JPS61266521A - Manufacture of hot rolled steel plate having superior low temperature toughness and low yield ratio - Google Patents

Abstract

PURPOSE:To manufacture hot rolled steel plate having both toughness and low yield ratio practically and inexpensively, by controlling hot rolling condition of steel of which Nb is added. CONSTITUTION:Steel contg. by weight, 0.01-0.2% C, 0.01-0.7% Si, 0.3-2.0% Mn, <=0.003% S, 0.001-0.003% Al, 0.005-0.020% Nb is hot rolled under the following condition. Namely, the steel is held at >=1,150 deg.C temp. to enter Nb carbonitride is steel into solid soln. completely. Next, finish rolling in hot rolling of the steel is ended in austenitic range, e.g. finish rolling in the range is carried out under >=1,100 deg.C rolling starting temp., <=1,000 deg.C finish rolling inlet side temp., >=850 deg.C finish rolling end temp. The steel after completion of rolling is cooled at <=15 deg.C/sec rate and coiled at 550-680 deg.C conventionally. In this way, steel having both low temp. toughness and low yield ratio is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing a low yield ratio hot rolled steel sheet with excellent low temperature toughness.

(Prior Art) Conventionally, methods using heat treatment such as normalizing, quenching and tempering are well known as means for increasing the toughness of steel. However, this method requires a heat treatment process in addition to the hot rolling process, which has the disadvantage of decreasing productivity, and also has the disadvantage that the resulting steel sheet has a high yield ratio. In addition, a method using controlled rolling is also known, but this method also has the disadvantage of reducing productivity as well as the yield ratio of the resulting steel plate because a waiting time occurs during the rolling process. There is a problem with high Furthermore, a method using two-layer zone rolling is also known, but when this method is used, texture tends to develop and the material properties have anisotropy. Furthermore, technical management for recrystallizing ferrite is not easy. Similarly, a method of refining the structure by accelerated cooling is known, but cooling control is not easy.

Furthermore, adding alloying elements such as MO and Ni to steel to improve toughness has been widely put into practical use, but the manufacturing cost is generally high.

On the other hand, as means for lowering the yield ratio of steel, methods such as increasing the carbon content, increasing the positive ratio in the two-layer region, and performing low-temperature winding are conventionally known. However, on the other hand, increasing the carbon content deteriorates the toughness and weldability of the steel, and as mentioned above, the method of increasing the positive ratio in the two-layer region is not necessarily satisfactory in terms of material properties and technical management. do not have. In addition, low-temperature winding requires a large load on the coiler for thick materials, and
Cooling control is not easy.

As described above, it is difficult to practically and inexpensively produce steel having both toughness and a low yield ratio using conventionally known methods.

(Objective of the Invention) As a result of intensive research to solve the above-mentioned problems, the present inventors have discovered that Nb-added steel is It was discovered that by controlling the hot rolling conditions, it is possible to obtain a steel with a steel structure mainly consisting of a fine ferrite/bainite structure, which has both toughness and a low yield ratio, by normal winding, and has developed the present invention. This is what we have come to.

Therefore, an object of the present invention is to provide a method for manufacturing a hot rolled steel sheet having both toughness and low yield ratio practically and at low cost.

(Structure of the Invention) The method for producing a low yield ratio hot rolled steel sheet with excellent low-temperature toughness according to the present invention includes, in weight percent, C 0.2% or less, Si 0.7% or less, Mn 0.3 to 2.0%, S 0 .003% or less, Aβ o, oso% or less, Nb 0.005 to 0.020%, the balance being iron and unavoidable impurities when hot rolling the steel at a temperature of 1150°C or higher, After finishing the finish rolling in the austenite region, it is characterized by cooling at a cooling rate of 15°C/second or less and winding at a temperature of 550 to 680°C.

First, in the present invention, the reason why the chemical components of the steel are limited will be explained.

C is an element necessary to ensure strength, and has a content of 0.
It is desirable to add 0.1% or more. However, when the amount added exceeds 0.2%, the solution temperature conditions for carbonitrides become severe and it becomes necessary to heat the steel at high temperature for a long time. The amount added is 0.2
% or less.

St acts as a deoxidizing agent for steel and is dissolved in the matrix to have the effect of improving the elongation and ductility of steel. In order to effectively exhibit such effects, it is desirable to add 0.01% or more.

However, if added in excess, toughness, weldability, cleanliness, etc. will deteriorate, and surface scale will occur, so the amount added is limited to 0.7% or less.

Mn is effective in improving strength and toughness. In order to effectively obtain this effect, it is necessary to add at least 0.3%. However, when added in a large amount exceeding 2.0%, microsegregation and abnormal structures occur, and weldability also deteriorates. Therefore, in the present invention, the amount of Mn added is in the range of 0.3 to 2.0%.

S combines with Mn to form A-based inclusions.

Therefore, when the content is excessive, the impact value particularly in the direction perpendicular to the rolling direction decreases significantly, so the upper limit of the content is set to o, oio%.

