JPH01283344A - Production of high strength steel plate excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To stably and smoothly produce a high strength steel plate having superior toughness at low temp. by specifying cooling conditions for subjecting a molten steel of the prescribed composition to transformation strengthening from cast and solidified state, etc. CONSTITUTION:A molten steel having a composition consisting of, by weight, 0.05-0.20% C, <=0.030% S, 0.03-0.80% Si, <=0.100% Al, 0.50-2.00% Mn, <=0.0100% N, <=0.025% P, and the balance iron with inevitable impurities is cast and solidified. This steel is continuously cooled through a temp. region from a temp. of >=Ar3 point down to at least the transformation-finishing temp. of lower bainite at 10-100 deg.C/sec cooling rate while the temp. of this steel lies at a temp. of the Ar3 point or above from the state of as-cast steel or while the temp. of this steel lies at a temp. of the Ar3 point or above after rolling. Further, it is desirable that tempering is carried out at a temp. of the Ac1 point or below after cooling. By this method, the high strength steel plate excellent in toughness at low temp. can be manufactured with superior heat economy, under smooth operation, in high yield.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention solidifies molten steel containing the required components, and then solidifies it from its still state without rolling it while the steel is at a temperature above Ar. Alternatively, after rolling the steel while it is at a temperature of R, point or higher, the steel is continuously cooled at a required cooling rate in a temperature range from the Ar3 point temperature or higher to at least the completion of transformation of lower heenite or martensite, and then the shipbuilding process is carried out. The present invention relates to a method for manufacturing a high-strength steel plate with excellent low-temperature toughness that stably obtains T rs -80°C level required for grade steel.

<Conventional technology> In recent years, continuous casting slabs have been directly hot rolled (
(hereinafter referred to as DR) is in practical use.

On the other hand, the thickness of the continuously cast slab is 50 mm, which is close to the product thickness.
Continuous casting methods for producing steel sheets with a thickness of ~100 mm are being put into practical use, and various manufacturing processes are being considered for producing steel plates by DR from slabs cast by the continuous casting method.

In this case, DR starts rolling from the coarse austenite at the same time as a, and can achieve the target stably and smoothly even at a reduction ratio of about 1 to 2, which is much smaller than the conventional reduction ratio applied to slabs with a thickness of 250 to 300 mm. It is desired to be able to obtain the desired shape and material.

One of the proposals to meet this demand is disclosed in Japanese Patent Laid-Open No. 60-56024. In the field of conventional reduction ratios, this proposal involves adding a small amount of Ti to steel to suppress the precipitation of TiN, which is a coarse large nitride, and to precipitate some of it as fine TiN, leaving the rest in a solid solution state. Most of the Ti is finely precipitated as Ti (C,N) during or after rolling, which prevents recrystallization of T crystal grains to ensure a fine structure and strengthens the Ti (C,N) by precipitation. It strengthens steel and requires Ti and N.

The other one is the proposal of Japanese Patent Laid-Open No. 61-213322. This proposal involves adding no A1 to the steel, keeping the amount of AI in the steel to 0.007% or less including the amount of unavoidable intrusion, deoxidizing the steel with Ti, and adding it to the steel during the melting and casting process. This method uses Ti oxide-based precipitates formed in the slab to refine the transformed structure of the steel sheet, and the transformation of the steel is not carried out from a single grain boundary, but from fine grains in the steel material independently of the γ grain boundaries. This method attempts to obtain a fine heenite structure by performing intragranular transformation from Ti oxide-based inclusions dispersed and precipitated in the steel, and requires no addition of AI, addition of Ti, and deoxidation of Ti.

<Problems to be Solved by the Invention> However, as is clear from Table 5, the proposal of JP-A No. 60-56024 is necessary for shipbuilding grade E, which is the object of the present invention, and Trs--80'C or less. It is not possible to stably obtain low-temperature toughness.

Furthermore, as is clear from Tables 1 and 2 of the proposal in Japanese Patent Application Laid-open No. 61-213322, it is possible to obtain low-temperature toughness of Trs = -80°C or less, which is necessary for the shipbuilding E grade targeted by the present invention. do not have.

The present invention does not involve the problems of the prior art described above, and is necessary for shipbuilding grade E, which the above proposal could not achieve.
The present invention provides a method for stably and smoothly producing a steel plate having excellent low-temperature toughness of s -80°C or less by transformation strengthening.

