JPH032322A - Manufacture of steel plate having excellent brittle fracture-propagation stop characteristics - Google Patents

Abstract

PURPOSE:T o economically manufacture the thick steel plate for structural purposes having excellent brittle fracture-propagation stop characteristics with good productivity by subjecting a slab having specified compsn. constituted of C, Si, Mn, Al, N and Fe to specified cooling treatment, finish rolling and recuperating. CONSTITUTION:A rolled steel for structural purposes contg., by weight, 0.01 to 0.30% C, <=0.5% Si, <=2.0% Mn, <=0.1% Al and 0.001 to 0.01% N, furthermore contg., at need, one or more kinds among <=0.5% Cr, <=0.1% Ti, <=1.0% Ni, <=0.05% Nb, <=0.5% Mo, <=0.0015% B, <=0.1% V and <=0.9% Cu and the balance Fe with inevitable components is subjected to continuous casting. Water cooling is started to the obtd. slab from the AC3 point or above at >=2 deg.C/sec cooling rate. The water cooling is stopped after the surface layer part from the surface and back faces to the 1/3 in thickness of the slab is cooled to the Ar3 point or below. Next, finish rolling is started before the recuperation of the slab is ended. After the end of the finish rolling, the one less than the 1/3 in thickness of the slab from the surface and back faces is recuperated to less than the AC3 point and the one 1/3 or more to the AC3 point or above. In this way, the thick steel plate for structural purposes having excellent brittle fracture-propagation stop characteristics and having excellent toughness to the center of the plate thickness can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing a structural steel plate having excellent brittle crack propagation arresting properties.

<Prior Art> When a brittle crack occurs in a steel plate, the main crack is connected to a preceding crack and propagates.

This connection is formed by ductile fracture called grain boundary tearing, and at this time, crack propagation energy is absorbed by the ductile fracture of tearing.

This absorptive capacity is enhanced by increasing the chance of the ductile fracture due to the refinement of the grain size, improving the crack slowing effect and improving the brittle crack propagation arresting property.

In fact, what greatly contributes to improving the brittle crack propagation arresting properties are the shear lip and plastic deformation that occur in the surface layer of the steel sheet during brittle crack propagation, and this shear lip is also caused by the brittle crack that propagates due to grain refinement. The ability to absorb propagated energy increases.

Therefore, various attempts have been made to effectively refine crystal grains.

In order to realize this, for example, Japanese Patent Application Laid-Open No. 61-235534
As shown in Figures 1 (a) and (d), the publication discloses that 1/8 of the thickness of the slab is extended from the surface of the slab whose temperature is above the Acx point to the center.
Start rolling with a temperature difference between the surface layer and the center of the slab by cooling the surface layer of the slab to a point of Ar3 or lower, and bring the entire thickness of the slab to a point of Ac2 or higher during or after rolling. Reheat and E
We are proposing a method for producing a W4 plate with an excellent brittle crack propagation arresting property at -20'C determined by SSO test and having a ca of about 460 to 960 kg/14''.

However, the proposal of JP-A No. 61-235534 is
Since the entire area of the slab is reheated to the Ac3 point or higher, the reheating time becomes longer and productivity decreases, and the area that is cooled to the Ar3 point or lower is small, so reverse transformation from austenite to ferrite and recrystallization are utilized. The resulting area is narrow, crystal grain refinement and structure refinement are insufficient, and the shear lip effect is not fully exhibited, which is a hindrance in practical use.

<Problems to be solved by the invention> The present invention solves the problems of the prior art described above, and
The object of the present invention is to provide a highly productive and economical method for manufacturing thick structural steel plates that have even better brittle crack propagation arresting properties and excellent toughness up to the center of the plate thickness.

