JPS62256915A - Production of high-tension steel plate - Google Patents

Abstract

PURPOSE:To produce a steel plate having excellent weldability and high strength and toughness with high productivity by hot rolling a high-tension steel specified in the contents of C, Cu, Nb, etc., under controlled conditions and subjecting the steel to an aging treatment after cooling. CONSTITUTION:The steel consisting of 0.01-0.10wt% C, 0.01-0.80% Si, 0.20-2.0% Mn, 0.6-1.5% Cu, 0.05-0.060% Nb, 0.05-0.080% SolAl, and the balance Fe and unavoidable impurities is produced. Such steel is heated to 950-1250 deg.C and is subjected to >=50% rolling in a 900-1,000 deg.C range. The finish rolling is ended at 800 deg.C or over and <=900 deg.C. The rolled steel is cooled immediately thereafter down to <=500 deg.C at a cooling rate of 2-50 deg.C/sec, then the steel plate is subjected to the aging treatment. Adequate rations of >=1 kinds among Ni, Cr, Mo, V, Ti, and B are incorporated into the above- mentioned steel according to need. The high-tensile steel plate having excellent weldability and toughness is obtd. at the improved rolling efficiency by the above-mentioned method.

Description

[Detailed Description of the Invention] "Object of the Invention" The present invention relates to a method for manufacturing high-strength steel plates, and is a method for producing high-strength, high-toughness steel plates with excellent weldability while maintaining high productivity without reducing rolling efficiency. The purpose is to provide a method that can be manufactured at low cost.

Industrial applications Manufacturing technology for high-strength structural steel that undergoes welding.

BACKGROUND OF THE INVENTION Typical examples of steels that utilize age precipitation strengthening of Cu to improve weldability include A710 and A736 steels standardized by ASTM. These are 1
．． It contains 0 to 1.3% Cu, and is imparted with the required strength and toughness mainly by aging after normalizing or quenching. This steel type exhibits excellent weldability due to its relatively low carbon equivalent content, but heat treatment such as normalizing or quenching is required to impart high toughness, which increases heat treatment costs and requires many steps. . Therefore, in order to improve this problem, it is necessary to use controlled rolling and controlled cooling (Therm) to strengthen Cu by aging precipitation.

-Mechanical Control Process
A Cu-added high-strength steel sheet using ss) has been proposed. The feature is that controlled rolling or controlled cooling is performed to impart properties such as strength and toughness, and the heat treatment step is omitted.

Based on this idea, 0.06%C
-1,40%Mn-1,2%Cu -0,03%Nb steel subjected to controlled rolling and controlled cooling [Tetsu to Hagane, 71 (
1985), 51507'l has been published.

Problems to be Solved by the Invention However, when using the conventional TMCP as described above,
In precipitation strengthened steel, it is necessary to finish rolling in the austenite low temperature range in the rolling process to ensure toughness.
Implementation of relatively strict controlled rolling is an essential requirement, which causes the following problems.

■ Decreased rolling efficiency due to controlled rolling ■ Increased load on the rolling mill due to increased rolling load.

Therefore, higher costs are unavoidable.

"Structure of the invention" Means for solving the problem C': 0.01 to 0. 10wt%, Si: 0.01~
0.80wt%, Mn: 0.20-2. 0wt%
, Cu: 0.6 to 1.5 wt%, Nb: 0.0
05-0.060 wt%, sol, Ajl! : 0.
005 to 0.080 wt%, with the balance consisting of Fe and unavoidable impurities, is heated to 950 to 1250°C, and then
The material is rolled by 50% or more in the temperature range of 100°C, and final finish rolling is carried out at 900°C/sec over 800°C, and then immediately cooled to a temperature range of 550°C/sec at a speed of 2 to 30°C, and then subjected to aging treatment. A method for manufacturing a high-strength steel plate, characterized by:
and C: 0.01-0.10 wt%, Si: 0.01-0.
30wt%, iAn: 0.21)-2.0wt%,
Cu: 0.6-1.5 wt%, Nb: 0.00
5 to 0.060wt%, S01. Eight β: 0.005
~0.080 wt%, Ni: 0.05~0.8 wt%, Cr: 0.08 wt%.
05-1.5 wt%, Mo: 0.03-Q, 5w
t%, V: 0.01-0.20wt%, Ti:Q, Q3
~0.10wt%, B: 0.0003~0.003
A steel containing one or more of the following wt% and the remainder consisting of Fe and unavoidable impurities is heated to 950 to 1250°C and then rolled to a temperature of 50% or more in a temperature range of 900 to 1000°C. A method for producing a high-strength steel sheet, which comprises performing final finish rolling at a temperature exceeding 800° C. and 900° C./sec, followed by immediately cooling to a temperature range of 2 to 2 b 2 or less, and then aging treatment.