<Means for Solving the Problems> The present invention is characterized by using the following means in order to secure a steel plate having the desired material without having the above-mentioned problems of the prior art. That is, (1) in weight%, C: 0.05-0.20! S: ≦0.030
XSi: 0.03-0.80Z Al: ≦
0.100” 1st n: 0.50~2.00χ N
, ≦0.0100χPz≦o, o25z, and the remaining iron and unavoidable impurities are cast and solidified, and the steel is removed from that state without being rolled while it is at a temperature above the Ar point, or when a political policy is established. After rolling while the temperature is at the Ar point or higher, the temperature range from the Ar 3 point temperature or higher to at least the transformation end temperature of the lower heenite is 10 to 10.
The first method is to cool continuously at a cooling rate of 100°C/sec. Si
: 0.03-0.80X Al: ≦0.100
”IKn: 0.50-2.OOZN:≦0
．． 0100χP; ≦0.025y.

The molten steel containing iron and unavoidable impurities is cast and solidified, and then rolled from that state without rolling while the steel wire is at a temperature of Ar3 or higher, or rolled while the steel is at a temperature of Ar, or higher. After that, the first means is to continuously cool the temperature range from 800°C to 300°C at a cooling rate of 10-100°C/see, (3) in weight%, C : 0.05~0.20χ S:≦0.030%Si
: 0.03-0.80Z Al: <0.10
0”AMn: 0.50-2.OOXN:
≦0.0100 After rolling while at the above temperature, the temperature range from Ar 3 point temperature or higher to at least the martensite transformation end temperature is 10 to 10%.
The second method is to cool continuously at a cooling rate of 100°C/sec, (4) in weight%, C: 0.05 to 0.20X S: ≦0.030
XSi: 0.03-0.802: Al:
≦0.100 "IMn: 0.50~2.OO
XN: ≦0.0100ZP: ≦0
．． The molten steel containing 025 (5) In weight%, Cr:≦1”X Zr:≦0.IZNi
: <IOZ Nb : ≦0.1”XMO=
≦1χ B:≦0.00:V:≦0.2y,
Cu:≦1z Ti:≦0. IX Ca: ≦0.00
The fifth to eighth means include one or more of 8Z in the first means, the second means, and the specific implementation means of each means, (6) Ac, point or more after cooling. The ninth to sixteenth means include tempering at a temperature of .

The reason for limiting each requirement constituting each means will be explained below.

C, Si, and Mn are each added to ensure strength, but upper limits are set for C to prevent deterioration of toughness and weldability, for Si=Mn to prevent deterioration of weldability, and for both P and S. These are impurities that are inevitably mixed in, and both impair toughness, so the above range is the upper limit.

A1 is a deoxidizing agent, but the upper limit is set for economic efficiency based on saturation of the effect, and N is an impurity that is inevitably mixed in like P and S, but it is used to ensure the toughness of the heat affected zone during high heat input welding. For-
The L limit is set.

Also, as an optional additive, C(N) increases the strength of the base metal and welded part.
7 of the base metal, from the viewpoint of deterioration of the toughness of the heat-affected zone; Ni, which increases the toughness and strength of the heat-affected zone; Ti, which is uneconomical due to saturation of the effect;
Zr improves ductility and notch toughness from the perspective of deteriorating the toughness of the weld zone, and Ca improves hardenability and toughness of the heat-affected zone from the perspective of preventing surface defects and deterioration of steel cleanliness, respectively. From the point of view of preventing hot cracking during the transformation process,
Upper limits are set for Cu, which improves corrosion resistance and strength, from the viewpoint of preventing hot cracking in the weld metal.

Also, when rolling is required unrelated to the purpose of the present invention, Δ
Finishing rolling at a temperature of R3 or higher is achieved by effectively utilizing the thermal energy of the steel billet, which is at a high temperature during casting following melting, and by maintaining the coarse austenite structure and subsequent controlled cooling. This is to economically and efficiently generate a structure essential to a steel plate that exhibits high strength and excellent low-temperature toughness, which is the object of the invention.

<Function> The present inventors have conducted experiments and tests on a method for producing high-strength steel sheets with excellent low-temperature toughness exhibiting vTrs of -80°C or less with good thermoeconomic efficiency and high yield under smooth operation. , have solved all the problems of the prior art described above, and have found a method for manufacturing steel sheets superior to those obtained using these methods.

The experimental conditions and experimental results are shown below.

The main components other than iron of the sample steel are as follows.

C: 0.08χ S: 0.010%Si
: 0.15Z Al :
0. O3XMn: 1.50X
N: 0.0035Zp: o, ol
oz Ar3 of the Naoki test steel is 738°C.