Means for Solving the Problems> The present invention achieves the above problems by: (1) In weight%, C: o,ot~0.30% SI: ≦0.5% Mn: ≦2.0% Al: ≦O01% N: 0.001-0.01% A slab obtained by continuous casting of structural rolled steel consisting of Fe and other unavoidable components was cooled at a cooling rate of 2° (/sec) from a temperature of Ac 3 or higher. With the above, water cooling is started, and the surface layer from the front and back surfaces of the slab to 1/3 of the thickness is cooled to below Ar 1 point, and the water cooling is stopped, and finishing is completed until the reheating of the slab is completed. Start rolling, and after finishing the finish rolling, reheat the slab from the front and back surfaces up to a thickness of 173 to below the AC3 point, and reheat the slab from the front and back surfaces to a thickness of 173 or more to an AC2 point or higher. The first method is to produce a steel plate with excellent brittle crack propagation arresting properties characterized by: (2) In weight%, C: 0.01 to 0.30% Si: ≦0.5% Mn: ≦ 2.0% Al: ≦0.1% N: 0.001-0.01% Furthermore, Cr: ≦0.5% Ti: ≦0.1% Ni
:≦1.0% Nb:≦0.05%MO:
≦0.5% B:≦0.0015%V:≦0.
A slab obtained by continuous casting of a structural rolled steel containing one or more of 1% Cu:≦0.9% and other Fe and other unavoidable components is cooled from a temperature of 2 or more Ac points. Water cooling is started at 2°C/sec or more, and the surface layer from the front and back surfaces of the slab to 1/3 of the thickness is cooled to Ar+.
The water cooling is stopped at a temperature below the Ac point, finish rolling is started until the reheating of the slab is completed, and after finishing the finishing rolling, the thickness of the slab from the front and back to the thickness 173 is reduced to less than the Ac2 point. As a second means, a method for producing a steel plate with excellent brittle crack propagation arresting properties is provided, which is characterized by recuperating heat and reheating a thickness of 173 or more from the front and back surfaces of the slab to an Ac2 point or more, (3) In weight%, C: 0.01-0.30% Si: ≦0.5% Mn: ≦2.0% Al=≦0.1% N: 0.001-0.01% Others Fe and unavoidable A slab obtained by continuous casting of structural rolled steel consisting of the following components is water-cooled from a temperature of AC2 or higher at a cooling rate of 2°/sec or higher, and the slab is cast from the front and back surfaces to 1/3 of the thickness. The surface layer is cooled to an Ar point of 3 or less, the water cooling is stopped, finish rolling is started until the reheating of the slab is completed, and after finishing the finish rolling, 174
A third method for producing a steel plate with excellent brittle crack propagation arresting properties, which is characterized by reheating up to below the Ac4 point and reheating 1/4 or more from the front and back surfaces of the STI slab to an Ac4 point or above.
(4) In weight%, C: 0.01-0.30% Si: ≦0.5% Mn: ≦2.0% Al: ≦0.1% N: 0.001-0. 01% Furthermore, Cr:≦0.5% Ti:≦0.1% Ni:
≦1.0% Nb:≦0.05 %Mo:≦0
．． 5% B=≦0.0015%V:≦0.1
%Cu:≦0.9% A slab obtained by continuous casting of structural rolled steel containing one or more of 0.9% and other Fe and other unavoidable components was cooled at a cooling rate of 2 from a temperature of Ac 1 point or higher. Start water cooling at ℃/sec or more, cool the surface layer from the front and back surfaces of the slab to 1/3 of the thickness, and stop the water cooling when the Ar point is 3 or less, completing the reheating of the 2% TFI piece. In the meantime, finish rolling is started, and after finishing the finish rolling, reheat up to 1/4 of the front and back surfaces of the slab to less than 3 Ac points, and at the same time bring 174 or more from the front and back surfaces of the slab to 3 Ac points or higher. The fourth means is a method for manufacturing a steel plate with excellent brittle crack propagation arresting properties characterized by reheating, and (5) the steel plate after completion of reheating described in the above means 1.2. The fifth to eighth means are to cool down to 650°C or less at a cooling rate of 0°C/sec or less.

The reasons for limiting the above-mentioned components will be explained below.

C is added as a reinforcing component to steel materials, and an upper limit is set to prevent deterioration of the toughness of the welded part.

No. 51 is added for the purpose of deoxidizing and maintaining strength, and the upper limit is set to prevent deterioration of weldability.

Mn is added for the purpose of improving low-temperature toughness, and the upper limit is set to prevent weld cracking.

N is added together with AI for the purpose of refining crystal grains by forming nitrides, and the upper limit is set to prevent deterioration of the toughness of the weld zone.

Cr, Ni, MoB, and Cu are all added for the purpose of improving hardenability and effective strength, and upper limits are set to suppress the formation of low-temperature transformation products and prevent a decrease in the ferrite area ratio. ing.

Ti and Nb are added for the purpose of refining crystal grains, and the upper limit is set to prevent reduction in toughness of the weld zone.

