JPH02282420A - Production of hot-rolled steel sheet to be worked and thermomechanical treatment of hot-rolled steel sheet - Google Patents

Abstract

PURPOSE:To produce the hot-rolled steel sheet to be worked capable of being reheated and strengthened by applying specified hot rolling, winding and cooling to a carbon billet having a specified content of C or further to a low-alloy billet contg. Ti, Nb, Cu, rare-earth elements, Ca and Zr. CONSTITUTION:A carbon billet contg. 0.005-0.20wt.% C or a low-alloy billet contg. 0.005-0.20% C, >=0.01% Ti and/or Nb in total where 0.005<=C-(12/48Ti+12/93Nb) and/or 0.5-3.0% Cu and/or >=1 kind among 0.002-0.10% rare-earth element, 0.002-0.01% Ca and 0.01-0.10% Zr is continuously hot-rolled at the finishing temp. of above the Ar3 transformation temp. The hot-rolled sheet is then wound at >=300 deg.C. The obtained coil is annealed at 250-150 deg.C for >=10hr or allowed to cool. Consequently, a soft hot-rolled steel sheet to be worked is obtained. The hot-rolled steel sheet can be heated at 400-750 deg.C for >=5 sec after forming and then cooled to <=200 deg.C at the rate of >=10 deg.C/sec to drastically increase its tensile strength.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a steel plate suitable for high-strength parts for automobiles or industrial equipment, specifically, a steel plate that is processed at a relatively low strength until it is subjected to forming processing. The present invention relates to a method for manufacturing a hot-rolled steel sheet for processing, which is easy to produce, and whose strength is increased by an appropriate heat treatment after processing, and a processing heat treatment method for the same.

(Prior Art) Steel plates manufactured by continuous hot rolling, so-called hot-rolled steel plates, are widely used as relatively inexpensive structural materials in various industrial equipment including automobiles. Many of its uses involve parts that are formed by press working, and therefore,
Hot-rolled steel sheets are often required to have excellent workability, while as structural members they are also required to have high strength, but it is usually difficult to achieve both high strength and excellent workability. be.

Therefore, various materials have been developed that have low strength and good workability in the raw material stage before processing, and that can be increased in strength by appropriate heat treatment after processing. Cold-rolled steel sheets are soft and easy to work with before being processed, and harden and increase yield strength during baking painting after processing, so-called bake-hardened high-strength steel sheets that have already been put into practical use. . Recently, studies have been progressing on baking-hardened hot-rolled steel sheets, and patents related to this have also been filed.

For example, Japanese Patent Publication No. Sho 62-180021 describes a method for manufacturing high-strength hot-rolled m-plates of which baking is hardening, in which steel with a specific chemical composition containing a large amount of N (nitrogen) is rapidly cooled after hot rolling. is disclosed. This method uses the strain aging of solid solution N to obtain hardening properties, but according to the experimental results of the inventors of this application, the baking properties obtained by this method are similar to hardening type high-strength hot rolling. Although the yield strength of the steel plate significantly increased after baking, the increase in tensile strength was small.

It has been reported that when the increase in tensile strength is small, the improvement in fatigue properties is small.Fatigue properties have a strong correlation with tensile strength, and as tensile strength increases, fatigue properties increase. Used; [Jinmin Metal J vol, 33 (
1981) Nu 4, P, 121) Therefore, if the increase in tensile strength is small, the effect of improving fatigue properties required for high-strength parts for automobiles and industrial equipment, which are the main uses of these steel sheets, will be small. , it has little practical value.

(Problem to be Solved by the Invention) The problem of the present invention is to significantly increase tensile strength, which is effective for improving fatigue properties, by reheating the soft material at a relatively low temperature after processing, which is soft before processing. An object of the present invention is to provide a method for producing a hot-rolled steel plate for processing, and a processing heat treatment method using the steel plate as a raw material.

(Means for Solving Problem ii) The present inventors used carbon steel slabs or low alloy steel slabs with at least a specified C content as raw materials, and continuously hot-rolled them under specific conditions. that a soft hot-rolled steel sheet can be obtained;
We have also discovered that after processing and forming this hot-rolled steel sheet into a predetermined shape, it is possible to increase the strength by subjecting it to an appropriate heat treatment. That is, (1) Rolling is completed at a temperature above the transformation point and then coiled at a temperature above 300°C, or the solid solution C is converted into cementite (PezC) as much as possible by winding at a temperature above 300°C and then slow cooling.
), it is possible to produce a soft hot-rolled steel sheet with excellent workability.

■Then, after processing and forming this steel plate into a predetermined shape,
If cementite is dissolved in solid solution by heating to a temperature of 0 to 750° C., and the solid solution C thus produced is allowed to remain by rapid cooling after heating, the strength will be significantly increased.

