JP2006070346A - Steel sheet for hot press having excellent hydrogen embrittlement resistance, automobile member and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having a tensile strength of ≥980 MPa, having high workability and formability, and further having improved delayed fracture properties caused by hydrogen embrittlement, to provide an automobile member using the same, and to provide a production method therefor. <P>SOLUTION: The steel sheet for hot press has a composition comprising, by mass, 0.01 to 0.40% C, ≤2.0% Si, 0.01 to 3.5% Mn, ≤0.1% P, ≤0.05% S, 0.005 to 4% Al and ≤0.01% N, and the balance Fe with inevitable impurities, and has a microstructure comprising martensite as the maximum phase in area ratio, and has a tensile strength of ≥980 MPa, wherein an old austenitic structure is composed of a martensitic structure or a bainitic structure with a grain size of 3 to 50 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention provides a steel material for manufacturing a part that requires high strength, such as an automobile underbody / collision safety reinforcing member, and an automobile part using the steel material.

  In recent years, there has been a strong demand for high strength steel sheets for automobiles from the viewpoint of reducing fuel consumption of automobiles due to global environmental problems and improving collision safety. In general, however, high strength is associated with a decline in workability and moldability, as well as delayed fracture due to hydrogen embrittlement due to hydrogen penetration during product manufacture and use. High strength, high moldability, and hydrogen embrittlement resistance There is a need for a steel plate comprising: One answer to high strength and high formability is TRIP (Transformation Induced Plasticity) steel using martensitic transformation of retained austenite, and its use is expanding in recent years.

  However, with this steel, it is possible to produce a 1000 MPa class high strength steel plate with excellent formability, but it is difficult to secure formability with ultra-high strength steel such as a higher strength, for example 1500 MPa. Moreover, in the processing of TRIP steel, there is a concern about the deterioration of delayed fracture characteristics due to the transformation of retained austenite into martensite during member processing. Therefore, hot press is recently attracting attention as a method for producing a member having high strength, high formability, and water embrittlement resistance. This is to form a steel sheet heated to a high temperature of 800 ° C. or higher, thereby eliminating the problem of formability of the high-strength steel sheet and obtaining a desired material by cooling after forming.

  However, since it involves heating in the atmosphere, an oxide is generated on the surface and needs to be removed in a later step. An improvement of this is the invention disclosed in Japanese Patent Laid-Open No. 2000-38640 (Patent Document 1), which suppresses oxidation during heating by aluminizing a steel sheet containing 0.15-0.5% carbon. I am trying. This makes it possible to reduce surface oxidation during heating to a high temperature. However, since a member manufactured by hot pressing is quenched at the same time as molding from a high temperature in order to obtain a strength exceeding 980 MPa, the microstructure has a martensite phase with a volume ratio of 60% or more. For this reason, there is a concern about delayed fracture due to hydrogen embrittlement in a member having a particularly high strength exceeding 980 MPa.

JP 2000-38640 A

  The present invention provides a steel sheet having a tensile strength of 980 MPa or more, high workability and formability, and improved delayed fracture characteristics due to hydrogen embrittlement, an automobile member using the same, and a method for producing the same. With the goal.

  In order to overcome the problems as described above, the present inventors have studied in detail the influence factors on the hydrogen embrittlement resistance when processing a steel material or an aluminized steel sheet by hot pressing. Obtained knowledge. That is, the hydrogen embrittlement resistance of steel materials is greatly improved by optimizing the steel material components and manufacturing conditions. Moreover, it discovered that the hydrogen embrittlement-proof characteristic improved further by heat-processing this automotive member after shaping | molding for 1-1000 minutes in the temperature range of 150-700 degreeC. Details are as follows.

As a result of various investigations, the present inventors, as a technique for improving hydrogen embrittlement resistance in a region where the tensile strength is 980 MPa or more, limiting the microstructure and the component range, while maintaining the strength of 980 MPa or more. It has been found that the hydrogen embrittlement resistance after molding by hot pressing can be improved.
The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) By mass%, C: 0.01 to 0.40%, Si: 2.0% or less, Mn: 0.01 to 3.5%, P: 0.1% or less, S: 0.05 %, Al: 0.005 to 4%, N: 0.01% or less, the balance is composed of Fe and inevitable impurities, the microstructure has martensite as the largest area ratio, and the tensile strength is A steel sheet for hot press excellent in hydrogen embrittlement resistance, characterized by having a prior austenite structure of a martensite structure or a bainite structure having a particle size of 3 to 50 µm, which is 980 MPa or more.