(2) is added for the purpose of precipitation strengthening, and the upper limit is set for economic reasons.

Structural rolled 112I slabs to which the present invention is directed include continuously cast high-temperature steel slabs, steel slabs that have been cooled and then reheated, furthermore continuous cast steel slabs that have the same shape as before, and Refers to steel billets, etc. that have been subjected to rolling such as continuous casting, punching, shape adjustment rolling, etc.

Further, the water cooling in the present invention can be sufficiently carried out using a water cooling means which has a cooling rate of 2° C/sec or more and is normally used in continuous casting.

<Function> In order to achieve the above-mentioned object, the present inventors prepared the following in weight%: C: 0°01 to 0.30% Si: ≦0.5% Mn: ≦2.0% Al; ≦0.1% N: 0.001 to 0.01% Furthermore, Cr: ≦0.5% Ti: ≦0.1% Ni:
≦1.0% Nb:≦0.05 %Mo:≦0
．． 5% B:≦0.0015%■2≦0.1%C
Various experimental studies were repeated using slabs obtained by continuous casting of structural rolled steel containing one or more of u: 50.9% and other Fe and other unavoidable components.

As a result, steel plates with a thickness of 50m+n or less were tested in the first (a) (
As shown in b), the slab is cooled from the front and back surfaces of the slab in the thickness direction to about 173 (hereinafter referred to as 1/3 L) of the thickness at a cooling rate of 2°C/sec or more to an Ar point of 3 or less. Confidence rolling is started with a temperature difference between the surface layer and the center of the slab, and after finishing the finish rolling, the surface layer of about 1/3L from the front and back surfaces of the slab is reduced to less than the Ac2 point. When more than 1/3 of the front and back surfaces of the ferrite are reheated to Ac3 point or higher, the ferrite is sufficiently recrystallized and the crystal grains become fine even though it is in the non-recrystallized area because the rolling is performed during the temperature rising process. Thickness: 50mm
As shown in Figures 1(a) and (c), super steel plates can be heated to 174E or higher by reheating approximately 1/4 (hereinafter referred to as 1/4t) of the thickness from the front and back surfaces of the slab to a point of Ac3 or higher. It has been learned that the portion smaller than 1/4 L is refined by recrystallization as described above, and the portion larger than about 1/4 L is refined by reverse transformation from ferrite to austenite.

Based on these findings, it has been found that the above method can accurately generate a fine structure in a wide range of areas throughout the steel plate in a short period of time without being affected by the thickness of the steel plate.

In addition, at this time, the steel sheet that does not contain Ti and Nb has ferrite with a grain size of 5 μl or less reaching an area ratio of 50% or more of the surface layer, and the steel sheet that contains Ti and Nb has ferrite with a grain size of 3 μm or less. It has been found that when the area ratio of the surface layer reaches 50% or more, and both kca (-70)≧600, extremely stable brittle crack propagation arresting properties are exhibited.

Also, these steel plates can be heated to 650℃ at a cooling rate of 40℃/second or less.
It has been found that forced cooling to below °C improves the strength and toughness of the base metal.

The present invention is based on the above findings.

<Example> 1. Test w4: Shown in Table 1.

2. Rolling conditions: Shown in Table 2.

3. Cooling conditions are shown in Table 2.

4. Cooling stop conditions 2 are shown in Table 2.

S, ul heat conditions 8 are shown in Table 2.

6. Controlled cooling conditions after hot sake: Shown in Table 2.

7. Area ratio and Kca by particle size are shown in Table 3.

Perfumes of the invention examples 1, 3 to 7, 9 to 11.13 to 17.1
9-21.24, 25.27-29 are ca (-7
0″C)≧600. In contrast, Comparative Example Perfume 2
, 8.12.18.22, 23.26 is Kca (-7
0°C)≦300°C was not obtained.

<Effects of the Invention> The present invention forms a fine structure throughout the thickness of the steel sheet by combining recrystallization and reverse transformation by rolling at elevated temperature in the non-recrystallized region, and effectively acts on the shear lip. Excellent brittle crack propagation arresting property Kca (-70)
≧600, the economic efficiency and productivity of manufacturing steel plates have been significantly improved, and the effect it brings to this type of field is extremely large.

[Brief explanation of drawings]

FIG. 1(a) shows the positions in the thickness direction of slabs and steel plates regulated by the present invention. (C) indicates the relationship between the position and the water cooling/recuperation temperature in the example of the present invention of claims 1 and 2.5, and (C) indicates the relationship between the above position and the water cooling/recuperation temperature in the example of the present invention of claims 1 and 2.5.
4.5 shows the relationship between the above position and water cooling/recuperation temperature in the present invention example. (d) shows the relationship between the above-mentioned position and water cooling/recuperation temperature in a conventional example (Japanese Patent Laid-Open No. 61-235534). Patent applicant Nippon Steel Corporation

Claims (4)

[Claims] (1) In weight%, C: 0.01-0.30% Si: ≦0.5% Mn: ≦2.0% Al: ≦0.1% N: 0.001-0.01% Other Fe A slab obtained by continuous casting of structural rolled steel consisting of unavoidable components is started to be water-cooled from a temperature of Ac_3 or higher at a cooling rate of 2°C/sec or higher, and 1/3 of the thickness is removed from the front and back surfaces of the slab. Cool the surface layer to below Ar_3 point, stop the water cooling, start finish rolling until the reheating of the slab is completed, and after finishing the finish rolling, reduce the thickness from the front and back sides of the slab. A steel plate with excellent brittle crack propagation arresting properties, characterized in that up to 1/3 of the slab is reheated to below the Ac_3 point, and at least 1/3 of the thickness from the front and back surfaces of the slab is reheated to the Ac_3 point or more. Production method. (2) In weight%, C: 0.01-0.30% Si: ≦0.5% Mn: ≦2.0% Al: ≦0.1% N: 0.001-0.01% Furthermore, Cr:≦0.5% Ti:≦0.1% Ni:≦1.0% Nb:≦0.05% Mo:≦0.5% B:≦0.0015% V:≦0.1% Cu :≦0.9% of one or more of the above, and other Fe and other unavoidable components.
Start water cooling at a cooling rate of 2°C/sec or higher from a temperature of C_3 or higher, cool the surface layer from the front and back surfaces of the slab to 1/3 of the thickness, stop the water cooling at a temperature of Ar_3 or lower, and Finish rolling is started before the reheating of the slab is completed, and after finishing the finishing rolling, the slab is Ac_
1 of the thickness from the front and back surfaces of the slab.
A method for producing a steel plate having excellent brittle crack propagation arresting properties, characterized by recuperating heat from /3 or more to Ac_3 point or more. (3) In weight%, C: 0.01-0.30% Si: ≦0.5% Mn: ≦2.0% Al: ≦0.1% N: 0.001-0.01% Other Fe A slab obtained by continuous casting of structural rolled steel consisting of unavoidable components is started to be water-cooled from a temperature of Ac_3 or higher at a cooling rate of 2°C/sec or higher, and 1/3 of the thickness is removed from the front and back surfaces of the slab. The surface layer is cooled to below Ar_3 point, the water cooling is stopped, finish rolling is started until the reheating of the slab is completed, and after finishing the finishing rolling, 1 /
A method for producing a steel sheet with excellent brittle crack propagation arresting properties, characterized by reheating up to Ac_3 point up to Ac_3 point and reheating 1/4 or more from the front and back surfaces of the slab to Ac_3 point or more. (4) In weight%, C: 0.01-0.30% Si: ≦0.5% Mn: ≦2.0% Al: ≦0.1% N: 0.001-0.01% Furthermore, Cr:≦0.5% Ti:≦0.1% Ni:≦1.0% Nb:≦0.05% Mo:≦0.5% B:≦0.0015% V:≦0.1% Cu :≦0.9% of one or more of the above, and other Fe and other unavoidable components.
Start water cooling at a cooling rate of 2°C/sec or higher from a temperature of C_3 or higher, cool the surface layer from the front and back surfaces of the slab to 1/3 of the thickness, stop the water cooling at a temperature of Ar_3 or lower, and Finish rolling is started before the reheating of the slab is completed, and after finishing the finish rolling, the front and back surfaces of the slab are reheated to less than 1/4 of the Ac_3 point, and the front and back surfaces of the slab are reheated to less than Ac_3 point. 1/4 or more from A
c_A method for producing a steel plate with excellent brittle crack propagation arresting properties characterized by recuperation to a point of 3 or higher. (5) Claims 1-
4. A method for producing a steel plate having excellent brittle crack propagation arresting properties, which comprises cooling the steel plate after completion of reheating according to item 4 to 650°C or less at a cooling rate of 40°C/sec or less.