The present invention was completed based on such knowledge, and
The gist is in (1) to (iii) below.

(i) A carbon steel piece containing 0.005 to 0.20% C by weight, or a carbon steel piece containing 0.005 to 0.20% C and further comprising the following groups 1, 2, and 3. A low-alloy steel billet containing one or more components selected from at least one group of the above is subjected to continuous hot rolling at a finish rolling finish temperature of at least the Arc transformation point, and then subjected to continuous hot rolling at a temperature of 3 (10"c or higher). Winding, 250
A method for producing temporary hot rolled W4 for processing, characterized by slow cooling in a temperature range of ~150°C over 10 hours or more.

[Group 1] The total amount is 0.01% or more, and (0,005≦C-
(12/48)Ti + (12/93)Nb) Ti or Nb.

[Group 2] 0.5-3.0% Cu.

[Group 3] Q, QO2~0.10% rare earth elements, 0.002~0
．． 01% Ca, 0.01-0.10% Zr.

(ii) The carbon steel billet or low-alloy steel billet of (i) above is subjected to continuous hot rolling with a finish rolling finish temperature of Ar and a transformation point or higher, and then coiled at a temperature of 300°C or higher and left to cool. A method for producing a hot-rolled W4 sheet for processing, characterized by:

(ij) After processing and forming the hot rolled steel sheet for processing obtained by the manufacturing method of (i) and (ii) above into a predetermined shape,
Heat to a temperature of 400-750°C for 5 seconds or more, then 4
20 at a cooling rate of 10°C/s or more from a temperature of 00°C or higher
A heat treatment method for processed products characterized by cooling to below 0°C.

Reason.

(Function) The excellent properties of the hot-rolled steel sheet of the present invention, namely, the property that it is easy to process with relatively low strength until it is subjected to forming processing, and that the strength is increased by reheating after processing, is mainly due to the effect of C. by. If the solid solution C is precipitated as cementite as much as possible during the production of hot-rolled steel sheets to make it soft, it will be easy to form it in that state, and in the reheating process after processing, the cementite will be hardened. If the solid solution C obtained by dissolving remains, the tensile strength will significantly increase. In order to sufficiently obtain such effects, it is important to adjust the C content in carbon steel slabs and the content of C and other alloy components in low alloy steel slabs, as described below.

Hereinafter, the reason why the C content and the alloy component content of the raw steel slab are limited as described above will be explained together with the effects.

C: C is an important element in the present invention, and exists as cementite before forming, and dissolves in solid solution during reheating after processing and forming, without impairing workability, to improve tensile strength. However, if the content is less than 0.005%, desired high strength cannot be expected after reheating, and if it exceeds 0.20%, weldability deteriorates. For these reasons, the C content of both carbon steel slabs and low alloy steel slabs is set at 0.005 to 0.20%.
That's what I did.

In the present invention, when using carbon steel pieces as the material, C
By adjusting the content to 0.005 to 0.20% and manufacturing by the method described below, it is possible to obtain a hot rolled steel sheet with workability and high strength after working. However, carbon steel slabs have limited workability and strengthening ability by solid solution C. Therefore,
In the present invention, other materials include Ti, Nb, and C.
A low-alloy steel piece containing one or more of the following is used: u, rare earth elements, Ca, Zr, etc.

The effects of these alloying elements are as follows.

First group of Ti and Nb: Ti and Nb combine with C in the steel and precipitate as carbides, thereby strengthening the steel sheet.

However, 0.005% or more of C that is not combined with Ti and Nb is required in the steel for the above-mentioned reason. That is, (
When C-(12/48)Ti + (12/93)Nb) is less than 0.005, C in the steel not combined with T1 and Nb becomes less than 0.005%. Also, N
If the content of b and Ti is less than 0.01% in total, the above effect cannot be obtained. Therefore, the content of Ti and Nb is (0,005≦C-(12/48) Ti
+ (12/93)Nb), and
The total content of one or two types must be 0.01% or more.

Second group of Cu: Cu precipitates independently without combining with C, and has the effect of strengthening the steel plate. If the content is less than 0.5%, the above desired effect cannot be obtained, and if the content exceeds 3.0%, the effect will be saturated and it will be economically disadvantageous. Therefore, its content is set to 0.5 to 3.0%.

In addition, when Cu is contained, 1.5% is added together with Cu.
The following Ni is preferably added. Adding Ni can prevent cracks caused by Cu during hot rolling.

