JPH04224623A - Manufacture of thick 50kg class low yield ratio-high tensile strength steel plate small in difference of hardness in plate thickness direction - Google Patents

Abstract

PURPOSE:To manufacture a thick 50kg class low yield ratio-high tensile strength steel plate small in the difference of hardness in the plate thickness direction by limiting chemical components, heating temp. and rolling finishing temp. as well as cooling starting temp., cooling velocity and cooling stopping temp. at the time of controlled cooling in a steel plate. CONSTITUTION:This is a method of heating a slab to >=1100 deg.C, thereafter finishing its rolling in the temp. range of 850 to 900 deg.C and executing cooling to the temp. range of 500 to 400 deg.C at 3 to 12 deg.C/sec cooling velocity in which its cooling starting temp. is regulated to (the rolling finishing temp. -50 deg.C) or above.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for producing high-strength steel plates, and more specifically, to the production of 50 kg-class low yield ratio thick-walled high-strength steel plates with small differences in hardness in the thickness direction for use in high-rise buildings, etc. It is about the method.

0002

[Prior Art] At present, TMCP (Thermo-Mechanical C
(ontrol Process) is a technology that combines controlled rolling and controlled cooling, and is capable of imparting superior strength, toughness, and weldability to steel sheets compared to as-rolled materials and normalized materials.

However, thick-walled high-strength steel plates for construction are manufactured by TMC.
When manufactured from P, there is a difference in hardenability in the thickness direction during controlled cooling, and the difference in hardness in the thickness direction tends to be larger compared to as-rolled materials and normalized materials.

On the other hand, when the cooling rate during controlled cooling is set to 1
5℃/sec or more, and the lower limit of the cooling stop temperature is 500℃.
A manufacturing method that achieves excellent strength and toughness and reduces the difference in hardness in the thickness direction by adjusting the temperature to
It is disclosed in the publication No.

[0005]

[Problems to be Solved by the Invention] However, with this method, when producing a thick high-strength steel plate having a thickness exceeding 50 mm, it is expected that the difference in hardness in the thickness direction will increase.

The present invention has been made to reduce the hardness difference in the thickness direction of the above-mentioned thick-walled high-strength steel plate, and the present invention has been made to reduce the hardness difference in the thickness direction of the above-mentioned thick-walled high-strength steel plate. The object of the present invention is to provide a method for manufacturing a 50 kg class low yield ratio thick-walled high-strength steel plate with a small difference in hardness in the thickness direction by limiting the cooling rate and cooling stop temperature.

[0007]

[Means for Solving the Problems] The present inventors have conducted various studies on manufacturing methods for reducing the difference in hardness in the thickness direction of thick high-strength steel plates, and as a result, they have adjusted the chemical composition,
The present invention was based on the knowledge that by appropriately controlling heating temperature, rolling conditions, cooling conditions, etc., it is possible to manufacture a 50 kg class low yield ratio thick-walled high-strength steel plate with a small difference in hardness in the thickness direction. This is what we have come to.

[0008] The first invention has C: 0.05 to 0 in weight%.
．． 20%, Si: 0.05-0.50%, Mn: 0
．． 80-2.0%, Al: 0.01-0.10%,
Nb: 0.005~0.050%, Ti: 0.005~
Ceq. containing 0.050% and represented by the following formula.
0.40% or less, and the remainder consists of Fe and unavoidable impurities, after heating it to a temperature of 1100°C or higher, 8
After finishing rolling in the temperature range of 50 to 900°C, set the cooling start temperature to (rolling end temperature - 50°C) or higher, 3
This is a method for manufacturing a 50 kg class low yield ratio thick-walled high tensile strength steel plate with a small difference in hardness in the thickness direction, which is cooled to a temperature range of 500 to 400°C at a cooling rate of ~12°C/sec. Ceq
=C+Si/24+Mn/6+Ni/40+Cr/5+
Mo/4+V/14 (%)

[0009] The second invention is
In weight%, Cu: 0.05-1.0%, Ni: 0
．． 05-1.0%, Cr: 0.05-0.50%,
Mo: 0.05-0.50%, V: 0.01-0.1
0%, B: 0.0003-0.0030%, Ca: 0
．． 2. The method for producing a 50 kg class low yield ratio thick-walled high-strength steel plate having a small difference in hardness in the thickness direction according to claim 1, wherein the steel plate contains one or more selected from 0005 to 0.0040%.

0010

[Operation] The reasons for limiting the chemical components in the present invention will be explained below.

[0011] C is an effective element for increasing strength, but if it is less than 0.05%, the strength will be insufficient;
If it exceeds 0.20%, weldability deteriorates. therefore,
The amount of C added is in the range of 0.05 to 0.20%.

[0012]Si is an element necessary for deoxidizing and strengthening steel during steel manufacturing, and for this purpose, it is necessary to add 0.05% or more. However, if added in excess of 0.50%, weldability deteriorates. Therefore, the amount of Si added is 0
．． The range is 0.05% to 0.50%.

