JPS58126923A - Production of unnormalized high tensile and high toughness steel plate by continuous casting - Google Patents

Abstract

PURPOSE:To obtain a unnormalized high tensile and high toughness steel plate having surface characteristics by casting high Nb steel of specific compsn. limited particulrly in the contents of T.N and Ti continuously and hot rolling the cast steel, then air cooling the same at a rate lower than air cooling. CONSTITUTION:The steel contg. 0.005-0.06% C, 0.02-0.50% Si, 1.5-2.5% Mn, <=0.005% S, 0.06-0.20% Nb, 0.005-0.10% SolAl, contg. <=0.006% T.N, and 0.005-0.030% Ti, and further contg. V, Cr, Cu, Mo, Ni, B, Ca, etc. according to need is cast continuously. After the slab is hot rolled, the slab is cooled at a cooling rate lower than air cooling. By such method, the degradation in the suitability to hot rolling, and the generation of surface flaws of the continuous casting ingot by the hot rolling are prevented, and the intended high Nb type unnormalized high tensile and high toughness steel plate is obtained easily.

Description

DETAILED DESCRIPTION OF THE INVENTION This is a method of manufacturing a high-toughness steel plate and a high-Nb heat-treated, high-strength, high-toughness steel plate from a continuously cast piece with good surface properties.

In the past, only strength was required as a characteristic for high-strength steel plates, but as welding has been adopted for steel structures, it has become necessary to have not only strength but also weldability. In response to this, there is a trend to lower CIt, which impairs weldability, and to increase strength and toughness with alloying elements. In particular, recently, requirements for weldability, notch toughness, workability, etc. have become even more stringent.
691 or less, and by effectively utilizing trace amounts of added elements such as Nb, V, and Ti, there is a tendency to improve toughness and toughness while ensuring a small amount of strength. Such low C1 steel (C
50,06 steel), high Nb nitrogen concentration and current steel (
C': 0.09~o.

16g6) It is possible to achieve high tensions that cannot be achieved with 16g6). In other words, Figure 1 shows low C steel.

-02-〇-1.7 The change in strength when changing the amount of Nb added in the 1100℃ heating 900'C or less 60-
The figure shows the case where the rolling was completed at 790'C, and the strength increased continuously until the amount of Nb exceeded p, o, and 1o@. This is because the island is heated up to a wide range of Nb addition amounts because of the low C! This is because precipitation strengthening can be used because it melts and precipitates during the subsequent cooling process.

As mentioned above, it is possible to increase the tensile strength of low C steel over a wide range of Nb additions, so it is thought that wood texture will be increasingly used. There is one problem. When continuous casting technology is applied to low C-high Nb steel, conventional Nb steel (Nb content 0.0
Compared to 4qb), the occurrence of scratches on the surface of continuous casting slabs is significant and requires complicated maintenance as a pre-rolling process.
This means that the benefits of continuous casting cannot be fully obtained.

It is well known that the occurrence of scratches on the slab surface is generally closely related to a decrease in hot ductility in the first low temperature range during the post-solidification cooling process. As a concrete measure, at most the casting temperature,
Controls such as the casting speed and the amount of cooling water in the cooling zone are still in place. However, these methods are not a sufficient solution for high Nb steels, and therefore, a decrease in hot ductility is considered to be the fate of high Nb steels, resulting in surface scratches and problems in the pre-rolling process. Care is considered to be inevitable and is a matter of mourning.

The present invention is based on the above-mentioned actual situation vCI! This product was developed after repeated research, and its purpose is to provide a method for producing high-Nb type, non-tempered, high-strength, high-toughness steel sheets with excellent slab surface properties during continuous casting.

To achieve this objective, the present inventors focused on the phenomenon of hot ductility reduction in the low temperature range.