Rare earth elements of the third group, Ca and zr: One or more of these elements are added to adjust the shape of inclusions and improve cold workability. However, the content of rare earth elements: less than 0.002%, Ca:
If the content is less than 0.002% and Zr: less than 0.01%, the desired effect by the above action cannot be obtained; on the other hand, if the rare earth element: 0.10%, Ca: 0.01% and Zr:
If the content exceeds 0.10%, on the contrary, inclusions in the steel will increase too much and cold workability will deteriorate.

The components from the first group to the third group are selected and added from at least one group depending on the purpose.

The material steel piece used in the present invention has a carbon content of 0.0 as described above.
Carbon steel containing 05 to 0.20%, or C o, oos to 0
．． 20% and further contains at least one component from the first to third groups, but as elements other than the above that may be contained in these carbon steels and low alloy steels. There are Si, Mn, , kl and Ni.

These desirable contents are: Si: 1.2% or less;
Mn: 2.0% or less, ^i is Sol, A1. 0 in quantity
．． 01 to 0.10%, and Ni: 1.5% or less.

If excessively contained, Si and Mn deteriorate weldability, but if they are contained in appropriate amounts, they have the effect of improving strength and ductility through solid solution strengthening. 80 is added as a deoxidizing agent and has the effect of increasing the cleanliness of the rope.

Ni has the effect of suppressing cracking caused by Cu during hot rolling.

In addition, PS, N, etc. are included as impurities.

P has an adverse effect on weldability, S forms MnS-based inclusions and reduces workability, and N causes aging deterioration, so the content is 0.0 for both P and S.
It is desirable that N be 5% or less, and N be 0.010% or less.

Carbon steel billet and CaT with specified C content as above
If a low-alloy steel billet with a specified content of i, Nb, Cu, rare earth elements, Ca, Zr, etc. is hot rolled under the conditions described below, it will be easy to work with low strength as hot-rolled, and it will be easy to work after working. By reheating, it is possible to obtain hot rolled steel sheets with increased strength.

The hot-rolled steel sheet manufacturing conditions and post-processing heat treatment conditions will be described below.

[Hot-rolled steel sheet manufacturing conditions]

(a) Continuous hot rolling The carbon steel slabs and low-alloy steel slabs to be subjected to hot rolling are hot slabs sent directly from the continuous casting or blooming process, or slabs that have been cooled and then reheated. Any of the prepared slabs can be used. However, hot rolling is carried out using Ar
It is important to finish finish rolling at a temperature above the s transformation point.

If the finish rolling end temperature is lower than the Ars transformation point, processing & IIm will be mixed into the ferrite grains generated by transformation, resulting in deterioration of workability, and the amount of strain recovered during reheating after processing will be too large. As a result, the increase in tensile strength is small.

Function) Winding Winding is performed at a temperature of 300° C. or higher. By doing so, the solid solution C can be reliably precipitated as cementite, so that the steel sheet becomes soft.

If winding is performed at a temperature lower than 300°C, a large amount of solid solute C remains in the hot rolled steel sheet, resulting in significant hardening and deterioration of workability, as well as an increase in tensile strength due to solid solute C after heat treatment. I can't wait.

(C) Cooling after winding After winding, it may be left to cool or slowly cooled in a temperature range of 250 to 150''C for 10 hours or more, preferably 15 hours or more.

If the slow cooling is carried out under the above conditions and the remaining solid solution C is further precipitated as cementite, the solid solution C will be reduced and a steel sheet will be soft and have good workability.

Incidentally, the slow cooling can be carried out by covering the wound steel plate with a heat insulating cover or by placing it in a slow cooling furnace.

Steel sheets manufactured under the above hot-rolled steel sheet manufacturing conditions have a large amount of precipitated cementite before being processed, so they have low strength and are easy to process, and when reheated after processing, the cementite dissolves into solid solution and the tensile strength increases. will rise significantly.

In order to greatly increase the tensile strength, reheating after processing is preferably performed under the following heat treatment conditions.

[Heat treatment conditions after processing and forming]

After forming the hot-rolled steel sheet produced by the above method into a predetermined shape, the formed product is heated at a temperature of 400 to 750°C for 5 seconds or more, and then cooled at a cooling rate of 10°C or more from a temperature of 400°C or higher. Cooling is performed to below 200°C.

Ru.

If the heating temperature is less than 400℃, sufficient solid solution C will not be obtained due to insufficient solidity of cementite, and the increase in tensile strength will be small.On the other hand, if the heating temperature is higher than 750℃, the processed product will be heated. Distortion occurs. Heating temperature is 400~750″C
However, if the heating time is less than 5 seconds, the solid solution of cementite will be insufficient and the solid solution C will not increase. Depending on the purpose of the processed product, heating may be performed locally only in areas that require high strength. 6 Local heating, in which the entire processed product may be heated, is easier to rapidly cool after heating, so the aim of the present invention is more easily achieved.