The present inventors subjected this steel to a continuous casting process to produce slabs, and then conducted the experiments described above under the various conditions described in Section 2.

The results are shown in FIGS. 1 and 2.

Figures 1 and 2 show that the slab is rolled until it reaches the Ar 3 point temperature, and then water, steam, a steam/water mixture, air, etc. are used to set the cooling start temperature, cooling end temperature, and By varying the cooling rate, 50 kg of TS was obtained from the copper plates obtained.
f 7 mm" or more, and the respective vTrs are shown based on the cooling rate determined from the continuous cooling time and cooling temperature and the rolling reduction ratio of the slab in the cooling temperature range shown below.

The cooling temperature range in Figure 1 is from 800°C to 300°C, and the cooling temperature range in Figure 2 is from 800°C to 50°C.
It is.

As predicted from the sample steel composition in this experiment, the former has a cooling temperature range from the Ar3 point temperature or higher to the temperature range at which the transformation of the lower heenite structure ends, and the latter has a cooling range of Ar3 point temperature or higher. The temperature range includes the temperature range from

That is, when the cooling rate is 20°C/sec and when the cooling rate is 40'C/sec,
Each time in sec is as follows.

The inventors set the above range in consideration of experimental conditions, especially the temperature variations in the thickness direction, length direction, and width direction of the material to be cooled, in order to completely capture the start and end of each transformation. However, from the results shown in FIGS. 1 and 2, it was confirmed that the cooling temperature range described above is the temperature range of controlled cooling that is necessary and important to obtain each target structure of the present invention.

In other words, the cooling rate is 10% in each of the above cooling temperature ranges.
When maintained above °C/sec, each objective m weave, that is, the example in Figure 1 is a lower heenite structure, the example in Figure 2 is a martensite structure or a structure in which a lower bainite structure mixed and coexisting with a martensitic structure as its main character, has a reduction ratio. Regardless of the temperature, the obtained steel plate exhibited high strength with a TS of 50 Kgf/mm2 or more, and excellent low-temperature toughness with a vTrs of -80'C or less.

On the other hand, the cooling rate is 10°C in each cooling temperature range mentioned above.
/sec When the groove was not formed, the steel structure became a ferrite-pearlite or upper heenite structure, and as a result, vTrs did not reach -80°C.

With current industrial technology, the cooling rate is 100'C/se.
Although it is difficult to achieve a cooling rate higher than c, it is thought that the effects of the present invention will not change in principle even if the cooling rate is higher than that.

The present invention has been made based on the above-mentioned knowledge, and after completing the rolling of the coarse austenite of the cast structure, in the subsequent controlled cooling process, the lower heenite structure, the martensitic structure, or the martensitic structure and the lower heenite structure are formed. It has been discovered that by efficiently generating a mixed coexisting structure, it is possible to solve all of the problems of the prior art described above and to produce the steel sheet that is the object of the present invention.

<Example> (Example 1) Table 1 shows the materials used in each of the first, second, fifth, sixth, ninth, tenth, thirteenth, and fourteenth invention examples and their respective comparative examples. Tables 2 to 9 show the composition of the test steel, the manufacturing conditions for each steel plate, and the resulting materials.

As is clear from the table, all of the inventive perfumes 1 to 82 have a vTrs of -80°C or less and a TS of 50Kgf/m.
A steel plate having the desired material and exhibiting m 2 or more was obtained.

Furthermore, when Perfume Compounds 42 to 82 were tempered at a temperature below the Ac+ point, the toughness was further improved, and steel sheets with excellent low-temperature toughness with vTrs of less than -100'C were obtained.

In contrast to these examples of the present invention, Comparative Examples 83 to 164 in which the cooling conditions were outside the range of the present invention did not have a lower heenite structure, and some had TS of 50 Kgf/mm2. However, vTrs did not even reach -80°C.

Furthermore, when perfumes 124 to 164 were tempered at a temperature below the Ac+ point, vTrs did not reach -80°C.

(Example 2) Table 10 shows the 3rd, 4th, 7th, 8th, 11th, 12th
Tables 11 to 18 show the composition of the test steel used in each of the 15th and 16th invention examples and each comparative example, and the manufacturing conditions of each steel plate and the resulting material.

As is clear from the table, all of the inventive perfumes 1 to 84 have a vTrs of -80°C or less and a TS of 50 Kgf/mm.
A steel plate having the desired material properties and exhibiting 2 or more was obtained.

Furthermore, when perfumes 43 to 84 were tempered at a temperature below the Ac1 point, they exhibited excellent low-temperature toughness with vTrs below -100°C.