We studied the phenomenon of hot ductility reduction due to the precipitation of N) and found that the reduction in hot ductility due to the increase in Nb content is due to the synergistic effect of the precipitation of AAN and the precipitation of Nb(C,N). Since it is a high Nb steel, it is not possible to reduce the amount of Nb added, but adding a small amount of Ti as a method of suppressing precipitation in the door significantly improves hot ductility, and furthermore,
We discovered that increasing the amount of Nb added within the range of b addition actually improves the hot ductility.Based on these findings, we determined the surface properties by using the method of im* of the chemical composition of steel to be continuously cast. By obtaining a continuous cast slab with high tensile strength and high toughness, and hot ductility of this slab, and if necessary accelerated cooling, it is possible to obtain a high Nb type non-temperature steel plate with high tensile strength and high toughness. This is what I did.

That is, the characteristics of the present invention are that C: o, o
o s ~ o, os o, Si: 0.02 ~ 0.
50%, Mn: 1.5-2.5%, s: o, oos
*Hereinafter, Nb: 0.06-0.20%, 8otkL
: When continuously casting steel containing 0.005 to 0.10% as a basic component, the amounts of T and N (same below the total amount of N) are set to o, ooso- or less, and Ti is 0.005%.
~0.03096, continuous casting is performed, and the obtained slab is hot rolled and cooled at a cooling rate higher than air cooling.

Further, the present invention is characterized in that the basic components include V
: 0.01-0.2%, Cr: 0.5% or less, Cu: o, sl or less, Mo: 0.5- or less, N
i: 2% or less, B: 0.002- or less, Ca: 0.
When continuously casting steel containing one or more types of gold of 0.002 to 0.007, the amounts of T and N are set to 0.006 or less, and Ti: 0.005 to 0.030 is contained.
The method involves continuous casting, hot rolling of the obtained slab tube, and cooling at a cooling rate higher than air cooling.

The invention will now be described in detail with reference to the accompanying drawings.

As mentioned earlier, the occurrence of slab surface scratches is closely related to the decrease in hot ductility in the γ low temperature range during the post-solidification cooling process, and naturally the higher the surface scratch occurrence rate, the higher the maintenance rate. . Therefore, the present inventors first conducted nine high-temperature tensile tests on continuously cast slabs, examined the relationship between hot ductility and cast hand strain, and found that It has been found that there is a relationship as shown in Figure 2 between the following temperature range (width) (the same applies hereinafter) and the casting hand penetration rate.

The thermal history used in the above-mentioned high-temperature tensile test is as shown in Figure 3. This is a simulation of the case in which stress is applied by rolls or rolls.

According to FIG. 2, when the low ductility temperature range is wide, some slabs become unusable due to the high maintenance rate and the need for weight insertion. As the low ductility temperature range decreases, the maintenance rate of the slab decreases, and when the low ductility temperature range is approximately within 100°C, the maintenance rate falls below FiSSi, and the maintenance process is also light.

Therefore, the present inventors next investigated conventional Nb steel (0°094
C-1,5% Mn-〇, 03% Nb-0,0
0,05% to 10,03 fi 5olAt) and low C-high Nb steel (0,02fbC-104Mn-0,10%Nb
Hot ductility for -0.00aN-0.03%5olAl)? We investigated the temperature range from near 00°C to 1000°C. The resulting tube is shown in Figure WC4, and when comparing the square steels mentioned above, the drawing (R) of the low C-high Nb steel
It can be seen that A) is low at all temperatures, and the low ductility temperature range where the reduction of area is 504 or less is wider at 290°C compared to 150°C for 0.03 modulus Nb steel.

The reason for this is thought to be that in the low C-high Nb steel (0.101Nb steel), the amount of Nb precipitated in the solid bath during heating and in the γ low temperature range is greater than in the 0.03Nb steel. . Nb precipitated at grain boundaries in the low temperature range
When the tensile stress exceeding a certain limit is applied near the surface in a state where the grain boundaries have become brittle due to (C,N), the detonation occurs in the form of Nb(C,N)t-. These voids coagulate and form a series of agglomerates, leading to final cracking. Moreover, the precipitation of traces acts synergistically with this. Therefore, these will fill & Nb
In the case of steel, the reduction of area is lower, and in continuous casting, the stress between the rolls in the cooling zone or the thermal stress caused by repeated cooling and reheating corresponds to the above-mentioned limit value and strain supply stress. ), scratches on the surface frequently occur.