(2) The steel sheet for hot press excellent in hydrogen embrittlement resistance as described in (1) above, further comprising W: 0.005 to 5% by mass% in the steel.
(3) The above (1) or (2), wherein the steel further contains one or two of Cu: 0.005 to 5% and Ni: 0.005 to 5% in mass%. The steel sheet for hot press excellent in hydrogen embrittlement resistance described in (1).

(4) The hydrogen embrittlement resistance according to any one of (1) to (3) above, wherein the steel further contains B: 0.0002 to 0.1% by mass%. Hot press steel plate with excellent properties.
(5) Further, in the steel by mass%, REM: 0.0005 to 0.01%, Y: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, Mg: 0.0005 The steel sheet for hot press excellent in hydrogen embrittlement resistance according to any one of the above (1) to (4), characterized by containing ~ 0.01% of one kind or two or more kinds.

(6) A hot press excellent in hydrogen embrittlement resistance, characterized by having a metal film mainly composed of Al on the surface of the thin steel plate having the composition according to any one of (1) to (5). Steel plate. (7) Excellent in hydrogen embrittlement resistance characterized by using the steel sheet according to any one of (1) to (6) above, wherein the steel structure has a martensite structure of 60% or more in area ratio. Automotive parts.

(8) A method for producing an automobile member having excellent hydrogen embrittlement resistance, wherein the automobile member according to (7) is heat-treated at a temperature range of 150 to 700 ° C. for 1 to 1000 minutes.
Here, the metal film mainly composed of Al is Al% plating containing Si: 1 to 15% and Mg: 0.5 to 10% in mass%. Zn may be added to the plating within a range of 1 to 60%.

  The present invention improves delayed fracture resistance after molding, which was a problem during hot pressing at high temperatures, and provides optimum strength as a reinforcement member for automobile bumpers and door impact beams, and delayed fracture resistance after processing. The steel plate and automobile member which improved the property can be obtained.

  Delayed fracture in tempered martensitic steel or the like is considered to be caused by the accumulation of hydrogen at the prior austenite grain boundaries and the like, resulting in fracture starting from that portion. Therefore, it is generally considered that the addition of various elements suppresses the coarsening of the austenite grain size during high-temperature overheating and improves the hydrogen embrittlement resistance of the steel material. By optimizing the slab reheating conditions and hot rolling conditions, the austenite grain size was reduced and the hydrogen embrittlement resistance was improved. Furthermore, it is possible to remove diffusible hydrogen in the steel material by heat-treating the member after molding by hot pressing in a temperature range of 150 to 700 ° C. for 1 to 1000 minutes. It was found that the concentration was decreased.

  Usually, a thin steel plate cold-rolled material is subjected to molding by press, so that the member has a high residual stress after molding and is considered to work against delayed fracture characteristics. Further, as the strength of the steel material increases, the amount of spring back after molding increases, making it difficult to obtain a predetermined member shape. Therefore, the present inventors performed hot press molding in order to improve workability and reduce the amount of springback after molding, and further, steel material components and slab reheating conditions and hot rolling conditions before hot heating. We succeeded in improving the hydrogen embrittlement resistance by optimizing the heat treatment and devising the heat treatment after molding.

That is,
(1) Austenite grain size refinement by optimizing slab reheating conditions and hot rolling conditions before hot rolling.
(2) Reduction of residual stress during molding by adopting a hot press method.
(3) Dehydrogenation treatment by heat treatment after hot press molding.
By controlling these, it is possible to improve the toughness of steel materials effective for hydrogen embrittlement resistance, reduce residual stress, and reduce the initial diffusible hydrogen concentration by dehydrogenation treatment. Can be improved. For this purpose, it is important to control the steel material components, slab reheating conditions before hot rolling, and hot rolling conditions.