[0013] Mn is at least 0.0% to ensure strength.
It is necessary to add 80%, but if it is added in excess of 2.0%, weldability deteriorates. Therefore, the amount of Mn added is in the range of 0.80 to 2.0%.

Al is an effective element for deoxidizing and refining crystal grains, and needs to be added in an amount of 0.01% or more.
Addition of more than 10% deteriorates weldability. Therefore, the amount of Al added is in the range of 0.01 to 0.10%.

Nb is one of the elements characteristic of the present invention, and is added to reduce the difference in hardness in the thickness direction of a 50 kg class thick high tensile strength steel plate. This element increases the hardenability of steel in a solid solution state and effectively works to improve hardness by promoting bainite transformation, but 0.0
Below 0.05%, this effect is small; on the other hand, 0.050
%, weldability deteriorates. Therefore, the amount of Nb added is in the range of 0.005 to 0.050%.

[0016] Ti is an element effective in improving the toughness of the weld heat affected zone by refining austenite grains in the heat affected zone of large heat input welds and promoting the formation of ferrite.
This effect is small below 0.05%, while 0.050
%, weldability deteriorates. Therefore, the amount of Ti added is in the range of 0.005 to 0.050%.

In the present invention, in addition to the above chemical components, Cu, N
One or more selected from i, Cr, Mo, V 2 , B 2 , and Ca can be added.

[0018] Cu is an effective element for improving the strength and toughness of the base metal without hardening the weld heat affected zone or deteriorating the toughness of the weld heat affected zone.
If it is less than 1.0%, this effect will be small, while if it exceeds 1.0%, cracks will easily occur during hot rolling. therefore,
The amount of Cu added is in the range of 0.05 to 1.0%.

Ni is an element that has almost the same effect as Cu, but if it is less than 0.05%, this effect is small, and on the other hand, if it is added in excess, it is an expensive element, leading to an increase in manufacturing costs. Therefore, the amount of Ni added is 0.05~
The range shall be 1.0%.

[0020] Cr is an effective element for improving hardenability and increasing strength, and in order to obtain this effect, 0.0
It is necessary to add 5% or more, but if it is added in a large amount exceeding 0.50%, weldability will deteriorate. Therefore, Cr
The amount added is in the range of 0.05 to 0.50%.

[0021] Mo is an element that has the same effect as Cr, and in order to obtain this effect it is necessary to add 0.05% or more, but if it is added in a large amount exceeding 0.50%, weldability deteriorates. This causes deterioration and increases in manufacturing costs. Therefore, the amount of Mo added is in the range of 0.05 to 0.50%.

V is an effective element for increasing strength, but if it is less than 0.01%, this effect is small;
If it exceeds 10%, weldability deteriorates. Therefore, V
The amount added is in the range of 0.01 to 0.10%.

[0023] B is an element that is effective in improving hardenability and increasing strength when added in a small amount.
Below 3%, this effect is small; on the other hand, 0.0030%
If it exceeds this, weldability will deteriorate. Therefore, the amount of B added is in the range of 0.0003 to 0.0030%.

[0024] Ca is an element that is effective in improving anisotropy and improving lamellar tear resistance by controlling the morphology of inclusions, but this effect is small at less than 0.0005%, while at more than 0.0040%. This increases the amount of nonmetallic inclusions in the steel and causes internal defects. Therefore, the amount of Ca added is in the range of 0.0005 to 0.0040%.

In addition to limiting the range of chemical components as described above, Ceq is limited to 0.40% or less in order to improve weldability.

Next, the reasons for limiting the manufacturing conditions of the present invention will be explained. The heating temperature of the steel slab is preferably low in order to prevent coarsening of austenite crystal grains. However, in order to form a solid solution of 0.005% or more of Nb, which is necessary to ensure strength, a certain heating temperature must be maintained. Therefore, in the present invention, the heating temperature of the steel piece is limited to 1100°C or higher.

[0027] In addition, the rolling end temperature is determined from the viewpoint of earthquake resistance.
In particular, the yield ratio [
(yield point/tensile strength) x 100%], the temperature should be set at 850°C or higher, and the temperature should be set at 900°C or higher to ensure toughness.
The temperature shall be below ℃. The reason for this is that at temperatures below 850°C, the increase in yield point exceeds the increase in tensile strength due to the refinement of ferrite grains, and at temperatures above 900°C, the refinement of ferrite grains is not sufficient.

Furthermore, the reason why the cooling start temperature after rolling is set to be higher than (rolling end temperature - 50° C.) is that if the temperature is lower than this, the cooling effect cannot be obtained and it becomes difficult to ensure strength. The reason why the lower limit of the cooling rate was set to 3℃/sec is that while Ceq is limited to 0.40% or less, the structure in the thickness direction of a thick steel plate with a thickness of 50 mm or more is a mixture of ferrite and bainite. uniformly distributed in the tissue,
This is to increase the strength at the center of the plate thickness. On the other hand, the reason why the upper limit of the cooling rate was set at 12°C/sec is that when the cooling rate is increased, the surface part of a thick steel plate with a thickness of 50 mm or more is supercooled, and the hardness difference between the center part of the plate thickness and the surface part is becomes larger,
This is because it becomes difficult to reduce the hardness in the thickness direction, which is the objective of the present invention.