Al, like Si, is an element necessary as a deoxidizing agent, and it is preferable to add 0,00% or more. Furthermore,
It is effective in improving toughness by refining crystal grains and fixing N. However, if added in excess, it will have a detrimental effect on toughness, so the amount added should be at most 0.0%.

Nb is an essential element for improving the strength and toughness of steel according to the present invention. In order to effectively exhibit this effect, it is necessary to add at least 0.005%. However, when adding too much, the effect of precipitation strengthening increases, and in particular, it becomes difficult to obtain the effect of improving toughness, so the upper limit of the amount added is set to 0.020%.

Furthermore, in the present invention, the steel contains, in addition to the above-mentioned elements,
It can contain Ca. Ca is effective for controlling the form and composition of sulfide inclusions in steel, and it is desirable to add at least 0.00% or more.

In particular, when the Ca/S weight ratio is 2 or more, the sulfide inclusions are completely spheroidized, so that the impact values in the rolling direction and in the direction perpendicular to the rolling direction can be made equal. However, when added in excess, cluster-like nonmetallic inclusions are generated and the toughness is deteriorated, so the amount added is limited to 0.0100% or less.

Next, hot rolling conditions in the present invention will be explained.

In the method of the present invention, when hot rolling steel having the above-described chemical composition, the steel is first maintained at a temperature of 1150° C. or higher to completely solutionize the Nb carbonitrides in the steel. Therefore, the heating holding time is preferably at least 0
．． It is 5 hours. When the heating temperature is lower than 1150° C., the solution treatment described above is insufficient.

After this, the steel is hot rolled, but in the present invention,
Finish rolling is completed in the austenite region.

That is, the rolling start temperature is 1100°C or higher, the finish rolling entry temperature is 1000°C or lower, and the finish rolling end temperature is 850°C.
The austenite grains can be made finer by performing rolling finishing in the austenite region with the rolling reduction ratio preferably set to 70% or more. When the rolling end temperature is lower than 850°C, ferrite processing is involved.

After completing the rolling in this manner, the material is cooled at a cooling rate of 15° C./second or less, and normal winding is performed at 550 to 680° C. These cooling conditions and coiling temperature are conventional conditions that have been conventionally employed in the production of hot-rolled steel sheets for relatively thick materials.

(Effects of the Invention) According to the method of the present invention, steel to which a small amount of Nb is added is heated to a predetermined temperature to completely dissolve Nb carbonitrides, and then heated under predetermined conditions. By performing inter-rolling, after Nb contributes to the refinement of austenite grains, it also significantly lowers the ferrite transformation temperature after finish rolling, so even by normal winding, a fine bainite structure is generated, and thus, Since the metal structure can be made into a mixed structure consisting of fine ferrite, fine bainite, and part of fine pearlite, it is possible to obtain a steel that has both low-temperature toughness and a low yield ratio.

(Examples) The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

Examples Steel having the chemical composition shown in Table 1 was heated and held at a predetermined temperature as shown in Table 2, and then hot rolled under predetermined conditions. A tensile test piece (C direction) and a Charpy impact test piece (thickness 71 mm, C direction) were taken from this hot rolled plate material,
It was used for each test. The results are shown in Table 2.

Since Comparative Steel C does not contain Nb, it is inferior in both yield ratio and low-temperature toughness. Since the comparative steel contains an excessive amount of Nb, its strength is high, but its yield ratio is low.

Next, Table 2 shows the results when the hot rolling conditions of Steel B in Table 1 were variously changed.

The hot rolled steel sheet B1 manufactured according to the method of the present invention has a low yield ratio and high toughness. However, in B2, the heating temperature prior to hot rolling is low and the solutionization of Nb carbonitrides is insufficient, so low-temperature toughness is not improved. Furthermore, in Comparative Steel B3, the heating holding time before hot rolling was insufficient for complete solutionization of Nb carbonitrides, and as a result, low-temperature toughness was not similarly improved.

Claims (2)

[Claims] (1) C0.2% or less, Si 0.7% or less, Mn 0.3-2.0%, S 0.003% or less, Al 0.050% or less, Nb 0.005-0.020%, balance iron When hot rolling steel containing unavoidable impurities, the steel is held at a temperature of 1150°C or higher, finish rolling is completed in the austenite region, and then cooled at a cooling rate of 15°C/second or less to achieve a A method for producing a low-yield ratio hot-rolled steel sheet with excellent low-temperature toughness, characterized by winding at a temperature of 680°C. (2) C0.2% or less, Si 0.7% or less, Mn 0.3-2.0%, S 0.003% or less, Al 0.050% or less, Nb 0.005-0.020%, Ca0. When hot rolling steel consisting of 0.0100% or less, balance iron and unavoidable impurities, the steel is maintained at a temperature of 1150°C or higher, finish rolling is completed in the austenite region, and then the cooling rate is reduced to 15°C/sec or less. A method for producing a low yield ratio hot-rolled steel sheet with excellent low-temperature toughness, the method comprising cooling the steel sheet and winding it at a temperature of 550 to 680°C.