Next, the microstructure will be described.
The reason why the phase having the largest area ratio is martensite is to obtain a tensile strength of 980 MPa or more, preferably 1200 MPa or more, and more preferably 1500 MPa or more. % Or more, preferably 80% or more, more preferably 95% or more and 100% or less. However, the area ratio of 100% mentioned here naturally has unavoidable impurities and inclusions in the steel, and is not strictly 100%. However, these unavoidable impurities and inclusions are not observed by observation with an optical microscope. Is 100% because it exists at a level that cannot be recognized.

The present invention is described in further detail below.
First, the reasons for limiting the chemical components of steel in the present invention will be described.
C is an element that can increase the strength of the steel sheet. In particular, it is an essential element for generating a hard phase such as martensite and bainite and increasing the strength, and in order to obtain a strength of 980 MPa or more, the mass% (the same shall apply hereinafter) of 0.01% or more is required. On the contrary, if it is contained in a large amount, the cementite that becomes the starting point of brittle fracture is increased, so that hydrogen embrittlement is likely to occur. Therefore, the upper limit was made 0.4%.

  Si is a substitutional solid solution strengthening element that hardens the material greatly, and is effective in increasing the strength of the steel sheet, and is an element that suppresses cementite precipitation. Since it is costly to remove the scale and is economically disadvantageous, 2.0% is made the upper limit. Although the lower limit is not particularly defined, it is desirable to add 0.005% or more because extremely low causes an increase in manufacturing cost.

Mn is an element effective for increasing the strength of the steel sheet. However, since this effect cannot be obtained if the content is less than 0.01%, the lower limit is set to 0.01%. On the other hand, if the amount is large, not only co-segregation with P and S is promoted, but also workability may be deteriorated, so 3.5% or less, preferably 3.0% is set as the upper limit.
P is an element that promotes grain boundary fracture due to grain boundary segregation, and a lower value is desirable, but extremely low is not preferable in terms of manufacturing cost. Moreover, since it is an element which degrades corrosion resistance, the upper limit is made 0.1%.

S is an element that promotes hydrogen absorption in a corrosive environment, and a lower value is desirable, but extremely low is not preferable in terms of manufacturing cost. In particular, in order to improve the workability, a lower value is desirable and the upper limit is set to 0.05%.
Al is added in an amount of 0.005% or more for deoxidation, but inclusions such as alumina increase and the workability deteriorates as the addition amount increases, so 4.0% is made the upper limit.
N is better because it contributes to workability deterioration and blowhole generation during welding. If it exceeds 0.01%, the workability deteriorates, so 0.01% is made the upper limit.

W is an extremely important element because it is effective for increasing the strength of the steel sheet, and precipitates and crystallized substances containing W become hydrogen trap sites. However, since these effects cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the other hand, if the content exceeds 5%, the workability deteriorates, so the upper limit was made 5%.
Cu is effective for strengthening, and the fine precipitation of confidence contributes to the improvement of delayed fracture, so 0.005% or more was added. Moreover, since excessive addition causes deterioration of workability, the upper limit was made 5.0%.

Ni has the effect that Ni sulfide suppresses hydrogen penetration and improves delayed fracture characteristics, and the effect of ensuring the strength of the steel sheet by enhancing the hardenability of the steel sheet. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. On the contrary, if it exceeds 5%, the workability deteriorates, so the upper limit was set to 5%.
B is an element effective for increasing the strength of the steel sheet. However, since these effects cannot be obtained if the content is less than 0.0002%, the lower limit is set to 0.0002%. On the other hand, if the content exceeds 0.1%, the hot workability deteriorates, so the upper limit was made 0.1%.

  Mg is not only effective for improving delayed fracture resistance of its own compound, but also controls to form composite precipitates with other elements or composite precipitates, and to contribute their morphology to improved delayed fracture resistance. Since it is an element necessary for this, the content is set to 0.0005% or more. However, if it exceeds 0.01%, coarse oxides and sulfides are produced, which is not effective for shape control, and lowers the workability, which is a basic required characteristic of a thin steel sheet, so the upper limit is 0.01%. It was.

