y, La, Ce and Ti: 0.005% to 1.0%, S: 0.0
01 to 0.02%, the balance being Fe and unavoidable impurities.The cooling rate between the liquidus temperature and the solidus temperature is 25% with the top surface of the molten steel open horizontally or at an angle of 10 degrees or less from the horizontal line. °C/
Continuously cast the slab at a cooling rate of 1°C/s or more and 50°C/s or less from a temperature of Ar_3 or higher to 550°C.
A method for manufacturing strong thick steel plates without segregation, which is characterized by cooling to a temperature below. 2. Nb≦0.10%, Cu≦1.0%, Ni≦2.0%, Cr≦1.0%, Mo≦1.0%, Co≦1.0%, W≦1 in weight% .0%, V≦0.10%, B≦0.0025%, and Ca≦0.005%. The method for producing a tough thick steel plate without segregation according to claim 1. 3. Direct hot rolling of the slab as it is at a temperature of Ar_3 or higher, and then rolling it at a temperature of 1°C/s or higher from a temperature of Ar_3 or higher5.
3. The method for producing a strong thick steel plate free of segregation according to claim 1 or 2, characterized in that the steel plate is cooled to 550°C or less at a cooling rate of 0°C/s or less. 4. Claims 1 and 2 characterized in that the thick steel plate is tempered.
Or the method for producing a strong thick steel plate without segregation according to 3.