Effect C: 0. Old% or more, iAn: 0.20% or more, Nt
): Strength is improved by containing 0.005% or more.

C: 0.10% or less, Si: 0.80% or less, Mn: 2
．． 0% or less, Nb: 0.060% or less, Cu: 0.
Toughness is achieved by containing 60% or more.

C is set to 0.10% or less to avoid deterioration of weldability and there is no need to add a large amount of alloying elements to ensure strength, and Sl is set to 0.8% or less to improve weldability.

sol, Aβ is set at 0.005% or more to deoxidize,
Moreover, the cleanliness of the steel is ensured by setting it to 0.08% or less.

With respect to the above basic components, Cr is 0.05% or more and MO is 0.05% or more.
B: By containing 0.0003% or more, the hardenability is improved and the strength improvement effect during accelerated cooling is increased, and by containing 0.05% or more Ni, the base material toughness is improved.

Precipitation strengthening is achieved by containing 0.01% or more of (2) and 0.03% or more of Ti.

Economic efficiency is achieved by setting Cr to 0.80% or less and MO to 0.50% or less.

As a manufacturing process, by heating to 950°C or higher, Nb is dissolved in solid form and favorable hot workability is obtained.
Further, the heating rate is set at 1250° C./sec to avoid coarsening of austenite grains.

By conducting the first stage rolling at 900° C. to 1000° C. at a rate of 50% or more, the grain refining effect due to recrystallization can be sufficiently obtained even by the inclusion of Cu, and grain growth after completion of recrystallization can be avoided.

A second stage rolling is performed at 800° C. to 900° C. to achieve a final finish, thereby making the transformed structure even finer.

Immediately after rolling, the steel is cooled to 2 to 550° C./sec to make the transformed structure mainly composed of fine-grained low-temperature transformed structures, thereby improving strength and toughness.

EXAMPLE To further explain the present invention as described above, a controlled rolling technique has been developed that strengthens rolling in the non-recrystallized region in order to improve toughness. For example, 800℃ as seen in
There has been a need for so-called austenite low-temperature range rolling in which a cumulative reduction of /sec is applied during rolling. In other words, for improving toughness by rolling, low temperature rolling (mainly 800℃/sec)
) is essential, but in this case, as mentioned above, disadvantages in terms of rolling efficiency are unavoidable.

Therefore, the inventors of the present invention conducted extensive studies from the standpoint of improving processes and components to obtain excellent steel sheet properties without reducing rolling efficiency, and found that Cu suppresses recrystallization after hot working of steel. For the first time, we have discovered a completely new finding that it has the property of However, as a result of reconsidering the relationship between rolling method and toughness based on this new knowledge, it was found that for steel containing 0.6% or more of Cu, rolling in the recrystallization temperature range and cooling conditions after rolling should be devised. By doing so, it was discovered that the controlled rolling conditions for obtaining the required mechanical properties could be significantly relaxed compared to conventional methods, resulting in significant benefits such as a significant improvement in rolling efficiency and an extension of the life of the rolling mill.

In other words, as a component characteristic, Cu is contained in an amount of 0.6% or more, and as manufacturing conditions, (1) rolling is carried out at a reduction rate of 50% or more in a temperature range of 900°C or more, which is the recrystallization temperature range, and (2) accelerated cooling is performed after rolling. By combining these two methods, it is possible to relax the controlled rolling conditions in the non-recrystallized region.

Describing the details of the constituent elements of the present invention below, the present invention first consists of 0.6% or more and 1.5%
Inclusion of the following Cu is an essential condition, and this has two effects described below. That is, the first is that it can be strengthened based on the precipitation of ε-Cu due to aging, and the second is that the toughness is improved based on the effect of suppressing recrystallization during hot rolling. In other words, the addition of Cu is effective in improving both strength and toughness.

Furthermore, rolling in a temperature range of 900°C or higher is specified because Cu has a recrystallization suppressing effect as mentioned above.
This is because unless rolling is performed in a temperature range higher than conventional manufacturing conditions, it will be difficult to ensure rolling in the recrystallization region. As is generally known, rolling in the recrystallization zone is effective in refining coarse-grained austenite during heating, and the magnitude of the rolling reduction in the recrystallization zone affects the mechanical properties of the steel sheet, especially its toughness. closely related. In Cu-added steel, recrystallization region rolling in a relatively high temperature range is essential for improving properties, and the prior art did not take this point into consideration at all.