Next, set the upper limit of the cooling stop temperature to 500°C.
The reason for this limitation is that for thick steel plates with a thickness of 50 mm or more, the cooling rate inside the plate is slow, and when the temperature exceeds 500°C, the center of the plate becomes a ferrite-based structure, which causes the hardness in the thickness direction to decrease. On the other hand, the reason why the lower limit was limited to 400℃ is that below 400℃, hydrogen defects are likely to occur in the steel sheet, and large residual stress is generated within the steel sheet, causing shape defects. It is to invite.

[0030]

[Examples] The present invention will be explained below with reference to Examples.
The present invention is not limited in any way by these Examples. The test steel plates were steel slabs containing the chemical components shown in Tables 1 and 2, and were made to a thickness of 50 mm according to the manufacturing conditions shown in Table 3.
It is finished to ~100mm. Test pieces were taken from these steel plates and subjected to a tensile test, impact test, hardness measurement in the plate thickness direction, maximum weld hardness test, and weld joint impact test. The results are shown in Table 4. The welding conditions for the welded joint impact test were submerged arc welding assuming a square joint of a box column, and the heat input was 400 kJ/cm.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

Tables 1 and 2 show the chemical components, Table 3 shows the manufacturing conditions, and Table 4 shows the test results. In Tables 1 and 2, steels A to E are steels having the chemical composition range of the present invention, and steel F
~H is a comparative method, in which steel F has no added Nb or Ti and has a high Ceq, steel G has no Nb added, and steel H has no added Ti. In addition, in Table 3, steel A1, A2, B1, B2, C1
, C2, D1, D2, E1, and E2 mean that the manufacturing conditions are different from steels A to E in Tables 1 and 2, respectively.

As is clear from Table 4, the steels A1, B1, C1, D1, and E1 produced by the method of the present invention all have a tensile strength of 50 kgf/mm2 or more and a yield ratio of 70 to 70.
It is stably low at 74%, vTrs is on the low temperature side and has good toughness, and the hardness difference in the thickness direction is also stably low at HV12-27, and has excellent mechanical properties. Furthermore, the maximum weld hardness is low and the weld joint toughness is also good.

On the other hand, steel A2 prepared by the comparative method has a high cooling stop temperature, so the hardness difference in the thickness direction is large. Steel B2 produced by the comparative method had a large cooling rate, so the difference in hardness in the thickness direction was large, similar to A2. Steel C2 produced by the comparative method has a low rolling finish temperature and therefore has a high yield ratio. Steel D2 produced by the comparative method has a high rolling finish temperature and therefore has low toughness. Steel E2 manufactured by the comparative method has a tensile strength of 50 kgf because the cooling start temperature is as low as 800°C.
/mm2, strength is insufficient. Steel F manufactured by the comparative method has a small hardness difference in the plate thickness direction due to air cooling after rolling, but due to its high Ceq, the maximum welding hardness is high, and the toughness of high heat input submerged arc welded joints is low. is low. Steel G produced by the comparative method has low tensile strength because Nb is not added, and the difference in hardness in the thickness direction is also large. Steel H manufactured by the comparative method has low toughness at high heat input submerged arc welded joints because no Ti is added.

[
0038

As explained above, the present invention improves the hardness in the thickness direction by limiting the chemical composition, heating temperature, rolling end temperature, cooling start temperature, cooling rate, and cooling stop temperature during controlled cooling. The present invention produces thick high-strength steel plates with small hardness differences in the thickness direction.
It is possible to manufacture thick-walled, high-strength steel plates with a low yield ratio of 0 kg.

Claims (2)

[Claims] [Claim 1] C: 0.05 to 0.20 in weight ratio %
%, Si: 0.05-0.50%, Mn: 0.80
~2.0%, Al: 0.01 ~0.10%, Nb:
0.005-0.050%, Ti: 0.005-0.
050%, and Ceq shown by the following formula is 0
．． After heating a steel piece satisfying 40% or less and the balance consisting of Fe and unavoidable impurities to a temperature of 1100°C or higher, 850
After finishing rolling in the temperature range of ~900°C, set the cooling start temperature to (rolling end temperature - 50°C) or higher, and perform 3~
A method for manufacturing a 50 kg class low yield ratio thick-walled high tensile strength steel plate with a small difference in hardness in the thickness direction, characterized by cooling to a temperature range of 500 to 400°C at a cooling rate of 12°C/sec. Ceq=C+Si/24+Mn/6+Ni/40+C
r/5+Mo/4+V/14 (%) [Claim 2]
In weight ratio %, Cu: 0.05-1.0%, Ni:
0.05~1.0%, Cr:0.05~0.50%
, Mo: 0.05-0.50%, V: 0.01-0.
10%, B: 0.0003-0.0030%, Ca:
50 kg class low yield ratio thick wall high tensile strength steel plate with small hardness difference in the plate thickness direction according to claim 1, characterized in that it contains one or more selected from 0.0005 to 0.0040%. manufacturing method.