After heating, 10°C/
If the steel is rapidly cooled at a temperature of s or more, the precipitation of cementite dissolved in the solid solution caused by heating is suppressed, and the steel sheet is significantly strengthened by the C generated by the solid solution of cementite.

Next, the present invention will be further explained by examples.

(Example) 50 kg of steel with the chemical composition shown in Table 1 is melted in a vacuum melting furnace and hot forged into an ingot of 300111Iφ, or directly cast into a mold to form a 60 m-thick slab. produced and hot rolled under the conditions shown in Table 2 to obtain 2
A hot-rolled steel plate with a thickness of 100 ms was manufactured.

The hot-rolled steel sheet was wound up and then allowed to cool or slowly cooled. In addition, the Arx transformation points of the steel plates in the examples are all 9.
It is less than 00℃.

A test piece was taken from the obtained hot-rolled steel sheet and the tensile strength (S) and elongation (EL) of the as-hot-rolled steel sheet were examined. Reheating treatment was performed under the conditions shown in the table, and the tensile strength after the reheating treatment was measured to determine the amount of increase in tensile strength (ΔTS). These results are shown in Table 3.

(Hereinafter, blank space) 3rd back (h) Hot-rolled steel sheets of Nα1 to 6 and Nα11 to 17 are produced using carbon steel slabs and subjected to slow cooling after winding, and heat-rolled steel sheets of Nα7 to kio Rolled steel plates use low-alloy steel pieces,
Also produced by slow cooling after winding, Sou 22~
The hot-rolled steel sheets in No. 24 are produced by cooling carbon steel pieces after winding, Na1B to N1121 and N(12
The hot-rolled steel sheets of Nos. 5 to 40 were manufactured using low-alloy steel slabs, which were similarly rolled up and allowed to cool.

The hot-rolled steel sheets of KL to KLO and Nos. 18 to 32 produced by the method of the present invention and heat-treated by the method of the present invention after processing have low strength and softness before processing, and have a strength of 8 after heat treatment. gf/am" or more. Particularly, the strength increase after heat treatment is large for those that are slowly cooled after winding.

On the other hand, as shown in the comparative example, products produced and heat-treated under conditions other than those specified in the present invention have poor workability or a small increase in strength after heat treatment, that is, slag 11 with a low finishing temperature. The hot-rolled steel sheets of Stone 33, Stone 12 and Ka4, which have a low coiling temperature, have high strength as hot-rolled, and have a small increase in strength after heat treatment. Nα15 with short retention time
and Nα37, Na16 with low C content and slow cooling rate after reheating, Na38, C
Hot-rolled mi with a low content of sago 40 does not have sufficient solid solution C after heat treatment, resulting in a small increase in strength; Temporarily, arc welding defects occurred while the as-hot-rolled strength was high, and the hot-rolled steel plates of floor 13 and floor 35, which had a high maximum heating temperature, suffered thermal distortion during heat treatment.

(Effects of the Invention) As explained above, the hot-rolled steel sheet produced by the method of the present invention is soft and easy to work when hot-rolled, and its strength increases significantly when reheated after working. Therefore, this steel sheet is ideal as a material for automobiles and other industrial equipment, which require both excellent workability and high strength.

Claims (3)

[Claims] (1) A carbon steel piece containing 0.005 to 0.20% C by weight, or a carbon steel piece containing 0.005 to 0.20% C and further comprising the following groups 1, 2, and 3. A low-alloy steel billet containing one or more components selected from at least one group of the above is subjected to continuous hot rolling with a finish rolling finish temperature of Ar_3 transformation point or higher, and then coiled at a temperature of 300 ° C. or higher, 250～
A method for producing a hot-rolled steel sheet for processing, characterized by slow cooling in a temperature range of 150° C. for 10 hours or more. [Group 1] The total amount is 0.01% or more, and [0.005≦C-
(12/48)Ti+(12/93)Nb]. [Group 2] 0.5-3.0% Cu. [Group 3] 0.002-0.10% rare earth elements, 0.002-0
．． 01% Ca, 0.01-0.10% Zr. (2) The carbon steel billet or low-alloy steel billet described in claim 1 is subjected to continuous hot rolling with a finish rolling finish temperature of Ar_3 transformation point or higher, and then coiled at a temperature of 300°C or higher. A method for manufacturing a hot-rolled steel sheet for processing, characterized by allowing it to cool. (3) After processing and forming the hot-rolled steel sheet for processing obtained by the manufacturing method according to claim 1 or 2 into a predetermined shape, heating it to a temperature of 400 to 750°C for 5 seconds or more, A process heat treatment method for a hot rolled steel sheet, which comprises cooling from a temperature of 400°C or higher to 200°C or lower at a cooling rate of 10°C/s or higher.