  REM, Ca, and Y are effective for controlling the form of inclusions and contribute to delayed fracture resistance, so 0.0005% or more was added. On the other hand, excessive addition deteriorates hot workability, so 0.01% or less was added. Here, REM is an abbreviation for Rare Earth Metal and is a general term for lanthanoid elements starting from La.

Next, the prior austenite grain size will be described. The grain size of the prior austenite structure of the steel sheet is defined as the former austenite grain boundary appearing using a picric acid alcohol solution and the average grain size of the prior austenite grains measured using a microscope and an image processing apparatus.
The reason why the range of the prior austenite grain size is 3 to 50 μm is that the upper limit is set to 50 μm and the lower limit is set to 3 μm because the contribution of the refinement of the prior austenite grain size to the hydrogen embrittlement resistance is small at grain sizes exceeding 50 μm. The reason for this is that it is difficult to produce a prior austenite grain size of less than 3 μm in heating at 700 to 1000 ° C. during the production of automobile parts by the hot press method.

Next, the manufacturing method of the steel plate of this invention is described. Here, the manufacturing method may be a generally used hot-rolled steel sheet, cold-rolled steel sheet, or Al-plated steel sheet manufacturing facility. In order to refine the prior austenite grain size, it is important to control the slab reheating conditions before hot rolling and the hot rolling conditions during steel plate production.
The slab adjusted to a predetermined component is directly or once cooled and then reheated to perform hot rolling. Control of the heating temperature in this reheating is important, and it is desirable that the temperature be 1050 ° C. or more and less than 1200 ° C. This is because when the reheating temperature becomes high, the austenite grain size becomes coarse and a thick oxide scale is formed, and the upper limit is set to 1200 ° C. On the other hand, since the rolling resistance is increased by low-temperature heating, the lower limit is set to 1050 ° C.

Next, conditions for hot rolling will be described. Accumulation of processing strain in the austenite temperature region is important for fine graining. Fine recrystallized austenite grains can be obtained by rapidly cooling to the ferrite region after large pressure reduction in the austenite region in the last two stands of hot rolling. In order to perform this control, it is necessary that the temperature of the steel sheet in the last two stands is in the range of Ar ₃ point or higher and Ar ₃ point + 100 ° C. and the rolling reduction is 25% or higher. The lower limit value of the temperature was Ar ₃ point, this is less than the temperature ferrite is precipitated from austenite, because it is not obtained a sufficient potency to grain refining of austenite, the lower limit was set to _3-point Ar. The reason why the upper limit is Ar ₃ point + 100 ° C. is that if the temperature during hot rolling is high, recrystallization and coarsening of austenite grains are promoted, and a sufficient refinement effect cannot be obtained. .

  Further, the hot rolling reduction ratio in the final two stands is set to 25% or more and less than 70%. The reason why the lower limit is set to 25% is that sufficient work strain cannot be accumulated in austenite at a rolling reduction of less than 25%. The reason why the upper limit is set to less than 70% is that the steel sheet may be broken during the manufacture at a rolling reduction of 70% or more. Desirably, the slab reheating temperature before hot rolling is 1100 ° C., and as the hot rolling conditions, it is desirable to finish rolling at a rolling reduction of 25% to 40% in the final two stands.

  After hot rolling, it is cooled at a cooling rate of 0.1 to 1000 ° C./second to a coiling temperature range of 400 ° C. to 700 ° C. Lowering the cooling stop temperature below 400 ° C increases the strength of the hot-rolled sheet and increases the load during cold rolling after pickling, so the lower limit is 400 ° C. If the upper limit of the cooling stop temperature is higher than 700 ° C, ferrite Precipitation may be insufficient, and the austenite grain size may become coarse.

  Moreover, since it takes a long time for the cooling time to be slower than 0.1 ° C./sec and it is difficult to operate, the lower limit of the cooling rate is set to 0.1 ° C./sec. Similarly, if the cooling rate exceeds 1000 ° C./second, it is difficult to operate, so this was made the upper limit. The cooling after hot rolling is preferably performed at about 550 ° C. at a cooling rate of 20 ° C./second or more in order to avoid coarsening of the austenite grain size.

In cold rolling after pickling, if the rolling reduction is low, it becomes difficult to correct the shape of the steel sheet, so the lower limit is preferably 30%. In addition, when rolling at a rolling reduction exceeding 80%, it is preferable to set the upper limit value to 80% because of the occurrence of cracks in the edge portion of the steel sheet and the disorder of the shape.
If the continuous annealing temperature is too low, it becomes non-recrystallized and hardens. On the other hand, if it is too high, the grains become coarse and sufficient improvement in hydrogen embrittlement resistance cannot be expected. Is desirable. Among these, annealing at 800 to 850 ° C. is most desirable.

  After annealing, it is preferable to cool to a temperature range of 25 ° C. to 500 ° C. at a cooling rate of 0.1 to 1000 ° C./second, and subsequently hold in the same temperature range for 1 second to 10000 seconds. Making the cooling rate slower than 0.1 ° C./second requires a huge amount of cooling time and is difficult to operate, so the lower limit of the cooling rate was set to 0.1 ° C./second. On the other hand, since it is difficult to operate at a cooling rate of 1000 ° C./second or more, the above range is specified. Further, since it is difficult to operate the cooling stop temperature lower than 25 ° C., the lower limit is set to 25 ° C., and if it is higher than 500 ° C., the crystal grains are coarsened. It is difficult from the performance of the production line to make the holding time at the cooling stop temperature shorter than 1 second, and it is difficult from the manufacturing cost to make it longer than 10000 seconds.

Next, the manufacturing method of the member for motor vehicles is described.
The hot press molding conditions are as follows. The temperature of a cold-rolled steel sheet or Al-plated steel sheet manufactured under the above-described conditions is raised to 700 to 1000 ° C., held for 10 to 6000 seconds, and immediately placed on a press die. Press. It is preferable that the temperature increase rate at this time shall be 1-100 degree-C / sec. This is because when the heating temperature is lower than 700 ° C., the structure before quenching does not become an austenite single phase, but quenching from two phases of ferrite and austenite, and a predetermined strength cannot be obtained. If it is high, the austenite grain size becomes coarse and there is a concern about deterioration of hydrogen embrittlement resistance after quenching. If the rate of temperature rise is slower than 1 ° C / second, the production efficiency may be lowered, and 100 ° C / second. Making it faster is not possible with normal furnace heating.

  Further, if the holding time at the maximum temperature reached is less than 10 seconds, the predetermined temperature is not reached inside the steel sheet, and the ferrite is not reversely transformed inside the steel sheet, and a predetermined strength cannot be obtained after quenching. Therefore, it is set to 10 seconds or more. Further, if held for longer than 6000 seconds, austenite grains become coarse, and sufficient hydrogen embrittlement resistance cannot be obtained after roasting. The cooling rate during pressing is preferably 1 to 500 ° C./second. The reason why the lower limit of the cooling rate at the time of pressing is 1 ° C./second is that, at a cooling rate slower than this, no quenching occurs and a predetermined strength cannot be obtained. On the other hand, since it is difficult to manufacture at a rate faster than 500 ° C./second, the upper limit is set to 500 ° C./second.

  Moreover, the more superior hydrogen embrittlement resistance can be obtained by heating the automobile member produced in the above process to a temperature range of 150 to 700 ° C. This is because hydrogen that has penetrated into the steel material during production escapes into the atmosphere by heat treatment at 150 ° C. or higher and works effectively against cracks. The reason for setting the heat treatment time to 1 to 1000 minutes is that if less than 1 minute, the dehydrogenation treatment is insufficient, so the lower limit is set to 1 minute. The upper limit was 1000 minutes.

Next, an Example demonstrates this invention in detail.
Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. Reference signs A to F are steels of the components according to the present invention, and reference signs G and H are components that deviate. These steels were reheated in a heating furnace under the conditions shown in Tables 2 and 3 and then hot-rolled to obtain hot-rolled steel sheets having a thickness of 3 mm. The hot-rolled sheet was made 1.5 mm thick by 50% cold rolling, and a cold-rolled steel sheet and an Al-plated steel sheet were produced. The heat history of the cold-rolled steel sheet was maintained at 5.6 ° C./second for 90 seconds after reaching the maximum heating temperature of 790 ° C., cooled to 490 ° C. at 40 ° C./second for 300 seconds, and then cooled to room temperature. The temperature history of the Al-plated steel sheet was maintained at a maximum heating of 790 ° C. for 90 seconds, then immersed in an Al bath at 490 ° C., and the amount of plating adhered was adjusted to 25 μm per side by gas wiping. The plating composition at this time contained Si, Cr, and 2% Fe in the table in addition to Al as the main component, but Fe is inevitable supplied from equipment and strips in the bath.

The cold-rolled steel sheet and Al-plated steel sheet thus manufactured were evaluated for tensile strength and hydrogen embrittlement resistance. The evaluation method of hydrogen embrittlement resistance was as follows: a 100 mm × 30 mm strip test piece was heated to 940 ° C., then U-bent to 10R, and a water-resistant strain gauge was attached to the surface, followed by 0.5 mol / l sulfuric acid. The sample was soaked in it and electrolyzed with an electric current to infiltrate hydrogen, and the occurrence of cracks after 2 hours was evaluated. Bending radius and 10 mm, the stress applied respectively as 60 kgf / mm ² and 90 kgf / mm ^2. In addition, the steel plates A to H, which were subjected to heat treatment at 200 ° C. and 500 ° C. for 1 hour after hot press molding, were also evaluated for hydrogen embrittlement resistance.

As shown in Tables 2 and 3, in the production conditions a and b of the steels A to F of the present invention steel, the tensile strength sufficient to be applied to automobile reinforcing parts is shown, and the hydrogen embrittlement resistance described above Since no cracks occurred in the characteristic test, the hydrogen embrittlement resistance is excellent. On the other hand, in the production conditions cf in the comparative steels G steel, H steel, and A to F steel, the predetermined strength is obtained because the components or the production conditions deviate from the scope of the present invention. The material has poor hydrogen embrittlement resistance. Furthermore, when a dehydrogenation treatment at 200 ° C. × 1 h was performed after hot pressing, the time until the occurrence of cracking was extended in all the test pieces. Therefore, it is clear that the initial diffusible hydrogen concentration is reduced by the dehydrogenation treatment, which effectively works to improve the hydrogen embrittlement resistance.


Patent applicant: Nippon Steel Corporation
Attorney Attorney Shiina and others 1

Claims (8)

% By mass
C: 0.01-0.40%,
Si: 2.0% or less,
Mn: 0.01 to 3.5%
P: 0.1% or less,
S: 0.05% or less,
Al: 0.005 to 4%
N: 0.01% or less, the balance is Fe and inevitable impurities, the microstructure has martensite as the largest phase in area ratio, the tensile strength is 980 MPa or more, and the prior austenite structure has a particle size of 3 A steel sheet for hot pressing excellent in hydrogen embrittlement resistance, characterized by comprising a martensite structure or a bainite structure of -50 μm. Furthermore, in steel,
W: 0.005 to 5%
The hot-press steel sheet having excellent hydrogen embrittlement resistance according to claim 1, comprising: Furthermore, in steel,
Cu: 0.005 to 5%
Ni: 0.005 to 5%
The steel sheet for hot press excellent in hydrogen embrittlement resistance according to claim 1 or 2, characterized by containing one or two of the following. Furthermore, in steel,
B: 0.0002 to 0.1%
The hot-press steel sheet having excellent hydrogen embrittlement resistance according to any one of claims 1 to 3, wherein the steel sheet is excellent in hydrogen embrittlement resistance. Furthermore, in steel,
REM: 0.0005 to 0.01%,
Y: 0.0005 to 0.01%
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%
One type or two types or more of these are contained, The hot-press steel plate excellent in the hydrogen embrittlement resistance of any one of Claims 1-4 characterized by the above-mentioned.   A steel sheet for hot press excellent in hydrogen embrittlement resistance, comprising a metal film mainly composed of Al on the surface of a steel sheet having the composition according to any one of claims 1 to 5.   An automotive member having excellent hydrogen embrittlement resistance, wherein the steel sheet according to any one of claims 1 to 6 having a martensite structure with an area ratio of 60% or more is used.   A method for producing an automotive member having excellent hydrogen embrittlement resistance, wherein the automotive member according to claim 7 is heat-treated at a temperature range of 150 to 700 ° C for 1 to 1000 minutes.