Furthermore, the cooling conditions after rolling are specified mainly for the purpose of ensuring the strength of the steel sheet. The implementation of accelerated cooling after rolling provides the necessary strength due to the lower carbon equivalent base metal content compared to air cooling, but it also provides improved base metal toughness and weldability.

That is, by adding an appropriate amount of Cu and accurately controlling the recrystallization region rolling and post-rolling cooling conditions, which are the steps before and after the non-recrystallization region rolling, controlled rolling in the low-temperature austenite region can be relaxed. This leads to improved efficiency.

The reasons for limiting the components and rolling/cooling conditions in the present invention are as follows.

0.01≦C≦0.10%.

Carbon is an element that is advantageous for improving strength, but if it exceeds 0.10%, it may impair weldability and toughness, so the upper limit should be set at 0.1%.
It was set at 10%. Furthermore, if the content is less than 0.0%, a large amount of alloying elements must be added to ensure strength, which is economically disadvantageous, so the lower limit was set at 0.01%.

0.01≦Si≦0.80%.

Si is an element useful as a deoxidizing element and a solid solution strengthening element, but if it exceeds 0.80%, it may cause deterioration of weldability and base material toughness, so the upper limit was set at 0.8%. Further, if the content is less than 0.01%, the effect cannot be expected, so the lower limit was set at 0.01%.

0.20≦Mn≦2.0%.

Mn is an element that can improve strength relatively economically, but
If it exceeds 2.0%, it may be disadvantageous to both weldability and base metal toughness, so the upper limit was set at 2.0%. On the other hand, if it is less than 0.2%, large amounts of other alloying elements must be added to ensure strength, which is economically disadvantageous.
It was set to 0%.

0.60≦Cu≦1.5%.

Addition of Cu is the main requirement constituting the present invention, and ε-
Although Cu effectively strengthens the steel sheet through aging precipitation strengthening and Cu's recrystallization suppressing effect, 0.6
If it is less than 1%, the required amount of precipitation strengthening and recrystallization suppressing effect cannot be obtained, and if it exceeds 1.5%, the amount of strengthening approaches saturation, and the property improvement commensurate with the addition may not be obtained. was set to 0.60≦Cu≦1.5%.

0,005≦Nb≦0.060%.

Nb is an element that can achieve precipitation strengthening with the addition of a small amount, and the addition of Nb further increases the effect of improving mechanical properties through controlled rolling and accelerated cooling, but if the addition exceeds 0.060%, these effects are almost completely lost. The upper limit was set at 0.060% because the content may become saturated and be disadvantageous in terms of toughness. On the other hand, if the content is less than 0.005%, the effect is small, so the lower limit was set at 0.005%.

0,005≦sol, Al≦0.080%.

Aβ is used as a deoxidizer, but if it is less than 0.005%, its effect is insufficient, and if it exceeds 0.080%, it may impair the cleanliness of steel, so the range of addition should be reduced to 0.
．． 005≦so1. Aβ≦0.080%.

In the present invention, one or more of the following components may be added to the above-mentioned basic components as necessary.

0.05≦N+≦0.080%.

N1 is an element added for the purpose of improving base material toughness and brittle crack propagation arresting performance, and the range was set to 0.05≦N+≦0.080% from the point of view of economy and effectiveness.

0.05≦Cr≦1.5%.

Cr is an element that improves hardenability and further increases the effect of improving strength during accelerated cooling, but in consideration of economy and effectiveness, the range is set to 0.05≦Cr≦1.5%.

0.03≦MO≦0.50%.

MO is an element that improves hardenability like the above-mentioned Cr and acts effectively against the strengthening effect caused by accelerated cooling,
From the point of view of economy and effectiveness, the range is 0.03≦MO≦0.
It was set at 50%.

0.01≦V≦0.20%.

■ is an element that effectively acts on precipitation strengthening when added in small amounts,
It works effectively to improve strength, but if it is less than 0.01%, the effect is small, and if it exceeds 0.20%, it may be harmful in terms of toughness, so the addition range was set as 0.01≦■≦0.20%. .

0.03≦Ti≦0.10%.