From the above points, it is clear that the decrease in hot ductility caused by increasing the Nb content is due to the synergistic effect of the precipitation of traces and the precipitation of Nb(C,N), but the amount of Nb added cannot be reduced as a countermeasure. Therefore, the present invention aims to suppress the precipitation, which is one of the causes of a decrease in hot ductility, and as a specific method, we have decided to add a small amount of Ti.

This resulted in a significant improvement in hot ductility without reducing the amount of fi Nb added. That is, Fig. 5 1i 0.0
2 tlrc-2,04Mn-Nb-0,0044T
, N steel (Ti-free steel) and 0.02-C-2, O-Nb-0,015-Ti-0,004-T, N steel (T
This figure shows the change in hot ductility at tPsoo°C and 1000°C when the amount of Nb added is changed in the case of (i-added steel).

As is clear from Fig. 5, at 1000°C, T
With i-free steel, the hot ductility (RA value) rapidly decreases at high Nb @ after reaching liO and 03%Nb, and becomes around 50 inches. On the other hand, in Ti-added steel, the reduction of area is approximately 100- regardless of the amount of Nb added. Also, in the case of 800℃,
In Ti-free steel, the reduction of area is extremely low at 20 inches or less regardless of the amount of Nb added;
In the case of additive steel, when the reduction of area is around 50 pl and the Nbs addition is 0.06% or more, the ductility improves. This difference in hot ductility is caused by the fact that Ir 1 fixes N and suppresses the precipitation of AtN in the γ low temperature range, and the grain refinement with TiN greatly improves υ hot ductility. It is obvious that this is due to the fact that at 800°C, 0.
At 60%Nb or more, the hot ductility improves as the Nb increases, mainly due to precipitated Nb(C).

This is thought to be due to differences in the coarsening process of N).

Based on these facts, the present invention fixes υN by adding Ti when continuously casting high Nb steel, and utilizes the Nb2O, 06% range where hot ductility improves. Great effects can also be obtained in terms of strength and toughness. That is, FIG. 6 shows the above-mentioned 0, 02-〇-2
．． 0 Chi pigeon - Nb - 0.015 Chi Ti - 0,004 Chi T
, N steel is heated to 1100°C, subjected to a cumulative reduction of 70% below 900°C, and rolled to 20wm at 770°C to show the change in toughness if %.

As shown in FIG. 6, the strength increases continuously with the amount of FiNb added, but the toughness begins to improve after 0.06 inch, and a high toughness of vTs = -100°C or less is obtained. Furthermore, when the amount of Nb added exceeds 0°10-, the toughness is further improved, and in the region of 0.10% Nb, vTs
An extremely excellent toughness of =-120 to -140°C was obtained. Therefore, it is desirable to add Nb to 0.06% or more, not only from the viewpoint of hot ductility but also from the viewpoint of toughness, especially when high toughness is required.
It can be said that addition or inclusion of one or more is desirable.

Therefore, the specific component composition for fully exhibiting the above %a according to the present invention is as follows.

(I)C: 0.005-0.06 m, Si:
0.02-0.50%, Mn: 1.5-2.5
%, s: o, ooss or less, Nb: 0.06-0.2
Q%, 8oIAL: 0.00S ~ 0.10
% balance iron and unavoidable impurities, or the above components include V: 0.01 to 0.2%, Cr: o, s% or less, Cu: 0.5% or less, Mo: 0.5% or less, N1
= 2- or less, B: 0.002% or less, Ca: O, 0
Using a steel containing one type of 02 to 0.007 or two or more types of Fi, Ql) co(r) steel t/C>Vj) bT, N amount i0.00
6% or less, Ti l O, 005 to 0.030%
Contain w within the range of .

The reasons for limiting the above ingredients are as follows.

C is required to be at least 0.005% from the viewpoint of ensuring strength.

However, if the amount is 0.06 or more, in a high Nb addition system like the present invention, most of the added Nb becomes undissolved during heating, and the tensile strength cannot be increased to a high Nb addition amount. Therefore c 1 o, o.

It was set to 5 to 0.06 inches. From a practical point of view, assuming heating in the normal heating range of 1,050 to 1,200°C, and considering the solubility product of CtN, the value is 0.109.
In order to effectively utilize Nb of G or more, it is particularly desirable to make Ct 0.04 or less.

Si is required as a deoxidizing element in an amount of 0.02 or more.
If it exceeds 5096'i, weldability deteriorates.

Pigeon is a low C type steel and requires at least 1.5 inches to achieve high tension, but if it exceeds 2.5 inches, weldability becomes poor.

S is 0.00 from the viewpoint of preventing deterioration of hot ductility due to Mn8.
5 inches or less is desirable.

As shown in Figure 5, when Nb is added in an amount of 0.06% or more, the hot ductility is improved and it is effective in preventing scratches on the surface of the continuously cast slab. Also, 0.06- or more is required. Therefore, the lower limit of the amount of Nb added is 0.06%, 7'c. -1qNb addition amount is o
, zo%, weldability is impaired. Therefore, the upper limit was set at 0.20.

5otAL is required as a deoxidizing element in an amount of 0.005 or more, but if it is 0.10 or more, weldability deteriorates.

■ is required to be 0.01% or more from the viewpoint of precipitation strengthening, and 0.
．． If it exceeds 2 dl, weldability will be impaired.

If Cu, Cr, Mo1C is added in an amount of 0.5 s or less as necessary, it is possible to improve tension, toughness, and S contact.

Ni is effective in terms of both strength and toughness, and from the economic point of view, an amount of zl or less is considered an excessive amount.

In order to improve hardenability and achieve high tensile strength, B must be 0.
0.0005 mm or more is required, and if it exceeds 0.002 mm, the weld toughness will be impaired.

Ca1j is required to be 0.00296 or more in order to control the shape of inclusions, but if it exceeds 0.007%, it will harm the multi-material properties of steel where the cleanliness is rapid.

Next, the reason why the upper limit of T' and N amount is set to 0.006 inch is that if the content exceeds this, weldability will be impaired and hot ductility will also deteriorate. The following conditions are essential for the present invention.

As previously mentioned, Ti is an extremely important component in the present invention. If the amount added is low, N
cannot be sufficiently fixed, and Aa precipitates at the γ grain boundaries in the γ low temperature range, reducing ductility. To sufficiently fix N, improve hot ductility, and prevent scratches in Table 1, at least 0.005-
The above is necessary, so this is set as the lower limit.

On the other hand, if the amount of Ti added is too large, the excess Ti becomes TiC.
As γ, it precipitates in the low-temperature range, which actually reduces hot ductility. Therefore, the upper limit of the amount added is 0.030%, and the desired effect can be sufficiently achieved within the range of 0.005 to 0.030%.

Then, the rest is high Nb@4f with the components adjusted as above.
Continuous casting is carried out, and no special conditions are required for the continuous casting (containing conditions, cooling conditions, etc.), and the operation can be carried out according to conventional methods. Then, using the continuous casting slab with good surface quality obtained by the above method, it is hot rolled, and then cooled by air cooling or accelerated cooling if necessary, to achieve the desired high Nb type non-thermal treatment. A steel plate with high tensile strength and toughness is obtained.

Next, specific examples of the present invention will be shown.

Example l.

In order to see the appropriate range of the amount of Ti added in the present invention, a low C
Figure 7 shows the hot ductility of high Nb steel at 700 to 1000°C.

As is clear from the 7th factor in Table 1, when the amount of Ti is o, 604 (Hagane), even at 1000°C, the tolerance is 50-
The low ductility temperature range is extremely wide at 310°C. This is because the amount of Ti added is low, so N cannot be fixed sufficiently, and r
In the low temperature range, a precipitate precipitates at one grain boundary, resulting in a decrease in ductility.

In addition, when the Ti addition amount is 0.035% (steel B), 90
The hot ductility at 0°C is as low as 35%, and the low ductility temperature range is 24%.
It shows a large value of 0°C. This is because, contrary to the case of Steel A, since the amount of Ti added was large, excess Ti precipitated as TiC in the γ low temperature range, resulting in a decrease in ductility.

On the other hand, the amount of Ti added is within the range of the present invention, 0, 01
In the case of 7-inch steel (Steel C), hot ductility is extremely high, and the low ductility temperature range is extremely narrow at 80°C. From this point of view, the amount of Ti added should be 0.005 to 0.030-
should be within the range of

Example λ In order to examine the limiting effects of each of the above-mentioned components, test steel D-H was manufactured by continuous casting, heated at 1100°C, and heated to 900°C or less at 70°C.
Table 2 shows the chemical composition, strength, toughness, and hot ductility data when the rolling was completed at 750° C. with a cumulative reduction of 1.

As is clear from Table 2, in the case of the sample steel,
The amounts of T, N, and Ti added are appropriate, but the amount of Nb added is 0.04 mm, which is outside the range of the present invention, so the strength is about 6 mm lower than steels E and F, which are the methods of the present invention. The toughness is also impaired by about 20 to 30°C compared to the method of the present invention.

On the other hand, the steels E and F of the present invention have high strength and high toughness) and have a very narrow low ductility temperature range of 80°C or less, and therefore have a casting hand change rate Fi of 1% or less. # C that did not require much maintenance In addition, although the amount of GFiNb and Ti added to the test steel was appropriate, the amount of TN was 0.0070.
In sample steel H, the amount of TiNb added and the amount of TN are appropriate, but the amount of Ti added is as low as 0.002-. Therefore, in both cases, the presence of free N reduces hot ductility, and the low ductility temperature range is as wide as 280 to 300°C. Therefore, the casting rate is extremely high at 30-33%.
1. Due to maintenance, it is almost unusable.

Example 3゜Table 3 shows that in addition to the basic components, V, Cr, Cu, Mo
.

Test steel I containing one or more of Ni, B, and Ca
- Applying a cumulative reduction of 65 inches under heating at 1100°C using N, and applying a cumulative reduction of 65 inches in the temperature range of 1100°C heating and below 900°C, and applying a cumulative reduction of 65 inches in the temperature range of 72 Chemical composition, strength, toughness, thermal relationship, and relationship when rolling is finished at 750°C and accelerated cooling is performed at 10°C/sec to 600°C after finishing air blowing.
- It shows the one-handed insertion rate.

As is clear from this third layer, in any of the high Nb steel with ii1 addition (1, J), the steel with Mo addition (K), and the steel with Cu, Ni, Cr, and Ca f additions (L), Since the steel sheet also satisfies the requirements of the present invention, it has high strength and high toughness both when air cooling is performed after interval rolling and when accelerated cooling is performed. In addition, the low ductility temperature range in which these steels have hot ductility of 50°C or less is as narrow as 80°C or less, and the continuously cast slabs also have good surface properties.

On the other hand, low Nb steel (MtN) has low hot ductility,
-Although there is not much difference in one-handed rolling rate from the present invention, the strength and toughness are significantly improved when air cooling or accelerated cooling is performed after hot rolling, compared to the above-mentioned ilj and Nb based cases according to the present invention. is deteriorating.

According to the method of the present invention described above, the effect of suppressing the precipitation of traces can be obtained by adding a small amount of Ti, the effective use of the hot ductility improvement region by increasing the amount of Nb added, and the improvement of the structure by adding Ti. It becomes possible to improve the hot ductility by the grain refinement of Mn8 and the shape control effect of Mn8. From this, according to the present invention, a series of low C-high Nb steels! It can accurately prevent the decrease in hot ductility that has been a problem in casting and the occurrence of scratches on the surface of continuously cast pieces due to K, and has a high Nb with good surface quality and high tensile strength and high toughness! [An excellent effect can be obtained in that E' can be easily manufactured.

[Brief explanation of the drawing]

Figure 1 d A graph showing the strength change when changing the amount of Nb added in O,02%C-1,74Mn-based low C steel, jI2E is a high temperature tensile test conducted on low C-high Nb steel. A graph showing the relationship between the low ductility temperature range and the casting hand penetration rate, an explanatory diagram of the thermal history used in the high-temperature tensile strength test in Figure IIa enlarged in Figure 2, and Figure 4 shows the relationship between the conventional Nb steel and the low C- Hot ductility of high Nb steel t A graph showing the change in drawing strength with changes in deformation temperature when examined in the temperature range of 700 to 1000°C. 0,
004%N steel and o, o21cm2.0chihato-Nb
-0,015%Ti -0,004%N steel nioite Nb
Graph W, 6 shows the change in hot ductility when the addition amount is changed at 800°C and 100°C, respectively.
Figure 7 is a graph showing changes in strength and toughness with the amount of Nb added when cumulative reduction and rolling are applied to the Ti-added steel. It is a graph showing a change in hot ductility at 700 to 1ooo°C. Patent applicant Nippon Steel Tube Co., Ltd. Viewer Sada Yamamoto Butsuma
Tadashi Shinkura Japanese Patent Attorney Shozo Yoshihara”, +” ”; '
(1.7-li' (zLLILLI/l)M) SJ-'S flavor (olo) ya)jY+Ichitake 4th flame scorching 'C) Takamiku 5th figure Nbj private addition ((%) No. 6m 0 0.05 0.10 0.15 0.2O
Nb addition + (%) Figure 7 Shi Keiyo/l (0C) Procedural amendment and ω Homare) Commissioner of the Patent Office Shima l” Mr. Haruki (Examiner of the Patent Office)
゛ Relationship to the case Applicant (41!l'
Nippon Steel Tube Co., Ltd. 4 Agent 5 Date of amendment order Showa Year, month, day Amendment Contents 1 In the middle page of the specification of the present application, page 5, line 19, it says "high tensile strength and high toughness", and then it says "j for steel sheet manufacturing". .2 Ibid., page 11, line 4, ro, ao*NbJ Correct ``to'' to ro, oschi Nb J, yo Dobetsu, page 13, 1
The second line says "Assuming 11Kr", then r Nb J
and join.

Claims (1)

[Claims] 1. C:Q, 005-G, 061jl, 8i:
0.02-0.50 qb, Mn: 1.5-2
．． 5%, 8: 0.005% or less, Nb: 0.06
~0.20-1So! When continuously casting steel containing 5 to 0.10% of At: o, oo, the amounts of T and N are o, oo
al or less, 'l'i: 0.005 to o, oao is continuously cast, and the obtained slab is hot-rolled and cooled at a cooling rate higher than air cooling. A method for manufacturing non-temperature high-tensile and high-toughness steel sheets by continuous casting. 2, C: 0.005-0.069G, 8i:
0.02-0.50-lMn: 1.5-L5%,
8: 0.005% or less, Nb: o, o s
〜0.20 嗟、8ojAA: 0.00 S〜
Contains 0.10%'i and further [V: 0.01~G. 2%
. Cr: 0.5% or less, Cu: 0.5% or less,
MO: 0.5s or less, Ni: 2% or less, B: 0-
002% or less, Ca: 0.002-0.007%
When manufacturing steel containing one or more of the following, T, Ntt-0,0 (less than l, Ti:
, oos~o, oaol'l is continuously cast, and the obtained slab is hot-pressed and cooled at a cooling rate higher than air cooling. manufacturing method.