On the other hand, Comparative Examples 85 to 168, in which the cooling conditions were outside the range of the present invention, did not have the structure required by the present invention;
Some TS has reached 50Kgf/mm2, but vTr
s did not even reach -80°C.

In addition, although perfumes 127 to 16B were tempered at a temperature below the Ac1 point, vTrs did not reach -80°C.

<Effects of the Invention> As is clear from the above description, the present invention transforms and strengthens the coarse austenite structure of steel by generating a structure that exhibits the excellent low-temperature toughness and high strength that is the object of the present invention. Since we have established a controlled cooling method, it is not necessary to add expensive Ti to the steel components, there is no internal quality deterioration due to oxide inclusions, and the rolling temperature is lower due to the superiority of the coarse austenite structure during casting. Hot rolling can be started without the need to set a waiting time for adjustment, and nuclear hot rolling ends at a temperature of Ar1 or higher, making the most of the enormous thermal energy that cast steel has. It has great effects in this field, such as the ability to perform hot rolling with high thermoeconomic efficiency and high productivity.

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 are diagrams showing the experimental results of the child's child. Patent Applicant Nippon Steel Corporation Agent Masu Nakahori (and 2 others) 1st Figure Pressure Lower Ratio → 2nd Figure Lower Ratio Naka

Claims (6)

[Claims] (1) In weight%, C: 0.05-0.20% S: ≦0.030% Si: 0
．． 03~0.80%Al:≦0.100%Mn:0.5
0-2.00%N:≦0.0100%P:≦0.025
% and the balance iron and unavoidable impurities is cast and solidified, and the steel is left in that state without rolling while it is at a temperature of Ar_3 point or higher, or while the steel is at a temperature of Ar_3 point or higher. A high-strength steel sheet with excellent low-temperature toughness characterized by being continuously cooled at a cooling rate of 10 to 100°C/sec in a temperature range from Ar_3 point temperature or higher to at least the transformation end temperature of lower bainite after rolling. Production method. (2) In weight%, C: 0.05-0.20% S: ≦0.030% Si: 0
．． 03~0.80%Al:≦0.100%Mn:0.5
0-2.00%N:≦0.0100%P:≦0.025
% and the balance iron and unavoidable impurities is cast and solidified, and the steel is left in that state without rolling while it is at a temperature of Ar_3 point or higher, or while the steel is at a temperature of Ar_3 point or higher. A method for manufacturing a high-strength steel sheet with excellent low-temperature toughness, which comprises continuously cooling the steel sheet in a temperature range of 800°C to 300°C at a cooling rate of 10 to 100°C/sec after rolling. (3) In weight%, C: 0.05-0.20% S: ≦0.030% Si: 0
．． 03~0.80%Al:≦0.100%Mn:0.5
0-2.00%N:≦0.0100%P:≦0.025
% and the balance iron and unavoidable impurities is cast and solidified, and the steel is left in that state without rolling while it is at a temperature of Ar_3 point or higher, or while the steel is at a temperature of Ar_3 point or higher. A high-strength steel plate with excellent low-temperature toughness characterized by being continuously cooled at a cooling rate of 10 to 100°C/sec in a temperature range from Ar_3 point temperature or higher to at least the martensite transformation end temperature after rolling. Production method. (4) In weight%, C: 0.05-0.20% S: ≦0.030% Si: 0
．． 03~0.80%Al:≦0.100%Mn:0.5
0-2.00%N:≦0.0100%P:≦0.025
% and the balance iron and unavoidable impurities is cast and solidified, and the steel is left in that state without rolling while it is at a temperature of Ar_3 point or higher, or while the steel is at a temperature of Ar_3 point or higher. A method for manufacturing a high-strength steel sheet with excellent low-temperature toughness, which comprises continuously cooling the steel sheet in a temperature range of 800°C to 50°C at a cooling rate of 10 to 100°C/sec after rolling. (5) In weight%, Cr: ≦1% Zr: ≦0.1% Ni: ≦10% Nb: ≦0.1% Mo: ≦1% B: ≦0.01% V: ≦0.2% Any one of claims 1 to 4, characterized in that it contains one or more of the following: Cu:≦1% Ti:≦0, 1%Ca:≦0.008% A method for manufacturing high-strength steel sheets with excellent low-temperature toughness. (6) The method for producing a high-strength steel plate with excellent low-temperature toughness according to any one of claims 1 to 5, characterized by tempering at a temperature of Ac_1 point or less after cooling.