Adding an appropriate amount of Ti effectively contributes to precipitation strengthening by TiC, but if it is less than 0.03%, the effect is small;
If it exceeds 10%, the effect is almost saturated, so set the range to 0.
．． 03≦Ti≦0.10%.

0.0003≦B≦0.0030%.

When added in a very small amount, B improves the hardenability of steel and strengthens it by improving its structure, but if it is less than 0.0003%, the effect is not expected, and on the other hand, if it exceeds 0.0030%, the effect is saturated. Therefore, the addition range is 0,0003≦B≦
It was set to 0.0030%.

Next, in the present invention, steel containing the above components is manufactured by the process shown below.

Heating temperature: 950°C or higher and 1250°C/sec.

If the slab heating temperature is lower than 950° C., problems such as Nb remaining undissolved and difficulties in rolling processing occur.

On the other hand, heating the slab above 1250°C significantly coarsens the austenite grain size during heating and deteriorates the mechanical properties after rolling and aging. Therefore, the heating temperature should be set to 950℃ or higher, 1
The temperature was set at 250°C/sec.

Rolling conditions: 50% or more rolling is performed in a temperature range of 900°C or higher and 1000°C/sec, and final rolling is performed in a temperature range of 800°C or higher and 900°C/sec.

The rolling conditions are a basic component of the present invention, and its feature is that it specifies two-stage rolling.

First stage rolling: 50% or more rolling is performed in the range of 900° C. or higher and 1000° C./sec.

This rolling regulation concerns rolling in the recrystallization zone. It is generally well known that rolling in the recrystallization region can refine austenite grains and improve mechanical properties, but the feature of the present invention is that the rolling temperature is specified at a higher temperature than the conventional temperature range. be.

That is, in a temperature range of less than 900° C., a non-recrystallized region occurs due to the recrystallization suppressing effect of Cu, and a sufficient grain refinement effect due to recrystallization cannot be expected. On the other hand, reduction in a temperature range exceeding 1000° C. causes the problem that although recrystallization occurs, the grain size after recrystallization does not become sufficiently fine, and grain growth occurs after completion of recrystallization. Therefore, the temperature range of the first stage was defined as 900° C. or higher and 1000° C./sec.

Regarding the rolling reduction amount, since sufficient recrystallization cannot be expected with a rolling reduction of less than 50%, rolling is performed at a rolling temperature of 50% or more in the above rolling temperature range.

Second stage rolling: Final finishing rolling is carried out in a temperature range exceeding 800°C and 900°C/sec.

This rolling regulation relates to rolling in a non-recrystallized region. By performing non-recrystallization area rolling after recrystallization rolling,
This aims to further refine the transformed structure. 800℃/
sec rolling is extremely disadvantageous in terms of rolling efficiency, and on the other hand, rolling in a non-recrystallized region cannot be achieved in a temperature range exceeding 900°C. Therefore, the temperature range of the second stage was set to exceed 800°C and exceed 900°C/sec.

Cooling conditions: Immediately after rolling, it is cooled to a temperature range of 550°C/sec at a cooling rate of 2°C/S or more and 50°C/S or less.

The purpose of cooling after rolling is to improve strength and toughness by making the transformed structure into a fine-grained, low-temperature transformed structure. That is, if the cooling rate is less than 2°C/S, sufficient toughness cannot be achieved, and if the cooling rate is more than 50°C/S, excessive cooling equipment is required, which is economically disadvantageous. On the other hand, if cooling is stopped at a temperature exceeding 550°C, transformation strengthening will be insufficient and toughening will not be achieved. Therefore, the cooling rate range is 2℃/S
Above 50℃/s or less, cooling stop temperature 550℃/s
Specified in ec.

A specific manufacturing example of the product according to the present invention will be described below.

The compositional structures of the steel according to the present invention used by the present inventors and its comparative steel are as shown in Table 1 below.

That is, 0.6, which is characteristic in terms of the steel composition in the present invention.
An example of a component to which Cu within the range of ≦Cu≦1.5% is C-F! ! I, H~0 steel, A, B.

G steel is an example of a component in which Cu is outside the scope of the present invention. Also, A-
Gl is a C-Mn-Cu-Nb system that does not contain alloying elements such as Ni, Cr, and Mo;
Steels have N3, Cr, and MO added individually, K steel and N steel are examples of Ni, Cr, and Mo added at the same time, and LSM and G steels have trace elements ■ and Ti5B. has been added.

In addition, the manufacturing conditions implemented and the mechanical properties obtained are the second
As shown in the table. Here, production examples 1 to 6 are characteristic examples when A-G steels with different amounts of Cu were produced under the same production conditions within the scope of the present invention, and C- All F steels have YS≧60kgf/mm2
, vTrs≦−120°C, which shows excellent strength and toughness. On the other hand, ASB steel with Cu < Q and 5% and G steel with Cu > 1.5% are inferior in strength or toughness. Further, Production Examples 7 to 13 are steels having compositions according to the present invention.
This is an example of characteristics manufactured using steel and changing manufacturing conditions. Production Examples 7.10.11 were produced using A steel within the scope of the present invention, and it can be seen that they were provided with excellent toughness. On the other hand, in Production Examples 8.9.12.13, the temperature range of recrystallization zone rolling, rolling reduction, cooling rate after rolling, and stop temperature are outside the range of the present invention, and all of them are v Tr
s≧-70°C, and sufficient toughness was not obtained. On the other hand, Production Examples 14 to 20 are steels of the present invention, H steel to O steel, produced within the scope of the present invention, and it can be seen that excellent strength and toughness were imparted to all of the steels.

The attached drawing summarizes the changes in strength, toughness, and rolling time when the finishing temperature of the steel D is heated at 1050°C and heated and cooled after rolling. It is clear that although rolling conditions exceeding .degree. C. are advantageous in terms of strength and efficiency, they are significantly inferior in toughness. On the other hand, when the temperature is less than 800°C, the rolling efficiency becomes suddenly disadvantageous, and by adopting the finishing rolling temperature of the present invention, it is possible to manufacture a steel plate that approximately satisfies these relationships appropriately and secures preferable productivity. becomes possible. In addition, the present inventors conducted a diagonal Y-type weld cracking test on the steels of the present invention listed in Table 1 above to examine their susceptibility to cold cracking, and found that even at 25°C without preheating, No cracking occurred. It was also confirmed that the Charpy and CTOD characteristics of the welded heat-affected zone welded under the condition of heat input of 40 KJ/cm were sufficient at low temperatures.

"Effects of the Invention" According to the present invention as explained above, it is possible to relax the controlled rolling conditions, improve the rolling efficiency, reduce the load on the rolling mill, and increase the strength of the steel with a low PCX composition. This invention is industrially highly effective, as it is possible to obtain a high-strength, high-toughness steel plate with improved weldability such as low-temperature cracking susceptibility and toughness of the heat-affected zone.

[Brief explanation of drawings]

The drawing shows the technical content of the present invention, and is a chart showing changes in strength, toughness, and rolling time when the finishing temperature of rolling is changed. Patent applicant: From Nippon Kokan Co., Ltd.
Author Ryudo Abe
Masa Hori
Yomuka Omori
Toshi Do procedural amendment (voluntary) Showa □□□, March 3rd

Claims (2)

[Claims] (1) C: 0.01-0.10wt%, Si: 0.01
~0.80wt%, Mn:0.20~2.0wt%, C
u: 0.6 to 1.5 wt%, Nb: 0.005 to 0.0
60wt%, sol. Al: 0.005~0.080w
t%, with the balance consisting of Fe and unavoidable impurities, is heated to 950-1250°C and then heated to 900-1000°C.
50% or more rolling in the temperature range of 100°C, final finish rolling at more than 800°C and less than 900°C, then immediately cooled at a cooling rate of 2 to 50°C/sec to a temperature range of 550°C or less, and then A method for manufacturing a high-strength steel sheet, characterized by performing an aging treatment. (2) C: 0.01-0.10wt%, Si: 0.01
~0.80wt%, Mn:0.20~2.0wt%, C
u: 0.6 to 1.5 wt%, Nb: 0.005 to 0.0
60wt%, sol. Al: 0.005~0.080w
t%, Ni: 0.05 to 0.8 wt%, Cr: 0.05 to 1.
5wt%, Mo: 0.03-0.5wt%, V: 0.0
1 to 0.20 wt%, Ti: 0.03 to 0.10 wt%
, B: 0.0003 to 0.003 wt%, and the remainder is Fe and inevitable impurities, heated to 950 to 1250°C, and then heated to 900 to 1000°C. Rolling of 50% or more in the range, final finish rolling at over 800°C and below 900°C, then immediately cooling to a temperature range of 550°C or less at a cooling rate of 2 to 50°C/sec, and then aging treatment. A method of manufacturing a high tensile strength steel plate, characterized by: